EPR IN THE 21StCENTURY: BASICS AND APPLICATIONS TO MATERIAL, LIFE AND EARTH SCIENCES
The participants of the Third Asia-Pacific EPR/ESR Symposium (APES’Ol), 29 October- 1 November 2001, Kobe, Japan.
EPR IN THE 21StCENTURY BASICS AND APPLICATIONS TO MATERIAL, LIFE AND EARTH SCIENCES Proceedings of the Third Asia Pacific EPRESR Symposium Kobe, Japan, October 29 - November 1,2001
Edited by
Asako Kawamori Faculty of Science Kwansei Gakuin University Gakuen 2-1 Sanda 669-1337, Japan Jun Yamauchi Graduate School of Human and Environmental Studies Kyoto University Kyoto 606-8501, Japan
Hitoshi Ohta Molecular Photoscience Research Center and Dept. of Physics Kobe University 1- 1 Rokkodai, Nada Kobe 657-8501, Japan
2002
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2002 Elsevier Science B.V.
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Editorial Note The Third Asia-Pacific EPR/ESR Symposium (APES'Ol), our first meeting in the 21" century, was held from October 29 to November 1, 2001 in Kobe, Japan. About 220 participants from 20 countries from Asia, Australia, Europe, North and South America presented 210 papers. The Proceedings not only record the Symposium presentations but are also aimed as a blueprint for development of EPR/ESR in the Asia-Pacific region in the 21Sfcentury. The Symposium reflected the variety of research fields developed over half of a century since Zavoisky's discovery of EPR in 1944. Especially the most recent developments, such as high-field and high-frequency EPR, are envisaged to be firther developed and applied to various fields in the 21Sfcentury. Special topical session were devoted to these techniques at APES'Ol. All sessions consisted of Plenary, Invited and Contributed presentations. Poster sessions were also organized. The Plenary presentations, given to all participants, aimed at summarizing the overall developments. Invited presentations, reviewing the most recent developments, and Contributed ones, dealing with original research recently carried out in the EPR/ESR area, were given in one of three parallel sessions. Researchers from AsiaPacific countries as well as Europe and the United States presented their unique research works, which covered various fields and reflected the existing diversity of applications of the EPR/ESR techniques. The additional new arrangement of this Symposium was the introduction of two satellite meetings, Symposium A: 2001 ESR Dosimetry & Dating and Symposium B: International Workshop on Advanced EPR Applied to Biosciences. The organizers express their gratitude to Prof. J. Pilbrow, President of the International EPR Society, for his participation. Prof John Pilbrow also presented Prof. S. Yamauchi of Tohoku University with the IES Silver Medal. The organizers regretted that some researchers cancelled their participation due to the terrorists attacks in USA on September 11. Prof. A.A. Romanyukha (USA) contributed his plenary lecture paper in absentia. This Proceedings volume includes the papers submitted within the time limit set by the publisher. All papers have been refereed by the members of the Review Panel who are experts in their own areas. The efforts of all the reviewers and Elsevier Science in achieving the high scientific standard and technical quality of the Proceedings are gratefully acknowledged. The editors express their gratitude to the Commemorative Association for the Japan World Exposition (1970) for the major financial support of this Symposium. Asako Kawamori Professor of Kwansei Gakuin University Jun Yamauchi Professor of Kyoto University Hitoshi Ohta Professor of Kobe University
The APES President's Message It is a great pleasure for me, on behalf of the Council of the Asia-Pacific EPIUESR Society [APES], to present the Proceedings of the Third Asia-Pacific EPR/ESR Symposium [APES'OI] to the scientific community. This Symposium has been organized, under the Society, by Kobe University and the Local Organizing Committee ably headed by Professor Asako Kawamori. As the Founding President of the Society and then an elected one for two more terms, it is particularly satisfying to me to see the Asia-Pacific EPIUESR Society and the AsiaPacific EPIUESR Symposia grow in strength and maturity. Continuing with the spirit of the First APES, held at the City University of Hong Kong in 1997, and the Second APES held due to the efforts of Professor Yuanzhi - at the Zhejiang University in 1999, this Symposium, although aimed primarily at the Asia-Pacific countries, has also been open to participants from all over the world. The main aims of our Symposia are to bring together as many EPIUESR spectroscopists as possible and to promote and facilitate collaboration among the EPIUESR community. For the first time two Satellite Meetings: 2001 ESR Dosimetry and Dating and International Workshop on Advanced EPR Applied to Biosciences, have also been arranged. This is a very positive development, which shows that the idea of the APE Symposia is expanding and attracting other specialized areas of applications of EMR (encompassing EPR and ESR). Thanks are due to Prof. M. Ikeya and Prof. A. J. Hoff for their effort in organizing, in coordination with Prof. A. Kawamori, the Symposium A and B, respectively. During the Symposium the 3rd Meeting of the Asia-Pacific EPR/ESR Society was held. We were privileged to have with us Prof. John R. Pilbrow, President of the International EPIUESR Society, who delivered an Address to the participants of the Meeting. It was decided at the APES Meeting that the fourth Symposium (APES'03) will be held at the Indian Institute of Science in Bangalore in November 2003, with Prof S. V. Bhat as the Chairman of the Local Organising Committee (LOC) for APES'03. An International School on EPIUESR Spectroscopy with tutorial sessions for students and researchers will be held at Bhabha Atomic Research Centre in Mumbai prior to the APES'03, with Prof. K.P. Mishra as the Chairman of the OC for the School. All materials related to the APES'O1 as well as the APES Meeting will be available on the APES Web site at http://~.ied.edu.hk/has/phvs/apepr. The tentative schedule (to be confirmed) of the future Asia-Pacific EPR/ESR Symposia is: APES'OS in Korea, APES07 in Russia (Vladivostok), and APES09 in Australia. Please visit the APES Web site for updated information on the Asia-Pacific EPIUESR Symposia and the Society. On behalf of the APES Council I would like to thank all hard working Members of Members of the APES'O1 Local Organising Committee for their dedicated effort. Under the skilful leadership of Prof. A. Kawamori, helped especially by Prof. H. Ohta (Secretary) and Prof. T. Takui (Treasurer), APES'Ol was a very successful Symposium. Support from the International Advisory Committee in nominating the invited speakers and maintaining the
vii
high standard of this Symposium is also much appreciated. I also wish to express the Society’s gratitude to all sponsors for their financial support. Thanks are due to all speakers and participants, who by attending this meeting have helped to make this Symposium a successful event. Special thanks are due to Prof. A. Kawamori for her dedicated two-terms service as the Vice-president of the Asia-Pacific EPR/ESR Society. Czeslaw Rudowicz President of The Asia-Pacific EPR/ESR Society Professor of City University of Hong Kong
CONTRIBUTORS to Proceedings 10 Plenary lectures:
Noboru Hirota (Japan) Zohn-Li Liu (Chaina) Masaki Oshikawa (Japan)
Lawrence Berliner (USA) Jack H. Freed (USA) Reiner. Grun (Australia) Larry. Kevan (USA) Shin-ichi Kuroda (Japan)
Czeslaw Rudowicz
Kong)
Tetsuhiko Yoshimura(Japan
18 Invited Lecutures:
Seiji Miy ashita(Japan)
C . A. h e r l a a n (Netherlands) Oswald Baffa (Brazil) Sergei A. Dzuba (Russia) Arnold J. Hoff (Netherlands) Sa-Oak Kang (Korea) Yang Liu (China) Wolfgang Lubitz (Germany) David J. Lurie (England) Sushi1 Misra (Canada)
John Pilbrow (Australia)
B.J. Reddy (India) Rafic G. Saifutdinov (Russia) Kev Salikhov (Russia) R.J. Singh (India) Yuri D. Tzvetkov (Russia) Hideo Utsumi (Japan) Nicolla Yordanov (Burgaria)
Reviewers: L. Berliner: Univ. of Denver, USA S.V. Bhat S. Demishef J. Freed: Cornell Univ,. USA R. Grun M. Ikeya: Osaka UniyJapan A. Kawamori: Kwansei Gakuin Univ., Japan
R. Kevan: Univ. of Houston, USA S . Kuroda: Nagoya Univ. Japan Y. Li W. Lubitz M. Mino: Okayama Univ. Japan K.P. Mishra S.K. Misra: Concord Univ., Canada
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R. Miyamoto S. Miyashita: Tokyo Univ., Japan M. Motokawa: Tohoku Univ., Japan T. Nakamura H. Ohta: Kobe Univ., Japan J. Pilbrow: Monash Univ.. Australia C. Rudowicz: City Univ., Hong Kong K. Salikhov
C. Shiomi H. S. So S. Tero-Kubota H. Utsumi: Kyushu Univ., Japan J. Yamauchi: Kyoto Univ., Japan S. Yamauchi: Tohoku Univ., Japan N. Yordanov T.Yoshimura
COMMITTEES of APES01 International Organizing Committee
Local Organizing Committee
President:
Chairperson:Prof. Asako Kawamori Treasurer: Prof. Takeji Takui Members: Prof. Hiroaki Ohya Prof. Jun Yamauchi Prof. Motoji Ikeya
Prof.
Czslaw
Z.
Rudowicz
OIongKong) Vice President: Prof. Asako. Kawamori (Japan) Members: Prof. Albert M. Ziatdnov (Russia) Prof. Hyunsoo So (South Korea), Prof. Yong. Li (China), Prof. Hitoshi Ohta (Japan), Dr. Y.Y Yeueng(Hong Kong), Dr. Tien Tai Nguyen (Vietnam), Prof. S.V. Bhat (India), Dr. Graem R. Hanson (Australia).
Local Advisory Members Prof. Keiji Kuwata Prof. Hiroshi Watari Prof. Noboru Hirota
Scientific Program Committee
International Advisory Members Prof. Lawrence J. Berliner: (USA) Prof. Muneyuki Date: (Japan) Prof. Arnold J. HOE(Netherlands) Prof. Come1is.A.J.Ammerlaan: (Netherlands)
Institutional Sponsors Commemorative Association for the Japan World Exposition (1970) Nakauchi Foundation for Convention Portonia. 8 1 Foundation
Prof. Jun Yamauchi Prof. Mitsuhiro Motokawa: Prof. Shozo Tero: Dr. Hiroshi Hori Prof. Chihiro Yamanaka Prof. Hideo Utsumi Dr. Tetsuhiko Yoshimura
Industrial Sponsors Bruker Biospin K.K. JEOL co.
Ltd
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CONTENTS Page Editorial Note President's Message Contributors Committees
V
vi vii
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Section 1. Physics and Magnetism Plenary Lectures Electron magnetic resonance (EMR) of the spin S>1 systems: an overview of major intricacies awaiting unwary spectroscopists ......................................... Czeslaw RUDOWICZ
3
Recent developments in low-temperature ESR in quantum antiferromagnetic chains .......... 15 Masaki OSHIKAWA Invited Lectures Direct numerical study on ESR line shape ............................................................................. Seiji MIYASHITA and Akira OGASAHARA
27
g Tensor of Er3+centers in axial symmetry ............................................................................ C. A. J. AMMERLAAN
33
Continuous wave and pulsed EPR spectroscopy of paramagnetic ions in some fluoride, silicate and metaphosphate glasses ................................................................ S. C. DREW and J. R. PILBROW
39
General Papers Frequency dependence of resonance in one-dimensionalantiferromagnetic Heisenberg chain .................................................................................................................... Akira OGASAHARA and Seiji MIYASHITA
48
ESR selection rules for direct transition of spin gap .............................................................. T8ru SAKAI, Nobuhisa OKAZAKI, Takashi OHNISHI and Shuhei TAJCEMURA
54
Spin solitons in the alternate charge polarization background of MMX chains .................... Makoto KUWABARA, Kenji YONEMITSU and Hitoshi OHTA
59
Full Monte Carlo and Fourier transformed Monte Carlo EPR spectral simulations of 8s state ions ....................................................................................................................... Tomoharu TAJCEYAMA, Takato NAKAMURA, Naoyulu TAKAHASHI Veltran BELTRAN-LOPEZ and Christopher C. ROWLANDS
63
X-Band ESR measurements of spin ladder system BIP-TEN0 ............................................. Keizo KIRITA, Takahiro SAKURAI, Hitoshi OHTA, Yuko HOSOKOSHI Keiichi KATOH and Katsuya INOUE ESR Studies of a spin1/ 2 antiferromagnetic tetramer chain ........................................................................................... Masayulu HAGIWARA
69
73
ESR Studies of quasi-one-dimensional halogen-bridged mixed-metal complexes ............... 79 H. TANAKA, K. MARUMOTO, S. KURODA, T. MANABE, S. FURUKAWA and M. YAMASHITA Microwave radiation from magnetostatic mode in high power FMR .................................... Michinobu MINO, Hideaki TSUKUDA, Masayuki TSUKAMOTO and Hitoshi YAMAZAKI
85
Parametrically excited magnetoelastic waves in FeB03 ........................................................ Kenji ISHIHARA, Michinobu MINO, Masayuki TSUKAMOTO and Hitoshi YAMAZAKI
89
Slow dynamics in chaotic magnon system ............................................................................ Jiang CAI, Yoshiyasu KATO, Atsusi OGAWA, Takayuki HIRATA, Meiro CHIBA and Yoshifumi HARADA
93
ESR studies of spin-polarized atomic hydrogen adsorbed on 3He-4Hemixture film ............ 97 A. FUKUDA, T. OHMI, H. TAKENAKA, Y. WAKI, A. MATSUBARA and T. MIZUSAKI
Section 2. Materiais Sciences Plenary Lectures Electron spin resonance studies of molecular photoionization in Cr-AlMCM-41 mesoporous oxide materials ................................................................................................. Sunsanee SINLAPADECHand Larry KEVAN
105
ESR and ENDOR spectroscopy of solitons and polarons in conjugated polymers ............. 113 Shin-ichi KURODA Invited Lecture ESR study of deoxygenated high-temperature superconductorsand their constituents ...... 125 R. J. SINGH, P. K. SHARMA and Shakeel KHAN General Papers Magnetic phase separation in Lal,Ca,MnO3 near half-doped composition observed by EMR ................................................................................................................. Alexander I. SHAMES, Andrey Yu. YAKUBOVSKY and Stanislav V. GUDENKO
127
xi
Temperature rises by microwave absorptions in superconducting materials and liquid nitrogen bubbling ....................................................................................................... T. LI, A. HASHIZUME, K. ITOH, H. KOHMOTO, S. IWASAKI, M. YAMASAKI, K. YAMAGUCHI, T. ENDO and M. SHAHABUDDIN Comparison of near-Tc behaviors of microwave absorption and resistance in bulk YBCO superconductors .......................................................................... H. KOHMOTO, S. IWASAKI, M. WAKUTA, H. SARATANI, T. ENDO, S. UTHAYAKUMAR and R. DHANASEKARAN Estimation of fluxon response-delay from magnetic field variations in superconductors using ESR system ................................................................................................................. R. D. KALE, M. TADA, K. ITOH, H. KOHMOTO, S. IWASAKI, Y. NAKAUE, T. ENDO and S. V. BHAT Coexistence of ferromagnetism with superconductivity in RuSr2GdCu208 from ESR measurements ...................................................................................................... Koji YOSHIDA, Masatoshi NAKAMURA, Naoto HIGASHI and Hajime SHIMIZU Ferromagnetic resonance and intragraidintergrain crystallinity in La-Ba-Mn-0 thin films ........................................................................................................ S. IWASAKI, J. YAMADA, H. KOHMOTO, M. TADA, Y. SAKURAI, T. ENDO and B. J. REDDY Temperature dependence of paramagnetic resonance in pure and doped ferrihydrite nanoparticles ..................................................................................................... A. PUNNOOSE and M. S. SEEHRA
133
139
145
151
157
162
ESR study of Fe-SiOz granular films ................................................................................... Kazuaki KANAZAWA, Kouichi MATSUDA, Seitarou MITSUDO, Toshitaka IDEHARA, Sigeo HONDA
168
Oxygen dependent evolution of C6: EPR signal in fullerene thin films ............................. Alexander I. SHAMES, Eugene A. KATZ, Svetlana SHTUTINA, Wojciech KEMPIkXI, Szymon LOS and Lidia Piekara-SADY
174
Molecular orientations in Langmuir-Blodgett and vacuum-deposited films of VO-phthalocyanine .......................................................................................................... Yuhei SHIMOYAMA Structural elucidation of vacuum deposited films of titanyl phthalocyanine by EPR Hiroyuki KAJI, Yuhei SHIMOYAMA
180
......... 186
ESR investigation of organic conductor with itinerant and local spins, (CHTM-TTP)zTCNQ ........................................................................................................... T. NAKAMURA, M. TANIGUCHI, Y. MISAKI, K. TANAKA and Y. NOGAMI
192
X-band ESR measurements of Et2Me~P[Pd(dmit)z]2........................................................... T. SAKURAI, H. OHTA, S. OKUBO, R. KATO and T. NAKAMURA
197
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The role of Li' and Na' charge compensators in Sm3+-dopedCaF2 and SrF2 ..................... M. YAMAGA, M. HONDA, N. KAWAMATA, K. SAMEJIMA and J.-P. R. WELLS
201
The HF and SHF interactions of V02+ions in KZnClS04.3H20 single crystals ................ 207 K. V. NARASIMHULU, B. DEVA PRASAD RAJU and J. LAKSHMANA RAO EPR study of several C? centers in K2MgC14 single crystal .............................................. H. TAKEUCHI, H. TANAKA, M. MORI, H. EBISU and M. ARAKAWA
2 13
...............................................
2 19
EPR study of Cr3+centers in T12MgF4 and T12ZnF4 crystals M. ARAKAWA, H. EBISU and H. TAKEUCHI
Single crystal EPR study of Cr(II1)-doped magnesium potassium Tutton's salt .................. 225 H. ANANDALAKSHMI, P. NEERAJA, R. VENKATESAN, T. M. RAJENDIRAN and P. Sambasiva RAO ESEEM study of 14Nnuclear quadrupole resonances in S=3/2 chromium(II1) complex .... 231 Shoji UEKI and Jun YAMAUCHI EPR investigation of inhomogeneous phases in improper ferroelastic MgTiF6.6H20:Mn2+ ............................................................................................................. A. M. ZIATDINOV and P. G. SKRYLNIK
236
EPR investigations on Fe3+ions in alkali borotellurite glasses ............................................ P. Giri PRAKASH, A. MURAL1 and J. Lakshmana RAO
242
EPR study of X-ray and UV irradiated GeO2 glasses prepared by the sol-gel method K. KOJIMA, F. OGURA, N. WADA, K. YAMAMOTO, T. FUJITA and M. YAMAZAKI
....... 247
Structural studies of the fresh water (Apple) snail, globosa shells .............................. K. V. NARASIMHULU, C. P. Lakshmi PRASUNA, T. V. R. K. RAO and J. Lakshmana RAO
253
ESR study of iron-sites on Fe-ZSM5 zeolite ....................................................................... Nguyen Tien TAI, Nguyen Huu PHU, Tran Thi Kim HOA, Hidenobu HORI and Makoto TAKI
259
CW/pulsed ESR studies of Euz+-dopedSrA1204 phosphor ................................................. H. MATSUOKA, K. SATO, D. SHIOMI, Y. KOJIMA, K. HIROTSU, N. FURUNO and T. TAKUI
264
Thermoluminescent mechanism of tridymite SiOz phosphors ............................................ Masatoshi OHTA, Takato NAKAMURA and Michiko TAKAMI
270
ESR and luminescence spectral properties of europium compounds with trifluoroacetic acid ............................................................................................................... I. V. KALINOVSKAYA, A. N. ZADOROZHNAYA, V. G. KURYAVYI and V. E. KARASEV
276
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ESR and luminescence studies on formation of ions through photochemical reactions in potassium halide crystals doped with sulfur and manganese ........................... SARJONO, M. BABA, K. OHTA, K. NISHIDATE, I. SOKOLSKA and W. Ryba-ROMANOWSKI
282
Hyperfine structure ofNd3+and Er3' ions in LiNbO3 crystals ............................................ 11-Woo PARK, Sung Ho CHOH, Sang Su KIM, Kuk KANG and Deok CHOI
288
The nature of coduction ESR linewidth temperature dependence in graphite ..................... A. M. ZIATDINOV and V. V. KAINARA
293
ESR measurement of heavily doped Si:Fe ........................................................................... Ryouhei KOYAMA, Junk0 YOSHIKAWA, Takashi KUNIMOTO, Susumu OKUBO, Hitoshi OHTA and Hiroshi NAJSAYAMA
298
ESR study of heavily doped GaAs:Er grown by organometallicvapor phase epitaxy ........ 302 J. YOSHIKAWA, S. OKUBO, H. OHTA, T. KOIDE, T. KAWAMOTO, Y. FUJIWARA and Y. TAKEDA Location of dangling bonds in ELA poly-Si ........................................................................ H. FURUTA, T. KAWASHIMA, H. HARIMA, T. HIRAO, M. FURUTA, Y. TSUCHIHASHI and A. YOSHIDA
306
ESR studies of BEDT-TTF organic conductors containing supramolecular assemblies ..... 3 12 Y. OSHIMA, H. OHTA, H. M. YAMAMOTO and R. KATO EPR spectral study of gadolinium (111) cryptate .................................................................. Ryo MIYAMOTO, Hiroyuki SAT0 and Susumu SUDOH
3 16
Ground-recognitionability of p- and y-cyclodextrins as studied by the high-pressure EPR .......................................................................................................... M. KASAHARA, H. TOBISAKO, Y. SUEISHI, S. YAMAMOTO and Y. KOTAKE
322
ESR Studies on a new phenyl t-butyl nitroxide biradical based on calix[4]arene ............... 326 Q. WANG, J.-S. WANG, Y. LI and G . 3 . WU
Section 3. Chemical Reactions Plenary Lecture Time-resolved EPR studies of excited states: some old and some new stories ...................333 Noboru HIROTA General Papers Time-resolved EPR studies of the excited triplet states of p-methylcinnamic acid and its deprotonated anion ................................................................................................... Ichiro YAMAMOTO, Kanekazu SEKI and Mikio YAGI
Quenching of singlet molecular oxygen ('Ad by vitamins and polyphenols studied by time-resolved ESR .............................................................................................. Akio KAWAI. Takahito FUSE and Kazuhiko SHIBUYA
344
349
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A time-resolved EPR study of weakly coupled triplet-doublet pairs of copper(I1)-free base porphyrin dimers .......................................................................................................... 355 M. ASANO-SOMEDA, A. JINMON, Y. KAIZU, P. RAGOGNA and A. van der EST Pulsed-ESR investigations of the photo-excited triplet state of naphthalene ...................... Kunio TAGUMA, Jun YAMAUCHI, Masaaki BABA Light-induced ESR studies of regioregular K. MARUMOTO, N. TAKEUCHI and S. KURODA
36 1
composites .......... 367
ESR study of photodecomposition mechanism of a long-lived radical ESR spectrum of trifluoromethyl radical formed during solid-phase photodecomposition at 77K in glassy matrix ........................... S. R. ALLAYAROV and D. A. GORDON ESR study and quantum-chemical calculations of alkyl radicals in the matrix of polycrystalline n-alkane irradiated at 77 K. Effects of intermolecular interactions and carbon chain length on the radical formation ................................................................ S. R. ALLAYAROV, S. V. KONOVALIKHIN and T. E. CHERNYSHEVA
373
378
ESR/ENDOR study for new radical dianion species of 6-oxophenalenoxyl derivative ...... 384 Y. MORITA, S. NISHIDA, J. KAWAI, K. FUKUI, S. NAKAZAWA, D. SHIOMI, K. SATO, T. TAKUI and K. NAKASUJI Spin labeling study of polymer chain motion in PEGPVP blend ....................................... Shiming CHEN, Guidong JIN, Zhenghua PING, Sizhao JIN and Xmin SHEN
389
EPR and UV-VIS studies on the influence of solute-solvent interactions on the self-redox reaction of bis (dithiophosphate) copper(I1) ............................................ N. D. YORDANOV and K. RANGUELOVA
395
Section 4. Environmental Sciences Plenary Lecture In and ex EPR spectroscopy and imaging of endogenously produced nitric oxide under physiological and pathophysiological conditions ................................... Tetsuhiko YOSHIMURA and Naoki KATO General Papers Molecular-electronic mechanism of the toxicity of Dioxin and ability of some natural structures to concurrently interact to inhibit its activity .......................................... Nguyen Van TRI, Pham The VUNG, Dinh Pham THAI, Ha Van M A 0 and Dinh Ngoc LAM
403
412
Section 5. Biology and Life Sciences Plenary Lecture Kinetic EPR study on reactions of vitamin E radicals ......................................................... Zhihua CHEN, Bo ZHOU, Huihe ZHU, Long-Min WU, Li YANG and Zhong-Li LIU
421
Invited Lectures ESR investigation on ROS initiated by visible light in PSI1 particles of high plants .......... 429 Yang. LIU, Jian SUN, Ke LIU, Qiyuan ZHANG and Tingyun KUANG EPR and theoretical investigations of PiFe] hydrogenase: Insight into the mechanism of biological hydrogen conversion .......................................... W. LUBITZ, M. BRECHT, S. FOERSTER, M. STEIN, Y. HIGUCHI, T. BUHRKE and B. FRIEDRICH
437
EPR studies on free radical generation by the reaction of methylglyoxal amino acids and protein ............................................................................................... 446 Hyung-Soon YIM, Cheolju LEE, P. Boon CHOCK, Moon B. YIM and Sa-Ouk KANG EPR monitoring on the quality of life .................................................................................. N. D. YORDANOV, G. PETKOVA and I. NAIDENOVA
456
General Papers EPR studies of manganese spin centers in the even-number oxidation states of water oxidizing complex in photosystem I1 ........................................................................ S. ARAO, S. YAMADA, A. KAWAMORI, J.-R. SHEN, N. IONNIDIS and V. PETROULEAS
466
Magnetic resonance studies on ascorbate binding to albumin ............................................. E. LOZINSKY, A. NOVOSELSKY, A. I. SHAMES, R. GLASER, G. I. LIKHTENSHTEIN and D. MEYERSTEIN
471
Electron magnetic resonance study on the effect of radioactive radiation on the photosynthesis of chlorophyll in lipid bilayers ......................................................... Y. S. KANG, D. K. LEE and S. M. PARK and K. W. SEO
477
Effects of tannin compounds on metal ion-hydrogen peroxide systems .............................. A. NAKAJIMA, Y. UEDA, N. ENDOH, K. TAJIMA and K. MAKINO
483
The [2Fe-2S] cluster in sulredoxin from the thermoacidophilicarchaeon sulfolobus tokodaii strain 7, a novel water-soluble Rieske protein ...................................... Toshio IWASAKI, Asako KOUNOSU and Sergei A. DIKANOV
488
EPR and saturation recovery investigations of spin probes in dispersions of hydrogenated castor oil .................................................................................................... Kouichi NAKAGAWA
494
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Section 6. Medical Sciences Plenary Lecture Advances in the spin labeling method ................................................................................. Lawrence J. BERLINER Invited Lectures Recent progress and future prospects of free radical imaging by PEDRI ............................ David J. LURIE, Margaret A. FOSTER, Wiwat YOUNGDEE, Valery V. KHRAMTSOV and Igor GRIGOR’EV
503
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525
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533
General Papers ESR studies on spin-clearance in GPxl -transgenic mice ........................................ K. MURAKAMI, 0. MIROCHNITCHENKO and H. UTSUMI
542
Non-invasive analysis of stress-induced gastric ulcer in rats ............................................... K. YASUKAWA and H. UTSUMI
548
Nitric oxide production and inducible nitric oxide synthase induction in iron-loaded rats ................................................................................................................ T. KAWABATA, A. I. HIDA, M. FUJISAWA, M. KAMEKAWA and S. OKADA
552
Electron paramagnetic resonance in medicine R. SAIFOUTDINOV
Development of in vivo ESWspin probe technique for oxidative injuries H. UTSUMI, J.-Y. HAN and K. TAKESHITA
Collapse of redox state by glutamate transporter inhibition in the rat’s hippocampus Y. UEDA, Y. HAYASHI, A. NAKAJIMA, H. YOKOYAMA, Y. MITSUYAMA, H. OHYA-NISHIGUCHI and H. KAMADA
........ 556
Non-invasive assessment of oxidative stress in the brain of small animal models by using electron spin resonance (ESR) imaging system ......................................... Masaichi-Chang-il LEE, Hirofumi SHOJI, Hiroyuki MIYAZAKI, Fumihiko YOSHINO, Kohki NAKAZONO, Kazunori ANZAI and Toshihiko OZAWA Possible production of hydroxyl free radical in the gastric legion of nitroso carcinogen-administrated rats .............................................................................................. T. MIKUNI, T. MORII, H. NAJIMA, M. EHARA and M. TATSUTA
562
567
Section 7. Geology Invited Lecture EPR and optical absorption spectroscopy on minerals ........................................................ B. J. REDDY, Jun YAMAUCHI, Y. P. REDDY, A. V. CHANDRASEKHAR and R. V. S. S. N. RAVIKUMAR
575
xvii
General Papers Thermoluminescenceand ESR centers of fluorapatite crystal from Brazil ......................... Henrique K. de FRANCAF, Luciana R. P. KASSAB and Sonia H. TATUMI
585
Spectral studies of divalent copper in antlerite mineral ....................................................... R. Rama Subba REDDY, S. Lakshmi REDDY, G. Siva REDDY and B. J. REDDY
589
Paramagnetic criterions of prognosis for oil and gas rocks content ..................................... K. KUDAIBAYEV, Sz. S. SZAMAROV, B. K. KUSPANOVA, A. S. KALAUOVA and R. N. NASIROV
595
Section 8. Dosimetry EPR dose reconstruction in teeth: Fundamentals, applications, problems and perspectives ................................................................................................................... 603 A. A. ROMANWKHA and D. A. SCHAUER
Plenary Lecture ESR dating applications in archaeology and earth sciences ................................................ Rainer G R m
613
Invited Lecture ESR and NMR dosimetry ..................................................................................................... 0. BAFFA, A. KINOSHITA, F. Chen ABREGO and N. A. SILVA
6 14
General Papers K-band ESR spectra of irradiated tooth enamel ................................................................... A. KINOSHITA, C. F. 0. GRAEFF and 0. BAFFA
624
Assessment of contribution of confounding factors to cumulative dose determined by EPR of enamel ............................................................................................. S. V. SHOLOM , V. V. CHUMAK AND E.V. BAKHANOVA
628
Retrospective EPR-dosimetry in Semipalatinsk nuclear test site region ............................. S. PIVOVAROV, A. RUKHIN, T. SEREDAVINA and A. ZHDANOV
634
Determination of total ionizing radiation dose on animals from west Kazakhstan by EPR method .................................................................................................................... R. N. NASIROV, B. K. KUSPANOVA, K. KUDAIBAYEV and M. B. KILIBAYEV
640
Section 9. Cross-Disciplinary and Methodology Invited Lectures Pulsed ESR double resonance (PELDOR) spectroscopy: Application to spin-labeled peptides .................................................................................... Yuri D. TSVETKOV and Alexander D. MILOV
647
xviii
The carotenoid triplet state in reaction centers. An EPR magnetophotoselectionstudy ................................................................................. Igor V. BOROVYKH, Irina B. KLENINA, Ivan I. PROSKURYAKOV, Peter GAST and Arnold J. HOFF Electron dipole-dipole interaction in ESEEM of biradicals ................................................. S. A. DZUBA and L. V. KULIK
659
669
The influence of label spins on EPR spectra of charge separated states in photosynthetic reaction center ...................................................................................................................... 678 Kev M. SALIKHOV, Stephan G. ZECH and Dietmar STEHLIK General Papers The structural analysis of photosystem I1 by PELDOR of three spin system ...................... H. HARA, A. KAWAMORI and N. KATSUTA
679
Application of pulsed ELDOR detected NMR measurements on the studies of photosystem I1 ...................................................................................................................... 684 H. MINO and T. ON0 Pulsed-ENDOR cavities modified from the CW-ENDOR TM-mode and pulsed-ESR TE-mode cavities ............................................................................................. Jun YAMAUCHI and Kanae FUJI1
688
Ferroelectric resonators for EPR spectrometers at 35, 65 and 125 GHz ............................. I. N. GEIFMAN and I. S. GOLOVINA
694
Fourier-transform ESR spectroscopy and observation of ultrafast spin-lattice relaxation by optical means .................................................................................................. T. KOHMOTO, Y. FUKUDA, M. KUNITOMO
700
Subnanosecond relaxation of optically-induced magnetization in aqueous solutions of transition-metal ions ....................................................................... S. FURUE, K. NAKAYAMA, T. KOHMOTO, Y. FUKUDA and M. KUNITOMO Detection of the internal electric field and relaxational magnetoelectric effect in chromium mesogen .......................................................................................................... N. E. DOMRACHEVA, I. V. OVCHNNIKOV, A. TLJRANOV and G. LATTERMANN
706
7 10
Section 10. High Frequency and High Field EPR Plenary Lecture Modern ESR methods in studies of the dynamic structure of proteins and membranes ........................................................................................................................... Jack H. FREED
719
Invited Lecture High-frequency single-crystal EPR application to multifrequency approach: Study of metalloproteins ...................................................................................................... Sushi1 K. MISRA General Papers EPR evidence of onset of the quantum critical point in CuGe03:Fe ................................... S. V. DEMISHEV, R. V. BUNTING, H. OHTA, S. OKUBO, Y. OSHIMA, N. E. SLUCHANKO
Millimeter and submillimeter wave ESR measurements of spin ladder system Sr(Cul.,ZnX)2O3 ................................................................................. S. OKUBO, K. HAZUKI, T. SAKURAI, H. OHTA, H. YOSHIDA, M. AZUMA and M. TAKANO High frequency ESR on quantum spin systems by using single shot and repeating pulsed fields ................................................................................................... H. NOJIRI ESR study on magnetic ordering of spin-hstrated antiferromagnet ZnCr204 single crystal ........................................................................................................................ H. KIKUCHI, H. OHTA, S. OKUBO, I. KAGOMIYA, M. TOKI, K. KOHN and K. SHIRATORI
73 1
741
747
75 1
755
ESR study of frustrated spin chain [Cu(bpy)HzO] [Cu(bpy)(mal)H2O](C104)2 .................. 759 T. KUNIMOTO, T. KAMIKAWA, S. OKUBO, H. OHTA and H. KIKUCHI Millimeter wave ESR measurement of diamond chain substance azurite ........................... Tomohisa KAMIKAWA, Takashi KUNIMOTO, Susumu OKUBO, Hitoshi OHTA and Hikomitsu KIKUCHI
763
High field ESR of(Ca~.,Sr,)~,CuO~with edge-sharing Cu02 chain .................................. K. KAWAKAMI, A. UEDA, H. OHTA, S. OKUBO, Z. HIROI, M. TAKANO
767
High field ESR measurements of(V0)2P207 ...................................................................... Yuta NAGASAKA, Takashi KUNIMOTO, Susumu OKUBO, Hitoshi OHTA, Touru YAMAUCHI and Yutaka UEDA
771
ESR measurements on triangular antiferromagnets CsCu~.,Co,Cl~.................................... Toshio ONO, Hidekazu TANAKA, Hiroyulu NOJIRI and Mitsuhiro MOTOKAWA
775
Gyrotron ESR in CsFeC13 up to 40 T ................................................................................... M. CHIBA, K. KITAI, S. MITSUDO, T. IDEHARA, S. UEDA and M. TODA
779
Magnetic properties of Fe12 ring: ESR and magnetization measurements ......................... Y. INAGAKI, T. ASANO, Y. AJIRO, Y. NARUMI, K. KINDO, H. NOJIRI, M. MOTOKAWA, A. CORNIA and D. GATTESCHI
784
High magnetic field ESR measurements of ACu2(P04)2 (A=Ba, Sr) .................................. 788 Masayuki HATA, Seitarou MITSUDO, Toshitaka IDEHARA and Mamoru MEKATA
ESR transmission experiments on P’-(ET)~SFSCF~SO~ and (ET)2SFsNN02, investigations of spin-Peierls systems .................................................................................. I. B. RUTEL, J. BROOKS, B. H. WARD, D. VANDERVEER, M. E. SITZMANN, J. SCHLUETER, R.W. WINTER and G. GARD
793
High field ESR measurements on molecular oxygen .......................................................... Shojiro KIMURA and Koichi KIND0
799
High field ESR measurements ofCuZ(CsH12N2)2C14 under high pressure .......................... M. SARUHASHI, T. SAKURAI, K. KIRITA, T. KUNIMOTO, S. OKUBO, H. OHTA, H. KIKUCHI and Y. UWATOKO
803
ESR at ultra-low temperatures and observation of new mode in Cu-Benzoate ...................807 T. SAKON, H. NOJIRI, K. KOYAMA, T. ASANO, Y. AJIRO and M. MOTOKAWA ESR spectrometer using frequency tunable gyrotrons as a radiation source ....................... Seitaro MITSUDO, Kazuaki KANAZAWA, Masayulu HATA, Isamu OGAWA, Tomohiro KANEMAKI and Toshitaka IDEHARA
813
High-frequency (W-band) EPR studies of biological samples ............................................ KBichi FUKUI, Tomohiro IT0 and Hiroaki OHYA
8 18
AUTHOR INDEX ................................................................................................................
825
Tribute Remembering Professor Arnold Jan Hoff (1939-2002) Just a few days before this volume was to be finally closed, we received the sad news that the EPR/ESR community has lost such an eminent researcher as Professor Arnold Hoff. He was one of the invited speakers at the Third Asia-Pacific EPR/ESR Symposium (APES’Ol) held in Kobe, the Proceedings of which form this volume. Although Arnold had not been a member of the Asia-Pacific EPR/ESR Society (APES) for ‘geographical’ reasons, we all in APES greatly appreciated his recent significant contribution to the APES activities. With Prof. Kawamori, Arnold co-organized Symposium B: International Workshop on Advanced EPR Applied to Biosciences, a Satellite Meeting which followed APES’O1. Arnold looked fine at both meetings and gave fruitful lectures concerning his recent work. A paper titled: “The carotenoid triplet state in Rhodobacter Sphaeroides reaction centers: An EPR magnetophotoselection stu+” appears in this book. After APES01 and Symposium B Arnold visited Kwansei Gakuin University at Sanda, Tohoku University in Sendai, the Institute of Molecular Sciences at Okazaki, and Kyoto. His family, friends, and colleagues have suffered a great loss. On behalf of the APES Council and all APES members we express our sincere sympathy to his wife, Zina, and his children. We will value his memory and his contributions to the APES activities. As a tribute to Arnold, on the following page we reproduce a note provided to us by Peter Gast, Hans van Gorkom, T h i s Aartsma, and Thomas Schmidt, who are at the Huygens Laboratory at the University of Leiden. Asako Kawamori Chairperson, Local Organising Committee, APES’O1 Czeslaw Rudowicz President, The Asia-Pacific EPRlESR Society President of APES
Dear colleagues and friends of Arnold Hoff Arnold Jan Hoff passed away on 22 April 2002 at the age of 62. Until the very last days he chose to ignore the fatal cancer he was suffering from and continued to work with all available energy. After his study in physics Arnold graduated under Johan Blok at the Free University in Amsterdam in 1969. In 1971 Arnold was introduced to the biophysics of photosynthetic reaction centers as a post-doc in George Feher’s group at the University of California in San Diego. Fully aware of the great potential of his specialization, magnetic resonance techniques, in the field photosynthesis research, in 1974 Arnold joined Lou Duysens (Biophysics Laboratory at Leiden University), which at that time used mainly optical techniques. In 1985 he was appointed full professor in Biophysics. The impact of Arnold’s efforts on the development and application of magnetic resonance techniques for the study of primary reactions in photosynthesis is hard to overestimate. With his team of graduate students and post-docs he worked on electron spin polarization phenomena studied with cw and time-resolved EPR and ESE. Around 1982 he developed the technique of ADMR, an ODMR technique especially suited for photosynthetic samples. In recent years his attention turned to isotopically labeled reaction centers which were studied by NMR and EPR. Also, with his Russian collaborators he worked on improving and re-evaluating magneto-photoselection and applied it to radical pair spin polarization. His future plans were the study of reaction centers with site directed spin labeling and developing the technique of CD-ADMR, but this was not to be. Arnold published more than 250 articles and 19 and students graduated under him. One of the things he loved most was to travel and to meet new people and to make new friends. He must have visited hundreds of symposia and congresses. Visitors to his office will remember the large collection of photographs on the wall of friends from all around the world. His joy for travel also led to a special relation with scientists in the field of EPR and photosynthesis from Russia. Since 1992 he was director of the Dutch-Russian Research Collaboration Network. In 1999 he was awarded the Voevodsky Gold Medal of the Russian Academy of Sciences. He was also the chairman of the highly successful ESF program: Biophysics of Photosynthesis, that, in his own words, forged a network of cordial links between a great number of groups in practically all European countries. At Leiden University he was the initiator of BIOSPEC, a collaboration between physics, chemistry and biology, which led to the foundation of the research school BIOMAC. This department and the biophysics community have lost a strong leader. We feel deep compassion for Zina and his children, who have lost much more.
Section 1 Physics and Magnetism
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EPR in the 2 lstCentury A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
3
Electron magnetic resonance (EMR) of the spin systems: an overview of major intricacies awaiting unwary spectroscopists C. Rudowicz Department of Physics and Materials Science, City University of Hong Kong, Kowloon Tong, Hong Kong SAR, China The aim of this paper is to bring about a better understanding of the intricacies of the spin Hamiltonian (SH) theory. A number of theoretical aspects underpinning the experimental EMR (encompassing EPR, ESR and related techniques) studies of paramagnetic species with the spin S21, especially transition ions, which require a thorough clarification, are critically reviewed. Examples of the intricacies awaiting unwary spectroscopists, drawn from recent EMR literature, are discussed in a nutshell in order to illustrate the potential pitfalls and their consequences.
1. INTRODUCTION Electron magnetic resonance (EMR), which nowadays encompasses electron paramagnetic resonance (EPR - the term dominating applications in physics) and electron spin resonance (ESR - the term dominating applications in chemistry) as well as related techniques, is a mature area with applications ranging from biology to materials sciences [l - 81. However, various theoretical aspects underpinning the interpretation of experimental EMR data, which are often confused in the EMR literature, can be identified. In this paper the major points concerning EMR studies of paramagnetic species with the spin S21, especially transition ions, which have recently been clarified, are reviewed. Examples of the intricacies awaiting unwary spectroscopists, drawn from recent EMR literature, are presented a nutshell in order to illustrate the potential pitfalls, e.g. usage of erroneous relations and misinterpretation of experimental EMR data. Thus we hope to bring about a better understanding of the intricacies of the spin Hamiltonian (SH) theory [9-141, especially the zero-field splitting (ZFS) Hamiltonian, &&s, and the crystal-field (CF) Hamiltonian, &CF, which are fundamental to EMR studies [1-81. Ramifications include the magnetic susceptibility, magnetic anisotropy, Mossbauer spectroscopy, which rely on SH, as well as the optical absorption spectroscopy, inelastic neutron scattering, and infrared spectroscopy, which rely on CF Hamiltonian. First we outline briefly the reference notations for &ZFS [I, 21 to provide background for the discussion in Sections 2 to 7. terms of the extended Stevens (ES) operators [9] the compact form of &FS (as defined in [lo]; to be distinguished from the expanded form using explicitly the pairs of the tensor operators with +q in &ZFS) is given as:
4
The uniform 'scaling'ofZFS parameters b l requires the factorsfk to be taken as [l, 2, 10, 141: 1/3,
1/60,
1/1260
(2)
For numerical convenience, the second form of &DS in Equation (1) has been more often used in EMR studies of transition ions. The conventional form of &ZFS most widely used in the literature, which is suitable for paramagnetic species with the spin S2 1 at sites with triclinic symmetry, is given by [ 1-7, 101:
whereas for the spin S22 the higher-order ZFS terms are required. For orthorhombic symmetry as well as for monoclinic and triclinic symmetry with &&FS expressed in the principal axis system, the conventional form of &ZFS is given by [I-3, 10, 141:
where the axes (x, y, z) may be chosen in different ways for orthorhombic symmetry [ 10, 111 (see Section 5), whereas for monoclinic and triclinic symmetry the orientation of the principal axes (x, y, z) with respect to the crystallographic axis system (X, Y, Z) must be provided [lo]. In Sections 2 to 6 various categories of the intricacies of the SH theory are briefly outlined; for details and references the reader shall refer to the respective original papers. In Section 7 possible remedies to some of the problems encountered by experimentalists in interpretation of EMR data are presented.
2. MAJOR INTRICACIES CONCERNING NOTATIONS AND NOMENCLATURE
2.1. Multitude of notations for the operators and the ZFS parameters There exist in the EMR (as well as CF) literature a multitude of notations for the operators and the associated ZFS (CF) parameters. Such an abundance hampers comparisons of data from various sources. All notations appearing in the EMR-related literature up to late 1986 were comprehensively surveyed in [lo], whereas recent literature will be summarized in [12]. A general pitfall concerning all SH notations consists in the misleading or often inconsistent nomenclature for various terms appearing in &~Fs. Historical developments have led to the existence of various SH notations classified briefly into three groups as follows [lo, 141. A. ConventionalSH notations Any notation using explicitly the 'spin' operators S,, S,, S,, or S,, S =S, & is,, like Equations (3) and (4), belongs to the conventional notations [lo]. In fact, the 'spin' may mean the physical spin S , or the effective spin, 3 , or the fictitious spin, S' (see Section 3). The conventional notations are particularly inconvenient for low-symmetry, e.g. monoclinic and triclinic, which require a large number of ZFS terms [lo]. Hence, these notations have been mostly used for higher-symmetry and low-spin cases [l-8, 101. In spite of the widespread
5
usage of the tensor-operator notations in the literature, some recent books still adopt only the conventional notation with the former notations receiving only scant remarks, see, e.g. [4-71. The major pitfall concerning the conventional SH notation is the existence of a number of conventions for operator combinations and symbols for ZFS parameters as reviewed in [lo]. One must be cautious when comparing SH parameter values from different original sources and reviews. B. Tensor-operatornotations Tensor-operator notations may be collectively denoted by the generic symbols: xlrn for operators and At, for the associated parameters [lo, 141. Apart from the most common product form: Alm~lm, alternate tensor forms of the ZFS Hamiltonian are sporadically used in the literature [lo, 141. The advantages of the tensor operators [lo, 141 have led to the development of several notations, which can be subdivided into two subgroups, each consisting of several different types of tensor operators interrelated by conversion coefficients. The general classification briefly outlined below may serve as a quick reference guide for practitioners. The most representative symbols originally introduced for the operators and their coefficients, defining the ZFS parameters, are given for each tensor-operator notation. A full review of the present status and a detailed survey of operator and parameter notations currently in usage in the EMR area, including a list of specific intricacies, will be provided in [ 121. A general pitfall concerning the tensor-operator notations is the fact that several symbols have been used for the same quantity and conversely, the same symbols have been used to denote different quantities. B l . Tesseral-tensoroperators (TTO) Historically [9, 141 Stevens [151 introduced an incomplete set of tesseral-tensor operators (TTO), 09,(L), being “operator equivalents” of real tesseral harmonics with the components q limited to k 2 q 2 0, which latter become known as the Stevens operators [l, 3, 9, 101. Some misleading statements concerning the ‘operator equivalents’ in [4] are discussed in [14]. The , . Stevens operators, 0; (S), with L replaced by S (in fact or S‘ - see Section 3) have been widely used in the EMR area. However, the lack of the negative q components hampered application of the Stevens operators for low-symmetry cases [lo]. The extended Stevens (ES) operators O:(S) including all components: -k q +k were introduced and their transformation properties were worked out in [9]. These are now the most widely used operators in the EMR area. Thus we adopt the ES operators as the reference notation in Equation (1). The usual Stevens operators [l, 151, however, suffer from a serious drawback in that they are not normalized [9, 101. This has led to the appearance of the corresponding normalized operator sets. The various tesseral-tensor operators (TTO) existing in the literature may be classified as follows [lo, 121. a) The usual Stevens operators, where the q values are limited only to the positive
s
integers and zero (k q 2 with the coefficients B: or the ‘resealed’ b: , see Equation (1). The major pitfall concerning this notation is the occurrence of other ‘scaling‘factors than those defined in Equation (2) as reviewed in [lo]. b) The extended Stevens (ES) operators, integers (-k
, where the q values include also the negative
q I +k) [9, lo]; with the coefficients B!
(or b:).
The early inconsistent
notations for the components q I 0 may pose apitfall as discussed in [lo].
6
c) The normalized Stevens (NS) operators, OF, [lo], which are, in fact, the ES operators, each multiplied by certain normalizing factor; with the coefficients B: . Similar pitfalls as for the operators (a) and (b) apply, to a certain extend, to the NS operators [ 10, 121. d) The normalized combinations of spherical tensor (NCST) operators, [lo, 121, which are simply related to the NS operators 0: and are linear combinations of the STO (see Section B.2); with the coefficients The usage in the literature of similar symbols for the NCST operators and their STO counterparts may easily lead to confusion [lo, 121. e) Five unnamed notations of ephemeral usage in the literature reviewed in [lo]. B2. Spherical-tensor operators (STO) Historically [101 the extension of the operator equivalents method to the spherical harmonics was made subsequent to introduction of TTO and has led to introduction of the sphericaltensor operators (STO): Tkq(X), X = L, J or s. The operators Tkq(L or J) have now become widely used in CF theory, where they are usually denoted as Ckq, whereas the extended Stevens operators ( S ) play the same role in EMR. The various spherical-tensor operators (STO) existing in the literature may be classified as follows [lo, 121. a) Buckmaster, and Smith & Thornley (BST) operators of);with the coefficients B:. Note that the BST operators are mathematically equivalent to the Wybourne operators c k q used in the CF theory [ 10, 121. b) Phase-modified BST operators - these are BST operators multiplied by a phase factor:
d(k)=ik o q( ~with ) ; the coefficients c) Koster & Statz and Buckmaster, Chatterjee & Shing (KS/BCS) TI,; with the coefficients B I ~ d) Ten unnamed notations of ephemeral usage in the literature reviewed in [lo]. Full discussion of the intricacies in the above category and interrelations between various notations used in the area of EMR and related spectroscopies [ 19, 141 will be provided in [12].
2.2. Incorrect relations for ZFS parameters Recently three cases of incorrect relations between the extended Stevens ZFS parameters and the conventional ones have been identified in the literature [16]. Case 1 concerns the conversion relations for the second-order rhombic ZFS parameters. Which relation is correct: 3E = b i = 3B: [ l , 101 or E =b; =3Bi [13, 17, 18]? It turns out [16] that the former relation is correct, whereas the latter one is incorrect. The substitution E -+bi in [13, 17, 181 has three implications. First, the maximum rhombicity limit is then: 0
1/3 (Reference 17, p. 112), whereas in fact it is: 0 I b ; (ES)/b: (ES) I 1 [ll].
Second, the relations: bz (= E ) and E (= b i ) are explicitly used in the extensive tabulation of transition ion data in [13], being a compilation of data mostly taken from other review articles and a few original papers. Since in majority of the original sources the well-established relation: b; (ES) = 3E is used, the substitution E -+ bz in [13, 17, 181 may render the tabulation of this parameter in [13] unreliable. This has been verified in [16] by comparing sample original data expressed in terms of either b i or E with those quoted in [ 131 after other reviews. Third, in the light of this, in order to avoid confusion, it is advisable to check the
7
values of E and b i in the original references cited in [13,17-191. Possible origin of this pitfall may be due to a misprint in Altshuler & Kozyrev [2] - both in Russian (1972) and English (1974) version. However, its usage in [13, 17, 181 is not a simple misprint. Note that the relation: E=b; /3 was used earlier in [20]. Other intricacies in [13, 17-19], as discussed in detail in [16], include: (i) the usage of a variety of symbols, correlated neither mutually nor with the existing notation for the ES operators [9, lo], (ii) the confusion between the properties of TTO and those of STO, and (iii) inappropriate nomenclature for the 'Steven'soperator equivalents'. Case 2 concerns the set of five relations [21, 221 between the extended Stevens ZFS parameters bjf and the conventional ones Du for triclinic symmetry:
The relations in Equation apart from the first one, are incorrect [16]. Moreover, they imply that the ZFS parameters with q expressed in the ES notation defined in Equation (l), are complex, whereas, in fact, the parameters B: (ES) as well as b: (ES) are all real [2, 9, 10, 121. The correct relations for triclinic symmetry in terms of B: (ES) are [23,24]:
Bi (ES) = DJ2,
Bi (ES) = 2D,,
B i (ES) = (Dn - Dyy)/2,
B; (ES) = Ow
B; (ES) = 2Dyz, (6)
Case 3 concerns the omission of the scaling factors& for bgk (ES) in Equation (1). Using the
form: 'b;'.
=
E:b: 0; introduces a serious ambiguity about the meaning of the parameters
Two options are possible: (i) the authors did in fact mean the 'capital B' parameters Bqk
defined in Equation (1) or (ii) they meant the 'small b parameters bz actually appearing in their equation, whereas the factors fk were inadvertently omitted. A number of recent examples of such omission have been identified in the literature - for references, see [16]. 3. CONCEPTUAL PROBLEMS
The major intricacies in this category consist in confusion concerning the distinction between the three existing spin Hamiltonian formalisms and misunderstanding of the various crucial notions underlying the SH theory. This is due to the lack of clear and widely accepted definitions, which are not provided even in major textbooks (see, e.g. [l-71). Below we provide an overview of the basic concepts. A full presentation would require a detailed consideration of the various SH formalisms and the relevant terms for the specific cases of transition-ion systems to which they apply. This is beyond the space limits of this paper. Instead we refer the readers for details to the review [lo] and the more recent one [14]. The microscopic SH (MSH) formalism, i.e. the historically first formalism [lo], is based on perturbation theory and treats the full physical Hamiltonian (H + V) as the solvable part H
8
and the perturbation V. MSH formalism enables 'derivation' of the explicit formulas for SH parameters in terms of the more microscopic ones [l, 21. For example, the MSH formulas for 3d4 and 3d6 ions derived in [25] enable calculations of Bqk (ES) [ bft (ES)] provided the values of the microscopic parameters, i.e. the energy-level splittings A, within the ground multiplet 2s+1 L and the spin-orbit coupling constant h and the (electronic - see Section 4) spin-spin constant p, are known. General derivation of MSH differs for the two major cases [lo, 141, namely, (i) a ground-state multiplet 2s+1Lof a transition-metal 3d" ion (see, e.g. [25]) and (ii) a full 3d" and 4f" configuration (much more cumbersome calculations are required in this case, especially for the S-state ions, for references, see [lo, 141). The generalized SH (GSH) formalism is based on group theory and enables 'construction'of a more complete form of SH, including the higher-order terms, which so far could not be 'derived using MSH approach [lo, 141. Two methods of construction of GSH exist namely, the matrix method and the method of invariants - for references, see [lo, 141. Unlike MSH, GSH formalism can provide no information on the values of the SH parameters. However, it has enabled first predictions of various higher-order SH terms, e.g. the higher-order electronic Zeeman terms and the higher-order hyperfine terms [14]. The phenomenological SH (PSH) formalism is not clearly defined. Various meanings have been assigned to this term. We adopt a definition of 'PSH as a 'spin Hamiltonian' postulated on the basis of a researcher's experience on an ad hoc basis to describe EMR spectra. Hence, no apriori prescription for the construction of the PSH can be provided [lo, 141. In the development of MSH and GSH formalisms specific questions, which may be difficult to comprehend by EMR experimentalists, have arisen. This includes, e.g. 'How is the effective spin, related the real electronic spin, S ?' and 'What is the direrence between the fictitious spin, S', and the effective spin, ?'. Let us briefly mention here that, regardless of the formalism by which it was obtained, the spin Hamiltonian (either 'derived' using MSH or 'constructed using GSH) has an effective nature as it describes the action of a given physical Hamiltonian = + V) within a restricted basis of states of the Hamiltonian ., It should be kept in mind that the spin Hamiltonian does not act within the eigenstates of (IrplS, MQ} but rather acts within its own basis of the states ( 15, G , ) } of the effective spin operator
s,
s
3. A succinct description of the 'effective' nature of SH can be found, e.g. in Reference 6, pp. 35 & 36. Note that for transition ions with an orbitally-nondegenerate ground state the spin quantum numbers s" and S are of the same value (s" = S), however, the nature of the operator S remains different from that of 3 . The distinction between the real physical quantities, i.e. describing the physical interactions, and the effective ones, i.e. describing only the restricted basis of states (e.g. the orbitally-nondegenerate ground state) of a particular physical Hamiltonian, have been discussed in [14] at the level comprehensible by EMR experimentalists. Thefictitious spin is the term most appropriate to describe a particular subset of N distinct lowest-lying energy levels of a paramagnetic ion out of the total manifold of Nt energy levels (N < NJ. This subset of energy levels may be regarded as equivalent to a 'spin' multiplet, to which a fictitious spin operator S', characterized by the spin quantum number S' and the magnetic quantum number Ms (-S' S Mst +S'), is ascribed in such a way that the number of energy levels N equals 2s' + 1. Conversely, the fictitious spin S' = (N-1)/2. The concept of fictitious spin for the three possible systems: (1) transition ions with an orbitally-degenerate ground state, (2) non-Kramers rare-earth ions with integral angular-momentum values in the ground state, and (3) projected or restricted 'spin' subsystems, has been discussed in [14].
9
4. MISCONCEPTIONS RELATED TO ZFS HAMILTONIAN The major intricacies in this category consist in confusion, of various degree, between ZFS Hamiltonian and three other physically distinct Hamiltonians, namely, (A) CF Hamiltonian, (B) spin-spin Hamiltonian, and (C) the nuclear quadrupole interaction Hamiltonian. By definition, the ZFS terms in SH, Equation (l), describe splitting within a given 'spin' multiplet in the absence of an external magnetic field, i.e. the fine-structure splitting. As discussed in Section 3, in general, a Hamiltonian, which describes the zero-field splitting of a particular 'spin' system, involves either the operator 3 (eflective spin) or S' (fictitious spin). Thus the nature of a particular ZFS Hamiltonian is determined by the nature of the spin operator appearing in it. In any case, the physical meaning of a ZFS Hamiltonian, as determined by its role in describing fine structure of a 'spin' multiplet, remains the same. This helps to distinguish ZFS Hamiltonians from other Hamiltonians discussed below. The type (A) confusion appears to be the most widespread in the literature [lo, 14, 261. It originates partially from the fact that the mathematical forms of both Hamiltonians are nearly identical for a given point-symmetry group, G, for the reasons outlined in [ 141. Some recent EMR papers reviewed in [26] imply the positive answer to the question: 'Can the EMR techniques measure the crystal (ligand)field parameters?' However, a critical examination of the relevant papers reveals that the authors do not fully realize the distinction between the actual crystalfield (CF) or equivalently ligandfield (LF) related quantities and the actual zerofield splitting (ZFS) or equivalently fine structure (FS) quantities. The origin of such incorrect terminology has been examined in [26]. Aspects concerning Hamiltonians, parameters, energy level splitting, and nature of the operators involved have been taken into account in the review [26]. A comprehensive literature survey has revealed a large number of cases of incorrect terminology as well as several inconsistencies. Implications of the confusion between ZFS Hamiltonian and the CF Hamiltonian go beyond the simple semantic issues. The two most serious categories of the type (A) confusion are [26]: (a) using the notation specifically applicable to the ZFS parameters for parameters which are, in fact, the CF parameters due to their physical nature or vice versa; (b) applying incorrectly the point-charge model (PCM), or any other electrostatic model, to derive relations between the structural parameters, e.g. the ligand-paramagnetic ion distances, and the ZFS parameters, e.g. B;f or b', in Equation (1). Thus, this confusion has lead also to erroneous numerical results and misinterpretation of the experimental EMR data as evidenced in [26]. Several examples of serious misinterpretations have been found in the books, reviews, and original papers [26]. Additionally, the incorrect terminology has contributed to misleading keyword classifications of papers in journals as well as references in scientific literature databases [26]. In order to prevent further proliferation of the incorrect terminology and thus to increase reliability of the published EMR data, a concerted effort within the EMR community is indispensable (see Section 7). The type ( B ) confusion is less widely spread in the literature [lo, 141. Proper meaning of the term 'spin-spin Hamiltonian', &SS, depends on the physical system being described. mononuclear paramagnetic centers of transition ions &SS means the real electronic spin-spin coupling, &ss(el-el), between the 3d" or electrons [27]. &ss(el-el) can be expressed in a simpler form within a given LS multiplet (see, e.g. [25, 271). Within the MSH framework &ss(el-el) yields additional contributions to the single-ion ZFS parameters, i.e. the terms involving the electronic spin-spin constant p [lo, 141 as well as the mixed terms involving both
10
the spin-orbit constant h and p, see, e.g. [25, 271. For organic molecules exhibiting triplet states [28-311 &SS means the real magnetic (dipole-dipole) spin-spin interactions, &ss(d-d). In a similar way as &,s(el-el) for the single-ion ZFS parameters, &ss(d-d), in fact, also contributes to the ZFS parameters but of the whole molecule [lo, 141. However, the Hamiltonian for organic molecules [28-311 in the form of Equations (3) & (4) has been inappropriately [26, 32, 331 called the 'spin-spin' Hamiltonian, whereas it represents just the conventional ZFS. For exchange-coupled complexes of transition ions, each with a total electronic spin Si, &SS means most commonly the Heisenberg exchange Hamiltonian: && = = Jij Si Sj [33]. Such complexes comprise, e.g., pairs (i , j = 1, 2), triads (i, j = 1-3), or tetramers (i, j = 1-4) [32]. The ZFS parameters for the whole complex [33] include contributions from the single-ion ZFS parameters, which depend themselves upon the spinorbit constant and the (electronic) spin-spin constant (p) [lo, 141, as well as contributions from exchange interactions between the constituent transition ions, which depend, e.g. on the Heisenberg exchange constants (Jij). However, the ZFS Hamiltonian for the whole complex has sometimes been inappropriately called the 'spin-spin' Hamiltonian - for references, see [26]. The usage of the name 'spin-spin'for a Hamiltonian, which, in fact, is meant to describe the zero-field splitting, either for an organic molecule or an exchange-coupzed system with the total spin S, raises a question: 'Can one spin interact with itself?'. The answer is definitely negative. Hence, such nomenclature should be avoided, not only for the sake of semantic purity, but to prevent confusion between the physically distinct quantities discussed above. The type (C) confusion has appeared in a few cases only - for references see [lo, 141.
.
5. SYMMETRY PROPERTIES AND LOW SYMMETRY ASPECTS The major intricacies in this category concern: (1) incompatible ZFS parameter sets arising from the usage of (i) non-standard or (ii) mixed axis systems for orthorhombic symmetry sites and (2) lack of full understanding of the symmetry properties of SH for monoclinic symmetry sites. The values of the ZFS parameters and gij components depend on the choice of the axis system for SH. Several options are applicable to various symmetry cases. Clear definition of the axis system used in a particular paper is indispensable to avoid misinterpretation of the SH parameters. Yet often no such definition is provided by the authors. This may be specially critical for some symmetry cases as discussed in detail in [lo]. The following intricacies may be noted in the literature. For tetragonal symmetry the tetragonal axes may be chosen in a way so that they do not coincide with the cubic axes, being rotated by 45"/0z [lo]. For trigonal symmetry three different axis systems have been used in the literature [lo]. These distinct choices result in an ambiguity of the sign of some Z F S parameters [lo]. For orthorhombic [lo, 111 (and monoclinic [34, 351 - see below) symmetry the situation is more troublesome. Two kinds of rhombic symmetry exist: I and 11, with the axes interrelated by a rotation 45"/0z. For the rhombic symmetry of the kind the x- and y- axes do not coincide with the ligand bonds. This has led to introduction by some authors of the mixed axis systems for orthorhombic symmetry sites, i.e. the cubic one for the fourth-order ZFS terms, whereas the rhombic one for the second-order ZFS terms (for references, see [lo]). This makes the comparison of the experimental values from various authors more cumbersome. Moreover, there is a lack of commonly accepted notation for the rhombic fourth-order ZFS parameter, which may effect the interpretation of the experimental values [lo].
11
is presented by the question do Such data still appear in the literature - for a review, see [ l 1, 36-38]. The answer is not a large distortion in the crystals studied but the lack of knowledge of the standardization idea. This idea, which applies for both the ZFS and CF parameters, consists in confining the ratio, i.e. the 'rhombicity' parameter, given in terms of the reference ES notation as:
A
to the standard range (0, 1) by properly choosing the axis system for orthorhombic, monoclinic and triclinic symmetry [ 11, 351. The standardization transformations Si (i = 1 to 6) necessary to obtain the standard ratio in Equation (7) have been defined in [ 1I]. Note that fitting SH parameters to experimental EMR spectra may yield (or any equivalent ratio) of any value between and +-. According to the original value of one can standardize the ZFS (CF) parameters by choosing the appropriate transformation Si as required [l 11. Several cases of the non-standard ZFS parameter sets resulting in inconsistencies and misinterpretation of EMR experimental data have been identified [35-361. one may choose the axes (X, Y, Z) with respect to the monoclinic direction (Cz) in one of the three possible ways: C211 Z, CzII Y, or CzII X [lo, 24, 34, 351. Each case corresponds to a different form of ZFS (CF) Hamiltonian and thus results in a different set of the ZFS (CF) parameters. A proper form of ZFS (CF) Hamiltonian must be employed for a given choice of the axis system with respect to the crystallographic axes. Hence lack of full understanding of the symmetry properties of SH for monoclinic symmetry sites may lead to usage of inappropriate forms of ZFS (CF) Hamiltonian [lo, 24, 34,351. The relations for converting the ZFS (CF) parameters corresponding to a given axes choice have been derived in [34]; note the misprints - the third relation in Equation (7) in [34] (and in ~ s i nI 2 Equation (8) in [24]) should read: ~ ~ ~ ] - ~ ~ ( ~ ~ ~ ( ~I ~ ~ ~ ~ a - l ) + - ( Bin~ Table -2
2
in [34]: in the third line for [B:]in columns S2 and S5 'B' should read 'B: ', in the sixth line for [B;'] in column S5 the factor '$' should be In practice the second-order ZFS (CF) monoclinic parameter may be first set to zero, by a proper rotation around the Z-, or Y-, or Xaxis, yielding an orthorhombic-like set. Similarly, for symmetry, the second-order ZFS (CF) Hamiltonian may be first diagonalized, i.e. expressed in the principal axes. However, in such cases the respective low-symmetry fourth-order ZFS (CF) parameters remain non-zero.
6. SPECIFIC THEORETICAL PROBLEMS The major intricacies in this category concern the following aspects: (1) incorrect admission of the odd-order SH terms, (2) confusion concerning derivation of the microscopic SH, and (3) truncated forms of ZFS Hamiltonian. characterized by 1 = 1, 3 or 5 in Equation (1) were introduced into spin Hamiltonian on theoretical grounds [39, 401. The incorrectness of the theoretical reasoning [40] due to semantic misunderstanding of the crucial terms (see, Sections 3 and 4) was discussed critically in [41]. It turns out [41] that the faults in the reasoning [40] give rise to the incorrect admission of the odd-order SH terms. Independently, Grachev [42] pointed out
~ + ( B ~ }
12
additional misunderstandings in the reasoning given in [40] in favor of including odd-order ZFS terms, and dismissed admissibility of odd-order ZFS terms in GSH. Other papers [43,44] dealing with the possible presence of odd-order ZFS terms in the SH for the S-state ions require caution. Chatterjee et al. [43] stated that their 'earlier derivation was flawed because it used spin ( S ) rather than total angular momentum (J)'. Buckmaster et al. [44] proposed using 'tesserul rather than one of the conventional tensor angular momentum operator formulations thus adding another intricacy to the SH theory. Moreover, the for the ZFS terms a earlier criticism [41, 421 of introduction of odd-order ZFS terms was not refuted in [44]. Instead, the authors [44] alleged 'errors and misconceptions' in [lo], which were not substantiated and explicitly listed in [44]. Confusion concerning the approach the derivation the microscopic proposed by Zhao et al. [4S, 461 has been clarified in [47]. The confusion consists in an erroneous identification (or 'approximate equivalence 7 of the wavefunctions of the two Hamiltonians, namely, the physical Hamiltonian and the effective spin Hamiltonian. As discussed in [47] this confusion has led to invalid relations for the zero-field-splitting parameters for the S-state 3d5 ions at axial symmetry and the spurious numerical results in [4S,46]. Truncated forms of ZFS Hamiltonian can be classified into three types: (i) some independent ZFS terms allowed by symmetry of a paramagnetic complex are arbitrarily set to zero, (ii) one of several interdependent ZFS terms is omitted, and (iii) some operator terms are omitted leading to a specific definition of ZFS parameters. The examples of the truncated Hamiltonians include, e.g. & = -AS: + SS: and & = -0s: - SS: . Such forms have been used, e.g. in high-frequencyhigh-field EPR and macroscopic quantum tunneling studies of the Mnlz complexes with spin S=10. The work is now in progress [48].
7. SOME REMEDIES AND CONCLUSIONS In view of the intricacies discussed in Sections 2 to 6, which may potentially pose pitfalls, two-fold approach to remedy the situation may be proposed. First approach relies on an international cooperation between EMR researchers and editors of EMR-related journals. As amply illustrated in Sections 2 to 6, there is an urgent imperative for adoption of unified guidelines for presentation of ZFS parameters [49, 501. Suitable options for unified guidelines in this regard include adoption of (i) the extended Stevens operators and the parameters b (ES) as the standard reference notation and (ii) units of cm-' or cm-' [lo, 14,49, SO]. Additionally for orthorhombic and lower symmetry cases, (iii) it seems useful to adopt in a uniform way the axis system conforming to the standard range of the ratio 0 = b i (ES)/b: (ES) I 1 (see, e.g., [ l l , 36-38]. The guideline (i), and partially (ii), have been adopted in several reviews dealing with EMR data (for references, see [ 161). Second approach relies on development of computational tools which may facilitate the work of experimentalists. Hence, the computer package CST (Conversions, Standardization and Transformations) has been developed for general manipulations of the zero-field splitting (ZFS) and crystal field (CF) parameters for various systems, especially for transitions ions at orthorhombic and lower symmetry sites in crystals [51, 521. An extended version of the CST package has been developed recently with several additional capabilities and improved features [S2]. The CST package [Sl, 521 is expected to satisfy fully the various needs of
13
researchers working in the EMR, optical spectroscopy and related areas. Recent applications of the CST package for the CF and ZFS parameters have been presented in [35-371. In conclusion, by presenting in a nutshell the intricacies, the related potential pitfalls and their consequences, as well as the proposed solutions, we provide a guide through the fundamentals of the spin Hamiltonian theory at the level accessible for EMR experimentalists. It is hoped that making the spectroscopists, working in the EMR and related areas, aware of the major intricacies will help reducing the proliferation of the incorrect relations and misinterpretations, which pose potential pitfalls.
ACKNOWLEDGMENTS This work was supported by the RGC and the City University of Hong Kong grant: SRG 7001099. Technical assistance from Mrs J. Qin and Miss H.W.F. Sung is gratefully acknowledged.
REFERENCES 1. 2. 3. 4.
5. 6. 7.
8. 9. 0. 1. 2. 3.
4.
5. 6. 7. 8.
A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Oxford: Clarendon Press, 1970; New York: Dover, 1986. S. Altshuler and B.M. Kozyrev, Electron Paramagnetic Resonance in Compounds of Transition Elements, New York, Wiley, 1974; Moscow, Nauka, in Russian, 1972. J.R. Pilbrow, Transition-Ion Electron Paramagnetic Resonance, Oxford, Clarendon Press, 1990. F.E. Mabbs and D. Collison, Electron Paramagnetic Resonance of d Transition-Metal Compounds, Amsterdam, Elsevier, 1992. J.M. Spaeth, J.R. Niklas, and R.H. Bartram, Structural Analysis of Point Defects in Solids. Springer Series in Solid-state Sciences, Vol. 43, Berlin, Springer, 1992. N.M. Atherton, Principles of Electron Spin Resonance, Oxford, Ellis Horwood, 1993. J.A. Weil, J.R. Bolton, and J.E. Wertz, Electron Paramagnetic Resonance: Elemental Theory And Practical Applications, New York, Wiley, 1994; Corrigenda (J.A. Weil, private communication, 1995). K.W.H. Stevens, Magnetic Ions in Crystals, Princeton, Princeton Univ. Press, 1997. C. Rudowicz, J. Phys. C: Solid State Phys., 18 (1985) 1415; Erratum ibidem, 18 (1985) 3837 C. Rudowicz, Magn. Reson. Rev., 13 (1987) 1; Erratum ibidem, 13 (1998) 355. C. Rudowicz and R. Bramley, J. Chem. Phys., 83 (1985) 5192. C. Rudowicz, A. Galeev, and T.C.Y. Chung (2001); in preparation. S.K. Misra, Handbook of Electron Spin Resonance, v01.2, Chapter VIII, (eds.) C.P. Poole Jr and H.A. Farach, New York, AIP Press, 1999. C. Rudowicz and S.K. Misra, Appl. Spectr. Rev., 36 (2001) 11. K.W.H. Stevens, Proc. Phys. SOC.,65 (1952) 209. C. Rudowicz, J. Phys. Condensed Matter, 12 (2000) L417. S.K. Misra, Handbook of Electron Spin Resonance, v01.2, Chapter VII, (eds.) C.P. Poole Jr and H.A. Farach, New York, AIP Press, 1999. S.K. Misra, Handbook of Electron Spin Resonance, v01.2, Chapter VI, (eds.) C.P.
14
19. 20. 21. 22. 23. 24.
25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51.
Poole Jr and H.A. Farach, New York, AIP Press, 1999. S.K. Misra, Handbook of Electron Spin Resonance, v01.2, Chapter IX, (eds.) C.P. Poole Jr and H.A. Farach, New York, AIP Press, 1999. S.K. Misra and J.S. Sun, Magn. Reson. Rev., 16 (1991) 57. J.M. Baker, J. Kuriata, A.C. O'Connell, and L. Sadlowski, J. Phys. Condensed Matter, 7 (1995) 2321. J. Kuriata, J.M. Baker, L. Sadlowski, I. Stefaniuk, and T. Bodziony, J. Phys. Condensed Matter, 10 (1998) 407. D.G. McGavin, J. Magn. Reson., 74 (1987) 19. T.H. Yeom, C. Rudowicz, S.H. Choh, and D.G. McGavin, Physica Status Solidi, (b) 198 (1996) 839. C. Rudowicz and Y.Y. Zhou, J. Phys. Chem. Solids, 55 (1994) 745. C. Rudowicz and H.W.F. Sung, Physica, B 300 (2001) 1. C. Rudowicz, Acta Phys. Polonica, A 44 (1973) 103. C.P. Poole, J.A. Farach, and H.A. Farach, The Theory of Magnetic Resonance, New York, Wiley, 1972. J.E. Wertz and J.R. Bolton, Electron Spin Resonance, New York, McGraw-Hill, 1972. W. Weltner Jr., Magnetic Atoms and Molecules, New York, Dover, 1989. A.J. Hoff, Methods in Enzymology, Vol. 227 (eds.) J.F. Riordan, B.L. Vallee, New York, Academic Press, p.290, 1993. A. Bencini and D. Gatteschi, Electron Paramagnetic Resonance of Exchange-Coupled Systems, Berlin, Springer, 1990. J. Owen and E.A. Harris, Electron Paramagnetic Resonance, (ed.) S. Geschwind, New York, Plenum Press, p. 427, 1972. C. Rudowicz, J. Chem. Phys., 84 (1986) 5045. S.K. Misra and C. Rudowicz, Phys. Stat. Solidi, (b) 147 (1988) 677. C. Rudowicz and S.B. Madhu, J. Phys. Condensed Matter, 11 (1999) 273. C. Rudowicz and S.B. Madhu, Physica, B 279 (2000) 302. C. Rudowicz, S.B. Madhu, N.M. Khasanova, and A. Galeev, J. Magn. Mag. Mat., 231 (2001) 146. H.A. Buckmaster, R. Chatterjee, and J.A. Tuszynski, J. Chem. Phys., 83 (1985) 4001. J.A. Tuszynski, H.A. Buckmaster, R.,Chatterjee, and J.M. Boteler, J. Magn. Reson., 63 (1985) 241. C. Rudowicz and R. Bramley, J. Phys. C: Solid State Phys., 20 (1987) L77. V.G. Grachev, Sov. Phys. JETP, 65 (1987) 1029. R. Chatterjee and H.A. Buckmaster, J. Phys. C: Solid State Phys., 3 (1991) 7079. H.A. Buckmaster and R. Chatterjee, Physica Status Solidi, (b) 209 (1998) 433. M.G. Zhao and M. Chiu, Phys. Rev., B 52 (1995) 10043. M.G. Zhao and Y. Lei, Phys. Rev., B 55 (1997) 8955. C. Rudowicz, Phys. Rev., B 63 (2001) electronic MS# 106401. T.C.Y. Chung and C. Rudowicz, 42nd Rocky Mountain Conference - 23rd International EPR Symposium, Denver, USA, July 2000 [abstract]. C. Rudowicz, Bull. Mag. Res., 12 (1991) 174. C. Rudowicz, Bull. Mag. Res., 16 (1994) 224. C. Rudowicz, Crystal Field Handbook, (eds.) D.J. Newman and B. Ng, Cambridge Univ. Press, p. 259,2000.
EPR in the 2 1’ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
15
Recent developments in low-temperature ESR in quantum antiferromagnetic chains Masaki Oshikawa Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro-ku, Tokyo 152-855 1 Japan
ESR in an antifen-omagnet is essentially a many-body problem. In particular, at low temperatures and in low dimensions the quantum fluctuation is strong, making previous theoretical methods invalid. We review a new theoretical approach to ESR in the S = 1/ 2 quantum antiferromagnetic chain, based on field theory methods which is reliable in one-dimensional systems at low temperature T . It is applied to the system with a transverse staggered field and that with an exchange anisotropy. In the lowest order of the perturbation theory, they lead to a linewidth proportional to 1/ T 2 and T , respectively. These behaviors reflect the strong many-body correlations in the quantum spin chain.
1. Introduction
Although we know, very precisely, the fundamental laws governing an electron and its spin, our understanding of magnetism is still far from complete. This is because a magnetic material often consists of a large number of interacting spins. Electron Spin Resonance (ESR) measurement provides a unique insight into the dynamical nature of magnetic materials. A theoretical approach to dynamics in many-body systems, including ESR, is rather difficult. Theories on ESR in such systems were nevertheless developed most notably by Kubo and Tomita [ 11, and by Mori and Kawasaki [2]. However, because of difficulties in solving the dynamics directly, they relied on several assumptions which would not always be justified. Moreover, a quantitative calculation is difficult when quantum fluctuations are dominant. The S = 1/2 Heisenberg antiferromagnetic chain at low temperature is an extreme example of a system with strong quantum fluctuations, in which traditional theoretical approaches becomes invalid. Interestingly, it is now one of the best understood strongly interacting many-body systems, thanks to the field theory methods. Here we report the recent developments [4,5] of theory of ESR in the S = 1/2 Heisenberg antiferromagnetic chain based on the field theory methods. The dynamical quantities can be calculated rather directly in a more rigorous framework which do not require the assumptions made in the previous theories. In this presentation we try to avoid technical mathematical details, which can be found in Ref. [5], as much as possible to explain the basic ideas and outcomes.
16
2. ESR in an interacting spin system For comparison, let us recall the basics of ESR in a single (or non-interacting) = 1/2. The static magnetic field is applied in direction so that the Hamiltonian of the system reads H=
There are two energy levels, corresponding to Sz = 1/2 and Sz = - 112, separated by the Zeeman energy By applying the electromagnetic wave to this spin, the absorption occurs only if causing the transition the angular frequency w matches with the Zeeman splitting hw = between the two energy levels. This is nothing but the ESR absorption. If one measures the absorption intensity as a fimction of the frequency w, it is given in the present case by 6(w In actual experiments, the frequency w is fixed and the absorption is measured varying the applied field Here we consider the opposite, which is more natural for theoretical analyses. One can translate w) to the experimentally measured spectrum as a function of the field This spectrum is simple but is useful in determining g, and hence gaining an information about the environment surrounding the electron. In the following, I put g = pB = h = 1 to simplify the formulae. They can be recovered when necessary. What if the system consists of two interacting S = 1/2’s? There are four independent states of the system, that is I TT), I TJ), I -IT) and I All physical states, and the energy eigenstates in particular, are given by linear superposition of these four. Thus there are four energy levels in the system, while the actual energy eigenstates of course depend on the interaction between the spins. In any case, having four states, we would have 4 x (4 - 1)/2 = 6 possible transitions. Therefore the the ESR absorption spectrum w ) consists of (at most) six delta functions. We can repeat the same argument for the system of N interacting spins. There are 2N independent states corresponding to T and for each spin. As a consequence, we would expect about 2”-’ sharp resonance peaks to appear in the spectrum. (Actually, there could be less, due to the selection rules.) In the limit of large system ( N + m), it is more useful to smear over the resonances to consider the absorption strength as a continuous function of the frequency w. (See Fig. 1.) This continuous spectrum, or the absorption lineshape, is what is measured in ESR experiments on strongly interacting spin systems, and hence what we want to calculate theoretically. Similar continuous spectra in many-spin systems are also measured in other experiments such as inelastic neutron scattering. However, there is an interesting peculiarity in ESR. That is, our previous statement that should be a continuous function of o notwithstanding, actually consists of a single, infinitely sharp (delta-function) peak exactly at the Zeeman energy, This applies for example to the isotropic Heisenberg spin system 0~
We take the direction of the applied static field as z-axis. The Zeeman term is then given as =
(3)
where Sz = C . S4 is the z-component of the total spin operator. The Hamiltonian of the system under the applied field then is given by 3 = Xo+ ZZ. For such a system, there is no apparent
17
#k smeared
I
H
Figure 1. ESR absorption in the single spin system (left) and in interacting many-spin systems (right). The absorption spectra I ( w ) are shown in the lower half of the figure. A resonant absorption occurs at a frequency w which matches the energy change associated to a transition between energy levels of the system. There are increasing number of the delta-function resonances for more number of spins N . We should introduce continuous absorption spectrum function by smearing it, in the limit of large N .
effect of the interaction on the ESR lineshape, regardless of the interaction strength and other conditions such as temperature. This, rather surprising, fact can be understood as follows. In the present experiments, the absorbed electromagnetic wave is in the microwave to milliwave regime, because the applied field is up to 10 Tesla. Its wavelength is much larger than any other lengthscale in the system, such as lattice spacing. Therefore the phase of the electromagnetic radiation can be regarded as uniform over the entire system. It means that the electromagnetic wave couples to the total spin operator in the system. In this paper, we restrict ourselves to the Faraday configuration, in which the polarization of the magnetic field of the radiation is perpendicular to axis. The coupling is given as Hrd = -HrcoswtSx,
(4)
where S" is the total spin operator defined by S" = z,S?. It is also useful to introduce the total spin raisingllowering operator S* = S? Therefore, the absorption rate for the frequency w at temperature T is proportional to
zj
+
where and labels the energy eigenstates of the Hamiltonian Z = Zo Zzunder the applied field, p = l/k,T where k, is the Boltzmann constant. Namely, the transition from to occurs only if they are connected by a non-vanishing transition matrix element
18
The crucial point to note is that both Xoand Xz commute with the magnitude of the total spin = S(S+ 1) and the z-component of the total spin Sz. Thus all the eigenstates can be classified by quantum numbers S and = Sz. In the absence of the applied field, any energy eigenstate of Xowith total spin quantum number S belongs to a degenerate (2S+ 1) multiplet. Introducing the magnetic field Xz,the multiplet split and the energy of each state is given by
E,, = E,” -
(6)
where E,” is the eigenvalue of Xoand is the eigenvalue of Sz. The operator Sx (which causes the transition) commutes with Xo,while it changes Sz by 1. Therefore it has a non-vanishing transition matrix element only between the two states which belong to the same multiplet (with same eigenvalue and differ in Sz by 1. Because of (6), it means that any transition accompanies the change of energy by As a consequence, the While the lineshape is apparently absorption spectrum has a delta-function peak at = unaffected by the interaction, the energy spectrum is governed by the strong interaction. We stress that ESR probes excitations in the many-body system. Many magnetic systems can be modeled by the isotropic Heisenberg model as a first approximation. However, in a real material, the isotropy cannot be perfect. There should be some kind of anisotropies in the system, the dipole-dipole interactions at least. Let us denote the anisotropic part of the Hamiltonian as X ’ . The total Hamiltonian of the system is now given by
+
+
X = Xo Xz 3’.
(7)
While a small anisotropy 2’would not change other measurable properties of the system drastically, it affects the ESR lineshape fundamentally. In the presence of the anisotropy, the argument leading to the delta-function spectrum breaks down. The absorption spectrum Z,(o) should now be a continuous function of w. It is evident that the calculation of is not easy. If one takes a “brute force” approach to a energy eigenstates by diagonalizing the Hamiltofinite system of N spins, one has to obtain x matrix. In order to calculate the spectrum one further needs to nian, which is a know the transition matrix elements between all the eigenstates. The calculation becomes complicated even for small N , but the computer technology has made the “brute force” approach actually meaningful. A direct numerical calculation of the ESR spectrum, on finite systems [6], who also report with up to N = 10 spins, has been pursued recently by Miyashita et at this meeting. While this type of calculation is surely useful, we still need a complementary analytical approach which directly deals with the thermodynamic ( N = -) limit. The spectrum Z(o), which involves the energy levels and transition matrix elements, is related to the dynamics of the system. Dynamics of a many-body system is generally a difficult problem. Nevertheless, analytic approaches to ESR were developed several decades ago, most notably by Kubo and Tomita [ 11, and subsequently by Mori and Kawasaki [2]. The following general picture has emerged for a small anisotropy X ’ . When the interaction among the spins in Zois small (compared to the applied field the lineshape takes a Gaussian form. On the other hand, when the interaction is strong (Zo >> the lineshape becomes generally Lorentzian. The linewidth 77 was given as
19
where xu is the magnetic susceptibility, = [%’,S+] and denotes the expectation value at temperature T under the unperturbed Hamiltonian (XO Xz).While this approach seems successful for several systems, there are still fundamental problems. Firstly, because of the difficulty in solving the dynamics of a many-body system, the Lorentzian lineshape was never derived rigorously. It was necessary to make some assumptions about the dynamics, which may be valid for some cases but not always so. The formula eq. (8) is valid (in the lowest order of the anisotropy provided that the lineshape is Lorentzian. [5] However, justification of the Lorentzian lineshape remains a problem. Secondly, even if the lineshape is indeed Lorentzian, the calculation of the lineshape by eq. (8) poses a problem, being a dynamical quantity itself. In the high-temperature limit, by further introducing assumptions, eq. (8) is reduced to a static correlation function which can be calculated rather easily. However, at lower temperature, many spins become correlated and the calculation of the correlation functions becomes difficult by traditional means. ESR in an = 1 /2 Heisenberg antiferromagnetic chain
+
(9)
at low temperature offers an extreme case where quantum fluctuations and many-body correlation becomes essential. 3. Field-theory approach to S = 1/2 antiferromagnetic chains
Interestingly, the = 1/2 Heisenberg antiferromagnet at low temperature, which is difficult for traditional theoretical approaches even for the calculation of static quantities, is one of the few quantum many-body systems which we understand the dynamics rather well. A powerful field theory approach to the S = 1/ 2 Heisenberg antiferromagnet has been developed, to provide us an asymptotically exact description of static and dynamical quantities in the low-energy limit. Here we will introduce a rough picture of the field theory approach to the S = 1/ 2 Heisenberg chain, avoiding technical and mathematical details. For the details the readers are referred, for example to Ref. [3]. A system of interacting S = 1/ 2 spins can be regarded as a quantum many-particle system in various ways. For example, “up spin” and “down spin” state can be identified with the site occupied by a particle (without spin degree of freedom). Because each site can have zero or one particle only, the system should be regarded as a system of hard-core (ie. kith infinite onsite repulsion) bosons. In one dimension, because the hard-core bosons never exchange their position, it can also identified with the system of fermions. The transverse part of the exchange interaction SS ;+;l = corresponds to a hopping of the particles to the nearest neighbor, while the longitudinal part S7Ss+I represents an interaction between the particles at nearest-neighbor sites. Another possible way is to consider the physical origin of the S = 1 /2 Heisenberg antiferromagnetic chain. Because the spins are carried by electrons in reality, we may consider the underlying system of interacting electrons (with spin). When the number of electrons is equal to the number of sites, a repulsive interaction between electrons leads to a Mott-insulator state, in which the electrons are localized so that each site is occupied by an electron. At sufficiently low energy scale, the charge fluctuation (which involves change in local number of electrons) is frozen. The spin degrees of freedom still remains alive, with an effective antiferromagnetic interaction between nearest neighbor spins. This is nothing but
+
+
20
the = 1/2 Heisenberg chain (9). Therefore, one can discuss the spin chain by studying an interacting electron system in the Mott insulator regime. In either case, the spin chain problem is mapped to a quantum many-particle problem with strong interaction. This is in clear contrast to more traditional approaches based on classical spin picture. The interacting fermions in one dimension is an important problem in its own. In real physical systems, electrons of course interact with each other via Coulomb force. However, in metallic systems a free electron model is used frequently, to describe various properties rather well at least qualitatively. This is because if one injects an electron to such a system, the electron drags other electrons around itself but this “dressed” electron - called as quasiparticles - behaves much like a free electron, except for its renormalized mass which is different from the original electron mass. Therefore the system is rather similar to (but of course not identical to) a system of free fermions, and is called as a Fermi liquid. Fermi liquid theory has been successfully applied to usual metals and to liquid 3He. However, in one dimension, quantum fluctuations are so strong that the Fermi liquid theory generally breaks down in the presence of interactions. Namely, a dressed electron is no longer a well-defined elementary excitation with a long lifetime. A one-dimensional system of interacting fermions had been often regarded as just a theoretical toy model. However, it is becoming more realistic, thanks to discovery of more quasi one dimensional materials and fabrication of “quantum wires” based on semiconductor technology. Actually, quantum spin chains, which can be regarded as interacting fermion systems in one dimension, has been studied experimentally for a long time. These real systems requires a theoretical framework which does not rely on the notion of independent fermions. It has become known [3] that an interacting fermion system in one dimension is rather better described in terms of bosons. The local density of the fermion fluctuates locally. The fluctuation of the density propagates in space-time, forming a “sound wave.” In quantum mechanics, sound wave consists of bosonic particles, namely phonons. While this argument may apply to any dimensions. the peculiar feature in one dimension is that whole (low-energy) excitation spectrum can be described in terms of the “phonons.” Thus it is possible to study the fermion system entirely in terms of the bosonic field. The beauty of this approach - bosonization - is that a certain class of interacting fermion systems, which includes the = 1/2 Heisenberg antiferromagnetic chain (9), is reduced to just a harmonic density “sound wave,” namely collection of non-interacting “phonons” which describes magnetic excitations. The interaction between the fermions is translated into the “rigidity” of the boson field. When there is a strong repulsive interaction between the fermions, the density fluctuation is suppressed, and the corresponding boson field becomes rigid. Even though the rigidity is renormalized by the interaction, the phonons are still non-interacting. This enables us to calculate various static and also quantities exactly. In the case of a system of interacting electron with spin, there are two kinds of density fluctuations: one with changes in the total density of electron (with up and down spins), and the other accompanies a difference in the densities between up and down spin electrons but with a constant total density. They correspond to charge and spin fluctuations, respectively. Each one is described by a corresponding boson field. In the Mott insulator phase, the charge fluctuations are frozen at a sufficiently low energy, and only the spin fluctuations described by a single boson field remain. Therefore the final description of the = 1/2 Heisenberg antiferromagnetic chain (9) is the same whether one identifies the system as an interacting spinless fermion or a
21
Mott insulator phase of an interacting electrons with spin.
4. Field-theory approach to ESR Let us consider ESR in = 1 / 2 Heisenberg antiferromagnetic chain (9). It turns out that the ESR absorption corresponds to creation of a single bosonic particle (“phonon”). As already discussed in Sec. 2, ESR measures excitations at zero momentum. However, it should be noted that ESR is measured in the presence of the applied field Zzwhich changes the spectrum. More specifically, the applied static field shifts the dispersion relation. To see how this happens, it is convenient to use the spinful electron model. In order to reproduce the spin chain, the charge degree of freedom is eventually frozen. For the moment, however, let us consider free electrons (with spin). Up-spin electrons and down-spin electrons fills the respective bands. At = 0, the dispersion curves for upldown-spin electrons are exactly the same. However, the magnetic field shifts the dispersion curves vertically by i H / 2 . The ground state is given by the up-spin and down-spin electrons filling the Fermi Sea of momentum -kL < k < k i and
-%
%
+%
%
0~
%,
22
Figure 2. The dispersion curves of (non-interacting) electrons in the magnetic field (left). Because they are shifted by in energy for spin-up and spin-down electrons, a spin-flip excitation at momentum zero has energy This spectrum of electrons translates to The dispersion curves of the boson describing the magnetic excitation, shown on right. It is shifted by the magnetic field so that the energy is at momentum k = 0.
zero, which means a boson at momentum &Ak with energy As a consequence, the ESR absorption in S = 112 Heisenberg chain (9) is given by the sharp delta-function at Zeeman After all, this is the result we already knew in Sec. 2. What is the energy I ( @ ) 0~ 6(0 point of introducing the whole business? The answer is that we are now prepared to discuss the lineshape in the presence of an anisotropic perturbation 2', In the absence of the anisotropy, the system can be mapped onto a system of free bosons and ESR measures a creation of a boson. At least certain kind of anisotropy 2' can be mapped to an interaction among the bosons. When there is a weak interaction, the boson is still a well-defined elementary excitation. However, it decays due to the interaction and hence has a finite lifetime z. Because of the finite lifetime, the energy of the boson becomes uncertain, according to the energy-time uncertainty relation AE x At 2 li. Therefore, the ESR spectrum corresponding to the energy of the boson also acquires a finite width 71, which is inversely proportional to the lifetime. The lineshape is can be also determined as Lorentzian, because it is given as the Fourier transform of ePiEt-'Jt. The field theory allows us to calculate the lifetime of the boson, in the lowest order of perturbation Z'.Therefore, we now succeed in formulating the calculation of ESR lineshape of S = 1/ 2 Heisenberg antiferromagnetic chain in the presence of an anisotropy 2'and at low temperature. The present new approach do not rely on the Kubo-Tomita or Mori-Kawasaki theories which depend on non-trivial assumptions.
5. Effect of a staggered transverse field Let us discuss a concrete example of transverse staggered field as the anisotropic perturbation. It means an additional magnetic field which is orthogonal to the uniform applied field and changes direction alternatingly along the chain. Taking the direction of the staggered field as
23
axis, it is written as
In the low energy effective theory of bosons, this perturbation is mapped to an interaction among bosons, and leads to a finite lifetime of the boson, and hence the broadening of the ESR absorption lineshape. For the present case of the transverse staggered field, we find in the lowest order of the perturbation theory, the linewidth at temperature T as
Is there any application of this result, for the apparently unphysical perturbation, to a real material? Cu benzoate has been studied as a typical = 1/2 Heisenberg antiferromagnetic chain. However, several experimental data show peculiar behavior in magnetic field, which cannot be understood as a standard Heisenberg chain (9). In fact, ESR linewidth exhibiting peculiar divergence at low temperature was reported in 1970s [7] for Cu benzoate. The linewidth increases rapidly at lower temperature, and the divergent part depends strongly on the direction of the applied field and also on the frequency (ie. the magnitude of the applied field). The width is larger for larger frequency (applied field). This experimental observation has not been explained for a long time. We have recently pointed out that this peculiar behavior of ESR in Cu benzoate is related to the effective transverse staggered field. Cu benzoate has a low crystal symmetry, in which molecules surrounding Cu2+ ion (which carries spin) are tilted altematingly along the chain. Because of this, when a magnetic field is applied to this material, the effective magnetic field felt by the spins is also tilted altematingly along the chain. This can be understood as a superposition of a uniform magnetic field and staggered transverse field. The effective staggered field h is where the coefficient c depends strongly on the proportional to the applied field H as h = direction of the applied field relative to the crystal axes. This, together with (1 l), basically explains the observed low-temperature anomaly in the linewidth as shown in Fig. 3. There are more theoretical results on Cu benzoate compared to the experimental data. For details the reader is referred to Refs. [4,5,8].
6. Staggered Dzyaloshinskii-Moriya interaction Precisely speaking, the effective staggered field in Cu benzoate is generated from two effects - a staggered component of g-tensor and the Dzyaloshinskii-Moriya (DM) interaction. While
both of them are allowed by the low crystal symmetry, the latter effect is less obvious. In Cu benzoate, the DM interaction is staggered, namely it is given as
For the illustrative purpose, for the moment let us take the direction of 6 as z-axis. The isotropic exchange term (9) and the DM term above are then combined into
24
1200
-
1000 800 -
-
E
400
Experiment (9 4GH.Z) Experiment (21 8GHz) Experiment 1GHz) Experiment (48 9GHz) Theory (9 4GHZ) Theory (21 8GHz) Theory 1GHz) Theory (48 9GHz)
+
0
x
-
1
10
Temperature (K)
Figure 3. The temperature and frequency dependence of the ESR linewidth for H 11 c" [7], after subtracting the frequency independent part which is presumably due to effects other than the staggered field. It is compared our theory (1 I). More details can be found in Ref. [ 5 ] .
where = J + iD. By rotating the spin by the angle + a 1 2 about axis alternatingly along the chain, this is reduced to
namely an antiferromagnetic chain with a "XXZ" type exchange anisotropy. Thus the DM interaction is reduced to an exchange anisotropy. There is another effect when there is an external field. If the external field is applied in direction on the original system, after the alternating rotation the field is tilted alternatingly in plane. This means an effective staggered field h DHI(2J) is generated. For general direction of the applied field and the staggered DM interaction 6, the effective staggered field is 8 x fil(2.T). This exact transformation raises a question in application of the standard Kubo-Tomita theory to the system with the DM interaction. (Recently a similar argument is also presented in Ref. [9] independently.) Namely, different results are obtained, if one applies the Kubo-Tomita theory before and after the transformation. The direct application, without using the transformation, has been discussed in the literature. However, within the Kubo-Tomita theory it is not clear which gives the correct result. In other words, it is not obvious a priori to which system the application of the KuboTomita theory is justified. On the other hand, in the field theory approach, we are somehow forced to make the transformation first, because the spectrum can be calculated without any nontrivial assumption for the staggered field, but not directly for the DM interaction. This would imply that the (staggered) DM interaction must be eliminated first by the exact transformation above before applying the Kubo-Tomita theory even at higher temperature. Since the field theory approach does not apply to the higher temperature, where we do not have a rigorous theory yet, we are not sure what is the correct answer. In any case, this example demonstrates that a naive application of the standard Kubo-Tomita theory cannot always be N
N
25
trusted. 7. Exchange anisotropy
An exchange anisotropy (or the dipolar interaction, which has almost the same effect) given by
is perhaps the most common anisotropy. In the effective boson field theory, this is again mapped onto an interaction among the bosons, but the interaction is different from that in the staggered field case. It is usually argued [2] that the linewidth in an antiferromagnet (in higher dimensions) caused by the exchange anisotropy increases when the temperature is lowered from infinity, and diverges towards the critical point of the ordering transition. In the one dimensional model (9), there is no phase transition at finite temperature. Nevertheless, zero temperature may be regarded as a critical point. Thus naively one might think that the linewidth due to the exchange anisotropy also diverges when the temperature is lowered to zero in the S = 1/2 Heisenberg antiferromagnetic chain. However, again we can calculate the linewidth more rigorously using the boson field theory, to obtain [4,5] a completely different result
in the lowest order of the perturbation 6. Namely, the linewidth rather decreases when the temperature is lowered to zero. We remark that the field theory is only valid as a low-energy effective theory, and thus the above result holds only for << J . This result presumably explains the temperature dependence of many S = 1/ 2 Heisenberg antiferromagnetic chains without the staggered field effect. The examples would include CuGeO,, KCuF, and NaV,05. [4,10,5] 8. Conclusions
ESR in an interacting magnetic system probes a dynamical information of the many-body system. It is a theoretically difficult problem and thus the existing theories relied on some nontrivial assumptions, which would be true for some cases but not always justified. Based on the field theory approach, which is asymptotically exact in the low energy limit, we have constructed a new approach to ESR in S = 1 / 2 Heisenberg antiferromagnetic chains. It allows us to calculate the dynamical quantity rather directly, without relying on the assumptions as in the previous theories, giving a more rigorous foundation to the calculation. The result explains the peculiar experimental data on Cu benzoate quite well. ESR measurement provides us a unique and very precise information on a strongly interacting magnetic system. On the other hand, it is also an interesting and challenging problem for theorists. We hope that the recent developments in S = 1/2 Heisenberg chain stimulates further progress in this exciting field.
26 2 1.8 1.6 1.4
1.2 1 0.8
06 0.4
0.2
n 0
0.2
0.4
0.6
08
1
1.2
1.4
Figure 4. The temperature dependence of the ESR linewidth in KCuF3, CuGe03 and NaV,O,. The experimental data are taken from Refs. [ 111. The horizontal axis is the temperature T normalized by the exchange coupling J , and the vertical axis is the normalized linewidth.
Acknowledgements I thank the organizers of the APES'O1 meeting, especially Professors Asako Kawamori and Hitoshi Ohta, for for inviting me to the plenary lecture and for their efforts in organizing the conference. I am also grateful to Prof. Ian Affleck for a fruitful collaboration on which this paper is based. This work is supported in part by Grand-in-Aid from MEXT of Japan. REFERENCES
1. R. Kubo and K. Tomita, J. Phys. SOC.Jpn. 9, 888 (1954). 2. H. Mori and K. Kawasaki, Prog. Theor. Phys. 27, 529 (1962); H. Mori and K. Kawasaki, Prog. Theor. Phys. 28,971 (1962). 3. A. 0. Gogolin, A. A. Nersesyan and A. M. Tsvelik, Cambridge University Press (1998). 4. M. Oshikawa and I. Affleck, Phys. Rev. Lett. 82,5 136 (1999). 5. M. Oshikawa and I. Affleck, preprint (2001). 6. S. Miyashita, T. Yoshino and A. Ogasahara, J. Phys. SOC. Jpn. 68,665 (1999). 7. K. Okuda, H. Hata and M. Date, J. Phys. SOC.Jpn. 33,1574 (1972) 8. T. Asano, H. Nojiri, Y. Inagaki, J. P. Boucher, T. Sakon, Y. Ajiro and M. Motokawa, Phys. Rev. Lett. 84, 5880 (2000). 9. J. Choukroun, J.-L. Richard and A. Stepanov, Phys. Rev. Lett. 87, 127207 (2001). 10. A. A. Zvyagin, Phys. Rev. B 63,172409 (2001). 11. I. Yamada, H. Fujii and M. Hidaka, J. Phys. Condens. Matter 1, 3397 (1989); I. Yamada, M. Nishi and J. Akimitsu, J. Phys. Condens. Matter 8,2625 (1 996); I. Yamada, H. Manaka, H. Sawa, M. Nishi, M. Isobe and Y. Ueda, J. Phys. SOC.Jpn. 67,4269 (1998).
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
27
Direct numerical study on ESR line shape S. Miyashita and A. Ogasahara Department of Applied Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan We have studied temperature dependence of the line shape of ESR for strongly interacting quantum spin systems. We have attempted to characterize the system by dependence on the relative angles among the axis of the system, static field and RF field. We exploit direct numerical estimation of the dynamical susceptibility with the Kubo formula. We have applied this method to several systems and confirmed validity of the method. We show several examples of temperature- and field-dependence of the line shape.
INTRODUCTION In low dimensional spin systems, the ESR line shape depends on the geometrical situations among the lattice axes, the static field and the oscillating filed, where the dipole-dipole interaction takes an important role. The famous example is the case of the one dimensional Heisenberg magnet. Models with pure Heisenberg interactions result in only the paramagnetic resonance regardless of the spatial structure of the interaction. Therefore we need some perturbations which violate the SU(2) symmetry to have line shift or broadening of the resonance. Usually, the dipole-dipole interaction takes place of this perturbation. Dependence of the line shape of the paramagnetic resonance due to the dipoledipole interaction has been studied. It depends on the the static angle field between chain axis and as the shown in Figure 1. The angle- and temperaturedependence of the shifts were studied by Nagata and Tazuke[l] with the effective mode method proposed by KanamoriTachiki.[2]. The shift also depends on the angle 4 (Figure 1) between the RF field and the chain axis when the static field is applied perpendicular to the chain.
-
p'c
J)Z2 p H
WlelWi
I:
Figure 1. Geometrical configuration among the fields and the lattice.
28
We have studied these dependences by an direct numerical method on the Kubo formula[3] with the full Hamiltonian of the spin system including dipole-dipole interaction. We have successfully reproduced the dependences. [4] We have applied this method to spin systems with various perturbations on the isotropic Heisenberg interaction, such as the dipole-dipole interaction, anisotropy of the interaction, the Dzyaloshinsky-Moria (DM) interaction, etc., which cause the shift of paramagnetic resonance. We have studied the effects of lattice shapes. Our method is exact although only small lattice can be treated, and we can discuss the temperature and angle dependence explicitly. 2. SYSTEM AND METHOD
In order to study the ESR line shape we study the imaginary part of the dynamical susceptibility[4]
Here we adopt simply that the autocorrelation function a pure quantum mechanics of the system Hamiltonian =
+
- (H,,
+ Hleiwt). M ,
(0)
is calculated by (2)
(3)
where is strength of the static field and is that of oscillating field (RF field). We diagonalize the Hamiltonian and obtain all of the eigenvalues and eigenvectors and calculate (1). Thus although the result is exact, we can treat only small sizes, i.e., 10 spins of = l / 2 at present. The resonant frequencies are obtained from the difference of eigen energies of the given Hamiltonian with a given value of Thus I1 XZZ for the given value of is obtained. In experiments, on the other hand, the frequency is fixed to be a value w and the field for a given value is swept. Thus x;Cz of w is obtained. In order to compare the peaks in the same definition we study the line shape as a function of the field as in the experiment. For this purpose, we calculate x;Z for various values of and make a Figure 2 . Field-frequency dependence kind of histogram in coordinate and of x ” ( w ) . convert them to for a given value of w as shown in Figure 2.
29
3. EFFECT OF ANISOTROPY
It is interesting problem how the anisotropy of the interaction affects on the shift. Here we study the dependence on the quantity A = First we show the case of XY anisotropy A = 0.9. Here we find almost the same shape as that of the pure Heisenberg model, but if we look at it carefully the sign of the shift is opposite. When we change the anisotropy little by little from A = 1.0 to 0.9, we find that the shifts for both case H o l c and Ho//c become small and merge at A = 0.97. Then they separate away and reach to the shape of A = 0.9. On the other hand the Ising anisotropy causes a complicated dependence. This case corresponds to the so-called antiferromagnetic resonance. [5] ESR of antiferromagnets has been studied extensively as the antiferromagnetic resonance (AFMR) in both theoretical and experimental sides.[6, 71 In the theoretical studies, properties of resonance in the ordered state of systems with weak anisotropic antiferromagnetic interaction have been studied by investigating the eigenfrequencies of the motion of the order parameter, i.e., sublattice magnetizations. Thus the resonant frequencies are obtained as functions of the external field. Such theories have succeeded to explain complicated structure of the resonance of antiferromagnets such as CuClz.[8] The corresponding resonance in full has been obtained by recent development of the range of the frequency w and the field high-field technology in an experiment of RbzMnCla. [6] The temperature dependence of the resonance could be obtained by studying the temperature dependence of the order parameter. Usually the temperature dependence is taken into account through the temperature dependence of the susceptibilities X I I and XI. So far mainly the properties in the ground state has been studied. We apply the present method to spin clusters in which spins align antiferromagnetically, and compare with the well-known AFMR.[9] Because of the finiteness of the lattice, the system does not show the phase transition. However, spins align antiferromagnetically in the system and we obtained field dependence of the resonance which agrees with the AFMR. Besides AFMR, we also found that the paramagnetic resonance (PMR) survives until rather low temperatures. Thus we found three peaks of resonance. PMR is originated in the motion of domain wall between the AF orders.[lO] 4. GEOMETRICAL DEPENDENCE
We study the temperature- and frequency-dependence of the shift on various lattices. 4.1. Ladder The lattice modeling the BIP-TEN0 has a generalized ladder structure. In Figure 3(a) the bond depicted by shin lines are antiferromagnetic, while the bold lines denote the ferromagnetic bond. The ferromagnetic bonds causes the symmetrization of the = 112 spins, and therefore the present model may be regarded as a ladder of = 1 spins. We studied the original = spin systems in a cubic lattice which is an elementary unit of the lattice.
30
JF
Figure 3. Box interaction (BIP-TENO): Bond configuration (a), and shifts for (b)d = 9.234A, (c)d = 12.000A, and(d)d = 18.468A.
As sown in Figure 3(b),(c) and (d), the amount of the resonance depends on the distance d which controls the strength of the dipole-dipole interaction along c-axis D,. In the shifts show the same sign as the case of the linear chain (say, the case of small Nagata-Tazuke like), while they show opposite sign when is large. In experiment of BIP-TENO[11], this latter case is observed. Here it should be noted that the shift begin at about 5K which is much lower that the temperature of the peak of the susceptibility 70K). It has been found that this large shift is due to the structure of the lowest triplet states. We also found that a small shift with the same amount of the experiment begins around which is due to the averaged effects of many levels. The latter shift is attributed to the cooperative effect of the lattice, and if we study a two-spin system, we find only the former shift. Actually if we study the systems with ten the shift at low temperature does not change much times larger exchange interaction because the shift is mainly due to the dipole interaction. Here we studied a complicated lattice motivated by BIP-TENO, but the same idea is valid for simple ladder system as well.
-
4.2. Zigzag Chain
We also study the shifts of a zigzag chain consisting of ferromagnetic bond and antiferromagnetic bond JAF. We consider the geometrical situation depicted in Figure 4( a). In Figure 4(b), we show the In the present paper, we only study the case J J p J= JJAFI.
31
shifts when the static field is parallel to the ferromagnetic bond and the antiferromagnetic bond when = 7r/2. Here we find that the antiferromagnetic bond gives the dominant contribution. The shift depends on the q 5 sinusoidal ~ way. We also studied the cases with different values of 27r/5,~/5),which causes the phase shift (Figure 4(c)). =
/
Figure 4. Zigzag chain configuration (a), and shifts for (b), (c) dependence on
and
and
Hexagon The shifts of the hexagon are also studied. We found that the shift is almost the same as one of the linear chain when the static field is applied perpendicular to the plane. On the other hand the amount of the shift is reduced when the field is applied in the plane. We found that the shift has little dependence on the angle in the plane. 5 . DISCUSSION
We have also studied the shift in the magnetization process and found that the shift shows a step-like structure according to the magnetization. When we increase the strength of the static field, that is at large frequency, the system has finite magnetization. In the case that the system size is small, the magnetization changes step-wisely. In such case the shift also changes step-wisely, although the shift linearly increases with the field at low field. Such frequency dependence is reported elsewhere.[12] The effect of magnetic interactions in the ESR line shape has been studied in the view point of the field theory[l3] and field- and temperature-dependence which gives the behavior due to the collective motion. It would be an interesting problem to interpolate their results with ours. As future problems, we are also studying the effect of spin density distribution and also the effect of electron hopping. In organic materials, the spin does not localize at an atomic site but distribute widely. In such case the role of dipole-dipole interaction becomes ambiguous. In a dimer system[l4] we expect that the theory and experiment agree with quantitatively. However, the temperature dependence does not agree yet. Furthermore, we expect the electron hopping may also affect ESR line shape, although the spin part and hopping term [15] are usually decoupled. Taking into account the dipole-dipole interaction this effect should be clarified. We would like thank Professors H. Ohta and K. Katsumata for encouraging and valuable discussions. This study is partially supported by the Grant-in-Aid from Ministry of
32
Education, Science, Sports, Culture and Technology.
REFERENCES K. Nagata and Y. Tazuke, J. Phys. SOC.Jpn., 32 (1972) 337. J. Kanamori and M. Tachiki, J. Phys. SOC.Jpn., 17 (1962) 1384. R. Kubo, J. Phys. SOC.Jpn., 12 (1957) 570. S. Miyashita, T. Yoshino and A. Ogasawara, J. Phys. SOC.Jpn., 68 (1999) 655-661. T. Nagamiya , Prog. Theor. Phys., 6 (1951) 342 and 350. M. Hagiwara, K. Katsumata and J. Tuchendler, J. Phys. Condens. Matter, 6 (1994) 545. 7. T. Nagamiya, K. Yoshida and R. Kubo, Adv. Phys., 4 (1955) 1-112. 8. C. J. Gorter and J. Haadel, Physica, 18 (1952) 285. 9. A. Ogasahara and S. Miyashita, J. Phys. SOC. Jpn., 69 (2000) 4043. 10. H. Shiba and K. Adachi, J. Phys. SOC.Jpn., 50 (1981) 3278. 11. H. Ohta private communication. 12. A. Ogasahara and S. Miyashita, 1P43 'Geometrical Dependence of Dynamical Shift', APES2001. 13. M. Oshikawa and I. Affleck, Phys. Rev. Lett., 82 (1999) 5136. 14. Y. Teki, Y. Ohmura, K. Itoh and Y. Miura, Mol. Cryst. Liq. Cryst., 306 (1997) 315. Phys. Rev. Lett., 85 (2000) 1742. 15. M. Lohmann 1. 2. 3. 4. 5. 6.
EPR in the 2 1* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
33
g Tensor of Er3+centers in axial symmetry C.A.J. Ammerlaan Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, NL 1018 Z Amsterdam, The Netherlands A scheme for the numerical calculation of Zeeman splitting factors for erbium ions in a crystalline environment is described. The examples of crystal fields of trigonal or tetragonal symmetry are presented in some detail. From the results it is concluded that the trace of the g tensors can be remarkably constant upon distortion fkom initially cubic symmetry to a lower axial symmetry.
1. INTRODUCTION Over the past forty years an impressive data base on the Zeeman effect of erbium ions in crystals has been brought about. For more than 70 such centers the g tensors for splitting of energy levels in a magnetic field have been reported. This paper discusses analysis of these data of which a summary with references is given in Ref. 1. The ion Er3' electronic configuration 4 f ' and possesses orbital momentum L = 6 and spin S = 3/2, resulting in a 52-fold degeneracy. By spin-orbit interaction moments couple to form levels characterized by quantum number J, which can take the values 15/2, 13/2, 11/2 and 9/2. For erbium, the J = 15/2 level is the ground state. In a crystal the still 16-fold degeneracy of the ground state is further reduced by formation of doublet and quartet levels and spin resonance is observed in these states. In cubic symmetry the resonance in states of r6 symmetry type has isotropic g value g = 6.8; for the resonance in the doublets the g tensor is an isotropic g = 6.0. These theoretical predictions have been abundantly confirmed by experimental observations. In axial symmetry the Zeeman splitting becomes anisotropic and will be described by a tensor interaction with the principal values g// and gl. Some fifty spectra of axial centers have been described [ 13. Applying perturbation theory, it was shown already in an early paper [2] that for small axial distortion the trace + 2gl of the g tensors remains constant. Also for a purely axial field the g tensors can be derived by analytical means. In an axial field the states quantize as 115/2,m~>with mJ = *1/2,.....+15/2. For instance, for the transition between states in the doublet 115/2,*1/2> the g tensor is normally quoted as g// = 1.2 and g l = 9.6. Its trace + 2 g = ~ 20.4 equals the trace of a I'6 state in the cubic symmetry, establishing an apparent relation between the two cases. In interpreting experimental data caution is in order, as both in theory and experiment the sign of a g value is not easily determined. In the theoretical calculation upper parallel spin state and lower antiparallel state should be identifiable. This is straightforward in few cases only, such for
34
instance, a magnetic field parallel to the axis of crystal field with axial symmetry. Also, in experiment normally only the energy difference between Zeeman split levels is measured and is taken positive. To establish the relation to spin quantization requires a dedicated set up [3]. If the principal g values of the doublet 115/2,k1/2> have opposite signs the calculated trace becomes s18.0, equal to the value of a doublet in cubic symmetry. Numerical calculations as presented in this paper allow the variation of g values continuously to be followed as a function of axial field between the extreme limits of pure cubic and pure axial field. This allows to follow the variation of the trace and to detect possible changes of sign. Analytical treatments which are restricted to the limiting cases do not have this feasibility.
2. OUTLINE OF COMPUTATIONAL METHOD Energy levels in zero magnetic field are obtained by solving the eigenvalue equation for the crystal field potentials in the basis set of the 16 states for the J = 15/2 spin-orbit ground state. A cubic field, valid for Td symmetry, has the forth- and sixth-order contributions
vcu4 = 35(x4+ y4 + z4)
-
2ir4
and
+ y6 + z6) - 3i5(X4+ y4 -k Z4)? + 90r6.
vcu6 = 23i(X6
Representative expressions for axial trigonal and tetragonalpotentials are, respectively, Vh = xy + yz + zx and
v, = 2. Equivalent crystal-field operators Kf acting on spin the states IJ,mJ>are derived fiom the potentials. A general expression will have the form
Kf=Vc~cosp(sina.Ku4+ cosa.KU6)+ sinfl.H@,te], Parameters a and p, with -90' 5 a$ 5 +90', describe the relative strengths of various contributionsto the potential and Vcfthe total strength. To obtain the Zeeman effect, the energy due to a magnetic field is added to the crystal field Hamiltonian. This energy is given, directly in operator form, by
For the ground state of Er3+the Land6 factor has value = 615, which is experimentally well confirmed to be the applicable value in the cubic r6 and r7 states. By this feature of not adding new fieely adjustable parameters, the Zeeman effect is a valuable tool in spectroscopy.
100
> Q)
30
60Graph2W"2'5a 90
Parameter CI (degrees) Figure 1. Crystal-field energies in cubic symmetry of the spin-orbit ground state J = 1512 of i= the E?+ ion. States are labeled with their symmetry type or F7 for the doublets and (r&, 1, 2 and 3, for the quartets. Parameter in the range -90" < < +90" controls the mixing of fourth- and sixth-order contributions to the cubic crystal field. Parameter Vcf> 0. 3. ENERGIES AND g VALUES In the absence of axial fields, the calculated energy level diagram for cubic symmetry is given in Figure 1. The results are equivalent to the classical data of Lea, Leask and Wolf [4], but are presented in a form matching the parameters V,f and introduced in Equation (5). The calculations predict a ground state of r7 symmetry for -90 5 -40.4", for instance, therefore, for the case of pure 4th-order cubic field = -90"), with corresponding g value g = 6.0. In the range -40.4' 5 5 +54.5" there will be a r6 type ground state, with g value g = 6.8. This includes the case of pure 6th-order cubic crystal field = 0"). In the remaining range +54.5" 5 a 5 +90" the ground state is a Ts quartet, to be described by an effective spin J = 312 with a Hamiltonian including cubic terms and with an anisotropic spectrum. With an axial field present energy diagrams are given in Figures 2(a) to 2(d), for two selected cases of cubic potential, = -90" and = 0", and including trigonal and tetragonal cases. In the lower axial symmetry all degeneracy is lifted and the energy spectrum consists of eight doublets. Crossings of levels a fbnction of the relative strength of the axial field, specified by parameter p, ffequently occur. Figures 3(a) to 3(d) present the calculated principal g values and g l for the ground states of the considered cases, as well as tensor trace g// + 2 g ~The . sudden changes in ground state properties are related to level crossings at particular values of p. Figures 3 indicate that for small values of the trace remains constant at the value 18.0 for states around the r7state and at 20.4 near the r6state. The numerical
36 500).
,
,
,
.I,,
,
I
, , I , ,
*9/2 +9/2 W
Parameter p (degrees)
I '
-60
30
Parameter p (degrees)
Parameterp (degrees)
~Tmi"""""'" ' I
"
I
"
' " ' '
I
'
30
'
*-h*Wll*ll."., I '
'
I
60
Parameter p (degrees)
Figure 2. Energies of the eight doublet levels for fourth- or sixth-order cubic crystal field together with a second-order trigonal or tetragonal crystal field calculated from Equation ( 5 ) for (a) a = -90", trigonal, (b) a = O", trigonal, (c) a = -90", tetragonal and (d) a = 0", tetragonal and positive V,f. Parameter p in the range -90" < p < +90" controls the mixing of the cubic and the axial crystal fields.
37
Trace .......... .................. -
..................................
....
..................................
-
........................
L
-5 -90
UMrmmm
Parameter 3! (degrees)
15
J
10
3
5
"
-90 Parameter p (degrees)
I
"
I
"
1 '
" '
tGJ
'
-60 Parameter p (degrees)
Figure 3. Principal g values and and trace + 2gl for transitions in the lowest-energy doublets. Illustrated cases, (a) a = -90°, trigonal, (b) a = O", trigOna1, (c) a = -90°, tetragonal and (d) a = O", tetragonal, correspond to the energy diagrams shown in Figure 2. Parameter in the range -90" < p < +90" controls the mixing of the cubic and the axial crystal fields.
38
calculations provide a full confirmation of the early result based on perturbation theory [2], for both trigonal and tetragonal axial fields, both for states derived fkom r6 or r7 states in the cubic symmetry, and for ground or excited states. Positive values of the parameter p correspond, according to Equation (5), to equal signs of the cubic and axial potentials. In the range 0" I p I +90" energy curves of the ground state do not tend to cross the first excited state in the crystal field. If so, then at p = +90" the doublet state 115/2,*1/2> is reached. Transitions within this doublet in the axial field have the g values = 1.2 and g l = 9.6. Inspection of Figure 3(b) for a = 0" shows that positive values are to be taken for both and resulting in the trace 20.4, equal to the trace 3g of the isotropic r6 state. For = -90" the Figures 3(a) and 3(c) show that upon mixing axial field into the cubic field the principal value g/l changes sign in the interval 0 p +90". At p = +90° the better choice for is therefore g/l= -1.2. The trace + 2 g = ~ 18.0 at this point is equal again to the trace 3g of the cubic symmetry state fkom which the axial state can be considered to be derived. For negative values of p, on increasing the axial potential, the doublet 115/2,&15/2>will be reached, or, in case a level crossing occurs, the doublet 115/2,*13/2>. The g tensors for transitions within these doublets have components = 18.0, = 0 and = 15.6, = 0, respectively. It is then immediately clear that for states derived fkom r6 in cubic symmetry, with trace 20.4, the trace cannot be a constant. In contrast, for the doublet related to the state, with trace 18.0 this might well be the case. Indeed, Figure 3(c) shows, the case of tetragonal distortion for = -90", reveals a remarkably constant value of its trace also for negative values of p. For this particular case, over the whole range of p, the trace can decrease a bit below its limiting value of 18.0, but never falls below 17.95. The more substantial reductions of trace occur in the region of adjoining p = -90". It will be noted, however, that for the corresponding tensors = 0. This implies that the states are EPR silent; these resonances with the reduced trace are not observable. It adds support to the empirical fact that observed resonances for the erbium ion its threefold ionized state are characterized by traces in the range fiom 18.0 to 20.4.
REFERENCES 1. C.A.J. Ammerlaan and I. de Maat-Gersdorf, Appl. Magn. Reson. 21 (2001) 13-33. 2. H.R Lewis and E.S. Sabisky, Phys. Rev. 130 (1963) 137c1373. 3. F.V. Strnisa and J.W. Corbett, Cryst. Lattice Defects 5 (1974) 261-268. 4. K.R. Lea, M.J.M. Leask and W.P. Wolf, J. Phys. Chem. Solids 23 (1962) 1381-1405.
EPR in the 21” Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
39
Continuous wave and pulsed EPR spectroscopy of paramagnetic ions in some fluoride, silicate and metaphosphate glasses S.C. Drewa* and J.R. Pilbrow” ”School of Physics and Materials Engineering] P.O. Box 27, Monash University] Victoria] Australia] 3800 Low concentrations of paramagnetic transition metal or rare earth ions often found in commercial glasses are ideal spin probes to explore the local structural disorder intrinsic to these materials. Investigations of the EPR spectra of glasses have raised the question as to the whether the spin Hamiltonian model provides a valid basis for interpreting spectra from disordered solids. This is because the connection between the spin Hamiltonian and its environment is subtle and indirect, the more so for the likely C1 spin-probe site symmetry in a glass. CW simulations of random network glasses is thought to require a distribution of spin Hamiltonians for C1 point symmetry sites (stochastic model) whereas powder simulations should be appropriate for micro-crystallite glasses. Our present understanding concerns transitions in lightly doped glasses where individual spins are well separated; it is ‘bottomup’ thinking rather than ‘top-down’ thinking normally applied to EPR simulation. 1. SIMULATION OF C W SPECTRA OF GLASSES CW-EPR of glasses has been modeled with a variety of levels of sophistication [l,2]. The typical transition metal ion EPR spectrum extends over many kilogauss at 9 GHz. The goal of simulation strategies down the years has been to explain features often observed in normal first derivative CW-EPR spectra, however the recent trend has been towards a characterisation of the entire spectrum. Most EPR in glasses is interpreted using powder models, which means that in effect one is assuming a micro-crystallite view of glass structure. Oxide and silicate glasses are believed to have random network structures. The currently accepted view is that the classic oxide glasses are an extended 3D network without symmetry or periodicity] with inter-atomic forces that are essentially the same as in crystals. That they are not microcrystallite structures follows from x-ray line broadening, which sets a limit on possible crystallite size, and the absence of intense small angle scattering [3]. X-ray scattering radial distribution functions (RDF) do not clearly discriminate between random network and micro-crystallite models of glass; this is due to the nature of statistical averaging inherent in the experiments [4]. In vitreous silica (SiO2), the average Si-Si distance is 0.36nm and no order exists for Si-Si distances 2 l n m [5, p.11. In silicate glasses, Si N
N
*Supported by the Monash Graduate Scholarship.
40
atoms are coordinated to four oxygens at the corners of imperfect tetrahedra [5, pp.25 f€l; estimates of the distribution of Si-0-Si angles give /3 = 160 20’ [6, p.282,284]. Short planes or chains of alkali or alkaline earth ion modifiers in multi-component silicate glasses define intermediate range order. The situation is not so clear for fluoride glasses although it is generally accepted that there exist octahedral sites for Cr3+ and Fe3+ in transition metal fluoride glasses (TMFG) [7,8] and higher coordination numbers for Gd3+ in TMFG that vary from site to site [9]. Nevertheless, the rare-earth ions are found in a more regular environment than in oxide glasses. Ce3+ ions in fluorozirconate glass are thought to occupy 8-fold coordinated square anti-prism sites [lo]. Griscom [ll]has this to say: “glasses differ from simple crystalline powders in a fundamental way: The intrinsic randomness of the vitreous state gives rise to statistical distributions of crystal fields that are reflected to various degrees in many spectroscopic properties”. That statement ignores the fact that randomness requires C1 point symmetry at each spin probe site. Thus ingenious explanations such as that due to Peterson [12,13] for the g 4.3 line based on the lowest Kramers doublet, assuming axial symmetry and a joint distribution function for correlated variables gll and g l is immediately ruled out. Brodbeck and Iton [14] provided helpful insights regarding the features for = 712 (Gd3+ and Eu2+) EPR spectra in glasses, particularly the g 6 feature which occurs for a range of values, provided the limiting value = 113 is included in the range. Their powder like simulation included only 2“d order spin Hamiltonian parameters with a very broad distribution and an arbitrary lineshape function of 80G width. To investigate the possibility of simulating the EPR spectrum of a random network glass, we have written a simulation program using the Eigenfields method [15] which has complete generality [16]. Any distribution of spin Hamiltonians and their parameters can be generated using a Monte Carlo approach and the EPR spectrum is built up from a summation over all randomly generated sites. Although the aim is to use delta function resonance lines without invoking an arbitrary linewidth, these lines must still be ‘binned’ to obtain a meaningful spectrum, because the resonance lines obtained using a Monte Carlo approach are not evenly spaced in the magnetic field dimension. Thus, a linewidth is involved in the sense that binning resonance lines with widths (typically 1-10G) imposes an effective residual linewidth of If we assume the glass is a network then our disorder model should be based upon site-to-site uniqueness and a random number generator should be used to generate the principal values and orientations of the interactions. The question which is hard to answer is ‘what choice of distribution of spin Hamiltonians is appropriate for a given glass and how can we physically justify our choice?’ A popular choice for high spin ions is the Czjzek function [7,8] which is used to correlate the probability densities of the and parameters of spin probes. The Czjzek method has been applied to = 312, = 512 and S = 712 transition metal ions in fluoride glasses [7,8], however a residual linewdith of up to 80G was still used. The Czjzek distribution was also trialled for S = 512 in an oxide glass by Yahiaoui it et al. (1994) but was in this case found to be unsatisfactory in comparison to a two-dimensional Gaussian distribution. Again the linewidths were up to 80G. Legein al.[8] responded to this inference by noting that Fe3+ coordination in oxide glasses is not yet clearly established. Thus there may exist more than one type of coordination site and so more than one distribution may be needed. N
N
41
Nonetheless, the Czjzek distribution is not expected to be appropriate for oxide glasses since it is based upon Newman's superposition model [17] and a point charge model of this kind is only expected to be of relevance when the structure is highly ionic as for the TMFG's. Even so, it has been noted recently that while the superposition model can be used to predict the 4th and 6th order crystal field terms for S-state ions, it is not reliable for the 2nd order terms as these require further correction [lo]. Thus, the ability to choose a set of fine structure parameters that adequately reflect the nature of the environment of the spin probe in a glass is not a simple task. For a system such as Cr3+, where g is essentially isotropic and the second order fine structure tensor is the only appreciable anisotropic interaction, the Monte Carlo method in triclinic symmetry is mathematically equivalent to a powder model with orthorhombic symmetry. This is because a set of randomly oriented 'non-coincidences with the lab frame' (C1-symmetry D tensors) is equivalent to an average over all Bo and B1 orientations with respect to the canonical directions of an orthorhombic D, provided one assumes there is no preferred orientation of D. However, the underlying physics of the two models is still different. For sites with symmetry lower than orthorhombic, there is no direct relation between the immediate surroundings of the ion and the second order tensor [18]. If there exist two or more anisotropic interactions then the stochastic and powder simulations will lead to different results since the stochastic model can have distributions of non-coincidences bewteen the different sets of principal axes in addition to a the random orientation of Bo and Trial runs for Cu2+ indicate that simulation of many spectra require that g and A have coincident principal axes and near axial symmetry (Figure 1). This is in accord with what is found in inorganic complexes, where, in a sense, copper usually constrains its own environment. So one presumes this means at the local level copper binding to the usual four oxygens in silicate glasses is near planar and not tetrahedral. This is a very interesting chemical inference on the basis of a few simulations. However, this must be weighed against the fact that these simulations have not so far taken into account any correlations between g and values or the tensor orientations. For = 512 and 712, inclusion of only an isotropic Zeeman interaction and the 2nd order fine structure tensor also leads to a powder-like result, for the same reasons as above. Furthermore, it produces sharp spikes on the main features in the simulated C W spectrum which naturally become more pronounced as the bin width is reduced. The inclusion of higher order terms removes the equivalence with a powder model by introducing another anisotropic interaction that can be randomised with respect to the 2nd order fine structure. The 4th order terms can smooth out the very sharp features obtained in their absence, as shown for = 712, for example (Figure 2). Note that the choice of distribution at this point is completely ad and has no physical basis. However we wish to indicate that the residual linewidth could in principal be substantially due to these higher order interactions. The pseudo-symmetry method of Michoulier and Gaite [19] represents one way of linking these terms to low site symmetries, but it is unclear whether this method could be meaningfully applied in our Monte Carlo model. There are perhaps too many random parameters to pursue these higher order terms in C1 symmetry.
42
Figure 1. Computer simulation of a Cu2+ system. = 1/2; = l/2; 9GHz; 300K; (Gaussian) (gz)avg= (gy)avg= 2.06; (gz)avg= 2.37; (standard deviation) Age = Agy = = = 0 . 0 0 2 ~ m - ~AA, ; = AA, = 0 . 0 0 0 5 ~ m - ~ ; Agz = 0.01; (Ax)avg = 0.014cm-'; AA, = 0.005cm-'; bin width=lG. Left: coincident g and A, 250000 sites; 4 000 000 transitions. __ Right: random relative orientation of the principal axes; 285 000 sites; 4 559 776 transitions.
2. EXPERIMENTAL Commercial silicate glass samples were provided by Mr Peter Lowe, Pilkington Glass, Melbourne. The ZBLAN glass samples were made by Dr Peter Newman and Professor D.R. MacFarlane within the Department of Chemistry, Monash University. Metaphosphate glass samples were obtained from Professor G. Saunders & Dr Richard Martin, University of Bath, UK. All EPR experiments were carried out with a Bruker ESP380E FT spectrometer equipped with an Oxford Instruments helium flow cryostat, a dielectric resonator and a 1kW TWT amplifier. 3. PULSEDEPR 3.1. Field-swept pulsed EPR of glasses The ideas presented above should also be directly applicable to the simulation of the pulsed field-swept spectra of glasses. However, the task is somewhat more complicated in instances of systems characterised by large fine structure interactions or large Zeeman anisotropy. Here transformation to the rotating-frame fails to remove the time-dependence from the Hamiltonian during a microwave pulse, due to the deviation of the electronic quantisation axis from the direction of the static magnetic field. The behaviour of the magnetisation is expected to be very complicated in such cases, with the apparent consequence that resonance features associated with large fine structure splittings are reduced or missing from the echo-detected EPR (ED-EPR) spectra. This has been observed for the geff 5 feature in Cr3+-doped ZBLAN [20], as well as the geff 4 resonance in Fe3+-doped silicate glasses, for example. [all showed that if the microwave (mw) field As a way around this problem, Oliete
-
-
43
0
2000
4000
6000
8000 (G)
0
2000
4000
6000
8000 (G)
Figure Computer simulation of an S-state ion system. = 9GHz; (Gaussian) = 0.056cm-'; (standard deviation) AD = 0.019cm-1; uniform 0 5 5 bin width=lOG. without addition of qth order fine structure; 55000 sites; 1071 transitions. Right: With 4th order terms, all -4 5 q 5 4 chosen randomly from same = Ocm-'; = 10-4cm-1; 500 sites; 999 transiGaussian distribution; tions. is a small perturbation and if the excited transitions do not share a common energy level, the problem can be reduced to a time-independent two-level system whose evolution can then be solved for using standard density matrix methods. One foreseeable drawback to this approach is the difficulty in relating the calculated magnetisation in the interaction representation at the time of echo formation with that measured by a phase detector synchronous with the mw field. For = every unitary transformation (rotation in Hilbert space) can be associated with a physical rotation in real space, because of the one-to-one correspondence between the real orthogonal group in 3-space and the unitary unimodular group in two dimensions. ie. a representation in the former corresponds to the introduction of a coordinate system in the latter. An example of this connection is afforded by the simple case of diagonalisation of the axially symmetric Zeeman Hamiltonian for an = system Appendix J.l.11, where the rotation of the coordinate system of the vector operator S through an angle about the y-axis in 3-space amounts to a rotation in the Hermitian vector space through = where is a Wigner rotation matrix. For a an angle ie. Hamiltonian expressed in the principal axis system of the g matrix, $ represents the angle in real space that the quantisation axis of the spin makes with the z-axis. however, a rotation in the complex vector space need not have anything For general to do with rotations in real space Only in special circumstances is it possible to associate a unitary operator R with a rotation in 3-space, the form of this operator being given by the Wigner rotation matrices in terms of the three Euler angles /3,
The Wigner rotation matrices possess the symmetry relation
R(J)* - (-1)
44
The rotating frame transformation represents a special case where
and = w t . However, a transformation to the interaction representation with = ePixot for general and ?to has no such rotational analogue in 3-space, since does not fulfill the symmetry property (2). This means that there is no simple way of relating the magnetisation in the rotating (detection) frame at the time of echo formation with that computed in the interaction representation except when g-anisotropy is small and when the high field approximation is valid. We conclude, therefore, that the method of Oliete [all is not a promising framework for the simulation of field-swept spectra in pulsed EPR for spin systems where the quantisation axis differs appreciably from the direction of the static magnetic field. 3.2. Quantum beat FID in glasses For most solids, a short, non-selective mw pulse will result in the decay of the freeinduction signal within the dead-time of the mw detection system. A selective mw pulse, however, burns a hole into a line whose width approaches the homogeneous linewidth. The frequency selectivity of the pulse has the advantage of generating a FID that can be used to obtain high resolution field-swept pulsed-EPR spectra in both ordered [24] and disordered systems [20]. Application of a mw pulse of appropriate duration and strength can result in electron polarisation (EP), electron coherence (EC) and nuclear coherence (NC). A number of two-pulse experiments are possible which exploit each of these to measure electron-nuclear cannot be applied to glasses, however, interactions [25-271. The method of Dzuba since it involves monitoring the FID following a non-selective mw pulse. In the present case, we observe superhyperfine (shf) interactions in disordered systems using a high-turning-angle microwave pulse. The free induction signal exhibits oscillations that are distinct from the oscillatory FID [28] observable in some inhomogeneously broadened systems. Figure 3 shows a rather good example with a 2ps rectangular pulse applied to a metaphosphate glass doped with 1%Ce3+, with oscillations appearing at the Larmor frequency of the phosphorous matrix nuclei as the pulse power is increased using the level potentiometer of the mw channel. The nominal flip angle Po = indicated next to each spectrum was estimated by comparing the (measured with the transmitter monitor) required to optimise a primary echo using a 7r/2 pulse of 1611s. This phenomenon is not unique to glasses. In polycrystalline y-irradiated alanine for example, the quantum beats due to remote protons can be seen superimposed upon the oscillatory FID (Figure 3(bottom right)). Here the nominal flip angle can easily be extracted from the number of transitory oscillations present in the FID [as]. Quantum beats have been observed for transition metal and rare earth ions in a number of silicate, fluoride and phosphate glasses at the frequency of remote matrix nuclei and should, in principal, be observable in any system possessing a large inhomogeneous linewidth. Like the FT-EPR detected NMR experiment [25,29], the quantum beat phenomenon relies on inhomogeneous broadening of the EPR line. A frequency selective mw pulse excites on-resonance forbidden transitions, which indirectly alters the population difference of the allowed transitions that share common energy level with forbidden ones. The
45
ensuing FID contains precession signals at frequencies characteristic of shf interaction splittings. A zero frequency component will also be present due to the excitation of spin packets with on-resonance allowed transitions. The depth of the oscillations is generally larger in the FT-EPR detected NMR experiment because the second non-selective mw pulse re-focusses the EP pattern created by the first pulse, taking advantage of the greater population difference that can be created for the allowed transitions by fully inverting the forbidden transition sharing the common energy level. The quantum beat FID is therefore expected to be less effective a t uncovering electronnuclear interactions than the FT-EPR detected NMR experiment. On balance, however, it is still an effect that requires some consideration, since it has the ability to distort a field-swept FID-detected spectrum in a manner analogous to the way in which the nuclear modulation effect distorts ED-EPR.
13n 30 (MHz)
1.3n
10
0
~-
<18dB
5on
,
0
500
1000
(ns)
20
0
,
,
,
,
500
,
,
,
,
,
,
,
,
,
1000
,
,
(ns)
Figure 3. Left: The quantum beat FID of 1%Ce3+ in metaphosphate glass. = 2000ns; 9.69GHz; 2.7K; = 7000G. __ Right(top): FT of Ce3+ for Po = 1 3 ~ .Right (bottom): y-irradiated alanine powder; = 1000ns; 9.70GHz; 100K; = 3430G. The (arbitrary) power attenuator setting is indicated. The nominal flip angle can be estimated from the number of transitory oscillations and is, from top to bottom, PO 7 r , 27r, 47r and 67r.
4. CONCLUSIONS
We are pursuing a method of simulation which has, in principle, the ability to distinguish between a micro-crystallite and a random network model of a glass. The former is clearly simpler to visualise and define, since it represents a model in which there exist
46
random ‘strains’ (distributed SH parameters) within a polycrystalline powder defined by a single spin Hamiltonian. The latter is far more difficult to define, because of the possibility of having a distribution of C1-symmetry Hamiltonians in addition to distributed SH parameters. Given the difficulty in estimating higher order SH parameters, to correlate all of these random parameters in order to adequately reflect the underlying structure, remains a daunting task. Pulsed EPR simulations, which should in principle complement the information obtained from CW simulations, are difficult to implement, regardless of the precise SH model used, for high spin systems possessing distributions of highly distorted sites within a glass. Ideally one would like to employ methods based on distributions of bond angles and bond lengths. While it would be possible to do a small number of such calculations, it would be impossible to do this with say Density Functional Theory for the vast number of sites generated in the stochastic model. It appears unlikely to be of much value in providing meaningful spin Hamiltonian parameters reliable enough for simulations, because the very small splittings observed in EPR (Wcm-’) would not be clearly resolved in an model which leads to large ligand field energies and splittings. For example, g-values are obtained from small differences between large numbers. Thus it would seem that at present SH modeling represents the only practical option.
REFERENCES 1. D.L. Griscom, J. Non-Cryst. Solids, 40 (1980) 211. 2. J. Kliava, phys. stat. sol., (b) 134 (1986) 411. 3. D.R. Uhlmann and N.J. Kreidl (eds.), Glass: Science and Technology, Academic, Orlando, 1990. 4. E.A. Porai-Koshits, ‘Overview’, Chapter 1, Vol. 4A of [3]. 5. R.H. Doremus, Glass Science, Wiley-Interscience, New York, 1990. 6. N.E. Cusack, The Physics of Structurally Disordered Matter: An Introduction, Adam Hilger, Bristol, 1987. 7. C. Legein, J.Y. BuzarC, J. Emery and C. Jacoboni, J. Non-Cryst. Solids, 161 (1993) 112. 8. C. Legein, J.Y. BuzarC and C. Jacoboni, J. Phys.: Condens. Matter, 7 (1995) 3853. 9. C. Legein, J.Y. Buzark, B. Boulard and C. Jacoboni, J. Phys.: Condens. Matter, 7 (1995) 4829. 10. A. Edgar, D. Giltrap and D.R. MacFarlane, J. Non-Cryst. Solids, 231 (1998) 257. 11. D.L. Griscom, Electron Spin Resonance, Chapter 3, Vol. 4B of [3]. 12. G.E. Peterson, C.R. Kurkjian and A. Carnevale, J. Phys. Chem. Glasses, 15 (1974) 52. 13. A. Carnevale, G.E. Peterson and C.R. Kurkjian, J. Non-Cryst. Solids, 22 (1976) 269. 14. C.M. Brodbeck and L.E. Iton, J. Chem. Phys., 83 (1985) 4285. 15. G. G. Belford, R. L. Belford and J. F. Burkhalter, J. Magn. Reson., 11 (1973) 251. 16. J.R. Pilbrow and S.C. Drew, Mol. Phys. Rep., 26 (1999) 109. 17. D.J. Newman, Austral. J. Phys., 31 (1978) 79. 18. J.M. Gaite, in Electron Magnetic Resonance of the Solid State, ed. J.A. Weil, Canadian Society for Chemistry, Ottawa, 1987, p.151.
47
19. J.M. Gaite and J. Michoulier, J. Chem. Phys., 59 (1973) 488. 20. S.C. Drew, J.R. Pilbrow, P.J. Newman and D.R. MacFarlane, J. Phys. D., 34 (2001) 2987. 21. P.B. Oliete, V.M. Orera and P.J. Alonso, Phys. Rev. B, 54 (1996) 12099. 22. J.R. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Clarendon, Oxford, 1990. 23. E. Merzbacher, Quantum Mechanics, 2nd ed., John Wiley & Sons, New York, 1970. 24. J.A. Coelho Net0 and N.V. Vugman, J. Magn. Reson., 125 (1997) 242. 25. Th. Wacker and A. Schweiger, Chem. Phys. Lett., 186 (1991) 27. 26. A. Schweiger, L. Braunschweiler, J.M. Fauth and R.R. Ernst, Phys. Rev. Lett., 54 (1985) 1241. 27. S.A. Dzuba, I.V. Borovykh and A.J. Hoff, J. Magn. Reson., 133 (1998) 286. 28. L. Braunschweiler, A. Schweiger, J.M. Fauth R.R. Ernst, J. Magn. Reson., 72 (1987) 579. 29. M. Willer and A. Schweiger, Chem. Phys. Lett., 230 (1994) 67.
EPR in the 2 1 Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
48
Frequency dependence of resonance in one-dimensional antiferromagnetic Heisenberg chain Akira Ogasahara and Seiji Miyashita Department of Applied Physics, Graduate School of Engineering, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
ESR in = 1/2 spin system is studied by a direct numerical method calculating the dynamical susceptibility expressed by Kubo formula. Frequency-dependence of the resonant field is calculated in onedimensional spin system In one-dimensional antiferr+ magnetic Heisenberg chain we find that the frequency-dependence of the resonant field changes according to relation among the amplitude of the interaction and the external parameters, e.g., temperature and field. 1. INTRODUCTION
We have developed a method for the direct numerical calculation of Kubo formula[l]. In the method, we diagonalize the Hamiltonian of the system under the external static field of fixed amplitude, and estimate Kubo formula directly. Then we obtain the frequencydependence of the imaginary part of the dynamical susceptibility[2],
/
1 ( w ) = - (1 - eWP")
2
00
(0)
ePbtdt.
-00
And by turning the data calculated for quasi-continuously swept amplitudes of the static field, we finally obtain the field-dependence of the susceptibility for given frequency of the oscillating field (RF field), as observed in experiments. We estimate the behavior of the absorption curve using the statistical mean value of finestructural absorptions with those absorption amplitudes as the statistical weights. Namely, we define the first-order moment of peak fields of fine-structural absorptions as the resonant field, and the deviation of that from the paramagnetic resonant field as the resonant shift, (2)
-h/gpB.
By this method we can study the dependence of the resonant field on geometrical configuration among the lattice, the static field Ho and the oscillating field H I ,i.e., the dynamical shift, and also the temperaturedependence of the line shape in the first principle way, although we can treat the system with only finite number of spins N 10. Actually N
49
we have successfully reproduced the dynamical shift in low-dimensional antiferromagnetic Heisenberg model[2, 31. In this paper, we deal with Heisenberg antiferromagnetic spin systems with isotropic g-tensor where only anisotropic spin interaction such as the dipole interaction violates the SU(2), i.e., the Heisenberg symmetry and causes other resonant peaks than the paramagnetic resonance (PMR), which result in complicated fine-structure in the absorption curve. In next section we investigate the origin in which the resonant field shifts from paramagnetic resonance field by studying the change which occurs in the energy structure. 2. LINEAR CHAIN SYSTEM 2.1. Frequency dependence of resonant shift ESR in one-dimensional antiferromagnet has been investigated both theoretically[2, 41 and experimentally[5-71, and the observed temperature-dependence of the resonant shift in such system is fitted by the equation of Nagata-Tazuke[4,7] and the internal parameters such as the exchange integrals are estimated. The dependence of the resonant shift on the oscillating-field frequency in such system is also observed[7] and the results are fitted by linear-increase so that the resonant shifts are proportional to the frequency. So in order to examine the fitting we numerically study the dependence of the resonant shift on the oscillating-field frequency in antiferromagnetic linear-chain system described by the Hamiltonian, = 3-10
+
3-1D -
+ Hlebt) . M ,
(3)
where
3-10 is the spin exchange interactions and 7 - i ~ the dipole interactions. The dipole interactions are considered for all couples of spins, ( i , j ) .Ho is the static field and Hle" is the oscillating field, M is the total spin of the system: M = Si. = (gPB)' using g-factor, g = 2 and Bohr magneton p ~ ri,j . is the inter-spin vector from to Sj. is the size of the system and in this paper we adopt = 8 system with the periodic boundary condition: S N +=~S1. From experimental observations the exchange integral and interspin length between nearest-neighbors are estimated as = 8.6K and ri,i+l = 3.15A[7]. In this model because of the dipole interaction the resonant field shows deviation from the paramagnetic resonant field. The deviation depends mainly on the angle between the direction of the system (c-axis) and that of the static field, and changes its sign for the angle. In Figure 1 (a) the frequency-dependences of the deviations (resonant shifts) are plotted in the case of = 4.2 K as in experimental observation[7]. The resonant shift is positive in the configuration of Ho c-axis and negative for Ho 11 c-axis. This is consistent with the result calculated by Nagata-Tazuke using classical spin approximation[4].
cE1
50
-4,W -1106
-
.??to
-
-4,m
-<115
N
x
Q
.....................................
..... .............................
-4,o -IlP .,IS -1140
Figure 1. (a) Frequency-dependence of resonant shift. Upper points corresponds to the case of Ho I c-axis, and lower points to the case of Ho 11 c-axis. Dotted lines are ones for linear-fitting. (b) Lowest S = 1-space energy structure. Solid lines correspond to the case of Ho I c-axis, and dotted lines to the case of Ho 11 c-axis. And the amplitudes of the shifts increase in proportional to the frequency except in low frequency region for Ho c-axis. Above results are explained by investigating the transitions among the lowest triplet states in the energy structure. In Figure 1 (b), we can see the field-dependences of lowest triplet states which are the first to the third excited states under low fields. As the ground state is singlet the transition is forbidden from the ground state by the selection rule except for the case that strong anisotropic spin interaction violates the SU(2) symmetry. In the range of the exchange integral ,-., 0 (lOO)K and the lattice constant T 0 (lOo)A, the ratio between the dipole interaction and the exchange integral is estimated negligible: 0 ( 1 0 - 2 ) , so the transition from the ground state does not yield remarkable signal, and actually the matrix element between the ground state IG) and the excited state I(G @I2, are sufficiently small compared to those among the lowest triplet states. So we can analyze the resonant shift, especially at low temperatures, by studying the field-dependences of the energy-differences among the lowest triplet states. Namely the energy-differences determine the peak points of the fine-structural absorptions which correspond to the transitions between the microscopic states. In the configuration of Ho c-axis (solid lines in Figure 1 (b)), the energy-difference between = 1 state (lowest solid-line) and S, = 0 (middle solid-line) is smaller than that in case of PMR, and that between S, = 0 and = -1 (highest solid-line) is larger in whole range of the static field. (Because of the dipole interactions is not a good quantum number, so in this paper we use S, as that in rotationally-symmetric model where the dipole interactions are not considered). So the transition between the former couple of states yields the fine structural absorption at larger field than PMR field, and the latter yields at smaller field. While in the configuration of Ho )I c-axis (dotted lines in Figure 1 (b)), above relation is reversed. Especially in low temperatures low-lying states have dominant statistical probability and the finestructural absorption between the lowest states and the middle state in this S-space determines the behavior of the resonant shift. Those changes in the N
N
51
energy structure are mainly attributed to that of the effective exchange integrals between nearest-neighbors in the coordinate where the direction of the static field is set as z-axis, namely
J," < Jf = Jiff in the configuration of Ho
c-axis, and
JZff= JZ > J:ff for Ho 11 c-axis, because of the dipole interactions included to the spin interactions. Especially in the configuration of Ho 11 c-axis the interactions between nearest-neighbors are reduced to so-called spin interactions, and actually the energy structure in the model because the dipole interaction configuration (Figure 1 (b)) is almost that of reduces as $ and the dipole interactions between the nearest-neighbors are dominant in constructing the energy structure compared to those between next-nearest-neighbors and more distant couples of spins. 2.2. Resonant shift in high frequency region In frequency region where the states which belong to the lowest = 1-space are lower levels in whole energy structure the corresponding fine-structural absorptions are dominant in the absorption curve, and the resonant shift is determined mainly by those states. While the magnetization, namely the value of to which the ground state belongs, increases as the static field increases. Then the energy structure of higher S-space mainly determines the resonant shift in higher frequency region. So we next investigate the behavior of the resonant shift in higher frequency region over that in experimental observation[7]. In Figure 2 (a) the resonant shifts in two different temperatures are shown. At = 4.2 K the resonant shift deviates from the linear fitting in lower frequency region (w 5 100 GHz: Figure 1 (a)). This means that S-space to which the ground state belongs changes following the increase of the resonant field corresponding to the oscillating-field frequency. This is reflected more clearly in the behavior of the resonant shift in lower temperatures where the ground state is dominant. At = 0.105 K the resonant shift increases in staircase-like way such as the plateau structure of low-temperature magnetization process. Recently by use of high-frequency ESR equipment the behavior of the resonant shift in high frequency is experimentally observed, and in the ESR measurement of the antiferromagnetic ladder system, C U ~ ( C S H I ~ N(abbreviated ~ ) ~ C ~ ~ as CHpC), the plateau-structure of the resonant shift is observed[8]. This dependence means that the g-factor of the material varies in the staircase-like way. While the magnetization process shows a rather smooth change even at lower temperatures[9, 101 than the resonant shift. So as a preliminary work we study a small system which has a staircase-like magnetization process in the ground state. There the staircase-like structure of the magnetization process is smeared out at finite temperatures and we see a smooth curve. While even at such temperatures we also observe the plateau structure in the frequency-dependence of the resonant shift, and we found that the ESR absorption is much sensitive to the energy structure than the
52
Figure 2. (a) Frequency-dependence of resonant shift. Thick solid lines correspond to T = 4.2 K, and thin solid lines to T = 0.105 K. Dotted lines are fitting lines for low frequency region. (b) Whole energy structure. magnetization. Thus we expect that the magnetization process of CHpC also have a hidden staircase-like structure and the observed frequency-dependence of the shift represents such structure. In summary we find the qualitatively different frequency-dependence of the resonant shift at various temperatures, namely the qualitatively different behavior of the temperature-dependence of that at various frequencies. And from the results at lower temperatures it is expected that ESR can be a fine method to investigate the ground state of the material.
ACKNOWLEDGEMENTS We appreciate Professors H. Ohta and his collaborators for their valuable discussions and experimental data. The present work is partially supported by the Grant-in-Aid for Scientific Research from Ministry of Education, Culture, Sports, Science and Technology of Japan. We also appreciate the facility of Supercomputer Center of Institute for Solid State Physics, University of Tokyo.
REFERENCES 1. R. Kubo, J. Phys. SOC.Jpn., 12 (1957) 570. 2. S. Miyashita, T. Yoshino and A. Ogasahara, J. Phys. SOC.Jpn., 68 (1999) 655.
3. A. Ogasahara and S. Miyashita, J. Phys. SOC.Jpn., 69 (2000) 4043. 4. K. Nagata and Y. Tazuke, J. Phys. SOC.Jpn., 32 (1972) 337. 5. M. Hagiwara, K. Katsumata and J. Tuchendler, J. Phys. Condens. Matter, 6 (1994) 545.
6. R. Dingle, M. E. Lines and S. L. Holt, Phys. Rev., 187 (1969) 643. 7. K. Okuda, H. Hata and M. Date, J. Phys. SOC.Jpn., 33 (1972) 1574. 8. H. Ohta, S. Okubo, T. Tanaka, H. Kikuchi and H. Nagasawa, 4th Int. Symposium on Advanced Physical Fields : Quantum Phenomena in Advanced Materials at High Magnetic Fields (Tsukuba, Japan, March 9-12, 1997) 9. G. Chaboussant, P. A. Crowell, L. P. Levy, 0. Piovesana, A. Madouri and D. Mailly, Phys. Rev. B, 55 (1997) 3046. 10. B. Chiari, 0. Piovesana, T. Tarantelli and P. F. Zanazzi, Inorg. Chem., 29 (1990) 1172.
54
EPR in the 21%' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR selection rules for direct transition of spin gap TBru Sakai", Nobuhisa Okazakib, Takashi Ohnishi" and Shuhei Takemura" "Tokyo Metropolitan Institute of Technology, Asahigaoka, Hino, Tokyo 191-0065, Japan bKyoto Miyama High School, Sasari, Miyama-cho, Kitakuwata-gun, Kyoto 601-0705, Japan "Himeji Institute of Technology, Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan The ESR direct transition from the ground state to the first excited state has been observed in some recent experiments on gapped spin systems, although such a transition is forbidden because of the conservation law of the spin quantum number. We consider the two possible mechanisms of the direct trasition between the singlet and triplet states; the Dzyaloshinsky-Moriya intraction and the staggered field due to the alternating g-tensor. The angle-dependent selection rules are presented for the two mechanisms, respectively. For the Dzyaloshinsky-Moriya interaction, non-zero intensity of the ESR can appear only in the two cases; ( i ) 6 11 i d or ( i i ) 6 11 &f, where 6, Z? and are the characteristic vector of the Dzyloshinskii-Moriya interaction, the external magnetic field and the magnetic component of the rf-field, respectively. For the longitudinal staggered field, the singlet-triplet transition is possible for i d . In contrast, the condition of the 11 for the transverse staggered field, which is realized appearance of the transition is in the Haldane antiferromagnet NENP. Based on the present results, the mechanism of the singlet-triplet transition observed in the recent experiment on CuGe03 is discussed. 1. INTRODUCTION
The spin gap in low-dimensional systems, such as the bond alternating chain, the spin 1-chain or spin ladders, has attracted much interest, both experimental and theoretical. Electron spin resonance (ESR) experiments at very low temperatures provide very highenergy resolution measurements of the spin gap. Tuning an external magnetic field, the field-dependent energy of the excited triplet state is adjusted to the frequency of the electromagnetic propagating wave. A direct transition from the singlet ground state to the gapped triplet state may be observed. Such a direct transition would be, however, forbidden by the conservation law of the total spin quantum number.
55
In the ESR measurement [l]for the Haldane system Ni(CzH8N2)2N02(ClOs), abbreviated NENP, direct transitions from the ground state to the first excited triplet state have been detected. The mechanism of the transition was successfully explained by the effective staggered magnetic field due to alternaitng g-tensor [2]. The recent ESR measurement for the spin-Peierls system CuGeO3 also detected the direct transition of the spin gap [3]. Since the observed intensity of the transition for CuGeOBexhibited little dependence on the external magnetic field, Nojiri et al. [3] concluded that the explanation may be provided by the Dzyaloshinsky-Moriya (DM) interaction, which had been suggested on the basis of electron paramagnetic resonance measurements [4]. However, the DM interaction which is expected from the crystal structure of CuGe03 could not explain the observed ESR transtion [5]. Thus the main origin of the transition is still an open problem. In this paper, we give some angle-dependent selection rules for the direct transition of the spin gap in the presence of the DM interaction and staggered field, respectively. Comparing the rules and the experimental result, we discuss on the mechanism of the direct transition for CuGe03. 2. ESR SELECTION RULES 2.1. Dzyaloshinsky-Moriya interaction
At first we briefly reviewed the selection rule of the direct transition of the spin gap based on the DM interaction which was given in the previous work [5]. When the magnetic field of the wave h is polarized along an axis = z,y,z), the intensity of ESR for quantum spin systems at zero temperature, assumed due to the magnetic dipole transitions, is determined by the matrix element = I(g1 CS~le)12,
(1)
j
where is the a-component of the local spin operator at the j - t h site, 19) and le) denote the ground state and the excited state with the energy difference corresponding to the frequency w of the electromagnetic wave. For an isotropic spin system, the dynamics conserves the total spin quantum number Stotal.Therefore, every component of the intensity (1) vanishes. In the presence of the DM interaction, however, Stotalis no longer a good quantum number and some intensities may become finite. To investigate the effect of the DM interaction on the intensity, we consider first the problem of two spins = 1/2 coupled interacting with Hamiltonian %(2)
=
9, .9;+
(9, x 9 2 ) - H . (9,+ 92),
(2)
where is the vector of the DM interaction and is the external magnetic field. Solving the two spin problem, we got the following selection rule. Non-zero intensity appears only when the has some component satisfying one of the two conditions;
56
( i ) i ( t ) I l?
11 6: the intensity is independent of
(ii);(t) I( the intensities depend on Il?I and the intensity of the transition = 0 at = 0) which vanishes at = 0 increases to the non-degenerate state for small The rule is also valid for the bulk system, if some conditions on the translational symmetry are satisfied. Indeed if we assume the interchain DM coupling with parallel to c-axis which is alternating along the chain, we can explain most of experimental results of the direct transition for CuGeOs. However, the crystal structure of CuGe03 indicated the interchain should be uniform along the chain, which could not explain any finite intensities of the direct transition. Thus we should consider another mechanism. 2.2. Staggered field
Next we investigate the staggered field. When the external field is applied along z-axis, there can be two types of the staggered field depending on the crystal strusture, described by the Hamiltonians %$ = =
C(-l)jSj", C(-l)jS,".
(3)
(4)
They are the staggered fields parallel and perpendicular to the external field, respectively. is proportional to and the ratio is determined by the amplitude of the alternation of the g-tensor along the chain. Based on the analysis on the simplest two spin cluster of = 1/2, we present the selection rule for the direct singlet-triplet transition of ESR depending on the relative angles among the external field the polarized magnetic component of the rf-field h and the staggered field as follows: (i)parallel staggered field %!t leads to finite intensity for h (ii) perpendicular staggered field I f s t ) . In both cases the intensity increases with the leads to finite intensity for h 11 external field as I H 2 . Since these rules depend only on the relative configuration of the vectors h, and HSt, and they are independent of the lattice structure, they are expected to be valid even in general bulk systems with a spin gap. The rule (ii) indicates that the transverse staggered field yields no intensity of the singlet-triplet transition for the Faraday configuration, where the wave vector k of the rf-field is along Such a transverse staggered field was revealed to appear in the the 11 c-axis for the Haldane antiferromagnet NENP [6]. In fact the experiment on case of NENP observed larger intensity for the Voigt configuration than the Faraday one [7]. N
3. NUMERICAL RESULT
Base on the selection rule in the previous section, we consider the effect of the staggered field on the singlet-triplet transition observed for CuGe03. The experimental result of
57
the angle-dependent ESR intensities is given in Table 1. and indicate the crystal axes and k is the wave vector of the rf-field. Note that the magnetic component h is not polarized, but only k is fixed. Thus we should consider the averaged intensity over the two magnetic components perpendicular to k to compare with the experimental results. We take y and axes as a, b and c axes of CuGeOB in the following. According to the crystal structure of CuGeOs, we can expect the parallel staggered field along z-axis and the transverse one in xy-plane [8], described by the Hamiltonian
The rules (i) and (ii) suggest that the intensity for the Faraday configuration where k 11 (the diagonal components in Table 1)should come only from the parallel staggered field. Thus the strong intensity for k 11 11 in Table 1 is expected to be due to ?I!~,. We also explain some strong or moderate intensities for the Voigt configuration with 11 z or y, based on However, neither of the two staggered fields leads to the moderate intensity for k 11 11 z. Thus we introduce a small parallel staggered field along x-axis 31!t,x = x(-l)jST. Including the three staggered fields, we perform the numerical diagonalization of the finite cluster to estimate the intensity of the direct transition between the ground and first-excited states of CuGe03. The 16spin cluster describing the weakly coupled bond-alternating chains under the periodic boundary condition [5], which is realistic for CuGe03, is investigated. The parameters of the Hamiltonian
31 = C ( l
+ (-1y+J6)Sl,j .
1, j
+
S1,j 43
. Sl+l,j-
.
$j,
1 ,j
are fixed as follows: = 1, 6 = 0.1, = 0.15. In addition the staggered fields are fixed as c’ = = 0.1, = 0.05. Based on the numerical calculation, we got a list of the intensities in Table 2 to compare with the experimental one. We also confirmed that the list is valid in a wide range of the external field (0 < < 0.4), which covers the region where the experimental result was obtained. The present numerical result well agrees with the experimental one. It suggests that the staggered field is one of important origins of the direct ESR transition for the spin gap. The staggered field, however, should lead to a strong field dependence of the ESR intensity, which is not consistent with the experimental result. Thus maybe we should also consider some other mechanisms, not only the staggered field, in future works. 4. SUMMARY
The Dzyaloshinsky-Moriya interaction and the staggered fields were investigated as an origin of the direct ESR transition of the spin gap. The angle-dependent selection rule
58
Table 1 The experimental result of the intensity for the direct transition between the ground and first excited states, which was observed in the measurement on CuGe03 by Nojiri et al. [3]. S, M, W show that the intensity of the transition is strong, moderate and weak, respectively Polarization ill a ill b ill c
gill a gll b gll c M M
M W
M S
s
w
s
Table 2 The numerical result of the ESR intensity for the direct transition between the ground and first excited states obtained by the present study. y and axes are corresponding to the a, b and c axes of CuGe03, respectively. Polarization kll x ill Y $11 z
x
iQl y 811 z
f
M S S
M W M
M M S
was presented for both cases. The numerical results of the ESR intensities of the gap based on some staggered fields well agreed with the experimental result of CuGe03. REFERENCES
1. H. Shiba, T. Sakai, B. Luthi, W. Palme and M. Sieling, J. Magn. Magn. Mat., 140-144 (1995) 1590. 2. T. Sakai and H. Shiba, J. Phys. SOC.Jpn., 63 (1994) 867.
3. H. Nojiri, H. Ohta, S. Okubo, 0. Fujita, J. Akimitsu and M. Motokawa, J. Phys. SOC.Jpn., 68 (1999) 3417. 4. I. Yamada, M. Nishi and J. Akimitsu, J. Phys.: Condens. Matter, 8 (1996) 2625.
5 . T. Sakai, 0. Cepas and T. Ziman, J. Phys. SOC.Jpn., 69 (2000) 3521. 6. M. Chiba, Y. Ajiro, H. Kikuchi, T. Kubo and T. Morimoto, Phys. Rev., B 44 (1991) 2838. 7. W. Palme, H. Kriegelstein, B. Liithi, T. M. Brill, T. Yoshida and M. Date, Int. J. Mod. Phys., B 7 (1992) 1016. 8. 0. Cepas, Ph. D. Thesis (2000) University Joseph Foulier, Grenoble, France.
EPR in the 21' Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
59
Spin solitons in the alternate charge polarization background of
MMX chains Makoto Kuwabaraa, Kenji Yonemitsub%'and Hitoshi Ohta" "Molecular Photoscience Research Center, Kobe University, Nada, Kobe 657-8501, JAPAN bInstitute for Molecular Science, Okazaki 444-8585, Japan 'Graduate University for Advanced Studies, Okazaki 444-8585, Japan
We study spin solitons in the alternate charge polarization background of the MMX chains, using the unrestricted Hartree-Fock approximation to a one-dimensional PeierlsHubbard model. The effects of the electron-lattice coupling and the electron-electron interaction on the shape of the soliton are discussed.
1. INTRODUCTION The halogen-bridged binuclear metal complexes (MMX chains) are quasi-onedimensional materials consisting of dimer units of transition-metal (M) ions bridged by halogen (X) ions. They show a variety of charge ordering states accompanied with lattice modulations caused by strong electron-lattice coupling and electron-electron interaction; an averaged-valence (AV) state, a charge-density-wave (CDW) state, a chargepolarization (CP) state, and an alternate-charge-polarization (ACP) state (Figure 1) [1,2]. In R4[Ptz(pop)4X] (pop=Pz05Hz), the AV, CDW and C P states are realized by choosing the counter ions (R=Na, K , NH4,CH3(CH2)7NH2,etc.) and the halogen ions (X=I, Br) [381. In Pta(dta)41(dta=CH3CS2), a metal-insulator transition has been found [2]. Above 300K it is a metallic AV state, and at least below 80K it is suggested to be the ACP state. Theoretical studies for the origins of these ordering states have been performed with the extended Huckel calculation [9,10], in the mean field approximation [11,12], and by the exact-diagonalization method [ 13-15], The ACP state observed in Ptz(dta)J is analogous to the spin-Peierls state because the electrons on the M sites bridged by an X site form a singlet pair. However, no spin gap has so far been observed at low temperatures. Spin solitons in the ACP state possibly account for the small but appreciable magnetic susceptibility (Figure 2). In this paper we study spin solitons in the ACP background, using the unrestricted
60 AV
(m) ~)x(-
)M -(
-x-
CDW - x - ( ~ ) - + ( ~ ) - x - ( ~ ) - ~
cp ACP - ~ - ( M ' f ) - + ( ~ ) - x - ( ~ ) +
Figure 1. Schematic structures of the MMX chains.
Figure 2 . Spin soliton in the ACP background.
Hartree-Fock (UHF) approximation to obtain the electronic states in one-dimensional quarter-filled Peierls-Hubbard models for MMX chains. The lattice displacement patterns and the spin distributions are discussed. We insist that the profile of the spin soliton provides an indication of the strengths of the electron-lattice coupling and the electronelectron interaction.
2. PEIERLS-HUBBARD MODEL We use a one-dimensional dimerized Peierls-Hubbard model for the 3/4-filled M d,z band with 2N sites;
i,o
z
where c : , ~ (, c~! , ~ creates ,~) an electron with spin at site in the i-th dimer, na,i,u= C & ~ C ~ , ~ , ,and , = Euna,i,,,.The unit cell consists of two M sites and b, forming a dimer, and an site. The intradimer transfer integral is denoted by = and the interdimer transfer integral through the X p , orbital by = ya,i+l), where is the bond length relative to that in the undistorted phase between the M site in the i-th unit cell and its neighboring X site. The strengths of the site diagonal respectively. is and off-diagonal electron-lattice interactions are denoted by /3 and the on-site repulsion. denotes the elastic constant between M and X.
x
+
61
1
~
1
'
1
'
1
site
Figure 3. Profile of the self-consistently determined spin soliton; (a) the lattice displacements and (b) the spin density. The parameters are t M M = I . O , t ~ x ~ = 0 . cu=0.4, 8, /3=3.0, and KMx=6.0. 2. RESULTS AND DISCUSSIONS We apply the UHF approximation to the 122-site (61-dimer) periodic system containing 183 electrons. The electronic ground state is determined in the self-consistent manner with the static lattice displacements. Figure 3 shows the lattice displacements and the spin density of a self-consistent soliton solution. The shape of the soliton sensitively de/3) and the electron-electron pends on the strengths of the electron-lattice couplings interaction The width of the soliton broadens out when the electron-lattice couplings become weak. Negative spin density appears by the effect of the on-site repulsion, just like as the soliton in the polyacetylene [16].The amplitude of the dimer-by-dimer alternation of the spin density becomes large with increasing It has been shown by the X-ray analysis that the amplitude of the lattice displacements of Pt2(dta)J in the ACP state is extremely small [17].This suggests that the ACP state is realized in the vicinity of the phase boundary of the AV state, so that the band gap is reduced. In such a case, the width of the soliton would be wide and the creation energy is considered to be small. The soliton is not observed directory so far in the MMX chains. The observation of the motional narrowing in the ESR signal would be one of the strong evidence of the existence of the spin soliton. The spin density with dimer-by-dimer alternation in spin polarization provides an indication of the strength of the electron-electron interaction. The profile of the spin soliton would have important information for estimation of the strengths of the inter actions.
62
ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (B) (No.10440109) and (C) (No.12640361) from Japan Society for the Promotion of Science, a Grant-in-Aid for Scientific Research on Priority Areas (A) (Nos.10149105, 11136231, 12023232) and (B) (No.13130204) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and the N E D 0 International Joint Research Grant Program.
REFERENCES 1. M. Kurmoo, and R. J. H. Clark, Inorg. Chem., 24 (1985) 4420. 2. H. Kitawagta, N. Onodera, T. Sonoyama, M. Yamamoto, M. Fukawa, T. Mitani, M. Seto, and Y. Maeda, J. Am. Chem. SOC.,121 (1999) 10068. 3. M. Yamashita, and K. Toriumi, Inorg. Chim. Acta., 178 (1990) 143. 4. N. Kimura, H. Ohki, R. Ikeda, and M. Yamashita, Chem. Phys. Lett., 220 (1994) 40. 5. T. Mitani, Y. Wada, M. Yamashita, K. Toriumi, A. Kobayashi, and H. Kobayashi, Synth. Met., 64 (1994) 291. 6. Y. Wada, T. Furuta, M. Yamashita, and K. Toriumi, Synth. Met., 70 (1995) 1195. 7. Yamashita, M., Miya, S., Kawashima, T., Manabe, T., Sonoyama, T., H. Kitagawa, T. Mitani, H. Okamoto, and R. Ikeda, J. Am. Chem. SOC.,121 (1999) 2321. 8. K Marumoto, H. Tanaka, S. Kozaki, S. Kuroda, S. Miya, T. Kawashima, and M. Yamashita, Solid State Commun., 120 (2001) 101. 9. M. Whangbo, and E. Canadell, Inorg. Chem., 25 (1986) 1726. 10. S. A. Borshch, K. Prassides, V. Robert, and A. 0. Solonenko, J. Chem. Phys., 109 (1998) 4562. 11. S. Yamamoto, Phys. Lett., A 258 (1999) 183; 261 (1999) 125 (E). 12. S. Yamamoto, J. Phys. SOC.Jpn., 69 (2000) 13. 13. M. Kuwabara, and K. Yonemitsu, Mol. Cryst. Liq. Cryst., 343 (2000) 47. 14. M. Kuwabara, and K. Yonemitsu, J. Phys. Chem. Solids., 62 (2001) 435. 15. M. Kuwabara, and K. Yonemitsu, J. Mater. Chem., 11 (2001) 2163. 16. A.J. Heeger, S. Kivelson, J.R. Schrieffer and W.P. Su, Rev. Mod. Phys., 60 (1988) 781. 17. C. Bellitto, A. Flamini, L. Gastaldi and L. Scaramunzza, Inorg. Chem., 22 (1983) 444.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
63
Full Monte Carlo and Fourier transformed Monte Carlo EPR spectral simulations of 8s state ions Tomoharu Takeyamaa*,Takato Nakamuraa, Naoyuki Takahashi", Veltran Beltran-Lopezb and Christopher C. Rowlandsc a
Faculty of Engineering, Shizuoka University, Hamamatsu 432-8561, Japan Instituto de Ciencias Nucleares, Universidad Nacional Aut6noma de MCxico, Ciudad Universitaria, MCxico D.F., MCxico
'Department of Chemistry, Cardiff University, P.O.Box 912, Cardiff CF1 3TB, UK
Lanthanide state ions, doped into powder and glassy materials have been identified by the use of EPR spectral simulation based on Monte Carlo calculations using second order corrected energy equations. High frequency EPR measurements satisfies the condition of Zeeman interaction >> zero-field splittings even if the zero-field splittings are of the same energy range of X-band. This means that a simple equation can be derived for the energies using second order perturbation theory. The procedure has been demonstrated assuming a large zero-field splittings of I I = 0.150 T and I I = 0.05 at W-band. It was found that a Fourier transformed Monte Carlo simulation is more than adequate for the simulation of high frequency EPR spectra as opposed to the full Monte Carlo method. An example is given in order to demonstrate the usefulness of this approach for the ' S state ions in low symmetry sites.
1. INTRODUCTION It has been known for some time that a variety of powder phosphors and glasses doped with lanthanide in low symmetry sites have practical applications because low symmetry enhances the luminescence intensity due to electric dipole transition = +_1.However, there have been very few EPR studies on low symmetry lanthanide sites even when occupied by 'S ions such as europium (11) and gadolinium (III), despite the fact that their EPR signals are easily detected. At X-band EPR spectral simulation is time consuming because of the complicated spectra obtained when the zero-field splitting is greater than the microwave frequency. They are also insensitive to changes in the zero-field splitting parameters. Fortunately, modern technology has made it possible to measure EPR
* To whom correspondence should be addressed. E-mail:
[email protected]
64
spectra with a frequency more than ten times higher than X band, which satisfies the condition that the microwave frequency is far greater than the zero-field splittings. This means that the spectral pattern for powder samples is significantly simplified so that the energy equation derived by second order perturbation theory is available for the spectral simulation [l]. In this paper, therefore we report the details of the simulation of the *S state ions by Fourier Transformed Monte Carlo method (FT-Monte Carlo) which have already applied to the simulation of europium (11) and gadolinium (III) in strontium aluminates phosphors, and successllly identified the sites being occupied by them [ 1-41 even if they occupy more than one site. 2. EQUATION USED FOR TEE SIMULATION EPR spectra of 8S state ions with an effectiveelectron spin quantum number = 7/2 in non-cubic sites are expected to show fine structure due to the seven allowed transitions. It has been shown that the energies for the respective to the second order can be expressed [11
+
+
=
+ 114 --
+
+ f
-
f
+C
O S ~ f~ ) ~
(1) in which and are gpH and h vo(pis the Bohr magneton, H is the applied magnetic field, h is Planck’s constant and is the microwave frequency respectively). is the electron spin quantum number, and are the uniaxial and rhombic zero-field splitting parameters, respectively,and Band 4 are the Euler angles. 3. SIMULATION PROCEDURE
It is apparent that if the values of 6, qj and the microwave frequency used are specified, and the energies for the respective MS’Sare calculated using equation (1) then the resonance positions are obtained. Therefore the magnetic fields, where the allowed transitions can occur, are estimated by the energy difference between and -1. Note that the transition probability function was not used for the intensity estimation. Alternately, the
Channel numbex
Figure 1. A plot of the magnetic field against the storage channel number
65
Monte Carlo method was applied for the estimation of the powder pattern. The most time consuming procedure in this simulation, when a large number of orientations are considered, is to construct a low-noise spectrum, hence a Fourier transformation was performed. In order to illustrate how the Monte Carlo technique works for an EPR spectral simulation, microwave frequency, isotropic g-value, zero-field splitting parameters and linewidth are assumed follows: v=92.500,g=2.0027andI =O.lSOT, I andAHpp=0.070T The magnetic field scan range over which the EPR spectrum is to be simulated is divided into 1023. For conveniencethe magnetic field intervals are converted into storage channels. In Figure 1, the relation between magnetic field from 2.5 to 4.0 T and the channel number are illustrated, in which the incremental magnetic field per channel is 0.01467 T. If the resonance magnetic field calculated using equation (1) a set of 4 orientation falls into a certain magnetic field interval, then +1 is accumulated in the corresponding storage channel. One can easily imagine that when the same procedure is repeated using a large number of 4 orientations, then the basic powder pattern (stick spectrum) would be obtained. This is just like the EPR measurement of randomly oriented crystals. Practically, cosBand 4 in equation(1) substitutedby 1 - 2p and 2nq, respectively,using a @, q ) set, in which Ospandqsl. For a rough estimation of the EPR parameters of g, I I and I I the basic powder pattern was obtained by samplingrandomly generated (p,q ) orientationsin 2 lo4for the each transition between MSand - I. There is no doubt about the fact that the intensity ratios are of 7 : 12 : 15 : 16 : 15 : 12 : 7 for the allowed transition between and Ms-1, in whch = 7,5,3.. . ., -5, respectively. Therefore, the intensity ratios were multiplied to the powder pattern corresponding to each transition in order to construct the basic powder pattern. Figure 2 shows typical powder patterns of the transitions between = 712 and 512 and between = 112 and -112 calculated using the program written by Mathematica ver.4.0. It is evident that the powder pattern for the outmost transition of = 712 - 512 100t
2000
R
1000
50
0
200
400
Channel number
800
lo00
0
200
400
600 800
1000
Channel number
Figure 2. Typical powder patterns of the transitions between(a) and (b) 112 and -112
712
512
66
attributes transition of Ms = 1/2 to -1/2, the number of channels is limited to a small region with strong intensity. Figure 3 shows the basic powder pattern (stick spectrum) obtained for the example, in which the simulation takes only 2 min utilising a Fujitsu FMV personal computer with randomly generated @, q) orientations in 2 lo4 for each transition. It is worth noting that the obtained basic powder pattern contains simulation noise, which is dependent upon the number of the @, q ) orientationsused. Improvement of the S/N ratio is important when comparing simulated and experimental spectra. Therefore, a Fourier transformation was performed using a Lorentzian line function in order to generate the envelope of basic powder pattern with a The Fourier transformation is incorporated in the software used as a built-in function. The powder pattern obtained and EPR spectrum are shown in Figures 4 and respectively, along with those constructed by the full Monte Carlo method for comparison. The latter powder pattern and EPR spectrum are directly convoluted by Monte
-
-
I -
1
0
Fourier transform significantly improves the quality of the simulated powder pattern. Even if the number of the (p, q ) orientations is reduced to 10,000, the S/N ratio is far better than that obtained by the full Monte Carlo method. Also, it is obvious from a comparison of the simulated spectra shown in Figure 5 that one can easily recognise shoulders and divergences appearing on both side of the central intense line in the simulated spectrum using the FT-Monte Carlo method rather than those obtained using the full Monte Carlo method. 4. CONCLUSION EPR spectral simulation for the 8Sstate ions in low symmetryhas been examined using energies correctedto the second order for the calculationof resonance magnetic field and Monte Carlo technique for the estimation of intensity. It was demonstrated that the FT-Monte Carlo method developed in this study is a reasonably fast and convenient method to simulatethe EPR spectra of 8Sstate ions using a small microprocessorand a consequence the method is more useful than the full Monte Carlo method.
67
0 Channel number
200
400 600 Channel numbex
Figure 4. A comparison of the powder patterns simulated by FT-Monte Carlo and full Monte Carlo methods, in which the number of the (p, q) orientations is 20,000.
2.5
Magnetic Field I T
3 3.5 Magnetic Field I T
4
Figure 5. Simulated EPR spectra by FT-Monte Carlo and full Monte Carlo methods, in which the parameters used are given in the text.
ACKNOWLEDGEMENT This work was supported by the Japan Society for the Promotion of Science through a grant-in-aid for the Scientific Research (A) No. 12355030. REFERENCES J.R. Pilbrow, “Transition Ion Electron Paramagnetic Resonance”, Clarendon Press, Oxford 1990, p.614. T. Nakamura, T. Matsuzawa, C.C. Rowlands, V. Beltran-Lopez, G.M. Smith and P.C. Riedi, J. Chem. Soc., Faraday Trans., 94,( 1998) 3009. T. Nakamura, K. Kaiya, N. Takahashi, T. Matsuzawa, C.C. Rowlands, V. Beltran-Lopez, GM. Smith and P.C. Riedi, Phys. Chem. Chem. Phys.,l, (1999) 4011. T. Nakamura, N. Takahashi, K. Kaiya, T. Matsuzawa, C.C. Rowlands, V. Beltran-Lopez, G.M. Smith and P.C. Riedi, J. Mater. Chem., 10, (2000) 2566.
68
5. T. Nakamura, K. Kaiya, N. Takahashi, T. Matsuzawa, M. Ohta, C. C. Rowlands, G M. Smith and P.C. Riedi, Phys. Chem. Chem. Phys., 3,(2001) 1721.
EPR in the 21' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
69
X-band ESR Measurements of spin ladder system BIP-TEN0 Keizo KIRITA".", Takahiro SAKURAI".",Hitoshi OHTAb.": Yuko HOSOKOSHId, Keiichi KATOHd, Katsuya INOUEd 'The Graduate School of Science and Technology , Kobe University, Kobe, 657-8501, Japan bMolecular Photoscience Research Center, Kobe University, Kobe, 657-8501, Japan 'Venture Business Laboratory, Kobe University, Kobe, 657-8501, Japan dInstitute of Molecular Science, Aichi, 444-8585, Japan
X-band ESR measurements have been performed on a single crystal of S=l spin ladder substance BIP-TEN0 at room temperature. The angular dependence of line width showed the typical behavior to the low dimensional antiferromagnet, while the detailed analysis of ESR line shape revealed that the system shows intermediate behavior. This might be attributed to the ladder nature of this system. 1. INTRODUCTION Exchange-narrowed ESR has remarkable characteristics in low-dimensional systems. Line shape and line width studied by ESR measurements have documented experimentally so far in quasi one and two dimensional materials[l,2], and also the theoretical investigations have generally been successful in giving the quantitative interpretation. In the low dimensional magnetic system, it is well known that the dynamic behavior of local fluctuations was dominated by the long time persistence of spin correlation on the basis of spin diffusion model in the high temperature limit, in which the system exhibits a paramagnetic behavior. This results in the divergence of spectral density at lower frequency part and is reflected to the ESR line shape such as non-Lorentzian type. The study of the spin dynamics of ladder system is an interesting problem because of its peculiar dimensionality, that is, it is regarded as the intermediate dimension between one succeeded in synthesizing the S=1 dimension and two dimensions. Recently, Katoh et antiferromagnetic spin ladder model substance 3,3' ,5,5'-tetrakis(N-tertbutylaminoxy1)biphenyl (abbreviated as BIP-TEN0)[3]. Its crystal structure is the orthorhombic space group and has antiferromagnetic double spin chain structure
, S=1/2 spin
112 spin
i
0
b
a-axis
Figure 1. (a) Schematic illustration of the chain structure of BIP-TEN0 along the c-xis. (b) Schematic illustration of BIP-TEN0 viwed along the b-axis. composed of S=1/2 spin pairs, each of which are strongly coupled with ferromagnetic interaction, as shown in Figure l(a) schematically[3]. So BIP-TEN0 can be considered as an 1 two-leg ladder. The temperature dependence of magnetic susceptibility of BIP-TEN0 showed a broad maximum at around 67 K[3]. This broad maximum in the susceptibility suggests the low dimensionality of the system. In view of the fact that the susceptibility, however, provides only an indirect information of the dimensionality, we report further evidence for the low dimensionality of S=l two-leg spin ladder system BIP-TEN0 through X-band ESR measurements at high temperature.
2. EXPERIMENTAL X-band ESR measurements were performed on ca. 0.1 mm' of a single crystal at room temperature with Bruker EPR spectrometer EMX081 using a TE103 rectangular cavity. The microwave frequency was approximately 9.4 GHz through the measurements and the explored field, which was monitored by the proton NMR gaussmeter, was ranging from 320 mT to 350 mT.
3. RESULTS AND ANALYSIS At room temperature, all ESR absorption lines we collected was single line. From the angular dependence of resonance field within plane and plane including c-axis (chain direction), we estimated the principal values of the g-tensor experimentally to be g,=2.00730, g,,=2.00673 and g,=2.00347 (Figure 2). These are in accordance with crystallographic three axes while the local symmetry around each spin within a BIP-TEN0 molecule does not coincide with any crystallographic axes. Therefore, the observed g-values are the consequence of the averaging due to exchange coupling within not only intra-ladder but also over inter-ladder, because two spin pairs within a molecule are equivalent to each other, while the molecular planes between neighboring ladders are aligned in opposite way as shown in Figure 1 (b).
71
3
2.0073 L 2.0072 L 2.0071 2.0070 2.0069 0, b-axis 2.0068 . 2.0067
'
1.4 1.2 0
'
8 .
"
~ ' ' ' " ~ ' ' ' ' $ &
0
a-axis
o%oi$oo 1 . .
, . I . . . . , I . ,
0
A A
0 :
0
..
. . . I . ,
. . , 1 ..
0.0
0
30 60 90 120 150 180 Angle (deg.)
Figure 3. The angular dependence of A Hp.pat room temperature in the chain plane : the measured results (circles), calculated results (triangles). Figure 3 shows the angular dependence of peak-to-peak derivative line width observed in a plane including c-axis (0'). Observation of minimum line width at about 55', which is known as magic angle, means that the origin of the width is mainly based on the dipolar contribution. Then we tentatively calculated the line width, assuming the interactions to be dipole-dipole interactions over 300 spins and applying three dimensional exchange narrowing theory. It seems that the observed line width is a factor of 5 larger than calculated one as shown in Figure 3. The degree of exchange narrowing effect might be weak due to the low dimensionality of the system. Figure 4 shows the detailed analysis of the line shapes at =O" (chain axis) and 55 (magic angle) by plotting the ratio Z(H,,)/Z(Hjas a function of where is the resonance field and is the amplitude of absorption at In this plot, the Lorentzian yields a straight line, while the other curves have the dependence shown in this Figure. The dotted line represents the Fourier transform of the relaxation function e ~ p ( - t ~based ' ~ ) on the one dimensional spin diffusion model. The line shape observed at magic angle seems to be almost straight line. This behavior reminds us the line shape for one dimensional magnet with only dipolar contribution along the chain, although the observed one has a slightly larger slope than that of Lorentzian. On the contrary the line shape for the chain direction is close to Lorentzian in the smaller part of abscissa and then it increases slowly with a finite This behavior is also similar to the one curvature as going away from resonance center dimensional diffusive behavior qualitatively, however, the deviation form Loretzian occurs at rather far region from compare to the diffusion model. For instance the ESR line shape itself is almost reproduced by the Lorentzian curve in rather wide field range around resonance center, compare to the pure one dimensional chain, but still fall off faster than Lorentzian in the wings. This means that the spectral density of local fluctuations has moderate intensity at lower frequency part, that is, in other word, the second moment of the line shape is not infinite as in the Lorentzian but finite due to the moderate decay of the
72
((H-HoYA HI1212 Figure 4. The detailed analysis of line shapesk, Applied field is along chain direction(circ1es) and magic-angle direction(triang1es).
correlation due to slow exchange originate from a low dimensionality of the system. As a consequence we can suggest that the system has the intermediate character between one dimension and three dimensions, and it does not contradict qualitatively with two-leg ladder model. In conclusion we performed the X-band ESR measurements on S=l spin ladder model substance BIP-TEN0 at room temperature. The angular dependence of line width showed a minimum at magic angle, and therefore the behavior is explained qualitatively by the dipolar origin with exchange narrowing, although it is broader than we expected. The detailed analysis of the line shape revealed that one dimensional chain model does not match our experimental data. The intermediate behavior between Lorentzian and Fourier transform of exp(-?’*) implies that the system possesses low dimensional character at some extent in agreement with ladder structure of BIP-TENO.
4. ACNKOWLEDGEMEMTS This work was supported by Grant-in-Aid for Scientific Research on Priority Areas (B) (No. 13 130204 “Field-Induced New Quantum Phenomena in Magnetic Systems”) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
REFERENCES
1. R. E. Dietz, F. R. Merritt, R. Dingle, D. Hone, B.G. Sillbemagel and P.M. Richards: Phys. Rev., 26 (1971) 1186. 2. P. M. Richards and M. B. Salamon: Phys. Rev., B 9 (1974) 32. 3. K. Katoh , Y. Hosokoshi, K. Inoue and T. Goto: J. Phys. SOC.Jpn., 69 (2000) 1008.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
73
ESR studies of a spin- 1/2 antiferromagnetic tetramer chain Masayuki Hagiwara RIKEN (The Institute of Physical and Chemical Research),
Saitama 351-0198 and
Graduate School of Integrated Science, Yokohama City University, Yokohama, Kanagawa 230-0045, Japan Electron spin resonance (ESR) measurements have been performed at X-band on a single crystal sample of Cu(3-Clpy),(N3), (3-Clpy=3-Chloropyridine) which is regarded as the spin 1/2
tetramer chain. Angular
dependence of the line width at half height of the absorption profile at room temperature obeys the diffusive type of angular anisotropy, which means that the minimum of the line width takes place at about 55 ' profile of the signal at 0
or 90
between the applied field and the chain direction. The line O
deviates from not only Lorentzian and Gaussian but also
the typical one dimensional line profile. The temperature dependence of the resonance field parallel to the chain shows a monotonical shift to low field side upon cooling, while that of the resonance field perpendicular to the chain shows the opposite behavior. Most of these findings indicate that this compound behaves as a typical one-dimensional system like an antiferromagnetic uniform chain irrespective of a peculiar exchange coupling.
1. INTRODUCTION Recently, there has been a considerable interest in quantum ferrimagnetic systems. In particular, spin alternating antiferromagnetic chain composed of spin 1 and 1/2 has been extensively studied as one of the quantum ferrimagnets [l-41. This system has a magnetic ground state and two kinds of excitation, gapless and with a gap [2]. The gapless excitation has a ferromagnetic feature and the excitation spectrum with a gap has an antiferromagnetic one, thus having two aspects of thermodynamic nature. Another remarkable theoretical
74
finding is that the two-spin correlation length is very short in this system. This fact probably affects a spin dynamics in this quantum ferrimagnet. Quantum ferrimagnets are constructed by not only spin alternating chains but also alternating chains with a novel fashion of bonding. For instance, if the spin 1/2 system has a (F-F-AF-AF) interaction, this must behave as the S=3/2 and 1/2 ferrimagnetic chain at low temperature. This case corresponds to our compound Cu(3-Clpy),(N3), (3-Clpy=3-Chloropyridine), hereafter abbreviated as CCPA [5]. In this paper, we report the results of ESR measurements on a single crystal sample of CCPA in order to investigate a spin dynamics of the S=1/2 F-F-AF-
AF chain system.
2. EXPERIMENTAL 2.1. Crystal structure and magnetic properties Single crystal samples of the compound CCPA were synthesized by the method according to Reference 6. This compound crystallizes in the monoclinic system (space group P2,lc). The lattice constants and
p
angle at room temperature are a=15.113(7)
A, b=13.483(6) 8,
c=15.801(9) 8, and p116.93(4)" . The chain structure of this compound is shown in Figure 1. Most part of the 3-Chloropyridine is omitted for simplicity. The structure consists of copper chains linked by azido bridges along the
axis. Two kinds of azido bridges are present in the
structure, namely end-on and end-to-end bridges. Three kinds of copper sites are linked by two kinds of bridging fashion, one of which is a double end-on fashion and the other an endon and an end-to-end fashion. The exchange interactions between Cu2' ions linked by azido bridges have been studied [7]. Ferromagnetic and antiferromagnetic coupling usually appears for the end-on and the end-to-end azido
bridge,
Antiferromagnetic
respectively. coupling
is
proposed for the end-on bridge, when the Cu-N-Cu angle is greater than 106" Figure 1. Chain structure of C u ( 3 - C l ~ y ) ~ ( N ~ ) ~ . Most part ofthe 3-Clpy is omitted for simplicity.
. In this compound, the
Cu-N-Cu angle for the end-on bridge on the latter bridging fashion is about 130" . Thus, we expect that this
75
compound is a tetramer chain with antiferromagnetic exchange interactions. The nearest interchain distance is about 10 A, so that the magnetic interaction between chains must be very weak and quite an ideal onedimensional system realizes in this compound. Magnetic properties of this compound were measured with a SQUID magnetometer (Quantum design's MPMS2) installed at RIKEN. The temperature dependence of the magnetic susceptibility times temperature shows a monotonical decrease with decreasing temperature from room temperature and reaches a minimum at about 10 K, then increases steeply upon cooling further [ 5 ] .This behavior is typical of a quantum ferrimagnet.
2.2. ESR studies ESR measurements were carried out with a conventional X-band ESR spectrometer (JEOL TE300) with a continuous He flow cryostat (Oxford Instruments). Figure 2 shows an ESR signal at room temperature for the designated angle between applied field and the chain direction. The resonance field and the line width largely changes with the angle. These field derivative signals are integrated and the signal absorption profiles are obtained. The angular dependence of the line width AH,,2 at half height of the absorption profile is displayed in Figure 3. We see the minimum of the line width AH,,, at about 55" that is generally accepted in a one dimensional system [ S ] and the angular dependence does not obey the simple l+cos2€J curve as commonly observed in three dimensional systems. The solid line in Figure 3 indicates a fit to the equation shown in the figure, which is expected, in onedimensional systems. In this equation, a and b are constants. The agreement is
satisfactory. The results of the line profile analysis are sown in Figure 4. The line profile at
Magnetic field (Oe) Figure 2. ESR signals of a single crystal sample of CCPA obtained at room temperature and X-band for the designated frequencies
Lorentzian,
which
contribution
from
is very close to means the
no
long-time
diffusive tail of the spin correlation function at this magic angle. The line profiles at 0"
// chain) and 90" (H
chain) are not very close to a typical
76 300
I
I
I
I
1D profile obtained by the Fourier
I
Room temperature
transform of exp[-At3’*] compared
-
with those of TMMC which is a classical
H I chain
200-
4
one
dimensional
antiferromagnet. This indicates that the short time contribution of the correlation
function
becomes
somehow large compared to the diffusive term which is usually 0
20
60
1 (10
80
Angle 8 (degrees) Figure 3. Angular dependence of the half width at half height AH,,2 of CCPA at room temperature
dominant in 1D systems probably
and X-band. The solid curve is a fit to the equation in this figure.
different copper sites.
due to
the
- ____ 20 15 25
. v X
Lorentzian _ Gaussian
/ /
1D
5 5 degrees
/
H//chain H I chain
-
/ -
/
exchange
interaction F-F-AF-AF or three Next, we show in Figure 5 the temperature
I
I
peculiar
resonance
dependence fields
of
parallel
the and
perpendicular to the chain. The resonance field perpendicular to the chain shifts monotonously to high
h
field side upon cooling, while that
v
parallel to the chain shifts to the
-
opposite field side. The geometrical mean 0
2
4 res
Figure 4. The inverse of the line profile for representative directions. 1D represents a line profile for Fourier tranform of the relaxation function exp(-At ) pertaining to 1D system.
the is
10
f 1R
[(H-H )/AH
3/2
of
resonance almost
fields
temperature
independent in the entire temperature region. These shifts of the resonance field are commonly observed in onedimensional
system
like
CsMnCl3*2H,O[9].
3. CONCLUSIONS We performed ESR measurements on a single crystal sample of CCPA which is regarded as
the S=1/2 F-F-AF-AF tetramer chain. The angular dependence of the half line width at half height of the line profile shows a diffusive type of angular
anisotropy,
commonly
observed
dimensional systems. profiles at 0"
which in
is one-
The line
// chain) and 90" (H
chain) largely deviate from not 0
50
100
150
200
Temperature
Figure 5. Temperature dependence of the resonance fields parallel and perpendicular to the chain. The geometrical mean of the resonance fields is almost temperature independent.
only Lorentzian and Gaussian but also the 1D profile obtained by Fourier transform of exp(-A?'*), although the line profile at 55" is Lorentzian Temperature
as
expected.
dependence of the
resonance field of this compound shows typical behavior of the one-dimensional system. That is to say, the resonance field perpendicular to the chain shifts to high field side upon cooling, while that parallel to the chain shifts to the opposite side. From these findings, we have concluded that CCPA shows a typical one dimensional behavior like an antiferromagnetic uniform chain irrespective of its peculiar exchange couplings except for the line profiles at 0" chain) and 90" (H
chain).
ACKNOWLEDGEMENTS This work is supported by the MR Science Research Program of RIKEN and by a Grant-inAid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science and Technology. Thanks are also due to Chemical Analysis Unit in RIKEN. REFERENCES 1. S. Brehmer, H.-J. Mikeska and S. Yamamoto, J. Phys.: Condens. Matter., 9 (1997) 3921. 2. S. Yamamoto, T. Fukui, K. Maisinger and U. Schollwock, J. Phys.: Condens. Matter., 10 (1998) 11033.
78
3. M. Hagiwara, K. Minami, Y. Narumi, K. Tatani and K. Kindo, J. Phys. SOC.Jpn., 67 (1998) 2209. 4. M. Hagiwara, Y. Narumi, K. Minami. K. Tatani and K. Kindo, J. Phys. SOC.Jpn., 68 (1999) 2214. 5. M. Hagiwara, Y. Narumi, K. Minami and K. Kindo, Physica B, 294-295 (2001) 30. 6. A. Escuer, R. Vicente, M. S. E. Fallah, M. A. S. Goher and F. A. Mautner, Inorg. Chem., 37 (1998) 4466. 7. L. K. Thompson, S. S. Tandon, M. E. Manuel, Inorg. Chem., 34 (1995) 2356. 8. R. E. Dietz, F. R. Merritt, R. Dingle, D. Hone, B. G. Silbernagel and P. M. Richards, Phys. Rev. Lett., 26 (1971) 1186.
9. K. Nagata and Y. Tazuke, J. Phys. SOC. Jpn., 32 (1972) 337.
EPR in the 2 1" Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
79
ESR studies of quasi-one-dimensionalhalogen-bridged mixed-metal complexes H. Tanakaa, K. Marumotoa, S. Kurodaa, T. Manabeb, S. Furukawab, and M. Yamashitab "Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan bGraduate School of Science, Tokyo Metropolitan University, 1-1Minami-Ohsawa, Hachioji 192-0397,Japan ESR measurements were carried out on two types of halogen-bridged mixed-metal complexes Ni,,M,(chxn),Br, (M=Pd, Co) to study the competition between magnetic Ni3+ mono-valence (SDW) state and nonmagnetic Pd2+-Pd4+ mixed-valence (CDW) state. Clear dependence of the low-temperature Curie-like spin concentration N, was observed for Ni-Co system whereas almost unchanged one for Ni-Pd system. This behavior is consistent with the fact that Co3+ion in Ni-Co system is nonmagnetic in contrast to the magnetic Pd3+ion, which is induced from Pd2+-Pd4+ state in Ni-Pd system. Such results are consistent with the observed enhancement of magnetic susceptibility and systematic variation of g-value in Ni-Pd system. 1. INTRODUCTION
Halogen-bridged metal complexes have been attracting much attention as typical materials of the quasi-one-dimensional system[1-31. In these materials, there are two well-known electronic ground states due to the relative strength between on-site Coulomb interaction (U) and electron-phonon interaction (S), e., (i) mono-valence state for U>S case, --M3+--X--M3+--X---M3+--X--and (ii) mixed-valence state for U<S case, M4+-,where M and X represent metal and halogen atoms, respectively. In the case of M=Ni, the system has former ground state with the X ion in the midpoint of two neighboring Ni ions. In this case, the unpaired electron on the Ni(3d:) orbital constructs the one-dimensional spin112 chain through the halogen p, orbital along the crystallographic b-axis. On the other hand, M=Pt, Pd complexes have latter dimerized ground state involving the deviation of X ions as represented schematically above. In this case, each metal ion is nonmagnetic.
80
Hereafter we define such states as spin-density-wave (SDW) and charge-density-wave (CDW), respectively. The SDW does not mean a three-dimensionally ordered state and this wording have been employed so far[4]. Recently, mixed-metal complexes Ni,,Pd,(chxn),Br, (chxn=lR, 2R-cyclohexandiamine) have been successfully synthesized and then it has become possible to investigate the competition between these two states. Many kinds of experimental and theoretical studies have been carried out and it is suggested that the CDW state of Pd2+-Pd4+ converts into Pd3+ions in the mixed crystals[5-81. We have been studying this system magnetically by using ESR. Spin susceptibility has shown clear enhancement from the linear summation of pure Ni and Pd complexes in the mixed crystals without showing any increase of Curie spin concentration as shown in Figure 1[9]. This provides strong evidence for the induced spin moments Pd3+ from the nonmagnetic CDW state. Furthermore, the principal g-values g, obtained from single crystalline samples show monotonic decrease as x increases as shown in Figure 2[10, 111. This indicates the contribution from the induced Pd3' ion with smaller g-value than that of Ni3' ion, which supports the spin susceptibility enhancement well. In this paper we report ESR measurements of newly synthesized mixed-metal complexes Ni,,Co,(chxn),Br, (OsxsO.118), where it is reported that the mixing of Co ion into Ni(chxn),Br, acts as the hole-doping in this system[l2]. Magnetically, Co3+ion is regarded as impurity in the Ni3+ spin chain. In this case, clear difference of the the magnetism between Ni-Pd and Ni-Co systems are expected. Then such comparison provides further evidence for the induced Pd3' ion in the Ni-Pd system.
0
0.2
0.4
0.6
0.8
1
X
Figure 1. Room temperature spin susceptibility of polycrystalline Nil. ,Pd,(chxn),Br,. The solid curve is a guide to eyes. The dashed line represents the linear summation of pure Ni and Pd complexes.
0
0.2
0.4
0.6
0.8
1
X
Figure 2. The x-dependence of the principal g-value g, at room temperature in Ni,,Pd,(chxn),Br,.
81
2. EXPERIMENTAL The polycrystalline samples were obtained by electro-chemical oxidation method as described in references [5] and [121 for Ni,,Pd,(chxn),Br, and Ni,,Co,(chxn),Br,, respectively. For Ni,,Co,(chxn),Br,, samples of x=O, 0.044, 0.093 and 0.118 have been used. The homogeneity of samples and metal mixing ratios had been previously confirmed by X-ray powder pattern analysis and ICP emission spectroscopy, respectively. ESR measurements were performed by using a Bruker E500 X-band spectrometer equipped with a gas-flow type cryostat Oxford ESR-900. Sample temperature was controlled by Oxford ITC 601. The absolute magnitude of the spin susceptibility and g-value were calibrated using CuSO,*SH,O and diphenylpicrylhydrazyl (DPPH) as a standard, respectively. 3. EXPERIMENTAL RESULTS AND DISCUSSION The crystal structures of Ni(chxn),Br, and Pd(chxn),Br, are shown in Figure 1 of reference [6]. These two complexes are isomorphous with each other except for the halogen deviation from the midpoint of two metal ions in Pd(chxn),Br,. The mixed crystals of Ni,,Pd,(chxn),Br, and Ni,,Co,(chxn),Br, are also isomorphous with Ni(chxn),Br,, except for the halogendeviation in Ni,,Pd,(chxn),Br, ( ~ 0 . 7 1 )[7, 121. Each metal ions are coordinated by four Nitrogen atoms of two chxn ligands in the planar fashion. These M(chxn), moieties are bridged by bromine ions and stacked along the b-axis, constructing linear chain structures. Figure 3 shows the temperature dependence of spin susceptibility Almost temperature independent behavior of 4 Ni(chxn),Br, (x=O) obtained for T d 0 0 K is attributable to the strong antiferromagnetic --interaction (151-2700 K) of one-dimensional system[9, 131. This behavior is also observed in Ni,,Co,(chxn),Br, indicating the existence v0 of linear spin chains even in the mixed X - 1 crystals which contain more than 10 percent X of nonmagnetic Co3+ion. On the other hand,
of Ni,,Co,(chxn),Br,.
systematic increase of is observed as increases in the low temperature region. This
200 T(K)
h
-x=O.O44
1
behavior should be attributable to the creation of odd-length chain-segments due to the scission of the Ni3+linear chain by the
0
100
300
Figure 3. Temperature dependence of spin susceptibility Ni,,Co,(chxn),Br,.
of polycrystalline
82
nonmagnetic Co3+ion. In this case, the spin concentration N , is expected to obey the following equation[l4],
To compare the experimental values of N, with equation (l), we use the following equation,
where deviates from 1 (Curie low) in the case of well-known Random-Exchange Heisenberg Antiferromagnetic Chain (REHAC)[lS]. This type of are actually observed with the values =0.86-0.90 typically, in Ni,,Co,(chxn),Br,, where weak exchange interaction effectively works between the segments through randomly distributed Co3' ions as discussed later. Figure 4 shows the comparison of experimentally observed N , (solid circles) with equation (1) (dashed line) in Ni,,Co,(chxn),Br,. Monotonic increase of N , with indicates the successful substitution of nonmagnetic Co3+ion for Ni3' ion. On the other hand, no such increase of N , were observed in Ni,,Pd,(chxn),Br, as reported elsewhere[9]. This indicates that no scissions of the spin chain occur in Ni-Pd system, that is, the magnetic Pd3' ion which is induced from nonmagnetic Pd2'-Pd4+state is substituted for Ni3+ion in this system. These results are consistent with the observed behavior of spin susceptibility enhancement and systematic variation of g-value as shown in Figures 1 and 2, respectively. Now we should note that experimental values of N, in Figure 4 are rather smaller than the expectation from I " Y ' I " / /equation (1). This implies that Co3' ions in Ni,,Co,(chxn),Br, make some cluster-like assembled mixing states in the chain. Thus the number of nonmagnetic impurities which effectively cut the chain reduces from the value of x. Then it is natural that even the complex of x=O.118 shows one-dimensional 0 0.05 0.1 linear-chain character in as shown in
t
Figure 3. The increase of N , in Ni-Co system also affects the ESR linewidth AH. Figure 5 shows the temperature dependence of AH for polycrystalline Ni,,Co,(chxn),Br, with the
1
X
Figure 4. Experimentally obtained N , (solid circles) and expected values from equation (1) (dashed line) in Nil. xCox(chxn),Br,.
83
definition of AH shown on the inset. Large temperature dependence observed in Ni(chxn),Br, at high temperature region is due to the dominance of spin-lattice relaxation time Tl to AH in this material[ll, 131. In the mixed crystals, temperature dependence of AH become weaker, resulting in the smaller room temperature value as x increases as shown in Figure 6 (solid circles). Such decreasing AH was also observed in the mixed crystals of Ni-Pd system as shown in Figure 6 (open circles), although the origins of the x-dependence are considered to be different between these two systems. In Ni-Pd system, observed x-dependence of AH is attributable to the contribution from the Pd3+ion which has smaller AH than that of Ni3+ion due to its longer TI-value[lO, 111.Then the systematic variation of AH in Ni-Pd system can be regarded as a modification of Tl due to the mixing of two different spin species. On the other hand, in Ni-Co system, only Ni3' ions are responsible to the ESR spectra as discussed before. In fact, the principal g-values g , in Ni-Co system obtained from the powder pattern spectra at 4K are almost unchanged within the experimental error in the mixed crystals. Then the xdependence of AH in Ni-Co system is not due to the modification of TI of the linear spin chain but the contribution from the increasing odd-length segments which show small and nearly temperature independent AH as shown in Figure 5 in the low temperature region. This process is possible by introducing the small exchange interaction between the segments and linear spin chains through Co3' ions. In addition, this consideration is consistent with the power low behavior of in equation (2). In this case, the total width of the exchange-narrowed ESR spectra depend strongly on the spin concentration N,. As already mentioned, N, shows monotonic increase as x increases in Ni-Co system. This result is consistent with the steep drop of AH shown in Figure 6. 500 1
I
I
I
400
400
300
300
2 200
2 200
100
100
0
-* - Ni-Co
'
Ni-Pd
% Q
200
100
300
T(K)
0
0.2
0.6
0.4
0.8
1
Y
A
Figure 5. Temperature dependence of the
Figure 6. The x-dependence of the
linewidth AH of Ni,,Co,(chxn),Br,
room temperature linewidth AH in Ni,,Co,(chxn),Br, (solid circles) and Ni,,Pd,(chxn),Br, (open circles).
the definition of AH on the inset.
with
84
4. CONCLUSION ESR measurements were carried out on two types of halogen-bridged mixed-metal complexes Ni,,Co,(chxn),Br, and Ni,,Pd,(chxn),Br,. The experimental data were analyzed in terms of the typical mixing cases of nonmagnetic- and magnetic-ion into Ni3+spin-l/2 chain, respectively. In the former materials, systematic increase of the Curie-like spin concentration N,, arises from odd-length chain segments, were observed as x increases. This result is consistent with the fact that Co3+ion in this system is nonmagnetic. The increasing N, also affects the temperature dependence of the ESR linewidth AH through the small exchange interaction between chain segments. On the other hand, in the latter materials, no remarkable change of N, have been observed, indicating the mixing of magnetic Pd3+ion into the chain. This result is consistent with the previously observed spin susceptibility enhancement and systematic variation of g-value, which suggest the conversion of Pdz+-Pd4+CDW state into Pd3+ion in this system.
REFERENCES 1. H. Okamoto, M. Yamashita, Bull. Chem. SOC.Jpn., 71 (1998) 2023. 2. K. Nasu, J. Phys. SOC,Jpn., 53 (1984) 427. 3. M. Yamashita, T. Manabe, T. Kawashima, H. Okamoto, H. Kitagawa, Coord. Chem. Rev., 190-192 (1999) 309. 4. K. Nasu, J. Phys. SOC.Jpn., 52 (1983) 3865. 5. T. Manabe, M. Yamashita, T. Kawashima, H. Okamoto, H. Kitagawa, T. Mitani, K. Toriumi, H. Miyamae, K. Inoue, and K. Yakushi, Proc. SPIE., 3145 (1998)106. 6. M. Yamashita, T. Ishii, H. Matsuzaka, T. Manabe, T. Kawashima, H. Okamoto, H. Kitagawa, T. Mitani, K. Marumoto, and S. Kuroda, Inorg. Chem., 38 (1999) 5124. 7. Y. Wakabayashi , N. Wakabayashi, M. Yamashita, T. Manabe, and N. Matsushita, J. Phys. SOC.Jpn., 68 (1999) 3948. 8. K. Iwano, J. Phys. SOC.Jpn., 68 (1999) 935. 9. K. Marumoto, H. Tanaka, and S. Kuroda, T. Manabe Phys. Rev. B, 60 (1999) 7699. 10. H. Tanaka, K. Marumoto, S. Kuroda, T. Manabe Synth. Met. 120 (2001) 949. 11. H. Tanaka, K. Marumoto, S. Kuroda to be published in J. Phys. SOC.Jpn. 12. M. Yamashita, K. Yokoyama, S. Furukawa to be published in J. Am. Chem. SOC. 13. H. Okamoto, K. Toriumi, T. Mitani, and M. Yamashita, Phys. Rev. B, 42 (1990) 10381. 14. T. Tonegawa, Phys. Rev. B, 14 (1976) 3166. 15. Z. G. Soos and S. R. Bondeson, Solid State Commun., 35 (1980) 11.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
85
Microwave radiation from magnetostatic mode in high power FMR Michinobu Mino, Hideaki Tsukuda, Masayuki Tsukamoto and Hitoshi Yamazaki Department of Physics, Faculty of Science, Okayama University, Tsushima, Okayama, 700-8530 Japan Microwave radiation from nonlinearly excited magnons in a ferromagnetic yttrium iron garnet (YIG) is studied. Experiments were carried out in the X-band range with a single crystal spherical YIG at liquid nitrogen temperature. Parametric resonance of magnons with a parallel punping configuration was employed. The spectrum of microwave radiation depends on a static field and the pumping power. At high pumping power, more than one frequency peak are observed simultaneously. These peaks correspond to magnetostatic modes.
1. INTRODUCTION A nonlinear effect of a ferromagnetic resonance is characterized by the threshold
microwave power
Pthr.
In parallel pumping experiments, where a microwave magnetic
field is applied parallel to a static field, a magnon (spin-wave) system keeps a thermal equilibrium state at pumping power P
Just above P t h r , the number of manons in a
very narrow region in a wave-number space, i.e., resonant magnon, grows exponentially[l]. On the other hand the population of nonresonant magnons can be considered as the thermal equilibrium level.
But, the level of overheating of
nonresonant magnon increases with the increasing of the pump power. Finally, autooscillations of magnon number are observed [ 2 ] . In this nonequilibrium dissipative region, nonlinear dumping of excited magnons is very important. We study a energy flow from parametrically excited magnons to magnetostatic modes in this report. For a ferromagnetic spherical sample, the disperssion relation is as follow;
86
where kwavenumber of the spin wave, y=gyromagnetic ratio, Ho=applied dc magnetic field, Mo=saturation magnetization, N,= demagnetizing factor along the applied field, H,=an exchange field, a=lattice spacing, and &=angle between k and the applied field. The result of Equation(1) is a band of dispersion curves bounded by the 8k=0 and E)k=Jd/2. Equation 1 will be applicable until the wave length is perhaps one-tenth the size of the spheroid, beyond which the effect of the boundaries becomes significant [3]. The spectrum will now be completed by the magnetostatic mode, whose location may be expected to be closely related to the k=O limits of Equation(1).
2. EXPERIMENTS Experiments were carried out in the X-band range with a single crystal sphere of YIG at liquid nitrogen temperature. The microwave was generated by HP 83624A synthesized sweeper and amplified upto 40W by a traveling wave tube. The pulse modulation of microwave power was employed. The measurement section was a short-circuited X-band waveguide, in which
open dielectric resonator (ODR) with
the sample YIG was installed. Both a microwave and a static magnetic field are applied along the [ 11I] direction, which is an easy magnetization axis. To study radiation signal from the sample, it was surrounded by a wire loop. This loop connected with coaxial cable to the measuring circuit. The plane of the loop was parallel to microwave field h,
so that the disturbance of the field in the ODR was minimal.
3. RESULT AND DISCUSSIONS As the pumping power is increased, a microwave radiation with 042 component is firstly observed just above Pthr [4]. Further increase of pumping power, more than one frequency peak are observed simultaneously. The radiation spectrum depends on the static magnetic field as shown in Figure 1, where a static field
changes between 1101
Oe to 2 110 Oe. Peaks shift to high frequency direction with an increasing of static field, except a 0 4 2 peak. A big component around 3.5GHz disappears above 1350 Oe. The radiation frequency is shown as a function of a static field in Figure 2. Most radiation peaks are in some lines, and parallel to the bottom frequency line of magnetostatic mode. This bottom frequency corresponds to the
and 8=0 limits of Equation(1).
87
?
?
L.
0
2
3
6
4
Frequency [GHz] Figure 1. Power of microwave radiation from YIG at various static field Ho. Pumping frequency is o p / 2 ~ 9 . O G Hand z its power is P=46dBm.
1000
1500
2000 (G)
-
1000
2000 Ho ( G )
1500
Figure 2. Radiation frequency as a function.
Figure 3. Magnetostatic mode
of a static field.
resonance.
88
All experimental points exist above this bottom frequency, i.e., locate in magnetostatic mode manifolds. Next, ferromagnetic resonant absorption patterns were studied using
the same
sample in a frequency range 2-6 GHz. Multiple absorption peaks are observed by the magneto static modes. Intensity of absorption by the magnetostatic modes deeply depends on a geometrical configuration, i.e.,
static and microwave fields directions,
sample orientation. Observed results are shown in Figure 3.
For the most part,
the
resonant points are consistent with radiation points in Figure 2. In conclusion, parametrically excited magnons are nonlinearly coupled magnet static modes at nonequilibrium region. A energy flow from parametrically excited magnons to magnetostatic modes are observed by detecting microwave radiation.
REFERENCES 1 . E. Schlomann, J.J. Green and U. Milano, J. Appl. Phys. 3 1 (1960) 3868. 2. Nnlinear phenomena and chaos in magnetic materials, ed. P,E. Wigen (World scientific,
Singapore, 1994)
3. L.R. Walker, Phys.Rev. 105 (1957) 390.
4. I. Kalinichenko, M. Mino and H. Yamazaki, Proceedings of the 8th International conference onferrites, (2000)900-902.
EPR in the 2lStCentury A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
89
Parametrically excited magnetoelastic waves in FeB03 Kenji Ishihara, Michinobu Mino, Masayuki Tsukamoto and Hitoshi Yamazaki Department of Physics, Faculty of Science, Okayama University, Tsushima-naka, Okayama, 700-8530 Japan Parametric excitation of magnetoelastic waves in the easy-plane antiferromagnet FeB03 is studied at room temperature. Measurements were performed at X-band microwave with a perpendicular pumping configuration. As increasing the pumping power, the microwave absorption by a quasimagnon
mode is saturated above the threshold level. Above this
threshold, the number of quasiphonons with a half pumping frequency grows exponentially. The microwave radiation from these quasiphonons is simultaneously observed. Further increase of microwave power causes auto-oscillations in the magnetoelastic system. The radiation spectrum shows side band peaks around a half pumping frequency.
1. INTRODUCTION
Magnetoelastic interaction makes a coupling of magnetic and elastic oscillations. The resulting normal modes (quasimagnons and quasiphonons) contain both magnetic and elastic components. A magnetic detection of elastic vibration is possible in a crystal with a strong magnetoelastic interaction [ 11.
The greatest changes in the magnon and phonon spectra
occur in antiferromagnets, because in these materials the magnetoelastic interaction is enhanced by the exchange interaction [2]. An iron borate (FeB03) is an easy-plane antiferromagnet with T~=384K.It has the rhombohedra1 crystalline structure with magnetization hard axis C3. The in-plane anisotropy field is negligible small (
ferromagnet
in
the
easy
plane.
The
spectra
quasimagnons(ok) and quasiphonons (!&) in FeB03 are as follows;
of
low-frequency
90 O~= y [ H ( H
Q,
=
+ H , ) + H i + (4)’f”,
c,k[l-(yH,E
/OJj1’*.
Where H is a static magnetic field directed in the easy plane; y is the gyromagnetic ratio; H~=250kOeis the Dzyalonshinskii Moriya field; yHAis the magneto acoustic gap in the magnon spectrum with HA=2.2kOe; and
Oe-’cm is a phenomenological exchange
constant that is proportional to the exchange field H~=1.6x10~0e.The sound velocity c,=4.8x105mlsec and the coefficient t-0.7, which describes the efficiency of the linear interaction between the magnons and phonons, depend on the direction of the wave vector and the polarization of the quasiphonons. Spectra of quasiphonons and quasimagnons are shown in Figure 1 [3]. When a pumping power is weak, the microwave field linearly excites the uniform mode precession of the magnetic moments with a pumping frequency cop. As increasing the pumping power, the microwave absorption is saturated above the threshold level P,. Above this threshold, the uniform quasimagnon precession decay nonlinearly into quasiphonon pairs, with 4 2 frequency and opposite wave vector +k and -k. This process analogous to the first order Suhl instability [4]. Parametrically excited quasiphonons in FeBO3 were observed by the Brillouin light scattering method. Quasiphonons with wave vector +k and -k of frequency 0,/2 propagate along hard magnetic axis
[S].
The microwave radiation from these quasiphonons is simultaneously observed above [6]. Further increase of microwave power causes auto-oscillations in the magnetoelastic system. The radiation spectrum shows side band peaks around a half pumping frequency.
-iX1o5
-sXio4
sXio4
iX1o5
Wavenumber[l /cm]
Figure 1. Quasimagnon and quasiphonon spectra in FeB03 at H=l700e.
91
2. EXPERTMENTS Experiments were performed with a X-band reflection-type setup with a rectangular TElol cavity at room temperature. The static magnetic field
and microwave field h, which had a
pumping frequency wp= 9.46GHz, were applied in the easy plane. The single crystal FeBO3 was attached to the bottom of the cavity. Microwave signals reflected from the cavity were detected by a tunnel diode and recorded by the digital storage oscilloscope. In order to detect another signal, i.e., the microwave radiation with wp/2 from the sample, a one-turn pickup coil encircled the sample. The received signals in the coil were low-pass filtered and amplified. It was sent to a spectrum analyzer. This filtering is necessary to prevent possible damage of the spectrum analyzer and it also suppresses interference of the pumping power.
3. RESULT AND DISCUSSIONS
At low pumping power, i.e., below the nonlinear threshold
microwave absorption by
quasimagnons linearly grows with increasing pumping power
The microwave radiation
from the sample in this region is too weak to be detected. Above
microwave absorption
saturates and microwave radiations with oP/2 from the sample are detected at a remarkable magnitude. The radiation keeps its center frequency of wp/2 even if the static magnetic field changes. By increasing the driving power, auto-oscillations of microwave absorption are observed.
ICJ
I
.
'
I
.
'
I
.
I
.
-1
Z X ~ O -4~~ 1 0 6 - ~~ 1 0 .8~~ 1 0 -1 ~ ~10.~
Time(s)
Figure 2. Real-time signals of absorption microwave power by quasimagnons at H=l600e. The pumping power is (a) P=24.2dBm; (b) P=24.3dBm; (c) P=24.4dBm.
92
-90
-
-106
I
4.72956 4.72958 4.72960 4.729 2 4.72964
Frequency(GHzf
Figure 3. Measured frequency spectra of quasiphonon. This result means that auto-oscillations of quasimagnon number occur a the onset power of auto-oscillations
By increasing the driving power further, an auto-oscillations frequency
increase significantly as shown in Figure 2. Above Pa, , a spectrum of microwave radiation from quasiphonons shows side band peaks as shown in Figure 3. A large peak at 4.7296 GHz is
radiation. Small peaks around
are side band peaks of 21 kHz amplitude modulation of 0 4 2 components. This frequency of auto-oscillations of quasiphonons increase also with increments of the driving power. These results suggest the observed auto-oscillations in a magnetoelastic system are caused by a nonlinear interaction between quasimagnons and quasiphonons. It is a pleasure to acknowledge
V.L.Safonov
and
Q.Zhang
for
valuable
discussions.
REFERENCES 1. L.E. Svistov, V.L. Safonov, J. Low and H. Benner, J. Phys:Condens. Matter, 6 (1994) 8051. 2. V.I. Ozhogin and V.L. Preobrazhenskii, Sov. Phys. Usp., 31 (1988) 713.
3 . W. Jantz and W. Wettling, Appl. Phys. 15 (1978) 399. 4. H. Suhl, J. Phys. Chm. Solids., 1 (1957) 209.
5. W. Wettling, W. Jantz and C.E. Patton, J. Appl. Phys. 50 (1978) 2030. 6. Q. Zhang, M. Mino, V.L. Safonov and H. Yamazak, J. Phys. SOC.Jpn. 69 (2000) 41.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
93
Slow dynamics in chaotic magnon system Jiang CAI", Yoshiyasu KATO", Atsusi OGAWAb,Takayuki HIRATAb,Meiro CHIBA" and Yoshifumi HARADAb *GraduateSchool of Engineering,AppliedPhysics,Fukui Univ., Fukui 9 10-8507,Japan beparhnent of Human & Artificial Intelligent Systems, Fukui Univ., Fukui 910-8507,Japan 'Department of Applied Physics,Fukui Univ.,Fukui 9 10-8507,Japan A new type of spin-wave instability and slow dynamics (nonlinear magnon relaxation) in YIG under a stepwise external pumping have been performed in perpendicular pumping for the first time. We observed the transition changed from quasiperiodic states to chaos, namely, collapse of torus.
1. INTRODUCTION Chaotic dynamics have been studied in ferromagnet (YIG) either under parallel pumping or perpendicular pumping [1,2]. Although many extensive works were performed, few works were performed in broad time domain chaos in magnon systems, especially for the chaotic transient phenomena. In our experiments, the process of the slow and long time magnon relaxation just above the threshold of spin-wave instability is affected by radiation damping due to the interaction of spin-wave turbulence. The microwave pumping frequency of X-band was used at about 9.4GHz.Experimentalmeasurements in YIG of the threshold of magnon instability were made as a function of time. After the aging under a stepwise pumping, the nonlinear coupling between two Hopf frequency modes can generate frequency locking and chaos. We observed the transition changed from quasiperiodic states to chaotic states, namely, collapse of torus. In order to describe the physical characters of chaos in this system, power spectrum and the largest Lyapunov exponents are obtained. We show that the largest Lyapunov exponents can be used to indicate the relaxation behavior of magnon creation. The spin-wave instability driven by microwave RF field constitutes a good physical system for studying nonlinear dynamic phenomena. We expect that there are some analogy between spin-wave instability and ultraslow mode in glass-forming system.
2. EXPERIMENTS High-power ferromagnetic resonance in a well polished 0.4572-mm-radius sphere of pure iron garnet (YIG) was performed with a conventional ESR spectrometer (JEOL
94
FAIOO). The X-band microwave pumping frequency at about 9.4GHz was used at room temperature. Owing to the resonant pumping condition in YIG spheres, was applied in 4 1 1 > orientation at 3300Oe. TEoll cylinder cavity (Q=18,000) was used to separate the high microwave input power from the weak time-dependent output signal. After carrying out phase detection of the output signal expanded with amplifier, it realizes digital data measurement on the AD board (ADVANTECH PC-LABCARD) of 12-bit decomposition ability carried in the computer. Experimental measurements in YIG of the threshold of spin-wave instability were made as a function of time. 3.
We changed suddenly the microwave power Pi, from 43dB to 4dB ( 0.1 w is listed P,,, = O d B throughout this paper) , obtained time series of the slow relaxation of magnons (Figure 1) .We constructed the strange attractor by using the method of embedding of Takens 131 (Figure 2 ,delay time s). A quantitative measure of strange attractor is represented by the largest Lyapunov exponents E [4] which characterize the average rate of exponential divergence of near by trajectories within the attractor. In our case, the largest Lyapunov exponent gets larger the relaxation time (Figure 3). This means the transient develops from quasiperiod to chaos .It dropped a hint that the special type of nonlinear coupling between magnon modes may lead chaos happened, even multiple scattering and magnon-phonon interaction. Figure 4(a) shows power spectrum of a part the transient (from 0 to 112.5 s). Two fundamental frequencies are distinguished at =4.19Hz and = 0.818Hz means the strange attractor has a construction of torus T2. Another strong spectral component is confirmed as mixing frequency: 5 =5.0 Hz , were identified to present mixing frequencies mfi+nh, n integer. Power spectrum of another part of the transient (from 787.5 to 900 s) was shown in Figure 4(b).
0
20
40
TIME, min
60
882
884
886
-,
888
890
s
Figure 1. A slow relaxation lasts over 1 hour (a). Expanding chart for a part of the original waveform shows detailed structure (b).
95
c
:t " I
'
I
'
I
'
I
I
t
3
0.1
8
i 0 0
200
400
600
800
TIME, s Figure 2. Three-dimensionalprojection of the strange attractor.
Figure 3. Lyapunov exponents saturated at E = 0.123.
The ingredient corresponding to the peak of the strongest spectrum in Figure 4(a) has fallen and extended widely. It shows that collapse of torus has happened. 4. CONCLUSIONS
In conclusion, we carefully examined transitions in YIG we decrease the microwave power suddenly. Firstly, the slow transitions for several minutes were observed for the first time. We compare it to the fast chaotic transients reported by T.L.Carrol1,F.J.Rachford,and
Figure 4. Power spectrum of different transient shown in Figure l(a).
(a: 0-112.5 s ;b: 787.5-900 s ) in the
96
L.M.Pecora [5] previously. In that case one observes that typical initial conditions initially behave in an apparently chaotic manner for a possibly long time, then rapidly move off to some other region of phase space, asymptotically approaching a nonchaotic attractor. All phenomena happened in hundreds milliseconds. As opposed to that, the results of our experiments describe a macroscopic chaos dynamics of magnon system, and the slow chaotic transients showed collapse of torus . Secondly, a large dimension of the slow transition is calculated as about 3.9 by the method proposed by Grassberger and Procaccia (GP) [6]. It indicates that not only a large number of magnon modes are involved in the chaotic dynamics, but also a lot of phonon modes are involved in. The effect of multiplex distribution and magnon-phonon conversion should be taken into consideration of the generating mechanism. We expect that there are some analogy between spin-wave instability and ultraslow mode in glass-forming system.
1. S. Mitsudo, M. Mino and H. Yamazaki, J. Phys. Jpn., 59 (1990) 423 1. 2. J. Becker, F. Rodelsperger,Th. Weyrauch, H. Benner, W. Just, and A. Just, Phys. Rev. E, 59 (1999) 1622. 3. F. Takens, in “Dynamical Systems and Turbulence”, Lecture Note in Mathematics 898, ed. By D. A. Rand and L. S. Young (Springer, 1981) 336. 4. S. Sato, M. Sano and Y. Sawada, Prog. Theor. Phys., 77 (1987) 1-5. 5. T. L. Carroll, L. M. Pecora, and F. J. Rachford, Phys. Rev. Lett., 59 (1987) 2891; L. Carroll, F. J. Rachford, and L. M. Pecora, Phys. Rev. B, 38 (1988) 2938. 6. P. Grassberger and I. Proccacia, Phys. Rev. Lett., 50 (1983) 346; Physica D, 9 (1983) 189.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors)
97
2002 Published by Elsevier Science B.V.
ESR studies of spin-polarized atomic hydrogen adsorbed on 3He-4Hemixture film A. Fukuda, T. Ohmi, H. Takenaka,
Waki, A. Matsubara and T. Mizusaki
Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
Spin-polarized atomic hydrogen (HA) under high magnetic field stays in a gas phase down to 0 K. In this study, we have searched for the two-dimensional (2-D) superfluidity of HA adsorbed on saturated helium film. We prepared a small and very cold surface, called a cold spot, in a sample cell whose temperature was kept much higher than the cold spot. High density 2-D HA gas was adsorbed and cooled on the cold spot. We introduced the 129 GHz mm-wave spectrometer and carried out the ESR measurement of HL. Our final goal is to observe directly the 2-D HA signal and to study the 2-D superfluidity of HA. In order to obtain the direct ESR signal of 2-D HA, we developed the mirror cavity, whose focal point was on the cold spot. We need the high resolution and very sensitive ESR measurement to separate the 2-D signal from the bulk HA signal. We measured the two-body recombination rate of HL adsorbed on the 3% 3He 3He-4He mixture film as a function of the cold spot temperature. We estimated the boundary thermal resistance between the helium mixture bath and adsorbed HA and concluded that 2-D HA adsorbed on the 3He-4Hemixture was cooled at same temperature as that of the cold spot down to about 40mK. We obtained the temperature dependence of the adsorption energy of HA on the mixture from the surface two-body recombination rate.
1. INTRODUCTION First we summarized here about the spin-polarized atomic hydrogen (H&)[l]. The HA is an atomic gas stabilized under high magnetic field and at low temperatures below 1K. Atomic hydrogen atoms are created from the hydrogen molecules by rf-discharge at low temperatures. The atomic hydrogen under the magnetic field has 4 hyperfine eigenstates and we denote them a, b, c and d from the lower energy level. In the high field of 4.5 T in this report and at low temperature below 1K only and b levels were occupied and both states are called spin-polarized atomic hydrogen. ESR transition from to d and b to c state are allowed by selection rule and thus we can observe the state-dependent ESR signals. The spin-polarized HA is a Bose particle and Bose Einstein condensation (BEC) is
98
expected under high density and at low temperatures. Recently BEC was achieved by MIT group by using the laser cooling and the evaporative cooling techniques[2]. Sample cell of H& was typically covered by superfluid 4He film to reduce the large recombination rate at a surface wall due to its large adsorption energy and magnetically inert surface. The H& adsorbed on the He film behaves as an ideal 2-D Bose gas and is expected the Kosterlitz-Thouless (K-T) transition and go into the 2-D superfluidity at very low temperatures. Our final goal is to achieve the 2-D superfluidity and observes it directly using ESR techniques. Recently 2-D quasi-condensate was reported by Turku group[3]. To quest the 2-D superfluidity of Hd, we developed a new method by which the 2-D surface density of H& was controlled by input Figure 1 Schematic drawing of the flux of H& and independently of the temperature sample cell. A: HA inlet, B: cold spot, (shown in Figure 1). We proved that more than C: Stycast 1266, D: Al-evaporated 98% of a huge recombination energy (50,000 K Mylar film, E: sintered metal, F: per one recombination) were carried away by an mirror, G: Kapton-acqadaq bolometer, excited Hz molecules from the 2-D H& system H: carbon-chip bolometer, into the bulk sample ce11[4-61. We also found that I: mm-wave inlet (waveguide) small amount of 3He impurity greatly affects the coupling - - between the 2-D H& and He film and thus the cooling mechanisms of 2-D H&[4]. We also studied the 2-body recombination rate of Hd adsorbed on 3% 3He3He-4Hemixture[7]. In this report, we describe our ESR experiments of H& at ultra-low temperatures (Chapter 2). We calculate the Kapitza thermal resistance between adsorbed HA and the 3He-4He mixtures and point out the importance of the mixture to cool the 2-D H& in comparison with a pure 4He film (Chapter 3). Then we show the temperature dependence of adsorption energy of H& on 3% 3He3He-4Hemixture as a results of recombination measurements (Chapter 4).
2. EXPERIMENTAL DEVICE AND ESR CAVITY The schematic drawing of the sample cell was shown in Figure 1. The HA produced in the low temperature dissociator was introduced through the H& inlet (A) into the sample cell. We prepared the small and cold surface (cold spot, B) with 4.4 mm diameter at the upper part of the cell. High density 2-D H& typically with about 10l2 sec-' was adsorbed on the cold spot. In the cell we placed a Cu mirror (F) which has 6 cm focal length. The cold spot was at
99
the half distance of the focal length from the mirror and therefore they formed a semi-confocal Fabri-Perot mirror cavity. The cavity was operated under TEMo,o,sl mode. The intrinsic quality factor Q of this mirror cavity, which has the Cu plate at the focal point, was several lo4 but when we replace the Cu plate to small cold spot, Q decreased to less than 1000. In order to keep Q high, Al-evaporated Mylar film (D) was glued to a Stycast piece (C) outside the cold spot to avoid the diffraction losses. In this way we could obtain the Q value of the cavity with the cold spot about 4000 and this value was limited by the conductivity of Al-evaporated Mylar sheet. The coupling between the cavity and waveguide (I) was determined by the size of the small coupling slit (0.12 mm x1.6 mm, capacitive coupling) at the center of the mirror and the reflection coefficient at the resonant frequency was approximately 0.5. We put the sintered silver (E) on the cold spot to get better thermal contact between He film and the cold spot. A bolometer (G) (acquadaq thermometer painted on Kapton film) was installed in order to detect ESR signal. The ESR absorption was measured by detecting the recombination heat triggered by the to d or b to c transitions. Another bolometer (H) was a Matsushita carbon resister. It was used to calibrate the absolute density of the H& by detecting the total heat production when all H& were recombined. A mm-wave was also detected by an InSb detector. InSb detector was situated outside the high magnetic field and thermally anchored to the 1K pot. We continuously measured the state-dependent H& density by using ESR and by fitting the time-dependence of H& during loading H& flux and after stopping loading, we extracted the two-body recombination rate Zffaa for various sample cell temperature Ts, cold spot temperature TC and input flux 4. The detail of this analysis was described in Reference 8.
3. KAPITZA THERMAL CONDUCTANCE BETWEEN RIPPLONS AND 'HE QUASI PARTICLES
The 2-D H& adsorbed on the film is expected to couple strongly with surface ripplons, which is the surface elementary excitations of He, and the coupling between ripplons and bulk excitations of the film is the weakest path of the heat flow from H& to the cold spot. In this section we present our calculation of the thermal Kapitza conductance between ripplons and the bulk excitations in 3He-4Hemixture, 3He quasi-particles (QPs). The most important feature of this work was to use 3He-4Hemixture film to ensure the better coupling between the ripplon and He bulk excitations. Reynolds et calculated the energy transfer between ripplon bath and phonon bath in liquid 4He[9]. Their model was based on the model formulated by Saam[101. They used ripplon velocity field VR as
where (T is the surface tension, p the bulk He density, A the surface area, w , = ( ~ q ~ / pthe )~'~ ripplon frequency with a 2-D wave vector q and b, the annihilation operator. They also proved that the two ripplons and one phonon process R+R++P dominantly contributed to the heat transfer. Here R denoted ripplon and P denoted phonon.
100
We calculated the coupling between ripplons and the 3He QPs in 3He-4Hemixture. We consider the coupling term between ripplons and QPs as
where p = -iAV is the momentum operator of 3He QP,
1
ly = -xexp(i(q. f
i
r,
+ Icz))uk the
k
field operator of QP, Vthe volume, aq,bthe annihilation operator of QP with 3-D momentum (9, k) where q and k are the component parallel and perpendicular to the surface, the effective mass of 3He QP and 6w2 the difference between effective mass of QP and bare mass of 3He. This term was adopted for calculation of QP and phonon (roton) scattering[111. We consider the process R+QP++QP.The thermal energy flux per unit area was given by d 2qd'q'dk'd2q"dk"-s(o, 2~ P(T,,T) = JV'A + €q>,k< - €q%.,k<.~o, (2d A (3)
+
- (1 Nq)(l- nq8)nq..]
where N,(TR) and n,(T) are the equilibrium occupation numbers for ripplons and 3He QPs, A E ~= ,_(q2 ~ + k 2 ) the QP's frequency, TRthe temperature of ripplons and T the 2m temperature of QPs. The Kapitza thermal conductance was defmed by
Calculation was straightforward by using Equation (1)-(4).
Here p'=plkBT and p is the chemical potential determined by the following equation.
where is the concentration of 3He and n3 is the number density of pure liquid 3He. In the high temperature limit, we can replace the Fermi distribution function by the Boltzmann distribution function in Equation 5, and G(T) is given by,
Then G(T) is proportional to 25" and this temperature dependence is much weaker than the result of ripplon-phonon scattering Results are shown in Figure 2. The solid lines
101
Figure 2 Calculated Kapitza conductance between He surface and bulk He as a function of temperature.
Figure 3 Adsorption e n e r a of HJ on 3% 3He 3He-4He mixture film. open circles are data taken at and open triangles, open squares and open inverted triangles were taken at Ts=l70, 150, and 110 mK, respectively, for # = 2 . 5 ~ 1 0sec-'. ~ ~ Solid squares, solid diamonds and solid inverted triangles were taken at Ts=150,130, and 110 mK, respectively, for .5x1012sec-'.
(1) and (2) are the exact G as a function of temperature for c=l (pure 3He) and 0.03, respectively. The dashed lines are the Boltzmann limit of G. Here we use the effective mass m*/m3=2.8 for pure 3He and 2.3 for 3% 3He 3He-4Hemixture. We use ~ 1 . 5 6 ~ 1Jm-' 0 ' ~for pure 3He and interpolated temperature dependent o(T) for 3% 3He mixture from 0.56% and 6.16% mixture data". The solid line (3) in Figure 2 is the results of ripplon-phonon coupling'. From this result, for example at mK, the coupling between the surface ripplon and the bulk 3% 3He mixture is more than 4 order of magnitudes bigger than that for pure 4He. This calculation indicates that the coupling of H& adsorbed on mixture is very strong and H& is cooled down to 40 mK despite the heat input of recombination and thermal energy of hot H& gas. We found the thermal boundary resistance between adsorbed H& and phonon bath in the case of pure 4He[4].
4. RESULTS AND FUTURE PROSPECTS We measured the 2-body recombination rate of H& adsorbed on both pure 4He and on 3% 3He 3He-4Hemixture films on the cold spot. Results and detailed analysis should be referred to Reference 8. From recombination data at low temperatures we can deduce the adsorption energy of HA on He film. Result for E.(T) was shown in Figure 3. The in the high temperature region is constant ( E ~=0.51 K). This value was consistent with the value for 1%
102
3He 3He-4Hemixture in Reference 13. But below 70 mK, was deviated to the lower value. This deviation may be attributed to the 2nd excited bound surface state of 3He on 4He but firther study is needed. As for the direct detection of 2-D HJ, We did not yet succeed in separating 2-D Hd signal from the bulk signal because of the spectrum broadening due to the magnet inhomogenities. We need a smaller size of mirror cavity for a better homogeneity. We are also improving the coupling of semi-confocal mirror cavity to improve the sensitivity.
REFERENCES 1 . For example, I. F. Silvera and J. T. M. Walraven, in Prog. in Low Temp. Phys. ed. by D. Brewer (North Holland, Amsterdam), Vol. X, p.139 (1986). 2. D. G. Fried, T. C. Killian, L. Willmann, D. Landhuis, S. C. Moss, D. Kleppner and T. J. Greytak, Phys. Rev. Lett., 81 (1998) 3811. 3. A. I. Safonov, S. A. Vasilyev, I. S. Yasnikov, I. I. Lukashevich and S. Jaakkola, J. Low Temp. Phys., 113 (1998) 201; A. I. Safonov, S. A. Vasilyev, I. S. Yasnikov, I. I. Lukashevich and S. Jaakkola, Phys. Rev. Lett., 81 (1998) 4545. 4. A. Fukuda, M. Yamane, A. Matsubara, T. Arai, J. S. Korhonen, J. T. M. Walraven and T. Mizusaki, Czechoslovak J. Phys., 46(S1) (1996) 541. 5. A. Matsubara, T. Arai, S. Hotta, J. S. Korhonen, T. Suzuki, A. Masaike, J. T. M. Walraven, T. Mizusaki and A. Hirai, in Bose-Einstein Condensation ed. by A. Griffin, D. W. Snoke and S.Stringari, (Cambridge University Press, N.Y.), p. 428 (1994). 6 . A. Matsubara, T. Arai, S. Hotta, J. S. Korhonen, T. Suzuki, A. Masaike, J. T. M. Walraven, T. Mizusaki and A. Hirai, Physica, B194-196 (1994) 899. 7. A. Fukuda, H. Takenaka and T. Mizusaki, J. Low Temp. Phys., 121 (2000) 737. 8. A. Fukuda, H. Takenaka, T. Ohmi and T. Mizusaki, to be published in J. Low Temp. Phys. (Proceedings of QFS2001). 9. M. W. Reynolds, I. D. Setija and G. V. Shlyapnikov, Phys. Rev. B 46, (1992) 575. 10. W. F. Saam, Phys. Rev. B 12, (1975) 163. 11. G. Baym and C. Pethick, in The Physics of Liquid and Solid Helium PART ed. by K. H. Bennemann and J. B. Ketterson, (Wiley, N.Y., 1978) chapter 2. 12. See Fig. 5, D. 0. Edwards and W. F. Saam in Prog. in Low Temp. Phys. ed. by D. F. Brewer, Vol. 7 (1978) chapter 4. 13. A. I. Safonov, S. A. Vasilyev, A. A. Kharitonov, S. T. Boldarev, I. I. Lukaschevich and S. Jaakkola, Phys. Rev. Lett., 86 (2001) 3356.
Section 2 Materials Sciences
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EPR in the 21' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
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Electron spin resonance studies of molecular photoionization in Cr-AlMCM-4 1 mesoporous oxide materials Sunsanee Sinlapadech and Larry Kevan Department of Chemistry, University of Houston, Houston, Texas 77204-5003, U.S.A. Photoionization of 10-methylphenothiazine (PC1); (TMB) and pyrene (Py) impregnated into mesoporous AlMCM-41 ion-exchanged with Cr(III), as an electron acceptor, to give Cr-AIMCM-41 was investigated. Cation radicals (PC,", TMBI", Pyl") are produced by 320 nm light at room temperature and characterized by electron spin resonance (ESR) and UV-VIS diffuse reflectance spectroscopy. The chromium ion concentration was varied from Si/Cr = 52 to 121. Cr-AlMCM-41 with the intermediate concentration of Si/Cr = 80 exhibits the greatest electron acceptor ability and shows the highest photoionization efficiency. The photoionization efficiency also depends on the type of photoionizable molecule impregnated into mesoporous Cr-AlMCM-4 1 with PCI being the most efficiently photoionized. The calcination temperature used before impregnation controls the oxidation state of chromium ions to Cr3+ or Cr5+ which also affects the photoionization efficiency. Cr5+ gives about four times higher photoionization efficiency than Cr3+ . Cr-AIMCM-41 is shown to be a promising heterogeneous host for the efficient formation of photoinduced cation radicals to achieve long-lived charge separation in solid state systems.
1. INTRODUCTION Photoinduced electron transfer reactions have been extensively studied within the last decade to develop systems for long-lived charge separation which may be useful for light energy storage. Photosynthesis is a known natural example of photoinduced electron transfer which converts light energy into usable chemical energy. Light absorption by a chemical system may cause electron transfer from a donor species to an acceptor species, generating a pair of high energy intermediates which usually are paramagnetic radical ions.[ 1,2] This charge separation reaction competes with back electron transfer which must be minimized to maximize the net production of photoproducts. It has been found that radical lifetimes in heterogeneous systems such as micelles, vesicles, silica gel or zeolites are longer than in homogeneous systems. Thus, heterogeneous hosts provide an effective environment to minimize back electron transfer and increase net photoyields. Recently, MCM-4 1 and AIMCM-4 1 mesoporous oxide materials have been explored for scientific and industrial applications. MCM-41 or modified MCM-41 materials are able
106
to catalyze several chemical reactions involving large reactants. Also, they can be used for selective adsorption of large molecule gases and liquids or heavy metal ions. Prior research has also shown that MCM-41 and AlMCM-41 materials are effective hosts for the net photoionization of incorporated molecules. The photoionization efficiency of incorporated molecules in silica and aluminosilica mesoporous materials can be enhanced by also incorporating reducible transition metal ions which act as stable electron acceptors.[3-91 Transition metal ions such as titanium (Ti4+)[3,4], copper (Cu2+)[7], manganese (Mn2+)[8] or vanadium (V5')[9] have been incorporated into either framework or extraframework (ion-exchange) sites of MCM-4 1. Previous studies showed that titanium ion (Ti4+) incorporated into framework sites showed high photoionization efficiency for incorporated photoionizable molecules. Nickel (Ni2+ ), copper (Cu2') and iron (Fe3') ions were also successfully ion-exchanged into AlMCM-4 1 and enhanced the photoionization of incorporated molecules relative to AlMCM-41 without such ion-exchange.[ 101 In this work, Cr3+ incorporated into AlMCM-41 by ion-exchange to form Cr-AlMCM-4 1 is used as an electron acceptor to enhance photoinduced electron transfer from three different incorporated photoionizable molecules, namely 10-methylphenothaizine (PC,), (TMB) and pyrene (Py). The structures, pore sizes and surface areas of AlMCM-41 with different amounts of A1 and Cr were characterized by powder x-ray diffraction and N2 adsorption. Cr-AlMCM-41 materials were treated at two different temperatures (100 OC and 550 OC) to vary the oxidation state of chromium. The chromium ion concentration was also varied. The experimental results show that the photoyields can be controlled by the type of photoionizable molecule, the chromium ion content and the oxidation state of the chromium ions.
2. EXPERIMENTAL SECTION Synthesis of AIMCM-41. Commercial trimethylammonium hydroxide (Aldrich), sodium silicate solution (27 wt % Si02 ; Aldrich), cetyltrimethyl ammonium bromide (Aldrich), fumed silica (Aldrich) and aluminum nitrate (Aldrich) were used as received for the AlMCM-4 1 synthesis. MCM-41 and AlMCM-4 1 materials with different A1 contents (Si/Al = 20, 40 and 80) were prepared hydrothermally using cetyltrimethyl ammonium bromide as the organic template. For MCM-41 synthesis, 10 g trimethylammonium hydroxide was mixed with 2.95 g sodium silicate solution and 15 g of distilled water. Cetyltrimethyl ammonium bromide (4.3 g) was dissolved in 43 g distilled water and slowly added to the mixture. To this mixture, fumed silica 2.26 was added and stirred for 1 h. The pH of the mixture was adjusted to be 11.5 using 2M H2SO4. For AlMCM-41 synthesis, 10 g trimethylammonium hydroxide was mixed with 2.95 g sodium silicate solution and 10 g of distilled water. Cetyltrimethyl ammonium bromide (4.3 g) was dissolved in 43 g distilled water and slowly added to the mixture. To this mixture, fumed silica 2.26 was added and
107
stirred for 1 h. Aluminum nitrate (0.96, 0.48 or 0.24 g) was dissolved in 15 g water and added to the mixture. The pH of the mixture was adjusted to be 11.5 using 2 M H2S04. All these samples were heated in teflon bottles for 2 days at about 100 OC. After crystallization, the solid product was filtered at room temperature. The solid product was washed with distilled water and dehydrated at 80 OC in air for about 3 h. The solid powder as calcined in static air at 550 OC for 12 h to remove water and organic templates. Characterization. X-ray powder diffraction patterns of AlMCM-4 1 were obtained with a Philips PW 1840 diffractometer using CuKa radiation of wavelength 1.541 A over the range 1.5' < 2q < 1.5'. The elemental compositions of AlMCM-41 and Cr-AlMCM-41 were determined with a Kevex 770 x-ray fluorescence spectrometer. A titanium target with an accelerating voltage of 25 kV and a current of 2.5 mA was used for the measurements. The concentrations of the elements were determined by calibration with standards of known concentrations. Nitrogen adsorption isotherms were measured at 77 K using a Micromeritics Gemini 2375 gas adsorption analyzer. The volume of adsorbed N2 was normalized to standard pressure and temperature. Prior to the experiments, samples were dehydrated at 280 OC for 3 h. The specific BET surface area was determined from the linear part of the BET equation. The Barrett-Joyner-Halenda (BJH) method[ 1 11 was also used to determine the pore sizes of the AlMCM-41 samples. The electron spin resonance (ESR) spectra were recorded at room temperature at Xband frequency using a Bruker ESP 300 ESR spectrometer with 100 k€Iz magnetic field modulation and microwave power low enough to avoid saturation and distortion of the +. +. +. spectrum. The photoproduced PC1 , TMB and Py radical yields were determined by double integration of the ESR spectra using the ESP 300 software. The diffuse reflectance (DR) UV-VIS spectra were recorded before and after different times of 320 nm photoirradiation at room temperature using a Perkin-Elmer model 330 spectrophotometer with an integrating sphere accessory. Ion-exchanged AlMCM-41. Cr(NO,), * 9 ~ (Aldrich) ~ 0 was used as received for ionexchange. Calcined AlMCM-41 (SilAI = 20, 40 and 80 gel composition) was liquid state ion-exchanged with chromium ion aqueous solution (0.1 M). The mixture was stirred at room temperature for 24 h. Finally, the material was washed with distilled water and filtered. The chromium ion contents in Cr-AIMCM-41 were determined by x-ray fluorescense (XRF) analysis using a Kevex 770 spectrometer. P hot oio niza t ion samples. 1 0 -Met h y Ip heno t hiazine (Aldric h) , N ,N ,N ' ,N 'tetramethylbenzidine (Acros) and pyrene (Lancaster) were used as received. AIMCM-4 1 and Cr-AlMCM-41 were heated at two different temperatures. One set of samples was heated at 100 O C for 3 h to remove water. The second set was heated at 100 OC for 1 h followed by 550 OC for an additional 3 h to remove water and oxidize Cr3+ to Cr5+. The solids were impregnated with 10-methylphenothiazine (PCl), N,N,N',N'tetramethylbenzidine (TMB) and pyrene (Py) at room temperature by mixing AlMCM-41 or Cr-AlMCM-41 (0.1 g) with 1 ml of 0.01M of PC1, TMB or Py
108
in benzene for 1 h in the dark. The benzene was evaporated by flowing dry nitrogen gas over the samples for 30 min. Each sample was transferred into a Suprasil quartz tube (2 mm i.d. x 3 mm 0.d.) which was sealed at one end. The sample tube was evacuated below 5 mTorr for 2 h and was then flame-sealed for ESR measurements. For diffuse reflectance measurements the samples were filled into a cylindrical quartz sample cell (20 mm diameter by 1 mm path length). AlMCM-41 was used as the reference solid. All samples were handled in the dark to minimize exposure to light. Each powder was photoirradiated at room temperature with a Cermax 150 W xenon lamp (ILC-LX150 F). The light was passed through a 10 cm water filter and a Corning #7-5 1 glass filter with 90 % transparency at 240 and 400 nm and a maximum at 320 nm. The samples were rotated during photoirridiation for uniform exposure to the light. Electron spin resonance was used to detect the photoproduced cation radicals.
+.
The cation radical yield (PC1 , TMB", P);' produced from the reaction was determined by ESR and by diffuse reflectance UV-VIS spectroscopy.
3. RESULTS XRD and N2 Adsorption. The XRD patterns of calcined siliceous MCM-41, AlMCM-41 with different Si/Al ratios and Cr-AlMCM-4 1 are consistent with previous reports. The well-defined reflections at 100, 110, 200 and 210 indicate a hexagonal structure for these materials. The adsorption isotherms exhibit a large increase in the P/Po range from 0.2-0.3 characteristic of capillary condensation in MCM-4 1 mesopore materials. Increasing the A1 content in AlMCM-41 results in a small decrease of the surface area. However, the pore sizes for different %/A1 ratios in AlMCM-41 are similar. This confirms that the AlMCM-41 materials are still mesoporous. After ion-exchanging AlMCM-41 (Si/Al = 47) with Cr3+, all surface areas are significantly reduced which seems to indicate some structural degradation. Comparison between the gel composition Si/Al ratio and the elemental ratio from XRF from the AlMCM-41 product are similar (20 gel versus 22 product, 40 gel versus 47 product and 80 gel versus 92 product). UV-VIS and ESR of Cr-AIMCM-41. UV-VIS spectra of Cr-AlMCM-41 (Si/Al = 47)(Si/Cr= 80) heated at 100 OC for 3 h show two broad peaks at 3+
440 nm and 610 nm characteristic of Cr . These peaks can be assigned to d-d 4 4 4 transitions described as A2g -+ 4T2g and Azg .+ Tlg.[12.13] When the sample is heated at 550 OC, C?+ is oxidized to Cr and the UV-VIS spectrum shows bands at 275 nm and 350nm, a shoulder around 440 nm and a very weak band around 610 nm. The bands at 275 nm and 350 nm are normally assigned to 0 -+ Cr6+ charge transfer absorption bands.[ 121 However, the d-d transition of Cr5+ occurs in the same spectral region.[ 141 ESR results show that Cr5+ is present in the sample after
109 5+
calcination. Thus, we assign the optical bands at 275 nm and 350 nm to Cr . The two bands at 440 and 610 nm imply that there is some unoxidized Cr3+ even after heating at 550 OC for 3 h. ESR spectra of Cr-AIMCM-41 (Si/Al = 47) (SriCr = 80) heated in air at 100 OC for 3 h show a broad line of Cr3+ at g = 1.985. After heating at 550 OC, the signal of Cr3+ disappears and a sharp line of Cr5+ appears at g = 1.970. These results are similar to those of a previous study on CrMCM-41.[12] Photoionization of electron donor organic molecules in MCM-41, AlMCM41 and Cr-AlMCM-41. PC1, TMB, and Py can be photoionized to produce cation radicals. The sizes of PCI, TMB and Py are about 6.5 8, 10 A, 5 8, x 13 8, and 6.9 10 8, respectively. This means that the 30 8, or larger channel openings of mesoporous MCM-41 materials should be big enough to incorporate these electron donor species. Incorporation of 10-methylphenothiazine (PC,) into AlMCM-41 and Cr-AlMCM-41 generates a thermally produced (dark reaction) ESR +. signal of methylphenothiazine cation radicals (PC1 ). The dark reaction for CrAlMCM-41 is much larger than for AlMCM-41. All samples show a photoyield intensity increase after irradiating by 320 nm light at room temperature for 60 min. The photoyield intensities are lower for MCM-41IPC1 and AlMCM41/PC, at different %/A1 ratios and are higher for Cr-AIMCM-41(Si/Al= 47)PCl at different Si/Cr ratios. The Cr materials show a larger dark reaction and larger photoyield intensity increases. This implies that Cr-AlMCM-4 1 provides a better system for charge separation than MCM-41 and AIMCM-41 although it is definitely desirable to minimize the dark reaction. After being irradiated with 320 nm light +.
at room temperature for 60 min, the photoyield intensity of PCl in MCM41/Pc1 is unchanged but AlMCM-41/PC1 and Cr-AlMCM-41/PC1 show increases. For AlMCM-41, the net photoyield intensities increase by 4.7, 7.5 and 10.1 units for Si/Al = 22, 47 and 92 respectively. The greater the aluminum content, the less is the net photoyield increase. This is not true for the Cr-AIMCM-41 samples. The net photoyield intensities increase by 8.5, 20.4 and 8.9 units for Si/Cr = 52, S X r = 80 and Si/Cr = 121 respectively. There is an optimum chromium concentration for the photoyield increase in the Cr-AlMCM-41/PC1 samples. The yields of PCl" in MCM-41/PCl, AlMCM-41/PC1 and Cr-AlMCM-41/PC1 slowly decrease during the first 24 h after photoirradiation and then are relatively stable for several days. After 24 h after photoirradiation the photoyield intensity decreases 23 YO for Cr-AlMCM-41/Py, 33 YO for Cr-AlMCM-4VTMB and only 7 % for Cr-AIMCM-4 l/PCI. Diffuse reflectance UV-VIS spectra of Cr-AlMCM-41(Si/Al = 47)(Si/Cr = 80)/PC1 +. show a 515 nm band characteristic of PC1 cation radicals.[4,5] This also supports +. that some PC1 is ionized to PCI thermally during sample preparation. When
110
Cr-AIMCM-41IPCl is irradiated by 320 nm light for 60 rnin the PCl" photoyield significantly increases while the absorption of PCI decreases and also the two absorptionpeaks at 275 nm and 350 nm assigned to Cr5+decrease. This shows that there is electron transfer from PC1 to Cr5+ during photoirradiation. Further study was focused on the effect of the chromium ion amount in J
Cr-AlMCM-41 on the photoyield. The photoyield of PC1 " increases with increasing irradiation time. The maximum net photoyield increase was obtained for Si/Cr = 80. At low Cr content (SiiCr = 121), the photoyield seems rather constant after 10 min photoirradiation at room temperature. For higher Cr content (Si/Cr = 52), the photoyield increases after 10 min photoirradiation and reaches a plateau after about 30 min. However, the net photoyield intensity increase after 10 rnin photoirradiation for Si/Cr = 80 is larger than that for SiKr = 52. After 60 rnin irradiation, the photoyield intensities of PC1 " in Cr-AlMCM-4UPCl increased 23 % for Si/Cr = 121, 36 % for Si/Cr = 80 and 20 % for Si/Cr = 5 2 .
4. DISCUSSION
XRD and adsorption experiments indicate that the cage sizes of AlMCM-41 are large enough to incorporate 10-methylphenothiazine (PC I), (TMB) and pyrene (Py). ESR and diffuse reflectance (DR) UV-VIS spectra of some 10-methylphenothiazine cation radicals (g = 2.006 for ESR or 515 nm for DR) were observed after sample preparation and before photoirradiation. This indicates that the impregnation procedure for PC1 into AIMCM-41 thermally oxidizes some of
the PC1 into PCl+'.
+.
This is also true of TMB and Py since ESR signals of
TMB" and Py at g=2.006 are likewise observed after sample preparation. The effect of heating Cr-AlMCM-41 before impregnation with PC1, TMB or Py on the photoionization efficiency is considerable. Heating to 550 OC before impregnation causes the oxidation of Cr3+ to Cr5+ which is a better electron acceptor according to their reduction potentials. This fact is consistent with the ESR results where the net photoyield of PC1 " in Cr-AIMCM-41/PC1 heated at 550 OC is about four times greater than when heated at 100 OC where Cr ion still exists as Cr3+. The photoionization efficiency clearly depends on the type of incorporated photoionizable molecule. Among the three different organic molecules studied, PC 1 is the best electron donor with the highest net photoyield intensity and also the longest radical lifetime. The high photoionization efficiency of PC1 is not explained by the ionization potentials determined by photoelectron spectroscopy. +. It might be related to greater radical stability for PC1 compared to TMB" and
111
Py'.. It is found photoirradiation.
that
TMB
+
decays several times
faster
than
PC1
+.
after
The photoyield of PC1" increases in the order MCM-41 < AlMCM-41 < Cr-AIMCM-4 1 indicating that the chromium ion in Cr-AlMCM-4 1 enhances the photoionization reaction. We previously found that the concentration of Ni2+ ion-exchanged into AlMCM-4 1 enhanced the photoyield of incorporated photoionizable molecules. 10 The photoyield in Cr-AlMCM-41/PCl initially increases with the amount of Cr3+ ion-exchanged into AlMCM-4 1. However, for Si/Cr = 52 the photoyield decreases. This may be explained by some structural degradation at higher Cr content and/or by the formation of secondary radicals. Therefore, the amount of Cr ions ion-exchanged into extraframework sites of AlMCM-41 has an optimal value to achieve maximum charge separation. One can also conclude that Cr5+ in Cr-AlMCM-41 enhances charge separation by accepting an electron from the electron donor species. The UV-VIS spectra support that electron transfer occurs between PC1 and Cr5+ . Incorporation of A1 into the MCM-41 framework creates Lewis and Bronsted acid sites. The more Al in AIMCM-41/PC1, the less the net photoyield. So the photoyield does not correlate with the formation of acid sites. However, for higher A1 content in AlMCM-41 the surface area is reduced as shown by N2 adsorption. So the photoyield seems to decrease with increasing A1 content due to less PC1 being adsorbed because of smaller surface area.
5. CONCLUSIONS ESR and UV-VIS spectroscopies were used to study photoionization of 10-methylphenothiazine (PC,), (TMB) and pyrene (Py) in Cr-AlMCM-41 mesoporous materials. It is found that PC, incorporated into Cr-AlMCM-41 is the best electron donor among these three photoionizable molecules. Formation of cation radicals within modified Cr-AlMCM-41 materials is due to electron transfer between the electron donor molecules and chromium ions with Cr5+ being a better electron acceptor than Cr3+. It is verified that the photoyield intensity depends on the amount of chromium ions ion-exchanged into the mesoporous AlMCM-4 1 materials, but an intermediate concentration is optimal. The temperature before impregnation controls the chromium ion valence state and the photoyield. Also the photoyield depends of the type of photoionizable molecule. ACKNOWLEDGMENT. This research was support by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy and by the Texas Advanced Research Program.
112
REFERENCES 1. G. J. Kavarnos and N.J. Turro, Chem. Rev., 86 (1986) 401. 2. F. D. Seava, Top.Curr.Chem., 156 (1990) 60. 3. H. M. Sung-Suh, Z. Luan and L. Kevan, J. Phys. Chem. B, 101 (1997) 10455. 4. R. M. Krishna, A.M. Prakash and L. Kevan, J. Phys. Chem. B, 104 (2000) 1796. 5. V. Kurshev, A. M. Prakash, R. M. Krishna and L. Kevan, Microporous Mesoporous Mater., 34 (2000) 9. 6. T. R. Koodali, Z. Chang, R.M. Krishna, A.M. Prakash and L. Kevan, J. Phys.Chem. B, 104 (2000) 5579. 7. Z. Luan, J. Xu and L. Kevan, Nukleonika, 42 (1997) 493. 8. J. Xu, Z. Luan, T. Wasowicz and L. Kevan, Microporous Mesoporous Mater., 22 (1998) 179. 9. Z. Luan, J. Xu, H. He, J. Klinowski and L. Kevan, J. Phys. Chem., 100 (1996) 19595. 10. S. Sinlapadech, R. M. Krishna, Z. Luan and L. Kevan, J. Phys. Chem. B., 105 (2001) 4350. 11. E. P. Barrett, L. G. Joyner and P.O. Halenda, J. Am. Chem. SOC.,73 (1951) 373. 12. Z. Zhu, Z. Chang and L. Kevan, J.Phys.Chem.B., 103 (1999) 2680. 13. B. M. Weckhuysen, I. E. Wachs and R. A. Schoonheydt, Chem.Rev., 96 (1996) 3327. 14. C. D. Garner, J. Kendrick, P. Lambert, F. E. Mabbs and I. Hiller, Inorg. Chem. 15 (1976) 1287.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
113
ESR and ENDOR spectroscopy of solitons and polarons in conjugated polymes S. Kuroda
Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Nonlinear excitations such as solitons and polarons in conjugated polymers carry unpaired electrons. In this case ESR and electron-nuclear double-resonance (ENDOR) provide unique methods to determine their wave function through the observation of proton hyperfine couplings on the conjugated chain. This information has two significant meanings. Firstly, it provides the spatial extension of these self-localized excitation that is essential in proving their existence. Secondly, the degree of electron correlation can be directly determined from the detailed form of spin distribtuion associated with negative spin sites. In this paper, the case of solitons in polyacetylene, CH,, and polarons in an electroleminescent polymer, poly(parapheny1ene vinylene) (PPV) are discussed as typical examples. High-resolution ENDOR spectra, obtained using stretch-oriented samples yield the half extenson the excitations of 18 carbon atoms and 4 phenyl rings for CH, and PPV, respectively, that are well desicribed by the theories in the case of moderate Coulomb interactions. In addition, light-induced ESR technique is shown to be useful in obtaining site-selective information of spin distribution in the case of PPV derivatives, as well as the excitation spectra of polarons.
1. INTRODUCTION Since the discovery of metallic conductivity upon doping of polyacetylene films, many conjugated polymers have been synthesized to develop conducting polymers [ 1,2]. Nobel Prize in Chemistry 2000 has been awarded to three scholars, Hideki Shirakawa, Alan Heeger and Alan MacDiarmid. In addition to metallic conductivity, another important property of conjugated polymers has been added in 1990, when intense electroluminescence has been discovered in a polymer, poly(parapheny1ene vinylene), PPV and its derivatives [3]. Now the displays using light-emitting polymers are approaching to the threshold of commercial application [4]. These conjugated polymers, at the same time, attract much attention as onedimensional electronic systems that can generate nonlinear excitations such as solitons, polarons and bipolarons. These excitations give rise to characteristic absorptions in optical, magnetic and other spectroscopic measurements [2]. Among these spectroscopic methods, ESR and electron-nuclear double-resonance (ENDOR) spectroscopies of protons on the conjugated chain can directly measure the spin density distribution of paramagnetic excitations through the study of hyperfine coupling [5,6]. In the case of solitons in polyacetylene, ENDOR studies of oriented polymers have been successful in revealing the detailed spin distribution in the case of finite electron
114
correlation[7-10]. The obtained spin distribution has the extension of about 18 carbon atoms in the full width at half maximum and is associated with negative spin sites, arising from electron correlation effect. Prior to the ENDOR analysis, n-electron nature of the soliton has been confirmed from the studies of the anisotropy of ESR and ENDOR spectra of stretchoriented samples, by noticing that K-electron shows characteristic anisotropy of g value and hyperfine coupling in solids [5,11]. The results have justified the use of the theory of electron in subsequent ENDOR analyses. The soliton can exist in the so-called degenerate polymer structure like trans-polyacetylene. On the other hand, most of other polymers have a non-degenerate structure that cannot support solitons and polarons become primary excitations. While the spin distribution of the soliton in polyacetylene has been established by the ENDOR studies by a few research groups [6], there had been no quantitative studies of polarons in conjugated polymers until recently. The first detailed studies of polarons have been carried out in a nondegenerate polymer, PPV [ 12,131. This polymer is important because of its electroluminescent properties and polarons attract much attention in relation to their possible roles in the mechanism of electroluminescence due to charge recombination. Using stretch-oriented samples, ENDOR of PPV has been successful in detecting the spatial extension of polarons trapped in the dark state of the polymer [12]. The observed spin distribution extending over several phenyl rings has been successfully reproduced by quantum-chemical modeling of the polaron wavefunction in the case of finite electron-electron (e-e) interactions [ 14,151. Furthermore, ESR spectrum simulation using the theoretical distribution has accurately reproduced the ESR spectra of polarons in PPV detected by light-induced ESR (LESR), which shows that LESR is an appropriate method to study spin distributions in conjugated polymers [ 12,13,16,17]. In this respect it would be important to study a range of different polymer structures where some of the protons are substituted by other groups, in order to confirm the detail of the spin distribution [18]. The substitution of protons bonded to a particular carbon sites with large spin densities should result in clear reduction of ESR linewidth due to proton hyperfine structures. Such site-selective information of the spin distribution would be important in understanding the processes of charge transport and recombination in devices such as polymer light-emitting diodes, since these processes depend not only on the local molecular configuration [ 191 but also on the details of the distribution of electron density of carriers on the chain. Recently, we have been successful in observing significant reduction of LESR linewidth in CN-PPV compared with that of MEH-PPV, where cyan0 groups are attached at the vinylic carbon sites in the former [18,20,21]. Chemical structures of these polymers are shown later in Fig. 4, together with that of PPV. The result has been well explained by the theoretical spin distribution of the polaron. Another feature was that the observed LESR action spectra of CN-PPV and MEH-PPV show a clear threshold, the behavior of which compares well with the photocurrent action spectra reported in PPV derivatives [22]. These results are discussed in this paper.
2. ENDOR STUDIES OF SOLITONS IN POLYACETYLENE
ESR and ENDOR spectra of unpaired n-electron interacting with nuclear spins in conjugated spins are well described by the following spin hamiltonian [6].
115
H
I
I \c/c\c/*\c/
I
I
H
I
C
C
I
H
\cg
I
H
Figure 1. Solitons in trans-polyacetylene. Definition of coordinate axes is also shown.
H = ~ $ 3g. . H - g,,,pNcIi.H + Z S . Ai .Ii I
I
The first and the second terms of the right-hand side are the electronic and nuclear Zeeman terms, respectively. The third term shows hyperfine interactions. g and Ai show g tensor of the electron spin and the hyperfine tensor of i-th nuclear spin, respectively. The chemical structure of trans-polyacetylene is shown in Figure 1. In the figure is also shown the definition of the coordinate axes used in this paper, where the and z axes are parallel to the C-H bond axis and the pn: orbital axis, respectively. A finite spin density p on a carbon pn orbital gives rise to a hyperfine coupling with the proton bonded to the carbon with the magnitude of pA. Here A is the hyperfine tensor of a Tc-electron due to a C-H proton. The tensor becomes diagonal in the above coordinate system and the principal components are given as,
Here A is the so-called McConnell s constant, having the magnitude of 56-84 MHz in the frequency unit. We adopt a typical value of A=70 MHz in the subsequent analysis. represents the relative magnitude of the anisotropic coupling. Among three tensor components, A,, has the largest absolute magnitude. ENDOR is the direct method to determine the hyperfine coupling constant. The hyperfine constant of proton bonded to a carbon with a spin density p is directly measured as the frequency shift of (1/2)pA,, with i =x.y.z, from the free proton NMR frequency of v, [6]. That is, a pair of lines centered at v, with the frequency separation of pAiiis observed. Thus, the spin distribution is directly observed as the distribution of the ENDOR frequency shift. The major advantage of ENDOR over ESR in determining hyperfine coupling is that for N nuclear spins of 1=1/2 interacting with an electron spin, only Npairs of lines are observed for ENDOR, while 2N hyperfine lines result in ESR spectra that make ESR analyses much more complicated. In the case of conjugated polymers, however, both the dark and light-induced spins are present, in particular, the latter cases are found in photogenerated polarons in electroluminescent polymers. Since light-induced ENDOR instrumentation has not been well developed so far, light-induced ESR spectra also provide equally important alternative method to study spin distribution. This is clearly demonstrated when hyperfine ESR linewidth is significantly modified due to substitution of protons by other substituent groups, as discussed in the next section for the ESR studies of some typical derivatives of electroluminescent polymers.
116
Figure 2 shows schematically the spin density distribution of the soliton in the case of finite electron correlation. The filled circles represent individual carbon sites. The full width at half maximum of the spin density of 18 carbon atoms is shown in the figure.
0.1 A
f
-0.1
Figure 2. Spin density distributon of the soliton in the case of a finite electron correlation
f
%
16
18
20
22
24
ENDOR FREQUENCY (MHz)
Figure 3. Detailed comparison of the frequency-derivative ENDOR spectra of a stretchoriented cis-rich sample of v, branch (v>v,). The external field is parallel and perpendicular to the stretch direction of the sample for the thick- and thin-line curvers, respectively. Dashed lines show the frequencies corresponding to hyperfine tensor components as indicated.
117
The soliton extension is determined by the balance between the localization energy and the condensation energy. As discussed below, this quantity is directly observable as the maximum ENDOR frequency. Another important piece of information contained in the soliton spin density is the effect of electron correlation. In the original Su-Schrieffer-Heeger model, where electron correlation is neglected, the unpaired electron density appears at every even carbon site from the center of the soliton. This reflcts the non-bonding character of the soliton whose energy is lacated at the center of the energy gap. On the other hand, in the case of finite electron correlation spin polarization due to correlation occurs and negative spins are induced at odd carbon sites. Its manitude directly provides the strength of the electron correlation energy of the system. This another important aspect in the study of the soliton spin density. Such occurrence of negative spin sites can be recognized as the spectral turning points resolved for the ENDOR spectrum of the stretch direction, as explained below. Figure 3 shows the anisotropy of the frequency-derivative ENDOR spectra at 12K in a 190% stretched cis-rich polyacetylene. In cis-rich sample, the short average length of trans segments in the cis matrix restricts the motion of the soliton at low temperatures, which is suitable for the measurement of spin distribution obtained from the static hyperfine couplings of the soliton. In Figure 3, thick line and thin lines show the spectra with the external magnetic field parallel and perpendicular to the stretch direction. The larger ENDOR shift observed for the stretch direction is the direct consequence of the x-electron nature of the soliton, since the y-axis component of the hyperfine tensor with the largest magnitude should be observed preferentially along the stretch direction. The most important quantity determined from the spectra is the maximum ENDOR frequency shift of the stretch direction, that is (1/2)p(O)A,,, determined from the wing of the spectra. ENDOR-induced ESR technique is useful to determine the maximum ENDOR frequency in a high accuracy and p(0) = 0.17 has been obtained. On the other hand, the spectral turning point arises from the signal of negative spin sites with smaller peak density of p( l), as shown in Figure 2. That is, the spectral turning point can be understood as the frequency at which the contribution from the ENDOR signal from the negative spin sites starts to overlap with those of the positive spin sites when one approaches from the maximum frequency side to the center of the ENDOR spectrum at v,. Its value has been obtained as p(l)= -0.075 and then p(*l)lp(O) = -0.44. The negative sign of p( 1) has been actually confirmed with the aid of electron-nuclear-nuclear triple (TRIPLE) resonance technique. More details are described elsewhere [9]. The functional form of spin distribution predicted by the soliton theory in the case of finite electron correlation is characterized by the sech form. Then the bound condition that the total sum of the spin densities to be unity yields the extension of the soliton to be 18 carbon atoms in the full width at half maximum. The obtained values of p(0) and p(1) and their ratio are quantitatively reproduced by the calculation by Yonemitsu et al. According to their results, on-site and near-neighbor Coulomb interaction of = 1.6t0, = 0.8t0 fit the observed results well by assuming a dimensionless electron-phonon interaction parameter of h = 0.1. Here shows the transfer integral whose magnitude is about 3eV in polyacetylene. Systems other than cis-rich Shirakawa polyacetylene have also been studied by ENDOR with the goal of determining the soliton spin distribution. Similar results as those in cis-rich samples, that is the similar spectra with resolved structures for the stretch-direction and similar spectral frequencies have been reported for stretch-oriented trans-polyacetylene prepared by the Durham route, where the unpaired electrons observed were conjectured to be
118
trapped solitons from nearly temperature independent ENDOR spectra. Discussion of this as well as other systems can be found elsewhere 3. ENDOR STUDIES OF POLARONS IN POLY(PARAPHENYLENE VINYLENE), PPV The chemical structure of PPV is shown at the bottom of Figure 4. Block letters from A to F show the eight carbon sites in one PPV monomer unit. The principal axes of proton hyperfine coupling of an unpaired n-electron are also shown for two proton sites of inequivalent bond orientations. In undoped PPV, the unpaired electrons exist in the dark condition with the spin concentration of about 1 spinl106PPV monomer unit. The resemblance between the dark and light-induced ESR spectra strongly suggests the dark spins are trapped polarons [12]. The origin of the dark spin may be presumably due to doping by remaining oxygen. ESR spectra in stretch-oriented samples show the anisotropy consistent with that of n-electrons, qualitatively similar as in the case of stretched polyacetylene films, by taking into account two inequivalent C-H bond orientations in PPV case. Proton ENDOR spectra of these trapped polarons have been successfully obtained at 4K in 10 times stretched polymer and the results have been well reproduced by theoretically calculated spin distribtuions in the case of finite electron-electron interactions [ 12-151.
CN-PPV
f-
MEH-PPV
q m a x . hfc
ppv
AXIS
B'c H
H
H
Figure 4. Chemical structures of CN-PPV, MEH-PPV and PPV. Principal axes of proton hyperfine coupling a n-electron are also shown for two inequivalent C-H bond orientations for PPV.
119
I
I
I
I
I
SiteA&D C
I
1i
Site E
F
r
I
I
I
II I
0.04
0.00
-0.021
I
2
I
4
I
I
I
10
I
12
I
14
Site / Unit Cell Figure 5. Spin density distribution of a polaron in a PPV chain calculated by the PPP model with 1 = 0.16, U = 2.5t and V = 1.3t. Vinyl sites ( sites E and F in Figure 4) are marked by the solid squares.
Figure 6. Comparison between the calculated ENDOR spectra using the polaron spin distribution shown in Figure 6 and the observed proton ENDOR spectra at 4K in the integrated form in a 10-times stretch-oriented undoped PPV film. The external field is parallel to the stretch direction for the curves in (a) and perpendicular to it in (b). Three distinct spectral turning points are marked as P1, P2 and P3 in the spectrum of the stretch direction.
Figure 5 shows the theoretically calculated spin density distribution of a polaron using PPP (Pariser-Parr-Pople) hamiltonian that explains well the observed ENDOR spectra [ 14,151. The half width of distribution amounts to about 4 phenyl rings. As for the parameter values of PPP hamiltonian used for the result in Figure 5 , they are on-site and nearest neighbor Coulomb interactions of and V=l.3t with being transfer integral, respectively, and more longrange interactions given by Ohno formula and dimensionless electron-lattice coupling
120
constant of h=O.16. These values are fairly close to those in the case of soliton in polyacetylene obtained from the ENDOR analyses and their magnitudes show that the systems in polyacetylene and PPV fall in the intermediate coupling regime. An important feature of the spin distribution shown in Figure 5 is that it consists of three distinct groups in magnitude of spin densities. The existence of these three different groups are recognized as three distinct spectral turning points, P1 to P3 in the observed spectrum of the stretch direction as shown in Figure 6. The above obtained half width of the polaron is nearly twice as large as that of the soliton in polyacetylene. This would be reasonably understood if we consider that polaron in conjugated polymers can be viewed as the bound soliton and anti-soliton pair, one neutral and the other charged resulting in the larger extension of the polaron than the soliton. In particular, the largest group containing the largest and second largest densities of 0.09 and 0.08, respectively, reside on the carbons of vinyl sites, as indicated by solid squares in Fig. 6. This quantity is inversely proportional to the extension of the polaron. It would be quite interesting if we can confirm that the largest densities reside on vinyl sites. In fact, such siteselective information could be obtained by our recent LESR studies of PPV derivatives and this is the subject described in the next section in some detail.
4. ESR SPECTRA OF PHOTOGENERATED POLARONS IN PPV DERIVATIVES The upper figures in Figure 4 show the chemical structures of poly(2-methoxy-5-(2’MEH-PPV, and a similar dialkoxy derivative, CNPPV, where cyan0 groups are attached to the carbons of vinylene sites. Figure 7(a) shows the comparison of the first-derivative light-induced ESR spectra of these two polymers. These light-induced ESR (LESR) signals were obtained by subtracting the dark ESR signals from the signals under illumination, although dark signals were almost negligible in these polymers. Time response studies show that LESR signals are transient in nature and ascribed to the photoinduced charge seperation and subsequent trapping of charges. Hence LESR were observable only at low temperatures and no signals were detected at temperatures higher than above 220K due to faster recombination time of charges [23]. LESR action spectra shown later further indicate that these LESR signals are due to photogenerated polarons. Asymmetrical line shape of the spectra in Figure 7(a), more pronounced in MEH-PPV results from the g and hypefine anisotropy of n-electron, as is also observed in PPV. Full-widths at half maximum of the integrated forms of these spectra in Figure 7(a) are 6.6 0.2 Gauss and 4.5 0.1 Gauss for PPV, MEH-PPV and CN-PPV, respectively, showing a clear reduction of the linewidth in CN-PPV. This fact alone shows that the hyperfine width of CN-PPV is significantly reduced compared with the case of MEH-PPV, consistent with the theoretical prediction that the spin density resides predominantly on the carbons at vinyl sites half of which protons are substituted by the CN group in CN-PPV (see Figure 4). The results are also consistent with previous optically-detected magnetic resonance (ODMR) studies of the polaron signal in PPV and CN-PPV, where a considerable reduction of ESR linewidth was observed for CN-PPV compared with PPV [24, 251. Since CN-PPV has not only substitution by CN groups but also by alkoxy groups at phenyl rings of PPV, the effect of CNsubstitution can be identified uniquely by the present comparison of CN-PPV and MEH-PPV, both of which have similar alkoxy subsituents at the same sites on the phenyl rings.
121
-10.0
-5.0
0.0
5.0
10.0
Magnetic Field (Gauss)
Figue 7. (a) First-derivative light-induced ESR spectra of cast films of MEH-PPV ( upper curve) and CN-PPV (lower curve) at 80K under 300 nm light illumination. (b) Calculated ESR spectra, in integrated from, of MEH-PPV ( solid line) and CN-PPV (dotted line), using the theoretical spin density distribution shown in Figure 5. The LESR linewidths in these polymers can be calculated by using the theoretical spin distribution of the polaron previously obtained for PPV by ENDOR. Hyperfine splittings due to the protons within the envelope of polaron spin distribution are calculated by assuming the theoretical spin distribution of the polaron previously calculated for PPV, shown in Figure 5, thus neglecting the effect of substituent groups on the spin density in the first approximation. The results are shown in Figure 7(b). The calculated width of 4.9 Gauss in CN-PPV is considerably reduced from the width of 6.1 Gauss in MEH-PPV, consistent with the observation. The relatively good agreement between the observed and calculated widths supports our picture of a spin distribution extending over approximately 4 phenyl rings. The experimental reduction in spectral width upon CN substitution from that of MEH-PPV (a factor of 1.4 - 1.5) is in fact rather larger than the calculated reduction (a factor of 1.24). A likely explanation for this is that the substitution of the CN group with its high electron affinity attracts more spin density away from the C-H sites [26] thus giving a larger reduction in spectral width than is predicted by assuming that the spin distribution is unchanged. The excitation spectrum of the LESR signal provides important information concerning the mechanism of charge separation. Figure 8 shows the variation of the normalized LESR intensity with the photon energy of incident light in (a) MEH-PPV and (b) CN-PPV films. The light intensity was adjusted to give the same photon flux at each wavelength. Thin solid
122 Wavelength (nm) 900
600 1
2
4
Photon Energy (ev)
Figure 8. Excitation spectrum of the LESR signal at 80K (closed circles) together with the optical absorption spectrum (thin solid line) for CN-PPV in the upper figure and MEH-PPV in the lower figure. Photocurrent action spectrum is also shown for MEH-PPV by a dotted line. line in each figure shows the optical absorption spectrum. It is seen that the LESR intensity becomes large at high excitation energies, as observed in PPV [13,16,17]. The distinct feature of the present case is that the action spectrum has a clear threshold at an energy of 2.72.8eV in CN-PPV and around 3eV in MEH-PPV, which is higher than the peak energy of the optical absorption of about 2.5 eV in both polymers. The threshold energy of the LESR action spectrum can be related to the energy above which weakly-bound electrons and holes are created which can be more readily dissociated into polarons. It is interesting to note that similar behavior with a distinct threshold around 3eV is reported in the action spectrum of photoconductivity in MEH-PPV, as indicated by dotted line in Figure 8(a) [22]. The observed resemblance between the action spectra of LESR and photoconductivity provides supporting evidence that the photoinduced spins are charged species, that is, polarons. Further analysis of the threshold cnergy may provide insight into the excitation energy and electronic structures of these polymers that are sensitively affected by the magnitude of e-e interactions [27].
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CONCLUDING REMARKS
As discussed in the present paper, ESR and ENDOR spectroscopy is usehl in revealing the spin distributions of solitons and polarons in conjugated polymers. Based on the anisotropic properties of unpaired n-electrons, the use of stretch-oriented polymers enables us to conduct ENDOR analysis of spin density distributions, associated with electron correlation effects, with the spectral resolution comparable to those of single crystalline systems. Furthermore, by employing the electroluminescent polymers with different substituent groups, site-selective information of spin distribution of photogeneratd polarons has been obtained. Light-induced ESR is also usefbl in revealing the excitation spectra of polarons. By considering the development of conjugated polymers for both basic science and applications, there would be more cases where ESR and ENDOR spectroscopy would play essentail roles in revealing the nature of nonlinear excitations in these polymers. ACKNOWLEDGMENT
This work was partially supported by NED0 International Joint Research Program 99MB1 and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The present author thanks H. Shirakawa, Y. Shimoi, S. Abe, K.Marumoto, H. Ito, N.C. Greenham, R.H. Friend for valuable discusssions. REFERENCES
1. H. Shirakawa, E.J. Louis, A.G. MacDiarmid, C.K. Chiang and A.J. Heeger, Chem. Commun., 1977 (1977) 578. 2. A.J. Heeger, S. Kivelson, J.R. Schrieffer, W.P. Su, Rev. Mod. Phys., 60 (1988) 781. 3. J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature, 347 (1990) 539. 4. R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradely, D.A. Dos Santos, J.L. Bredas, M. Loegdlund, W.R. Salaneck, Nature, 397 (1999) 121. 5. S. Kuroda and H. Shirakawa, Solid State Commun., 43 (1982) 591. 6. S. Kuroda, Int. J. Mod. Phys., B9 (1995) 221. 7. S. Kuroda, H. Bando and H. Shirakawa, Solid State Commun., 77 (1984) 893. 8. S. Kuroda and H. Shirakawa, Phys. Rev., B 35 (1987) 9380. 9. S. Kuroda and H. Shirakawa, J. Phys. SOC.Jpn., 61 (1992) 2930. 10. M. Mehring, A. Grupp, P. Hoefer and H. Kaess, Synth. Met. 28 (1989) D399. 11. S. Kuroda, M. Tokumoto, N. Kinoshita and H. Shirakawa, J. Phys. SOC.Jpn., 51 (1982) 693. 12. S. Kuroda, T. Ohnishi, T. Noguchi, Phys. Rev. Lett., 72 (1994) 286. 13. S. Kuroda, K. Murata, Y. Shimoi, S. Abe, T. Noguchi, T. Ohnishi, in ed. K. Kajimura and S. Kuroda, Springer
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Proceedings in Physics 81 (Springer, HeidelbergTokyo, 1996) 256. 14. Y. Shimoi, S. Abe, S. Kuroda, K. Murata, Solid State Commum., 95 (1995) 137. 15. S. Kuroda, Y. Shimoi, S. Abe, T. Ohnishi, T. Noguchi, J. Phys. SOC.Jpn., 67 (1998) 3936. 16. K. Murata, S. Kuroda, Y. Shimoi, S. Abe, T. Ohnishi, T. Noguchi, J. Phys. SOC.Jpn., 65 (1996) 3743. 17. K. Murata, Y. Shimoi, S. Abe, S. Kuroda, T. Ohnishi, T. Noguchi, Chem. Phys., 227 (1998) 191. 18. S. Kuroda, K. Marumoto, H. Ito, N.C. Greenhm, R.H. Friend, Y. Shimoi and S. Abe, Chem. Phys. Lett., 325 (2000) 183. 19. E.M. Conwell, J. Perlstein, S. Shiak, Phys. Rev., B 54 (1996) R2308. 20. S. Kuroda, K. Marumoto, N.C. Greenhm, R.H. Friend, Y. Shimoi and S. Abe, Synth. Metals, Synth. Met., 119 (2001) 655. 21. S. Kuroda, K. Marumoto, Y. Shimoi and S. Abe, Thin Solid Films, 393 (2001) 304 22. Koehler, D.A. Dos Santos, D. Beljonne, Z. Shuai, J.L. Br das, A.B. Holmes, A. Kraus, K. Muellen, R.H. Frined, Nature 376 (1998) 903. 23. S. Kuroda, K. Murata, T. Ohnishi, T. Noguchi, J. Phys. SOC.Jpn., 64 (1995) 1363. 24. N.C. Greenham, J. Shinar, J. Partee, P.A. Lane, 0. Amir, F. Lu, R.H. Friend, Phys. Rev., B53 (1996) 13528. 25. L.S. Swanson, J. Shinar, A. Brown, D.D.C. Bradley, R.H. Friend, P.L. Bum, A. Kraft, A.B. Holmes, Phys. Rev. B46, (1992) 15072. 26. D.A. dos Santos, D. Beljonne, J. Cornil, J.L. Bredas., Chem. Phys. 227, (1998) 1. 27. M. Chandross, S. Mazumdar, S. Jeglinski, X. Wei, Z. Vardeny, E.W. Kwock, T.M. Miller, Phys. Rev. B50, (1994) 14702.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
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EPR study of deoxygenated high-temperature superconductors and their constituents R. J. Singh, P. K. S h a m and Shakeel Khan Physics Department, Aligarh M u s h University Aligarh-202002 E-mail:
[email protected] EPR spectra of deoxygenated high temperature superconductors and their constituents have been investigated. These compouuds are (1) CuO (2) BaCuOz (3) CaCuOz (4) YZCUZOS (5) YBazCu30.1 (6) Bi-based 2201,2212,2223 (7) TI-based 2223 (8) Hg-based 1223 (9) LazCu04 and (10) La~-~Cu04 (M=Sr,Ba). One thing common to all these materials is the CuOz plaue and is considered to be the seat of superconductivity. This plane, on deoxygenation, gets broken into fragments of various sizes, the bigger ones do not give any signal because the original antifeiromaguetic order remains intact in them, the intermediate ones give signals but without any structure aud hence their identification becomes di%cult and in the smaller fragments the signals due to Cu-octamers, tetramers, diners and monomers have been identified. These species produce signals when they are magnetically isolated from the bulk. Same representative spectra have been reproduced, aualyzed and discussed. Cu-tetramer (CUO)4 is the most frequently observed species aud is incidentally the unit cell of the all - important CuOz plane. It has been discussed how the (CUO)~units can be the basis for the mechanism of high temperature superconductivity. 1.INTRODUCTION
High temperature superconductors are EPR silent [l-51. But on deoxygenation, they give spectra. The EPR study of the following deoxygenated high temperature superconductors and their constituents have been carried out [6-141. (1) CuO (2) BaCuO2 (3) CaCuO2 (4) Y&uzOs (5) YBa2Cu307 (6) Bi-based 2201,2212,2223 (7) TI-based 2223 (8) Hg-based 1223 (9) LazCu04 and (10) La~.~M,Cu04(M=Sr,Ba). It is believed that on deoxygenation, the Cu02 plane which is common to all these compounds and is considered to be the seat of superconductivity is broken into fiagments of various sizes. The big fragments do not give any spectra, probably because the oi-iginal antiferromagnetic ordeiiug seems to remains intact in them, the iuteimediate ones give signals but without any structure and hence their identification becomes dficult; in the smaller fragments Cu-octamers, tetramers, diners and monomers have been detected. It is expected that when these fragments become magnetically isolated from the bulk, they give signals. these fragments, the spins of all the Cu-ions involved get ferromaguetically coupled through super exchange interactiou via the intervenbg oxygen ions. There is also the ferromagnetic coupling of the nuclear of all the Cu-ions. The spectra in geueral are veiy complex aud not easily reproducible. Different samples of the same compound gave different spectra. Some of which were repetitive but some spectra of the samples of different compouuds were also similar. different samples of the same compound,
126
different combinations of the spectra of octamers, tetramers, dimers aud mouomers appeared. It is expected that in the violent process of deoxygenation which shatters the Cu02 plane, the pieces will not be identical all the time and at all the parts in the sample. The signal giving species may have different type of attachments with the additional oxygen ions or the crystal field around these species may also be different. These signal giving species are not ur&ordy distributed over the whole specimen but occur as islands, some small and some big. Each compound was studied several times by varying the deoxygenation process, preparation method, initial ingredients etc and a few huudred spectra were recorded of these compounds to collect good statistics. A few representative spectra are reproduced, aualyzed and discussed. Cu-tetramer or (CUO)~ unit has been fouud to be most ftequent. As it is the unit cell of the allimportant Cu02 plane, it should reflect the property of the bulk. Attempt has been made to explain the mechanism of high temperature superconductivity in terms of (CUO)~complexes. Problems, which remain to be tackled, are also mentioned. REFERENCES 1 2 3 4 5 6 7 8 9
10 11 12 13 14
F. Mehran, S.'E.Baiues, T.R. Mcguhe, W.J. Gallagher, R.L. Saandstorm, T.R. Dinger and D.A. Chance, Phys. Rev. B, 36 (1987) 740. R.N.Schwartz et.al; Bull Am.Phys.Soc., 33 (1988)807. F. Mehran P.W. Anderson, Sol. Stat. Comm., 71( 1990)29. T.G.Castner and M. S. Seehra, Phys. Rev. B 47( 1993)578. P.Simon, J.M.Basset, S.B.Oserof€, Z.Fisk, S.W.Cheong, A. Wattiaux and S. Schultz, Phys. Rev. B, 48(1993)4216. Alex Puuuoose, B.P.Mamya, M.Umar aud RJ.Siugh, Mod.Phys.Letts., B 6( 1992)1043. Alex Pumoose, B.P.Maurya, J.Mathew,M.Umar, M.LHaque and RJ.Singh, Sol.Stat. Comm., 88( 1993)195. R.J. Singh,A. se,J.Mathew,B.P.Maurya,M.Umar Phys.Rev.B,49( 1994)1346. RJ.Siugh, Mohd. Ikram, Alex Puuuoose, B.P.Maurya, Shakeel Khan, Phys.Letts., A 208( 1995)369. Alex Pumoose, and RJ.Siugh, Jnt.Jour.Mod.Phys., B 9( 19991123. Shakeel Khan, h t i Shgh aud RJ.Singh, Sol. Stat.Com., 106(1998)621. Shakeel Khan, Mohd.kam, &ti Shgh and R.J.Singh,PhysicaC, 281( 1997)143. Shakeel Khan, h t i Shgh and R.J.Singh,PhysicaC, 325( 1999)165. R.J. Siugh, P.K. Shaima, A.Sin& and Shakeel Khan, PhysicaC, 356(2001)285.
EPR in the 2Ist Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
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Magnetic phase separation in Lal,CaxMn03 near half-doped composition observed by EMR Alexander I. Shames ', Andrey Yu. Yakubovsky ', Stanislav V. Gudenko a
Department of Physics, Ben-Gurion University of the Negev, P.O.Box 653, 84 105, Be'er-Sheva, Israel
'Russian Research Centre "Kurchatov Institute", Moscow 123182, Russia
EMR measurements have been carried out in two La1,CaxMn03
compositions
(x = 0.5 and 0.6) on loose packed powdered ceramic samples within the temperature range 100 - 500 K. For x = 0.5 the low temperature magnetic phase is a charged ordered
antiferromagnetic insulator (CO AFMI) = = 170 K) originating from the ferromagnetic metal phase (FMM) = 215 K). In the x = 0.6 compound the same ground state = 150 K) originates from the charged ordered = 260 K) paramagnetic (PM) state. Within the PM state the double integrated intensity of EMR signal changes obeying the Arrhenius law. At critical temperature the long range magnetic ordering appears that causes significant changes in all of of EMR spectra parameters (DIN, resonance field, line width and shape). For x = 0.6 the only EMR line is observed. At = both line broadening and decrease in intensity occur. On crossing the EMR line position does not change, but at = the line starts to move towards high fields. For x = 0.5 EMR spectrum below consist of three overlapping lines attributed to PM, FMM and FMI phases. Its sharply drops on crossing which evidences the existence of a cluster glass state. Below the FMI line moves toward high fields. EMR data clearly points out the single phase nature of the x = 0.6 compound, where the high temperature PM phase transforms into CO PM phase at and then into CO AFI at On the other hand, the x = 0.5 sample shows an evident phase separation in the range of 2 5 where three magnetic phases (PM, FMM and FMI) coexist. 1. INTRODUCTION
Magnetic phase separation or coexistence of competing magnetic phases is a very general and intrinsic feature of manganites. In spite of the recent considerable progress in this field it is clear that more work should be devoted to the detailed study of this mixed-phase systems. A reliable identification of inhomogeneous magnetic states may be obtained only by the local techniques such as electron microscopy, neutron diffraction and nuclear magnetic resonance (see a review paper [I] and references therein). One more local technique, electron
123
magnetic resonance (EMR), which combines ferromagnetic resonance (FMR) below and electron paramagnetic resonance (EPR) above represents a powerful tool for observation and study of minor magnetic phases in manganites. Magnetic phase diagram of Lal,CaxMn03 is well established [2]. However, subtle details of different coexisting electronic andor magnetic phases as well as their mutual interactions are still within the scope of rapt interest. In this paper we present results of EMR study of two Lal,CaxMn03 compositions (x = 0.5 and 0.6).
2. EXPERIMENTAL
EMR measurements were carried out using Bruker ESP-300 and EMX-220 X-band (v spectrometers, equipped with ER-4121VT nitrogen continuous-flow temperature control systems (accuracy 1 K). EPR spectra processing and parameters' calculations were done using Bruker WIN-EPR software. Two ceramic samples of Lal,CaxMn03 (x = 0.5, 0.60) were prepared by conventional technique described elsewhere [3, 41 . The metal content was checked by EDX and found to be homogeneous and stoichiometric within the method accuracy. In the former compound a low temperature magnetic phase is a charged ordered antiferromagnetic insulator (CO AFMI) = = 170 K) originating from the ferromagnetic metal phase (FMM) = 215 K) [3]. In the latter one the same AFM ground state = 150 K) originates from the charged ordered = 260 K) paramagnetic (PM) state [4].
- 9.4
*
3. RESULTS AND DISCUSSION
We measured the temperature dependencies of the line position, width and intensity within the range of 100 - 550 K. All the results were obtained for loose packed ceramic powders. Within the PM region > EPR spectrum for both samples showed a singlet Lorentzian line with g-value approaching g = 1.98 0.01 at high temperature limit, that is typical for all manganese perovskites. With decreasing temperature, the intensity of EPR To' . . ' ' I line increases and the line width AHppdecreases passing through a minimum at = l.lTc,co. Figure 1 shows the temperature dependence of IE 100 the EPR line width within the PM region and below, on crossing Solid lines represent the best fits to the data using the expression [5]: '
'
'
-
150 200 250
450 500 550
Temperature (K)
Figure 1. Temperature dependence of the line width. Solid lines represent the best fits according to Equation (1).
The following fitting parameters for the activation energies were found: Ea = 0.109 0.005 eV (x = 0.5) and Ea= 0.072 0.005 eV (x
129
0.6). Equation (1) was proposed in [5] for the description of the EPR line width temperature behavior within the framework of the of small polaron adiabatic hopping model. The hopping rate of the charge carriers limits the lifetime of the spin state favoring the condition of the bottleneck regime. A broadening of the EMR line arises due to the hopping of the eg electrons via the spin-orbital coupling. Figure 2 shows a temperature dependence of the double integrated intensity (DIN of EMR signal. For both compositions the DIN behavior in the PM state obeys the Arrhenius law (Figure 2, inset). Such a description is based on the CCA model of resonating centers [6]. The latter describes the progressive clustering of all Mn3+-Mn4+ions, mediated by the hopping of the electron. From the best linear fit the activation energies was found as = 0.117 0.002 eV (x = 0.5) and = 0.091 0.002 eV (x = 0.6). It should be noted that for x = 0.5 sample the Arrhenius law holds well from the highest temperature down just to the temperature = 215 K, which was determined by the distortion of the symmetric Lorentzian EMR 2.0 line shape. It is worthwhile to emphasize here a very reasonable agreement between the values 1.5 obtained from the best fits of two independent EMR line parameters (AHpp, see Figure 1 and DIN, see Figure 2, inset). The same (or even better) compliance was found for the "classical" FM La2/3Cal/3Mn03 composition = 265 K): = 0.140 0.002 eV (from AHpp)and = 0.143 0.001 eV (from DIN). These facts may promote a 150200250300350400450500550 hrther interest to development of an adequate Temperature (K) consistent model of the PM centers in manganites within the PM region. It is clearly seen that the Figure 2. Temperature dependence DIN values reach their maxima in the vicinity of of the DZNvalues, normalized to the the transition points (Figure 2) : Tmax= 205 K (x sample's weight. Inset shows the = and T~~~= 270 K (x = Taking into ArrheniusplotsforthePMregion. account that the DIN values plotted in Figure 2 Lines are the best linear fits. were normalized to the samples' weight, the maximal value of EMR susceptibility for the CO x = 0.6 sample is about 20 times smaller compared with the FM x = 0.5 sample. Below the first transition point the EMR signal intensities drop down till the lowest temperature available. Here it is worthwhile discussing an origin of the EMR signals observed within the different magnetically ordered regions. Above the only EMR active centers are exchange coupled Mn3+-Mn4+entities. Just the latter are responsible for all EMR signals of magnetically ordered phases observed within the range of < < During our study of a variety of manganite systems showing an AFM transition, we have never observed zeroing of the EMR signal intensity just below as it was reported for the AFM LaMnO3 compound [7]. On the other hand, in our case of the DC field < 1 T, the true AFM resonance in La-based manganites may be observed only at frequencies of the order of hundreds [S, 91. Thus, the EMR signals observed at < are definitely due to some minor FM phases. In general, the observation of these
=
130
signals does not evidence the presence of any additional phases in the AFM state at all. These signals may naturally appear due to partially non-compensated FM moments. The latter coincides with dc-susceptibility data for AFM systems. On decreasing temperature, both EMR signal intensity and dc-susceptibility go down zeroing far below (see, for example, [7]). In the view of these speculations, the drop of the values above in both compounds should have another explanation. Thus, for the CO x = 0.6 sample the decrease of observed at may be due to the local AFM coupling of PM entities originated from the charge ordering. On the other hand, the sharp drop of the value in the = 0.5 compound is observed far above = = 170 K and may not be explained by CO or AFM correlations. Such a behavior of the high frequency susceptibility is a characteristic feature of the cluster glass state and is usually observed for systems where entities, characterized by different types of magnetic ordering, coexist. Carefbl analysis of the EMR spectra recorded within magnetically ordered regions 2 allowed us drawing conclusions on the magnetic phase homogeneity for samples under study. .Z Below the second transition point, the AFM != phase in the = 0.6 compounds seem to be a homogeneous one. As it was mentioned above, m .the disappearing traces of the initial PM phases, observed here by the very sensitive EMR technique, should not point out any magnetic inhomogeneity. On the other hand, the FM signal, observed in the x = 0.5 sample within the AFM 5 region, remains to be at least of an order of 'zz magnitude stronger than the corresponding one in the = 0.6 sample (see Figure 2). Thus, the EMR data may evidence of a certain coexistence of FM and AFM phases below a Nee1 temperature, found for the half-doped composition by various w o 200 400 600 800 Magnetic Field (mT) techniques [lo, 111. The amount of that FM phase progressively decreases at temperature dropping Figure 3. (a) EMR absorption spectra off T N . The most interesting magnetic structure of Lao,5cao5Mno3 sample at various was found in the = 0.5 sample within the temperatures; (b) deconvolution (open intermediate temperature region, < < circles) ofthe experimental spectrum at Figure 3 clearly shows that the EMR absorption 195 K (solid line) by 2 Gaussians spectra below the point consist of at least two (open squares and triangles). components (see Figure 3b): one line remains at the resonance field of the initial PM line (Figure 4a) and a second line appears at the low field wing of the PM line and shifts towards low fields, reaching its lowest resonance field value at - 200 K (Figure 4b). The intensity of the latter progressively increases whereas the PM line diminishes becoming practically undetectable at = 175 K. For the correct determination of the second line let us apply for the magnetic phase diagram of Lal,CaxMn03 [2]. For a precise half-doped composition the magnetically ordered phase appearing just below
-
h
5
e-
131
originates from the PM insulating phase and was determined as FM metallic @MI@ phase. But the same diagram concedes the existence of the FM insulating (FMI) phase above Taking into account such an ambiguity as well as both the line shape and the resonance field temperature dependence (Figure 4a), one can attribute the EMR line under consideration to FMI phase. On the other hand, in the vicinity of 190 K an additional very broad EMR line centered at low (or even zero) field starts to be distinguishable. The resonance parameters of such an EMR line could not be reliably determined. Nevertheless, the spectra' deconvolution shows that the intensity of that line reaches its maximum at - 150 K and then decreases on approaching (see, for instance, Figure 3a, spectra at = 100 K and 120 K). The EMR lines of the latter type are usually observed in powdered magnetic metal samples, characterized by a strong uniaxial anisotropy. Following the analogy, we may relate the EMR line appearing at 190 K to FMM phase. The existence of two FM phases observed perfectly corresponds to the detailed magnetic phase diagram for near half-doped compositions, determined by neutron-powder-diffraction [ 101. For the x = 0.6 sample the only EMR line is observed within the whole temperature range. On crossing the decrease in the intensity (Figure 2) and the progressive change in the line shape occur whereas the line position does not change at all. The line shape change is revealed in a developing low field wing that results in an effective line broadening. At = the EMR line starts moving towards high fields (Figure 4b). Since this line reveals all aforementioned characteristic features of EMR FMI line, it may be also related to the FMI phase. It is worth noting that the amount of such a phase within the PM CO region does not exceed 10% of the main PM phase. One may attribute this EMR line to short-range FM correlations earlier observed around [2]. Below this minor phase (which does not still involved into CO and AFM ordering) belongs just to FMI that is observed, as it was mentioned, in disappearing trace amounts. The temperature dependence of the EMR line positions - resonance fields, Hr, show that the FMI signal in the = 0.5 sample shifts towards low field till - 200 K and then starts moving in opposite direction. It may evidence that the magnetic anisotropy parameters of FMI entities undergo significant changes in the vicinity of this temperature (Figure 4b). It is interesting, that the position of the PM line (Figure 4a), which depends on the internal magnetic field induced at a PM center embedded into magnetically ordered phases, does not follow the same behavior. It reaches its minimum in the vicinity of and then comes back
-
132
-
to its initial position at It may be supposed that an internal field affecting PM center does not originate from a single FM phase. Interplay between internal fields, induced by different FM phases (FMM and FMI), may be responsible for such a behavior, which could be an additional indirect evidence of phases' coexistence. Below the only FMI line observable in both compounds behaves in the same manner: it starts shifting toward high fields (Figure 4b). It may be supposed, that the main AFM phase strongly affects magnetic state (or even local geometry) of minute FMI regions that is manifested in the change of the magnetic anisotropy parameters. In conclusion, the EMR technique allowed revealing evident magnetic phase separation in the L~,5Ca,,5Mn03compound within the intermediate temperature region between transition points < < Two magnetically ordered phases, and FMI, coexist here with the PM phase. Below a presence of a non-traced amount of FM phases is still observed. This fact is in a good agreement with the position of the half-doped compound at the crossover boundary line on the magnetic phase diagram. On the contrary, the La0,4Ca&hO3 compound was found to be practically single phased. However, the sensitivity of the EMR techniques allows reliable observation of FM correlations at temperatures below the CO transition. REFERENCES 1. E. Dagotto, T. Hotta, A. Moreo. Phys. Reports, 344 (2001) 1. 2. S-W. Cheong, H.Y. Hwang. In: Y. Tokura (Ed.), Contribution to Colossal Magnetoresistance Oxides (1999), Monographs in Condensed Matter Science. Gordon & Breach, London; C. N. R. Rao, J. Phys. Chem, B 104 (2000) 5877. 3. S. Mori, C.H. Chen and S.-W. Cheong, Phys. Rev. Lett., 81 (1998) 3972. 4. B. Raveau, A. Maignan, C. Martin, R. Mahendiran, and M. Hervieu, J. Solid State Chem., 151 (2000) 330. 5. A. Shengelaya, G.-M. Zhao, H. Keller, and K.A. Muller, Phys. Rev. B, 61 (2000) 5888. 6. M. Causa, M. Tovar, A. Caneiro, F. Prado, G. Ibanez, C. Ramos, A. Butera, B. Alascio, X. Obradors, S. Pinol, F. Rivadulla, C. Vazquez-Vazquez, M. Lopez-Quintela, Rivas, Y. Tokura, and S. Oseroff, Phys. Rev., B58 (1998) 3233. 7. M.T. Causa, G. Alejandro, R. Zysler, F. Prado, A Caniero and M. Tovar, 196-197 (1999) 506. 8. S. Mitsudo, K. Hirano, H. Nojiri, M. Motokawa, K. Hirota, A. Nishizawa, N. Kaneko and Y. Endoh, JMMM, 177 - 181 (1998) 877. 9. L. E. Gonchar, A. E. Nikiforov, and S. E. Popov, JETP, 91 (2000) 1221. 10. Q. Huang, J. W. Lynn, R. W. Erwin, A. Santoro, D. C. Dender, V. N. Smolyaninova, K. Ghosh and R. L. Green, Phys. Rev. B, 61, (2000) 8895 11. H. Wakai, J. Phys.: Condens. Matter, 13 (2001) 1627
EPR in the 21' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
133
Temperature rises by microwave absorptions in superconducting materials and liquid nitrogen bubbling T. Lia, A. Hashizumea, KI. Itoha, H. Kohmotoa, S. Iwasakia, M. Yamasakia, K.Yamaguchia, T. Endoa and M. Schahabuddinb aFaculty of Engineering, Mie Univeristy, Tsu, Mie 5 14-8507, Japan bDepartment of Physics, University of Jamia Millia Islamia, Jamia Nagar, New Delhi-110 025, India Microwave absorption was measured on various forms of high temperature superconductors in liquid nitrogen, and resulting sample temperature rises were investigated. A bulk Bi2212 shows characteristic big signal fluctuations caused by temperature drop induced by nitrogen bubbling. The signal decay time reflecting the temperature recovery is shortened by increasing microwave power due to rapid heating. The bubbling is triggered by ESR microwave absorption in the substrate of YBCO thin film. 1. INTRODUCTION
High temperature superconducting (HTS) wires for electricity transport will be used in liquid nitrogen. Sometimes these wires are heated by the low frequency transport current, resulting in the liquid nitrogen bubbling [ 1-31. In such practical applications, these problems of the bubbling cannot be neglected. On the other hand, high frequency properties of HTS materials have obtained attention due to two reasons. One is pure physics interests such as Josephson plasma in HTS materials [4]. In some cases, the Josephson plasma resonant microwave absorption is detected by a bolometric device utilizing a temperature rise in HTS due to the microwave absorption 61. The other is HTS microwave filter devices used at base stations of mobile communication system [7, 81. In such an application, the temperature rise of HTS microwave devices is a very serious problem for high power microwave use. Moreover, there is strong expectation that HTS can be used as bolometers for far-infrared radiations [9, 101.
134
Therefore, the temperature rises in HTS materials are important problems to be solved in various fields. We have been facing to these kinds of temperature rises in various HTS materials during the microwave absorption experiments. First one is anomalous signal fluctuations induced by ambient liquid nitrogen bubbling [ll], and second one is characteristic evolutions of the microwave absorption spectrum with varying the microwave power on Bi2Sr2CaCu20x single crystals [12]. In this paper, we mainly report the former subject.
2. EXPERIMENTAL PROCEDURES Several forms of HTS samples were used in this experiment, bulk granular Bi2Sr2CaCu2OX(Bi22 12), bulk granular YBa2Cu30, (YBCO), a-axis oriented YBCO thin film on SrLaGaO4 substrate and Bi2212 single crystal. A sample in a quartz tube was cooled to liquid nitrogen temperature in the TEl02 cavity as shown in Figure 1. Field modulated microwave absorption (S) was measured on the sample at 9.3 GHz using a cavity perturbation method under external filed (Ha) sweeps up and down [13], and at a fixed Ha=50 G. In the case of fixed field, the signal was recorded as a function of time (t). The sample temperature (T,) was monitored using a Cu-Constantan thermocouple directly attached to the sample as shown in Figure 1. In one series of the experiments, the liquid nitrogen temperature T was also directly measured by another thermocouple as shown. The coolant temperature T was increased intentionally by dissolving atmospheric oxygen into the liquid nitrogen. 3. RESULTS AND DISCUSSION
Field sweep microwave absorption signals S are shown in Figure 2 for the bulk Bi2212 sample at two sample temperatures (T,) of 77.5 and 79.8 K. At sufficiently low T of 77.5 K, the spectrum of S-Ha shows a quite monotonous behavior from Meissner to the mixed states with increasing Ha. This is a typical normal signal. However at some higher Ts, say 79.8 K, there observed a large signal fluctuation besides tiny fluctuations.
~i~~~ 1. Schemes for the sample, quartz tube and liquid nitrogen dewar.
We have suggested that this big fluctuation is induced by the liquid nitrogen bubbling. In this work, we confirmed this origin by a unique experiment. The sample temperature and the liquid nitrogen temperature were measured simultaneously with time as shown in Figure 3. Sometimes the liquid nitrogen temperature drops and rises to the equilibrium state. This drop is caused by the bubbling, because when the bubbling takes place it absorbs latent heat from ambient liquid nitrogen. Then the ambient liquid nitrogen is cooled down. While, the heat is transferred through a dewar and the quartz tube from the atmosphere, resulting in the temperature rise to the equilibrium state in a short period. It should be noticed that the sample temperature is synchronously changed due to the heat transfer through the quartz tube. We found that the big signal fluctuation takes place always according to this sample temperature variation as shown in Figure 4 as a function of time. Therefore, it could be clarified that the big signal fluctuation is induced by the bubbling. Next we have to elucidate the characteristic structure of signal fluctuation, i.e., (1) sudden drop and recovery to the original intensity at the beginning, (2) the following steep rise and (3) the gradual decay. (1) The signal suddenly drops each time when the “big” fluctuation occurs. This is induced by “out-of-resonance’’ in the cavity when the big bubbling occurs. Probably it is caused by sample displacement triggered by the bubbling shock. (2) The following steep signal rise corresponds to the temperature drop [14]. (3) After showing the maximum, the signal decays gradually. This behavior corresponds to the gradual
4
Ts=77.5K
..
77.45
E
Y
v)
t
t
F
0
Ha (G) Figure 2. Signal S vs Ha on the bulk Bi2212.
Time (s) Figure 3. The sample temperature Ts and liquid nitrogen temperature
T as a function of time.
136
temperature rise indicated in Figure 4. Therefore, the maximum position of the signal is located at the minimum position of the temperature. Now we discuss the decay time constant of the signal. It is defined as the half-width of the signal peak as indicated in Figure 4 (a). This must correspond to the recovery of cooled-down temperature induced by the bubbling. It also implies that must correspond to the temperature lowering ATs between the equilibrium and minimum temperatures as defined in Figure 4 (a). Then the series of experiments were done to investigate the relation between and ATs. A part of the results are shown in Figure 4 (a) to (c). As increasing ATs, namely, as increasing the bubbling scale, the value of increases. These relations are plotted in Figure 5 for many fluctuation events for various microwave powers PW indicated. Although there are large scatterings of the plots, it is clear that 21 increases with increasing ATs for all Pw values. The result obviously indicates that it takes longer time for the coolant to recover the equilibrium temperature for the bigger bubbling consuming more latent heat. Here we discuss the power dependence. The result notably shows that is smaller for the higher Pw at the same ATs, that is, it takes shorter time for the sample to recover the equilibrium temperature when the microwave power is higher even if the bubbling scale is the same. This indicates that the sample is more rapidly heated up by the larger microwave absorption in the sample due to the higher power. Then the absorption signal intensity decays faster. The large scattering of the plots is caused by the fact that the data include the various fluctuations occurring at different absolute temperatures. We have investigated the fluctuations on various forms of HTS materials at around 80
(4
-.-
-.-
g
G
Y
I
b-
0
20 40 60
I-* 7
K
TI = 48.8 S
C
-'
81
v)
80
I- 79
20 40
20 40 60 80
t [SI Figure 4. Fluctuating signal S vs time t for different temperature drops ATs.
137
K. The characteristic big signal fluctuations cannot be observed on the bulk YBCO, -but observed on the aYE3CO thin film as shown in Figure 6. The reason is that the signal intensity drastically changes around these temperature ranges in the a-YBCO thin film but it does not in the bulk YBCO. If we carefully observe some of the rising signals, derivative-shaped signals are superimposed on the microwave absorption signal, for example, at Ha-230 G and 300 G. These derivative signals are ESR signal originated from SrLaGaOq substrate. The substrate is locally heated up by this ESR microwave absorption, resulting in the bubbling. Then the ESR can be sensitively detected by the microwave absorption in the adjacent superconducting layer. We tried next on the Bi2212 single crystal. It shows a characteristic “second broad peak” in the microwave absorption signal S in a certain temperature range [ 151. When the microwave power was raised, the broad peak was shifted to the lower field. This indicates the rise up of real sample temperature due to the microwave absorption. 4. SUMMARY
The absorption signals on the bulk Bi22 12 reveal the characteristic big fluctuations during the field and time sweeps. This fluctuation is caused by the temperature drop induced by the oxygewdissolved liquid nitrogen bubbling. The bigger bubbling induces the larger temperature drop, which in turn results in the longer signal decay time, i.e., the longer recovery time to the equilibrium temperature. This decay time is
Figure 5. Recovery time 21 as a function of AT, for various powers PW.
Figure 6. Signal S vs Ha on the YBCO thin film on SrLaGaOq substrate.
138 shortened with increasing microwave power, indicating that the temperature recovery is
The bubbling is triggered by the rapid heating effect due to ESR microwave absorption in the YBCO film substrate. This fact might indicate that the bubbling is also induced by some triggers in the bulk Bi2212. enhanced by the stronger heating due to the larger microwave absorption.
REFERENCES 1. 0.Tsukamoto, T. Uyemura, T. Uyemura, Adv. Cryogenic Eng., 25 (1980) 476.
2. S. Fuchino, I. Ishii, N. Tamada, 0. Tsukamoto, M. Furuse, T. Takao, Proc. 15" Int. Conf. on Magnet Technology, Part one (1998) 526. 3. S. Fuchino, N. Tamada, I. Ishii, M. Okano, IEEE Trans. Appl. Supercond., 10 (2000) 1526. 4. M. Tachiki, T. Koyama, S. Takahashi, Phys. Rev., B50 (1994) 7065.
5 . 0. K. C. Tsui, N. P. Ong, Y. Matsuda, Y. F. Yan, J. B. Peterson, Phys. Rev. Lett., 73 (1994) 724. 6. Y. Mastuda, M. B. Gaifullin, K. Kumagai, K. Kadowaki, T. Mochiku, Phys. Rev. Lett., 75 (1995) 4512. 7. A. Lander, K. E. Myers and D. W. Face, Adv. Mater., 10 (1998) 1249. 8. Z. Y. Shen, C. Wilker, P. Pang, D. W. Face, C. F. Carter, and C. M. Harrington, IEEE Trans. Appl. Supercond., 7 (1999) 2446. 9. H. Kraus, Supercond. Sci. Technol., 9 (1996) 827. 10. M. M. Kaila, J. W. Cochrane, and G. J. Russell, J. Supercond., 11 (1998) 463.
11. KI. Itoh, A. Hashizume, M. Tada, J. Yamada, V. V. Srinivasu, M. Ashida, T. Itoh, and T. Endo, Adv. Supercond. XI1 (2000) 239. 12. V. V. Srinivasu, KI. Itoh, A. Hashizume, V. Sreedevi, H. Kohmoto, T. Endo, R. Ricardo da Silva, Y. Kopelevich, S. Moehlecke, T. Masui and K. Hayashi, J. Supercond., 14 (2001) 41. 13. K. W. Blazey, K. A. Muller, J. G. Bednorz, W. Berlinger, G. Amoretti, E. Buluggiu, A. Vera and F. C. Matacotta, Phys. Rev., B36 (1987) 7241. 14. T. Endo, S. Nagase, S. Sugiura, N. Hirate, M. Horie, and S. Yamada, Physica C, 282-287 (1997) 1591. 15. A. Hashizume, J. Yamada, H. Kohmoto, Y. Yamada, T. Endo, and M. Shahabuddin, Physica
C, 357-360 (2001) 481.
EPR in the 21’‘ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
139
Comparison of near-Tc behaviors of microwave absorption and resistance in bulk YBCO superconductors H. Kohmoto’, S. Iwasaki’, M. Wakutaa, H. Saratania, T. Endo’, S. Uthayakumarb, R. Dhanasekaranb a Faculty
of Engineering, Mie University, Tsu, Mie 5 14-8507, Japan
bCtystalGrowth Center, Anna University, Chennai-600 025, India
Microwave absorption measurements were done on a bulk YBCO sample at various temperatures T. The signal intensities S were plotted a function of T together with critical current density JC and resistance R. The low-field S appears blow 94 K and increases rapidly with decreasing T below 90 K. While, JC appears below 90 K and increases in the same manner with the low-field S. R appears above 90 K, then Tc(R)=90 K. The low-field S arises from the sample periphery screening current below 90 K whereas it arises from the intragrain screening current above Tc(R)=90 K.
1. INTRODUCTION To realize a “flux trap superconducting magnet”, and improve its performance, various problems lying in grain boundaries should be solved [I-31. For example, it is easy to enter fluxons into a superconducting bulk sample by application of external magnetic field, while it is also easy for the entered fluxons to get out from the sample through the grain boundaries. Then only a part of the fluxons are trapped in the sample which are effective to the magnet function. In this sense, the fluxon kinetics and accompanying screening current behaviors in the bulk superconductors including the grain boundaries, must be investigated thoroughly a function of applied field sweeps up and down [4-61. The fluxon kinetics and screening currents should also be investigated as a function of sample temperature, because the temperature is considerably raised up from the ambient temperature during the pulse-like field application. As a fundamental research of these problems, we study in this work microwave absorption behaviors in bulk high temperature superconductors as hnctions of applied field and
140
temperature. The temperature dependence of microwave absorption is compared with a temperature dependence of a transport resistance in the same sample. This can clarify a difference between a long-distance transport current and an intragrain screening current. We have briefly reported this subject previously [7]. In this paper, we report the same subject in more detail in terms of the intragrain and intergrain screening currents, and a role of grain boundary.
2. EXPERIMENTAL
Superconducting bulk samples of yBa~Cu,Oxwere prepared by usual solid state reaction method. The sintered sample were again annealed at 930°C in oxygen atmospherejust before measurements. A dimension of a main sample was 13.7 mmx1.8 mmx1.7 mm. The sample was put in the center of TElo2 cavity and cooled down to 77 K in a liquid nitrogen cryostat without applied magnetic field (zero-field-cooled). Modulated microwave absorption (h4A) measurements were done on the samples under the dc field (Ha) sweeps upward and downward, reported by Blazey et al. [8]. The modulation field Hmwas superimposed on Ha and it was fixed at very small value of 0.05 G at 100 kHz. Microwaves were introduced into the cavity and their reflected power which is proportional to the absorbed power in the sample, was detected by a phase sensitive detection system. Then the field derivative microwave absorption signal S was recorded a function of H,. The microwave frequency was 9.3 GHz and power was 1 mW. Detailed measurement processes can be referred in ref. MA measurement was also conducted at various temperatures T using a cryostat of cooled nitrogen gas flow. Some samples were crashed into granules and powders, and MA measurement was also performed on these granule and powder samples. We are standing on MA mechanism of the “excess screening current” and neglecting the “vortex mechanism” in this paper because H, is sufficiently small. This mechanism is explained follows. When a resultant current density of the screening current and microwave-induced current exceeds over some critical current densities at weak superconducting regions or weak links, a voltage is generated then the microwaves absorbed excessively.One should refer ref. [4-61 for the detailed explanation. Electrical transport measurements were done on another piece of the bulk samples by a four probe method with sweeping temperature T. The sample resistance R and the critical current density were obtained a function T. This sample was again annealed at a low temperature of after the above transport measurement, then again the same transport measurements were done with different transport currents I after this low temperature annealing.
141
3.
DISCUSSION
3.1. MA spectra
MA spectrum (S-&) measured at 77 K is shown in Figure 1 (a) on the zero-field-cooled bulk sample against the field (Ha) sweeps up and down between -50 G and +50 G. The initial signal S shows zero at Ha=O because it is derivative, and it steeply increases initially with increasing Ha due to an increase in Meissner S(arb.units) screening current. It shows an initial peak (intensity is T defined by Pi) then decreases with increasing Ha to 50 G. On a reversal of Ha, the signal shows a hysteresis down to -50 G. On a second reversal of H, at -50 G, it shows the same hysteresis up to a certain positive Ha (-10 G) until the “hysteretic signal” merges in the “initial signal”. During this process, the hysteretic signal also shows a second peak (called the “hysteretic peak”) at a slightly different Ha from Pi. The intensity of this peak is defined by Ph. The magnitude and position of these two peaks are different, because these are no trapped fluxons on the initial sweep while there are trapped fluxons on the hysteretic sweep in the sample. Such trapped flwons affect on flux distribution and surface screening current, then MA signal is varied. We propose here that the signal in the very low field region near the peaks arises from Meissner S(arb.units) surface screening current circulating at the sample periphery shown schematically in Figure 2. There must be strong superconducting regions (strongly linked: SL) and weak superconducting regions (weakly linked: WL) in the sample. Therefore, the low-field signal arises from both of SL and WL current loops at the periphery. With increasing Ha up to 50 G, the sample goes form Meissner to mixed states and WL current loop is decoupled by the field I (“magnetic decoupling”). Then the signal at 50 G (c) only arises fiom SL current loop. This results in the Figure 1. MA spectra for (a) rapid decrease in S because the sample is granular bulk, (b) granules (1-2 mm) and involves much grain boundaries. and (c) powder (-10 p).
142
In order to confirm the above idea, the bulk sample was crashed into the granules with 1-2 mm diameter, and into the powder with -10 pm diameter. MA was measured on these samples, and the results are shown in Figure 1 (b) for the granules and (c) for the powder. Entire spectra are almost the same with that for the bulk sample shown in (a). However, there observed one outstanding difference, namely, the difference in a ratio of Pi and Ph. The ratios of Pi / Ph are 1.15 for the bulk, 1.09 for the granules and less than 1 for the powder. Figure 2. Model sample and With progressing this division, a relative contribution of current loops. SL: strong link, the sample periphery intergrain current is reduced, and WL: weak link, G: grain. that of the intragrain current increases. Therefore, the low-field signal mainly arises from the periphery current. The hysteretic signals for the bulk sample are shown in Figure 3 for various temperatures. With increasing T, the signal intensity becomes smaller, and it disappears at 96 K. Then TC value evaluated by MA is around Tc(h4A)=94 K. The signal shape, however, does not vary so much. Then it is yet applicable that the low-field signal mainly arises from SL and WL currents and the high-field (50 G ) signal arises only from SL current even at the high temperatures. However, we propose here that the signal at the highest T comes from the 96 intragrain SL and WL currents as -50 shown in Figure 2. Because, the periphery intergrain current loops are Ha(G) decoupled by thermal degradation of Figure 3. MA spectra for the bulk at various weak regions (“thermal decoupling”). temperatures indicated. Recorder gains are Therefore at the highest T, the different, the signal intensity decreases low-field signal arises from SL and considerably with increasing T. WL currents while the high-field
143
signal arises only from SL current in the grains or clusters due to the magnetic decoupling. Comparison of MA and near Tc Characteristic values of S in MA spectrum were obtained shown in the inset of Figure 4. They are plotted in Figure 4 a h c t i o n of T. S, and are large at lower T and decrease rapidly with increasing T up to Tc(MA)=94 K. This decrease is caused by the thermal decoupling mentioned above, and the current contribution on the signal changes from the periphery SL and WL to the intragrain SL and WL. The critical current density JC decreases almost in the same manner with S, and So near Tc. This can clearly support the above suggestion that the intergrain current is thermally decoupled in this temperature region. However, Tc(Jc) evaluated by JC curve (double-dotted dashed) is 90 K. Then this supports that MA signal above 90 K till 94 K comes from the intragrain current mentioned above. The signal intensity S50 is much smaller than S, and SOeven at the lower T. This decrease is caused by the magnetic decoupling mentioned above. Then with increasing T, the current contribution on S ~ isOvaried from the periphery SL for T < 90 K to the intragrain SL for T > 90K. The R-T curve measured with I=5 mA is also shown in Figure 4 by single-dotted dashed 90 K again. This indicates that the curve. By this curve, Tc(R) is evaluated superconducting current pass is 16 not connected over long distance n above 90 K. Then MA signal 12 comes from the intragrain current above 90 K. U 8 The sample was annealed at A VI after the first transport measurement, then R-T was measured again with I=5 mA. The 3 result is shown in Figure 4 by a v1 0 dashed curve denoted by R(a). 80 90 100 110 Obviously Tc(R(a)) is shifted from T (K) 90 K to 93 K by this annealing. Figure 4. Comparison of MA signal intensities, The non-superconducting grain JC and R a function of T for the bulk. S,, SO boundaries are improved. This and SSOare defined in the inset. R(a) indicates clearly indicates that there were the resistance after the annealing. Resistance non-superconducting grain was measured with 5 mA. A bold solid arrow boundaries between 90 and 93 K and a bold dashed arrow indicate the “thermal before the annealing. The decoupling” and “magnetic decoupling”, periphery screening current had respectively.
-3
U
144
been thermally decoupled by these grain boundaries in the region of 90 < T < 94 K, leading to the intragrain screening current. When the transport current is increased from 5 mA to 150 mA, R-T curve is drastically changed shown in Figure 5. The result indicates that there are grossly two grades of grains. One TC of 90 K and the other has TC of 77 K. As increasing T, the weak links are decoupled above 77 K, and the strong links remaining above 77 K up to 90 K. This is a verification that there are strong links and weak links near TCin the sample even after the annealing.
Figure 5. Resistance vs T for the annealed bulk measured with 150 mA.
4. SUMMARY
MA measurements were done on the bulk sample. The low-field signal at the lower T decreases with increasing T due to the thermal decoupling of the periphery current. Then the signal arises from the periphery current at the lower T while from the intragrain current at the higher T than 90 K. This is supported by Jc-T curve and R-T curve both of which show Tc(Jc) and Tc(R) of 90 K. The high-field signal is much weaker than the low-field signal even at the lower T. This is caused by the magnetic decoupling.
REFERENCES 1. T. Takizawa, K. Kanbara, M. Morita and M. Hashimoto, Jpn. J. Appl. Phys. 32 (1993)
L774. 2. Y. Tanimura, T. Itoh, T. Takizawa, K. Kanbata, M. Morita and M. Hashimoto, Physica C 235-240 (1994) 3019. 3. Y. Itoh and U. Mizutani, Jpn. J. Appl. Phys. 35 (1996) 2114. 4. T. Endo and H. Yan, Jpn. J. Appl. Phys. 33 (1994) 103. 5. T. Endo, H. Yan, S. Nagase and H. Shibata, J. Supercond. 8 (1995) 259. 6. T. Endo and H. Yan, Studies of High Temperature Superconductors 14 (1994) 65. 7. T. Endo, S. Nagase, S. Sugiura, N. Hirate, M. Hone and S. Ymada, Physica C 282-287 (1997) 1591. 8. K. W. Blazey, K. A. Milller, J. G. Bednorz, W. Berlinger, G. Amoretti, E. Buluggiu, A. Vera and F. C. Matacotta, Phys. Rev. B36 (1987) 7241.
EPR in the 21* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
145
Estimation of fluxon response-delay from magnetic field variations in superconductors using ESR system a R. D. Kalea, M. Tadaa, KI. Itoha, H. Kohmoto , S. Iwasakia, Y. Nakauea, T. Endoa and b S. V. Bhat
%acuity of Engineering, Mie University, Tsu, Mie 514-8507, Japan bDepartment of Physics, Indian Institute of Science, Bangalore-560 012, India Fluxon delay time zr from external field-rising is estimated by a sweep-reversal method of microwave absorption in superconducting YBa2Cu30, bulk sample at 77 K. 7,. initially increases and turns to decrease with increasing field sweep rate R,. The values of zr are estimated to be 20-120 msec depending upon R,. The initial rise reveals the true nature of fluxon delay while the following decrease is apparent one. The decrease in zr is caused by rapid reduction of the external field. 1. INTRODUCTION
One of promising large-scale applications of high temperature superconductors (HTS) is a high field magnet. A magnetic levitation is induced by high density of flux trapped in HTS bulks through application of external field [l-31. The trapped field is larger for the application of pulsed-fields than for that of slow-increasing and static fields [l-31. Therefore, in order to develop the highest superconducting magnet, we have to clarify the mechanisms of increasing and decreasing manners of flux in HTS bulks corresponding to the field increase and decrease. Especially the dynamic mechanism of flux trapping is the most important subject to be solved. The problem is not a flux creep but flux dynamics during the field variations and just after the field changes [4-61. Now ramp rates of the applied field have become an important experimental parameter in magnetization measurements [7-81. Yeshurun et al. [8] found the very important magnetization relaxation. Under the fast field sweep, so called second peak can be observed in Bi2Sr2CaCu20x crystals. While, this second peak gradually disappears with time. They suggest that there is a field region of fast flux relaxation. Takizawa group [2,9] and Ikuta and Mizutani group [1,10] have reported the delay times in detail. Judging from their results, an order of the fluxon delay time is 1-40 msec. These values are much smaller than an order of 0.5 sec estimated by a signal delay of microwave absorption after a field sweep-stop [4]. There must be some essential differences between these two kinds of experiments.
146
2. EXPERIMENTAL A sintered bulk sample of YBazCusO, (YBCO) was used in this work. Its T, is 93 K. Field modulated microwave absorption (MA) technique called “cavity perturbation method” was employed to estimate fluxon delay. The sample was placed in the center of cavity, and cooled down to 77 K. A dc magnetic filed Ha and modulation field H, were applied to the sample in the same direction. H, was fixed at 0.5 G and 100 kHz. Besides the dc and modulation fields, the microwave magnetic field H, is applied perpendicularly to Ha and H,. Therefore, screening currents I, induced by Ha and Hm flow near the surface of the sample as well as microwave-induced current. If the resultant current density exceeds some critical current densities in weak superconducting regions or weak links, reinforced microwave absorption takes place in the sample. See references [11,121 for the details. We have tried sweep-reversal method. The microwave absorption signal S is recorded against the increasing Ha with a sweep rate R,. The field is quickly reversed at some reversal fields H,, and the field is decreased with the same sweep rate R,. We can evaluate the transient field interval AH, and the corresponding fluxon delay time Tr by Tr=AH/ R,.
3. RESULTS The microwave absorption signals S are recorded under the upward and downward sweeps as shown in Figure 1 as a function of Ha for various R,. The value of H, is very small as 0.5 G. A horizontal dotted-dashed line shows the signal level when the field is reversed at H,, = 55 G. The whole signal behaviors are shown in references [4,6,11]. When R, is small enough as 3.3 G/s (a), there is no signal drop just after the field reversal. However, when R, is increased to 33.3 G/s (b), there observed the signal drop from the level just after the reversal. Then, the downward signal rises again with decreasing Ha according to the normal signal behavior as observed in (a). This drop behavior obviously evolves with increasing R, as observed in Figure 1. The evolution can be estimated by a depth of the drop AS (Figure l(e)) as a function of R, as shown in Figure 2 (a). AS steeply increases with increasing R, at the beginning, then increases gradually. We can judge that this drop certainly corresponds to a transient signal induced by delayed fluxon entrance into the sample from the external field. Because, when the field sweep is “stopped” at this field of H, the signal suddenly drops and decays with time. This signal decay is clearly shown in Reference [4], and its delay time constant z, is evaluatedas 0.5 sec. Therefore, we can estimate the delay time constant zrby this transient “drop” behavior for the sweep-reversal method. The transient time is defined as the time interval corresponding to the transient field interval AH between the reversal field Hma, and the field of signal minimum as indicated in Figure 1 (e). The value of z,. can be calculated by zr = AHRs. A time is scaled after the reversal in Figure 1 (d) along Ha axis. The zr is plotted in Figure 2 (b) as a function of R,. It increases rapidly at the beginning, shows a peak, then decreases with increasing R,.
147
Time ( s e d 0.1
0
(d)b 0
40
HdG) H = ,
[GI
3
Figure 1. The traces of signal S before and after the sweep reversal at H, = 55 G for various R, indicated. The time is scaled only for (d) R, = 133 G/s. The definitions of AS and zr are shown in (e).
4. DISCUSSION
4.1. Mechanism of signal drop and definition of z,. First of all, we have to explain why the signal drops sharply when the field sweep is stopped [4] or reversed in this work. There is a certain “viscosity” for a fluxon to move in a superconductor. If a static field is applied to the superconducting sample, a flux density distribution has U-shape inside the sample due to an exclusion nature. This is called “equilibrium distribution” hereafter. However, if the field is under rising with a certain sweep rate R,, the flux distribution inside the sample is different from the equilibrium distribution as schematically shown in Figure 3. It has “sharper” U-shape because the fluxon entering into the sample is delayed due to the viscosity. When R, is smaller as 33.3 G/s, the sharpness of U-shape is smaller. Whereas, with increasing R,, the sharpness should become larger as shown in the figure because of the larger delay of fluxon entering. This enhanced U-shape is called “dynamical distribution” hereafter. There must be screening current to exclude the flux, and its intensity is larger for larger R, because the stronger current is needed to exclude more fluxons from the sample. This can be understood also by a relation that the screening current intensity I, is proportional to the slope of flux distribution dB/dx. Then the intensity of “surface screening current” is proportional to the “surface slope”. The surface slope is larger for larger R, as mentioned above, then the signal intensity S must be larger for larger R, and I,, because our microwave absorption mechanism here is based on the “screening current
I
I
~
~
I
r
‘“*L i =, ,H ,
55G
0.04 0
0
200
400
600
Figure 2. The transient characteristics of (a) AS and (b) 7,. as a function of R,. mechanism” as mentioned in section 2. As a matter of fact, the signal S strikingly increases with increasing R, as shown in Reference It should be reminded, however, that these surface current and surface slope are exactly the dynamical ones. Then if the dynamical filed rising is ceased, the state with the dynamical surface slope and current gets back to the equilibrium one with the smaller surface slope and current. This leads to the smaller signal S within a certain delay time (the order of 0.5 sec) [4]. When the field is “reversed”, once the signal drop takes place in this transientperiod, then the signal recovers to its intrinsic nature after this transient period. These behaviors are clearly revealed in Figure 1. We characterized this transient time by the definition of in Figure 1. 4.2. Why z,. decreases According to the above interpretation, zr must keep increasing with increasing R, because the fluxon delay must be continuously enhanced. The experimental results of shown in Figure 2 (b), however, obviously differ from this expectation. It certainly increases steeply at the beginning but decreases in the larger R, region. AS shown in Figure 2 (a) also shows a kind of saturation in the larger R, region. It implies that there are some additional mechanisms which are responsible for the unexpected behaviors. We have two reasons for them, technical and thermodynamic ones. Technical reason : If the field sweep is stopped at the delayed fluxons can enter deep into the sample even in short time either in long time under the static field. This means that we can obtain the “true” delay time constant and it has considerably longer time of the order of 0.5 sec [4]. However, in this experiment, the external field Ha is suddenly swept back to the lower side as shown in Figure 3 by a dashed arrow. Due to this, two phenomena take place. The already entered fluxons near the surface in the sample lose force of magnetic pressure exerting them to the inside direction. Then they cease to move into the center of the sample. Furthermore, there is less supply of fluxons by the external field because Ha is less than in a short period after the reversal. Then the fluxons cannot enter into the sample
149
Figure 3. Model flux density (B) distributions in the sample for various sweep rates R,. The solid curves show the U-shape distributions as a function of sample position x when Ha reaches H, (the reversal field). The dashed curves show the distributions just after the reversal. The delayed fluxons can still enter into the sample just after the reversals as shown by arrows for the smaller R,. Whereas, the delayed fluxons cannot enter into the sample, rather they get out from the sample as shown by an arrow for the larger R,. Thus the delayed fluxon entrance is interrupted by the reduction of field for the larger R,. The upward sweep of Ha is shown by a solid curve and the following downward sweep by a dashed curve at the right hand side. from the outside additionally which ought to be entered if Ha is fixed at Hma. For these two circumstances, the real delay time is hidden. With increasing R,, these two phenomena are more enhanced, leading to the decrease in This indicates that the decreasing manner of is apparent and not the “true” nature. Thermodynamic reason : The fluxons are always moving in the viscous medium to the inside direction in the sample during the upward field sweep. Then there must be some heat generation in the sample. This heat generation is larger for larger R, due to the larger velocity of fluxons. Then the real sample temperature is more raised with increasing R,. The temperature rise reduces the viscosity of fluxons, resulting in the more gradual surface slope of flux distribution. This indicates the smaller difference between the equilibrium and dynamical distributions, then the shorter delay time of z,. From the above two discussions, the initial steep rise of zr in Figure 4 (b) shows the true nature of delay phenomenon, however, the following decreasing manner of z,. is the apparent nature. Thus the real delay time must be much larger in the larger R, region as 0.5 sec as obtained by the sweep-stop experiment. In the larger R, region, say R, = 533 GIs, the
150
value of zr is around 20 msec. This value well corresponds to the values of 1 - 40 msec obtained by pulsed-field experiments [1,2,9,10]. This implies that the delay time of fluxon estimated by the pulsed-field method has some uncertainty caused by the same technical reason.
5. Summary The fluxon delay time was estimated on the superconducting YBCO bulk sample by the sweep-reversal method of the microwave absorption. The signal S suddenly drops just after the field reversal of the upward sweep due to the entrance of delayed fluxons into the sample. Then it recovers to the inherent signal behavior under the downward sweep. The delay time 7,. is defined by this transient signal behavior, and increases initially but turns to decrease with increasing Rs. The initial increase in zr reveals the true nature of fluxon delay but the following decrease in is the apparent nature. There are two reasons for this decrease, the technical and thermodynamic reasons.
REFERENCES 1. Y. Itoh and U. Mizutani, Jpn. J. Appl. Phys., 35 (1996) 2114. 2. T Takizawa, K. Kanbara, M. Morita, and M. Hashimoto, Jpn. J. Appl. Phys., 32 (1993) L774. 3. Y. Tanimura, I Itoh, T Takizawa, K. Kanbara, M. Morita and M. Hashimoto, Physica C, 235-240 (1994) 3019. 4. T. Endo, S. Yamada, M. Horie, N. Hitate, K.I. Itoh and Y. Tsutsumi, Adv. Supercond., X (1998) 179. 5. J. Yamada, V.V. Srinivasu, M. Tada, K.I. Itoh, A Hashizume, I. Kometani, K. Anwar and T. Endo, Adv. Supercond., XI1 (2000) 353. 6. K.I. Itoh, A Hashizume, H. Kohmoto, M. Matsuo, T. Endo and M. Mukaida, Physica C, 357-360 (2001) 477. 7. S. Anders, R. Parthasarathy, H.M. Jaeger, P. Guptasarma, D.G. Hinks and R. van Veen, Phys. Rev. B, 58 (1998) 6639. 8. Y. Yeshurun, N. Bontemps, L. Burlachkov, A. Kapitulnik, Phys. Rev. B, 49 (1994) 1548. 9. T. Itoh, Y. Tanimura, T. Takizawa and K. Kanbara, Jpn. J. Appl. Phys., 34 (1995) L810. 10. T. Terasaki, Y. Yanagi, Y. Itoh, M. Yoshikawa, T. Oka, H. Ikuta and U. Mizutani, Adv. Supercond., X (1998) 945. 11. T. Endo and H. Yan, Jpn. J. Apl. Phys., 33 (1994) 103. 12. T. Endo and H. Yan, A.V. Narlikar (ed.) Studies of High Temperature Superconductors, 14, Nova Science, New York, 1994, pp. 65-106.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
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Coexistence of ferromagnetism with superconductivity in RuSr2GdCu208 from ESR measurements Koji Yoshida, Masatoshi Nakamura, Naoto Higashi and Hajime Shimizu Department of Electronics and Computer Science, Tokyo University of Science in Yamaguchi, Onoda, Yamaguchi 756-0884, Japan
We have investigated the magnetic properties of RuSr2GdCu2Os by means of ESR measurements. The Gd EPR spectra suggest the predominant antiferromagnetic correlation with a small ferromagnetic moment. We found that the Ru FMR spectra consist of two components, suggesting the mixed valency in the Ru sites. In addition, we will report the coexistence of superconductivity with the magnetic order on a microscopic scale.
1. INTRODUCTION A possible coexistence of a long-range ferromagnetic order with superconductivity in RuSr2RCuzOs = Gd, Eu, Y Ru1212) has provided us a fascinating opportunity in the study of the high-T, cuprates. The hybrid ruthenium-copper oxides have an analogous crystal structure to YBazCu307, where the CuO chains are replaced by the RuO2 planes. The superconductivity is realized predominantly in the CuO2 planes, while the magnetic order is present in the RuOz planes. However, there is still no consensus for the magnetic structure, which even seems to depend on experimental probes. In the first stage of the study for Rul2 12, results of the pSR and magnetization measurements indicated ferromagnetic order of the Ru moments [I]. On the other hand, neutron diffraction measurements revealed the antiferromagnetic correlation along the c axis among the Ru ions, together with a field-induced ferromagnetic component [2]. Recently, from the NMR measurements, the magnetic ordering with a ferromagnetic component exists within the RuOz planes even in the zero field [3]. In order to settle this problem, we investigate the magnetic and superconducting properties of RuSrzGdCuzOs using an ESR technique. ESR is a good probe to study the magnetism on a microscopic scale because there is no uncertainty for a position where the internal magnetic
152
field exists. 2. EXPERIMENTAL
Polycrystalline samples of RuSrzGdCu20~were prepared by the solid-state reactions. The stoichiometric mixtures of RuO2, SrCO3, Gd203 and CuO is calcined in air at 960°C. After being palletized, they were preheated in a flowing nitrogen atmosphere for 48 h to minimize impurity inclusions. And then, the palletized samples were sintered in a flowing oxygen atmosphere at 1020, 1030, 1040 and 1050°C for a total of 236 h with intermediate grindings. The X-ray diffiaction analysis indicates no impurity phase. The onset of the superconducting transition determined by resistivity @) measurements is 45 K, as shown in fig.1. At TCo=28K,the zero resistivity is obtained. We observed a small hump structure in p at the magnetic ordering temperature the X band (- 9GHz) were performed between diffused in parafin.
= 133 K. ESR measurements using and 300 K for fine powder samples
3. RESULTS AND DISCUSSION
Figure 2 shows the typical magnetic resonance spectra of RuSr2GdCuzOg for various temperatures. The spectra can be roughly classified into two groups. One is an absorption signal near 0.32 T seen at all temperatures, and the other is an absorption structure at low fields below about We will show that Ru1212 exhibits the antiferromagnetic correlation with a small ferromagnetic component and the coexistence of superconductivity with the magnetic order on a microscopic scale. 3.1. Gd signal
First, we consider the resonance signal around 0.32 T (g = 2). This absorption originates from the electron paramagnetic resonance (EPR) of Gd3+ ions. Its derivative line can be fitted well by a Lorentz resonance curve, suggesting importance of the exchange interaction, in addition to the dipole-dipole interaction.
We plot the temperature
dependence of the peak-to-peak line width AHppin Figure 3. It follows a relation of AHpp(T) = (l+B/T)AH(w)+bT, where B is the Weiss temperature and AH(w) the high-temperature limit of Hpp(T). The second term is based on the Korringa relaxation process through the exchange interaction between Gd3+ ions and conduction electrons within the Cu02 planes. This term is absent in the previous work by Fainstein el [4]. The first term describes a divergent increase of AHppat low temperatures as a slowing down of the Gd spins toward the antiferomagnetic transition = 2.5 K) of Gd3+[2].
153
0.15 I
h
E
I
7&290K
I
I
0.10 -
.
v
0.05
0
RuSr,GdCu,O,
+
28K
0
100
300
200
Temperature (K)
1 OK
Figure 1. Temperature dependence of resistivity in Ru1212.
0.2 0.4 0.6 0.8 Magnetic Field (T)
0
Figure 2. Typical magnetic resonance spectra of RuSr2GdCu208for various temperatures.
25001 1000, , , I
I
I
~
I
120
RuSr,GdC%O,
~~
0 I
0
I
I
I
I
I
50
100
150
200
250
Temperature(K)
0 300
200
100
300
Temperature(K)
Figure 3. Temperature dependence of the peak-to-peak line width AHpp.It is replotted
Figure 4. Temperature dependence of the Gd3+ internal field f i which
on an enlarged scale in the inset.
is defined as
=
ff,Gd(300 K) -
Here, ffr,Gd is the Gd3' EPR resonance field.
ffr,Gd(T).
154
It is meaningful that AHppexhibits a negative jump at as shown in the inset of Figure 3. We assume that it is caused by a decrease of where and correspond to the dipolar and the exchange field, respectively. The change of the dipolar field at the Gd sites accompanied by the magnetic ordering of the Ru moments could be mainly responsible for the discontinuity. In this case, because antiferromagnetic order in the Ru sites reduces the dipolar field at the Gd sites, our results suggest the predominant antiferromagnetic correlation in the Ru sites. A similar decrease in AHpp at has been reported in Gdz,CexCu04 [ 5 ] . Figure 4 shows the temperature dependence of the Gd3+internal field which is defined as = H,~d(300K) - Hr,Gd(T). Here, Hr,Gd is the Gd3' EPR resonance field. The appearance of HIbelow indicates the possible existence of a ferromagnetic component because each dipolar field induced by just antiferromagnetic ordered Ru moments cancels out at the Gd sites which are located in the body-center position of the Ru sublattice. is at most 60 Oe, which is about 10 times as small as that reported by Fainstein et [4]. We guess that such the small HI is due to a weak ferromagnetic correlation, for example, which is due to a canted spin structure induced by a rotation of the Ru06 octahedra. In fact, neutron measurements revealed the rotation of the octahedral around the c axis. However, we cannot rule out other possibilities, for example, of a ferrimagmetism. As for superconductivity, we cannot observe large changes in the Gd3+ ESR spectra through the superconducting transition. 3.2. signal It is of great importance to investigate the magnetism at the Ru sites directly. We assign the resonance structure at low fields developed below (Figure 2) to the Ru ferromagnetic resonance. It could correspond to the excitation of the k = 0 magnon of the weak ferromagnetic component. An important finding is that the resonance consists of the following two components, which is clearly Separated at around 110 K, as shown in Figure 5. (i) One is a sharp resonance signal near the zero field, and (ii) the other is a broad signal at higher fields. These signals are also observed in RuSr2EuCu208 (not shown), suggesting a common feature among Ru1212 compounds. Figure 6 shows the temperature dependence of the resonance fields for the two absorption spectra. In the signal (i), decreases with lowering temperature, together with broadening of the line width. Below about 80 K, the signal changes to a non-resonant zero-field absorption, indicating the microwave energy below the magnon gap. On the other hand, of the signal (ii) increases with lowering temperature. Below about 100 K, the signal (ii) is difficult to confirm with our resolution. The most fascinating scenario is a possibility of the mixed
155
135K 130K 127K
800
1
t
1
I I
(. ii .)
' 1
110K 79K 60K
2 o o ~ l $ * l
30K 28K 1OK
0
0 60
0.05 0.10 0.15 0.20 Magnetic Field (T)
80
I
1
100 120 7M140 Temperature (K)
Figure 6. Temperature dependence of the resonance fields in the Ru FMR.
Figure 5. Variations of the Ru FMR spectra with temperature.
T=50K 40K
30K 28K 16K 13K 1OK
I
I
I
I
I
,
0
0.08 0.12 Magnetic Field (T)
Figure 7. Variations of derivative curves with across
156
valence states of the Ru ions. NMR measurements show the charge segregation of the RuSt (60%) and Ru4’ (40%) sites as a result of the charge transfer of holes from the Ru02 planes to the Cu02 planes [3]. We conjecture that two kinds of the Ru resonance signals result in the difference of the anisotropic field due to the different valence states, while the mechanism still remains unclear for the anomalous temperature dependence of Hr,~”.In addition, the FMR signal appears below 140 K > This suggests a field-induced ferromagnetic component. Figure 7 shows the variations of Ru derivative curves with below 50K. While there is = 45 K, we found an anomalous structure below Tco = no obvious change across 28K. It could be relevant to the motions of self-induced vortices which originats from fluctuations of the FM component [3]. The superconductivity clearly affects magnetic properties in the Ru sites, suggesting the coexistence of the magnetic order with superconductivity on a microscopic scale. 4. SUMMARY
We have investigated the magnetic properties of RuSr2GdCuzOg by means of ESR measurements. We suppose that the discontinuous drop of AHppat and the small in the Gd EPR are caused by the antiferromagnetic correlation with a small ferromagnetic component. The two kinds of the Ru FMR near zero-magnetic field suggest the mixed valence states of the Ru ions. In order to clarify the magnetic structure of Ru1212, we are planning to perform ESR measurements using single crystals. The change of the dx’’ldH curve below TCoclearly shows the coexistence of superconductivity with the magnetic order on a microscopic scale in Ru1212. REFERENCES 1. C. Bernhard, J. Tallon, Ch. Niedermayer, Th. Blasius, A. Golnik, E. Brucher, R.K.
Kremer, D.R. Noakes, C.E. Stronach and E.J. Ansaldo, Phys. Rev., B 59 (1999) 14099. 2. J.W. Lynn, B. Keimer, C.Ulrich, C. Bernhard, J. Tallon, Phys. Rev., B 61 (2000) R14964. 3. Y. Tokunaga, H. Kotegawa, K. Ishida, Y. Kitaoka, H. Takagiwa and J. Akimitsu, Phys. Rev. Lett., 86 (2001) 5767.
A. Fainstein, E. Winkler, A. Butera and J. Tallon, Phys. Rev., B 60 (1999) R12597. 5. H. Shimizu, S. Suzuki and K. Hatada, Physica C, 282-287 (1997) 1379.
EPR in the 2 1* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Ferromagnetic resonance La-Ba-Mn-0 thin films
and
157
intragraidintergrain
crystallinity
in
S. Iwasaki", J. Yamada", H. Kohmotoa, M. Tada", Y. Sakurai", T. Endo" and B. J. Reddyb aFaculty of Engineering, Mie University, Tsu, Mie 5 14-8507, Japan bDepartment of Physics, Sri Venkateswara University, Tirupati-5 17 502, India
La-Ba-Mn-0 thin films were deposited by ion beam sputtering at 750°C with supply of oxygen molecules or plasma at oxygen partial pressures PO of 0.5, 1.O and 1.5 mTorr. Intergrain crystallinity (mosaicity) is the best at 1.O mTorr while intragrain (plane-distance) crystallinity is worst at 1.O mTorr. The as-grown films show ferromagnetic resonance only for Po=l .O mTorr. Therefore, the ferromagnetic property can be obtained in the film with aligned crystalline planes among grains (higher mosaicity) rather than in the film with equal plane-distances.
1. INTRODUCTION
Perovskite manganite oxides are extremely unique system in materials so called "strongly electron-correlated system". In the system, various physical properties such as magnetism, conductivity and lattice distortion, are entangling with each other. One of the resulting appearances is colossal magnetoresistance (CMR) which is a magnetically-induced metal-insulator transition [ 11. It is strongly expected that tunable microwave filters composed of stacked manganite/superconducting oxides can be applied for mobile communication system. To realize these devices, the as-grown manganite thin films should be ferromagnetic, and problems existing in interface should be cleared. There have not been so many papers reporting the as-grown ferromagnetic manganite thin films. Most of the papers showed that the ferromagnetic property can be obtain by annealing [2-41. In this work, Lal.,Ba,MnO, (LBMO) system was selected from the many of its
158
family. The reason is that this material has high potential for device applications, because it has an extremely high Curie temperature of 355 5 K then it must be easy to control its temperature-dependent magnetic properties. As the first stage of LBMO/YBCO double layer fabrications, in this work, LBMO thin films were grown at a high temperature aiming to get as-grown ferromagnetic films. To elucidate the magnetic properties, ferromagnetic resonance (FMR) measurement was done on these thin films. Then, correlation between FMR and film crystallinity was investigated, especially in terms of intergrain and intragrain crystallinities. We discuss ferromagnetic anisotropy arising from in-plane and out-of-plane FMR signals.
2. EXPERIMENTAL LBMO thin films were deposited on MgO (1 00) substrate by ion beam sputtering. A target of Lao 94Bao 39Mn10, was sputtered by 4 keV Ar’ ion beam. The substrate was heated by lamp heaters, and substrate temperature Ts was monitored during the deposition using a thermocouple. In this experiment, Ts was fixed at a high temperature of 750°C. During the deposition, either oxygen molecules (ML) or oxygen plasma (PL) was supplied. The oxygen plasma was produced in a plasma source by discharge of oxygen gas at -1 kV at 60 Hz, and it was emitted fiom nozzles to the substrate. The oxygen molecules were supplied through the same plasma source without the discharge. The flow rate of oxidants was controlled then an average oxygen partial pressure PO in a chamber was adjusted to be 0.5, 1.O and 1.5 mTorr. The deposited films were characterized by X-ray diffraction (XRD) using Cu K line in two manners, 0 -2 8 scan and -scan (rocking curve). Then, two sorts of crystallinity were estimated. One is called “intragrain crystallinity” hereafter in this paper, it indicates a distribution of crystalline plane-distances in grains whose crystallographic plane is parallel to the substrate plane. This was estimated by 8 -2 8
XRD peak half-width A
The other is called “intergrain crystallinity” or “mosaicity” hereafter, it indicates a distribution of grains which have different directions of crystallographic plane from the substrate plane. This was estimated by XRD rocking curve half-width A Ferromagnetic resonance (FMR) was measured on the as-grown films to characterize their ferromagneticy and spin properties using ESR apparatus. The sample was cooled to liquid nitrogen temperature (77.3 K) in the TE102 cavity, and dc magnetic field Ha and modulation field H, ( 5 G) were applied on the sample. Ha was swept with
159
its direction parallel (Ha 11 plane) and perpendicular (H,lplane) to the film plane. Microwave frequency was 100 kHz and power was 0.1 mW.
3. RESULTS AND DISCCUSION First of all, we characterized the two sorts of film crystallinities, A and A w. The results are shown in Figure 1 as a function of Po both for the molecular (ML) and plasma (PL) supplies. However, we do not mention here about exact differences between ML and PL, rather grossly grasp behaviors of Po-dependences of A and A for ML and PL in a bundle. The films show a “V-shape” behavior of A ”, i.e., the minimum of A at Po=l.OmTorr. It indicates that the mosaicity is the best at 1.0 mTorr. Whereas, the films show a ‘‘A -shape” behavior of A B , i.e., the maximum of A at 1.0 mTorr just except for the ML-film at 1.5 mTorr. It indicates that the intragrain crystallinity is the worst at 1.O mTorr. As a result, the intragrain and mosaic crystallinities reveal “inverse correlation”. This must be arised from lattice mismatching between the substrate and film. The grains on the substrate grow independently with each other reflecting no exact substrate lattice information. Therefore, the optimum oxygen partial pressures for the intragrain and mosaic crystallinities are not necessarily the same FMR measurements were done on these films. The results are that the films deposited at 1.0 mTorr both for ML and PL show FMR signals thought the films at 0.5 and 1.5 mTorr do not at IFMR: x o x all, as indicated by a circle and crosses 0.3 at the top of Figure 1. These results surprisingly correspond well to the 9 4 3 above mentioned behaviors of 0.1
I
2 . 4
0
0
0
0.5
1
1.5
Po [mTorr]
2
Po-dependences
of
the
two
cristallinities. Then the appearance of film ferromagneticy must strongly related to the film crystallographic
Figure 1. and A, as a function of Pofor ML and PL-films. O...FMR
structure. The films which show FMR signals
yes, X . .FMR no.
have poor intragrain crystallinity but
160
excellent mosaicity. Then the important factor must be the mosaicity rather than the intragrain crystallinity. This indicates that the film with smaller deflections of the crystalline plane can obtain the ferromagneticy. If the crystalline plane is aligned, spins must also be aligned in one direction, originating the ferromagneticy. If there are grains with much deflected crystalline plane, these spins must disturb the spins in the aligned grains. Then the film loses the ferromagneticy. On the contrary, the spins in the grains with slightly different plane-distances do not affect so much on the aligned spins in the regular grains. Resonance fields H, in the two films are plotted in Figure 2 for the field directions of Ha 11 plane and H,Iplane. The apparent H, is lowered from the true resonance field for Ha 11 plane due to a flux concentration, and the apparent HI is raised from the true one for HaIplane due to a strong demagnetizing effect. Comparing ML-film and PL-film, H, for Ha 11 plane is smaller and HI for H,Iplane is larger for PL-film. This indicates that PL-film has the stronger ferromagnetic property, and this is directly known also from FMR peak intensity I p shown in Figure 3. The both of Ip for Ha 11 plane and H,Lplane are distinguishably larger for PL-film than for ML-film. A mechanism of this plasma effect is not known at present. Half-widths r p p of FMR signals are plotted in Figure 3 together with Ip for Ha I( plane and H,Iplane. The half-widths are not so much different between ML and PL, and between Ha (1 plane and H,Iplane. Then Ip directly reveals spin orientation ability. The spins are more easily oriented when the field is applied perpendicularly to the film plane. This anisotropy must arise from the crystalline structure and orientation to the substrate. By the way, it is not shown here but effective magnetization becomes larger
-
4
1.5 7 51
Y
1
II
Figure 2. H, against Ha 11 plane and H a l plane for ML and PL-films.
Figure 3.
rpp
and Ip against Ha 11 plane
and Ha plane for ML and PL-films.
161
contrarily for Ha 11 plane than for H,Iplane when the measurement temperature is raised. An exact origin is now under investigation.
4. CONCLUSION LBMO thin films were fabricated by IBS on MgO at 750°C with supply of oxygen molecules or plasma at Po=0.5, 1.0 and 1.5 mTorr, and their intragrain and mosaic crystallinities were estimated. The mosaicity is the best at 1.O mTorr while the intragrain crystallinity is the worst at 1.O mTorr. The as-grown films deposited at 1.O mTorr only showed FMR signals, indicating that the ferromagnetic nature can be preferably obtained in the film with higher mosaicity. The resonance field of FMR is lowered for
Ha 11 plane but it is raised for H,Iplane, and the indication that PL-film has larger ferromagneticy than ML-film was obtained. Actually the FMR peak intensity Ip is much larger for PL-film than for ML-film. Its origin is now under investigation. Ip, then ferromagneticy is notably larger for HaIplane than for Ha 11 plane. This indicates that the spins are more easily oriented perpendicularly by the field application.
REFERENCES 1. K. Miyano and Y. Tokura, Solid State Physics, 34 (1999) 637 (in Japanese). 2. X. Zhu, W. Si, X. Xi, Q. Li, Q, Jiang, and M. Medici, Appl. Phys. Lett., 74 (1999) 3540. 3. K. Choi and Y. Yamazaki ,Jpn. J. Appl. Phys., 38 (1999) 56. 4. R. Helmolt, J. Wecker, B. Holzapfel, L. Schultz, and K. Samwer, Phys. Rev. Lett., 71 (1993) 2331. 5 . J. Yamada, M. Tada, A. Hashizume, H. Kohmoto, E. Takahashi, S. Shiomi, T. Endo, J. Nogues, J. S. Munoz and T. Masui, Trans. Mat. Res. SOC.Jpn., 26 (2001) 1049. 6. J. Yamada, M. Tada, H. kohmoto, A. Hashizume, Y. Inamori, D. Morimoto, T. Endo, J. M. Colino and J. Santamaria, Trans. Mat. Res. SOC.Jpn., 26 (2001) 1053.
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EPR in the 2 1'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Temperature dependence of paramagnetic resonance in pure and doped ferrihydrite nanoparticles' A. Punnoose and M. S. Seehra Physics Department, West Virginia University, Morgantown, WV 26506-63 15, U. S. A. Temperature variation (5 - 300 K) of the magnetic susceptibility and electron paramagnetic resonance (EPR) spectra are reported for nanoparticles of ferrihydrite (FeOOH nH2O) and the ferrihydrite (FHYD) doped with 5% of Ni, Mo and Ir. The FHYD nanoparticles (nominal size = 4 nm) order antiferromagnetically at TN = 350 K and carry a magnetic moment = 300 p~ per particle due to uncompensated surface Fe3+spins. EPR data for FHYD taken at 300 K and 9.24 shows a single line at g = 2 with linewidth H = 750 Oe. Doping with Ni lowers AH whereas AH is increased substantially on doping with Mo and Ir. The temperature variations of AH and the resonance field shift SHr were measured for FHYD and Ni/FHYD. Both samples show that AH increases and line shifts to lower magnetic fields according to SH, = (AH)3, in agreement with the predictions for randomly oriented superparamagnetic particles. Below the blocking temperature Tp(= 71 K for FHYD and 48 K for Ni/FHYD), more rapid changes in SH, and AH are observed due to progressive freezing of the uncompensated spins.
1. INTRODUCTION The nature of magnetism in antiferromagnetic nanoparticles (AF-NP) is expected to differ substantially from their bulk counterparts because of the dominant role of the surface magnetic ions with lower coordination and broken exchange bonds [l]. Because of this and the potential applications of NP, the physics of systems has become a major area of focus in recent years [ 2 ] . Ferrihydrite (FHYD) nanoparticles with the generic formula FeOOH nH2O occur naturally in the 3-5 nm size range and they are easily synthesized [3]. Recent neutron diffraction and magnetic studies have shown that the FHYD-NP orders antiferromagnetically at TN 2: 350 K and carry a magnetic moment = 300 pB/particle, due to the uncompensated Fe3+surface spins [4]. Using the XAFS (x-ray absorption fine structure) spectra of FHYD, Zhao et al [5] pro osed that Fe3+ions in the core of FHYD-NP are coordinated by six groups with Fe3+-FeP+distance of 3.01.k On the other hand, Fe3+on the surface have only tetrahedral coordination, leading to
1
This research was supported in part by the U.S. Department of Energy (contract #DE-FC2699FT40540).
163
the surface uncoordinated sites. In the FHYD-NP, nearly 30% of total Fe3+ ions are on the surface. In recent papers, changes in the magnetic properties of FHYD-NP produced by doping FHYD with Si (for x = Si/(Si + Fe) = 0.05, 0.10 and 0.15) have been reported using magnetometry [6], Mossbauer studies [7] and electron paramagnetic resonance (EPR) [8]. Since Si favors the tetrahedral bonding characteristic of surface Fe3+,these changes could be explained by Si substituting for surface Fe3+ions. Here we report studies on the temperature variation of the magnetization and EPR spectra in FHYD doped with 5% of Ni, Mo and Ir.
2. EXPERIMENTAL DETAILS The procedures for preparing these samples are described elsewhere [7]. The room temperature x-ray diffraction of all our samples yielded the familiar two broad lines characteristic of 2-line FHYD [3,4], about which various earlier studies have been reported [3-81. For EPR studies, we have used a standard reflection-type X-band (9.25 GHz) spectrometer, with a Varian microwave cavity and a variable temperature cryostat system obtained from Oxford Instruments. Measurements of magnetization as a function of temperature were done on a commercial superconducting quantum interference device (SQUID) magnetometer.
3. RESULTS AND DISCUSSION The temperature variation of the magnetic susceptibilities for undoped FHYD and FHYD samples doped with 5% Ni, Mo and Ir is shown in Figure 1. In these measurements, the samples were zero-field cooled (ZFC) to 5 K and a magnetic field H = 100 Oe was then applied and data
4
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6.
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2-
0
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150 200 250 TEM PERATUR E(K)
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4
0
Figure 1. Temperature variations of the magnetic susceptibility under the zero-field-cooled conditions at H = 100 Oe for FHYD (pure) and FHYD doped with 5% Ni, Mo and Ir.
164
were then taken with increasing temperatures, after temperature becomes stable at each point. A detailed analysis of the magnetization data on these samples as a function of temperature (T) and magnetic field (H) will be reported elsewhere [9]. Here it suffices to note that and T, (the temperature at which peaks) change on doping with Ni, Mo and Ir. This is likely related to the decrease in the magnetic moment per particle, as the dopants replace Fe3+[9]. For T, < T < TN,the vs. T data can be explained by superparamagnetism[lo].
I
1
.
8
,
1
I 5800
I ,800
MAGNETICFIELD (Oe) Figure 2. Room temperature EPR spectra of the four samples at 9.24 GHz. The inset shows the measured peak-to-peak linewidths AH.
From the room temperature EPR spectra for the four samples shown in Figure 2, it is evident that AH (the peak-to-peak linewidth in the absorption derivative) is lowered for Ni/FHYD as compared to pure (undoped) FHYD, whereas AH is increased substantially for doping with Mo and Ir. Since the core of FHYD-NP orders antiferromagnetically(AF) below TN = 350K [4], no EPR signal is expected from the core Fe3+ spins for T < TN. Therefore, the observed EPR signals must be due to uncompensated surface Fe3+ spins which are also responsible for the observed vs. T variations of Figure 1 [4, 91. In magnetic materials, AH H:mx where Ha and &, are respectively the strengths of anisotropy (e.g. dipole-dipole interaction) and exchange interactions [ll]. For NiRHYD, Ha is expected to decrease because Ni2+has a lower moment ( ~ 3 . 2 compared to that for Fe3+( ~ 5 . 9 resulting in lower AH even if there is no change in &,. A similar decrease in AH has been observed in thin Fe films on
-
165
coating with Ni [12]. For Mo and Ir doped FHYD, decrease in between Fe3+-Fe3+due to intervening Mo and Ir ions is likely to be the reason for the observed increase in AH. This is because in insulators is considerably shorter in range than the dipole-dipole interaction.
"11
- 3200
3000
h
-2800 Z
2600-
g
%
8
-rn
2000-
W
-2400
1500-
6 2
i..
A
1000
-
500~
'
I
0
50
'
I
100
'
150
I
'
200
I
'
I
'
I
~
250
I
TEMPERATURE (K) Figure 3. Temperature variations of the linewidth and resonance field for the FHYD and Ni/FHYD samples at 9.24 The lines joining the points are for visual aid. Temperature variations of the EPR spectra were investigated for two samples, pure FHYD and NiEHYD, both showing similar variations. Large increases in the linewidth AH and shifts of the resonance field H, to lower fields is observed with decreasing temperatures for both samples (Figure 3). The linewidth of Ni/FHYD is lower than that of pure FHYD throughout the temperature range but the resonance field differs only at lower temperatures. On approach to the peak temperatures T, of the samples (71 K for FHYD and 48 K for Ni/FHYD), the signals shift extensively to the lower fields and only the high field part of the signal could be observed below T,, making accurate determination of AH and H, impossible. A dramatic increase in the width of the signal could be inferred from the extensive changes observed in the position of the right peak, illustrated in Figure 4. This may be due to the progressive freezing of the surface spins expected below T, if T, is the blocking temperature. Nagata and Ishihara [13] have shown that
166
0
50
100
150
200
250
300
TEMPERATURE (K)
Figure 4. Temperature variation of the right peak position (shown in the inset) for pure FHYD. For comparison, the temperature variation of magnetic susceptibility is also shown, demonstrating the rapid broadening of the EPR line below Tp. the shift in the resonance field 6Hr and the linewidth AH of a superparamagnetic system with a statistical distribution of sizes and shapes varies as
6H,
(AH)'.
A power of n = 2 is predicted for partially oriented particles and n = 3 for randomly oriented particles. Plots of log AH verses log 6H, using the data above Tp yield n = 2.97 for pure FBYD and n = 3.19 for Ni/FHYD (Figure 5), clearly suggesting randomly oriented particles. In summary, the results presented here show that doping of FHYD-NP produced substantial changes in the measured EPR spectra and magnetic susceptibility due to the substitution of the
167
.
I
2.8
-
I1
Pure
-B-- Ni'FHY[ FHYD
II
,
I
I
3.4
.
Log AH
Figure 5. Plot of log SH, vs log AH for the FHYD and Ni/FHYD samples. Slope yield n = 3 in 8H, (AH)".
-
dopant for surface Fe3+. The temperature variation of the EPR spectra fit the model of Nagata and Ishihara for randomly oriented superparamagneticparticles.
REFERENCES 1. R. H. Kodama and A. E. Berkowitz, Phys. Rev. B, 59 (1999) 6321. 2. J. L. Dormann and D. Fiorani (eds.), Magnetic Properties of Fine Particles, Elsevier Science, Amsterdam, 1992; G. C. Hadjipanayis and R. W. Siege1 (eds.), Nanophase Materials: Synthesis, Properties, Applications, Kluwer, Dordrecht, 1994. 3. See the review by J. L. Jambor and J. E. Dutrizac, Chem. Rev., 98 (1998) 2549. 4. M. S . Seehra, V. S. Babu, A. Manivannan, and J. W. Lynn, Phys. Rev. B, 61 (2000) 3513. 5. J. Zhao, F. E. Huggins, Z. Feng, F. Lu, N. Shah and G. P. Huffman, J. Catal., 143 (1993) 499. 6. P. Jena, S . N. Khanna and B. K. Rao (eds.), Cluster and Nanostructure Interfaces, World Scientific, Singapore, 2000 ( pp. 229-234). 7. J. Zhao, F. E. Huggins, Z. Feng and G. P. Huffman, Phys. Rev. B, 54 (1996) 3403. 8. M. S . Seehra, A. Punnoose, P. Roy and A. Manivannan, IEEE Transactions on Magnetics, 37 (2001) 2207. 9. A. Punnoose, M. S . Seehra, N. Shah and G. P. Huffman, Phys. Rev. B (to be published). 10. M. S . Seehra and A. Punnoose, Phys. Rev. B, 64 (2001) 132410. 11. T. G. Castner and M. S . Seehra, Phys. Rev. B, 4 (1971) 38. 12. P. Lubitz, M. Rubinstein, D. B. Chrisey, J. S . Honvitz and P. R. Broussard, J. Appl. Phys., 75 (1994) 5595. 13. K. Nagata and A. Ishihara, J. Magn. Magn. Mater., 104-107 (1992) 1571.
168
EPR in the Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR study of Fe-Si02 granular films Kazuaki Kanazawa ‘, Kouichi Matsuda ‘,Seitarou Mitsudo Sigeo Honda a
Toshitaka Idehara
Faculty of Engineering, Fukui University, Fukui 910-8507, Japan
Research Center for Development of Far-Infrared Region, Fukui University, Fukui 910-8507, Japan Interdisciplinary Faculty of Science and Engineering, Shimane University. Fe-SiOZ granular films having Fe volume fractions smaller than 45 % are known to be characterized by the tunneling giant magnetoresistance and superparamagnetic nature [ l ] . The magnetization and magnetoresistance (MR) curves can be fitted by the summation of two Langevin functions [2]. On the other hand, the ferromagnetic thin films and ferromagnetic ultrafine particles are studied with electron spin resonance (ESR) measurement technique. Because granular films exhibit properties of both thin films and ultrafine particles, X-band ESR measurements have been performed in Fe-SiO2 granular films with Fe volume fractions of 21, 25, and 29 vol.%. Based on the results of angular dependence measurements of ESR absorption line, films with Fe volume fractions of 25 and 29 vol.% have shown behavior of a typical ferromagnetic film. However, based on the results of temperature dependence measurements of ESR absorption line, the film with Fe volume fraction of 21 vol.% could be characterized by superparamagnetism.
1. INTRODUCTION There is extensive interest in studying magnetoresistance after the discovery of the giant magnetoresistance (GMR) in Fe/Cr multilayers [3], and in magnetic heterogeneous alloys with ferromagnetic granules embedded in a nonmagnetic metallic matrix [4]. In Fe-Si02 granular films, magnetic Fe particles are imbedded into the insulator film of Si02 at the nano scale. The bias voltage effect, the temperature effect, etc. have also been reported for Fe-Si02 granular films. Experimental magnetization curves of
169
Fe-SiOz granular films can be fitted by assuming that there are magnetic particles of 2 or 3 different sizes present in these films. To further investigate magnetic nature of Fe-SiOz granular films, it is important to perform ESR measurements.
2. EXPERIMENTS X-band (9.159 GHz) ESR measurements have been performed on Fe-Si02 granular films with Fe volume fractions of 21, 25 and 29 vol.%. The films of about 8000 A thickness were used in the experiments. The diameter of particles is estimated to be 9 -16 A based on the magnetization measurements. To study thin film properties of Fe-SiOz granular films, the angular dependence of ESR absorption was measured. Magnetic field applied parallel to the sample plane corresponds to the angle of 0" in Figurel, and magnetic field applied perpendicular to the sample plane corresponds to the angle of 90" in Figure 1. Angular dependents were measured at the room temperature. On the other hand, to study properties of ultrafine particle for Fe-SiOz granular films, the temperature dependence of ESR absorption was measured. The temperature was varied from 50 to 300 K, and the magnetic field was applied parallel to the sample plane.
Fe-SiO,
/A
f = 9.159 GHz
film plane
29 vol.% 25 vol.%
a
vol.%
Y
1
a
0 2 vl 1
Figure 1. Orientation of the dc magnetic field H and of the magnetization M with respect to the coordinate system used in the calculations. The film is parallel to the x-z plane.
20
40
Angle (degrees) Figure 2. The angular dependence of ESR resonance field.
170
3. RESULTS AND DISCUSSION 3.1. The angular dependence The results of the angular dependence measurements of ESR absorption for all three Fe volume fractions are shown in Figure From these results it follows that the long-range interaction works between Fe particles, and overall granular films have ferromagnetic nature. A quantitative analysis of the results was also performed. The geometrical configuration of the problem is shown in Figure 1. The energy density function corresponding to a system with hexagonal structure, up to the second-order term in the magnetic anisotropy, is then given by the expression 1 cos(p, - r p e q ) + Z ( 4 ~ 2 ) s i nrpcq 2 - (K, + 2K,)sin2 rpeq + K, sin4 p e q ,
=
(1)
where the first term represents the Zeeman energy, the second term is magnetic energy, and the last two terms represent the axial anisotropy energy with the c-axis parallel to are the first- and second-order anisotropy constants. The static the y-axis. Kl and equilibrium position of the magnetization is given by the relation, sin(q, - qcq) =
-
The resonance field and Beljers [6],
-
)sin rpeqcosqeq+
sin’ rp, cos
.
(2)
can be calculated by using the general equation derived by Smit
[s]:=L[e.e-(&)2]. ae2 aq2
If Equation (1) is substituted into Equation
2I:[
=
cos(rpH - rpeq
+ ( ~ J ~ J-M
)cos 2 p +
sin2 peq cos2 peq- sin 4 cpeq)]x
cos(rp, - pCq) - ( 4 n -~
)sin2 peq -
]
sin 4 rpcq
where This gives a general formula of the resonance field for a ferromagnetic film. For the granular film, it is thought that crystal axes were randomly oriented. Therefore, substituting into Equation (2) and Equation the following equations for the balance of magnetic field and moment,
and resonance condition,
171
Table 1. The g-value and value of 4xM Fe volume fractions 29 vol.% 25 vol.% 21 vol.%
n-value 2.05 2.06 2.05
4x M 0.616 T 0.437 T 0.272 T
are obtained. Equation (5) and (6) were used to fit experimental data of the angular dependence of the resonance field, and theoretical curves are plotted together with experimental data in Figure 3. The g-value and value of 4 x M were obtained from Equation (6) as shown in Table 1. The obtained g-value is the same value of pure cx -Fe bulk. It means that there is no enhancement of a Fe moment. As can be seen from Figure 3, the angular dependence of ESR absorption line for granular films with Fe volume fractions of 25, 29 vol.% are well fitted by equations for a ferromagnetic thin film. However, the results obtained for the Fe 21 vol.% film are not in good agreement with a thin ferromagnetic film model.
1.0
2
1.0 -
I,::::::.;::::-/--25vol.%
,
Q 1.0 2.5
,
,
,
,
,
,
,
,
u
--&-
29vol.%
0
20
40
60
80
100
cp,,,(degrees) Figure 3 . Comparison of the experiment data 0 (Experiments) and fitting results (Calculations) for the angle dependence of resonance field.
100
200
300
temperature Figure The results of the temperature dependence of resonance field 0 ( I f r ) and line width ( ,,) in ESR measurements. ~
172
3.2. The temperature dependence The results of the temperature dependence of ESR absorption line measurements for all three Fe volume fractions are shown in Figure 4. As the decrease of temperature, the resonance field H, shift to low field side and the line width H,, increases. These features can be explained qualitatively by assuming superparamagnetic character of Fe-SiOz granular films. Figure 5 shows relation between the shift of resonance field 6H, and half line widthAHpp on double logarithmic scale. As seen in figure, the relations for the samples with Fe volume fractions of 29, 25, 21 vol.% are on a straight lines with slopes =4.6, 2.5 and 1.7, respectively. According to results reported by Nagata et al. [7], for the partially oriented elliptical particles, the resonance field shift will be proportional to ( A HPp)’. However, it particles are randomly oriented, the shift will become as ( A Hpp)3.For the films with Fe of 21 vol.%, the same dependence for the partially oriented elliptical particle was observed. However, for the films with Fe of 25 vol.% and 29 vol.%, it seems that the exponent qualitatively can’t be understand by same point of view. Because, the results of angular dependence experiments suggest that the ferromagnetic interaction ranges overall a films, for these Fe volume fraction films.
I
I
A
3000 4 0 0 0 Hpp(Gauss)
Figure 5. Experimental 6 Hr vs. line width a Hppdependence and straight line fitting results. is exponential
173
REFERENCES 1. S.Honda, T.Okada, M.Nawata, M.Tokumoto, Phys. Rev., B 56 (1997) 14566 2. T.Okada, T.Umemoto, M.Nawate, S,Honda; Tec. Rep. IEICE. MR96-87. 3. . N .Baibich, J.M.Broto, A.Fert, J.Nguyen Van Dau, F.Petroff, P.Etienne, G.Creuzet, A.Friederich and J.Chazelas, Phys. Rev. Lett., 61 (1988) 2472. 4. S.Mitani, H.Takanashi, H.Fujimori, Solid. phys., 32 No4 (1997) 231. 5. C.Chappert, K.Le Dang, P.Beauvillain, H.Hurdequint, Phys. Rev., B 34 (1986) 3192. 6. J.Smit, H.C.Beljers, Philips Res. Rep lO(1955) 113 7. K.Nagata, A.Ishihara, J.Magn.Magn.Mater., 104-107 (1992) 1571
174
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Oxygen dependent evolution of C ~ OEPR + signal in fullerene thin films Alexander I. Shames ', Eugene A. Katz Svetlana Shtutina ', Wojciech Kempinski ', Szymon Lo6 ' and Lidia Piekara-Sady ' a
Department of Physics, Ben-Gurion University University of the Negev, P.O.Box 653, 84 105 Be'er-Sheva, Israel The National Solar Energy Center, The Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, 84 990 Sede Boqer, Israel
'Institute of Molecular Physics, Polish Academy of Science, ul. Smoluchowskiego 17, 60-179 Poznan, Poland
Time evolution of c60+EPR signal (g = 2.0026) under the air/t-oom-light exposure was studied on two identical c 6 0 films of 200 nm thickness on a glass substrate. One of these fullerene films was covered by thin Au layer. For both samples the time-dependent signal growth may be divided into at least two clearly distinguished regions: the region of the fast growth at 0 < t < 1 hour and slow growth region at t > 1 hour. The slow growth rate for the Au covered sample was found about 3 times slower in comparison with the non-covered one. The rate of the EPR signal growth depends on the rate of oxygen diffusion at both early stage (fast grain-boundary diffusion) and following stage (intra-grain diffusion). The long time kinetics measured on the covered sample is one of the best evidences that the evolution of C6[ EPR signal depends only on the rate of oxygen diffusion but not on any other external or internal factor. EPR signal kinetic curves were recorded during in-sifu pumping of the noncovered film sample. The removal of air from the hllerene film leads to a decrease in the amount of EPR active C6[ centers. Opening to air caused an immediate jump of the signal's peak height. No signal restore under the nitrogen pressure was found. The role of oxygen and various models of fullerene-oxygen paramagnetic centers in solids are discussed.
1. INTRODUCTION
Solid c 6 0 is a molecular crystal with Cso molecules occupying, at room temperature, the sites of a face-centered cubic (fcc) lattice [l]. While the carbon atoms within each CSo molecule are held together by strong covalent bonding, weak van der Waals interactions are the dominant intermolecular forces [2]. Therefore well structured c 6 0 crystals and thin films in as-grown state are practically EPR silent [3 - 61. On the other hand, an aidlight exposure leads to the appearance of a sharp (AHpp- 0.1 mT) EPR signal characterized by the Lorentzian lineshape and g = 2.0026. This signal was attributed to c60+centers located in the bulk of the film
175
and generated due to oxygen diffusion from air into c 6 0 film [3]. In a joint EPWSPV experiment we demonstrated recently [4] that the airhight exposure of c 6 0 films leads to the generation of the c60+paramagnetic centers (PCls) and deep acceptor states at = + 1.3 eV. These acceptors act as recombination andor scattering centers. The paramagnetic and recombinatiodscattering centers were suggested to have the same origin. Furthermore, the time development of that EPR signal under airhight exposure of the hllerene film was found to be strongly dependent on the film crystalline structure and consists of two clearly distinguished regions of "fast" and "slow" growth. Improvement of the film structure leads to a deceleration of the "fast" growth. The results were explained assuming the EPR signal growth is controlled by oxygen diffusion, along grain boundaries and into grains, during the "fast" and "slow" periods, respectively [5]. In the present paper we suggest additional experimental arguments directly proving the hypothesis that the c60+Pck appear due to oxygen diffusion into the hllerene bulk. On the one hand, we demonstrate that covering of CSOfilm by a thin Au layer, which slows down the actual amount of air reaching the film, considerably reduces the EPR signal growth. On the other hand, by an in-situ experiment we clearly show the effect of air pressure on the EPR signal while no effect of nitrogen partial pressure was found.
2. EXPERIMENTAL c 6 0 thin films with a thickness of about 200 nm were grown on optical glass substrates of 3.5 - 20.0 mm2 area using a vacuum evaporation of CSOpowder (Hoechst AG 'Super Gold Grade', 99.9%). The vacuum chamber pressure was maintained at about 8 x Torr. Two samples (S1 and S2) were grown under identical conditions: the deposition rate was 6 3 s and the substrate temperature 300 K. However, for the sample S2, on completion of the vacuum deposition process the hllerene film was covered by the thin (1 10 nm) Au layer. Immediately after the sample preparation completed, both film samples were placed into EPR silent Wilmad quartz tubes (5mm 0.d.). Then the tubes were filled with Ar gas and sealed. It should be noted, however, that during this procedure the samples were exposed to air for a short time (of about 3 minutes). All procedures of the films' growth and transferring into the EPR tubes were done under the dark conditions. Finally, the tubes were kept in the dark before and at the first stages of EPR measurements. X-ray diffraction (XRD) and Atomic Force Microscopy (AFM) characterization shown that both samples (S1 and S2) have the same highly dispersed polycrystalline structure with average grain sizes of about 20 nm. EPR spectra were recorded using a Bruker EMX-220 X-band digital spectrometer at room temperature. The amplitude of 100 modulation and the microwave power level were chosen as 0.1 mT and 200 KW, correspondingly, for preventing saturation and obtaining better signal-to-noise ratio. Kinetic EPR measurements were carried out in Time Scan mode when the external magnetic field was fixed at the peak of the corresponding EPR signal. EPR signals' processing was done using Bruker WIN-EPR Software.
3. RESULTS
In accord with our recent results [3 - 61, c 6 0 samples in as-grown state were found to be practically EPR silent. Further aidlight exposure leads to the appearance of a sharp (AHpp=
176
0.124 f 0.005 mT) Lorentzian EPR signal with g = 1 2.0026 k 0.0002. Meanwhile no other EPR signals 1200 were observed. Since both line-shape and line- v m width of this signal are found to remain the same during the exposure, the peak-to-peak intensity of this signal is an adequate measure of the total amount of PC's in the samples under study. Figure m s o 1 displays peak-to-peak intensities of the EPR 0 0 20 40 60 80 .signal as a function of the light/air exposure time v) Time (davsl for samples S1 and S2. The first experimental points (at time, t = 0, Figures la, b) were measured in the dark conditions for the samples, sealed into the Ar-filled tube and. It is clearly seen that the S1 sample demonstrates weak, but quite distinguishable, EPR signal while the S2 sample 40 Noise Level (covered by the Au layer) exhibits a very weak EPR signal with the intensity slightly surpassing 20 0.0 0.2 0.3 0.5 0.7 over the noise level. The second experimental point Time (hours) was obtained after the film samples, remaining in Ar filled tubes, were exposed to roomlight Figure 1. Long time kinetics of the irradiation for several minutes. The following c60+EPR signal intensity obtained points were obtained after the Ar gas was removed on samples S 1 (open circles) and S2 from the tubes and both samples were open to air (closed circles) : (a) whole kinetics; and kept under the same ambient room-light (b) zoom of the first hour. conditions. Starting from this point, the EPR signals from both samples smoothly increase. However, the rate of such a growth was found to be significantly slower for the Au-covered sample S2. As we previously reported [5], the removal of oxygen from fullerene film leads to the decrease in the intensity of the c60+EPR signal. For a further clarification of the role of oxygen in creation of c60+ centers, we performed the long time in-situ pumping experiment. Sample S1, exposed to airllight for about half a year, was placed into the quartz tube connected to a diffusion pump Torr) through a three-way valve. The third input of that valve was connected to a vessel containing pure NZ gas, or could be opened to air. Then the quartz tube was placed into the cavity of the EPR spectrometer and strictly fixed there. The kinetics of the peak EPR signal intensity was recorded before, during and after the in-situ pumping (Figure 2). The whole spectrum of c60+signal was recorded before and after the pumping as well as at several intermediate points between each 45 minutes time scans. Figure 2a clearly demonstrates the moments, when the pump was turned on and when the sample was opened to air. The EPR signal intensity begins declining immediately after the pump was connected to the sample tube (f = 0 hours). During the first 1.5 hours of pumping the peak of the EPR signal drops for about 30 % of its initial value. For justification that such kinetics corresponds to the real decrease in the amount of PC's, we plotted at the same graph data, obtained by the double integration of EPR spectra, recorded at intermediate points (open circles). The latter shows that the decline of the EPR signal peak is mostly governed by the decrease in amount of EPR active centers rather than by the instability of resonant conditions during a long time experiment (without using fieldlfrequency lock accessories). Then the I
t
+
h
L
177
sample was pumped for additional 20 hours. Before the new kinetic measurement, the EPR spectrum was recorded again and the new kinetics has begun. One can clearly see that the long 20-hour pumping did not lead to the firther decrease of the amount of c60+ PC's. Five minutes later the pump was disconnected from the sample tube and the sample was open to air. It resulted in an immediate jump of the signal peak intensity. The EPR signal practically 20 40 60 Time (min) reaches its initial (before pumping) value Figure 2. kinetics of the c60+EPR signal and remained constant till the pumping started again. Then the cycle intensity obtained on sample S 1: (a) long time pumpinglopening - to - air was repeated. pumping (line - normalized intensity measured The EPR spectrum, recorded after all open circles - normalized double cycles had been done, indicates the integrated intensity, obtained between kinetics initial amount of PC's is instantly recorded in-situ); (b) under pumping and restored when the sample is subjected to nitrogen pressure. the ambient air pressure. The latter happens disregarding the number of preceding pumpinglopening-to-air cycles. Another experiment was undergone for elucidation of the effect of partial pressure of nitrogen on the EPR signal kinetics. Result of this experiment is presented in Figure 2b. Before the pump was opened, the EPR signal had not changed. Just after the rotary pump (- lo-' Torr) was opened, the signal declines for about 6% for 30 minutes. Then the pump was closed and the sample exposed to the nitrogen gas at pressure of 75 Torr. The signal kinetics shows no changes in general trend of the signal decay. In a few minutes the nitrogen pressure was removed and the tube was opened to air. It is interesting to note that in this case, contrary to the previous experiment, the kinetics curve did not jump upward. Then the pump was opened again and the pumping continued for another 15 minutes to remove the rest of oxygen and air. The effect of the next two pumpinglopen-to-air cycles looks the same as the result of the previous experiment shown in Figure 2a. Surprisingly, at each cycle the kinetics curve jumps to the level that is a little bit lower than the signal level at zero moment. On the other hand, the previous double integration data show that the EPR signal hlly restores its initial value. Such a decrease in the maximum peak value, obtained after the multiple cycling during the kinetics measurement, may be explained, presumably, by minor changes in the resonance conditions, originating in changes of the sample's position because of the pressure cycles. 4. DISCUSSION
Analysis of the first points in the long time kinetics of the EPR signal justifies the model of PC's with g = 2.0025 2.0026 in hllerene solids, which has been proposed in [3 - 61. Summarizing all discussions, such a PC's may be described as a hole localized in the close vicinity of the hllerene molecule, namely c60+The . conclusion on the hole nature of the radical
178
observed is based on the g-factor value which is higher than that for a free electron g, = 2.0023 [3], as well as on the fact of the quenching of this signal in result of the intercalation of our samples c 6 0 by such strong electron donor as an alkali metal [6]. The aforementioned c60+ center appears when the fullerene solid (either film or powder) is exposed to the simultaneous impact of air and visible light. (Here we will not discuss appearance of EPR signals due to photopolymerization of fullerenes under strong irradiation). Let us consider the starting points for the kinetics shown in Figure 1. The first, dark measurements reveal weak but non-zero EPR signals. However, even non-zero initial signals have acceptable explanation within the framework of the proposed model. Indeed, both technological and EPR recording processes assume that all samples were undergone to a short contact with air and, possibly, exposed to weak light (from the computer display, for instance). Accordingly, we suggest that initial non-zero signal may be due to oxygen diffusion during that short period (between the film growth and the beginning of the EPR measurement). As we have mentioned above, the rate of the EPR signal growth depends on the rate of oxygen diffusion at both early (fast grain boundaries diffusion) and following (intra-grain diffusion) stages. In turn, the diffusion rate depends on the crystalline structure of the film sample, i.e. grain sizes [ S ] . However, both samples under study have the same crystalline structure. The only difference is a thin Au layer covering the surface of the S2 sample, which slows down oxygen diffusion into the fullerene film. The latter results in a significant difference in the initial signals as well as the long time kinetics for both samples. The long time kinetics (Figure la) of the S1 sample is very similar to that observed for a lot of c 6 0 thin films studied [3 - 61. EPR signals in all these samples had tendency to reach saturation in about 80 days. It was supposed [2, 31 the latter reflects mostly intra-grain oxygen diffusion. On the other hand, the long time kinetics for the S2 sample is one of the best evidences that the growth of C6: EPR signals depends only on the rate of oxygen diffusion but not on any other external or internal factor. Indeed, both samples of the same crystallinity (S1 and S2) were kept together and, correspondingly, undergone the same ambient conditions. Nevertheless, in the Au-covered sample S2 the actual amount of air reaching the film was considerably less. This hampers the diffusion rate of air oxygen. Let us analyze the in-situ kinetics. Figure 2a shows the decrease in air pressure causes reduction of the amount of c60+centers. It was reliably proved by consequent detection of the signal’s integral intensity. The rate of this reduction depends on the pumping efficiency and, as it was shown in Reference [ S ] , sample’s crystallinity. However, in a few hours the number of C6: centers reaches levels down and the signal remains the same even after very long time pumping. In the present experiment (the vacuum of Torr) the amount of c 6 0 + centers dropped for about 30% of its initial value. In the recent experiment, described in [S], the vacuum was of about lo-* Tom and the signal was found to be leveled after the signal dropped for 17%. It seems us that at each actual pressure value the fullerene-air system exists in an equilibrium that is manifested in a certain value of C60+ centers detected by EPR. The experimental results presented at Figure 2b evidences that only oxygen, but not nitrogen, affects the amount of c60+centers. Indeed, when the sample, undergone to a certain pumping, was open to the pressure of nitrogen, the signal reduction rate slowed down but the tendency remained. Moreover, when, after the nitrogen pressure, sample was opened to air, no drastic fast increase in signal intensity was observed, as observed, as ill previous experiments on opening to air. Only the additional pumping of excess nitrogen turned the system into its “normal” condition and then opening to air led to its usual effect (Figure 2b). We can suppose
179
this result may be explained in terms of nitrogen that blocks internal pathways for the diffusion of oxygen and prevents oxygen molecules restoring their initial locations. Another conclusion, which may be reliably drawn from the results presented by Figure 2 is that, on opening the sample to ambient air, the EPR signal restores its initial value for a few seconds - practically immediately (within the time scale of the appearance of this signal). The actual time of signal recovery depends, most likely, on the length of the pathway (tube, valves) needed for air to reach the sample. This fact sounds the most paradoxical in the present study. Indeed, several months were required for the oxygen molecules to occupied (by the diffusion at normal atmospheric pressure) those specific sites, which are suitable for the hllereneoxygen system to produces (26: centers. And only a few moments were needed to restore the initial situation, which were created during very long time! The simplest model, which may describe the EPR signal reduction on pumping, is that the pumping significantly removes oxygen from the hllerene system. Though, the fact of the fast recovery of the EPR signal strongly contradicts that model. It is hard to imagine that oxygen returns into the film bulk. Such a situation does not look a relevant one. The most probably, the main role in the appearance of c60+ centers plays a partial pressure of oxygen. The latter creates such conditions that are energetically more favorable for the localization of holes, generating by ambient light irradiation, in the vicinity of hllerene molecules. Taking into account a very long lifetime of c60+PC's (years at ambient conditions, as stated in [I]), the positively charged hllerene-oxygen complex is very stable one but only in condition when oxygen pressure is stable as well. Any change in a partial pressure of oxygen leads to the destruction of the complex and release of the localized hole. When the localization conditions are restored, the hole trapping is favorable again and the sharp increase in EPR signal intensity is observed. The last point should be mentioned here, is that in all pressure experiments we did not succeeded in reduction of all C6: PC's. It means the airAight exposure creates also more stable hllerene oxygen complexes that could not be destroyed by changes in a partial oxygen pressure.
REFERENCES 1. R. Tycko, G. Dabbagh, R.M. Fleming, R.C. Haddon, A.V. Makhia and S.M. Zahurak, Phys. Rev. Lett., 67 (1991) 1886. 2. J.P. Lu, X.-P. Li and R.M. Martin, Phys. Rev. Lett., 68 (1992) 1551 , 3 . A. Shames, E.A. Katz, S. Goren, D. Faiman and S. Shtutina, Mater. Sci. Eng., B 45 (1997) 134. 4. E.A. Katz, D. Faiman, B. Mishori, Yoram Shapka, A. I. Shames, S. Shtutina and S. Goren, J. Appl. Phys., 84 (1998) 3333. 5 . E.A. Katz, A.I. Shames, D. Faiman, S.Shtutina, Y. Cohen, S. Goren, W. Kempinski and L. Piekara-Sady. Physica B, 2738~274(1999) 932. 6. W. Kempifiski, L. Piekara-Sady, E. A. Katz , A. I. Shames and S. Shtutina, Solid State Commun., 114 (2000) 173.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
180
Molecular orientations in Langmuir-Blodgett and vacuum-deposited films of VO-phthalocyanine Yuhei Shimoyama
*
Department of Physics, Hokkaido University of Education, 1-2 Hachiman-cho, Hakodate 040-8567, Japan Langmuir-Blodgett (LB) and vacuum-deposited (VD) films of vanadyl tetra t-butyl phthalocyanine (VOTBP) were investigated by electron paramagnetic resonance (EPR) to elucidate their film structure and molecular organization. Angular variation of the EPR spectral position revealed that VOTBP in the LB and VD films orients perpendicular and parallel to the substrate, respectively. The EPR linewidth indicated a two-dimensional spin chain in the LB films and a three-dimensional in the VD films. The order parameters evaluated from the EPR magnetic parameters demonstrated a moderate degree of ordering in both the films.
1. INTRODUCTION Phthalocyanines have long been used as active materials for molecular devices such as chemical sensors [ 11. The thin film techniques for phthalocyanines include vacuum deposition (VD), the Langmuir-Blodgett (LB) method [2], and spraying of a fine suspension, some of which involves procedures with low reproducibility [3, 41. The physical properties of vanadyl phthalocyanine, however, has received less attention than other transition metal phthalocyanines, mainly because of the complex vanadyl cation and the difficulty in obtaining well-defined thin films. Electron paramagnetic resonance (EPR) has been used for the evaluation of thin film structure and physical properties of transition metal phthalocyanines The angular dependence of EPR spectra yields information on the molecular orientation of paramagnetic moieties in the thin films. The EPR linewidth also provides information about the molecular interactions, where the mode of the spin-spin interaction determines the dimensions of the spin chain in the thin films [6-81. The dimension of spin chain in organic molecules have been utilized to analyze the anisotropic physical properties. EPR spectra have been used to evaluate the order parameter of organic molecular systems, such as liquid crystals [9, 101. The order parameter makes it possible to estimate the molecular ordering in the aligned molecular system. In the present paper, we report on the variety of molecular orientations, the spin chain systems and the order parameters found in the LB and VD films of vanadyl phthalocyanine.
"E-mail:
[email protected]
181
2. EXPERIMENTAL Vanadyl tetra t-butyl phthalocyanine (VOTBP: Wako Chemical) of 99% purity was used as the film material. The organic solvents were all spectral grade (99.9%, Kishida Chemical). The substrate used for the LB and VD films was high purity quartz glass (spracil, Eikou-sya) chosen to minimize EPR signals from impurities. The LB films were prepared using a Langmuir trough (Kyowa Kaimenkagaku: LB-5). Ultra pure water (Organo: puric-Z) with a purity of 18 MQcm was used for the subphase. A benzene solution of VOTBP was used as the developing solution that was spread onto the subphase. Since the breaking pressure is 30 mN/m, we transferred the monolayer onto a glass substrate at 20 mN/m by the standard vertical dipping method at a speed of 2.5 mm/min. Y-type deposition was achieved in all cases with transfer ratios of 0.7-0.8 for both the up and down strokes. The transfer ratio was calculated from the area of transferred Langmuir (L)-monolayers divided by the moving area of the substrate on the water surface. The VD films of VOTBP were prepared in a vacuum of Pa using a conventional bell-jar system (Hitachi: HUS-5GB). The VOTBP powder was placed in a tungsten boat with a perforated cover. We controlled the heating current using a stabilized power supply. The VD films were deposited onto the glass substrates at room temperature. The thickness of the VD films was approximately 100 nm as measured by optical absorption. Electron paramagnetic resonance (EPR) measurements were carried out by an EPR spectrometer (JEOL: JESFELXG) operating at the X-band (9.3 GHz) with a homemade goniometer. The resonant magnetic field was 3202100 mT and the microwave power was 196 mW. The angle between the film normal and the applied magnetic field 8 was varied from 0 to 2x. All the EPR measurements were carried out at room temperature. Since the EPR lines were strongly overlapped, we used the line intensities instead of the linewidths. Since all our EPR spectra show Lorentzian lineshape, we employed the reciprocal relationship between the line intensity and the square of its linewidth.
3. RESULTS AND DISCUSSION 3.1. EPR spectral feature Figures l(a) and (b) show the EPR spectra of the LB films of VOTBP. The eight peaks from the hyperfine structure (hfs) of vanadium ion (I = 7/2) were exhibited in both directions of the magnetic field (0 = 0" and 90") [5]. A wider hyperfine coupling was observed at the magnetic field parallel to the film surface, whereas the narrower hfs was observed at the perpendicular position. This indicates that VOTBP is oriented in the LB films with a significant degree of order. In contrast, the EPR spectra of the VD films [Figures l(c) and 1(d)] showed the reverse relationship between the hfs coupling and the magnetic field direction. The wider hfs coupling was found at the magnetic field normal to the film surface. Those anisotropic hfs couplings suggest that the difference in the molecular orientations is ca. 90" between the LB and VD films. In the VOTBP molecule, the VO axis is located at the center of macrocycle and is normal to its plane. The hfs tensor yields the maximum splitting when the magnetic field is parallel to the VO axis. The parameters of the LB and VD films gave similar & and go values, although the magnetic parameter resulted in different values at various orientations. Since the hfs of the LB films yielded the maximum splitting when the magnetic field was applied parallel to the films, hence VOTBP is oriented perpendicular to the substrate. The molecular orientation in the VD films has parallel orientation, in contrast to the LB film.
182
fH
20mT
Figure 1. EPR spectra of the LB (a: 90" and b: 0") and the VD (c: 90" and d: 0") films of VOTBP. 0 is defined as an angle between the film normal and the applied magnetic field.
3.2. Spin chain systems The linewidth of the EPR spectra is relates with the spin-chain system [6]. In the spin diffusion model, the spin chain in the two- (2D) and the three-dimensional (3D) systems were defined by the following equations [6-81. AH1,2 is the half-width at half-height, and 8 is an angle between the molecular axis and the magnetic field.
2D:
= (3cosz0 -1)'
3D:
= (1+ cos20)
The angular dependence of the linewidth the line intensity defined in the experimental section) of the EPR spectra of the LB film is shown in Figure 2(a). The open circles are the experimental linewidths and the solid line is the best-fit curve. The fitting to the angular variation of the linewidths of LB films reveals the 2D spin chain system. This means that the LB films have a strong in-plane interaction and a weak interlayer interaction. The angular dependence of the linewidths of the VD films is shown in Figure 2(b). The curve fitting resulted in the 3D spin chain.
3.3. Order parameter The angular dependence of the g-value of the EPR spectra may reveal the molecular orientation and distribution in the thin films. Therefore, when the g-value was hard to determine, we instead utilized the ordcr parameter to define the molecular ordcring. The spin Hamiltonian is a linear combination of Zeeman term, hyperfine structure and spin-spin
183
1 0.9 -u N 0.8
2 0.7
m .4
0.6 v
0.5
Y
2 0.4
.4
5
0.3
2 0.2 .0.1
0 -3.2 -2.4 -1.6 -0.8 0 0.8 1.6 2.4 3.2
Angle [rad] Figure 2. Angular variations and the best-fit curve of the linewidth of EPR spectra recorded from (A) the LB and (b) the VD films of VOTBP.
interaction. The hyperfine structure and the Zeeman term are linearly independent of the spin-spin interaction. Thus, we can treat the order parameter, even that from the broadening signal, through the spin-spin interactions. The degree of order, the order parameter of liquid crystals as introduced by Saupe [9], for the first time, defines the following equation
s
t
=-(3 c o s * q -1). 2
(3)
Here 0i is an instantaneous angle between the i-th principal axis and the director, and < > denotes an ensemble average. The order parameter of angular order ranged from zero, the random orientation, to unity, axial orientation. We employed the same definition for the ordering in the thin films [lo]. Experimental order parameters are summarized in Table I. The LB films indicated an order parameter of 0.71, which means good ordering along the director, an ensemble-averaged axis. The VD films exhibited an order parameter of 0.72, which is the same order of magnitude as that for the LB films. It is quite possible that the cluster of VD films possess similar order as the domains of LB films. The order parameter indicated that the VOTBP molecules align along a certain axis.
3.4. Molecular orientation In the process of L-monolayer formation, VOTBP eventually forms a monolayer even if it does not have a hydrophilic moiety in the molecule. In fact, the limiting area of L-monolayer is equal to the cross section of VOTBP, indicating that the molecule in the monolayer is Table 1 Order parameters and director angles of LB and VD films as determined by EPR. Films SII 0 (deg) SI 0 (deg) VOTBP-LB 0.72 26 -0.44 78 0.71 26 -0.46 81 VOTBP-VD
184
oriented perpendicular to the water surface. Transfer and stacking of the L-monolayers by the vertical dipping method leads to the formation of the LB films. This process conserved the perpendicular orientation inherited from the L-monolayers. The EPR spectra also indicated that molecular orientation in the LB films is perpendicular, which is illustrated in Figure 3(a). The spin chain network in the LB films yielded a 2D system, which reflects an in-plane organization. The EPR measurements of the VD films revealed the parallel orientation of VOTBP. Figure 3(b) illustrates the plausible structure. Similar structures have been reported in the epitaxial films of phthalocyanines other than VOTBP deposited on the surfaces of KCI and MoSz single crystals. Although we used a glass substrate, which has an amorphous structure and no dipole moments, the resultant structure is identical to those of the epitaxial films. The molecular orientation of VOTBP in the VD films is inferior to that of epitaxial films. During the formation of the VD film, VOTBP molecules in the vapor phase attached to the substrate as a cluster. On the cold substrate, one may expect an amorphous film. At a higher temperature, however, the clusters rearrange into a crystalline film. Simultaneously, the dipole moment of the substrate may control the molecular orientation through electrostatic interactions. It has been reported that the VD films of phthalocyanine containing metal ions other than VO are in a perpendicular position on the glass substrate. However, our VD films of VOTBP possess an parallel orientation. We attribute the parallel orientation of VOTBP to the intermolecular force being weaker than the surface force from the substrate. The 3D spin chain was found in the VD films whose structure consisted of bulk micro crystals.
3.5. Cooperative ordering Although we have discussed the molecular orientations in the thin films, as a kind of condensed phase, the three of thin films is governed by a many body problem where the collective properties of constituent molecules, such as ordering, play a crucial role. There are two concepts in molecular ordering [11]. One is orientational ordering and the other is positional ordering. The order parameter St depicts the orientational order that determines the angular correlation between two individual molecules [9, 101. A spin chain implies positional
Figure 3. The pictorial model of the molecular packing and orientation in (a) the LB and (b) the VD films of VOTBP.
185
ordering which governs the translational degree of freedom [ 111. We shall therefore discuss molecular ordering in terms of orientational and positional order in both films at the level of various hierarchies, macroscopic, microscopic and mesoscopic structures [111. The spin chain systems of both films determine their spin dynamics. The spin chain yields mesoscopic information for both films. The LB films have 2D spin chains, and the VD films have a 3D spin chain system. Since VOTBP in the LB films possesses a 2 0 spin chain, the positional order in the LB films is confined to the layer. On the other hand, VOTBP in the VD films possesses a 3D spin chain, which indicates three-dimensional degrees of freedom, such as for molecules in a bulk crystal. However, since the order parameter St shows identical values, the orientational order is nearly the same in both films. Therefore, the mesoscopic structure is a rather unique state that hardly reflects the microscopic or macroscopic structure in terms of their phase or molecular organization [11].
4. CONCLUSIONS EPR measurements revealed the molecular orientations in both LB and VD films. VOTBP in the LB films is oriented perpendicular to the glass substrate, whereas that in the VD films is oriented parallel to it. The spin chain structure was determined by the EPR linewidth measurements. The LB films have a two-dimensional spin chains, whereas the VD films have three-dimensional one. The local structures among the nearest-neighbor molecules are the same in both LB and VD films. The difference between the LB films and the VD films is due to the balance of the molecular aggregation force and the surface force of the substrate, which eventually determines the films structure.
ACKNOWLEDGMENTS This research has been supported by the Grant-in-Aid Program (11875019 to Y. S.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
REFERENCES 1. C. Leznoff and A. B. P. Lever, Phthalocyanines: Properties and Applications, VCH Publishers, New York, 1989. 2. A. W. Snow and N. L. Jarvis, J. Am. Chem. SOC.106 (1984) 4706. 3. A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly, Academic Press, San Diego, 1991. 4. R. H. Tredgold, Order in Thin Organic Films, Cambridge University Press, London, 1994. 5. J. H. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Clarendon, Oxford, 1990. 6. R. E. Dietz, F. R. Merritt, R. Dingle, D. Hone, B. G. Silbernagel and P. M. Richards, Phys. Rev. Lett. 40 (1978) 246. 7. P. Turek, Mol. Cryst. Liq. Cryst. 233 (1993) 191. 8. B. Doscher, P. Priess and W. Gunsser, Organ. Magn. Res. 22 (1984) 658. 9. A. Saupe, Z. Naturforsch 199 (1964) 161. 10. Y. Shimoyama, M. Shiotani and J. Sohma, Jpn. J. Appl. Phys. 16 (1977) 1437. 11. P. M. Chaikin and T. C. Lubensky, Principles of Condensed Matter Physics, Cambridge University Press, New York, 1995.
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EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Structure elucidation of vacuum deposited films of titanyl phthalocyanine by EPR Hiroyuki Kaji and Yuhei Shimoyama* Department of Physics, Hokkaido University of Education, 1-2 Hachiman-cho, Hakodate 040-8567, Japan
Using x-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy, we revealed structure and spin-chain interaction of vacuum deposited (VD) films of titanyl phthalocyanine (TiOPc). The XRD and EPR spectra demonstrated that the molecular orientation is dependent upon the deposition periods. Thermal annealing for two hours induced an orientation change in the films where TiOPc reorients from the amorphous to perpendicular positions with respect to the substrate.
1. INTRODUCTION Titanyl phthalocyanine (TiOPc) has light sensitivity at the infrared region, which has attracted much attention due to the applicability of photoelectric conductivity, and toner materials for printers and copying machines [l, 21. TiOPc with Y-type has been prominently used in electro photoreceptor devices. The molecule has polymorphs of a-form (phase 11) and @-form(phase I), and structures of their single crystals have been fully elucidated [ 3 ] .Thin films of TiOPc have been an intriguing target in the recent research of nonlinear optical devices because of their functional properties [4, 51. Thus far, there are many reports on the structure and physical properties of thin films of TiOPc [1-5]. However, it is very rare to find reports on EPR studies of the thin films of TiOPc and its powder, except the work done by Enokida a1 [6]. They have reported, for the first time, EPR spectra of TiOPc powder. The isotope effects were found in hyperfine structures due to 47Tias well as 49Tinuclei. EPR has revealed the molecular interactions where the mode of spin interactions determines the dimensions and anisotropy of the spin-chain system in the film [7-111. Therefore, It is worth while to define the structure and the spin-chain property of TiOPc thin films via EPR spectroscopy. In the present paper, we will report on the fabrication method as well as structure of ultrathin films of titanyl phthalocyanine. We will further demonstrate that EPR spectra may reveal organization of the molecular orientation and the spin-chain of titanyl phthalocyanine films.
*
To whom all correspondence should be directed. E-mail:
[email protected]
187
2. EXPERIMENT The film material in the present study was titanyl phthalocyanine (TiOPc, C32H16N80Ti, Aldrich). We used the material without further purification. For substrates of the films, we used quartz glass (supracil, Eiko-sya). The substrate is highly pure, and is free from the background EPR signals. The substrate was washed several times in an ultrasonic bath of films of TiOPc were prepared in a ethanol prior to use for the vacuum deposition. The Pa) with a conventional heat evaporator. The TiOPc powder was placed in a vacuum (1.3~10.~ tungsten boat. As soon as the temperature of deposition source reached the desired value, the shutter was opened and the source started to deposit on the substrate. The temperatures of deposition source and the substrate measured by thermocouples were controlled at 210°C and 45" during the deposition, respectively. The deposition was stopped every 1 h, and repeated for three times (3 h). The distance between the deposition source and the substrate was ca. 9 cm. The thickness of TiOPc thin films was fixed at ca. 150 (220) nm. X-ray diffraction of the TiOPc films was performed by an X-ray diffractometer (Rigaku, RINT-1200). EPR measurements were carried out using an EPR spectrometer (JEOL, JES-FEIXG). All the EPR spectra were recorded at the X-band (9.3 GHz). The microwave strength was varied between 1-64 mW. Magnetic field modulation was operated at 100 kHz. The angle between the film normal and the applied magnetic field 8 was varied over 0-211, with a homemade goniometer. All the measurements were made at a room temperature (ca. 20°C).
3. RESULTS AND DISCUSSION
3.1. Molecular organization in vacuum deposited films of TiOPc Figure 1 shows XRD patterns of the TiOPc films of h deposition. A weak peak was films of 2 h deposition, whose intensity was severely interfered detected at ca. 6.3" in the by the background scattering. We therefore enhanced the signal-to-noise ratio by the moving average method, and obtained the plane separation of 1 4 A . This value is indicative that TiOPc orients perpendicular to the substrate. The corresponding plane separation is 14A, 10000 which is approximately identical with lattice 9000 constants of phase I. The data eventua8000 lly reflect the perpendicular orientation of 7000 TiOPc In our TiOPc films, the crystal structure was modified by the deposition ,X .- 6000 peaks were period. No significant 5000 observable from the films of and 3 h 4000 deposition. These films may take the 3000 amorphous structure. Only the films of 2000 h deposition exhibited peaks.The 1000 peaks seen in the films of h deposition 0 disappear in the films of 3 h deposition. This 0 10 15 20 25 30 35 40 evidence suggests that the annealing effect is induced by the substrate temperature. By Angle (degree) keeping the deposition of TiOPc for 2 h, we Figure 1. pattern of the vacuum obtained the films with a better crystallideposited films of TiOPc as annealed for 2 h.
8
188
nity by the reorganization through thermal agitation.
3.2. Spectral features of EPR Figure 2 shows EPR spectra of the TiOPc powder, and the VD films at various deposition periods. The EPR spectra of the TiOPc VD films of 1 and 3 h deposition sorely indicated a sharp signal at g = 2.003 which is attributable to the free electron spin (ge = 2.0023). Peaks of the EPR spectrum of the TiOPc VD films of 2 h deposition were found at g = 2.006 and 1.970. The EPR signal indicated a wide hyperfine structure (hfs). However, the peak intensity was relatively weak so that the hfs coupling constant (hfcc: A) was undetermined. Appearance of the hfs suggests that the distance between the central titanium ion is more than 6 A which may prevent line broadening due to the spin exchange interactions [9]. Figure 3 shows EPR spectra of the TiOPc VD films of 2 h deposition at 0 = 0 and 90". Table 1 summarizes the data from the EPR spectra with respect to the applied magnetic field. The anisotropy was described by the 11- (parallel) and I-(perpendicular) components, since TiOPc molecule has a four-fold symmetrical axis along Ti=O double bond. Similar anisotropy has been also found in the VD films of VOPc that also possesses the shuttlecock shaped structure [9]. The anisotropy g-value satisfies the following conditions, 811< g, [9, 121. In the present spectra, the I-and 11-components were evaluated from those at 0 = 0 and go", respectively. These components predict TiOPc being normal to the substrate in the VD films. In principle, the angular variation of the peak position or g-value reflects the orientation distribution of molecules in the spatial confinements. Let be the statistical tilt, and (J the deviation (or distribution) angle. We may define an extended normal distribution function by g=2.006 g~1.970
a) 0 = 90"
g=2 006
I
@)For 1 hour
(c)For 2 hours
!/
g = l 966
j
j
Figure 2. EPR spectra of TiOPc (a) powder, and VD films annealed for (b) 1 h, (c) 2 h, and (d) 3 h. All the spectra were recorded at the film surface normal to thc applied magnetic field.
Figure 3. EPR spectra of VD films (for 2 h) of TiOPc. The spectra were recorded at the film surface (a) parallel and (b) perpendicular to the applied magnetic ficld.
189
Table 1 g-values and linewidths of the powder and the vacuum deposited films of TiOPc at the deposited time for 2 h powder g-value Linewidth (mT)
2.006 0.51
1.970 3.2
vacuum deposited films (for 2h)
no
9no - .,
2.006 1.1
1.966 2.4
2.006 1.1
1.972 3.0
the following equation:
Figure 4A shows an angular variation of the g-value for the high field peak of the TiOPc VD films of 2 h deposition (open squares) and the theoretical fitting curve (solid line). The 0 = 22.1 rad. The best-fit values for the tilt and deviation g-value hit the minimum at angles were 12" and 26", respectively. EPR spectra recorded at 0 = 90 and 0" yielded 11- and I-components of the g-tensor, respectively. A simple analysis of the tensor components predicts that phthalocyanine aligns perpendicular to the glass substrate. In fact, the theoretical fit of the g-value variation for the high field peak of the VD films suggested a tilt angle of 12". The phthalocyanine ring orients along the substrate normal with a tilt angle of 12". This result coincides with those of the XRD measurements. The difference in the tilt angle between XRD and EPR results may be due to the time scales of those detection principles. Deviation angle (0 = 26") yields a spread of molecular orientation around the tilt. Therefore, TiOPc in the VD films oriented within a cone of the spread. 1
1
0.9
0.9
9 0.8
$0.8
-
N ._ 0.7
0.7
2 0.6
0.6 v
-
0
0.5
0.5
I :
4 0.4 3 0.3
gj 0.4 KJ
?
.4
0.3 0.2
2 0.2
0.1
0.1
0
0
-3.2 -2.4 -1.6 -0.8 0
0.8 1.6 2.4 3.2
Angle (rad)
-3.2 -2.4 -1.6 -0.8 0
0.8 1.6 2.4 3.2
Angle (rad)
Figure 4. Angular variation of (A) the g-value, (B) the linewidth of EPR spectra of TiOPc VD films as annealed for 2 h.
190
3.3. Spin-chain systems The spin-chain interaction that relates to the positional order of a spin-nctwork affects the linewidth and lineshape of EPR spectra. One may define the spin-chain interaction in one(lD), two- (2D) and three-dimensions (3D). In the 3D system, spins interact equally over all directions. On the other hand, 2D and 1D systems may havc the plane- and line-wise interactions, respectively. According to the spin diffusion model, complete 2D and 1D systems may exist. Howevcr, these are weak interactions in practice. Even if exchange interactions are negligibly small, dipolar interactions still remain. Therefore, the ideal 2D and 1D systems never exist. Thus, the linewidth (AH) is governed by the flowing expressions [7, 131.
2D: AH
(3cos’O -1y
3D: AH
(l+cos’H)
Figure 5. Pictorial diagrams of the molecular organization of VD films of TiOPc; a) side and b) top views.
(4) Figure 4B shows the angular variation of the normalized linewidth of the high field peak of the VD films of the 2 h deposition (open circles). Both the linewidths of the low and high field peaks varied in a periodical fashion with the minimums at 0 = 21.0 rad, k2.1 rad and the maximums at 8 = 0 rad, rad, respectively. We successfully simulate the linewidth variation of the TiOPc VD films by Eq. 3. The best-fit curve (solid line in Figure 4B) implies that the magnetic interactions of the TiOPc VD films are dominated by the in plane or 2D-order. The TiOPc VD films take the phase 1 structurc in which titanyl groups faced and interdigitated each other. Such a cofacial arrangement between titanyl groups aligns oxygen atoms along a straight line, and eventually promotes strong in-plane interactions. We depicted a model in Figure 5 that shows 2D alignment of the TiOPc VD films. On the othcr hand, in the phase I1 where titanyl moiety directs along a certain axis, every neighboring molecule locates at an equal distance, and undergoes isotropic interactions in the 3D space. Thus, 3D interactions bccome dominant in the phase 11 structure. Wc conclude that the TiOPc VD films take the slipped-stack structurc with 2D order.
191
4. CONCLUSIONS We revealed that TiOPc orients normal to the substrate in the VD films by XRD and the angular variation of g-value of EPR spectra. The VD films of TiOPc molecule may take the phase I structure in which the oxygen moiety faces each other and interdigitates simultaneously. Angular variation of the EPR linewidths revealed that the TiOPc VD films possess a 2D spin-chain. A plane-like network is promoted by the spin-chain interaction.
ACKNOWLEDGMENTS This research has been supported by the Grant-in-Aid Program (11875019 to Y. S.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
REFERENCES 1. B. Dunn, A. R. West and J. W. Goodby, Phthalocyanine Materials Synthesis, Structure and Function, Cambridge University Press, Cambridge, 1998. 2. A. Kakuta, Y. Mori, S. Takano, M. Sawada and I. Shibuya, J. Imag. Tech. 11 (1985) 7. 3. W. Hiller, J. Strahle, W. Kobe1 and M. Hanack, Zeitschrift fur Kristallographie 159 (1982) 173. 4. H. Yanagi, S. Chen, P. A. Lee, K. W. Nebesny, N. R. Armstrong and A. Fujishima, J. Phys. Chem. 100 (1996) 5447. 5. Z. D. Popovic, M. I. Khan, S. J. Atherton, A. M. Hor and J. L. Goodman, J. Phys. Chem. B 102 (1998) 657. 6. T. Enokida, R. Hirohashi and T. Nakamura, J. Imag. Sci. 34 (1990) 234. 7. T. Takamura, M. Moriyama, T. Komatsu, Y. Shimoyama, Jpn. J. Appl. Phys. 38 (1999) 2928. 8. J. H. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Clarendon Press, Oxford, 1990. 9. S. Palacin, A. Ruaudel-Texier and A. Barraud, Mol. Cryst. Liq. Cryst. 156 (1988) 331. 10. M. Brinkmann, C. Chaumont, H. Wachtel and J. J. Andre, Thin Solid Films 283 (1996) 97. 11. R. E. Dietz, F. R. Merritt, R. Dingle, D. Hone, B. G. Silbernagel and P. M. Richards, Phys. Rev. Lett. 40 (1978) 246. 12. P. Turek, Mol. Cryst. Liq. Cryst. 233 (1993) 191. 13. J. E. Wertz and J. R. Bolton, Electron Spin Resonance, Elementary Theory and Practical Applications, McGraw-Hill Book Company, New York, 1972.
192
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR investigation of organic conductor with itinerant and local spins, (CHTM-TTP)zTCNQ T. Nakamuraa, M. Taniguchi', Y. Misakib, K. Tanaka' and Y. Nogamic aInstitute for Molecular Science, Okazaki 444-8585, Japan 'Dep. Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan 'Dep. Science, Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
ESR investigation was carried out for the organic conductor with itinerant and local spins, (CHTM-TTP)2TCNQ. This salt shows two drastic successive phase transitions at low temperatures. The contributions of the two spin species of the magnetic properties are separated using with the g-tensor analyses. The electronic phases of the title salt are discussed by microscopic point of view. 1. INTRODUCTION
The recent discovery of superconductivity in BEDT-TSF salts with iron counter ions under high magnetic fields has expanded the interest in possible cooperative phenomena between spins of donors and counter ions [l]. It is interesting to investigate the electronic phases of organic charge transfer salts that are candidates with possible cooperative phenomena between two different spin species, since we can expect much stronger interactions between spins on the donor and accepter molecules. CHTM-TTP is an organic donor recently synthesized by Kyoto Univ. group [2]. Its cation salt, (CHTM-TTP)2TCNQ, is a new organic conductor that is composed of segregated donor (CHTM-TTP) and acceptor (TCNQ) layers (Figure 1). CHTM-TTP molecules stack to form one-dimensional columns. On the other hand, there is little interaction between TCNQ molecules. This salt is metallic around R.T., and its resistivity shows an abrupt jump around
193
Figure 1. Crystal structure of (CHTM-TTP)2TCNQ. 225-245K. But it remains conductive with a slight maximum around 150-180K down to 30K. At 30K, the resistivity turns to increase. In order to clarify the mechanism of the anomalies mentioned above, we performed magnetic resonance measurements. The low temperature electronic phases of the two-component system, (CHTM-TTP)2TCNQ, are discussed by microscopic point of view.
2. EXPERIMENTAL The ESR measurements were carried out for a single crystal using an X-band spectrometer, Bruker ESP-300E, with a rectangular cavity: TM110. The temperature range of the ESR measurements was between 300K and 4K. The ESR signal is a single Lorentzian in the whole temperatures.
3. RESULT AND DISCUSSION
Figure 2 shows the temperature dependence of the spin susceptibility, xspln, of (CHTM-TTP)2TCNQ determined by the EPR signal intensity. Between 240K and 300K, zspln increases gradually as temperature decreases. At 240 K, xspln shows an abrupt decrease, indicating a phase transition. In this xspln jump, we do not observe any hysteresis behavior between in cooling and in heating processes. Between 195 K and 240 K, the increase of xspln with temperature became weak. Around 180 K, another abrupt decrease of xspln was observed. As for the low-temperature phase transition, xspln shows a clear hysteresis behavior: In the case of the cooling process, xspln abruptly drops around 170 K, while it jumps around 210K for the heating process. These observations indicate that the transition is of first order. Below
194
L . L
I
I " ' " "
0
50
200 250 Temperature (K)
100
150
300
Figure 2. Temperature dependence of xspin of (CHTM-TTP)2TCNQ determined by EPR.
0
50
' I "
' I
"
1 ' ' -
200 250 Tempeartaure (K)
100
150
Figure 3. Temperature dependence AHpp of (CHTM-TTP)2TCNQ.
300
of
170K, zspin of (CHTM-TTP)zTCNQ shows a gradual decrease and no obvious anomalies down to the lowest temperature.
Temperature dependence of the EPR linewidth, AHpp, is shown in Figure 3. Clear anomalies are also observed in the temperature dependence of the EPR linewidth, AHpp: The linewidth anomalies are associated with the both 240 K and 195 K phase transitions. Abrupt increases of AHpp suggest drastic changes of the relaxation mechanism of the electron spins. These observations indicate that the spins with different character exist, and that the fraction of them changes below the phase transitions. Especially the 240 K anomaly probably corresponds with the abrupt increase of the electric resistivity. As for the 195K phase transition, the EPR parameters exhibit clear hysteretic phenomenon, indicating that the transition is of first order. Most of phase transition associated with abrupt decreases of xspln, the EPR linewidth generally decreases: It is mainly due to reduction of the spin-spin interaction associated with spin-gap formation. On the other hand, in the case of (CHTM-TTP)2TCNQ, AHpp shows opposite behavior in both phase transition in which xspin suddenly decreases. Broadening of AHpp indicates increase of the averaged EPR relaxation rate, suggesting enhancement of itinerant nature. Below 170K, AHpp shows monotonically decrease down to 12 K, turns to increase slightly
195
below 12K. It should be noted that the enhancement of AHpp below 12K is moderate and weak. Moreover zspln of (CHTM-TTPhTCNQ shows no obvious anomalies down to the lowest temperature. Hence the insulating behavior below 30K is considered to be extrinsic. The enhancement of AHpp below 12 K is considered to be inhomogeneous broadening: A possible weak localization seems to be very likely. Further investigations are now going on. Figure 4 shows the temperature dependence of g-values of (CHTM-TTPhTCNQ applying the external static field along the orthogonal three axes. The anisotropy of the g-values at R.T. is very small: These features are quite different from those of typical TTF cation radical salts. The g-values for all the directions indicate stepwise increase at two phase transitions. The principal values of the g-tensor change their absolute values at them. It cannot be explained within the framework of one spin model. It is an obvious evidence of the existence of TCNQ spins besides CHTM-TTP spins. Moreover the average of the principal values also shows significant temperature dependence. Detailed of the g-tensors analyses enable us to decompose zspln of systems with two different spin species [3]. It indicates that contribution of the TCNQ spins to the total spin susceptibility also changes; the effective local moments on TCNQ decrease at 240K, and that disappear perfectly below 170K. These considerations are consistent with the results of the temperature dependence of the 'H-NMR relaxation rate. Detailed analyses of them will be discussed elsewhere.
2.012 2.010
-F
I
'
2.008 2.006
2.002
0
50
100
150 200
250
300
Figure 4. Temperature dependence of the g-values of (CHTM-TTP)*TCNQ applying the external static field along the crystal axes.
196
4. CONCLUSION In summary, we investigated the low temperature electronic state of (CHTM-TTP)2TCNQ by the ESR measurements. The principal values of the g-tensor change their absolute values at them. It is an obvious evidence of coexistence of TCNQ spins besides CHTM-TTP spins. The effective local moments on TCNQ decrease at 240K, and that disappear perfectly below 170K.
5. ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (B) of "Molecular Conductors and Magnets" (Area No. 730h1224213) from the Ministry of Education, Science, Sports and Culture of Japan.
REFERENCES
1. S. Uji, H. Shinagawa, T. Terashima, T. Yakabe, Y. Terai, M. Tokumoto, A. Kobayashi, H. Tanaka and H. Kobayashi, Nature, 410 (2001) 908. 2. M. Taniguchi, Y. Misaki and K. Tanaka, in preparation for publication. 3. T. Nakamura, T. Takahashi, M. Taniguchi, Y. Misaki and K. Tanaka, Synth. Met., 103 (1999) 1900, and references therein.
EPR in the 21" Century A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
197
X-band ESR measurements of Et,Me,P[Pd(dmit),], T. Sakurai"., H. Ohta..', S. 0kubob,', R. Katodand T. Nakamurae "The Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan bVenture Business Laboratory, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan 'Molecular Photoscience Research Center and Department of Physics, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan dTheInstitute of Physics and Chemical Research, Wako, Saitama 351-0198, Japan 'Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
X-band ESR measurements of aligned single crystals of EGMep[Pd(dmit),],, which is considered to undergo antiferromagnetic transition at T, = 18 K, were carried out with the static magnetic field up to 1 T. ESR signal was observed below room temperature. The linewidth at room temperature is almost 100 mT and it is relatively large compared to general radical organic compounds. The linewidth shows the minimum at 25 K and then increased as the temperature approached T,. The g-values for all axes start to change below 25 - 35 K. These behaviors of the linewidth and the g-value above T, are the typical behavior in lowdimensional antiferrornagnet and they suggest that the short range order starts to develop below 25 K.
1. INTRODUCTION A series of ~'-(cation)[Pd(dmit),], salts (cation = Me,Z and Et,Me&; Z = P, As and Sb) is well known as an interesting system which has the unique two band electronic structure and shows various T-P phase diagrams depending on cations [l]. These salts have almost the same crystal structure regardless of cation, and the crystal structure of these salts is based on the stack of strongly dimerized Pd(dmit), molecules. These salts are considered to be MottHubberd insulator with half-filled HOMO band under ambient pressure. Nakamura performed X-band ESR measurements on these single crystals and proposed that the ground state of these salts is antiferromagnetic state except for E&Me,Sb satls [ 2 ] .And they revealed that the T,'s of these salts depends strongly on the cation. For instance, the T, of the title
198
compound Et$le,P salt, which is known as a superconductor under pressure, is 18 K and that of Me,P salt is 35 K, while an antiferromagnetic transition is not observed for Et$le,Sb salt. This crucial cation dependence of T, has been explained by the effect of the magnetic frustration in this system Although the X-band ESR measurements of Pd(dmit), salts their measured temperature region was limited were already performed by Nakamura et below about 100 K due to the broad linewidth and the weak intensity of these salts at higher temperature. We have performed X-band ESR measurements on larger amount of Et,Me,P[Pd(dmit),], aligned single crystals than Nakamura et in order to gain more detailed information and extend measurable temperature region.
2. EXPERIMENTAL X-band ESR measurements have been performed on EGMe$"Pd(dmit),], single crystals using a conventional X-band spectrometer EMX081 (Bruker Co. Ltd.) with a continuous He-flow cryostat. 40 - 50 single crystals were aligned along each axis. The static magnetic field is up to about 1 T. Observed temperature region is from 3 K to room temperature.
3. RESULTS AND DISCUSSION
ESR signal of EGMe,P[Pd(dmit),], was observed below room temperature. Figure 1 shows the temperature dependence of the integrated intensity normalized by that obtained at 50 K for B//c. The integrated intensity decreases abruptly below 50 K for all axes as the temperature is decreased. Below TN= 18 K, the signal starts to be distorted by an asymmetric signal which comes from the quartz glass of the sample stage. As the intensity of this asymmetric component gets stronger as the temperature is decreased, small upturn of the
h
r
n
.
'a 1.2: Y
.
P
b +
'0 ' !! '
0
199
integrated intensity below 10 K is caused by the quartz glass and it is not intrinsic. An anomaly of the integrated intensity in the temperature region from 50 to 100 K is observed for does not show such anomaly clearly due to the Blla and b, while the result by Nakamura et scattering of data. In X-ray measurements, the superlattice reflections with the reciprocal wave vector (0 114 0) which equals to superstructure having a periodicity with a+4b+c were observed below 70 K [4]. Although the origin of this superlattice reflection is not clear at the moment, the anomaly of the integrated intensity may be related with this superlattice reflection. Figures 2 and 3 show the temperature dependence of the peak-to-peak linewidth AHpp and the g-value, respectively. These results below 100 K are consistent with the results of Nakamura [2]. The linewidth decreases as the temperature is decreased. It shows the minimum around 25 K and increases as the temperature approaches T,. This temperature dependence of the linewidth shows the typical feature of low dimensional antiferromagnet [5]. The accuracy of the g-value is not good at high temperature because of the broad linewidth and the weak intensity, while it gets better as the temperature is decreased because the linewidth gets narrower. As the temperature is decreased, the g-value starts to shift below 25 K where the linewidth shows the minimum. These results seem to suggest the occurrence of the short range order around 25 K. Moreover, the g-value for B//b increases, while those for Blla and c decrease as the temperature is decreased. The typical one dimensional antiferromagnet shows that the g-value for B//chain increases and that for Blchain decreases as the temperature is decreased below the short range ordering temperature As the stacking direction of Pd(dmit), molecules is a+b direction, the temperature dependence measurements for different direction may be required. As shown in Figure 2, the large linewidth of about 100 mT as compared with general organic compounds was observed at room temperature. Due to its weak intensity and the large linewidth, the signal is affected by the background. Therefore, we can not analyze precisely its lineshape and discuss what kind of interaction is dominant. However, it is worth comparing
0
2.06
* 2.04
? 2.02 MI
IF++++
+
O O
+
2.00 1.98 50 100 150 200 250 300
0
50
100 150 200 250 300
T (K) Figure 2. Temperature dependence of the peak-to-peak linewidth.
Figure 3. Temperature dependence of the g-value.
200
the experimental result with the estimated linewidth considering the dipole-dipole interaction and the exchange interaction. First, we made the order estimation of the linewidth of this salt in the case when only the dipole-dipole interaction exists. The obtained linewidth was about 15 - 30 mT on the assumption that the spin is S = 112 and an unit of Bohr magneton is located at the center of the Pd(dmit), dimer when the external magnetic field is applied in the ab plane. Second, in the case when there exist not only the dipole-dipole interaction but also the exchange interaction, the linewidth becomes sharp to the order of 0.03 mT using the exchange field of 47 T [3]. Neither obtained linewidths can not explain the experimental result. The followings can be the origin of the discrepancy between the experimental result and the estimated linewidth; 1) The assumption that the spin exist only on the center of Pd(dmit), dimer may not be adequate. 2) The magnetic frustration may affect this large linewidth. 3) Although the temperature dependence of the electric resistance shows semiconductive behavior, the conductivity at room temperature is about 10 S/cm [7] and this value is not so low. Therefore, the broad linewidth may reflect the effect of the conduction electron. In summary, X-band ESR measurements of the aligned single crystals of Et$ie,P[Pd(dmit),], have been performed and ESR signal was observed below room temperature. The temperature behaviors of the linewidth and the g-value above TN show typical behaviors of the low-dimensional antiferromagnet and they suggest that the short range order starts to develop below 25 K. The origin of the linewidth observed at room temperature has been also discussed.
ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Scientific Research on Priority Area (A) (No. 11 136231, 12023232 Metal-assembled Complexes) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
REFERENCES 1. R. Kato, Y. -L. Liu, Y. Hosokoshi and S. Aonuma, Mol. Cryst. Liq. Cryst. 296 (1997) 217. 2. T. Nakamura, H. Tsukuda, T. Takahashi, S. Aonuma and R. Kato, Mol. Cryst. Liq. Cryst. 343 (2000) 187. 3. M. Mori and K. Yonemitsu, Synth. Metals 120 (2001) 945. 4. S. Rouzihe, J. -I. Yamaura and R. Kato, Phys. Rev. B 60 (1999) 3113. 5. Y. Ajiro, S. Matsukawa, T. Yamada and T. Haseda, J. Phys. SOC.Jpn. 39 (1975) 259. 6. K. Nagata and Y. Tazuke, J. Phys. SOC.Jpn. 32 (1972) 337. 7. R. Kato, Y. Kashimura, S. Aonuma, N. Hanasaki and H. Tajima, Soild State Commun. 105 (1998) 561.
EPR in the 21' Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
20 1
The role of Li' and Na" charge compensators in Sm3+-dopedC a F 2 and SrF2 M. Yamaga", M. Hondab, N. Kawamatab,K. Samejimab,and J.-P. R. Wells" "Department of Electrical and Electronic Engineering, Faculty of Engineering, Gifu University, Gifu 50 1- 1193, Japan b
Faculty of Science, Naruto University of Education, Naruto 772-8502, Japan
'FELIX Free Electron Laser Facility, FOM-Institute for Plasmaphysics 'Rijnhuizen', P.O.Box 1207,3430 BE Nieuwegein, The Netherlands We have examined the role of co-doping of Sm3+-dopedCaF2 and SrF2 with Li+ and Na' using electron paramagnetic resonance (EPR). Three Sm3+centers in CaF2:Sm3+:Li' are identified as tetragonal(C4v). trigonal(C3v),and orthorhombic ((224 symmetry centers, whereas two Sm3+centers each in CaF2:Sm3+:Na+,SrF2:Sm3+:Li+ and SrF2:Sm3+:Na+are identified as C4v and CzVsymmetry centers with correspondingly different g values. The CzVcenters correspond to Li+/Na+-substitutionat Ca2'/Sr2' sites along the [1101 direction. The absence of cubic symmetry centers, observed in optical measurements of the same samples studied here, can be explained in terms of fast spin-lattice relaxation between the levels of the 6H5/2multiplet of Sm3+. split ground state (4)r~ 1. INTRODUCTION
The alkaline-earth fluoride crystals show space group 02, where the alkaline-earth ions are eightfold coordinated. Trivalent rare-earth ions (RE3') readily substitute for the divalent alkaline-earth cations and charge compensation is required. In CaF2:Sm3+and SrF2:Sm3+crystals, the well-known C4V(F3center is predominant [I]. This center is composed of a Sm3+-F-pair with the charge-compensating fluorine ion located in the nearest-neighbor position along the [OOl] direction from the Sm3+ ion [l-41. After oxidization, these samples have trigonal C3dO2-)centers, consisting of a Sm3+-02'pair with the charge-compensating oxygen ion located in the nearest-neighbor position along the [ 11 13 direction from the Sm3+ion [ 1,2]. The effect of co-doping CaF2:RE3+and SrF2:RE3+crystals with LiF, NaF or KF was reported during the 1960's by groups in the former Soviet Union, see for example [5], and
202
more recently by Jones, Reeves and co-workers [6,7]. From a combination of infrared absorption and laser selective excitation spectroscopy, the later workers illustrated profound changes to the defect distribution; namely that the population of regular C44F') centers is reducedeliminated, cubic centers with remote charge compensation are significantly enhanced, and new orthorhombic symmetry centers with monovalent alkali ions located in the Ca2+or Sr2+site in the [1101 direction from the RE3+ion are created. This paper presents electron paramagnetic resonance (EPR) spectroscopy upon Sm3+ centers in CaF2:Sm3+and SrF2:Sm3+crystals co-doped with Li+ or Na+ in order to better understand the microscopic structure, since the anisotropy of the g tensors yields indirect information on the possible ligand positions. From this, we propose models of the Sm3+-Lif ma+ centers.
2. EXPERIMENTAL PROCEDURES CaF2 and SrF2 crystals co-doped with up to 0.04% Sm3+and up to 1.2% of Li+ and Na' were grown in graphite crucibles by the Bridgmann-Stockbarger method in an RF-induction furnace [6]. As LiF and NaF melt around 400 'C lower than the melting points of CaF2 and SrF2, there is a loss of these dopants during crystal growth and Li' is more readily prone to evaporation as LiF. To counter this, crystals can be prepared in an inert atmosphere such as argon gas. The EPR measurements of Sm3+in these crystals were performed at temperatures between 5-50 K using a Bruker EMX10/12 X-band spectrometer with microwave frequencies of 9.695-9.700 GHz, a microwave power of 0.1 mW and 100 KHz field modulation. The angular variations of the EPR spectra were measured by rotating the sample in the cavity. The full range of the applied magnetic field was between 0-1.5 T. 3. RESULTS
Figure 1 shows the typical Sm3+EPR spectrum measured for CaF2:Sm3+:Li+with Bll[OOl] at 5 K. The spectrum consists of three groups of intense resonance lines accompanied by several weak hyperfne lines due to the 14'Srn and '49Smisotopes with a non-zero nuclear spin of 1=7/2 and natural abundances of 15.1% and 13.8%, respectively. Figure 2 shows the angular variations of the g values in the (010) and { 1 iO} planes for the Sm3+ions with zero nuclear spin. As the full range of the magnetic fields is 0-1.5 T, the g values below 0.46 are not observable. The curves in Fig. 2 are calculated using a spin Hamiltonian, including only the Zeeman interaction, of the form [S]:
203 1.o
1
-1.0 0.5
I
I
I
I
0.7
0.9
1.1
1.3
1 1.5
Magnetic field ( T ) Figure 1 The EPR spectrum for Sm3+:Li+:CaF2with B//[OOl] and 5 K. The letters of C4v,C3vand C2v denote tetragonal, trigonal and orthorhombic centers.
1.o
1.o
0.5
0.5
Q)
(D
0
0
60
Angle ( degree )
90
0
0
60
90
Angle (degree )
Figure 2 The angular variations of the resonance lines (I=O) observed in Sm3+:Li+:CaF2 with T=5 K in the (010) and (1 iOf-planes
204
where is the Bohr magneton, is the applied magnetic field, and (=1/2) is the effective spin. The patterns show tetragonal, trigonal and orthorhombic symmetry. The principal x, y , and axes of the GVcenters are defined as the [loo], [OlO], and [OOl] directions, whereas those of the C3., centers are defined as the [ l iO], [i i2], and [ l l l ] directions. The principal x and y axes of the orthorhombic ((22") centers are rotated by an amount from the [loo] and [OlO] crystallographic directions in the (001) plane, respectively, and the axis is parallel to the [OOl] direction. Cubic (Oh) symmetry centers, observed in laser selective excitation measurements of the same samples studied here, could not be detected using EPR. We tentatively ascribe this to fast spin-lattice relaxation between the magnetically split ground state and consequently broadened EPR linewidths. Analogous measurements were made for the CaF2:Sm3+:Na+,SrF2:Sm3+:Li+, and SrF2:Sm3+:Na+ as discussed below. Table 1 summarizes the optimized fit parameters to the Hamiltonian above. The g values Table 1 The spin-Hamiltonian parameters of Sm3+in alkaline-earth fluoride crystals
~~~
CaF2:Sm3+:Li+
CaF2:Sm3+:Na+
SrF2:Sm3+:Li+
gI1=O 05 gi=O 821
~
~
g11=0 415 g,=o 934
gx=O583 g,=O 558 g,=O 30 9 =+32'
g 11 =0.764 glz0.35
gI1 =0.05 gi4.822
g,=0.660 g,=0.477 g,=O. 30 e =&45' g,=0.637 gy=O.577 g,=O .33 e =+34'
gx=0.606 gy=0.537 g,=O. 34 e =+45'
205
of the C4V(F-) centers observed in CaF2:Sm3+:Li+,SrF2:Sm3+:Li+ and SrF2:Sm3+:Na+ are in agreement with those measured by Newman and Woodward [2]. The tetragonal C4,(F-) and trigonal C3,(o2-) centers consist of Sm3+-F-and Sm3+-02pairs, respectively, with the charge-compensating fluorine and oxygen ions located in the nearest-neighbor interstitial positions along the [ 1001 and [ 1111 directions from the Sm3+ion [ 1-31, As the magnitude of the g-values for the tetragonal CrV(Na) center observed in CaF2:Sm3+:Na+is the opposite of the regular C4V(F-)centers in CaF2, it is clear that the charge compensation configuration is also different. It is likely that this center consists of a second nearest neighbor Na' ion which has substituted for a Ca2' ion along the [OOl] direction. Such a configuration reduces axial distortion along the [OOl] direction, resulting in reduction of the anisotropy of the g values. We denote this center as C4dNa). The orthorhombic CzVsymmetry centers are strongly associated with Li+and Na' charge compensators. The principal x-axis of the Li+ orthorhombic center (hereafter denoted C2v(Li))is tilted with an amount of about 12O from the [ 1101 direction, whereas that of the orthorhombic C2,(Na) center is parallel to the [ 1101 direction. The difference is due to the differing ionic radii of Li+ (0.088 nm) and Na' (0.130 nm) [9]. As the ionic radius of Na' is close to those of Ca2+(0.126 nm) and S?' (0.139 nm) [9], Na' ions substitute for Ca" with minimal relaxation of the surrounding ions. On the other hand, Li+ ions are smaller in size and located slightly off the substitutional position, corresponding to two minima of the chemical potential. The relative EPR intensities, I(C4v(F-))>I(C2,(Li)),in CaF2:Sm3+:Li+suggest that the dominant charge compensators of Sm3+ are interstitial F- ions. The reduction in the intensity of the C4.,(F-) centers and enhancement in the intensities of the C4,(Na) and the CzV(Na) in CaF2:Sm3+:Na+is consistent with more efficient substitution of Na+ ions than Li+ ions, which are more readily prone to evaporation as LiF during crystal growth. The anisotropy of the g values for the C2V(Na)in CaF2/SrF2:Sm3+:Nafisslightly larger than that for the C2,(Li) in CaF2/SrF2:Sm3+:Li+. This fact indicates that the non-axial distortion of the C2V(Na) center is a little larger than that of the C2,(Li) center. The difference in the non-axial electrostatic potential is due to differences in effective size and charge of the Li+ and Na+ ions. The magnetic hyperfine coupling constants of the '47Sm and I4'Sm isotopes having a non-zero nuclear spin of 1=7/2, are estimated to be (Al,A.)=(183MHz, 571MHz) and (271MHz, 692MHz) respectively, for the C4,(F-) center in CaF2:Sm3':Li+. A comprehensive report of the spectroscopic properties of these materials including optical and EPR spectroscopies, and appropriate crystal- and magnetic-field analyses will be presented in a more complete account of this work.
206
ACKNOWLEDMENTS
One of the authors (M. Yamaga) is indebted to Iketani Science and Technology Foundation for a Research Grant Award. We would like to thank Dr Glynn D. Jones from the University of Canterbury, New Zealand for supplying the crystals used in this work.
REFERENCES 1. J.-P.R. Wells and R.J. Reeves, Phys. Rev. B., 61 (2000) 13593. 2. R.C. Newman and R.J. Woodward, J. Phys. C: Solid State Phys., 7 (1974) L432. 3. M.J. Weber andR.W. Bierig, Phys. Rev., 134 (1964) A1492. 4. A.A. Antipin, 1.1. Kurkin, L.D. Livanova, L.Z. Potvorova and L. Ya. Shekun, Sov. Phys.-Tech. Phys., 11 (1967) 821. 5. F.Z. Gil’fanov, L.D. Livanova, A.L.Stolov, Sov. Phys. Solid State, 8 (1966) 108. 6. G.D. Jones and R.J. Reeves, J. Lumin., 87-89 (2000) 1108. 7. S.P. Jamison, R.J. Reeves, P.P. Pavlichuk and G.D. Jones, J. Lumin., 83-84 (1999) 429. 8. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions (Clarendon Press, Oxford, 1970) Chaps. 3 and 5. 9. A.A. Kaminskii, Laser Crystals (Springer-Verlag, Berlin, 1990) Table 2.1.
EPR in the 21” Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
207
The HF and SHF interactions of V02+ions in KZnC1S04.3H20single crystals K.V.Narasimhulu, B. Deva Prasad Raju, J. Lakshmana Rao Department of Physics, Sri Venkateswara University, Tirupati - 5 17 502, India
The hyperfine (hf) and superhyperfine (shf) structural patterns exhibited by V02+ions in KZnClSO4.3HzO single crystals were studied using EPR technique. The angular variation of EPR spectra reveals the presence of four magnetic complexes, which correspond to two distinct sites of V02+ ion. From the angular variation EPR data, the spin-Hamiltonian parameters were evaluated. From the EPR and optical data, the molecular-orbital bonding coefficients were evaluated. The five line superhyperfine (shf) structure is seen in all the planes of the crystal, when the magnetic field is away from the crystallographic axes. This has been attributed to the four protons from H20 ligands, where the metal ion is surrounded by H20 and SO4 environment.
1.
INTRODUCTION
The ’lV nucleus (99.8 % abundant) has a nuclear spin I=7/2 and a large magnetic moment. V4+ ion has the electronic configuration [Ar] 3d’ which thereby leads to paramagnetism in V02+.Vanadyl ion (VO”> is the most stable cation among a few molecular paramagnetic transition metal ions which has been used extensively as an impurity probe for EPR studies. Vanadyl complexes have been the subject of interest to a number of workers for EPR studies over recent years, as the V02+ is found to have different behaviour in different host lattices; it may freely rotate or stay rigid in the lattice depending on the host and the environment. Such studies will provide information regarding the symmetry of crystalline electric fields [ 1-71. Potassium Zinc Chloro Sulphate Trihydrate (PZCST) is an isomorphous crystal of original kainite mineral O(MgClS04.3HzO) studied earlier [81. In continuation of our work on this host [9], the present paper is aimed at the hyperfine and superhyperfine interactions of the V02+ ion in PZCST single crystals and to get the information regarding the metal ion environment. 2. CRYSTAL STRUCTURE
PZCST single crystal is considered to be the zinc analogue of the original kainite. Kainite crystallises in monoclinic symmetry with space group Cz; (Cz,,,,). A detailed crystal structure of PZCST is not available. However, the unit cell dimensions were reported for
208
original kainite single crystal in early fifties [lo]. Later Robinson et a1 [S] studied the detailed crystal structure of kainite and reported the monoclinic symmetry with the same space group with the lattice parameters as a=l.972 nm, b=l.623 nm, c =0.953 nm p = 94" 55" and Z=16. Recently, Wolfgang Voigt's [ l l ] work on kainite confirmed the crystal structure with a slight fkactional change in the number of water molecules (2.75 H20 instead of 3 H20) present.
3. EXPERIMENTAL Single crystals of KZnClS04.3HzO were grown by the controlled evaporation technique to which about 0.5 mole % VOS04.5H20 was added as a dopant. A single crystal of about 0.3 mmz was selected for EPR measurement. For optical absorption studies, the crystals with high concentrations (1 mole %) were used. The other experimental details of the spectral measurements were given elsewhere [7, 91 in detail.
4.
RESULTS AND DISCUSSION
4.1 EPR Studies - Hypertine interactions Figure 1 shows the EPR spectrum of V02+ions doped PZCST single crystals at room temperature for an arbitrary orientation in the zx plane. The EPR spectra of V02+ions in PZCST are very complex and show more than 24 lines in any orientation away from the crystallographic axes. However, when the magnetic field is along any one of the crystallographic axes, they merge into 2 octet hyperfine lines. The presence of not less than two octets along any crystallographic axes indicates the presence of four complexes corresponding to at least two vanadyl ion sites. The above results can be explained as follows. When the vanadyl ion enters the divalent zinc site, the V=O orients itself along any of the three mutually perpendicular Zn-HZO directions within a given metal water octahedron (Zn-HzOoctahedron) with different populations. The EPR spectra were recorded for the single crystal in three mutually perpendicular planes at 5" regular intervals. As the V02+ions easily enter the divalent host ion, the Zn2+ is the most probable site for the vanadyl ions. it may be possible for the site I of V02+ complexes to occupy either the substitutional site of Zn2+or the interstitial position. The comparable ionic radii of vanadyl ion (0.63 A") and Zn" (0.74 A") make it possible for the vanadyl for substitution into Zn2+sites.
I
I
DCOB
2300
MAGNETIC FIELD (GAUSS)
43(
Figure 1: EPR spectrum of V02' ions in PZCST single crystal in the zx plane at RT.
209
The observed V02' spectra can be fitted to the spin-Hamiltoniancontaining the Zeeman and hyperfine term [ 121
where the terms have their usual meaning. The and values were calculated at each 5" interval from the angular variation spectra in the three mutually perpendicular planes. The diagonalization procedure was carried out following the standard procedure given by Schonland [ 131 for obtaining the principal values of the and tensors. The g,, and A,, values (where i = x, y or z), so calculated for V02+ ions doped PZCST single crystals are given in Table.1. The spinHamiltonian parameters indicate that the symmetry of V02+ ions in this lattice is orthorhombic or still lower. The observed spectral features and the mirror symmetry of the crystal lattice indicate that the symmetry will be lower than orthorhombic and thus demonstrates the features of monoclinic symmetry of the crystal lattice. The EPR spectrum has also been studied for the polycrystallinesample. Figure 2 shows the EPR spectra of a polycrystalline sample at room temperature. The spin-Hamiltonian parameters (SHP's) obtained from polycrystalline data will be useful as a crosscheck to the original single crystal SH parameters but of lower accuracy (see Table 1). The EPR data from the polycrystalline sample indicate that the vanadyl ion has only one type of Zn2+site axial symmetry parameters correspondingto a single site. in the lattice at RT, giving a The optical absorption spectrum of V02+doped PZCST exhibits of three bmds centred at 14405, 16497 and 25765 cm-' and they have been assigned to 2B2g-+ 2Eg, 2 B ~and g 2A~g transitions respectively [ 1,141.
Table 1: Spin-Hamiltonian parameters of V02+:PZCST single crystals, A and P values are in units of 10-4cm-'. The errors in g and A values are 0.002 and 3 10-4 cm-' respectively.
+
Site-I g
Single Crystal
Site-11 A
g
&z= 1.920 Azz=173 gyy = 1.972 Ayy = 108 gm = 1.998 Am = 61
A
*
Molecular Orbital parameters
gzz= 1.920 Azz= 174 g y y = 1.983 Ayy = 114 gm = 1.995 Axx = 52
p2= 0.91 Powder Sample
gzz = 1.936 gxx = gyy = 1.988
Azz = 173 A m =A, = 64
E~ = 0.68 P =128 k = 0.78
210
MAGNETIC FIELD ( G A U S S )
43
Figure 2: EPR spectrum of polycrystalline sample of VO2+:PZCSTcrystals at RT Using the EPR and optical data, the molecular orbital coefficients PI2, E', the dipolar hyperfine coupling parameter P and the Fermi contact interaction parameter k have been calculated using the expressions given by Kivelson and Lee [ 1, 5, 14, 151 and are given in Table 1. The values of molecular orbital coefficients obtained are pl2=0.91and s2=0.68. The parameters (I-Pl2) and ( I - E ~ )are the measures ofthe covalencyrates [l, 6,7, 141, the former (0.09) gives an indication of the influence of o-bonding between vanadium atom and equatorial ligands while the latter (0.32) indicates the influence of n-bonding with the vanadyl oxygen. The calculated values of PI' and e2 indicate that the in-plane o-bonding is ionic and out-of plane n-bonding is significantly covalent. The parameter k indicates extreme sensitivity to the deformations of the electrm orbitals of the central vanadium ion. The large value of k (0.78) indicates a large contribution to the hyperfine constant by the unpaired s-electron and also probably a contribution from spin polarization. The standard value of P for a free ion is 160 10-4cm-I [ 1, 141. The calculated value of P in the present system is 128 x cm-I, which is considerably reduced by (78 %) when compared to the free ion value ( 1 6 0 ~ 1 0cm"), - ~ which indicates a significant amount of covalent bonding in the complex. The values of P and k are close to the values reported for V02+ions in other lattices 6, 71.
4.2 The Superhyperfine Structures At certain orientations of the crystal with the magnetic field, i.e. when the crystal makes 20" or more with the magnetic field, a few sharp lines have been observed within the ml lines. Figure 3 shows five line SHF lines observed in V02+PZCST single crystals. These lines have been identified and are expected due to superhyperfine (SHF) structure [2,3]. This SHF structure must be due to interaction of unpaired electron (impurity) with the neighbouring nuclei having nuclear spin I=2. These observations show that this ligand hf structure should result from SO-: and/or the H20 molecules in the present case. Waplak [ 161 has observed a three line SHF structure in case of V02+doped K H 3 (SO& where the ligands are SO:-. He attributed these SHF lines to the three protons from so4 ligands. Jain and Venkateswarlu observed [ 171 the SHF structure containing a quintet SHF lines in case of V02' ions in Tutton salts, where the V02+ions are surrounded by both water and sulphate ligands. Hence it can be safely assumed that these five line SHF lines result from four protons of the surrounding nuclei having I=1/2. On comparison of the other such V02+systems in literature [2, 171, it is found that the SHF arises from four protons of the surrounding water coordination, which is responsible to form the vanadyl octahedra.
21 1
.
3225
,250
MAGNETIC FIELD (GAUSS)
327
Figure 3. The observed superhyperfine lines of V02+:PZCST at 296 K and 103 K. The angular variation studies indicate that these superhyperfine transition lines are confined to two octets in the zx and planes, whereas only one set of mr lines exhibit these SHF patterns in the xy plane. This indicates that superhyperfine lines have mirror symmetry in the xy plane corresponding to two vanadyl complexes and demonstrates the mirror symmetry of the crystal lattice itself. From this, it is expected that the V02+ions enter the substitutional positions of the Zn2+ions. The populations of the observed SHF lines are found to be 1 : 6 : 9 : 6 : 1. Hence the five line SHF interactions correspond to four protons in the surrounding HzO nuclei. The ligand hyperfme splitting parameter A ~ M has been computed as 4x104 cm-'. This is in good agreement with the values of V02+ligand hyperfine separation existing in literature [2, 16, 171. The most probable ligand nuclei that are responsible for SHF interaction are from HzO ligands. The linewidth (2.2 G) as well as Asm of the ligand hyperfine lines is in good agreement with those of observations [ 14, 16, 171where the metal ion is surroundedby SO:and/or the HzO ligand molecules. EPR spectra of V02+ions doped PZCST have been studied at various temperatures fiom 103-373 K. No appreciable linewidth variation is observed in both hyperfine (hf) and superhyperfine (shf) lines, whereas the intensities of both hf and shf are found to vary linearly with temperature and is in accordance with the Boltzmann law [ 181 as expected in most of V02+complexes in diamagnetic host lattices. 5. CONCLUSIONS
(i) EPR spectra of V02+ions in PZCST have been studied in the temperature range 103373 K. Four vanadyl complexes have been identified among which, two sets of each are found to have distinct orientations. The detailed EPR analysis indicates that the two sets are magnetically inequivalent and occupy the Zn" substitutional site in the lattice. (ii) From the spin-Hamiltonian parameters and the spectral features, the lattice symmetry is found to be monoclinic.
212
(iii) The molecular orbital coefficients have been calculated fiom EPR and optical data. It is found that the inplane o-bonding is ionic, whereas the out of plane n-bonding is significantly covalent in nature. (iv) A five line superhyperfine structure patterns were observed for certain orientations of the V02+:PZCST single crystals with ratios 1: 6 : 9 : 6 : 1. These have been attributed to four surrounding protons of H20 ligands in presence of SO4 environment. (v) The angular variation of the EPR spectra indicate that only a single set of octet lines exhibit superhyperfine structure in the xy plane whereas two octet hyperfine lines exhibit shf interaction in the yz and zx planes. This shows the mirror symmetry of monoclinic crystal lattice. (vi) The intensities of the HF and SHF lines are found to be linear with temperatures in accordance with the usual Boltzmann law.
Acknowledgements: One of the authors KVN is highly thankful to CSIR, New Delhi for the award of Research Associateship.
REFERENCES 1. Prem Chand, V.K. Jain, G.C. Upreti, Magn. Reson. Rev., 14 (1988) 49. 2. Geetha Jayaram, V.G. Krishnan, Phys. Rev.(B), 49 (1994) 271. 3. G.J. Edwards, O.R.Gilliam, R.H. Bartram, A. Watterich, R. Voszka, J.R. Niklas, S. Greulich-weber, J.-M. Spaeth, J. Phys. Condens. Matter., 7 (1995) 3013. 4. S. Waplak, W. Bednarski, I.V. Stasyuk, J.Phys. Condens. Matter., 10 (1998) L 373. 5. H. Kalkan, F. Koksal, Solid State Commun., 105 (1998) 307. 6. R. Tapramaz, B. Karabulut, F. Koksal, J. Phys. Chem. Solids, 61 (2000) 367. 7. N.O. Gopal, K.V. Narasimhulu, J. Lakshmana Rao, Physica B, 307 (2001) 117. 8. P. D. Robinson, J.H. Fang, Y. Ohya, Amer. Miner., 57 (1972) 1325. 9. K.V. Narasimhulu, C.S. Sunandana, J. Lakshmana Rao, J. Phys. Chem. Solids, 61 (2000) 1209. 10. H. Linstedt, Naturwissenshaften, 38 (1951) 476. 11. Wolfgang Voigt, ICSD Code No. 26003, Freiberg, Germany, Private Coinmun., January 2000 (e-mail :
[email protected]). 12. A. Abragam and B.Bleaney, Clarendon Press, Oxford, 1970, p. 175. 13. D. S. Schonland, Proc. Phys. Soc., 73 (1959) 788. 14. A. Kasi Viswanath, J. Chem. Phys., 67 (1977) 3744. 15. D. Kivelson, S.K. Lee, J. Chem. Phys., 41 (1964) 1896. 16. S. Waplak, Acta Phys. Pol. A, 86 (1994) 939. 17. V. K. Jain, P. Venkateswarlu, J. Chem. Phys., 73 (1980) 30. 18. J. A. Weil, J. R. Bolton, J. E. Wertz, Electron Paramagnetic Resonance, Vol. 1, Wiley Intersci., NY, 1994, p. 176.
EPR in the 21'' Century A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
213
EPR study of several Cr3+centres in K2MgC14 single crystal H. Takeuchi", H. Tanakab, M. Mori', H. Ebisud and M. Arakawae aDepartment of Advanced Science and Technology, Toyota Technological Institute, Tenpaku-ku, Nagoya, 468-85 11, Japan bFaculty of Science, Tokyo Institute of Technology, Meguro-ku, Tokyo, 152-0033, Japan 'School of Science and Informatics, Nagoya University, Chikusa-ku, Nagoya, 464-8601, Japan dDepartment of Electrical & Computer Engineering, Nagoya Institute of Technology, Showa-ku, Nagoya, 466-8555, Japan eDepartment of Materials Science & Engineering, Nagoya Institute of Technology, Showa-ku, Nagoya, 466-8555, Japan EPR measurements have been made at room temperature on K2MgC14 crystals doped with chromium. Spectrum observed has been analysed by direct dia onalization method of spin-Hamiltonian matrix. Ligand octahedra in the uncompensated Cr centre are compressed tetragonally along the c axis although those in the pure matrix are elongated. By the use of the spin-Hamiltonian separation anal sis, other low-symmetry centres are ascribed to the Cr3+ ions associated with the nearest Mg2v' vacancy, the nearest K' vacancy and the next nearest K+ vacancy.
q+
1. INTRODUCTION KzNiF4-like crystals A2MX4 with the space group I4/mmm are interesting in their low symmetry properties due to two-dimensional structure composed of AX and MX3 layers as shown in Figure 1. In the fluoride crystal K2MgF4 the anion octahedron is tetragonally compressed about 1% along the axis with the Mg-F distance 1.9628 along the axis and 1.9848 along the axis [l]. It was confirmed from the negative values of and gll-g, parameters obtained by EPR study of the uncompensated Cr3+ centre in the crystal that the fluorine octahedron surrounding a Cr3+substituted for a Mg2+ion is compressed tetragonally along the axis [2]. Together with the uncompensated centre, several charge-compensated Cr3+ centres were found in K2MgF4, where the spin-Hamiltonian separation (SHS) analysis was proposed to identify the observed centres by separating the ligand configuration [2]. Later, this SHS analysis has been extended to the Fe" and Gd3+ centres in K2NiF4-like fluorides [3-61. Rudowicz [7,8] proposed a net charge compensation (NCC) model and applied it to the orthorhombic Cr3+centres in layered perovskite fluorides. We showed that
214
(a)As-growncrystal
(b)X-rayed crystal
0
Figure 1. Unit cell of A2MX4 crystal.
1
2 3 4 H(k0e) Figure 2. EPR spectra at 290K with
5
11 c.
the NCC parameter b2’2 expresses the deviation of the “separated tetragonal configuration from that for the uncompensated centre [4]. It was shown using the electrostatic theory for a 3d3 ion at orthorhombic sites [9] that the two “separated” axial terms in b2(1: and b2(2: obtained by the SHS analysis from the orthorhombic fine structure terms with lbt/b21<1 can be regarded as the axial fine structure terms for the tetragonal centres about they and x axes, respectively. The SHS analysis is useful to identify the magnetic impurity centres with low symmetry. In this paper we report the results of EPR measurements on C?’ centres formed in K2MgCI4at room temperature. To our knowledge, no EPR studies have been reported for magnetic impurity centers in K2MgCI4 crystal. Unlike in fluorides, ligand octahedron of isomorphous chloride K2MgC14 is reported to be elongated about 1% along the c axis with the Mg-Cl distances 2.492a along the c axis and 2.470a along the axis [lo]. The spin-Hamiltonian parameters are determined by the matrix-diagonalization method for several kinds of centres observed at room temperature. 2. EXPERIMENTAL PROCEDURES AND RESULTS
Single crystals doped with chromium with a nominal concentration of 1 mol% were grown using the Bridgman technique. EPR measurements were made at room temperatures using a X-band spectrometer with 100kHz-field modulation. Depopulation effects are observed at 1.6K using a Q-band spectrometer to determine the signs of b 2 parameters. The X-ray irradiation were performed using a Cu tube operating at 40kV and 20mA. A recorder trace of EPR signals with H I / c at 290K is shown in Figure 2(a) for an as-grown crystal. Signals from two tetragonal centres (centres I and V), a monoclinic centre (centre 11) and an orthorhombic centre (centre 111) of Cr3’ ions are marked with roman numerals. Figure 2(b) shows the recorder trace of EPR signals with H 11 c at 290K after X-ray irradiation of the sample for 7h at room temperature. The signals from the centre were weakened by irradiation. The signals from the centre I were enhanced significantly. The signal intensities from the centres I1 and V were almost unchanged. After irradiation new strong signals from a centre with S = 512 appeared. In Figure 3 signals observed at 290K are
215
5
4
8& *3 2 -1
30 60 90 Angle(deg.) Figure 3. Angular variation of spectrum with H in (0 10) plane. 0
Figure 4. Principal axes directions for the centres (a)I , V, @)I1and (c)III.
plotted against external field direction in a plane including the and (90") axis. Open circles, open triangles, open squares and closed circles indicate respectively the centre I, the centre 11, the centre 111and the centre V. The spectra from the Cr3+ions in the centres I, I11 and V were described by the following spin Hamiltonian with S=3/2:
ft=gxI-l
Hx+gy1-1 BS,H, +gz1-1 BSZ& + (11 3)( b?
b:
0221,
(1)
where 0; and 0; are the Stevens operators given in Ref.[ll]. For the centers I and V with tetragonal symmetry, 622-0. The principal z axes are parallel to the axis and the x axes are parallel to the axis for these centers (Figure 4(a)). We denote the components of g tensor as gll=g, and gi= g,=g,,. For the centre 111, the axis is selected to be parallel to the axis direction where the spectrum shows maximum splitting. The x axis is selected to be parallel to the b axis (Figure 4(c)). The spectrum from the monoclinic centre I1 showed maximum splitting in a declined direction from the c axis in the (-110) plane. Spin Hamiltonian used to fit the spectrum from the centre I1 is selected to satisfy the conditions lb?/b?1<1 and vanishng bz'. This means that we describe the spectrum by the spin Hamiltonian tensor (1) in its principal axes system. The from the c axis in the main principal axis (z axis) direction is declined by an angle (-110)-symmetry plane (Figure 4(b)). The z-axis direction coincides with the hrection where the spectrum shows the maximum splitting. The angle can be determined experimentally as shown in Table 1. Using the principal axes system, the spectrum from the centre I1 was described well within the experimental errors by the spin Hamiltonian (1). Spin-Hamiltonian parameters obtained for the Cr3+centres at 290K are listed in Table 1. All the absolute signs of the b? parameters were determined by depopulation effect at 1.6K.
216
Table 1. Spin-Hamiltonianparameters for K2MgC14:Cr3+. brim in units of lo4 cm''. Centre gz 9x SY b? b22 (deg) 1.9831(3) 1.9846(2) 1.9846(2) - 583.5(3) Centre I Centre 1.9832(2) 1.9643(4) 1.9752(6) - 851.3(5) +578.3(8) 16.29(1) Centre lJI 1.9867(3) 1.9828(5) 1.9845(5) +455.1(5) +387.8(5) Centre V 1.9821(2) 1.9862(2) 1.9862(2) -855.5(3) 3. DISCUSSIONS
3.1 Centre I The signal intensities of the tetragonal centre I were enhanced largely by X-ray irradiation. This fact shows that in the as-grown crystal large of chromium may exist in the Crz' state at the Mg2' site although no EPR si als from any Cr2' centre have been observed even at 1.6K. A C3' ion changes into a Cri'on by losing one electron due to the irradiation and another C?' ion changes into a Cr" ion by capturing the electron [12]. We can therefore safely ascribe the centre I to an uncompensated Cr3+ion substituted for a Mg2+ion. The new centre with S=5/2 and lb:1=75.4 X 104cm-' observed after irradiation may be the uncompensated Cr' centre at Mg2+site. We may say from negative value of b: for the centre I that ligand octahedron is compressed along the c axis in the substitutional Cr3' centre in spite of tetragonal elongation (about 1% ) of octahedron in host crystal. Metal-ligand distances in cubic perovskite A M F 3 and those in layered perovskite A2W4 have almost the same values in the c plane. Bond length in the MX layer in A2MX4 is determined almost by the kinds of M2' and X- ions. Ligand ions in the plane may not relax so much in the impurity centre. On the contrary, two ligand ions on the axis may relax toward the Cr3' ion more easily. The ligand octahedron in the centre I therefore distorts in compressed configuration mainly due to attractive interaction of trivalent central ion with two ligands on the axis irrespective of elongation or compression in the pure matrix. 3.2 Centre 11 As the main principal axis (z axis) is declined from the crystalline c axis in the (-110) plane in the centre 11, some charge compensator may exist in the (-110) plane. Most probable compensator is a nearest K' vacancy at the [ l l l ] or [-1-11] site. Therefore, we separate the fine structure terms approximately into two uniaxial terms as follows:
where the respective axes are indicated explicitly in parentheses. In Eq.(2), 11 c and the z" axis is assumed to be declined by an angle 4 =*cos-'(l/J3)=k54.7" from the c axis toward the [110] axis. Using the transformation properties of the Stevens Operators [13], we express Eq.(2) in the x'y'z'-coordinate system where z' 11 c and x' 11 [110]. When we equate the coefficients of 020,02',022 of both sides in Eq.(2), the following relations are obtained: b2(1?=[b:(3~os2
-l)+b22(sin2 )]/2-[3b2sin2 +b?(cos2 +1)] (3cos2 -1)/(6sin24 ), (3)
217
b2(2:= sin2
[3 b;sin2 = (3
+b?(cos2 + 1)]/(3 sin2 ), sin2 /(3b2(2:).
The parameters b2(':, b2(2:, 4 can be calculated from the experimental values of b?, b2 and But within the framework of fixed value of r$=+54.7', above relations hold only approximately. Using the experimental values of b?, and a=16.29 ', we obtain from Eqs.(3) and (4) b2S? = -728 X lo4 cm-' and b2(2:= +455 X lo4 cm-'. The b2(1: has the same sign that of b2 for the center I and its magnitude is a little larger. Using above b2(2:, approximate relation (5) predicts that positive is obtained for negative 4 (-54.70). So, the compensator may be the nearest K' ion at [-1-11] site (Cr3"-V, centre). The signal intensity of this centre may be almost unchanged by X-ray irradiation because the excess monovalent positive charge on Cr3' is compensatedjust by excess monovalent negative charge on the K' vacancy (just compensation case). Thus, in spite of farely different values of and ,the centre I1 can be identified to be the same kind of Cr3+centre as the Cr3'-V,~ centre in K2MgF4 = -1861 X 10-4cm-', =43.1'). A characteristic difference in K2MgC14 is the positive value of b2(2: and therefore the vacancy is in the opposite direction to the axis relative to the c axis. The electrostatic model [14] within the 4F term predicts that the contribution to the parameter for the Cr3+-V, centre in KZnF3 from the Kf vacancy is positive and the contribution from the compressed ligand octahedron is negative. The positive b2(2: in the resent centre in K2MgC14 suggests the smaller effect of the ligand angular distortion on b2(2)8 than the effect by the point charge on the K+ vacancy. 3.3 Centre The fine structure terms in the spin Hamiltonian for the orthorhombic centre I11 can be separated into two uniaxial terms about they and x axis as follows
where and are defined in the xyz-coordinate system defined as shown in Fig.4(c). Equation (6) holds when the following conditions are satisfied: by:
=
1/3)b;,
b2(2:
= -b?+(
1/3)b?.
(7)
Using the experimental values for and we obtain -584 X lo4 cm-' for b2(1: and -326 X 104cm-' for b2(2:. The b2(1)Ovalue is in good agreement with b:= -583.5 X 10' cm-' for the uncompensated centre. This shows that the centre I11 is strongly perturbed by some charge compensator along the x axis. Most probable charge compensator is a nearest Mg2+ vacancy on the x axis. It results effectively in the increase of negative surrounding charge along the x axis relative to the assumed regular octahedral distribution. Also the negative surrounding charge on the c axis may increase due to the compression along this axis. Both negative charges effectively result in a positive char e along the axis relative to regular octahedral distribution of ligands. Positive value of b2 for the centre I11 with 1 1 4 is consistent with this ligand configuration elongated along the axis. Hence, the centre I11 can be ascribed to a substitutional Cr3' ion associated with a nearest M T vacancy along the x axis. Although the centre 111 is dominant in the as-grown crystal, signal intensity of this
218
centre decreases largely after X-ra irradiation. The instability may be explained by its over-compensated charge on the Mg vacancy. 3.4 Centre V The spin Hamiltonian for the tetra onal centre V can be separated into two uniaxial terms about the axis b: b:(I) 4 + b2(2: 020, where the coordinate system is defined as shown in Figure 4(a) and b:(I)= -583.5 X lo4 cm-' for the centre I. This holds when b:= b:(I)+ b2(2:. Then, we obtain b2(2:= -272 X lo4 cm-' for the perturbation in the centre V. There may exist some charge compensator along the axis. The perturbation for the uncompensated centre is smaller than that by the Mg2+vacancy (b2~: = -326 X 104cm-'). Most probable compensator is a next nearest K" vacancy lying on the c axis. This compensator may repulse the intervening chlorine toward the Cr3+ ion, resulting in enhancement of negative charge along the c axis effectively and therefore the stronger compression of octahedron along the c axis. As the charge compensator is monovalent in this ~ case, the perturbation may be smaller than that in the divalent case of the Cr3'- V M centre. Hence, we can ascribe the centre V to a Cr3+ionassociated with a next nearest K+ vacancy on the axis (Cr3+-V& centre). The signal intensity of this centre is stable during X-ray irradiation.
4
4. CONCLUSIONS
It was found from EPR results for K2MgC14 doped with chromium that there exist several Cr3+centres.These centres were identified by spin-Hamiltonian separation analysis and X-ray irradiation as an uncompensated Cr3' centre and three Cr3+centres associated with the nearest M e vacancy, the nearest K+ vacancy and the next nearest K' vacancy. In the uncompensated centre ligand octahedron is compressed along the c axis. REFERENCES 1. Yamaguchi, Researches of the Electrotechnical Laboratory, No.734 (1972) 1. 2. H. Takeuchi, M. Arakawa, H. Aoki, T. Yosida and K. Horai, J. Phys. SOC.Jpn., 51 (1982) 3 166. 3. H. Takeuchi, M. ArakawaandH. Ebisu, J. Phys. SOC.Jpn., 56 (1987)4571. [Errata, 59 (1990) 22971. 4.M.Arakawa, H. Ebisu and H. Takeuchi, J. Phys. SOC.Jpn., 57 (1988) 2801. 5. H. Takeuchi, M. Arakawa and H. Ebisu, J. Phys. : Condens. Matter, 3 (1991) 4405. 6. H. Takeuchi, H. Ebisu and M. Arakawa, J. Phys. SOC.Jpn., 60 (1991) 304. 7. C.Rudowicz, Phys.Rev., B37 (1988) 27. 8. C.Rudowicz, Solis State Commun., 65 (1988) 63 1. 9. H. Takeuchi and M. Arakawa, J. Phys.: Condens. Matter, 6 (1994) 3253. 10. C. S. Gibbons, V. C. Reinsborough and W. A. Whitla, Can. J. Chem., (1975) 114. 11. A.Abragam and B.Bleaney, "Electron Paramagnetic Resonance of Transition Ions", (Oxford: Clarendon Press, 1970). 12. J.J.Davies andK.Horai, J. Phys.C: Solid St.Phys., 4 (1971) 671. 13. C.Rudowicz, J. Phys.C: Solid St.Phys., 18 (1985) 1415. 14. H.Takeuchi, Research Bulletin (The College of General Education, Nagoya Univ.), B37 (1993) 11.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
EPR study of Cr3' centres in T12MgF4
219
TI2ZnF4crystals
M. Arakawaa,H. Ebisuband H. Takeuchi aDepartmentof Materials Science and Engineering, Nagoya Institute of Technology, Showa-ku,Nagoya, 466-8555, Japan Deparbnent of Electrical and Computer Engineering, Nagoya Institute of Technology, Showa-ku, Nagoya, 466-8555, Japan Department of Advanced Science and Technology, Toyota Technological Institute, Tenpak~-k~, Nagoya, 468-85 11, Japan Room-temperature EPR measurements have been made on as-gown single crystals of T12MgF4 and T12ZnF4. In the T12MgF4 crystals doped with C3', a spectrum with tetragonal symmetry (centre I) is observed. In crystals of T12MgF4and T12ZnF4 d o p e d with C3' and Li', signals h m a new centre with orthorhombic symmetry (centre IV) is observed together with those &om centre I. The spectra of centre I in both crystals exhibit anomalously large fine structure splittings, about two times larger than those in K2w4 and Rb$vlF4 (M=Mg, Zn). From s in Hamiltonian - associated separation analysis, centres IV ascribed to the C3' ions substituted for a MP+ ion with Li' at the nearest M2" sites in the c plane. This result shows that centres I are uncompensated centre in spite of the anomalously large fine-structure splittings.The other separated parameters for centres IV also discussed by comparing with those observed in other perovskite crystals.
1. INTRODUCTION KzNiF,-like layered perovskite crystals with space group I4/mmm are interesting of their close relationship to the KNiF3-likecubic perovskite crystals. Figure 1 shows the unit cell of the structure, which has a two-dimensional network of M F 6 octahedra sharing comers in contrast with the three-dimensional network in cubic perovskite structure. It is well known that the structure of ABF3 crystals is classified by the tolerance factor, Crystal structure is expected to change at about e l .OO &om the cubic perovskite structure to the hexagonal BaTiO3-type one with increasing the tolerance factor [l]. On the other hand, A2BF4 crystals keep the layered p v s k i t e over the wide range of the tolerance fxtor. The crystals of TI2MgF4and T12ZnF4crystals have the layered perovskite structure in contrast with the hexagonal structure for TMgF3 and T W 3 (F1.01)crystals. In ABF3 crystals the C2' ions known to substitute for host divalent cations. Several kinds of tetragonal C?" centres be formed in these crystals codoped with C3' and Li', where the excess positive charge on the Cr3' ion is just com locally by a Li" (the Clz'-Li' centre) at the nearest B2' site [2,3]. In A2BF4 crystals, C p i o n s are expected to substitute for B2+ions,
220
Figure 1. Unit cell of T12MF4crystals.
Figure 2. EPR spectra at room t e r n p e w .
similar to the case for ABF3 crystals. As reported in previous works, in the -doped crystals with C? and Li+ ions, EPR spectra of the Cr3'-V, and the C?-Li+ centres with orthorhombic symmetry have been observed [4,5]. For these orthorhombic centres the relationship between the parameters &"' (m=0,2) and the local environments around C?' ions were anaiysed using the spin-Hamiltonian separation method [6J by separating the fine terms in the spin Hamiltonian into two uniaxial terms along the c and along the C1-3'- Li' pair direction in the c plane. Each separated parameter found to be comparable with the uniaxial parameter for the corresponding uncompensated centre observed in the same host or the uniaxial parameter for the C?+- Lif centre in the corresponding ABF3 In paper, we report EPR results for the C? centres observed in T12MgF4 and TIzZnF4 single crystals. In TIzMgF4 crystals doped with C? the spectrum with tetragonal symmetry (centre I) was observed. In crystals co-doped with C1-3' and Li' a new spectrum with orthorhombic symmetry (centre IV) was observed. In TlzZnF4 both the spectra of the centres I and observed in codoped crystals. The spectrum of the centre I in TlzMF4 (M=Mg, have an anomalous fine structure splitting about two times larger than those of the uncompensated centres in K2MF4 and Rb&F4. For the centres IV the second-mnk fine structure terms will be d y s e d using the spin-Hdtonian separation method. The separated parameters will be discussed by comparing with the fme structure parameter for centre I in the same host crystal and with those for the corresponding centres observed in cubic perovskite 2.
PROCEDURES AND RESULTS
Single crystals of TI2MgF4 and Tl2ZnF4 were grown in graphite crucibles by the Bndgman technique. CrF3 were added to starting mixtures of MgF2 (or ZnF2) and TlF. For some crystals, powders of were added to the starting mixtures. The crystals obtained are cleaved easily in the c plane. The measurements were made at room temperature using a ESR spectrometer operating in the X band (JES-FElXG) at the Center for Instrumental Analysis Nagoya Institute of Technology.
22 1
'
1
n
$
2.5
2.0
Figure 3.
variation of the signals I and
marked in Figure 2 with H in the (001) plane.
Signals from magnetic centre with S=3/2 (centres I) were observed in T12MgF4 doped with C? ions. Figure 2(a) shows a recorder of the EPR signals with H 11 c at room temperature. Figure 2(b) shows a recorder trace of the EPR signals for the Li+-cedopedcrystals. Signals h m a new centre (labeled IV) appeared together with those h m the centre I(labe1ed I). For an TI$ZnF4 crystal co-doped with C? and Li*, signals h m the centres I and observed similarly to the case of TI2MgF4. In Figure 3 signals observed in d o p e d T12MgF4 plotted against external field direction in the (001) plane by open circles for the centre I and by open for the centre Signals fiom the centre I show no fielddirection dependence in the c plane and shows the maximum fine structure splitting in the [OOl] direction. This indicates that the centre I tetragonal symmetry about the crystalline c axis. The spectra of the centres I can be described by the following spin Hamiltonian with tetragonal symmetry: ?# = g 11pHSz+ gLp(Hx&+HyS.) + (1/3)
(1)
The spin Hamiltonian is described in the coordination system where the principal z and x axes respectively chosen to be parallel to the crystalline c and a axes. For the centre IV there are three sets of two branches. Each set shows maximum fine structure splitting in the [1001 direction and crossover at the [1 101 direction shown in Figure 3. It was confirmed each branch also has crossover point at the [OOl] direction. This indicates that the centre orthorhombic symmetry. From this symmetry of the spectrum the C? centre has some charge compensator along the crystalline axis in the c plane. The spectra of the centres IV can be described by the following spin Hamiltonian:
222
Tablel. Spin-Hamiltonian parameters for several kmds of Cr3' centres observed in T12MgF4 and TI2ZnF4crystals. Units are in 104cm-' for b: and b?. Crystal
Centre
g,
T12ZnF4
I IV I IV
1.9724(6) 1.971(1) 1.9732(4) 1.972(1)
T12MgF4
gz
QY
1.9724(6) 1.970(1) 1.9732(4) 1.9712(7)
1.9715(6) 1.9734(8) 1.9717(4) 1.9733(6)
6:
622
-866.1(5) +684.6(7) -1041.7(2) +843.6(6)
+529(1) +598( 1)
The z is chosen to be parallel to the crystalliie a having maximum f i e structure splitting. Thex is chosen to be parallel to the b axis. The spectra observed were fitted to the spin Hamiltonian by the matrix diagonalization method. The signs of bofor the centre I selected to negative the values of g//- g1 negative [4,5]. Then, the relative signs among parameters for the centres IV can be uniquely determined. The positive signs of bofor the centres IV selected the values of g& + , gJ2 positive. The spin Hamiltonian parameters obtained listed in Table 1. Dotted curves in Figure 3 show the theoretical resonant field calculated using the parameters in Table 1. Good agreement of the calculated values of the resonant fields with experimental ones is obtained. 3. DISCUSSIONS
3.1. Spin-Hamiltonian separation analysis The orthorhombic symmetry of the centres IV shows that these centres are perturbed by some charge compensators existing in the c plane. Here, we to analyse the orthorhombic centres IV using the spin-Hamiltonian separation method. We separate the second-rank f i e structure terms into an uniaxial term with the parameters parameter along the crystalliie c axis (the y axis) and into an uniaxial term with bza(2) parameter along the crystalline b axis (the x in the c plane. Then, the fine structure terms in the coordinate system can be separated follows: + b2O?(x,y) = b2a(I)O2OO)+bM2)oZ0(X).
The b2a(@;(y) denotes the uniaxial term along the y axis, where Quation (3) is valid when the following conditions are satisfied: bl)=-b?-(1/3)b22,
hl,=-b20+(1/3)b2.
(3)
0 2 0 =3q2-S(S+l). (4)
We can calculate the separated axial parameters b ~ land ) b2a(2) from the experimental values of bo and h2in Table 1. Results listed in Table 2 together with those for other layered perovskite fluorides reported previously. 3.2. Centre I The parameters b: for T12h@4(M=Mg, Zn) about two times larger than that for RbMF4 although the lattice parameter a is almost the same for both crystals shown in Table 2. of the anomalously large magnitudes of the bofor TlzMgF4 and TlzMgF4, it may be considered that some kind of charge compensator exists nearby on the c axis in this of C?'
223
Table 2. Values of &I) and h2)derived for the centres and bofor the centre I in several layered perovskite fluorides. Units in104cm-' for &&V) and b 2 @ 9 . Crystal KzMgF4 T12MgF4 RbMgF4 KzZnF4 T12ZnF4 RbM4
.(a) 3.977 4.007 4.055 4.055 4.105 4.136
c(
a)
13.160 14.430 13.790 13.096 14.100 13.706
c/u
3.31 3.60 3.40 3.23 3.43 3.31
b?(I)
b2.4V)
-419 -1041.7 -526.1 -381 -866.1 -369.0
-413.7 -1042.9 -564.0 -386 -860.9 -410.8
b2~2)(IV) Reference -470.3 -644.3 -610.2 -374 -508.3 -455.4
This work [51 [21 Thiswork [41
centre. However, the sepamted parameters for centre in good agreement with those of the bofor centre I in T12MF4 shown in Table 2. This result shows that the centre I can be to a CIf' ion substituted for the Zn2* site without any charge compensators in its immediate neighbourhood. Thus, the spin-Hamiltonian separation analysis is useful to idenw newly observed centres and to investigate the relationship among the centres formed in crystals. The ionic radius of T1+(1.47A)is the same that of Rbf(1.47A). The difference of the bofor TlzMF4fiom that for RbMF4 arises from the difference of the monovalent cation. In host crystals the characteristic of T1+ion in the lattice parameter c. The ratio of the lattice pamneters c/u for T12MF4 is larger than that for RbMF4, seen from Table 2. The fine structure bo for the uncompensated centre is significantly dependent on the ratio c/u in the layered perovskite fluorides.
33. Centre It must be emphasized that TlMF@l=Mg, Zn) do not possess the cubic perovskite structure but rather the hexagonal structure at room temperatures. However, using the spin-Hamiltonian separation analysis, we can estimate the uniaxial parameters for the Li+-associated centres formed in virtual cubic perovskite TlMF3 crystals by the separated parameters h 2 ) for the centres IV in Tl2MF4. The values of &2) for the centre IV in Tl2MF4 is close to those obtained previously for the Li+ centres (the centre in other layered perovskite fluorides seen fkom Table 2. From this fact it may be safely said that the centre T12MF4 is to a C? ion associated with a Li+at the nearest M2+site in the c plane. 4. CONCLUSION
In TI2MF4@4=Mg, Zn) crystals d o p e d with Clz' and Li', strong signals fiom the tetragonal C? centre have been observed together with those from the orthorhombic C? centre having weak intensity. The spectra of the tetragonal centers in both crystals have an anomalously large fine structure splittings, which times larger than those in &MgF4 and RbZnF4. For the orthorhombic centres the fine structure parameters analysed using the spin-Hamiltonian separation method. The separated parameters &I) in good agreement with the for the tetmgonal centre in the same host crystals. The results show that the tetragonal centre has been identified to be the uncompensated centre without any charge compensators nearby. The large magnitude of bofor T12MF4 is considered to come fiom some effect of T1' ion. It clarified that the parameter for the uncompensated centre varies with depending on the of the lattice
224
parameters c/u. For the orthorhombic centre, newly observed TlzMF4 cod0 C? and Li', the separated parameter is found to have close value to those for the C Li' centre obtained previously in the other layered perovskite fluorides. For reason, the orthorhombic centre is ascribed to the C? ion associated with a Li' at the nearest M2+site in the c plane. REFERENCES
1.D. Babel, Struct. Bonding, 3 (1967) 1. 2. H. Takeuchi and M. Arakawa, J. Phys. Soc. Jpn., 52 (1983) 279. 3. H. Takeuchi and M. Arakawa, J. Phys. Jpn., 53 (1984) 376. 4. M. Arakawa, H. Ebisu and H. Takeuchi, J. Phys. Jpn., 55 (1986) 2853. 5. M. Arakawa, H. Ebisu and H. Takeuchi, J. Phys. Jpn., 57 (1988) 2801. 6. H. Takeuchi, M. Arakawa, H. Aoki, T. Yosida and K. Horai, J. Phys. Jpn., 51 (1982) 3166. 7. H. Takeuchi and M. Arakawa, J. Phys.: Condens. Matter, 6 (1994) 3253.
EPR in the 21” Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
225
Single crystal EPR study of Cr(II1)-doped magnesium potassium Tutton’s salt H. Anandalakshmi, P. Neeraja, R. Venkatesan, T. M. Kajendiran and P. Sambasiva Rao* Department of Chemistry, Pondicheny University, Kalapet, Pondicheny - 605 0 14, India.
Single crystal Electron Paramagnetic Resonance studies of Cr(II1) doped into magnesium potassium sulphate hexahydrate have been investigated at room temperature. Even though, the host lattice contains two molecules per unit cell, a large number of resonances have been observed. These resonances have been explained on the basis of charge compensating vacancies in the lattice, as a trivalent chromium ion has been incorporated in place of a divalent magnesium ion. The number of vacancies depends on the dopant concentration and analysis has been done for a low concentration system. The zero-field splittings have been estimated for four such vacancies and correlated with the direction cosines of the vacancies. The powder spectrum indicates a minimum of six vacancies and the D values have been compared with the single crystal data. The resonances observed for lower concentration complex are present in the higher concentration one also.
1. INTRODUCTION In continuation of our work to understand the effect of doping of paramagnetic ions such as Cu(I1) [l], Mn(I1) [2], Fe(II1) [3] and VO(I1) [4] into a Tutton’s salt, MgKz(SO&.6H20, magnesium potassium sulphate hexahydrate (MPSH), we are reporting here the EPR results of yet another ion. Jahn-Teller distortion was observed for Cu(I1) ion in MPSH in elongated octahedral environment with a :-d: ground state even though the diamagnetic magnesium ion in MPSH is in a compressed state. However, when Mn(I1) impurity is incorporated into MPSH [2], the results indicate that the zero-field splitting parameter (D) is very large and shows a linear temperature dependence behavior between 77 and 370 K. In this case, the tetragonal distortion is along one of the Mg-0 bond directions. Further, the Fe(II1) results suggest that these trivalent ions are subjected to a strong tetragonal crystal field from the six oxygen-water molecules. The various possible charge-compensating vacancies superposed on this tetragonal crystal field dictate low symmetry. Also resonances from all the three Kramer’s doublets have been observed and successfully analysed [3]. When VO(I1) was
226
doped into MPSH, the results indicate that the impurity enters the lattice both substitutionally and interstitially with the direction of vanadyl ions at approximately right angles to each other [4]. The calculated admixture coefficients match fairly well with the reported values. In addition, in one of our previous communications, we have noticed that the number of sites seems to depend upon the dopant concentration, when Cr(II1) was incorporated into hexaimidazole zinc@) dichloride tetrahydrate (HZDT) lattice Hence, we have decided to dope Cr(II1) ion in the host lattice MPSH in order to study its influence on the nature of charge compensating vacancies through EPR spectroscopy. In the present investigation, our aim is two fold: first to find out the effect of charge compensating vacancies on the nature of the spectrum and second to study the effect of dopant concentration on the number of sites. Cr(II1) has been studied quite extensively by EPR spectroscopy in octahedral crystal fields due to its importance as MASER ion in ruby as well as tanning agent in leather industry [5-81. The studies are important and interesting when this ion is introduced into a divalent host lattice site. From our laboratory, we have reported a few systems when Fe(II1) and Cr(II1) are incorporated into a diamagnetic divalent host lattices [3,5,9]. In all these case, the charge compensating vacancies play an important role in deciding the value and orientation of zero-field tensors. 2. EXPERIMENTAL Single crystals of Cr(II1) doped into MPSH were grown by slow evaporation of a saturated solution containing equimolar amounts of MgSO4 and K2SO4 [lo]. To this solution, a small amount of the paramagnetic impurity (say 1 to is added in the form of chromic sulphate. Good colored crystals were obtained within two weeks. EPR spectra were recorded on a JEOL JES-TE100 ESR spectrometer operating at X-band frequencies, having a 100 KHz field modulation to obtain a first derivative EPR spectrum. DPPH with a g value of 2.0036 was used for g-factor calculations. Angular variations were performed at room temperature by rotating the crystal along the three mutually orthogonal axes a, b and c*. 3. CRYSTAL STRUCTURE
Tutton’s salts have the general formula M”M2’(X04)2.6H*O, where M” is a divalent cation like Co, Cu, Ni, Mg, Zn; M’ is a monovalent cation like K, Cs, Rb, NH4 and X is S or Se. Tutton’s salts have monoclinic crystal structure with space group P21/,. The lattice parameters of MPSH (M’ is Mg, M’ is K and X is S ) are: a = 0.613, b = 1.223, c = 0.909 nm, p = 104.78’ and Z = 2 [lo]. Six water molecules in the form of a compressed octahedron surround the magnesium ion in MPSH. In all Tutton’s salts, the shortest Mg-0 bond is unique, whereas the longest bond depends on the nature of M’ and M’.
4. RESULTS AND DISCUSSION Single crystals of MPSH doped with Cr(II1) with proper axes and size are selected for EPR measurements. During the crystal growth, Cr(II1) impurity is introduced in
227
percentage molar concentration of 1 to 5. When crystals are selected with different concentrations, we observed different spectra, a situation observed earlier when Fe(II1) was introduced into the same host lattice [3] and Cr(II1) was doped in HZDT [5]. Hence, two single crystals are selected, one with low concentration (Type I) and another with high concentration, say 5% (Type 11). A typical EPR spectra of Cr(III)/MPSH for type I and I1 crystals are given in Figures l(a) and (b) respectively. The weak centre multiplet lines are due to Mn(I1) impurity. The number of resonances is more in type I1 crystals. The same observation was noticed for Fe(III)/MPSH also. The angular variations of these resonances are followed in three mutually orthogonal planes for type I and I1 crystals. However, due to the complexity nature, road maps are given only for the low concentration sample. Figure 2(a) corresponds to isofrequency plot in bc* plane for type I crystals. One can clearly see four sets of lines, indicating four magnetically different sites. These split further into eight lines during crystal rotation. As expected, they are symmetrical around the centre of symmetry. However, the resonances arising from !-1/2> to !+1/2> have not been shown, as they are least variant for angular rotations. We have plotted similar plots for the other two planes also. In these cases also, we noticed four pairs of lines, indicating the presence of more than two sites, expected from X-ray data of the host lattice (Z = 2 for MPSH). As it was impossible to follow Schonland's [l 11 procedure, in a case such this, as there are 2744 (3x3) matrices, without considering the ambiguity of not performing a positive right handed rotation, we have followed a different procedure, similar to the one adoDted in our
0=.0
Figure 1: Room temperature EPR spectra of Cr(III)/MPSH in different planes. l(a) corresponds to Type I crystals in ac* plane (v = 9.08979 GHz) and l(b) corresponds to Type I1 crystals in bc* plane respectively (v = 9.09293 GHz).
Figure 2: Isofrequency plot for Cr(III)/MPSH in bc* plane. One can see a set of four lines indicating the presence of four vacancies. The angular variation of 1+1/2> to 1-1/2> is not drawn since it is almost invariant. v = 9.08353 GHz.
228
As we are doping a trivalent ion, in place of a divalent ion, for charge compensation, whenever two ions enter a lattice, three divalent ions are replaced, creating a vacancy in the host lattice. These vacancies can be anywhere in vacancies. They may be along any principle crystal axis, such as
, <010>, <001> or any other direction such as <1 lo>, <101>, <001>, 4 11> etc. We have calculated the direction cosines and distances for 15 such vacancies from the X-ray data of MPSH and are given in Table 1. In order to get the number of chemically and magnetically distinct sites, we have recorded a powder spectrum of the sample. One can notice a minimum of six sets of sites. As Cr(II1) belongs to d3 ion, the separation between the extreme sets of lines, i.e, 1-3/2> ++ -1/2> and 13/2> ++ 11/2>, in powder spectrum corresponds to 4D, where D is the zero-filed splitting parameter. From the powder spectrum, six such D values have been calculated. They are 136.5, 118.4, 100.3, 76.5, 68.9 and 41.5 mT. This powder spectrum clearly indicates the presence of charge the host lattice. For our present calculations, we have considered fifteen such compensating vacancies as we have noticed only one magnetically distinct site when divalent copper is doped [l]. Table 1 The distances and direction cosines of various charge compensating vacancies obtained from the X-ray data of MPSH [ 101 in the orthogonal framework of abc* Vacancy Distance (nm) Direction cosines a b C* 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0.6130 0.6840 0.9090 0.9580 1.0734 1.1043 1.2230 1.2726 1.3680 1S238 1S536 1.8599 2.0356 2.0521 2.1473
1.oooo 0.4481 -0.255 1 0.3979 0.0696 0.8327 0.0000 0.5404 0.4481 -0.1521 0.2454 0.1648 0.0367 0.4481 0.3203
0.0000 0.8940 0.0001 0.0001 0.5698 0.5537 1.oooo 0.4806 0.8940 0.8026 0.7872 0.9863 0.9012 0.8940 0.8543
0.0000 0.0000 0.9669 0.9174 0.8189 0.0000 0.0000 0.6907 0.0000 0.5768 0.5657 0.0000 0.43 18 0.0000 0.4093
In order to get spin Hamiltonian parameters from the isofrequency plots, the following procedure is adopted. The Spin Hamiltonian used for the purpose is H = 0.B.g.S + D[Sz2- (1/3)S(S+l)] + E (Sx2- Sy2)
(1)
229
Here, the terms have their usual meanings. We have selected four lines in each plane and labeled the pair of lines as A, B, C, D. By calculating the line positions from the EPR spectra and using the EPR-NMR program [12], we obtained the components of g and D tensors for each set of resonances. For example, in the xy (ab) plane of rotation, one can obtain only three of the six g values, i.e., g,, g, and g,, and two of five D-values, as D is a traceless tensor. The same procedure has been adopted for the other three planes of rotation. After obtaining 12 such sets, we compared the values and obtained four Dtensors. The values thus obtained are given in Table 2, along with eigen-values and eigen-vectors. The g value is almost isotropic with 1.98(3). The principal D values obtained from single crystal data agree fairly well with powder spectrum results, indicating the accuracy of single crystal analysis. Comparing the direction cosines of the principal D values (Table 2) with the direction cosines of vacancies (Table 1) indicate that these four vacancies are at a distance of 0.6130, 0.6840, 1.0734 and 1.1043 nm, i.e, within a unit cell and hence show up in the powder EPR spectrum. The agreement is very good, indicating the accuracy of our procedure. However, the other two values found in the powder spectrum could not be seen in the single crystal rotation, due to the reason that they may be beyond one unit cell distance. One can see from the isofrequency plots that a few points are not included for analysis. They may correspond to these two values. In addition, these can be found with higher intensity in higher concentration crystals. Work is in progress to analyze the EPR results of Type crystals. Table 2 D-tensors (mT) and their direction cosines, obtained from the single crystal analysis of Cr(III)/MPSH. Eigenvalues Direction Cosines a b C* D-tensor (1) -34.0 -40.6 -0.7 44.1 -0.0542 0.5959 0.8012 -9.9 2.5 20.2 0.0876 -0.7965 0.5983 43.8 -64.3 0.9947 0.1026 -0.0090 D-tensor (2) 13.8 4 . 7 20.2 52.0 0.4600 0.8819 0.1030 -27.6 4 4 . 2 4.4 -0.45 17 0.3323 -0.8280 13.7 0.7644 -0.3343 -56.4 -0.55 12 D-tensor (3) 76.4 -6.6 -41.4 37.8 -0.1114 0.6390 0.7605 -3.9 -35.0 -29.4 -0.7799 -0.5306 0.3319 -47.0 0.6158 -0.5562 10.5 0.5580 D-tensor (4) 42.7 -3.2 -41.0 68.8 -0.845 1 -0.4865 0.22 15 -43.6 -16.2 -16.2 -0.0524 0.4879 -0.8713 0.9 -52.6 0.5320 -0.7248 -0.4378
230
5. CONCLUSIONS In continuation of our work to study the effect of paramagnetic doping into Tutton’s salt (MPSH), we have obtained interesting results when Cr(II1) was introduced into the lattice. Analysis of single crystal data revealed the presence of four charge compensating vacancies, whose direction cosines indicate their distances from the central metal ion. The extra two sites found in powder spectrum may correspond to far away vacancies, which can be found in crystals, having higher dopant concentration. Acknowledgements The authors (RV, TMR and PSR) thank DST, UGC, AICTE and CSIR for financial assistance. PSR thanks Pondicheny University, Pondicheny for providing partial financial assistance to attend the Third Asia-Pacific EPWESR symposium, held on October 29-November 1,2001 at Kobe, Japan. REFERENCES 1. P. Sambasiva Rao, A.K. Viswanath and S. Subramanian, Spectrochim. Acta, Part A,
48 (1992) 1745 (Part 1). 2. P. Sambasiva Rao, Spectrochim. Acta, Part A, 49 (1993) 897 (Part 2). 3. P. Sambasiva Rao, Spectrochim. Acta, Part A, 52 (1996) 1127 (Part 3). 4. H. Anandalakshmi, T. M. Rajendiran, R. Venkatesan and P. Sambasiva Rao, Spectrochim. Acta, Part A, 56 (2000) 2617 (Part 4). 5. P. Sambasiva Rao and S. Subramanian, Mol. Phys., 39 (1980) 935 (Part 11) and references therein. 6. B. R. McGarvey, J. Chem. Phys., 37, (1962) 3020. 7. W. Low and J. T. Suss, Physics Lett., 11, (1964) 115. 8. W. Low, Phys. Rev., 105, (1957) 801. 9. P. Sambasiva Rao and S. Subramanian, Mol. Phys., 54 (1985) 415. 10. K.K. Kannan and M.A. Viswamitra, Z. Kristallogr., 122 (1965) 16. 11. D. Schonland, Proc. Phys. SOC.,73 (1959) 788.
EPR in the 21* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
ESEEM study of 14Nnuclear quadrupole resonances in chromium(II1) complex
23 1
= 3/2
Shoji Ueki and Jun Yamauchi Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, 606-8501 Japan
The 14N Electron Spin Echo Envelope Modulation (ESEEM) spectra of = 312 hexaamminechromium(III), [Cr(NH3)6I3' were measured. The zero-field splitting of the complex is smaller than the Zeeman interaction in this system, and three EPR transitions can be observed at the X-band frequency. The stimulated ESEEM spectra consist of a superposition of the nuclear quadrupole resonance (NQR) of the ligand '%. We concluded that these NQR are generated from different EPR transitions: one from the transition between = +1/2> and = -1/2>, and the other from the transition between = +1/2> and = +3/2>. The quadmpole coupling constant = 1.2 MHz, q = 0.4 and the isotropic hyperfine coupling constant = 3.3 MHz were estimated.
1. INTRODUCTION There have been many 14N ESEEM studies in = 112 spin systems. However, few experiments have been carried out in the systems of > 112. There has been an 14N ESEEM study of the = 312 system of the iron-molybdenum cofactor [1]. However, the zero-field splitting is larger than the Zeeman interaction in that system, and only the transition between = *1/2 Kramers doublet is possible in the X-band EPR measurements. In this study, we the measure the stimulated ESEEM of the [cr(M&)6l3+ complex, which has an = 312 spin system with a small zero-field splitting relative to the Zeeman interaction.
2. EXPERIMENTAL 2.1. Sample preparations The complex [cro\sH3)6](No3)3 was synthesized according to Mori [2] with slight modifications. The complex was dissolved in HzO/ethylene glycol (1: 1 v/v) mixed solvent and the concentration was adjusted to 5 mM for EPR measurements.
2.2. EPR measurements X-band CW EPR and ESEEM measurements were carried out on a JEOL RE3X spectrometer equipped with a JEOL JES-PX1050 pulse EPR unit. The three-pulse sequence (x/2-z-x//2-T-x12-~-echo) was applied with pulse widths of tdz = 30 ns, and z = 300 ns. In
232
order to eliminate the influence of unwanted two-pulse echoes, a combination of four threepulse sequences with different pulse phases were used [3]. All ESEEM measurements were carried out at ca. 4.5 K because of the rapid spin relaxation of the complex that is typical for the high spin system. The CW EPR spectra were measured at 120 K. 3. RESULTS AND DISCUSSION 3.1. CW EPR
The CW EPR spectrum of the [Cr@&)6]3+ could be interpreted in terms of the following spin Hamiltonian,
Ei, = p s . ~ . H + s . D . s .
(1)
Considering the axial symmetry, this Hamiltonian can be rewritten as,
1
I
I
280
290
300
mT
200
250
300
350
400
niT
Figure 1. (a) EPR spectra of [Cr(NH3)6l3+, solid line: frozen solution spectrum; dotted line: solid solution spectrum in hexaammine cobalt(I1) nitrate. (b) Resonance positions as a function of magnetic field orientation.
The spectrum of the complex is shown in Figure 1. The complex cation has an octahedral symmetry, and its zero-field splitting parameter is expected to be small and nearly zero. There is a discernible shoulder around 290 mT in the spectrum (see the inset of Figure la). This is attributed to one of = +1/2> and the transitions between = +3/2> compared with the solidsolution spectrum of the complex in analogous [Co(NH3)6](NO3)3 matrix. The spin parameters were estimated as g = 1.988, I I = 0.03 cm-', and = 0, and these values are consistent with the parameters estimated by McGarvey for Cr(II1) in [Co(NH3)6]13 [4]. The angle 0 dependence of the resonance positions is also shown in Figure lb, where the 0 is the angle between the molecular z axis and the external magnetic field. The transition between = -1/2> and = +1/2> is almost field-independent, and it appears around 320 mT. In contrast, the = +1/2> and = transition between +3/2> are much more field-dependent and occur in the range of 250 to 400 mT.
233 3.2. Stimulated ESEEM
The total magnetic interactions of this system is described by the following Hamiltonian, (31
Hc+H,,
where Heis mentioned above (equation ( Z ) ) , and = -g,&I.
H + 1. A . S + 1.P . I.
(4)
The 14N ESEEM spectra are governed by the nuclear magnetic interaction expressed by Equation (4). In the case of an isotropic g and a hyperfine tensor Equation (4) leads to the following Hamiltonian matrix [ 3 , 51, v&nOsing? vgin Osin ip v,&osO iv,sinOcosip
veFosO -iv g i n K(l - T J )
ip
1
(5)
-
in the basis, l/h(m, =l)+l-l)), 10) , and l/&(~l)-~-l)). Hear, vef = 1 VI + 1, = TJ is an asymmetry parameter, and O and represent the orientation of the external magnetic field with respect to the nuclear quadmpole axis system. In the condition of vef < the nuclear Zeeman and the hyperfine interactions are largely canceled out. This means that the off-diagonal elements go to nearly zero, and the nuclear states are mostly determined by
1500
. .$
500
8
............. . . .: .._. .. . ._. . . . . . . . . . . . . . . . .............. r :
Y
0
-
I
'~ % -
0
2
4
6
310
'i....29'.0.
mT
8 1 0 1 2 1 4 MHZ
Figure 2. Field-dependent stimulated ESEEM spectra of Hexaamminechromium(II1) complex in a range of 290 to 350 mT at 4.5 K. These spectra were measured with 30 mW microwave power.
234 the nuclear quadrupole coupling. As a consequence, three peaks appear in the I4N ESEEM spectra at the NQR frequencies, ~ + = K ( l + r ) ,v _ = K ( l - r ) , v0=2Kv.
(6)
In contrast, with the condition of v,f > the nuclear Zeeman and the hypefine interactions are dominant, and the dm1 = + 2 double quantum transition appears at the frequency vdq = 2Jvtf + K Z(3 + q 2 ) .
(7)
Figures 2 and 3 show the stimulated ESEEM spectra of the [Cr(NH3)6I3' complex. There are seven or eight peaks originating from the NQR in the range of 0 to 4 MHz. Some spectra have a peak at 6.8 MHz, which is assigned to one of the double quantum transitions. The field dependence of the 14N ESEEM spectra (Figure 2) clarified that the spectra are composed of at least two NQR sets. After a comparison with the resonance positions in Figure lb, we assigned the peak at 3.1 MHz to one of the NQR and the peak at 6.8 MHz to the double quantum transition generated from the EPR transition between the = -1/2> and = +1/2> sublevels because these peaks are apparently most intense at 320 mT. The peak at 0.6 MHz was assigned to the NQR in the transition between the = -1/2> and = -3/2> sublevels because this is clearly observed at high and low field. The microwave power dependence of the spectra (Figure 3) and the graph of the intensities of the three peaks vs. square root of microwave power (Figure 4) confirms this assignment. The nutation frequency is expressed as
j&.l 0
2
I
4
6
8 MHZ
, ,
10 mW
1 0 1 2 1 4
Figure 3. Pulsed microwave powerdependent stimulated ESEEM spectra of Hexaamminechromium(II1) complex at 4.5 K. All spectra were measured at 320 mT
The w1 is proportional to the square root of the microwave power, and the coefficients ,/S(S+1)-rns(m8-1) are 2 in the = -1/2> to jm, = +1/2> transition and in = +1/2> to = *3/2> transition. The the magnetization vector in the = -1/2> to = +1/2> transition rotates more easily by a microwave pulse than that in the = *1/2> to = =t3/2> transitions. Therefore, the echo intensity of the = -1/2> to = +1/2> transition is expected to be larger than that of the = +1/2> to = *3/2> transition at weak microwave power, and the
235
later is expected to become more intense with increasing the microwave power. This is -t+ consistent with the experimental results and the peak assignments. According to these assignments, the Hamiltonian parameters in the EPR transition between = -1/2> and = +1/2> were estimated as = 1.2 MHz, = 0.4, and = 3.3 MHz from Equations 6 and 7. This estimation leads to the conclusion that the peaks at 1.0, 3.1, and 4.0 MHz are the NQR in the = -1/2> to = +1/2> transition. The other peaks, except the 0.6 MHz peak, have almost the same dependence of the microwave power as the peak at 3.1 and 6.8 MHz. If vef is 4 5 6 1 8 not zero, the off-diagonal elements have some (MW Power)’’’ I mWln value, and the peak frequency becomes angular-dependent. These peaks are in a range Figure 4. Peak intensities at various of 1.0 to 4.0 MHz, which is between the square root of microwave power. In minimum and maximum frequencies of the each frequency, the intensity at 10 mW calculated NQR frequency. Therefore, these is taken as a unit. peaks might be generated in the = -1/2> to = +1/2> transition and appear at some orientation. As indicated in Figure 4, the behavior of the peak at 0.6 MHz on the microwave power is apparently different from that of other peaks. We concluded that this peak is generated from the = +1/2> to = +3/2> EPR transition. Considering the three-times-bigger hyperfine = &3/2> sublevels, v,f becomes larger than and a two-line pattern is interaction in the expected in the ESEEM spectrum. However, any double quantum transition is impossible at 0.6 MHz using Equation (7) with the estimated parameters. Therefore, the peak at 0.6 MHz might be one quantum NQR transition in the = *1/2> to = *3/2> EPR transition and accidentally have the intensity. +3-
-B-
0.6 MHz 3.1 MHz 6.8 MHz
REFERENCES 1. M. Mori, J. Chem. SOC.Jpn., 74 (1953) 253. 2. H.I. Lee, K.S. Threshre, D.R. Dean, W.E. Newton, and B.M. Hoffman, Biochem.,
37(1998) 13370. 3. S.A. Dikanov and Y.D. Tsvetkov, Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy, CRC Press, Boca Raton, Florida, 1992. 4. B.R. McGarvey, J. Chem. Phys., 37 (1962) 3020. 5. H.I. Lee, P.E. Doan, and B.M. Hoffman, J. Magn. Reson., 140 (1999) 91. 6. A. Abragam, The Principle of nuclear magnetism, Clarendon Press, Oxford, 1961.
236
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
EPR investigation of inhomogeneous phases in improper ferroelastic MgTiF6.6H20:Mn2+ A.M. Ziatdinov and P.G. Skrylnik Institute of Chemistry, Far Eastern Branch of the Russian Academy of Sciences. 159, Prosp. 100-letiya, 690022 Vladivostok, RUSSIA
New approaches to EPR study of inhomogeneous phases in improper ferroelastic crystals MgTiF6.6H20:Mn2' are presented. On the basis of analysis of temperature and angular dependences of experimental parameters and numerical calculations the conclusion has been drawn that above Tc+=311k0.3 K (Tc-=300f0.3 K) crystals investigated undergo first order transitions from the monoclinic ferroelastic phase to a structurally inhomogeneous phase. In this phase two types of regions characterized by homogeneous and inhomogeneous structural organization exist in the crystals. In the latter both the angle of Mg[H20]62+ octahedra rotation around crystal C3 axis and their axial distortion along C3 are modulated parameters. It was assumed that at T,2=370 K the crystals investigated may undergo phase transition from an inhomogeneous phase to an incommensurate phase, being related to that observed in MgSiF6.6H20.
1. INTRODUCTION
The crystals MgTiF6.6H20 belong to family ABF6.6H20 (where A and B are divalent and fourvalent elements, respectively). In these compounds complex ions A[H20]? and [BF6I2octahedra form rhombohedrally distorted CsC1-type lattices and can be distributed between two orientations around the 3-fold axis [l]. Single crystals of MgTiF6.6H20 undergo a first order structural transition to the low-temperature monoclinic phase at T=300 K [2]. After the discovery of a structurally inhomogeneous phase in MgBF6.6H20 (B=Si, Ge, Ti) crystals, existing between their rhombohedra1 paraelastic phase and monoclinic ferroelastic one, by Ziatdinov et [3-51, most attention has been directed to the nature of this phase ('intermediate' phase). Up to now, for the interpretation of experimental data different models of intermediate phases in these crystals have been suggested: some modifications of structural-incommensurate model [3-61, model of static and dynamic disordered structure fragments [7], and a domain model [8]. However, none of these models explains all known experimental data. In this paper, on the basis of analysis of MgTiF6.6H20:Mn2+single crystals EPR experiments, we suggest a model for their structural organization in the inhomogeneous phase, which relies in part on our previous ideas regarding the incommensurate structure of this phase [6] but is free of some their disadvantages.
237
2. EXPERIMENTAL The EPR measurements were carried out using an X-band spectrometer ESR-231 (production of Germany) and a Q-band spectrometer RE-1308 (production of Russia) in three mutually perpendicular crystal planes. Single crystals MgTiFg6H20 doped with -0.1% Mn2+ ions have been grown from aqueous solution at room temperature according to the method presented in Ref. [9]; the fluorotitanates were purchased commercially. The temperature range of experiments was 100-380 K with a measurement accuracy -0.3 K and stability -0.1 Khour. At Tc =300+0.3 K, the MgTiF6.6H20 crystals upon cooling undergo a first order phase transition from the inhomogeneous phase to monoclinic phase with the temperature hysteresis -11 K. Investigation of MgTiF6.6H20 crystals with polarizating microscope, analysis of angular dependences and variation of relative intensities of EPR lines (from sample to sample and from experiment to experiment for the sample) demonstrate that the crystal under study below Tc consists of orientational domains of three kinds related by 120" rotations about the trigonal crystal axis C3 and each domain contains two inequivalent Mn2' sites. Therefore, the MgTiF6.6H20 crystal is an improper ferroelastic at T
-',
238
T (K) 372
FAH,,--l
-
5
367
A
357
2
338
6.0
U
320 I
6.5
'
I
'
I
'
I
'
I
1216 1220 1224 1228 1232 H 0 (mT) Figure I . The temperature evolution of the Mn2' EPR low field HFS line for
MgTiF6.6H20:Mn2'crystals at Q-band and Ho((C3. Dots and solid lines correspond to experimental and theoretical spectra,
respectively.
-- 5.5 h
v
2
5
300
320
340
360
380
Temperature (K) Figure 2. The temperature dependence of the Mn2+EPR low field HFS lineshape parameter AH11 for MgTiF6.6HzO:Mn2+crystals at Q band and H&Z,. 1 and I ' - 6=0", 2 - 6=55". The black and white dots refer to parameters at heating and cooling of the crystals, respectively.
Ti1>380 K. Therefore the highest temperature discontinuity has been denoted as Ti2. It is worth mentioning that the transition at Ti2 is the most prominent. The step-wise discontinuity in parameter AH12, not only in the slope of dependence AH12(T), has been observed at Ti2, as well as in MgSiF6.6H20 crystals. The temperature decrease below Ti2 leads to the sequence of step-wise changes in the slope of temperature dependences of lineshape parameters (Figure 2), though these changes are small by comparison with those at Ti2. The temperatures, at which slope discontinuities Ti, (n=2+6) occur, depend on the direction of the temperature change and vary from sample to sample within -4 K, but they occur at practically the same values of lineshape parameters. The amplitude of these variations in lineshape parameters increases as temperature decreases. At T>Tc, the low-field broad component of the HFS line narrows at first and the inner structure of this component becomes observable, when the polar angle 8 tends to 'magic' angle value -54' (Figure 3). Simultaneously, the narrow high-field component of HFS line broadens (Figure 3) and splitting between components decreases. Above Tc, with rotation of the single crystal around C3 (varying azimuthal angle 9) which makes a certain angle &O", 90" with the Ho direction, the 120'-angular dependence of Mn2+ EPR lineshape has been observed (Figure 4).
3. DISCUSSION MgTiF6.6H20:Mn2+ crystals temperature evolution of EPR lineshapes, temperature dependences of lineshape parameters, angular dependences of spectra at rotating along 0 and 4 angles demonstrate a prominent similarity with those for experiments on MgSiF6.6H20 crystals at temperatures Tc=297 K
239
4 mT
3 I
0
1
'
1
.
1
100 200 4 (degrees)
Figure 3. Angular dependences of the Mn2+ Figure 4. Dependences of Mn2+EPR HFS EPR low field HFS lineshape for lineshapes ( I , a - @lo,b - +30° , c MgTiF6.6H20:Mn2+crystal at rotating along and lineshape parameter AHl2(2) on +angle the @angle in the planes containing the C3- (at rotating the sample around the C3-axis) axis. The Q-band, T=320K. for MgTiF6.6H20:Mn2+ crystals (the X-band; &/lC3; 8=55", T=320 K). Solid line (3) corresponds to calculated dependence. corresponding to components increases, as well as the linewidth of broad component, while the narrow component remains almost unchanged. Therefore, it is reasonable to apply theoretical consideration presented in our previous work on MgSiF6.6H20 crystals [101 to this investigation of MgTiF6.6H20crystals. Taking into account that the symmetry and the principal axis direction, related to the crystallographic axes, of the Mn2+ EPR spectrum for MgTiF6.6H20:Mn2+ above Tc do not change, and the &angle dependence of lineshape and spread of inhomogeneous HFS lines observed in the inhomogeneous phase may be reached only with variation of the axial fine structure parameter D, one may conclude, that in the crystals investigated at T>Tc the trigonal distortion of [Mn(H20)6I2+octahedra is a lattice displacement modulated over space. In the inhomogeneous phase the resonance field H, is a function of the small lattice displacements. The resulting spectrum shape may be calculated as a convolution of some single line shape (Lorentzian in most cases) and a distribution function of H,. The observation of a 4-angle dependence of the EPR lineshape in MgTiF6.6H20:Mn2+ crystals above Tc (Figure 4) is evidence for the occurrence of disorientation of cubic crystal field axes for different Mn2+ ions, i.e. rotation of complex ions Mg[H20]62+ around trigonal axis in inhomogeneous phase. The amplitude of this disorientation increases as the temperature is decreased. It is noted that in the ferroelastic phase within each orientational domain two spatially inequivalent complexes Mg[H20]62+, rotated with respect to each other around C3 axis by the certain angle, are present [5,6]. The facts cited allow us to conclude that disorientation angle 64 of octahedra Mg[H20]? around the trigonal axis with respect to their position in the paraelastic phase can be a primary modulated parameter in MgTiF6.6H20. Obviously, the temperature dependent trigonal distortion of octahedra Mg[H20]62+ cannot be a primary modulated parameter of this transition, because such an assumption leads to at least
240
Table I The parameters used for theoretical spectra calculation within the frameworks of model of coexisting systems of domains and disordered soliton-like regions.
320 338 357 367 372
5.85 5.78 5.40 5.15 5.05
0.80 0.76 0.76 0.82 0.82
0.93 0.93 0.78 0.46 0.63
0.40 0.38 0.32 0.60 0.49
two Mg[H20]62+ configurations with different values of trigonal distortion (and D values) in the ferroelastic phase, which does not agree with the EPR [5,6] data. Further, one may suppose that rotation of octahedra around the trigonal axis, consequence of change in lengths and orientations of hydrogen bonds, should be accompanied by a change in the trigonal distortion of complexes and, therefore, by variation of the fine structure parameter, i.e. D=Do(T)+AD(G$), where Do(T) is a value of fine structure parameter for complex not distorted by modulation, AD@$) is the contribution to the fine structure parameter by the modulated distortion. For the reasons of symmetry (the presence of symmetry axis and inversion centre), one should infer that AD@$) is an even function. Hence, by neglecting modulation of other spin Hamiltonian parameters, H,(G$) at HollC3 should contain only even terms, as far as in this orientation H, does not directly depend on the crystalline cubic axes orientation. For the theoretical description of the Mn2+HFS lineshape above Tc the following approach has been tested. As a possible model of the crystal structure at T>Tc, a reversible division of the crystal at this temperature into a subsystem of microscopic domains and a subsystem of disordered (single-soliton-like) regions may be introduced [lo]. Under such assumption above Tc, broad component of the HFS line should be referred to soliton-like regions and narrow component to domains, respectively. This conclusion is based upon the following experimental data. The linewidth of the broad component has considerable dependence on temperature, it decreases as 8 angle increases, and is greater than the corresponding HFS linewidth in ferroelastic phase. The linewidth of the narrow component barely changes with temperature, it increases as 8 angle increases, and is approximately equal to the corresponding HFS linewidth in the ferroelastic phase. We should note that the above model also agrees with the data from structural investigations on related MgSiF6.6H20 crystals [I I ] reported that the crystals immediately above Tc consists of 'stripe' domains with low-temperature monoclinic unit cell. The calculated lineshapes using the Gaussian distribution of parameter D as appropriate for disordered regions are presented in Figure 1. Corresponding calculation parameters are presented in Table 1 : the difference in H, values for the lineshape component positions AHc, the narrow component linewidth W, the ratio of domain-like regions volume to the disordered regions volume AI/A2,and a parameter of Gaussian distribution of resonance magnetic field within disordered regions OH. The theory provides a good fit to the experimental spectra, though some deviations at T>360 K take place. Thus, the model of coexisting systems of domains and disordered soliton-like regions for crystals of MgTiFs.6H20:Mn2+ from Ti2 to Tc is a model, which approximates the experimental Mn2+
24 1
EPR spectra well. The model for MgTiF6.6H20 crystal structure presented by us makes it possible to describe the angular - dependence of the Mn2' HFS lineshape. Results of calculations for angular - dependences of the lineshape parameter AH12 are presented in Figure 4 (AD(&$) was assumed to be a simple even function, the quadratic dependence 6D=d2.g2, $ is Gaussian distributed with 0+=2"in soliton-like regions, and the value of (I in domains differ from the mean value in soliton-like regions by 61)=10", d2=0.05x104cm-l /(degree)2). The analysis of temperature dependences of the EPR lineshape parameter AH12 (Figure 2) shows, that as the temperature is decreased below T,2, in soliton-like regions the structural ordering takes place and Ti, (n=3-6) may be the temperatures of structural phase transitions in these regions, some of these transitions at Ti4 and Ti6 have been detected with calorimetric techniques [12]. It is worth noticing that the step-wise dependence of the AHI~(T)slope becomes more prominent at &55" compared to that for 8=Oo(Figure 2). Taking into account the fact that the spin Hamiltonian terms containing are different from zero at 8+Oo, 90°, we may suppose the peculiarities previous mentioned are associated with discontinuities in the temperature dependence of rotations of Mg[H20]62+ octahedra around C3 axis. We suppose that Ti2 is the temperature of the phase transition from the low temperature inhomogeneous structure (which characterized by distinct ordered domain regions and disordered ones) to a structure with modulated lattice displacements, resulting in a continuous spectral distribution between dominant components of the EPR lineshape. This distribution arises above T,2 and is evidence for lattice parameter distribution between dominant values. This leads us to the conclusion that Ti2 may be the temperature of the transition to a phase with structure organization being related to an incommensurate phase. Thus, in the present paper, based on an EPR study of MgTiF6.6H20:Mn2+single crystals, structural organization motifs for their inhomogeneous phases between ferroelastic monoclinic and supposed paraelastic rhombohedra1 ones, are proposed, which results in a successful description of the experimental data. The authors would like to thank Prof. R.L. Davidovich and Dr.T.F. Antohina for providing us with MgTiF6.6H20 crystals and Dr.V.G. Kuryavyi for help in EPR measurements. REFERENCES
1. 2. 3. 4. 5. 6.
D.E. Price, Can. J. Phys., 65 (1987) 1280. R.S. Rubins and K.K. Kwee, J. Chem. Phys., 66 (1977) 3948. A.M. Ziatdinov, V.G. Kuryavyi and R.L. Davidovich, Sov. Phys. Solid State, 27 (1985) 1288. A.M. Ziatdinov, V.G. Kuryavyi and R.L. Davidovich, Sov. Phys. Solid State, 28 (1986) 3549. A.M. Ziatdinov, V.G. Kuryavyi and R.L. Davidovich, Sov. Phys. Solid State, 29 (1987) 120. A.M. Ziatdinov, V.G. Kuryavyi and T.F. Antohina in: Modern Applications of EPWESR (Ed. Cz. Rudowicz, Hong Kong, 1997) 554. 7. R. Hrabanski and A. Kassiba, Ferroelectrics, 172 (1995) 443. 8. W. Zapart and M.B. Zapart, Bull. Magn. Res., 19 (1999) 34. 9. I.V. Tananaev and K.A. Avduevskaya, Zhumal Neorganicheskoi Khimii (Russia), 5 (1960) 63. 10. P.G. Skrylnik and A.M. Ziatdinov, Ferroelectrics, 249 (2001) 279. 11. G. Chevrier and G. Jehanno, Acta Cryst., A35 (1979) 912. 12. I.N. Flerov, M.V. Gorev, S.V. Melnikova, M.L. Afanasyev and K.S. Aleksandrov, Sov. Phys. Solidstate, 33 (1991) 1921.
242
EPR in the 21* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
EPR Investigations on Fe3+ions in alkali borotellurite glasses P. Gin Prakash, A. Murali and J. Lakshmana Rao Department of Physics, Sri Venkateswara University, Tirupati 517 502, India
Electron paramagnetic resonance (EPR) spectra of Fe3' ions in alkali borotellurite glass systems have been studied. The EPR spectra of all the glasses exhibit three resonance signals at g = 2.0, g = 4.2 and g = 6.4. The EPR spectra of 3 mol % Fe,O, doped potassium borotellurite glass system were studied at various temperatures. The paramagnetic susceptibility was calculated fkom the EPR data at various temperatures and was used to calculate the Curie constant. The optical band gap energy was found to decrease with increase in iron content.
INTRODUCTION Teiiur;rr: gla,.qesare cowidered to be important because they have numems ryplications is semiconductors in microelectronics [ 11. The distinguishing factor of the matrix of this glass is that the tellurium atoms have unshared pairs of electrons which do not take part in bonding. On the other hand, their relatively low temperature of crystallization and melting makes these types of glasses as active candidates for CD memory devices. EPR spectrum of transition metal ions in glasses is an interesting research subject and affords a method to investigate glass structure. d5 ions, such as Fe3' and Mnz' are known to be good probes for EPR spectral studies. In EPR spectra of Fe3' ions in various glasses, it is found that trivalent iron ions can take two different coordination sites, i.e., tetrahedral [2,3] and octahedral [4,5] in glasses. Since the EPR spectra of Fe3' ions in alkali borotellurite glasses have not been reported so far, the authors studied in detail the EPR spectra of Fe3' ions in these glasses.
2. EXPERIMENTAL The glass samples were prenared by the melt quenching method with the composition 85 M,B,O,. (1 5 - X ) TeO,. X Fez 0, (M = Li, Na or K; 0.5 X I 8 mol %). Electron paramagnetic resonance spectra were recorded at room temperature on & EPR spectrometer (JEOL FE 1X) operating in the X-band fkequencies with a modulation frequency of 100 kHz. A powdered glass specimen of 100 mg was taken in a quartz tube for EPR measurements. Polycrystalline DPPH with a g value of 2.0036 was used as standard field marker. The EPR spectrum of the CuS0,S H,O powdered substancewas also recorded at 9.205 GHz as a reference to calculate tile number of spins. The EPR spectrum of 3 mol % Fe203doped potassium borotellurite (KBT3Fe) glass sample was recorded at different temperatures (123 - 433 K) using a variable temperature controller (JES UCI
243
Ultraviolet absorption edges of glasses were recorded at room temperature on a JASCO -UVVIS-NIR spectrophotometer from 360 to 500 nm. Glass samples of thickness 2 to 3 mm were used for optical absorption measurements. 3.
AND ANALYSIS
3.1. studies When Fe3' ions are introduced into the glasses, all glass samples under investigation exhibit three resonance signals at g = 4.2, g = 2.0 and a weak resonance signal at g = 6.4. Figure 1 shows typica EPR spectra of 3 mol % Fe3' ions in alkali borotellurite glasses at room temperature.
-TH
0.
w
-
G r
Y)
IE I 250
___
MAGNETIC FIELD ' m T 1
Figure 1. EPR spectra of alkali borotellurite glass samples containing 3 mol % Fe,03 at room temperature. EPR spectra of lithium borotellurite glasses with different mol % of Fe3+ions were also studied and the number of spins participated in the resonance at g= 4.2 was calculated for different concentrations. The number of spins (N) has been calculated with the help of a reference (CuSO,.SH,O) by using the formula given by Weil et a1 [6]. The EPR spectra of 3 mol % Fe3+ions doped in potassium borotellurite (KBT3Fe) glass sample were recorded at different temperatures (123 - 433 K). The number of spins participated in the resonance at g = 4.2 was calculated at various temperatures. Figure 2 shows a plot of log N against 1/T for this sample. The paramagnetic susceptibility can be calculated fiom the EPR data using the formula :
N g' p*J ( ~ + i )
3 k,T where N is the number of spins per m3, the rest of the symbols have their usual meaning. N and g are taken fiom EPR data. Paramagnetic susceptibilitywas calculated at various temperatures.
244
16.6
0.0025
0.0075
1/T (K-‘)
Figure 2. A plot of log N against 1/T for a potassium borotellurite (KBT 3Fe) glass containing 3 mol % Fe,O, Optical band gap energy Q,J The absorption coefficient can be determined
(v)
= 2.303
a fimction of frequency using the formula:
A/d
(2)
where A is the absorbance at fiesuency v and d is the thickness of the glass sample. The optical band gap for an indirect transition can be determined using the relation [7]
E,
=
hv -
hv / B)”’
(3)
where B is a constant and E,, is the band gap. Plots were drawn between
5J
I
hv
and hv
I”
a
-
I
@ 0.5 ---- LBT LBT Fo
- .-.-
I I
I I I
1
LB7 3 Fo
n 3:
>
:6-
0
I”
>I
3-!
i
5
4
f
2I- b -
/. /
,/
2 2-
1
500
0
I
2
2.5
3
WAVELENGTH(nrn)
Figure 3. (a) Ultraviolet absorption edge, (b) A plot between LBTO.SFe, LBTlFe and LBT3Fe glasses.
hv
and hv for
245
shown in Figure 3. The optical band gap is obtained by extrapolating the linear region of the curve to the hv axis.
4. DISCUSSION 4.1. EPR studies Fe3’ ions belong to d5 configuration and it has a 6S ground state. When Fe3’ impurity complexes are situated in a crystal field with a large axial component, the free ion 6S state splits into three Kramers doublets I 5/2 ), I 3/2 ) and I 1/2 )with their separations usually greater than the microwave quantum. Normally, the selection rules permit EPR transitions in the 1 1/2 ) doublet with g = 2.0 and g = 6.0 [8]. In the present work, the authors observed a sharp peak at g = 4.2 and a broad signal at g = 2.0 and a weak signal at g = 6.4. The resonances at g = 4.2 and g = 6.4 have been attributed to Fe3+ions in rhombic and axial symmetry sites respectively. The signal at g = 2.0 is attributed to paramagnetic ions involved in clusters. The g values thereby the symmetry around the paramagnetic Fe3+ions are found to be independent of alkali ions present in the samples. From the concentration dependence of the intensity of the resonance signals at g = 4.2, it is observed that the number of Fe3’ ions participating in the resonance increases with concentration of Fe3’ ions upto 3 mol % and after that it decreases. The decrease in the intensity of the resonance signal at g = 4.2 for x > 3 mol % has been reported in several systems of iron :Itailling glasses [ 1,9,10]. The decrease in intensity may be beca-.,ae of the isolated ions are eventually incorporated into growing clusters as the concentration increases. From Figure 2, we can observe that as temperature is lowered, the number of spins increases and further we can observe a linear relationship between log N and 1/T, a phenomenon that can be expected from the Boltzmann law. From this graph, the activation energy can be calculated from the slope of the straight line. The activation energy, being the energy needed to alter the number of spins (Fe3’ ions) participating in resonance and is found to be 2.9787 x 10.’’ J (0.0186 eV). The Curie constant ( 1 9 . 6 1 ~lo”) and Curie temperature (1 16 K) have been calculated from the temperature dependence of the reciprocal of susceptibility. The Curie constant calculated in the present work is of the same order to that of measured value (1OOx 10” mol) reported for Fe3’ ions in Te0,-B,O,-PbO glasses by Ardelean et a1 [l]. 4.2. Optical band gap energy (EE,,J The optical ene;,? gap values are calculated for lithium borotellurite glass samp1.s with 0.5, 1.0 and 3.0 mol % of iron content LBTO.SFe, LBTl Fe and LBT3 Fe and the values are found to be 3.26, 3.14 and 2.82 eV respectively. The values obtained in the present work are of the same order reported for iron containing borate glasses [ 113. It is found that as the concentration of iron ion increases, the energy gap value decreases as reported in the literature [7,11]. This indicates that with increase in iron concentration, the nature of the glasses become more semiconducting.
5. CONCLUSIONS 1. In all the investigated samples, the EPR spectra exhibit three resonance signals at ~ 4 . 2 ,
246
~ 2 . and 0 ~ 6 . and 4 these are attributed to Fe3' ions in rhombic and axial symmetry sites respectively. The site symmetry around Fe3' ions is found to be independent of alkali ion present in the glass. 2. The spectra exhibit a marked concentration dependence on iron content. The decrease of intensity with increase of iron content for more than 3 mol % has been attributed to the formation of clusters in the glass samples. The intensity of the resonance signals decreases with increase in temperature according to the Boltzmann law. 3. The optical band gap energy was found to decrease with increase in iron content which indicates that the glasses become more semiconductingnature with increase of iron content.
ACKNOWLEDGEMENTS One of the authors (JLR) is thankful to University Grants Commission (New Delhi) for financial support. REFERENCES 1. I. Ardelean, M. Peteanu, S. Filip,V. Simon and G. Gyro@, Solid State Commun., 102 (1997) 341. 2. C. R. Kurkjian and E. A. Sigety, Phys. Chem. Glasses, 9 (1968) 73. 3. K. Hirao, N. Soga and M. Kunugi, !. As:. Ceram. SOC.,62 (1979) 109. 4. R.P.S. Chakradhar, A. Murali, and J.L. Rao, Opt. Mater., 10 (1998) 109. 5. D. Legein, J. Y. Buzare, and C. Jacoboni, J. Non-Cryst. Solids, 161 (1993) 112. 6. J.A. Weil, J.R. Boltan, and J.E. Wertz, Electron ParamagneticResonance-ElementaryTheory and Practical Applications, Wiley, New York, 1994, p. 498. 7. M.A. Hassan and C.A. Hogarth, J. Mater. Sci., 23 (1988) 2500. 8. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon, Oxford, 1970, p. 203. 9. R.G. Gupta, R.G. Mendiratta, S.S. Sekhon, R. Kamal, S.K. Suri and N. Ahmed, J. NonCryst. Solids, 33 (1979) 121. 10. N. Iwamoto, Y. Makino and S. Kasahara, J.Non-Cryst. Solids, 55 (1983) 113 11. A. A. Kutub, J. Mater. Sci., 23 (1988) 2495.
EPR in the 2 1'Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
EPR study of X-ray and method
247
irradiated Ge02 glasses prepared by the sol-gel
K. Kojimaa,F. Ogura", N. Wadab, K. Yamamotoa, T. Fujita' and M. Yamazakid aDepartmentof Applied Chemistry, Ritsumeikan University, Kusatsu, Shiga 525-8577 Japan bDepartment of Materials Science and Engineering, Suzuka National College of Technology, Suzuka, Mie 5 10-0294 Japan 'Nikko Co. Ltd., Matto, Ishikawa 924-8686 Japan dSumitaOptical Glass Inc., Saitama, Saitama 338-8565 Japan X-ray irradiation of sol-gel derived GeO2 and 5Na20-94Ge02-1Er203 glasses generated many EPR signals: GeE' center (g = 1.993-1.994, 1.996-1.997, 2.000); four-coordinated germanium electron trapped center (GEC, or Ge(2); g = 1.987-1.988); self-trapped hole center (g = 2.009-2.015); and another weak signals (g = 1.965, 1.971, 1.977, 1.982, 1.983, 2.015, 2.019, 2.021, 2.025, 2.033). W irradiation yielded GeE' in the Ge02glass and GEC as well GeE' in a 99.5Ge02-0.5Er203 glass. A GeOz gel and a 0.5Na20-98.5GeO2-1Er203glass, both of which contained residual OH groups and water, were relatively unaffected by irradiation.
1. INTRODUCTION Rare-earth doped oxide glasses and films have received great attention due to the possibilities of using these materials for optoelectronics applications. Among the oxide glasses, GeOpbased glass is one of the promising hosts for waveguide amplifiers and lasers, and frequency up-conversion devices because of its lower phonon energy and high optical transparency in a wide wavelength range. We have already synthesized Er3'- and Nd3'-doped Ge02-based glasses by the sol-gel method and investigated their spectroscopic properties including absorption, fluorescence, up-conversion fluorescence and EPR spectra [ 1-41. Here we report EPR studies of X-ray and ultraviolet irradiation effect on GeO2-based glasses
248
prepared by the sol-gel process. There have been no such studies, although irradiation effects on GeO2 glasses produced by the melt-quenching [5-81 and vapor-phase axial deposition [9] methods have been reported. This report will contribute to fundamental properties of GeOa-SiOz waveguide amplifiers prepared by the sol-gel method [ 101. 2. EXPERIMENTAL
Ge02-based glasses were prepared by the sol-gel method as described elsewhere [l-41. X-ray irradiation was carried out with a Rigaku IKF-3064 X-ray fluorescence spectrometer (Rh target, 50 kV, 50 mA, dose rate: 5 X lo5Wmin) at room temperature. irradiation was done with an Ushio high-pressure Hg lamp (500 W) at room temperature. EPR spectra were measured using a Jeol JES-FE2XG X-band spectrometer with 100-kHz field modulation at room temperature. 3. RESULTS AND DISCUSSION
Figure 1 shows EPR spectra of an X-ray irradiated GeO2 glass. The original glass was transparent. Spectral intensities increased with the irradiation time. A well-known GeE’ center with g values of 1.994, 1.996 and 2.000 appeared [7-91. A resonance at g = 2.011 can be attributed to self-trapped hole (STH) [ll-131. A signal of g = 1.987 may be due to four-coordinated germanium electron trapped center (GEC, or Ge(2)) as observed in Ge02 glasses [7-91 and GeOz-SiOz glass preforms [12, 131. It is found that the signals of GeE’ and STH grow simultaneously with irradiation time, while the GEC signal tends to be saturated at the early stage of irradiation. The same will be observed below in Figs. 5, 6 , and 7. K. Ichii et al. [14] have pointed out similar tendency of the GEC signal in Ge02-SiO2 fiber preforms. Figure 2 gives the sample dependence of EPR spectra for X-ray irradiated Ge02 glasses. A spectrum (a) was obtained from a somewhat translucent glass, showing many EPR signals compared with a spectrum (b). In the spectrum (a), the centers of GeE’, STH, and GEC are observed. Signals with g = 1.983 to 1.965 may tentatively be ascribed to six- or five-coordinated germanium electron trapped centers, because such highly-coordinated germanium atoms are suggested to be formed in sol-gel derived Ge02-based glasses [15]. On the other hand, signals with g = 2.018 to 2.033 may be attributed to hole-trapped centers such non-bridging oxygen hole center W O H C ) and peroxy radical (OZ-).According to Tsai and co-workers [7], there is the possibility that those centers with small and large g values are complementary electron and hole centers. EPR spectra of a irradiated Ge02 glass are shown in Figure 3. A weak resonance at
249
330
335
340
345
350
Magnetic field / mT
Figure 1. EPR spectra of an X-ray irradiated GeOz glass. Irradiation time (in min) : (a) 0 ; (b) 1 ; (c) 5 ; (d) 10 ; (e) 30 ; (0 60 ;(g) 120.
330
335
340
345
Magnetic field / mT
Figure 2. Sample dependence of EPR spectra for Ge02 glasses X-ray irradiated for 30 min. A spectrum (b) is the same as that of Figure 1 (e). A spectrum (a) was obtained from another sample.
g = 2.003, which is observed in an unirradiated sample (a), is due to a carbon-related center This signal was not found in the transparent sample (Figure l(a)), but observed in the translucent one that gives the EPR spectrum of Figure 2(a) by X-ray irradiation. It is interesting that GeE' (g = 1.993) is produced by W irradiation. This GeE' signal was notreduced with 20 min passage of time after stopping irradiation (Figure 3(f)), but reduced with 60 min passage of time (not shown). GEC as well GeE' were observed in a W irradiated 99.5Ge02-0.5Er203 glass (not shown). In some unirradiated samples, GeE' is detected together with the carbon-related center, both of which were produced in the sol-gel reaction process. EPR spectra of a W irradiated GeOz gel are shown in Figure 4. Besides the somewhat increase in the intensity of the carbon-related signal, the spectra are hardly changed by irradiation, indicating that the gel sample, which contained residual OH groups and water, is unaffected by irradiation. The changes in EPR spectra of an X-ray irradiated 99Ge02-1Er203glass (Figure 5) are similar to those of the Ge02 glass (Figure l), since the glass composition is close to each other. Figure 6 shows EPR spectra of an X-ray irradiated OSNa~0-98.5GeO~-lEr~03 glass. The
250
I
2.003
320
I
I
I
I
325
330
335
340
320
Magnetic field mT
325
330
335
340
Magnetic field / mT
Figure 3. EPR spectra of a Ge02 glass before (a), under ( (b) 0.5 min ;(c) 5 min ), and after ( (d) - ) irradiation. Spectra (d), (e), and ( f ) were measured with 0.5 min, 10 min, 20 min passage of time, respectively, after stopping irradiation.
Figure 4. EPR spectra of a irradiated GeOz gel. Letters (a) have the same meanings as those in Figure 3.
~
330
335 340 345 Magnetic field 1
350
Figure 5. EPR spectra of an X-ray irradiated 99Ge0, - 1Er203glass. Irradiation time (in min) : (a) 0 ; (b) 30 ; (c) 60 ;(d) 120.
330
335
340
345
350
Magnetic field / mT
Figure 6. EPR spectra of an X-ray irradiated 0.5Na20- 98.5Ge02- 1Er203 glass. Letters (a) - (d) have the same meanings as those in Figure 5.
25 1
330
335 340 345 Magnetic field / mT
350
Figure 7. EPR spectra of an X-ray irradiated 5Naz0 - 94Ge02- lErz03glass. Letters (a) - (d) have the same meanings as those in Figure 5.
irradiation effect is found to be small, the intensity in the carbon-related center being reduced (the brownish color of the sample faded) and weak defect centers including GeE’ being produced. This is because the sample, like the GeOz gel shown in Figure 4, contained residual OH groups and water. EPR spectra of an X-ray irradiated SNa20-94Ge02-1Er203 glass are given in Figure 7. A resonance at g = 2.004 grows by irradiation, though it is rather overlapped with the signal of the carbon-related center. The g value of 2.004 is very close to that of the “k complex” which Purcell and Weeks [6] have observed in a y -ray irradiated GeOz rutile (tetragonal) powder. They proposed interstitial Ge3+ a possible model for this complex.
4. CONCLUSIONS In the sol-gel derived GeOz-based glasses, X-ray radiation induced GeE’, GEC, OHC such as STH, and weak EPR signals with small and large g values. UV radiation generated GeE’ and GEC. The sol-gel derived GeOZ-based glasses are considered to be suitable for the study of radiation-induced defect centers. This study will Contribute to the development of sol-gel derived GeOz-Si02 glass fibers.
REFERENCES 1. M.Yamazaki and K. Kojima, J. Mater. Sci. Lett., 14 (1995) 813. 2. K. Kojima, T. Fukuda and M. Yamazaki, Chem. Lett., (1997) 931. 3. K. Kojima, T. Fujita and M. Yamazaki, J. Non-Cryst. Solids, 259 (1999) 63. 4. K. kojima, K. Tsuchiya and N. Wada, J. Sol-Gel Sci. Tech., 19 (2000) 511. 5. R. A. Weeks and T. Purcell, J. Chem. Phys., 43 (1965) 483.
252
6. T. Purcell and R. A. Weeks, Phys. Chem. Glasses, 10 (1969) 198. 7. T. E. Tsai, D. L. Griscom and E. J. Friebele, Diffusion and Defect Data, 53-54 (1987) 469. 8. T. E. Tsai, D. L. Griscom, E. J. Friebele and J. W. Fleming, J. Appl. Phys., 62 (1 987) 2264. 9. H. Itoh, M. Shimizu and M. Horiguchi, J. Non-Cryst. Solids, 86 (1986) 261. 10. A. Martucci, M. Guglielmi, J, Fick, S. Pelli, M. Forastiere, G. C. Righini and C. Battaglin, in Sol-Gel Optics V, p2 (2000). 11. D. L. Griscom, Phys. Rev. B, 40 (1989) 4224. 12. J. Nishii, N. Kitamura, H. Yamanaka, H. Hosono and H. Kawazoe, Opt. Lett., 20 (1995) 11 84. 13. J. Nishii, K. Fukumi, H. Yamanaka, K. Kawamura, H. Hosono and H. Kawazoe, Phys. Rev. B, 52 (1995) 166 1. 14. K. Ichii, M. Takahashi and T. Yoko, Abstract of the 41st symposium on Glass and Photonics Materials of Japan, p94 (2000). 15. K. Kamiya, M. Tatsumi, H. Nasu and J. Matsyoka, J. Ceram. SOC.Jpn., 101 (1993) 1201.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
Structural studies of the fresh water (Apple) snail
253
shells
K.V.Narasimhulua,C.P.Lakshmi Prasunab, T.V.R.K. Raob, J. Lakshmana Raoa aDepartmentof Physics, Sri Venkateswara University, Tirupati - 5 17 502, India bDepartmentof Physics, Sri Krishnadevaraya University, Anantapur-5 15 003, India The (apple) snail shells containing three layers (periostracum, ostracum and hypostracum) were characterized by EPR, FT-infrared (FT-IR) and optical absorption techniques. The EPR spectrum of the organic (protein) layer periostracum shows a sharp resonance signal at g=2.032 due to a free radical namely, chonchiolin and two signals characteristic of Fe3+ions. The EPR spectra of the other two layers are found to contain Mn2+resonance signals. From the EPR spectra of ostracum and hypostracum layers, the SHP's and the ZFS parameter (D) have been evaluated and discussed with regard to the symmetry of the material. The FT-IR spectral results give strong support on the structural details of shells discussed by EPR technique. From the optical absorption spectrum of the ostracum, the crystalfield parameters Dq, B and C have been evaluated.
1. INTRODUCTION Carbonate minerals or carbonaceous shells in various forms such as calcite, aragonite, dolomite etc constitute the earth's largest COz reservoir. The marine and fresh water mussel shells in particular form themselves as the carbonate shells by absorbing much of the atmospheric COz [I]. The transition metal ions such as Fe3+,Mn2' and V4+ display a rich biocoordination in biominerals. In such cases, EPR and optical absorption techniques are the useful analytical tools for the identification and characterization of such paramagnetic cations in biological and related systems and is often used to investigate their behaviour [2-51. CaC03 exists as calcite, aragonite and vaterite. Aragonite materials and/or deposits undergo slow metamorphism into calcite [2,6] whereas the vaterite form is found to be unstable [4]. The present work is aimed at the spectroscopic studies of the shells of the edible apple snail, to know their structural details. An overview of the shells and their description and composition are given elsewhere in detail [5].
2. EXPERIMENTAL The fresh living animal shells of were collected from the fresh water ponds located around Tirupati in South Indian origin. The soft parts were removed by putting the specimen in warm water and washed them thoroughly. The shells were then dried at 40°C in a hot air oven. The three layers of the shells were separated out as
254
described in our earlier paper along with other experimental details [5]. For y-irradiation, 6oCosource was used at IGCAR, Kalpakkam, India. 3. RESULTS AND DISCUSSION EPR studies Figure 1 shows the EPR spectrum of periostracum at room temperature (RT) and at liquid nitrogen temperature (LNT). The EPR spectrum of periostracum layer at RT shows a sharp signal at g = 2.032. The intensity of the signal increases when the sample is dried at 368 K. This signal has been attributed to chonchiolin, associated with an organic protein matrix [7]. Besides this, a Dysonian signal centred at gx2.0 has also been appeared around this free radical signal on either sides. This Dysonian signal at gx2.0 and another clear signal with linewidth AHx68 gauss appearing at gx4.1 have been attributed to Fe3+ ions, which are expected to be involved in biological systems in transportation of oxygen to the tissues [8]. The EPR spectrum of fresh sample of the ostracum from the shell at RT gives a complex spectrum containing number of Mn2+ signals of at least 3 sets of six line hyperfine (hf) lines arising from the central sextet I -%> ++I +%> transition. Besides this, the forbidden doublets were also observed in between the two hf lines. The g values for the three sets varies as g =2.01f0.04. This indicates the strong deposition of Mn2+ions in this layer at random orientations. The innermost of the shell namely hypostracum is carefully collected from the shell and the operculum as well separately. The EPR spectrum of hypostracum of the shell recorded at RT exhibits a broad signal centred at gx3.45 and a sextet hf spectra of the Mn2+centred at g FS 2.01 1 as shown in Figure 1. No significant change is observed in the sextet hf lines of Mn2+ ions of this sample recorded at 103K, except an increase in intensities of the hyperfine lines at low temperatures. A broad peak appears at 153 K centred at g ~ ~ 3 . 7 7with 2 a linewidth AH = 750 gauss. As the temperature increases, the g value as well as the linewidth decreases steadily as shown in Figure 2. This can be attributed to the exchange interaction of the impurity Mn2+ions. The EPR spectrum of hypostracum sample collected from the operculum recorded at room temperature exhibits two broad resonances centred at g = 3.3 and g x 2.4 and sextet hf spectrum of Mn2+ions centred at g = 2.0.
Figure 1: EPR spectra of the fresh sample of (a) periostracum and (b) hypostracum layers of shells.
25 5
I
I
C
500 200
Temperature (K)
400
Temperature (K)
Figure 2. The variation of g m3.45 resonance (observed for hypostracum layer) and its linewidth with temperature. The EPR spectrum of hypostracum sample collected from the operculum recorded at room temperature exhibits two broad resonances centred at gm3.3and gm2.4 and sextet hyperfine spectrum of Mn2+ions centred at gz2.0. The g and A parameters calculated for all layers of the shells are given in Table 1. The D value has been calculated from the forbidden transitions as given by Nedelec et a1 [9]. A clear examination of the zerofield splitting parameter D, reflects the extent to which the coordination sphere of Mn2+ions deviate from perfect cubic symmetry and is consistent with most of the Mn2+:CaC03 lattices in calcite and aragonite symmetries. The y -irradiation of the shell species produced a change in the EPR spectrum in the free radical region giving resonances at g=2.0006, 1.9996 and 1.992 respectively. Based on their linewidths also, these signals can be attributed to CO3- and C02- radicals which are in good agreement with the reported results for y-irradiated CaCO3 materials [ 3 , 101. Table 1 The spin-Hamiltonian parameters (g and A) and the zerofield splitting parameter (D) for different layers of shells (The errors in g, A and D values are 0.002, 3 x 10"' cm-' and 4 x 10"' cm-' respectively)
ern-')
A
Material of the layer
g
1. Periostracum (Fe3') of the shell (free radical)
g = 2.0, g m 4.1 g m 2.032
2. Ostracum of the shell (Mn")
g1=2.006, g2=2.017 g3=2.030
A1=81, A 2 4 4 A3=85
108
3. Hypostracum of the shell (Mn")
2.01 1
87
137
4. Ostracum of the operculum (Mn2')
2.003
84
143
5. Hypostracum of the operculum (Mn2')
2.006
83
147
--
D
--
ern-')
256
3.2 Infrared spectral studies The infrared technique has been used to know the symmetry of CO?- molecule in different layers of the shells. The carbonate ion being D3h symmetry exhibits normally four vibrational modes as discussed elsewhere [2,6,11]. Among the four, one of the vibrational frequencies namely vl(A1), the symmetric vibration of CO': ion is reported to be infrared inactive [4,7,11]. The FT-IR spectra have been recorded in the range 2000-400 cm-' for all the two layers of the shell and is shown in Figure 3. The infrared band positions of co32and their tentative assignments in different layers of the shells are presented in Table 2. From the nature of the IR band positions, the planar bending v3(E) and the asymmetric stretching v ~ E at ) 1478 cm-' and 714 cm-' respectively in addition to the presence of infrared inactive mode vl(A1) observed at 1082 cm-' clearly indicate the deviation from the perfect calcite symmetry. From the above observed bands and their nature, it can be concluded that the carbonate ion in the shells layers is in aragonite whereas the material of the operculum is composed of both calcite and aragonite symmetries indicating the active metamorphic processes in the operculum.
Figure 3. FT-IR Spectra of ostracum and hypostracum layers of shells and the operculum of Table 2 Infrared band positions and their tentative assignments of shells (m = medium, sp=sharp, w=weak, b=broad) Band positions for Ostracum of Shell Operculum 1788(m) 1478(b) 1452 (w) 1082 (SP) 862 (w) 7 14 (SP) 700 (sp)
1798 1424
-876 712
shells in cm-'
Hypostracum of Shell Operculum 1788 (w) 1476 (b) 1455 (w) 1082 (m) 862 (m) 714 (SP) 700 (SP)
co32-molecular
Band Assignment
1798 1418
(v1 + v4) V3(E)
1084 864 714 710
v 1(A) V2(B) V4(E)
ion in
257
3.3 Optical absorption studies The optical absorption spectra observed at RT for the ostracum layer of both the shell and the operculum have been assigned to the d-d transitions of the Mn2' ions having ground state 6A&3). The observed and calculated band positions and their assignments have been given in Table 3. By diagonalising the energy matrices for d5 configurations inclusive of Tree's correction [12], the crystal field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated and are included in Table 3. The magnitude of these parameters indicates that the Mn2+ion is in a distorted octahedral environment [2, 131.
Table 3 Optical absorption spectral data of the ostracum layer of the shells and ostracum layer of the operculum Shell material
Operculum material
6Aig(S) to
4TI g(G) 4T2g(G) 4EdG> 4~2g(~) 4Eg(D>
observed (cm-')
calculated (crn-')
17694 21271 22618 26308 27389
17373 21355 23242 26027 27483
B = 750 cm-', C = 2850 cm-' Dq = 750 cm-'
observed (crn-')
17998 21277 23474 25963 27168
calculated (cm-')
17731 21568 23492 26142 27558
B = 725 cm-', C = 2950 cm'' Dq = 750 cm''
4. CONCLUSIONS
(i)
(ii)
(iii)
From the EPR spectra of the periostracum, the observed sharp line at g = 2.032 has been attributed to chonchiolin and the resonances at g 2.0 and 4.1 have been attributed to Fe3+ ions in a rhombic symmetry. The presence of Mn2+ions in ostracum and hypostracum layers has been established. The hyperfine splitting constant A and the zerofield splitting parameter D are well comparable with those of Mn2+:CaCO3 materials in both aragonite and calcite symmetries. It is found that y-irradiation of the snail shells induces changes in the EPR spectrum in the free radical region, producing resonance signals characteristic of both CO3- and COz- radicals.
258
(iv) (v)
From the infrared spectral studies, the symmetry of the C032- ion has been predicted. The slight variations in the band frequencies have been attributed to the positional changes in the C032-and crystallization processes in the layers. The optical absorption studies have been performed for ostracum of the shell and the operculum. The crystalfield and the Racah interelectronic parameters Dq, B and C have been evaluated. The magnitudes of Dq, B and C in these shells suggest that Mnz+ions are in a distorted octahedral site.
ACKNOWLEDGEMENTS One of the authors, KVN is highly thankful to the CSIR, New Delhi, for the award of Research Associateship. The authors are grateful to Mr. D. Ponraj, HASD, IGCAR, Kalpakkam, India for their help in the y-irradiation of these samples.
REFERENCES 1. Margaret-Gail Medina, Gilliam M.Bond, John Stringer, J. Electrochem. SOC., Interface, 10 (2001) 26. 2. Y.Nagaraja Naidu, J.Lakshmana Rao, S.V.J.Lakshman, Polyhedron, 11 (1992) 663. 3. R. Dejehet, S. Idrissi, R. Debuyst, J. Chim. Phys. (Fr.), 96 (1999) 741. 4. A.Boughriet, B.Mouche1, B. Revel, L.Gengembre, J.Laureyns, Phys. Chem. Chem. Phys., 1 (1999) 4051. 5 . K.V.Narasimhulu, J.Lakshmana Rao, Spectrochim. Acta, 56A (2000) 1345 and references therein. 6. K.Nakamoto, Infrared and Raman spectra of Inorganic and Coordination Compounds (lothEd.), John-Wiley & Sons, Inc., New York, 1986, p.87, 124. 7. E.L. Jordan, P.S.Verma in : Invertibrates Zoology, S.Chand & Co. Ltd., New Delhi, 1992, p. 813. 8. J.B. Neilands in : Advances in Experimental Medicine and Biology, Vol. 40; Metal ions in Biological Systems, Edited by Samat K. Dhar, Plenum Press, NY 1973, p.13. 9. J.M.Nedelec, M.Bouazaoui, S.Turrel1, Phys.Chem.Glasses, 40(1999) 264. 10. Toshikatsu Miki, Ayaku Kai, Jpn. J. Appl. Phys., 29 (1990) 2191. 11. S. Bhagavantham, T. Venkatrayudu, Proc. Ind. Acad. Sci., A9 (1939) 224. 12. A. Mehra, J. Chem. Phys., 49 (1968) 3516. 13. Roger Clausen, Klause Petermann, IEEE J. Quant. Electron., 24 (1988) 1114.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
259
ESR study of iron-sites on Fe-ZSMS zeolite Nguyen Tien Tai ', Nguyen Huu Phu a, Tran Thi Kim Hoa a, Hidenobu Hori b, Makoto Taki Institute of Chemistry, National Centre for Science and Technology, HoangQuocViet Road, Hanoi, VIETNAM
a
Japan Advanced Institute of Science and Technology, 1-1Asahidai, Tatsunokuchi, Nomi-gun, Ishikawa 923-12, JAPAN Fe-ZSM-S zeolites with Si/Fe ratio varying from 25 to 90 were hydrothermally synthesized. ESR technique at room temperature and 4.2K was used to obtain information mainly on iron species. Two groups of X-band ESR signal were observed from the zeolites. They are attributed to at least two kinds of Fe-center, namely framework and extra-framework iron. The ESR signal assignment is supported by data of vacuum treatment and Fe doped dependence. The framework iron is identified as Fe species at strong rhombic distortion of the tetrahedral coordination of Fe3+, while the extra-framework iron is identified as the antiferromagnetic binuclear ions with bridging oxygen, located in the channel system or at the surface of the zeolite. The catalytic activity of each Fe-sites on the zeolites is determined.
1. INTRODUCTION 3d transition metals as dopants in catalyst are of considerable interest in catalysts. Among them, iron in zeolites is very attractive to researchers in term of both catalytic activity and selectivity enhancements. Fe silicalite is able to catalyze the one-step oxidation of benzen to phenol with nitrogen monoxide as oxidant [l], while Fe-ZSMS zeolite is the promising catalyst for the phenol oxidation [2, 31. In a previous publication, based on IR, XRD, SEM and ion exchange technique data [4], we have reported on the synthesis, characterization and catalytic activity of Fe-ZSMS zeolite system, however the nature and magnetic behaviour of iron, doped in the zeolite lattice, were only sketchily suggested. The present contribution aims at giving a deeper understanding of Fe behaviour by ESR experiments. As high sensitive technique and the most effective tool for the characterization of iron species in different lattices, ESR technique was used for study on the magnetic behaviour of Fe-centers. The discussion is mainly focused on the interpretation and understanding of X-band ESR spectra, taken in Fe-ZSMS zeolites, synthesized by hydrothermal crystallization.
260
Table 1 Fe-ZSM5 synthesized zeolites
(%)
1 2 3 4
90 60 43 25
0. 782 0.2562 0.3676 0.6307
89 87 64 44
2. EXPERIMENTAL Fe-ZSM-5 zeolite samples with Si/Fe ratio varying from 25 to 142 were hydrothermally synthesized (Table 1). Iron concentration data, given in the table, are determined by a atomic absorption spectrometry (AAS 3300 Perkin Elmer). The Fe-ZSM-5 samples were crystallized in autoclave at 443K for 72h. The obtained solids were washed with distilled water until reaching pH=7, then dried at 393K, before calcined at 723 in airflow for 4h. Synthetic procedure is described elsewhere [4]. ESR ESR was used to obtain information mainly on iron species. X-band ESR spectra were recorded on a JES-RE3X spectrometer (School of Materials Sciences, Japan Advanced Institute of Science and Technology), at room temperature (RT) and 4.2K. For comparison of different ESR spectra, the acquisition parameters were kept constant and the microwave power was maintained below the saturation level.
3. RESULTS AND DISCUSSION 3.1. ESR observation The X-band ESR spectra of sample 2 (Si/Fe=90), recorded at RT, are given in Figure la. Main spectral features for all series of synthesized samples are rather similar and can be characterized as follow: The spectra consist of two major signal groups. The signals of the low-field spectral group, designated as Lsignals, is determined by the overlapping of various spectral lines at g>3, while a high-field group, designated as H-signals, consists of resonance transitions at g-2. According to a number of publications on similar subjects [5-71, the commonly accepted assignment of ESR signals have been as follow: The H-signals are attributed to octahedral coordination of Fe3+ ions in cationic exchangeable sites, having C3" symmetry. They are probably octahedrally coordinated by oxygen and located in the channel pore system of the zeolite. The L-signals are caused by Fe (111) ions in sites of strong rhombic distortion of the tetrahedral coordination of Fe3+.These ions substituted to Si (IV) in the zeolite lattice. Some further spectral structures are observed in very low temperature experiments or in the spectra of the vacuum treated samples. These spectral features will be described in more detail below. Fe-centers can be characterized in more detail by some additional experiments, which are described just below.
26 1
Figure 1. X-band ESR spectra of Fe-ZSMS zeolite, sample 1: Si/Fe=90, recorded at RT. a. After hydrothermal synthesis b. After the first vacuum treatment at 373K c. After exposition of sample to air d. After the second vacuum treatment at 300K
3.2. Vacuum treating cycle The sample is treated firstly in vacuum at 3 7 3 y and then ESR spectrum is taken (Figure 1b). After the vacuum treatment, the spectral profile changes strongly. The H-signals, attributed to Fe in pore, are obviously reduced, while the L-signals, attributed to Fe inside lattice, are remained almost unaffected. In subsequent step, the sample is exposed to air; ESR spectrum is acquired (Figure lc). The spectrum obtained is almost the same as one before the treatment. Finally, the sample is treated once more in vacuum and the ESR spectrum is recorded (Figure Id). The spectral profile observed after the second vacuum treatment is similar to that after the first one. The unchanged of L-signals may be explained by the stability of framework iron, while the reversible change of H-signal may be related to the transformation of Fe3+ and Fe3+/Fe2+complex, which is produced by partial reduction of Fe3+ to Fez+ at air condition. Although Fez+ions are not active in X-band ESR and measurements at temperatures of T>4& but if Fez+ is combined with Fe3+,a broad signal can be observed [S]. The reversibility of oxidation-reduction cycle of Fe ions, according to the H-signals, reveal the reversible redox cycle of Fe3+ Fez+, and may allow the preparation of suitable catalysts for oxidation reaction.
-
3.3. Temperature dependence When lowering the sample temperature to 4.2K, the spectral intensity of two groups of spectral lines changes in quite different way (Figure 2). The intensity of L-signals increases as normally when temperature goes down, but H-signal almost does not change (Figures 2a and 2b), even it decreases in the case of high-doped sample (Figures 2c and 2d). This effect firmly suggests antiferromagnetic coupling of the Fe3+ions in a (Fe3+-0-Fe3+)complex ions inside the pore system of the of the zeolite. This type of temperature dependence in the range of RT down to 77K has been observed and reported on calcined Fe-ZSMS [9] and sublimated Fe-ZSMS zeolite system
POI.
262
a
em
2w
am
Magnetic field (mT)
Figure 2 . x-band ESR s p e c t r a of Fe-ZSM5 Z e o l i t e . 21 a . Sample recorded a t RT;
Si/Fe=60,
c.Sample 4, recorded a t RT
Si/Fe=25 I
b. Sample
Si/Fe=60,
d.Sample 41 recorded a t 4 . 2 K
Si/Fe=25,
21
recorded a t 4 . 2 K
3.4. Catalytic activity Correlation of catalytic activity with magnetic and structural properties was determined, based on the oxidation reaction of phenol [4]. It is revealed that both the framework and extra-framework Fe play a role of active catalytic centers for the oxidation reaction of phenol in aqueous solution. However, the catalytic activity of framework Fe is higher than that of the extra-framework Fe. This conclusion on oxidation of phenol on Fe-ZSMS zeolite seems to be opposite to the case of the dehydrogenation of ethylbenzen by using impregnated Fe-silicalite, where the high catalytic action is attributed to the extra-framework iron, but not to the framework one [12]. cst.iyt1s s.1.ctlvity
to
co,
1
20.
0
I+a
7 0
50
100
150
2w
Reastion time (min)
Figure 3. The time dependence of selectivity to carbon dioxide a. FezOs-silicalite b. Fe-ZSMS zeolite, 17%intra-framework Fe c. Fe-ZSMS zeolite, treated by EDTA
250
0
263
4. CONCLUSION Iron doped in ZSM-5 zeolite exists in two different forms: framework and extra-framework iron. X-band ESR signals of Fe-ZSM5 zeolites with g > 3 exhibit Fe species, located at framework sites, while signals at g = 2 - 2.3 are related to Fe at extra-framework positions. Both the framework and extra-framework irons exhibit catalytic activity to the oxidation reaction of phenol in aqueous solution but not equally. The Fe inside zeolite lattice can catalyze to oxidize phenol completely than the extra-framework iron does. The authors gratefully acknowledge professor Roduner E., Institute of Physical Chemistry University of Stuttgart, for helpful discussions.
REFERENCES 1. A. S. Kharitonov, G. A. Sheveleva, G. I. Panov, V. I. Sobolev, Y. A. Paukhus, V. N. Romanikov, Appl. Catal., 98 (1993) 33. 2. K. Fajenverg, H. Debellefontaine, Appl. Catal. B: Environ., 10 (1996) 229. 3. J. C. Jansen, E. J. van der Gaag, H. van Bekkum, Zeolites, 4 (1984) 369. 4. Nguyen Huu Phu, Tran thi Kim Hoa, Nguyen Van Tan, Hoang Vinh Thang, Pham Ha, Appl. Catal. B: Environ., 901 (2001) 1. 5 . G. Calis, P. Frenken, E. de Boer, A. Swolfts and M. A. Hefni, Zeolites, 7(1981) 319. 6. K. G. Ione, L. A. Vostrikova, V. M. Mastikhin, J. Mol. Catal., 31(1985) 355. 7. L. M. Kustov, V. B. Kazansky, P. Ratnasamy, Zeolites, 7(1987) 79. 8. G. Scholz, R. Stosser, T. Grande, S. Asland, Ber. Bunsenges. Phys. Chem., 101 (1997) 1291. 9. A. Bruckner, R. Luck, W. Wieker, B. Fame, Zeolites, 12 (1992) 380. 10. El. M. El-Malki, R. A. van Santen, W. M. H. Sachtler, J. Phys. Chem. B, 103 (1999) 4611. 11. F. Grafe, Ph. D., Technical University, Dresden, 1989. 12. 0. Kan, Z. Wu, R. Xu, X. Liu, J. Mol. Catal., 74 (1992) 223.
264
EPR in the 21' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
CW/pulsed ESR studies of Eu2"-doped SrA1204phosphor H. Matsuoka", K. Sato", D. Shiomib, Kojima", K. Hmtsu",
Furunoc, and T. Takui"
a p a r t m e n t of Chemistry, Graduate School of Science, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-Ku, Osaka 558-8585, Japan bepartment of Materials Science, Gmduate School of Science, Osaka City University, Sugimoto 3-3-1 38, Sumiyoshi-Ku, Osaka 558-8585, Japan "Fine Clay Co., Ltd., Osho-Kita 1-3-8,
Hyogo 660-0063, Japan
In work, X and W-band ESR measurements of a long-lasting luminescent material SrAl~O4:Eu,Jlywere carried out in order to determine precise spin Hamiltonian parameters for the eledronic ground state. The spectral analysis including higher order fine-structure parameters reproduced the observed spectra better than the previous analysis neglecting the higher order terms. We also observed the X-baud ESR spectra of SrAlzO4:EupY whose particle are controlled in the micron range in order to examine a relationship the particle size and fine-structure parameters, showing significant differences between the observed spectra for the SrAlzO4:EupY with different particle sizes. 1. INTRODUCTION
Since it was found out that SrAl~O4:Eu(II),Dy(III)shows long lasting luminescence [l], much attention has been paid to the new luminescent material [1-6]. Two proposals made for the mechanism of the long lived luminescence by several workers [2, but the detailed mechanism has not been filly clarified yet. Electron Spin Resonance (ESR) spectroscopy provides us with information on a local setucture around the Eu(I1) ion, which is the luminescent center, well its electronic structure. High-fieldhigh-frequency powder-pattern CW-ESR measurements and a analysis neglecting higher-order fine-structure terms have been carried out for the luminescent material by et ul., showing that the Eu(@ ions in non-stoichiometric SrAlz04:EQy occupy four different sites and have intermediate zero-field splitting (ZFS) parameters [3,4]. We however, that the spectral simulations using the spin Hamiltonian parameters determined by et ul. do not reproduce completely the correspondingobserved X-band CW-ESR [6]. In this work, we reconsidered the spin-Hamiltonian parameters of SrA1~04:EUpyin terms of the combined use of X- and W-band CW-ESR spectroscopies and a spectral analysis including higher order fme-structure parameters. Spectral simulationswere carried out by the numerical exact diagonalization of the spin Hamiltonian based on a hybrid eigenfield
265
approach. In addition, X-band powder-pattern CW-ESR measurements of SrAlzO4EqDy whose particle sizes controlled in the micron range were performed in order to examine a relationshipbetween the particle size and the long lasting luminescentproperty.
DISCUSSION
2.
2.1 Reconsiderationof spin-Hamiltoninn parameters of Sr&O4;Eu,Dy
Figures l(a) and 2(a) show X and W-band powder-pattern CW-ESR spectra observed for SrA1204:Eu,Dy at room temperature, respectively. For Eu2* ion with *S7n ground state orthorhombic site, fme-structure ESR spectra can be analyzed in of the following spin Hamiltonian:
In the case of sites with lower symmetry such monoclinic or lriclinic symmetry, other terms, etc., should be considered. However, the number of which we can determine h m analyses of powder-pattern ESR spectra is limited due to problematic overparameterhation. Therefore, in this work the orthorhombic Hamiltonian were employed and the sixth order terms except were neglected. the order approximation, differences resonance fields of A4.s ++ A& -1 and - A& ++ A& +1 transitions give for & // z by the following equations, / 2 ++ +5 /
/ 2 ++
-
=-(12b,O+40b,O+12b,O)/gPe,
B 0 ( + 3 / 2 t , +1/2)-B0(-1/2t, -3/2)
+
-
=
/ gPe
and for & z by the following equations, B0(+7/2t,+5/2)-B0(-5/2t,-7/2) =
-
/ 2 ++ =
+ / 2 ++ +I
+ /
cos 29 + 5(b; cos 29 -
/ 4-
/ 2-
f)
/ 4-
cos 441)]/gP,,
-5 /
cos 29 -
cos 29 -
COSQ)/~]/
gPe7
f)
+9b: + 3 5 b , O / 4 - 2 b ~ ~ 0 ~ 2 ~ - 3 ( b ~ c o s 2 c p - b , " ~ 0 ~ 4 4 I ) ] / g P ~ , where b2q= b4q= b6q= and E= A of equations to ours has also been derived by Reynolds et ul., but the term was omitted in their [7$ In this work, two approaches for spectral analyses were employed in order to reconsider
266
,, 0
,
I
,
,
I
0.1
,
I
I , , . , , , , . , LLLILLLLCl 0.2 0.3 0.4 0.5 0.6 0.7 Magnetic Field I T ,
1
I
,
1
,
I
Figure (a) Observed and (b) calculated X-band ESR spectra for SrAhO4:Eu,Dy at room temperature. v = (bl) approach (bz) approach
2.8
3
3.2 3.4 3.6 3.8 Magnetic Field I T
4
Figure (a) Observed and (b) calculated W-band ESR spectra for SrAl~04:Eu,Dy at room temperature. v = (bl) approach (bz) approach
the spin Hamiltonian parameters of SrAlzO4:Eu,Dy. (i) We determined a set of the spin Hamiltonian parameters for SrA1204:Eu,Dy in terms of a spectral analysis based on the spin Hamiltonian including fourth and sixth order terms (termed approach First, a set of approximate parameters are obtained using the equations (2) and (3) from the observed W-band ESR spectrum. Next, the approximate parameters are refined by minimizing the followin groot-mean-squareparameter R with the help of a hybrid eigenfield method:
Bobsemed(n), &alculated(n), and stand for the number of observed data points, where observed fields, calculated fields, and weights. (ii) We also determined another set of the spin Hamiltonian parameters for SrAlzO4:Eu,Dy in terms of a spectral analysis based on the spin Hamiltonian neglecting the higher order fine-structure terms (termed as approach The best set of parameters, g, D, and E values, are directly determined by minimizing the root-mean-square parameter R in terms of a numerical calculation based on the eigenfield method, not on the perturbation approach. Finally, all spectral simulations were performed based on the eigenfield method. The resonance fields were calculated by solving numerically an eigenfield Hamiltonian [8The fine-structure transition intensities were calculated by diagonalyzing the spin Hamiltonian energy matrices with exact resonance fields acquired by the eigenfield method [8, This hybrid approach enables us to save a great deal of CPU time well to avoid technical difficulties in the numerical computation. Figures 2(bl) and (b2) show the respective spectral simulations using the spin Hamiltonian parameters determined by approaches 1 and indicating that the Eu(I1) ions occupy four
267
Table 1 Spin Hamiltonian parameters and R values obtained for SrAl204:Eu,Dy Approach 1 sl
s2
s3
1.9943
1.994
1.994
1.9944
1.9932
b:
1379
b;
Approach 2 sl
s2
s3
1.9941
1.9949
1.9936
1.994
1.994
1.994
1.9941
1.9941
1.9935
1.994
1.994
1104
1028
926.3
1377
1082
1005
898
1195
944
1028
926.3
1091
1042
1005
898
b:
0.47
6.6
5.8
8.7
bl
40
-33.2
-23.8
-36.5
b:
-57.8
78.3
-3 1.2
-59
b,"
0.7
2.2
2.4
1.3
R
0.9
1.3
1.1
0.4
4.8
4.5
2.6
3
~~
g//
* 1 All fine-structure terms and R values
given in units of lo4 an-' and mT, respectively. *2 The R values were acquired from the spectral analyses of the W-band ESR different sites. The calculated spectra were acquired by superimposing all the spectra fiom the four different sites with the intensity ratio of 6:6:2:1. The number of the Eu sites and h e The intensity ratio obtained in this work are consistent with the results by Nakamura et four different sites denoted by site 1,2,3, and 4 in Figure 2(a), which correspond site I and 11 of Sr4A1~025:Eu,and site I and I1 of stoichiometricSrAI204;EuDy in the l i t e m [4], respectively. The calculated spectra both approaches seemingly consistent with the observed W-band ESR spectrum, but the R values shown in Table 1 indicate that the spectral analysis by approach 1 reproduced the observed specbetter than by approach 2. The spectral simulations given in Figures l(b1) and (bz) reveal the higher order fme-structure terms crucial for the determination of the spin Hamiltonian parameters of SrAl204;EuDy. In order to acquire a agreement the observed and simulated X-band ESR spectra, Q-band ESR measurements and a spectral analysis considering a frequency dependence of linewidth underway. 2.2 X-band CW-ESR spectra of SrAlzOd;Eu,Dy whose particle sizes are controlled in the micron range Recently, diverse quantum effects have been observed for particles with semi-macroscopic sizes. Sharma et ul. observed that decreasing the particle size of Eu20fl203 powders h m pm to nm scale yields the enhancement of the emission intensity for fluorescence of Eu(5Do + 7F2 transition) [15]. In this work, long-lived luminescent materials, SrAI204;EUDy, whose particle sizes are controlled in the micron range were newly prepared. Their emission life time and color depend on the particle sizes. We carried preliminary X-band ESR
268
-
u--u--,
0
0.1
0.2 0.3 0.4 Magnetic Field / T
0.5
Figure 3. Observed X-band ESR spectra of SrAI204:Eu,Dy with the mean particle diameter of 6.8 p.(a) v = 9.79034 GHz, at room temperature. (b) v = 9.60478 GHz, at 4.0 K.
I , ,
0
.,
I , ,
0.1
..
L-LLU-
0.2 0.3 0.4 Magnetic Field / T
0.5
Figure 4. Observed X-band ESR spectra of SrAlzO4:EUpy with the mean particle diameter of 1.8 p.(a) v = 9.79642 GHz, at room temperature. (b) v = 9.60427 GHZ, at 3.0 K.
measurements of them with different particle sizes at room and liquid helium temperatures in order to examine a correlation between the particle size and luminescentproperties. Figures 3 and 4 show the observed X-band ESR spectra of SrAlz04;EuDy with the mean particle diameters of 6.8 and 1.8 p,respectively. There are signilkant particle size dependences in the observed spectra well the temperature dependence. It is suggested that the long-lived luminescent properties may depend on local structures around the Eu@) ions a d o r the site occupation ratio the Eu(II) ions well a particle size effecf though the homogeneity of the surface may also influence them. Q- and W-band ESR measurements of them underway in order to determine appropriate spin Hamiltonian parameters and to examine a relationship between the particle size and long-lived luminescent properties. ground-state polarized ESR spectroscopy is underway in order to acquire information on the luminescent sites.
CONCLUSIONS In work, we reconsidered and refined the spin Hamiltonian parameters of the long-lived luminescent material, SrAlzO4:Eu,Dy, in terms of X and W-band ESR spectroscopies and the spectral analysis including higher order fine-structure parameters. It was concluded that the higher order fme-structure terms required for the determination of the appropriate spin Hamiltonianparametem of SrAlz04;EuDy. X-band ESR measurements of SrAlp04:EupY whose particle sizes controlled in the micron range were also carried out in order to examine a relationship between the particle size and fine-structure parameters, showing the significantparticle-size and temperature dependences in the observed spectra.
269
ACKNOWLEDGMETNS We thank Professor T. Nakamura of Shizuoka University, Japan, for providing us with valuable discussions. We are also grateful to Professor T. Kato of Institute for Molecular science, Japan, for the W-band ESR measurements. This work been partially supported by Grants-in-Aid for General Scientific Research and for Scientific Research on Priority “Molecular Magnetism” No. 228/04 242 103 and 04 242 105), “Delocalized Electronic Systems” (Area No. 297), “Molecular Conducting and Magnetic Materials”, Grants-in-Aid for Encouragement of Young Scientists (Grant No. 12 740 385 (D.S.) and 12 740 324 (K.S.)) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and also by the Ministry of International Trade and Industries (NED0 project ‘‘Organic Magnets”). REFERENCES 1. T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, J. Electrochem. Soc., 143 (1996) 2670. 2. M. Ohta, M. Maruyama, T. Hayakawa, and T. Nishijo, J. Ceram. SOC.Japan, 108 (2000) 284 [in Japanese]. 3. K. Kaiya, N. Takahashi, T. Nakamura, T. Matswwa, G.M. Smith, and P.C. Riedi, J. Lumin., 87-89 (2000) 1073. 4. T. Nakamura, K. Kaiya, N. Takahashi, T. Matswawa, C.C. Rowlands, V. BeWn-Lbpez, G.M. Smith, and P.C. Riedi, J. Mater. Chem., 10 (2000) 2566. 5. M. Kamada, J. M& and N. Ohno, J. Lumin., 87-89 (2000) 1042. 6.’K. Sato, H. Matsuoka, J.Y. Bae, D. Shiomi, T. Takui, K. Kaiya, N. Takahashi, H. Fujiyasu, T. Matswawa, T. C.C. Rowlands, V. BeltrAn-Lbpez, and G.M. Smith, Proc. of 2“dAsia-Pacific EPRESR symposium, (1999) 76.
R.W. Reynolds, L.A. Boatner, C.B. Finch, A. Chatelain, and M.M. Abraham, J. Chem. Phys., 56 (1972) 5607. 8. K. Sato, H. Matsuoka, D. Shiomi, and T. Takui, Mol. Cryst. Liq. Cryst., 335 (1999) 333. 9. H. Matsuoka, K. Sato, D. Shiomi, and T. Takui, Synthetic Metals, 121 (2001) 1822. 10. N. Banwell, and H. Primas, Mol. Phys., 6 (1963) 225. 11. G.G. Belford, R.L. Belford, and J.F. Brukhalter, J. Mag. Reson., 11 (1973) 251. 12. K.T. Mcgregor, R.P. Scaringe, and W.E. Hatfield, Mol. Phys., 30 (1975) 1925. 13. K. Sato, Doctoral Thesis, Osaka City University, 1994. 14. Y. Teki, I. Fujita, T. Takui, T. Kinoshita, and K. Itoh, J. Am. Chem. Soc., 116 (1994) 11499. 15. P.K. Sharma, M.H. Jilavi, R. Nass, and H. Schmidt, J. Lumin., 82 (1999) 187.
270
EPR in the 2lStCentury A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Thermoluminescent mechanism of tridymite SiO, phosphors Masatoshi OHTA", Takato NAKAMURAb,Michiko TAKAMI" "Department of Material Science and Technology, Faculty of Engineering, Niigata University, 8050, Ikarashi 2-no-cho, Niigata, 950-2 181, Japan bDepartmentof Materials Science and Technology, Faculty of Engineering, Shizuoka University, Hamamatsu, 432-8561, Japan "Graduate School of Science and Technology, Niigata University, 8050, Ikarashi 2-no-cho, Niigata, 950-2 181, Japan
Stability of traps induced in SiO, by Al-, Eu- and Tb-doping has been examined by means of ESR and TL spectroscopy in order to elucidate relation between the depth of traps and emissions; the phosphors were prepared by a sol-gel method using optically high-pure SO,. For A13'- and Eu3+-dopings the dependence of the ESR and TL intensities on the storage time after X-ray irradiation suggests formation of a deep hole trap. Also, a shallow electron trap was found in the Eu3'-doped SO,. As for Tb-doping, TL measurements indicate that the emissions are due to Tb3', while the ESR spectrum suggests the formation of Tb4+.Furthermore, TL characteristics of the Eu3+-doped phosphors using ultra fine SiO, prepared by the hydrolysis of water glass were examined since the process is cheap and commercially available for the preparation of the phosphors with blue emission.
1. INTRODUCTION It has been well known that SiO, crystals are useful as a matrix for thermoluminescent (TL) phosphors, especially for the TL dating of earthern vessel and lava [l]. This is because they show a variety of emissions in the visible region depending on the doped metal ions. In the previous papers we have clarified the mechanisms of the TL of A13+and Eu3'-doped SiO,, in which a blue emission is due to A13' and other emissions between blue and red arise from Eu3+[2-41. In this paper, therefore, we report the results
27 1
of the investigations of the stability of traps formed in the A13'-, Eu3'- and Tb-doped SiO, prepared by a sol-gel method [5-61. the TL properties of Eu3'-doped ultra fine SiO, prepared using a reaction of water glass with hydrochloric acid were described in comparison of those prepared by the sol-gel method in order to develop cheap process for the industrial production of the phosphors.
2. EXPERIMENTAL SiO, gel was prepared as follows: lOOg of water glass were hydrolyzed with 0.6 mol/l hydrochloric acid. The obtained gel was washed thoroughly with dilute hydrochloric acid and then with deionized water in order to remove acid and watersoluble components. Various SiO, samples doped with A13', Eu3' or Tb3+were prepared by the following procedure: Measured amounts of A1,0,, Eu,03 or Tb,O, were mixed homogeneously using acetone, and then mixed with the SiO, prepared both by the solgel method (Mitsubishi Chemical Co., Ltd.,) [6] and by the hydrolytic method mentioned above. The mixture was fired at 1573 K for 6 hours in air. It was confirmed by the X-ray diffractometry that the resulting phosphors have tridymite structure. The detailed procedure was described elsewhere [2-41. The doping concentrations were selected to obtain the maximum ESR and TL intensities, respectively. The apparatus used for the measurements of TL spectra consists of a black box with a projection lamp heater, optical quartz fiber, a spectrometer with multi-channel analyzer and intensifier (Hamamatsu Photonics Co., Ltd., PMA-lo), and a personal computer for data analysis, as previously described [2-41. The SiO, sample, irradiated by X-ray (about 0.8kGy, Cu K,, 8keV), was flatly spread on a silver tray in the black box and was then linearly heated from 298K to about 680K by the projection lamp with a constant heating rate. The TL emission was guided to the spectrometer equipped with a multi-channel analyzer and intensifier using an optical quartz fiber, and the spectrum was recorded on a PC. A heating rate of 0.4 deg. s' (estimated error: was controlled by an automatic temperature regulator (Simaden, FP21) equipped with a CA thermocouple. The ESR spectra were measured on the SiO, sample irradiated by X-ray (about l.SkGy, Cu K,, 8keV) using an X-band ESR spectrometer with a 100 kHz magnetic field modulator and a phase-sensitive detector (JEOL Ltd., JES-FEXlXG).
272
3. RESULTS AND DISCUSSION
Figure 1 shows the storage time dependence of ESR signal intensity of A13+-and Eu3'-doped SiO, after X-ray irradiation, in which the A1 and Eu contents are 100 and 10 mmol%, respectively (1700 and 559 ppm for A1 and Eu, respectively). It is seen that the ESR signals start decreasing after X-ray irradiation and then become constant. Based on the g-values observed, the observed signals are due to trapped holes in the host. The decrease in ESR signal
I000
suggests that the electrons and holes produced Figure1. Storage time dependence by X-ray irradiation recombine slowly. Such of ESR spin concentration and ESR slow recombination arises from electron and holes thermally released from the traps, so that signal of (a) SiO, : (1OOmmol%) and (b) of SiO, : Eu3' TL should be observed. TL spectra of the A13+-and Eu3'-doped SiO, (1Ommol%) after X-ray irradiation. are shown in Figs.2 and 3, respectively, in which the A1 and Eu contents are 100 and 1 mmol%, respectively. For the A13'-doped SiO, a broad TL peak due to A13' was observed centered around 500 K, and showed little change in intensity with respect to the storage time. On the other hand, sharp TL peaks including two strong ones at 570 and 610 nm due to Eu3+appeared both in low and high temperature regions. Also, it was found that the TL decreased rapidly soon after X-ray irradiation, and then became constant. This trend is explained in terms of two different type of Figure2. (a) The typical TL spectraps: one is shallow traps which decrease the TL of SiO, : A13' (1 00 mmol%) rapidly, while the other is deep traps whose TL is and (b) storage time dependence independent of the time being. of TL intensity after X-ray irradPreviously, we have reported the ESR of the iation. 50 I00 Storage time I hr
I50
273
Figure3. (a) The typical spectrum of SiO, : Ed' ( l m o l % ) and (b) storage time dependence of TL intensity after X-ray irradiation.
SiO,, and found that the E,' center due to oxygen defect in the SiO, matrix [7-91 is formed. For the A13+-dopedSiO, the ESR signal is assigned to a A13+-centre consisting of non-bridging oxygen and A13+on which a hole is captured [lo]. On the contrary, Eu3+-dopingleads to the ESR signals of a hole and an electron. From the comparison of the ESR spectra of the samples heated at various temperatures and their TL spectra it was deduced that both electron and hole detected by ESR contribute to TL [2]. This implies that for the A13'-doping it occupies the Si4+site, which forms a stable hole. For the E d + doping, however, both an unstable electron trap and a stable hole trap are formed although it occupies the Same Si4+site.
Taking account of the facts described above, the difference between the storage dependence of the ESR and TL intensities shown in Figs. 1 , 2 and 3 is interpreted follows: For the A13'-doped SiO,, the TL intensity for the hole trap remain constant with the storage time although ESR signal intensity decreases gradually until ca. 80 hours after X-ray irradiation. For the Eu3+-dopedSiO, the ESR intensity decreases gradually until ca. 100 hours after X-ray irradiation, and little change was observed for the hole trap like that for A13+doped SiO,. However, it is worth noting that the TL assigned to electron trap decreases in intensity rapidly until 4 hours after X-ray irradiation. This discrepancy between the TL and ESR fading phenomena is explained as follows: As is generally known, ESR signals arise from the radicals existing both in the surface layer and bulk of SiO,, while the TL is due to the radicals existing in surface layer only. Therefore, it is presumed that although the radicals in the surface layer surely quench, they are supplied by migration from bulk of SiO,. The same thing happens for the electron trap formed by Ed+-doping. This brings that ESR signal continue decreasing after the TL quenching stopped. The ESR and TL spectra of the Tb (100 mmol%, 12300 ppm) -doped SiO, are
274
332
334
336
338
shown in Fig.4. For the ESR spectrum a broad signal with hyperfine lines, assigned to Tb4+, was observed. On the contrary, TL spectrum showed the TL peaks assigned to Tb3+at low and high temperature regions, ca. 380K and ca. 480K, respectively. Therefore, it is deduced that both Tb3+ and Tb4+occupythe Si4+site in the SiO, with less distorting the crystal lattice of SiO, for crystal optics.
340
Magnetic field I
Figure4. (a) The ESR signal and (b) the typical TL spectrum of SiO, :Tb (1OOmmol%).
Figure5. Typical TL spectrum of the Eu3'doped (10mmol%) sample using prepared by hydrolytic method.
TL spectrum showed blue emission for the Eu3+doped SO, (10 mmol%, 5590 ppm) using ultra fine SiO, prepared by cheap hydrolytic method of water glass with acid (Fig.5). Based on the results mentioned above, it is suggested that the Eu easily substitutes for Si site because of the increase of the surface free energy due to microcrystalization,
SiO, 4. CONCLUSION
Behaviors of the radicals formed in SiO, by A13+,Eu3+and Tb-dopining were examined by means of ESR and TL spectroscopy. It was found that they substitute for Si4+with less distorting the crystal lattice of SiO, for crystal optics, in which Tb has two oxidation states of Tb3+and Tb4'. Phosphors with blue emission were obtained using the
275
ultra fine SiO, prepared by cheap hydrolytic method of water glass.
Acknowledgments The authors are grateful to Mitsubishi Chemical Co. for providing the SiO, quartz prepared by sol-gel method and to Shin-Etsu Chemical Co. for providing the rare earth oxide. This work was supported by the Japan Society for the Promotion of Science through a grant-in-aid for the Scientific Research (A) No.12355030.
REFERENCES 1. Hujimura, R. and Yamashita, T., Tokyo Co., Youkendou 248-254(1985) 2. Ohta, M, Yasuda, M. and. Takami, M., J. of Alloys and Compoun., 303-304,320-324 (2000). 3. Ohta, M. and Takami, M., J., of Sol-Gel Sci. and Tec., 19, 737-740 (200). 4. Ohta, M. and Takami, M., J. of Lumin., be in press. 5. Hayashi, A., Ceram., 20 (4) ,274-279 (1985). 6. Shima, K. and Akira Utunomiya, Ceram.,33 (l), 39-42 (1998). 7. Griscom,D.L., Rev. Solid State Sci., 4, 565-599 (1990). 8. Halliburton, L.E., Appl. Radiat. Isot. 40,859-863 (1989). 9. Weil, J.A., Phys. Chem. Minerals, 10, 149-165 (1984). 10. Lell, E., Kriedl, N.J. and Hensler, J.R., Progr. Ceram. SOC.,4,3 (1966).
276
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR and luminescence spectral properties of europium compounds with trifluoroacetic acid I.V. Kalinovskaya, A.N. Zadorozhnaya, V.G. Kuryavyi, V.E. Karasev. Institute of Chemistry, Far East Division, Russian Academy of Science, pr. Stoletiya Vladivostoka 159, Vladivostok, 690022, Russia Europium trifluoroacetates of the composition Eu(TFA)32DnH20, where TFA trifluoroacetic acid anion, D - 1,lO-phenanthroline (phen), n=l; 2,2 '- bipyridil (bipy), n=3 were proven to be resistant to UV - light in crystalline state and in polyethylene matrix. Increase of luminescent emission of europium at irradiation by UV - light was revealed. The analysis of X -ray, ESR, IR and luminescence spectra of photochemical process products was carried out. The ESR study showed that increase of the luminescent emission of europium complexes is symbiosisly to increase of the contents of bipyridil anion - radical. The ESR and IR spectroscopy data allow to make the conclusion about the mechanism of the increase of europium luminescent emission in trifluoroacetates with nitrogen-containing neutral ligands.
1. INTRODUCTION
The rare earth elements ( W E ) trifluoroacetate complexes with interesting magnetic and spectral - fluorescence properties, are few studied. In works [I-31 such properties of lanthanoid compounds with trifluoroacetic acid, as solubility, density, crystalline and fluorescence character were investigated. The REE trifluoroacetates stability to the luminescent emission has not been tested as yet. At the present study the analysis of spectral - fluorescent properties and photochemical decomposition of europium trifluoroacetates with the nitrogen - containing neutral ligands with composition Eu(TFA)32DnH20, where TFA - anion of trifluoroacetic acid, D - 1,lOphenanthroline @hen), n=l; 2,2 '- bipyridil (bipy), n=3 was carried out.
2. EXPERIMENTAL Europium trifluoroacetates were synthesized according to the procedure [4]. The IR spectra in the region 400-4000 cm-' were record at a Perkin - Elmer FT - IR device using KBr. The accuracy of vibrational frequencies definition was k2.0 cm". The low-temperature luminescence spectra were recorded at a SDL - 1 spectrometer. For this purpose the samples were lowered into a Dewar vessel with liquid nitrogen. A DRSh 250 mercury lamp was used for excitation of the complexes. The error of measurements of wave numbers for the transition bands 5Do-7F, (i=0-4) did not exceed k3.0 cm-' (number of
217 definitions not less than The excitation luminescence spectra were recorded on a device made on the basis of a SDL - 1 spectrometer and a MRD - 23 monochromator, the excitation source was a xenon lamp "Tungsram" with power 2,5 kW. The luminescence spectra of the obtained compounds have an individual nature and differ by value of Stark cleavage and ~,~ (Table 1). As the main part of Eu3'radiation is necessary to intensity of ' D o - ~ F o ,transitions 5Do_7F2 electric dipole transition (h=615 nm), the excitation spectra were recorded at h,,=615 nm . For analysis of photochemical properties of compounds the samples were subjected to irradiation by unfiltered light of high - pressure mercury DRT - 250 lamp during 7 hours. The distance between the lamp and the sample was 26 cm. The X - ray research of the compounds was carried with the out both powder and singlecrystal methods. The powders were studied at a DRON - 2.0 diffractometer in CuK, radiation. The ESR - spectra were recorded at a ESR - 231 spectrometer in X - band at room temperature.
3. RESULTS AND DISCUSSION The europium trifluoroacetates obtained complexes with nitrogen - containing neutral ligands fluoresce by red light at room temperature. The luminescent spectra of this compounds have discrete structure. The compounds in crystalline state and, inserted in polymer matrix, Table 1 The wave numbers v/cm'l, relative intensities I/%, and splitting values of 'Do-~F,(i=O, 1, 2, 4) levels (AF) in the luminescence spectra of the mixed-ligand TFA europium compounds before and after irradiation Eu(TFA)32bipy Eu(TFA)32bipy Eu(TFA)32phen Eu(TFAh2phen Trasi3HzO(unirradiated) 3HzO(irradiated) HzO(unirradiated) HzO(irradiated) tion v(I) AF v(1) AF v(I) AF v(1) 5Do-7Fo 17265 (12.0) 17269(12.3) 17107(6.9) 17282(4.4) 16925 (2 1.3) 16928 (37.1) 16879 (30.6) 16918 (22.1) 5Do-7Fl 16835 (13.8) 90 16887 (30.7) 71 16841 (16.7) 72 16875 (28.3) 82 16857 (26.3) 16807 (20.8) 16836 (23.0) 16345 (6.4) 16345 (55.4) 16238 (79.2) 16228 (88.5) 16196 (100.0) 16289 (100.0) 16317 (100.0) 16179 (100.0) 5Do-7F2 16208 (42.4) 290 16292 (65.7) 200 16109 (34.7) 129 16168(92.9) 150 16152 (85.8) 16137 (2.3) 16206 (46.2) 16078 (28.3) 16105 (18.7) 16145 (25.5) 16055 (10.8) 14548 (8.3) 14550 (10.6) 5 ~ o - 7 ~ 4 14284 (41.1) 14461 (12.5) 204 14401 (142) 14354 (23.8) 445 14373 (3 1.9) 14304 (1 1.5) 14344 (27.8)
278
were tested to UV - light stability. As it is shown at Figure 1, UV - light irradiation of finecrystalline compounds for 2-3 hours 2-4 times increases europium luminescent emission, in the same conditions the intensity of activated polymer films grows in 3-4 times. Thus, there is a noticeable redistribution of intensities of some lines, in luminescence spectra as well as the ~ electric dipole considerable changes in the structure splitting of magnetic dipole ' D o - ~ Fand 'Do-'Fz levels (Table 1) are observed. The evident changes after UV - light irradiation were also detected in the excitation spectra for studied europium trifluoroacetates. Figure 2 the excitation spectra of the investigated complexes are shown. At the moment of an enhancement of luminescence of the mixed ligand complexes after 3-4 hours of UV - irradiation) intensity of the peaks increases greatly in the region 330-470 nm. The comparison of the roentgenograms of trifluoroacetates of europium complexes before and after UV - irradiation showns (Table 2), that they are different. After irradiation there are new bands. In the roentgenograms, the intensity of some reflexes and their interplanar spaces changes. The enhancement of Eu3+luminescent emission in europium trifluoroacetate with bipyridil is the most evident (Figure la). This compounds was studied by the ESR method. In the ESR spectrum of UV - unirradiated Eu(TFA)32bipy3H20 complex at room temperature one can distinguish the two components (Figure 3). 1. Narrow symmetrical peak with AH = 1.6 mT and the center at g = 2.0038+0.0002. 2. Board intensive asymmetrical line of the width AH = 20 mT and the center, defined at the interception with the zero line, disposed at g = 2.28f0.02. I"/%
I" 1 %
a
2
4
6
8 hours
1
0
2
4
8
a
i
o
TIME, hours
Figure 1. Luminescence intensity of the Eu3+ion vs. UV irradiation time for the complexes Eu(TFA)32bipy3H20 (a) and Eu(TFA)32phenH20 (b). - fine crystalline, o - in polymer matrix. The narrow peak with g = 2.0038 in all probability concerns to a signal of a radical. On the assumption of the structure of the complex and IR - spectroscopy data, this radical can be defined bipy anion - radical. Indeed, according to the X - ray crystal analysis data of
279
Table 2 X-ray diffraction data from the europium trifluoroacetates with nitrogen - containing neutral ligands studied Eu(TFA)32phenH20
Eu(TFA)32bipy3H20 Unirradiated
VIO 10 100 32 15 15 27 17 16 17 23 15 40 10 17 30 12 12 25 12 10 10 25 17 8 33 12
d, '4 13.11 10.91 9.46 7.90 7.49 6.86 6.60 6.44 6.28 5.52 5.35 5.24 5.06 4.86 4.50 4.39 4.12 4.04 3.91 3.73 3.64 3.41 3.37 3.30 3.18 3.09
irradiated (3h) VIO
6 100 23 7 10 18 12 10 12 7 14 9 26 6 7 15 5 20 6 6 9 24 11 9 9 7 15 13
d, '4 13.23 10.91 9.46 7.87 7.49 6.86 6.58 6.4 1 6.28 5.66 5.53 5.37 5.24 5.09 4.90 4.84 4.67 4.50 4.37 4.21 4.10 4.05 3.91 3.74 3.64 3.46 3.40 3.37
Unirradiated
VIO 10 100 46 48 42 34 8 6 6 6 18 11 9 9 9 12 8 14 6 14 18 12 9 9
d, '4 14.25 12.04 1 1.75 11.30 10.30 9.71 9.27 8.11 7.53 7.17 6.23 5.70 4.90 4.79 4.40 4.23 4.06 3.91 3.77 3.72 3.54 3.37 3.29 3.18
irradiated (3h) 1/10
4'4
15 18 100 44 34 39 10 13 13 13 10 19 10 10 11 11 10 11 16 10 15 18 18 8 27
14.39 13.11 12.20 11.30 10.35 9.71 9.36 8.1 1 7.60 7.19 6.46 6.23 5.75 4.81 4.26 4.18 3.92 3.79 3.73 3.66 3.58 3.54 3.37 3.30 3.18
europium trifluoroacetate with bipyridil, coordination environment of the central ion Eu 3+ includes three oxygen atoms from three monodentate trifluoroacetates ligands, two nitrogen atoms form bipyridil molecules and three oxygen atoms from water molecules. The second bipyridil molecule is not directly coordinate by the central ion and is disposed between two other bipyridil molecules of the adjacent complexes, forming a peculiar "sandwich". There are bands in the IR spectrum assigned to a "free" molecule of a neutral
280
ligand (1596, 1567, 1427, 1250, 1016, 763 cm-'), as well as of bipyridil anion - radical of (1496, 1439, 1200, 1130, 794, 646 cm-') [8,9].
'
Et? 7F0. 'L,
'Fa- 'L,
I
a
il
EI?
1 2
360
350
400
450
500
300
350
400
450
500
Figure 2. Luminescence excitation spectra of Eu3+ from Eu(TFA)32bipy3H20 (a) and Eu(TFA)32phenH20 (b) (1 - irradiated 3 hours, 2 - unirradiated).
2.0mT
1 g=2.0038
-
50mT
g=2.28
Figure 3. ESR - spectrum unirradiated Eu(TFA)32bipy3Hz0. Powder sample. X - band
Attribution of the second signal is not evident and could not be determined exclusively from ESR data. We made this attribution to Eu2+ as a probable variant. Ed' is S ion and, therefore, it can produce an intensive signal at room temperature. Eu2+ could be present in Eu(TFA)32bipy3HzOas non - stoichiometric admixture. The value of g-factor of the center of ESR spectra of S ions could differ significantly from g-2.0023, if the ion is located in ligand
281
strong fields or in low-symmetry fields or in ferromagnetic matrix. A reliably attributed experimental spectrum is, for example, in [ 101 for Eu2+in the matrix SrA1204.
8
Figure 4. Intensity dependence of anione - radical ESR signal versus time of UVirradiation. It is characteristically, that the signal intensity from the radical permanently increases with UV - irradiation during more then 4 hours (Figure 4). Thus, it was stated by the ESR method that the enhancement of the luminescent emission observed in the europium complexes is symbiosisly to increase of bipyridil of anion - radical contents. That is why we suggest an unconventional model of luminescence development. According to this model, Eu2+is photoionized at UV - irradiation, then Eu2' is transformed into excited Eu3+ that makes an additional contribution to luminescence. Electron from Eu2+is transferred to neutral bipy - additional aion - radical bipy is formed. The new bonds in roentgenogram of long time UV-irradiated sample are connected with its partial destruction.
REFERENCES. 1. V.E. Kavun, T.A. Kaidalova, V.I. Kostin etc. Koord. Khim., 10 (1984) 1502. (In Russian) 2. M Singh, S.N. Misra, R.D. Verma. J. Inorg. Nucl. Chem., 40 (1978) 1939. 3. K.W. Rillings, J.E. Roberts. Thermochemica acta., 10 (1974) 285. 4.1.V. Kalinovskaya, V.E Karasev, A.N Pyatkina. Russian J. Inorg.. Chem., 44 (1999) 432. 5. Y.G. Klyava. ESR - spectroscopy of irregular solids. Zinatne, Riga, 1988. (In Russian) 6. L.E. Iton, C.M. Brodbeck, S.L. Suib, G.D. Stucky. J. Chem. Phys., 79 (1983) 1185. 7. F. A. Cotton, G. Wilkinson. Advanced inorganic chemistry. J.Wiley & Sons, NY, 1972. 8. M.N. Bochkarev, I.L. Fedushkin, V.I. Nevodchikov, V.K. Cherkasov, H. Schumann, H. Hemling, R. Weimann. J. Organomet. Chem., 524 (1996) 125. 9. W.J. Evans, D.K. Drummond. J. Am.Chem. SOC.,111 (1989) 3329. 10. Hideto Matsuoka, Kazunobu Sato, Daisuke Shiomi, Yoshitane Kojima, Ken Hirotu, Nobuo Furuno, Takeji Takui. Third Asia-Pacific EPRESR Symposium. Abstracts. (2001) 2P44.
282
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR and luminescence studies on formation of S i ions through photochemical reactions in potassium halide crystals doped with sulfur and manganese Sarjonoa, M. Babaa, K. Ohtaa, K. Nishidatea, I. Sokolskab and W. Ryba-Romanowskib aDepartment of Electrical and Electronic Engineering, Iwate University, Morioka 020-8551, Japan bInstitute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw 2, Poland Remarkable photochemical reactions in potassium halide crystals doubly-doped with sulfur and manganese were investigated by electron spin resonance (ESR) and photoluminescence (PL) measurements. It was found from the PL excitation spectrum that manganese ions were dispersed into lattices as divalent Mn2+ ions and it was further determined by ESR measurements that they were displaced with potassium ions in the form of an Mn2+-vacancy in the quenched sample. After irradiated with deep uv light, it was found by observation of the characteristic vibronic structure of PL emission spectrum, these Mn2+-vacancy ESR centers were disappeared and in turn S2- molecular ions were effectively formed. Then, it was concluded from the analysis of different types of ESR spectra that monovalent Mn' ions and neutral Mno atoms existed at the same time in the lattice. Furthermore, the isolated Mnz+ions were formed from these Mn+ ions and Mno atoms by irradiation of visible light. Such photochemical reactions exhibited the extremely reversible behavior between uv and visible light irradiations. It was confirmed from the angular dependence of corresponding ESR spectra that these three types of manganese centers except for Mn2+-vacancy centers were all isotropic. By the present method, Sy molecular ions were efficiently introduced into potassium halide crystals for the first time.
*Corresponding author: Email address:
(Sarjono)
283
1. INTRODUCTION
We often observe the formation of complexes consisting of impurity ions with an opposite charge in doubly-doped crystals. If one of the ions in a complex center is paramagnetic, then we can study the nature of the electron configuration of such paramagnetic ion and also the structure, formation and decomposition processes of complexes in detail through the ESR method.') In the previous paper, it was found that Mn2+-S2-complex centers in an as-grown KC1 crystals doped with MnS were disappeared by ultraviolet (uv)-light radiation, accompanied with the formation of S< molecular ions2)On the other hand, in a quenched KCl-MnS c stal, different type of photochemical reactions may occur, because it is known that the Mn -S2complex centers are dispersed and in turn Mn2+-vacancycenters appear in such a crystal. In this paper, it is aimed that we follows in detail the photo-conversion processes from Mn'+-vacancy centers to Mn' ion and Mno atom centers and their hoto-reverse processes by ESR measurements. We also investigated the effective role of Mn ions on formation of S; molecular ions through photochemical reactions by ESR and PL measurements, comparing the two types of doubly-doped crystals prepared with different methods.
R
2. EXPERIMENTAL Double doping of S and Mn ions into lattice crystals was carried out by two different methods. One is direct doping using Czochralski method from KC1 powder with a trace of MnS (hereafter denoted by KC1-MnS). The other is the separate and sequential doping of Mn and S ions in which a trace of Na2S was first introduced into lattice powder by Czochralski method and then Mn was introduced by diffusion from MnC12 deposited on a surface of crystals (hereafter denoted by KC1-, KBr- and KI-S:Mn). The samples were annealed at 600'C and then quenched to room temperature on a copper plate. ESR measurements were carried out at room temperature by using 9.43 GHz X-band spectrometer. Calibration of g-value was done by comparison with the resonance line of polycrystalline with g=2.0036 (DPPH). A mercury (argon) or a tungsten lamp was used for ultraviolet or visible light irradiation, respectively. For the PL spectra measurements, a sample was set in a cryostat and temperature-controlled between 8.7K to room temperature using a closed-cycle He refrigerator, and then excited by light coming out from Xe or D2 lamp through a monochromator or filter. The luminescence light was detected by a photo-multiplier tube R-928P through another monochromator. The emission and excitation spectra were obtained by a computer-aided photon-counting system.
3. RESULTS AND DISCUSSION 3.1 ESR spectra of KCI-MnS crystals Figure 1 shows a typical change of the ESR spectrum caused with uv-light in the quenched KC1-MnS crystal. The spectrum of Figure l(a) before uv-irradiation is the well-known spectrum originated from two different types of Mn2+-vacancycenters, namely type 1111 and 1112 denoted by Watkin.3' The spectrum of Figure l(b) after uv-irradiation seems to be very
284
complicated. By careful observation, however, we can read three types of spectra. Namely the quenched first-type spectrum is widely and irregularly spread over the magnetic field. The second-type spectrum I that we call it a narrow-spacing sextet is a set of six lines with a relatively narrow equidistant spacing. The third-type spectrum is another sextet with a wide spacing, called a wide-spacing sextet. When this uv-irradiated sample was further illuminated with a white light from a tungsten lamp, it showed a very drastic change as shown in Figure 2. The ESR spectrum is composed of only six lines, 2&0 2Sb0 3&o 3& A0 45bo which is the same to the third-type spectrum of Figure l(b). After that, the sample was uv-irradiated (b) re-irradiated with uv-light. Then the complicated spectrum shown in Figure l(b) appeared again. Here, it should be emphasized that the -,'.-,.+ photochemical reactions (Figure l(b) S Figure 2) observed between uv-light and white-light irradiation are reversible quite well. I I I I I I In the next observation of three types of ESR 2000 2500 3s00 4M)o 4500 Magnetic field (G) spectra, we found that six lines of the spectra did not show any angular dependence about both of the Figure '. ESR spectra Of a quenched KC1-MnS crystal, (a) before and (b) after shape and position of resonance lines. Furthermore, uv-irradiation. the third-type spectrum surprisingly did not show any angular dependence in spite of its complicated shape. The above observations indicate that all of these spectra are isotropic and make a sharp contrast with the remarkable angular dependence of the type 1111and I112 spectra. From analysis of three types of ESR spectra related to manganese shown in Figure l(b) and Figure 2, we assigned the first-type, the second-type and the third-type spectrum to a monovalent Mn' ion, a neutral Mno atom and an isolated Mn2' ion, respectively. As described later, this assignment is very reasonable to explain the formation of the molecular S2- ions which is another and important product through white-light irradiated the photochemical reaction in the .KCl sample doubly-doped with S and Mn. Concerning this Mno ESR center, it was reported by Ikeya et. that the Mno atom occupies a C1- ion The g-value and hfs parameter A for the isolated Mn2+ ion and the neutral Mno atom in KCI-MnS crystal are shown in Table 1. ESR parameters of the monovalent Mn' ion are under computational calculation. Although we tried to I I I I I I do preliminary ESR measurements of the crystals 2000 2500 4ooo 4500 Magnetic field prepared by diffusion method, only a Mno atom center has been observed among the three types Figure 2. ESR spectrum of a quenched KCI-MnS crystal after white-light irradiation. of manganese ESR centers described above.
% ~
2 --
285
Table 1 ESR mrameters for the isolated Mn2+ion and the neutral Mno atom in KCl-MnS crvstal
ern-')
Center
g-value
A
Isolated Mn2' ion Mno atom
2.0043 1.9991
82.9 20.8
3.2 Luminescence spectra of KCl-S:Mn, KBr-S:Mn and KI-S:Mn crystals In the previous work6,'), luminescence properties of quenched KCl-MnS crystals have been investigated, mainly on the vibronic emission of S; ions. In order to make the existence of Mn2+ ions clear optically'), A Emission luminescence measurements of the sulfur-doped potassium halide crystals where Mn2+ions were 8 introduced by diffusion method were g systematically carried out. Figure 3 shows orange emission and its excitation spectra at 8.8K in a quenched KCl-S:Mn crystal. The 500 Wavelength characteristic excitation spectrum composed of Figure 3. Emission and excitation spectra six bands between 300 - 550 nm is very similar of Mn2+ions in a KCI-S:Mn crystal. to the absorption spectrum of the KC1:Mn ~ r y s t a l .Therefore, ~) it was concluded that this orange emission is due to Mn2+ions, and A to F bands are assigned to the transitions from 6Al(S) to 4Tl(G), 4T2 (G), 4Al (G) / 4E (G), 4T2 (D), 4E (D) and 4T1 (P) levels, respectively. Similar orange emission and its excitation spectra were observed in KBr-S:Mn and KI-S:Mn crystals. When the KCl-S:Mn crystal was irradiated by uv-light at room temperature, S; molecular ions were efficiently produced, similar to the direct-doping KCl-MnS crystal. Then the sample was colored with the formation of F-centers. In addition, these SY molecular ions were also observed in the KBr-S:Mn and KI-S:Mn crystals with and sometimes without uv-irradiation. Figure 4(a) and (b) show the vibronic emission and excitation spectra of SY molecular ions in KBr-S:Mn and KI-S:Mn crystals, respectively. I
-
0
I
I
I
I
I
I
I
I
I
KI-S:Mn crystal
I
'
I
T=8.8K
h
'9 k 0 5
d 0
Wavelength
Figure Vibronic emission and its excitation spectra of S; molecular ions in (a) KBr-S:Mn and (b) KI-S:Mn crystals irradiated with uv-light, measured at 8.8K.
286
3.3 Photochemical reactions responsible for formation of S i molecular ions When alkali halide crystals singly doped with sulfur were irradiated with uv-light, any S2molecular ion couldn't be produced. This means that there is not any effective electron trap for capturing an electron released from divalent S2' ions by uv-irradiation. A negative ion vacancy is one of such electron traps, but not much effective. On the other hand, in alkali halide crystals doubly-doped with sulfur and manganese, Mn2+ and Mn' ions are very effective for electron capture. First, release of an electron from S2-ions, or ionization of S2-ions occurs by uv-irradiation,
s2-.[-I
+
S-
-+
e-
+
so +
[-I -+
2e-
+ 1-1 ,
(1)
and partly e-
+ [-I
F.
-+
(2)
These released electrons are captured by the following reactions, Mn2+ . [+I
+
e-
-+
Mn+
+ [+I,
(31
and further into Mn'
+
e'
(4)
Mn'.
-+
As a result, a reaction into formation of Sy molecular ions proceeds,
s- + so
-+
s2-
+ [-I.
(5)
Here, F, [-I and indicate an F center, a negative-ion vacancy and a positive-ion vacancy, respectively. As described above, the reversible photochemical reactions between a group of the Mn' and Mno centers and the other Mn2' ion center can be explained by a mechanism of capture of an electron by Mn2+ion under uv-light irradiation and release of an electron by the Mnt ion and the Mno atom under visible-light illumination as follows, Mn2+
f
e-
+ visible-light uv-light -+
Mn'
f
e'
+ visible-light uv-light -+
Mno .
Detailed analysis of isolated Mn2', Mn' and MnoESR centers and also detailed theoretical treatment of excitation spectra for Mn2' emission will be given elsewhere.
REFERENCES 1. M. I. Kornfel'd and Yu. N. Tolparov, Sov. Phys. Solid State, 9 (1968) 1607 and ibid. 2. M. Baba, H. Saga, K. Nishidate, L.O. Schwan and D. Schmid, J. Phys. SOC.Jpn., 67 (1998) 3275. 3. G. D. Watkins, Phys. Rev. 113 (1959) 79. 4. M. Ikeya and N. Itoh, Solid State Commun., 7 (1969) 355. 5. M. Ikeya and N. Itoh, J. Phys. Chem. Solids, 32 (1971) 3569. 6. R.Ye, H. Tazawa, M. Baba, K. Nishidate, L.O. Schwan and D. Schmid, Jpn. J. Appl. Phys.,
281
37 (1998) 1154. 7. R. Ye, M. Baba, K. Nishidate, L.O. Schwan and D. Schmid, Journal of Luminescence, 87-89 (2000) 542. 8. I. Sokolska, phys. stat. sol. (b), K33 (1992) 172. 9. A. Mehra, phys. stat. sol., 29 (1968) 847.
288
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Hyperfine structure of Nd3' and Er3+ions in LiNb03 crystals 11-Woo Park', Sung Ho Choh", Sang Su Kimb, Kuk Kang' and Deok Choi' 'Seoul Branch, KBSI, 126-16 Anam-dong, Sungbuk-Ku, Seoul 136-701, Korea bDepartment of Physics, Changwon National University, Changwon 64 1-773, Korea 'Department of Physics, Myongji University, Yongin, Kyunggi-Do 449-728, Korea Rare earth ions such as Nd3+ and Er3+ in congruent LiNb03 and nearly stoichiometric crystals have been investigated to identify their local symmetry and structures with an X-band electron spin resonance spectrometer at liquid helium temperature. The spin Hamiltonian parameters of Nd3+ and Er3+in LiNbO3 could be precisely determined with nearly stoichiometric crystals by the reduction of intrinsic defects.
1. INTRODUCTION Lithium niobate (LN) crystals have usually been grown nonstoichiometric and congruent, of the same composition with the melt. Consequently they contain many defects such as the Nb antisite due to Li-deficiency [l]. However, we are able to obtain nearly stoichiometric samples by the vapor transport equilibrium (VTE) treatment [2, 31 or an addition of small amount of K20 powder in the starting mixture [4]. Rare earth doped lithium niobate has been known to widespread potentials for laser devices such as laser frequency converter [5] and optical parametric oscillator [6]. In this study, we have investigated electron spin resonance (ESR) of Nd3+and Er3' in congruent (CL) and VTEtreated LiNbO3 (VL) crystals, respectively, to identify their local symmetry and structures.
2. EXPERIMENT Neodymium and erbium doped CL crystals were grown by Czochralski method, doped with 1.3 x wt% and 0.19 wt%, respectively. The space group of the crystal is R3c with the hexagonal unit cell of a=5.15A and c=13.86A. The crystal structure
289
maintains after adding small content of rare earth impurities. To obtain the nearly stoichiometric samples they were heated in Li-rich powder, LiNbOs + PLi3Nb04, at 1100°C for 60 hours and slowly cooled down to room temperature in 24 hours. Then thin samples were prepared with the thickness of 0.5 mm for the easy incorporation of Li-atoms into CL. Similar sample preparation was reported previously [3, 71. ESR measurements were carried out by employing a Bruker ESP300 spectrometer and an Oxford ESR900 cryostat.
3. EXPERIMENTAL RESULTS AND DISCUSSION It has been reported that usual ESR linewidths of the paramagnetic centers in VL become sharper than those in CL [3] without the change of spin Hamiltonian (SH) parameters. Therefore, VL samples are desirable to determine precise SH parameters in LN. The results are characterized using the usual SH of the following form with axial symmetry:
where the notations are common. ESR spectra for both rare earth ions showed axial symmetry in the crystallographic aa-plane within experimental uncertainty, and the principal z-axis of g- and A-tensor turns out to be coinciding with the crystallographic caxis. Figure 1 shows stacked ESR spectra of Nd3+in CL measured at 4 K. The spectrum at the top is obtained when the magnetic field is parallel to the c-axis, and that at the bottom being when the magnetic field is parallel to the a-axis. The angular interval between the spectra is 6 degrees. As shown clearly, ESR signals overlap and are hard to resolve one another. However, ESR spectrum of Nd3+ in VL for B 11 c-axis was resolved well, as shown in Figure 2. Also we obtained only two strong signals with the g-values of 1.443 and 1.323, and with the peak-to-peak linewidths of 2.65 mT and 3.17 mT, respectively. The disappearance of additional signals may be due to the reduction of the intrinsic defects in CL crystals by VTE treatment. The signal at gll = 1.443 is about ten times stronger than that at gll= 1.323 by the double integration. Weaker signals near the two strong ones are originated from the hyperfine transitions of 1432 '45 Nd nuclei (both I=7/2). The g-values of Nd3+centers are summarized in Table 1, together with previous reports.
290
Table 1 g-values of Nd3+centers in LN at 4 K
present work
1
1.443( 1)
1.323( 1)
2.963(4)
Ref. 8
1.43*0.02
1.33*0.02
2.95*0.05
Ref. 9
1.42
3.136( 1 )
2.94
I
T=4K
Y
0 II c
~
= 9.4 GHz 11 c-axis
g,, = 1.443
(b)
\
\ 0 II a 1
,
1000
.
,
2000
.
,
.
,
.
4000
Magnetic Field
,
5000
.
,
6000
lo4T
Figure 1. Stacked ESR spectra of Nd3+ in CL at 4 K in the crystallographic ca-plane.
g, = 1.323 1000
2000
3000
4000
Magnetic Field
5000
6000
T
Figure 2.Comparison of ESR spectra for Nd3+in (a) CL, and (b) VL.
Er3+centers in CL showed similar behavior of Nd3+centers, the splitting of the signals, as shown in Figure 3. At least, two Er3+ signals for B 11 c-axis are recorded at the same position of gll= 14.44 and clearly show hyperfine structure due to 16'Er (Figure 4). But evenEr3+ signals start to split as the direction of magnetic field deviates from the c-axis, meaning Er3+centers have different g,-values. ESR parameters of two strong traceable Er3+centers in CL are determined as in Table 2. However, we failed to obtain relatively precise ESR parameters like Nd3+ in VL, since Er3+signals were superposed with those originating from Fe3+[ 11, an unintentional impurity resulting stronger signals after VTE treatment than before. Nd3+and Er3+ions have relatively large ionic radii of 0.995 and 0.88A, respectively, compared with the cations in LN. From our axial symmetric results and large ionic radii
29 1
for the both rare earth ions, we may propose that they are located along the c-axis in the crystal. This proposal is consistent with previous reports [ 10, 111 which argue that rare earth ions shift downward along the c-axis from the Li site to the center of oxygen octahedron. 4. SUMMARY
We observed Nd3’ and Er3+signals in LN crystals, and determined their SH parameters. Two rare earth ions in CL have two or more centers, respectively. But the number of Nd3+ centers reduces to only two centers and their linewidths become narrower after VTE treatment, while we failed to trace the Er3+ signals due to the unintentional impurity remaining in the sample.
B II
c 4K v = 9.4 GHz
B I I c-axis
P”enEr( I = 0 )
H
B II a
I
I
T I 167
I 4000
Magnetic Field / 10-4 T
Figure 4.ESR spectrum of Er3+in CL.
Figure 3. Stacked ESR spectra of Er3+ in CL at 4 K in the crystallographic ca-plane. Table 2 ESR parameters of Er3+in CL g 11
I6’Er: A II (mT)
g,
1.2
1
2
present work
14.44
2.11
0.57
Ref. 10
15.14
2.15
7.35 7.73
292
ACKNOWLEDGEMENT This work has been supported by the National Research Laboratory Program of MOST. REFERENCES 1. S. H. Choh, I.-W. Park and S. S. Kim, Proc. of the lstAsia Pacific EPR Symposium, Springer (1998) 335. 2. P. E. Bordui, R. G. Nonvood, D. H. Jundt and M. M. Fejer, J. Appl. Phys. 71 (1992) 875. 3. Y. N. Choi, S. H. Choh, I.-W. Park, E. K. Koh and S. S. Kim, J. Korean Phys. SOC.32 (1998) S643 and therein. 4. G. I. Malovichko, V. A. Grachev, L. P. Yurchenko, V. Y. Proshko, E. P. Kokanyan and V. T. Gabrielyan, phys. stat. sol. (a) 133 (1992) K29. 5. J. Capmany, J. A. Pereda, V. Bermudez, D. Callejo and E. DiCguez, Appl. Phys. Lett. 79 (200 1) 1751. 6. J. Capmany, D. Callejo, V. Bermudez, E. Dieguez, D. Artigas and L. Tomer, Appl. Phys. Lett. 79 (2001) 293. 7. Y. N. Choi, I.-W. Park, S. S. Kim, S. S. Park and S. H. Choh, J. Phys.:Condens. Matter 11 (1999) 4723. 8. N. F. Evlanova, L. S. Kornienko, L. N. Rashkovich and A. 0. Rybaltovskfi , Soviet Phys. JETP, 26 (1968) 1090. 9. G. Bums, D. F. O’Kane and R. S. Title, Phys. Rev. 167 (1968) 314. 10. D. M. B. P. Milori, I. J. Moraes, A. C. Harnandes, R. R. de Souza, M. Siu Li, M. C. Terrile and G. E. Bareris, Phys. Rev. B 5 1 (1995) 3206. 11. A. Lorenzo, H. Jaffezic, B. Roux, G. Boulon and J. Garcia-Sol&,App. Phys. Lett. 67 (1995) 3735.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
293
The nature of conduction ESR linewidth temperature dependence in graphite A.M. Ziatdinov and V.V. Kainara Institute of Chemistry, Far Eastern Branch of the Russian Academy of Sciences, 159, Prospect 100-letiya, 690022 Vladivostok, Russia For all orientations of the external constant magnetic field, Ho, relative to the graphite plate c-axis the linewidth of graphite conduction ESR (CESR) signal increases first with decreasing temperature, forms a distinct peak at -20 K and then falls off. The value of g-factor for HO along the c-axis increases with lowering temperature by a manner similar to that of the CESR linewidth, but for HOalong the basal plane its value does not depent on temperature. Up to the present, the nature of graphite conduction ESR linewidth temperature dependence and origin of its low temperature peak were not clear. In this work we show that a low temperature peak in the CESR linewidth temperature dependence is predictable, if the surface spin relaxation effects for graphite current carriers were taken into consideration. INTRODUCTION
The first systematic study of temperature dependences of graphite Conduction ESR (CESR) signal parameters was carried out as early as 1960 by Wagoner [l] using a natural single crystal specimen in the temperature range from 77 K up to 600 K. After Wagoner a number of authors [2-71 conducted similar studies on a variety of well-defined specimens of graphite, and have obtained nearly the same results. In particular, in all samples investigated and for all orientations of the external constant magnetic field, Ho, relative to the c-axis the graphite CESR signal linewidth increases first with decreasing temperature. According to the data of Matsubara et [7], the AH(7)-dependence forms a distinct peak near 20 K and then falls off. At present there is no consensus between researchers on both the origin of graphite CESR linewidth and of its temperature dependence. Kawamura et [4] showed that at Hollc the Elliot [S] expression for the CESR linewidth, due to carriers interacting with phonons and/or impurities, which for (& is Debye temperature) can be written as: constx(AgJ'/ym
(1)
(Agc=gc-go,where gc and go are the g-factor values respectively for graphite current carriers and for free electron, y is the electronic gyromagnetic ratio, * is the carriers effective mass, and is the carriers mobility), describes the graphite CESR linewidth dependence in the interval 77+300 K qualitatively at least. Matsubara et [7] considered the temperature variation of graphite CESR linewidth at Hollc as a direct consequence of motional narrowing
294
effect through an averaging process of g-values of scattered carriers over the Fermi surface in the limit of incomplete line averaging. In this limit the g-shift is averaged over all energy states of current carriers in k-space, but the linewidth contains the components which are proportional to the square of the microwave frequency. Kotosonov [3] pointed out that the small spectral linewidth in graphite may result from complete averaging of the g-factors over all energy states of current carriers during the spin-lattice relaxation. Thus, for example, in synthetic graphite samples the temperature change from 40 K to 100 K leads to the g, changing by -0.2, which agrees with the resonance field shift by - 3 ~ 1 0T,~ whereas ~ the CESR linewidth remains within the limits of several oersteds. According to the literature data [S, 91 the Debye temperature of graphite is about 400 K. Therefore, the description of the graphite CESR linewidth temperature dependence by Eq. (l), proposed by Elliot for is not obvious. Furthermore, this expression does not explain the presence of linewidth temperature dependence at Holc even at a qualitative level since in this orientation of HO the value of g-factor, ga, does not depend on temperature. The independence of the CESR linewidth on the microwave frequency shows that Matsubara et [7] interpretation of the linewidth temperature dependence as a result of the motional narrowing of incomplete averaging line is not correct also. Besides, the presence of lowtemperature peak in AH(7') curve also at Holc, where g-factor is temperature independent, shows that the origins of low-temperature peaks in and dependences are different. The Kotosonov's [3] point of view does not contradict to the experimental data, but he did not consider the nature of linewidth temperature dependence. We have studied the dependences of CESR signal linewidth in highly oriented pyrolitic graphite (HOPG) on sample dimensions and on temperature for different orientations of HO and have shown that all obtained experimental data on CESR linewidth in graphite may be explained well, if the surface spin relaxation of current carriers was taken into consideration. 2. EXPERIMENTAL
The CESR measurements were carried out using an X-band E-line spectrometer in a rectangular cavity with TE102 mode. The frequency and amplitude of HOmodulation were 2.5 and 0.1 mT, respectively. All experiments were carried out on samples in the shape of rectangular parallelepipeds with the dimensions: width (1)xheight (h)xthickness (4,where hxl is the area of basal plane. At the experiments, the basal Zxh and lateral dxh sides were parallel to the magnetic component, Hfi, of the microwave field and the c-axis was perpendicular to it. Note, that the rectangular resonator, the structure of electromagnetic field of TEIOZmode has such a form that, at a conventional setting of the resonator, HO)IEfi(the electrical component of the microwave field). The study of dependences of graphite CESR lineshape parameters on sample dimensions were carried out on HOPG plates with dimensions: lxO.355x0.072 cm3. The accuracy in the cm. determination of the sample dimensions was 5x The temperature studies of CESR spectra of the samples investigated were carried out in the temperature range from 100 K to 350 K. The temperature was varied by regulating the rate and temperature of nitrogen or helium gas flow through the quartz Dewar tube with the sample. The temperature was maintained and measured with an accuracy of -0.1 K/h and -0.5 K, respectively.
-
295
Figure 1. Temperature dependence of g-factor (a) and linewidth (b) for graphite. In (b), the theoretical curve 1 (2) was calculated with constant (determined by the Exp. (4)) value of intrinsic conduction ESR linewidth. 3. RESULTS
For HOPG plate investigated the CESR line is of typical Dyson [l 11 form and indicates a large g-factor anisotropy. The g,-value is about 2.047 at room temperature and first increases monotonously with lowering temperature so as to exceed 2.16, forms a peak at -20 K, and then steeply falls off (Figure la). The g,-value shows almost no shift from the free-electron value irrespective of temperature ( Aga=ga - go - 3 ~ 1 0 ~(Figure ) la). When HO is in the c-direction, the linewidth as narrow as - 6 ~ 1 0 - T~ near room temperature and first increases @ @ @ p E lremarkably * with lowering temperature, 4and then the rise of the is followed 0 1 2 3 also by a distinct peak at -20 K similar to that of the gc-shift (Figure lb). When I, lo-' cm HOlc the linewidth also increases with lowering temperature (Figure lb), by a Figure 3. The experimental (dots) and manner similar to that of the despite theoretical (lines) values of CESR linewidth, the fact that Aga does not depend on AH, in graphite vs. sample width 1. The line 1 temperature (Figure la). The value of was calculated using the value of G=180 (0) CESR linewidth monotonously changes cm-'. K; H ~ l c . with on the constant magnetic field
296
orientation. At all temperatures the is larger than the The value of CESR linewidth tends to the infinity, while 1 (HOPG plate size in a basal plane) tends to zero (Figure 2). At all temperatures the microwave field power and frequency, and the frequency of Ho modulation had no observable effect on the CESR linewidth. 3. DISCUSSION
The character of temperature dependence of CESR linewidth on 1 (Figure 2) unequivocally specifies the presence of the contribution of surface spin relaxation into total spin relaxation of current carriers in HOPG plates investigated. Indeed, while the experimental linewidth tends to the infinity, the corresponding theoretical values calculated using the well-known Dyson [l 11 expression for CESR line shape, which is not containing the surface spin relaxation parameter G,=3~~/4/1, ( E ~is the mean probability of spin reorientation during the collisions of current carriers with the lateral graphite surfaces and A, is a mean free path of current carriers in a basal plane) tends to the finite value, which differs from that for wide plates by -10% only. At the same time, the theoretical curves M(I)with the value of Dyson [l 13 surface spin relaxation parameter Ga=180 cm-' describes the experimental data well (Figure 2). Basing on this fact, we also considered the temperature dependence of CESR linewidth in HOPG studied in the frameworks of the model including surface spin relaxation effects of current carriers. Moreover, we suppose the presence of a small amount of the localized spins (-1% of the current carrier concentration or near one localized spin per lo6 carbon atoms) and complete averaging of g-factors of the conduction electrons and localized spins in HOPG studied. In such case, the CESR linewidth AHi (i=a, c) can be presented in the following form:
where AH,, and AH,, are the linewidths of CESR signal due to conduction electrons and localized spins, respectively; AH,,= f AH,:tr , where and AH::tr are contributions to the total conduction electron linewidth due to their interactions with sample surface and inner imperfections, respectively; xe and xs are the Curie and Pauli paramagnetic susceptibilities, respectively. At the calculations we assumed that
where is a constant depending on physical properties of the sample surface. Because the Elliot's expressions [8] for the intrinsic spin relaxation of current carriers were calculated for the simple isotropic metals, their applications to graphite is not obvious. Therefore, the calculations of AH, were carried out by us with values of both independent, and dependent on temperature according to the Elliot [S] law for << @I: AH,:m= constx(Ag)2@I /ym*,u(T)
?.
(4)
297 Basing on the analysis of literature data on the temperature dependence of current carriers mobility in graphite basal plane [ 121 pai(T) was approximated by the following expression pai(T)
=u
+
+g'.2 ,
where a, b and c are the variable parameters; at calculations for Ho c, their initial values were taken equal to -1.8 m2N.s, 2 . 6 ~ 1 0m2.K'.'/V.s ~ and 17 K, respectively (at these values of parameters approximately describes the in-plane mobility of carriers in HOPG sample of average quality). Taking into consideration the data of irradiated graphite CESRmeasurements [ 131 the values of g-factor for localized spins and dHi, were taken to be equal to 2,0023 and 0,25 mT, respectively. The values of a,i in Eq. (3) and constants in Eq. (4) were calculated using the literature data on the value of pai(T) in HOPG [12] and surface and intrinsic spin relaxation times at room temperature obtained from the analysis of experimental data (Figure 3), respectively. The results of approximation of experimental CESR linewidth at HO c by Eq. (2) are presented in Figure 1b. As is clear from this figure, for both forms of temperature dependence of intrinsic spin relaxation the theoretical curve AHi(T) contains the distinct peak near 20 K. At the same time, the theoretical analysis of Eq. (2) has shown, that this peak is absent if = 0. Thus, in this work we have shown that a low temperature peak in the curve of CESR linewidth temperature dependence appears, when the surface spin relaxation effects of graphite current carriers are taken into consideration. The authors are grateful to L.B. Nepomnyashchii (State Scientific Research Centre for Graphite, Moscow) for providing the HOPG. This work was supported by the Russian Foundation for Basic Research (grant No. 00-03-32610).
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
G. Wagoner, Phys. Rev., 118 (1960) 647. L.J. Singer and G. Wagoner, J. Chem. Phys., 37 (1962) 1812. A.S. Kotosonov, JETP Lett., 29 (1979) 307. K. Kawamura, S. Kaneko, and T. Tsuzuku, J. Phys. SOC.Jpn., 52 (1983) 3936. A.M. Ziatdinov, and N.M. Mishchenko, Phys. Solid State (St. Petersburg, Russia), 36 (1994) 1283. A.S. Kotosonov, Carbon, 26 (1988) 189. K. Matsubara, T. Tsuzuku, and K. Sugihara, Phys. Rev., B44 (1991) 11845. R.J. Elliot, Phys. Rev., 96 (1954) 266. K.A. Jr. Gschneider, Solid State Phys., 16 (1964) 275. V. Mizutani, T. Kondow, and T. B. Massalski, Phys. Rev., B17 (1978) 3165. F.J. Dyson, Phys. Rev., 98 (1955) 349. M.S. Dresselhaus and G. Dresselhaus, Adv. in Phys., 30 (1981) 139. M. Miyake, W. Saiki, T. Daimon, P. Son, C. Miyake, and H. Ohya-Nishiguchi, J. Nucl. Mat., 187 (1992) 138.
298
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR measurement of heavily doped Si:Fe Ryouhei KOYAMAa,', Junk0 YOSHIKAWAasb,Takashi KUNIMOTO', Susumu OKUBO', Hitoshi OHTAhsC, Hiroshi NAKAYAMAd "The Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai, Kobe, 657-8501, Japan 'Venture Business Laboratory, Kobe University, 1-1 Rokkodai, Kobe 657-8501, Japan "Molecular Photoscience Research Center and Department of Physics, Kobe University, 1-1 Rokkodai, Kobe 657-8501, Japan dDepartment of Applied Physics, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan
X-band ESR measurements of the heavily doped Si:Fe have been performed 300K and at the frequency of 9.45 GHz. We observed the ESR signal whose intensity becomes stronger as the Fe concentration increased. The angular dependence of this signal at room temperature was observed and compared with that of the Fe thin film. The observed signal is different from the ferromagnetic resonance of Fe thin film. The observed g-value suggests that one of ESR signals originates from Fe' situated on the interstitial site.
1.INTRODUCTION
The development of the new crystal growth technique opened a new field of research to grow heavily doped 111-V semiconductors with percent order transition metal magnetic impurities, and the ferromagnetism in heavily doped GaAs:Mn was discovered at low temperature [l]. Since then, the studies of the heavily doped magnetic semiconductors with transition metal ions have attracted much interest due to the possibility to apply these materials to the spin controlled new devices. Si based magnetic semiconductor is important from the application point of view. Heavily doped Si:Mn was prepared by Nakayama and millimeter wave ESR measurements of heavily doped Si:Mn have been performed and the observed resonances turned out to be cyclotron resonances [2]. Then heavily doped Si:Fe has been prepared by Abe recently using a chemical vapor deposition (CVD). In order to study its magnetic properties, we performed X-band ESR measurements of this heavily doped Si:Fe at Venture Business Laboratory, Kobe University.
299
2. EXPERIMENTAL
The preparation procedure of Si:Fe is using solid state reaction as follows. At first Fe was deposited on B-doped Si( 111) substrates by CVD (substrate temperature: about 600'C). Next the Fe films grown on Si(l11) substrates were annealed in UHV chamber (base pressure: (1.0-2.6)~10-~ Torr, annealing time: lh, annealing temperature: 600°C) [3]. Then the Fe layer left on the surface was taken away by laser ablation. From Auger Electron Spectroscopy analysis, Fe concentration of our samples were estimated to be 3.8%, 11.8% and 38.2%. X-band ESR measurements have been performed at 300K and the frequency is approximately 9.45 GHz. Angular dependence of resonance field has been performed by rotating the sample. 3. RESULTS AND DISCUSSION
Figure 1 shows typical ESR spectra of Si(1ll) substrate and Si:Fe samples (Fe:3.8%, 11.8%, and 38.2%). The magnetic field was applied parallel to the (1 11) plane. Two ESR signals were observed in Si:Fe at about 340 mT and 190 mT. The signal around 340 mT corresponds to the defect in silicon because the signal is also observed in Si substrate. The other signal around 190 mT has Asymmetric line shape which is observed in the conducting material. Since this signal is not observed in Si substrate and its intensity becomes stronger as the Fe concentration increases, this signal is related to Fe.
r-----
Si(ll1
0
Figure 1 . ESR spectra of Si( 1 1 1) substrates and Si:Fe samples (Fe:3.8%, 11.8%, arid 38.2%) observed at room temperature. Considering the preparation method of our samples, there is a possibility that the signal is originated from a Fe thin film remained on the Si substrate. In order to confirm whether Fe thin film exists or not, angular dependence measurements of the annealed sample is performed. We compare the angular dependence of annealed film with Fe film deposited on Si substrate. We also compare these angular dependences with the theoretical angular dependence of ferromagnetic resonance of thin film described as follows Using the coordinate system defined in Figure 2, the equilibrium condition is
300
MHsin(~ ~ - ~ , ~ = 4 ~ M ' s i n ~ ~ , , c o s ~ , ~ ,
(1)
where Hand M i s applied magnetic field and total magnetic moment, respectively. By solving the torque equation, the resonance condition can be obtained as (o/y)'=[Hcos(
(2)
&-q5e,,)-4nMsin2q5e,,],
where yis the gyromagnetic ratio. The results of experimental and calculated angular dependence of resonance field are shown in Figure 3.
0
700
film
x calculated value
B(deg.1
Figure 2. Coordinate system showing the orientation of the applied field H and the magnetization M . The film is in the xy-plane.
Figure 3. The angular dependence of' resonance field of the film, the main resonance in Fe 38.2% sample at room temperature and the calculated value.
The angular dependence of Fe film deposited on Si is in good agreement with the calculated angular dependence ferromagnetic thin film as shown in Figure 3. In contrast, the obtained angular dependence of Si:Fe is quite different from others. Consequently Fe thin film does not remain in the annealed sample. It suggests that the signal around 190 mT originated from Fe impurities diffused into the Si substrate. Figure 4 shows the detailed angular dependence of ESR spectra and g-values of the Fe 38.2% sample. Four ESR signals were found along the [ l l I] axis(8=0") g-2.8 (#l), g-1.9 (#2), g 4 . 3 (#3), g-1.1 (#4). For 8>40", the intensity of signal Pt2 drastically increases, the other three signals therefore cannot be distinguished because of their weak intensity in comparison with #2. The g-value of #2 is about 3.6 at 8=90". The value is close to g=3.524 of interstitial Fe' ion in Si. From the discussion of Gehlhoff and Segsa for Fe' ion in Si using the theory of Abragam and Price, the anisotropic g-values of Fe' (3d') were estimated to be g,,52 and g,14 [5]. The angular dependence of g-value for #2 is consistent with this anisotropy. Therefore the signal #2 observed at about 190 mT (e=90°) is considered to be due to the Fe' ion situated on a interstitial site in Si. Since Boron is doped into Si substrate, there is a possibility that centers of Fe and B complex were built up in Si substrate. The g-value were obtained as 2.895 and 8.899 for FeFeB complexes in Si Comparing the g-values, the signal #1 probably arises from
30 1
FeFeB complexes in Si. The signals #3 and #4 are unclear at present. Detailed studies for the angular dependence of Fe impurities and complex centers in Si is necessary to identify all the ESR signals.
Figure 4 The angular dependence of ESR spectra (a) and g-values of the Fe 38.2% sample (b) at room temperature. 4. CONCLUSION
X-band ESR measurements of the heavily doped Si:Fe have been performed at 300K and at the frequency of 9.45 GHz. We observed the ESR signal whose intensity becomes stronger as the Fe concentration increases. The angular dependence of this signal at room temperature was observed and compared with that of the Fe thin film. The observed signal is different from the ferromagnetic resonance of Fe thin film. The observed g-value suggests that one of ESR signals originates from Fe' situated on the interstitial site. Acknowledgment
The authors are grateful to the financial support from the Venture Business Laboratory of Kobe University. REFERENCES
1. H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto and Y. Iye, Appl. Phys. Lett., 69 (1996) 363. 2. H. Ohta, S. Okubo, J. Yoshikawa, Y. Nakashima, C. Urakawa, H. Nakayama, and T. Nishino, Physica B, 298 (2001) 449. 3. S. Abe, H. Nakayama, T. Nishino, H. Ohta and S. Iida, J. Cryst. Growth., 210 (2000) 137. 4. H. Ohta, H. Ohta, S. Imagawa, M. Motokawa and E. Kita, J. Phys. SOC.Japan, 62 (1993) 4467. 5. W. Gehlhoffand K. H. Segsa, Phys. Status Solidi B, 115 (1983) 443. 6. A. A. Ezhevskii and C. A. J. Ammerlaan, Sov. Phys. Semiconductors, 24 (1990) 851.
302
EPR in the 2 1* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR study of heavily doped GaAs:Er grown by organometallic vaporphase epitaxy J. Yoshikawa"', S. Okubo', H. Ohtab,', T. Koided, T. Kawamotod,Y. Fujiwarad,and Y. Takedad 'The Graduate School of Science and Technology, Kobe University, 1-1 Rokko-dai, Nada, Kobe 657-8501, Japan 'Venture Business Laboratory, Kobe University, 1-1 Rokko-dai, Nada, Kobe 657-8501, Japan 'Molecular Photoscience Research Center and Technology, Kobe University, 1- 1 Rokko-dai, Nada, Kobe 657-8501, Japan dDepartment of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan X-band ESR measurements of heavily doped GaAs:Er, whose Er concentration was 1 . 7 ~ 1 0 ~ ' cm-3 grown without additional 0, flow by organometallic vapor phase epitaxy (sample 11), have been performed at 3.5K. Although we observed an isotropic ESR signal around g=6 in the GaAs:Er sample whose Er concentration was 3 . 6 ~ 1 0~' ~r ngrown - ~ with additional 0, flow by OMVPE (sample I), we did not observe isotropic ESR signal around g=6 in sample 11. It suggests that Er centers at the tetrahedral sites decreased in sample 11. Several ESR signals with anisotropic g-values, which are different from those in sample I, were also observed in sample 11, suggesting the formation of new Er centers with oxygen in sample 11. The origin of these ESR signals will be discussed in connection with PL results.
1. INTRODUCTION As the photo luminescence (PL) spectra of rare-earth doped 111-V semiconductors are sharp and temperature-stable, a possibility to produce temperature-stable optical devices with rareearth doped 111-V semiconductors has attracted much interest. Particularly , the wavelength of the luminescence due to the 41,,,, +41,5,2 intra-4f-shell transition of Er" ions is around 1.54pm, which corresponds to the minimum loss region of silica-based optical fibers, Er atoms in semiconductors are very interesting. Fujiwara performed PL measurements of GaAs:Er in which Er(DPM), was used to dope Er by organometallic vapor phase epitaxy [I]. They observed strong emissions lines from only the Er-20 center, which was reported previously [2], in the GaAs:Er sample whose Er concentration was 3 . 6 ~ 1 0cm" ' ~ grown with the additional 0, flow. We performed the ESR measurement of this GaAs:Er sample (sample I) and observed isotropic signal around g=6 together with several anisotropic signals with C,, and C,,, symmetries [3]. The signal with C,, symmetry can be considered as coming from the Er-20 center. Recently Fujiwara prepared heavily doped GaAs:Er whose Er concentration was 1 . 7 ~ 1 0 cm-3 ~ ' grown without additional flow by OMVPE. In this sample, they observed strong but broad emission lines which are different from the reported emission lines from Er-20 centers [4]. The aim of our study is to get information about the local structure around Er in heavily doped GaAs:Er by X-band ESR.
303
2. EXPERIMENTAL
GaAs:Er epitaxial layers were grown on the (001)-oriented Si-doped GaAs substrates by OMVPE with (sample I) and without (sample 11) an additional O2 flow [l]. The growth temperatures were 543°C for samples I, and 530°C for sample 11. The thickness of the epitaxial layers were about 0.8 ym for samples I and about 1 pm for sample 11. Er(DPM)3, which contains six 0 atoms bonded to one Er atom in one molecule, were used as an Er source. The flow rate of the Er source was 4 times as much in the case of sample I1 than sample I. The Er concentration of sample I was 3.6~10''cmA3andthat of sample I1 was 1.7x102' ern-', determined by a secondary-ion mass spectrometry (SIMS). ESR measurements were performed by a Bruker EPR spectrometer EMX081 at Venture Business Laboratory of Kobe University using a TE,03 rectangular cavity. The microwave frequency was approximately 9.4 GHz and the magnetic field was swept from 5 to 900mT. The samples were cooled down to about 3.5K with an Oxford Instruments He flow cryostat.
3. RESULTS AND DISCUSSION Typical ESR spectra at 3.5K are shown in Figure 1. Sample I whose Er concentration was isotropic ESR signal around g=6. 3.6~10"cm-3grown with the additional 0, flow shows On the other hand, sample I1 does not show an isotropic ESR signal around g=6. An isotropic ESR signal around g=6 were reported previously in InP doped with Er [5,6] and GaAs doped with Er [7-91. By the crystal field analysis, it can be considered as the signal from E12' ions in tetrahedral site [lo]. The results suggest that Er centers which have tetrahedral symmetry decreased in sample 11. Several anisotropic ESR signals were also observed in sample I and 11. Particularly, we observed a lot of new sharp and anisotropic ESR signals in the g-value region from 6 to 16 in sample 11. As the analysis of the angular dependencies of ESR in
9.42GHz 3.5K
extrinsic
+a +b t c l +c2 +c3
c4 c5
16 14 12
+
a
10
8
I
m
E
6
4
I extrinsic
2
a
b
Figure 1. ESR spectra of sample I and I1 at 3.5K. The angle between the magnetic field and (001) plane of sample I and I1 is 45 . Extrinsic signals are coming from the cryostat.
Figure 2. Angular dependencies of anisotropic ESR signals in sample I1 at 3.5K. Lines are fitted curves calculated by eq. (1).
304
sample I suggested that the ESR signals with the principal g-values less than 1 can be considered as coming from Er-20 centers with C,, symmetry [3], which is consistent with the PL result [l], we performed the angular dependence measurement of sample I1 at 3.5 K. Figure 2 shows the angular dependencies of the anisotropic ESR signals observed in sample 11. We analyzed these angular dependencies by using the same effective spin Hamiltonian (1) (effective spin S=1/2) used in Ref. [9].
H= pBH g S
(1)
As a result the principal g values (g,, g,, g3) for the a, b, cl, c2, c3 and c4 centers are g,sl=O,
gc,,2=13.65, gcl,3=10.78~ g~2,1=~, g,,=4.012 ga,,=3.21, gb,l=o, 8,,2=3.47, gb,3=3.29, gcl g,,,,= 13.42, gC2,,= 12.87, gc3,,=0,gc,,,=l 5.15, gc3,,=l0.4t, and g,,,=O, gc,,,=l 4.83, g,,,=12.48. The analyses suggest that the a, b, c l , c2, c3 and c4 centers can be interpreted by C,, symmetry. The c l , c2, c3 and c4 centers are new resonances observed in sample and no resonance with C2, symmetry is observed. These results are consistent with the PL results [4]. On the other hand, the EXAFS result for sample I1 shows similar profile with sample I [4]. As the EXAFS is sensitive to the distance but not so sensitive to the angle structure, we suppose that the C,,, centers observed in our ESR are the angle deformed Er-20 centers with C,,,symmetry. We observed the temperature dependence of resonances a and b from 3.5 to 14K. The integrated intensity of resonance a is shown in Figure 3 as a typical example and its temperature dependence shows its maximum at 6K. It suggests that resonance a and b are coming from the excited states. The integrated intensity is analyzed by eq. (2) as shown by the solid line in Figure 3. I=
(1/T) e(-uKn
(2)
A [cm-'1 is the difference of energy between the ground state and the excited state. The analysis shows that the ground state is about 4 cm-' below the excited state which gives the ESR signal. We also observed the temperature dependence of resonances c from 3.5 to 11K. Figure 4 shows the integrated intensity of resonance cl and c2, and it shows that the integrated intensity decreases as the temperature increased. It suggests that resonances c are coming from the ground state.
Figure 3. Integrated intensity of resonance a as a function of temperature.
Figure 4. Integrated intensity of resonances c as a function of temperature.
305
4. SUMMARY
X-band ESR measurements of heavily doped GaAs:Er with the Er concentration of 1 . 7 ~ 1 0 ~ ' cm'3 (sample 11) have been performed. As we did not observe isotropic ESR signal around g=6 in sample 11, we can say that Er centers, which have tetrahedral symmetry, decreased. We also observed new anisotropic ESR signals with C,, symmetry. From the analysis of these angular dependencies and the result of EXAFS, we suppose that the Clhcenters observed in our ESR are the angle deformed Er-20 centers with C,,, symmetry. The temperature dependence of integrated intensity of the anisotropic ESR lines in sample I1 shows that there exist both ESR lines coming from the excited states and from the ground states. ACKNOWLEDGEMENTS The authors are grateful to the financial support from the Venture Business Laboratory of Kobe University. REFERENCES
1. Y. Fujiwara, T. Koide, T. Kawamoto and Y. Takeda, extended abstracts of the 5th Symposium on the Physics and Application of Spin-related Phenomena in Semiconductors (1999) 240. 2. K. Takahei, A. Taguchi, J. Appl. Phys. 74 (1993) 1979. 3. J. Yoshikawa, C. Urakawa, H. Ohta, T. Koide, T. Kawamoto, Y. Fujiwara, Y. Takeda, Physica E 10 (2001) 395. 4. Y. Fujiwara and Y. Takeda, private communications. 5. V. F. Masterov, K. F. Shtel'makh and L. F. Zakharenkov, Sov. Phys. Semicond. 21 (1987), 223. 6. C. Urakawa, Y. Nakashima, H. Ohta, T. Ito, Y. Fujiwara and Y. Takeda, Appl. Magn. Reson. 19 (2000) 3. 7. P. B. Klein, F. G. Moore and H. B. Dietrich, Appl. Phys. Lett. 58 (1991) 502. 8. M. Baeumler, J. Schneider, F. Kohl and E. Tomzing, J. Phys. C 20 (1987) 963. 9. T. Ishiyama, E. Katayama, K. Murakami, K. Takahei and A. Taguchi, J. Appl. Phys. 84 (1998) 6782. 10. M. Oohigashi and K. Motizuki, Physica E 10 (2001) 403.
306
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Location of dangling bonds in ELA poly-Si H. Furuta? b, T. Kawashimac,H. Harima' and T. Hiraoc, M. Furuta4 and Y. Tsuchihashid and A. Yoshidae "New Energy and Industrial Technology Development Organization (NEDO), 3- 1-1 Higashi-Ikebukuro, TOKYO 170-6027 Japan bJapan Fine Ceramics Center, 6F, Center for Advanced Research Projects, Osaka Univ., 2-1 Yamadaoka, Suita, OSAKA 565-0871 Japan 'Department of Electrical Engineering, Osaka Univ. 2-1 Yamadaoka, Suita, OSAKA 565-0871 Japan dMatsushita Electric Industrial Co., Ltd, LCD Division, 26-2 Kawakita, Nomi, ISHIKAWA 923-1296 Japan eMatsushita Electric Industrial Co., Ltd, 3-4, Hikaridai, Seika, Souraku, KYOTO 619-0237 Japan The existence ratio of paramagnetic centers in the intra grain, grain boundary and natural Si/SiOz interface was estimated 65 : 24 : 11 from the result of ESR measurement for Secco etched Excimer laser anneal (ELA) poly-Si prepared by the laser power of 350mJ/cm2. Raman measurements on Secco etched ELA poly-Si showed that stresses and disordered area were localized on the grain boundaries on ELA poly-Si. Dominant dangling bonds were located at the intra grains with high crystallinity.
1. INTRODUCTION The method of the excimer laser annealing (ELA) crystallization is widely used to fabricate poly-Si film with large grains. ELA poly-Si has grains that can be controlled by the power of the excimer laser [l, 21. The issue of TFTs prepared from ELA poly-Si film is a lowering of mobility caused by a trapping at the defects in the intra grains, grain boundaries and surface. Surface morphology can also reduce the mobility in the direction parallel to the surface of film. Many efforts have been paid to make large grains on ELA poly-Si in order to
307
reduce the numbers of grain boundaries. The knowledge of the location of dangling bonds in the ELA poly-Si would show us a next step to improve the ELA poly-Si film. In this paper, we report the result of separation of the location of dangling bonds in the ELA poly-Si using Secco etch [ 3 ] .
2. EXPERIMENTAL RESULTS AND DISCUSSION ELA poly-Si films were prepared by the ELA crystallization of PE(p1asma enhanced)-CVD amorphous silicon (a-Si). The ELA crystallization was carried out with XeCl = 308 nm) excimer laser at the fluence of 350 mJ/cm2for 35 consecutive pulses. Before ELA, specimens had been annealed under vacuum at 450 "C for 1 hour for the purpose of dehydrogenation which suppress the generation of hydrogen bubbles in the film during the ELA. The sample configuration was ELA poly-Si / TEOS-Si02 (400 or 600nm) / Glass Coming #1737. Chemical wet etching by HF treatment or Secco etching was employed to separate the defects in grain from those in natural Si/SiOz interfaces or grain boundaries in ELA poly-Si on the SiO2 (600nm) / Glass substrate. The etchant for the HF treatment was (46%HF) : (H20) = 1 : 9. Secco etching was carried out with the etchant of (0.46%HF) : (0.15M K2Cr207) = 2 : 1 under the environment of ultrasonic agitation. After these etchings, specimens were cleaned by distilled water with ultrasonic agitation and evacuated in sample tubes to suppress surface oxidation before ESR measurements.
Figure 1. (a) SEM image of ELA poly-Si. Substrate of ELA poly-Si was TEOS Si02(400nm)/Glass. Grains with the size of about 300 nm were observed. (b) TEM image of ELA poly-Si film on TEOS Si02(400nm)/Corning#1737. The layer of ELA poly-Si was constructed from single layer. The grain boundaries were rose up to the direction vertical to the film.
308
Figure l(a) shows SEM image of ELA poly-Si film on SiO2 (400nm)/Glass. Formation of grains with the average size of 300 nm was observed. Figure l(b) shows TEM image of ELA poly-Si film on Si02 (400nm)/Glass. The poly-Si film was constructed from single layer of silicon. Grain boundaries of the poly-Si film rose up to the direction perpendicular to the film. Figure 2(a) shows ESR spectra of ELA poly-Si. Angles between magnetic field and the film were changed from 0 to 90 degree. The g-value showed angular dependence of the magnetic field to the film, which indicates the poly-Si has a preferential orientation in the film. The ELA poly-Si film had a (1 11) preferential orientation, which was confirmed by the XRD diffraction. That was explained (1 11) texture minimizes the energy of the Si free surface [4, 51. Candidates of the paramagnetic centers in this film are Si dangling bond and Pb center which is a Si dangling bond located at Si/SiO2 interface. Pb center has axial symmetric g-value of g//-2.0020 and gi-2.0090 while Si dangling bonds have an isotropic g-value of g = 2.0055 [8, 91. Calculated g-values and experimental results of Pb centers in ELA poly Si are plotted in Figure 2(b). ESR intensity (peak to peak of 1st derivative line shape) of ELA poly-Si had maximum point at g = 2.0055 when the angle of H to film (9 was 45 degree. Powder spectrum of Pb center is surely not the candidate for this absorption at g = 2.0055 because the powder Pb center should have maximum absorption at g = 2.0088. Experimental data can be explained as isotropic dangling bonds, which have g-value of 2.0055, were overlapped on the axial Pb center which had preferential orientation to the direction perpendicular to the film.
film
II
H to film agle [degree]
Figure 2. (a) ESR spectra of ELA poly-Si on TEOS The spectra showed angular dependence of magnetic field. (b) Observed g-values of ELA poly-Si and calculated g-values of Pb-centers on ELA poly-Si with the preferential orientation of (111). HF or Secco etchings were employed to identify the position of dangling bonds on ELA poly-Si. The result of HF treatment on ELA poly-Si is shown in Figure 3(a). During the initial dissolution of natural SiOz layer for 5 min, 11 % of paramagnetic centers was removed. The rate of the dissolution of the poly-Si film was approximately constant. The result of the Secco
309
etching are also shown in Figure 3(a). ESR intensity was decreased in two stages I and 11. The etching rate in stage I was larger than that in stage 11. 25 of paramagnetic centers were removed by initial etching for 30 min. Paramagnetic centers were decreased in three stages. In first stage Ia, natural Si02 layer on the ELA poly-Si was removed within 13 min, which was estimated from the result of the HF etching experiment. In second stage Ib, grain boundaries and grains were dissolved by the Secco etching. In the process of Secco etching, Cr20:- ions protect crystalline silicon, which has good crystallinity, against HF etching. The high selective etching of grain boundaries can be controlled by the ratio of Cr20$-/HF. Grain boundaries, which can be dissolved in the etchant with the ratio of this experiment, were removed during the stage Ib. In third stage 11, grains dissolved slowly in the etchant.
HF etching time [min] 40
80
S e a etching time [sec]
Figure 3. (a) ESR intensity of ELA poly-Si treat by HF etchant (46%HF:H20 = 1:9) and Secco etches. (b) SEM analysis image for the ELA poly-Si after 240 min Secco etch. The removed grain boundaries are traced by thick lines. The length of grain boundaries were estimated as 3.3 lo4 cmkm2. of the boundaries were removed by the 240 min Secco etch. The length of grain boundaries on the ELA poly-Si was estimated 3.3 x lo4 cm/cm2 from SEM image. 55% of the boundary was removed by the Secco etching for 240 sec, which was estimated from the SEM images before and after the Secco etching (Figure 3b). We estimated the ratio of those paramagnetic centers located in (A) grains, grain boundaries and natural Si/SiOz interfaces. When total spins of these paramagnetic centers is defined as 1; A + + = 1 (Eq. 4). From the result of HF etching, spins in the natural Si/Si02 interface were 11%; C = 0.1 1 (Eq. 5). From the result of Secco etching, 13% of spins (= 0.89 - 0.76) in grain boundaries without good crystallinity were removed by the Secco etch.
3 10
From the result of analysis of SEM images after Secco etching for 240 sec, 55% of grain boundaries was removed; B x 0.55 = 0.13 (Eq. 6). By combining Eq. 4 - 6, spins in (A) grain, grain boundary and natural Si/SiOz interface could be estimated as A = 0.65, = 0.24 and = 0.1 1, respectively. Raman measurements were carried out for the ELA poly-Si sample before/after the Secco etching. Ar laser = 488,) was used for induced beam. Figure 4 shows Raman shift of ELA poly-Si, 240 sec Secco etched ELA poly-Si and crystalline Si. ELA poly-Si had broader line width than that for crystalline Si, which means ELA poly-Si had bad crystallinity than that for crystalline Si. ELA poly-Si showed stronger Raman shift to the low wave number than that of crystalline Si, which indicates that Si bonds in ELA poly-Si suffers stress. After the Secco etching for 240 sec, line width of Raman shift decreased and Raman shift recovered to the higher wave number. It can be explained as poly-Si with low crystallinity was removed by the Secco etching, therefore, average Raman shift shows narrower line width and recovery of the shift to the higher wave number. These results of Raman measurement mean that stresses and disorder area were localized on the grain boundaries on ELA poly-Si. It is noted dominant dangling bonds were located in the intra-grains with high crystallinity.
520.25cm-'
Ex. 488nm 400mW. x50, NAa.8 Exp.6Osec slit 200nm
3.11cm-'
0.4
10 Secco etoh (240sec)
El A D o l r S i 50mW slit lOOnm
(b) 525
._ 31.0
520
515
?
505
ELA oolv-Si 50mW slit IOOnm
06
(a)
510
0.4
0.2
zoo 5
Raman shift [cm-I]
Figure 4. shift of (a) ELA poly-Si, (b) Secco etched ELA poly-Si for 240 sec and (c) crystalline Si (100).
311
3. CONCLUSION
In conclusion, axial Pb-centers, dangling bond at Si/SiOz interface, were observed in the 350 mJ/cm2 35 shot ELA poly-Si film by ESR measurements with the preferential orientation of (1 11). The ESR signal of isotropic dangling bonds (g = 2.0055) were overlapped on that of the Pb centers. Ratio of the location of those dangling bonds in grain, grain boundary and natural Si/SiOZ interface was estimated as 65 : 24 : 11 from the result of ESR measurement for the HF and Secco etched ELA poly-Si. From Raman measurements, stresses and disordered area were localized on the grain boundaries on the ELA poly-Si. It can be suggested from these results that making larger grains on ELA poly-Si can reduce grain boundaries which reduce the mobility but it is also necessary to reduce defects located in the intra grains to make mobility high. ACKNOWLEDGEMENT The authours appreciate Professor M. Ikeya (Osaka Univ.) for valuable discussions on ESR, Dr. H. Ikeda (Ion Engineering Research Institute Corporation) for his discussion and Dr. T. Kamino (Hitachi Science Systems, Ltd.) for valuable discussion on TEM measurements. REFERENCES
1. C. A. Dimitriadis C. T. Angelis, M. Miyasaka, F. V. Farmakis, G. Kamarinos, J. Brini and J. Stoemenos, J. Appl. Phys., 86 (8) (1999) 4600. 2. A. Marmorstein, A. T. Voutsas, R. Solanki, J. Appl. Phys., 82 (9), (1997) 4303. 3. F. Secco d’Aragona, J. Electrochem. SOC.,119, No. 7. (1974) 948. 4. C. V. Thompson and H. I. Smith, Appl. Phys. Lett., 44, (1984) 603. 5. H. Kuriyama, T. Nohda, S. Ishida, T. Kuwahara, S. Noguchi, S. Kiyama, S. Tsuda and S. Nakano, Jpn. J. Appl. Phys., 76 32 Pt. 1, No. 12B. (1993) 6190. 6. P. J. Caplan, E. H. Poindexter, B. E. Deal, R. R. Razouk, J. Appl. Phys., 50 (9) (1979) 5847. 7. A. Stesmans and V. V. Afanas’ev, J. Appl. Phys., 83 (5), (1998) 2449. 8. M. Stutzmann and D. K. Biegelsen, Phys. Rev., B 40, (1989) 9834. 9. T. Umeda, S. Yamasaki, J. Isoya and K. Tanaka, Phys. Rev. B, 59, (1999) 4849.
312
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR studies of BEDT-TTF organic conductors containing supramolecular assemblies
Y. Oshima","', H. Ohtah,',H.M. Yamamotod and R. Katod
* The Graduate School of Science and Technology, Kobe University,
1-1 Rokkodai, Nada, Kobe 657-8501, Japan Venture Business Laboratory, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan ' Molecular Photoscience Research Center and Department of Physics, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan RIKEN (The Institute of Physical and Chemical Research), Hirosawa, Wako, Saitama 351-0198, Japan We have performed X-band ESR measurements of (BEDT-TTF),Br(pBIB). This is the first metallic 3: 1 BEDT-TTF salts and the nature of the cation radical salt with the formal charge of +1/3 is worth further study. We have observed some anomaly around 50 K which is consistent with resistivity measurements. We will report on the angular and temperature dependence measurements of this salt.
1. INTRODUCTION Recently, a new type of BEDT-TTF organic conductor containing supramolecular assembly (BEDT-TTF),Br(DIA) where DIA = Diiodoacetylene, was synthesized by Yamamoto et [ 11. One of the interesting features in this new salt is that it has a simple electronic structure. And we have succeed to observe high order of harmonic cyclotron resonance which comes from a simple quasi-two-dimensional elliptic Fermi surface [2]. Another interesting features in this salt is that the supramolecular ...Br-...DIA ... one-dimensional (1D) chains are formed and the donor molecules (i.e. BEDT-TTF salts) fit into the channels formed by 1D chains which may suggests possibility of "fractional band-filling control" by changing ID supramolecular assemblies with a different period. The organic metal (BEDT-TTF),X(pBIB) (X=Br,Cl) where are one of these interesting salts which has the 3:l pBIB = *
E-mail address: [email protected]
313
donodanion ratio achieved by a longer 1D supramolecular chains period. These salts are the first metallic BEDT-TTF salts where formal charge of donor is +1/3 and the nature of the cation radical salt with the formal charge of +1/3 is worth further study, especially in comparison with the well-known +1/2-charged system. All these salts exhibit metallic behaviors down to 1.6K. We have performed X-Band ESR measurements of (BEDT-TTF),Br(pBIB), angle and temperature dependence measurements will be discussed in this paper.
2. EXPERIMENTAL ESR measurements were performed on a Bruker EPR spectrometer EMXO81 with TE,,, rectangular cavity. The samples were cooled to about 3 K with an Oxford Instruments He continuous-flow cryostat. The modulation width was always kept much less than the linewidth to prevent distortion of line shape due to overmodulation. The typical sample size used for this study was about 1xlxO.l mm’ where conducting plane corresponds to the a*c*-plane and the least conducting direction corresponds to the b*-axis. 3. RESULTS AND DISCUSSION The g value and the peak-to-peak linewidth are found to be anisotropic. Figure 1 shows the angular dependence of the g value for (BEDT-TTF),Br(pBIB).The solid circles represent the g
a*
2.000
0
50
100
150
200
angle 8, d) (degree) Figure 1. Angular dependence of g value for (BEDT-TTF),Br(pBIB); Solid and open circle represent the a*c* plane ($) data obtained at 295K and 3.5K, respectively. The triangle shows the rotation from g ,, to b* direction (0). Calculated g values for each rotation are shown by solid and dashed lines (see text for detail).
3 14
value measured by rotations of magnetic field B in the a*c* conducting plane ($) at room temperature. The solid curve shows least-square fitting curve for the observed g values. The obtained maximum g value g,,,,, is 2.0122 and the minimum g value g,,, is 2.0028 from line fitting. The g value anisotropy in the a*c* plane at 3.5 K are shown by the open circle in Figure 1 and is qualitatively similar to that at room temperature. This result is quite similar from those reported for p-(BEDT-TTF),I,
However, a maximum of 2.01 17 and a minimum of 2.0018
are smaller to some extent than those at room temperature. Despite of applying microwave electric field (E-field) parallel to the conducting plane (i.e. a*c* plane), observed signals did not exhibit the Dysonian line shape. This is due to the positioning of the thin plate-shaped sample which was put in the relatively weak E-field position. On the other hand, we have observed the Dysonian line shape when rotating from the g,,, axis to the b* axis of which position is difficult to prevent penetrating the E-field to the conducting plane. Therefore, the g value was reconsidered by using Dyson's skin-depth theory [4] which are shown by the solid triangles in Figure 1. We note that we see slight difference of the g,,, value when rotating the a*c* plane ($) and from g,,,, to b* axis direction (0) which means the Dysonian correction is not good enough for 8 direction. Therefore, we have fitted the curve assuming that it has the same g,,, value at the same g,,, axis (dashed line in Figure 1). The minimum g value for this rotation is 2.0077. The principal values and axes of the g tensor are determined by the line fitting of each rotation where g,=2.0014, g,=2.0135, g,=2.0073. These results show good agreement with p-(BEDT-
0
2.000
0
50
100 150 200 250 300 Temperature ( K )
Figure 2. Temperature dependence of value for (BEDT-TTF),Br(pBTB). The circle, square and triangle correspond to g,,,, g,,, and b* axis, respectively.
0
50
100 150 200 250 300 Temperature (K)
Figure 3. Temperature dependence of linewidth for Br(pB1B) salts (see text for details). The inset shows linewidth for lower temperature region.
315
TTF),I, results which suggest the principal axis directions(g,, gz, g3) are nearly parallel to the direction normal to the molecular plane, the molecular short axis, and the long axis, respectively [3]. Therefore, the angular dependence of g value of the crystal is well interpreted by the manner of the molecular stacking of BEDT-TTF in the crystal. Temperature dependence of g values are shown in Figure 2. The circle, square and triangle corresponds to g,,,, g,,, axis of a*c*-plane and b* axis, respectively. The g value of g,,,,, g,,, have a tendency to decrease while the b* axis is increasing with decreasing the temperature. An anomaly is seen at around 150 K which is consitent with other measurements [3] and may suggest the local displacement of BEDT-TTF molecules. The temperature dependence of peak-to-peak linewidth are shown in Figure 3 where the symbols are the same as in Figure 2. It is clear that the g,,, and g,,,, axes have larger linewidth than the b* axis in the low temperature region. We see again some anomaly from 150 to 200 K which are consistent with g values. But there is also an anomaly for each axis at around 50K which was not so clear for the temperature dependence of g value. The resistivity of (BEDT-TTF),Br(pBIB) drops sharply at around K, however the relation between the resistivity and relaxation mechanism is still under discussion and remain as a future problem. Apart from these anomalies, the linewidths seem to decrease linearly in medium temperature region which suggests that the spin-lattice relaxation of conducting-electron spin is dominant than the modulation of spin-orbit coupling by the lattice vibrations The inset in Figure 3 shows the linewidth of g,,,, and g,,,, axis at the low temperature region. The linewidth plots are well fitted by a TZcurve. However, these results could not be explained by the wellknown Elliott mechanism [6] and it remains as a future problem . In summary, we have performed X-Band ESR measurements of (BEDT-TTF),Br(pBIB). Principal axes are determined and the obtained values of g tensor are g,=2.0014, g2=2.0135, gs=2.0073 from the angular dependence of g values which are in good agreement with the molecular stacking of BEDT-TTFs. Linewidth anomalies were found around 50K and 150K where the former may suggest the local displacement of BEDT-TTF molecules.
ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (B) (No.10440109) and Scientific Research on Priority Areas (A) (No. 11 136231, 12023232 Metal-assembled Complexes) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
REFERENCES 1. H.M. Yamamoto, J. Yamaura and R. Kato, J. Am. Chem. SOC.,120 (1998) 5905. 2. Y. Oshima, H. Ohta, K. Koyama, M.Motokawa, H.M. Yamamoto and R. Kato, in press. 3. T. Sugano, G. Saito and M. Kinoshita, Phys. Rev., B 34 (1986) 117. 4. G. Feher and A.F. Kip, Phys. Rev., 98 (1955) 337. 5. Y. Yafet, in Solid State Physics, Academic, New York, 1963, Vol 14, pp. 1. 6. R.J. Elliott, Phys. Rev., 96 (1954) 266.
316
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
EPR spectral study of gadolinium(II1) cryptate Ryo Miyarnoto', Hiroyuki Sato, and Susumu Sudoh Faculty of Science and Technology, Hirosaki University, Hirosaki, 036-8561, . IP N
X-band EPR spectra of the gadolinium(II1) cryptate and related compounds were observed in frozen solution. The obtained spectra of the cryptate were different from any spectra obtained before. Furthermore, the line-shape of the spectra changed when the solvent was changed. These results suggest that the cryptand ligand in solution, when attacking another ligand, is flexible and reconstructable in its coordination structure.
1. INTRODUCTION Much attention has been paid recently to supra-molecular chemistry, in which the study of self-organizing structure and molecular recognition are frontier subjects of molecular science. Macrocyclic ligands, especially cryptands and crown ethers, are attracting interest, as they are known to be the simplest molecules capable of selecting the metal ions that they will include [l]. Rare-earth elements have been widely used in magnetic and optical devices and in reaction catalysts [ 2 ] . Gadolinium(II1) complexes have also been known as contrast agents for magnetic resonance imaging (MRI) [3]. It has been necessary to develop stable complexes in biological systems. Macrocyclic ligands might be able to play an essential role in the chemistry of rare-earth complexes in solution. So it is very interesting and important to investigate the properties of the Gd(II1) complexes with macrocyclic ligands in the solution state and how these complexes interact with solvent molecules and with other molecules. In previous studies, we reported the electron paramagnetic resonance (EPR) spectra of some Gd(II1) complexes; the obtained spectra were different with different ligands [4, 51. Those results showed that EPR is a powerful means of directly observing the properties of the 4f-electrons of rare-earth, although they are shielded by the outer 5s- and 5p-electrons. So EPR can be useful in investigating the structure and properties of the Gd(II1) complexes. Hence, the Gd(II1) complexes of cryptand and related compounds were synthesized here, and their EPR spectra were observed. The obtained spectra were compared to investigate the complex structures in the solution and the behavior of the 4f-electrons in the encapsulating ligand.
'Corresponding author.
317
c222
2. EXPERIMENTAL
18C6 Scheme 1
Cryptand and crown ether+ were commercially obtained, and no further purification was done (Scheme 1). Gadolinium(II1) nitrate anhydrate, which was prepared by drying hydrate at 90 "C, 2 hours in vuco, was used for synthesis. The Gd(1II) complexes were synthesized according to the literature [6-91. The obtained compounds were confirmed by IR spectra. The solutions of the complex Gd-C222, which were used for observing the EPR spectra, were prepared in ether of two ways: In the first case (case-i), the powder of the synthetic compound was dissolved in the solvent, and in the second (case-ii), the solution of Gd(II1) ion and that of C222 were mixed at a cu. 1:l ratio. The stability constants for the rare-earth cryptates were thought to be large enough to make an encapsulated complex by the case-ii procedure [ 10, 111. EPR spectra were recorded in the X-band region by a JEOL RE3X X-band EPR spectrometer at liquid nitrogen temperature. Microwave frequency and magnetic field strengtf; were measured by using an EMC-14 microwave frequency counter and an EFM2000AX NMR field meter (both from Echo Electronics Co., Ltd.), respectively. Frozen solutions of the Gd(II1) complexes with various solvents, such as acetonitrile, ethanol, and a mixture of the two, were used to observe the EPR spectra, where the concentration of the complex was cu. 2 mmol dm-3 or less.
3. RESULTS AND DISCUSSION Figures 1 and 2 show the EPR spectra of the Gd-C222 and the Gd-18C6, respectively. The observed spectra of these Gd(II1) complexes have different line-shapes from previously obtained complexes having P-diketonate as a ligand. In the former complexes, the signal around 300 mT is broad and dominant, while in the latter complexes, the signals at cu. 100 mT and cu. 200 mT are the largest [4]. The figures also show that the line-shapes of the corresponding EPR spectra are change when the ligand is changed. The EPR spectra of the Gd-C222 was also different when the solvent of the solution was changed, while these phenomena were not seen in the Gd-18C6 system. This means that the different coordination structures around the Gd(II1) ion take +Abbreviations:C222 = cryptand[2.2.2], 18C6 = element
or or 18-crown-6, RE = rare earth
318
a
I
I
0
200
I
I
I
I
I
600
800
1000
0
200
I mT
Figure 1. X-band EPR spectra of the GdC222 frozen solution at 77K. Solvent: acetonitrile (a, b), ethanol (c, d), methanol (e). Sample preparation: case-i for a, c and e, case-ii for b and d (see text).
I
I
600
800
I
I
61 mT
Figure 2. X-band EPR spectra of the Gd18C6 frozen solution at 77K. Solvent: acetonitrile (a, b), ethanol (c, d). Sample preparation: (i) for a and c, (ii) for b and d (see text).
different EPR spectra, or vice versa. This suggests that EPR must be a powerful method of investigating the local environment around Gd(II1) ions. Furthermore, it is surprising that the effects of the solvents seem larger in the Gd-C222 complex than in Gd-18C6, though the cryptand ligand is considered to encapsulate the Gd(II1) ion and to isolate it from solvent more effectively (vide infru). It is interesting that different EPR spectra were obtained for the different preparation methods of the sample solutions of the Gd-C222: Whereas case-i had a strong absorption at 8-2 without any structure, case-ii resulted in complicated structures (Figure 1). On the other hand, in the Gd-18C6 system, the observed spectra did not change (Figure 2). Changing the constitution of the acetonitrile-ethanol mixed solvent also changed the lineshapes of the EPR spectra for the Gd-C222 complex. It is notable that adding only 10 % more ethanol in acetonitrile produced a large change in the line-shape (Figure 3). The structure of RE-C222 was reported for some rare-earth ions, in which two kinds of RE unit are included [RE(C222)(N03)]2+and [RE(NO3)s(Hz0)l2- [8, 12, 131. So the dissolved solution of the Gd-C222 compound may contain cryptate along with ether atecomplex or solvated ion. Hence, one can expect to observe the overlap spectra of these species. Another possibility is that the interaction between the positively charged
319
[REl(C222)(N03)]2+ moiety and the negatively charged [RE(NO3)5(H20)l2group reduces the spin-spin relaxation time, thereby producing a structureless single line. Here, we cannot determine which is the case. Regardless, the differences in the spectra of Gd-C222 upon the preparation procedure of the sample solution (case-i or case-ii) may cause the differences in the species in the solution. An interesting behavior in the EPR spectra of the Gd-C222 complex has been observed, in which the effect of the solvent seems to be larger for the cryptate than for the crown ether complex. This means that the ethanol molecule interacts with the Gd(II1) ion and probably change the structure of the cryptate, though the C222 ligand is considered to encapsulate the 1 I I I I I Gd(II1) ion and to isolate it from the solvent 0 200 400 1000 more effectively. So it is suggested for the B l mT Gd-C222 complex in ethanol that the Figure 3. X-band EPR spectra of Gd-C222 solvent molecule may penetrate and in the acetonitrile-ethanol mixed solvent at combine with the inner Gd(II1) ion to 77K. Constitution of the solvent increase the coordination number, or that (acetonitrile vs. ethanol): (a) 10:0, (b) 9:1, (c) the nitrate anion may be substituted by the 1:1, (d) 0:lO. ethanol molecule to bind the central ion (Scheme 2), whereas the acetonitrile molecule cannot behave in those ways. Though it can hold a metal ion closely in its cavity, the cryptand ligand is found to be more flexible and reconstructable than expected in its coordination structure when attacking another ligand. The structure of the RE-18C6 complex of the lightweight rare-earths, with nitrate as a
““I/--
*
Scheme 2
320
counter anion, has been reported as follows: the crown ether ligand occupies the equatorial plane and the three nitrate anions are coordinate at the upper and the lower sides of the plane, the coordination number of which is 12 [14, 151. This is large enough to preclude another ligand from being accepted. However, for the heavier rare-earth nitrate, [=(NO&(Hz0)3].18C6 complexes were reported, where the rare-earth ion was not included inside the crown ether ring but was coordinated by three bidentate nitrate groups and three water molecules [14]. Though we have no information about the structure of the crown ether complex of the Gd(II1) nitrate solution, it can be said that the observed EPR spectra of the complex did not coincide with the solvated rare-earth ion. Thus, it is supposed that the Gd(II1) ion is interacting with the crown ether in solution, such that the etheric oxygen atoms are hydrogen-bonded to the water molecules coordinated on the Gd(II1) ion [14]. In the latter structure, the coordination number is only nine but the surrounding ligands inhibit another molecule from coordinating to the central ion. In ether structure of the Gd-18C6 complex, the solvent’s effect is thought to be small.
4. CONCLUSION The Gd(II1) complexes of cryptand (C222) and crown ether (18C6) were synthesized, and their X-band EPR spectra were observed in various solvents at 77K. The obtained spectra differed according to the ligand. Furthermore, changing the solvent had interesting effects on the behavior of the Gd-C222 complex. The spectra of the Gd-C222 complex in acetonitrile solution and in ethanol were largely different, whereas changing the solvent was less effective for the Gd-18C6 complex, though the cryptand ligand is considered to encapsulate the ion more effectively than the crown ether does. It is suggested for the Gd-C222 complex in ethanol that the solvent molecule may penetrate and combine with the inner Gd(II1) ion to increase the coordination number, or the nitrate group may be substituted by the ethanol molecule to bind the central ion. Though the cryptand ligand can hold a metal ion closely in its cavity, its coordination structure is considered to be flexible and reconstructable when attacking another ligand. Therefore the cryptate may be useful as a catalyst in a homogeneous solution, a drag delivery system or a contrast agent for MRI.
REFERENCES 1. V. Alexander, Chem. Rev., 95 (1995) 273. 2. P. Maestro, in “Rare Earths”, R. Sanz-Puche, and P. Car0 (eds.), Editorial Complutense, Madrid, 1998, p. 317. 3. P. Caravan, J. J. Ellison, T. J. McMurry, and R. B. Lauffer, Chem. Rev., 99 (1999) 2293. 4. R. Miyamoto and S. Sudoh, Bull. Chem. Soc. Jpn., 68 (1995) 3439. 5. R. Miyamoto and S. Sudoh, in “Modern Applications of EPR/ESR: From Biophysics to Materials Science - Proc. of the 1st Asia-Pacific EPIUESR Symposium”, C. Z. Rudowicz, K. N. Yu, and H. Hiraoka (eds.), Springer-Verlag, Singapore, 1998, p. 256. 6. M. Ciampolini, P. Dapporto, and N. Nardi, J. Chem. Soc. Dalton Trans., (1979) 974. 7. A. Seminara and A. Musumech, Inorg. Chim. Acta, 39 (1980) 9. 8. J. Mao and Z. Jin, Polyhedron, 13 (1994) 319.
321
9. R. D. Rogers, A. N. Rollins, R. D. Etzenhouser, E. J. Voss, and C. B. Bauer, Inorg. Chem., 32 (1993) 3451. 10. J. H. Burns and C . F. Baes, Jr., Inorg. Chem., 20 (1981) 616. 11. F. Amaud-Neu, E. L. Loufouilou, and M. -J. Schwing-Weill, J. Chem. SOC.Dalton Trans., (1986) 2629. 12. J. H. Burns, Inorg. Chem., 18 (1979) 3044. 13. G . Yang, S. Liu, and Z. Jin, Inorg. Chim. Acta, 131 (1987) 125. 14. J. D. J. Backer-Dirks, J. E. Cooke, A. M. R. Galas, J. S. Ghotra, C. J. Gray, F. A. Hart, and M. B. Hursthouse, J. Chem. SOC.Dalton Trans., (1980) 2191. 15. G . Bombieri, G. de Paoli, F. Benetollo, A. Cassol, J. Inorg. Nucl. Chem., 42 (1980) 1417.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
322
Group-recognition ability of pressure EPR
p-
and y-cyclodextrins as studied by the high-
M. Kasahara", H. Tobisako", Y. Sueishi", S. Yamamotoa and Y. Kotakeb "Department of Chemistry, Faculty of Science, Okayama University, 3-1 -1 Tsushima Naka, Okayama 700-8530, Japan bFree Radical Biology and Aging Research Program, Oklahoma Medical Research Foundation, Oklahoma City, Oklahoma 73 104, Pressure effects on the bimodal inclusion equilibrium (t-butyl-in and substituent-in complexes) of tert-butyl nitroxide with p- and ycyclodextrins(CDs) have been studied by means of a high-pressure EPR technique. Pressure behavior on functional group recognition of CD is discussed from the viewpoint of volume. 1. INTRODUCTION
Cyclodextrins have attracted widespread interest as a model for studies of molecular recognition and enzyme-substrate interaction [ 1,2]. When a guest molecule has more than two different functional groups having the enough volume to fill the CD cavity, each group-in complex may be formed. Kotake and Janzen have suggested the possibility of CD including molecules from two different directions (i.e. bimodal inclusion) [3,4]. For example, in inclusion complexation of diphenylmethyl t-butyl nitroxide (DPBN), the equilibrium between two distinct inclusion complexes, phenyl-in and tbutyl-in complexes, has been monitored by using EPR spectroscopy (Scheme 1) [5,6]. In this study, four kinds of tert-butyl nitroxides are used as probes of bimodal inclusion complexes. By using high-pressure EPR technique, we have examined the pressure effects on the bimodal inclubenzyl sion of tert-butyl nitroxides with
p- and
y-CDs, and obtained the characteristic pressure effects. Based on the results, the ability of the CD cavity to recognize functional groups is discussed in detail.
&!Z3
+
(ypJ t-butyl-in
phenJ'l-in Scheme 1
2. EXPERIMENTAL SECTION Four nitroxide probes
tert -butyl nitroxide)
323
were synthesized by the reaction of 2,4,6-
HBCO
(M0,PBN) with appropriate Grignard reagent or organolithium, respectively H@ (Scheme 2 ) [7]. The high-pressure EPR cell and procedure of the observation of 6 “ O EPR signals at high pressure were identical with those described elsewhere [8]. EPR measurements were performed using a JEOL FE3XG spectrometer at room temperature.
1: 2: 3: 4: -
R Cyclohexyl CYcloPentYl Phenyl n-Hexyl
CHB
Scheme 2
3. RESULTS AND DISCUSSION
3.1. Bimodal inclusion for probes 1-4. The representative EPR spectra of probe 2 in excess y-CD aqueous solution under 1 and 637 bar are shown in Figure 1. The EPR spectrum obtained for 2 exhibits separated EPR peaks for two kinds of included radicals. Since the 2,4,6-trimethoxyphenyl group has a larger radius than the end of y-CD IS], inclusion of probes 1-4 by CD occurs from either the a-substituent or the t-butyl side, which are assigned to “substituent-in” and “tbutyl-in” complexes (bimodal inclusion complexes). Though the EPR spectrum is altered with increasing external pressure, the EPR spectrum can be reproduced with computer spectral simulation by adjusting the relative abundance of the two inclusion complexes. The equilibrium constants K for the bimodal inclusion complexes were determined from integration of each component of the simulated spectrum (Table 1). The magnitude of the K value reflects the difference in the stability between bimodal inclusion complexes, which is determined by hydrophobic interaction and fitness of the functional group in the CD cavity. The values for bimodal inclusion complexes with 0-CD increase with increasing external pressure except probe 1, while those with y-CD decrease with pressure. It should be noted that the equilibrium of bimodal inclusion complexes with p-CD for 2-4 shifts to the t-butylin side with pressure, while the equilibrium with y-CD for 1-4 shifts to the substituent-in side. The dift-butyl-in (2) simulation ference in volume between bimodal (1) at 1 bar&+& inclusion complexes (reaction volume, A V ) at 1 bar can be estimated according to the following equations. cyclopentyl-in 1nK =
AV
=
-
+
(1)
(dlnK
(2)
Using these equations, the reaction for the were estimated, as shown in Table 1.
at 637 bar
simulation
Figure 1. (1) EPR spectra of probe 2 in excess y-CD aqueous solution at 1 and 637 bar. (2) Computer-simulated spectra for the spectra (1).
324
Table 1 The Equilibrium constants and reaction volumes for bimodal inclusion complexes of 14 in water at 298 K - AV CD Probe 1 343 637(bar) cm3 mol-' p-CD 1 0.301 0.264 0.249 7.6 0 2 0.660 0.705 0.740 -4.0 0 3 4.50 5.00 6.50 -14.0 0 4 0.780 0.980 1.16 -16.7 0 y-CD 1 0.430 0.360 0.250 20.0 0.7 2 1.85 1.55 1.30 13.7 1.o 3 0.980 0.927 0.870 4.4 1.o 4 4.80 4.29 4.01 6.9 1.3
3.2. Reaction volume for the equilibrium of bimodal inclusion The reaction volume (AV,-J for the CD inclusion complexation can not be explained in terms of the primitive idea of a decrease in volume accompanied by inclusion of guest in the CD cavity. The following volumetric contributions upon inclusion complex formation with p- and y -CDs are likely [9].
where AY,,cl, denotes a decrease in volume related to inclusion of guest in the CD cavity. AVd,,,,, is the change in volume accompanying desolvation around guest upon inclusion. In CD cavity, water molecules are known to be involved. Upon inclusion, all or a part of the water molecules in CD cavity are repelled out according to the size of guest molecule. denotes an increase in volume caused by water molecules repelled out from the CD ca;ity. The change in volume for inclusion of 1-4 into CDs can be expressed as follows (Scheme 3). AVl and =A Vt-bu+A vdesolv(2) A vl=A vR+A vdesolv( 1) denote the volume vH20 change accompanying the inclusion of substituent R and t-butyl sides, respectively. The reaction volume for the equilibrium of bimodal inclusion corresponds to AV =AV, Since R-in t-butyl-in probes 1-4 are neutral radicals, the reaction Scheme 3
325
volume for the bimodal inclusion equilibrium can be expressed as follows. AV = AV2 - AV, = AVt-bu - AVR +(a- b)VH20
(4)
where a and b are the number of water molecules repelled out from the CD cavity, and thus AAV,,,,, =(a-b) VH20. VHz0 is molar volume (=18 cm3 mol-' ) of one water molecule. Based on the pressure effect on the bimodal inclusion of diphenylmethyl t-butyl nitroxide(DPBN) with p-CD, we have determined the molar volume included from the tbutyl side as AV,,,= - 105 cm3 mol-' [lo]. In the p-CD cavity, an average of 6.5 water molecules are situated [ l l ] . Judging from the molecular size of the Corey-PaulingKoltun (CPK) space-filling models, the p-CD cavity is almost fully occupied upon inclusion from t-butyl and substituent R sides, and then all water molecules situated in p-CD are repelled out. In the equilibrium of bimodal inclusion with 0-CD, a=b=6.5 and thus aVH20=bVH20=117 cm3 mol-'. Using the AV values for bimodal inclusions and AV,,,= - 105 cm3 mol-', the molar volumes included from the a-substituent sides can be calculated as 112.6, 101.0, 91.0, and 88.3 cm3 mol-' for 1,2, 3, and 4, respectively. In the case of y-CD, 12 water molecules are originally involved in its cavity [12]. Judging from the size o f t -butyl and substituents R, a part of the water molecules remain in the y-CD cavity upon the formation of bimodal complexes with y-CD. By using the AV values and the volumes of functional groups estimated above, the value for bimodal inclusion equilibrium with y-CD can be calculated for probes 1-4 (Table 1). The a-b value decreases with increasing the bulkiness of a-substituent R, since the number of water molecules repelled out is dependent of bulkiness of included asubstituent R. The above results suggest that about one more water molecule is repelled out by inclusion of the t -butyl side, compared with those of the a-substituent side. The number of water molecule repelled out from the CD cavity plays an important role for pressure behavior of bimodal inclusion equilibrium with y-CD.
REFERENCES 1. M.L. Bender, M. Komiyama, In Cyclodextrin Chemistry; Springer-Verlag: New York, 1978. 2. J.L. Atwood, J.E.D. Davis, D.D. MacNicol, In Inclusion Compounds, Academic Press, New York, 1984. 3. Y. Kotake, E.G. Janzen, Chem. Phys. Lett., 150 (1988) 199. 4. Y. Kotake, E.G. Janzen, J. Am. Chem. SOC.,114 (1992) 2892. 5. Y. Kotake, E.G. Janzen, J. Am. Chem. SOC.,111 (1989) 2066. 6. Y. Kotake, E.G. Janzen, J. Am. Chem. SOC.,110 (1988) 3699. 7. Y. Kotake, E.G. Janzen, J. Am. Chem. SOC., 111 (1989) 5138. 8. Y. Sueishi, N. Nishimura, K. Hirata, K. Kuwata, Bull. Chem. SOC.Jpn., 62 (1988) 4253. 9. Y. Sueishi, N. Nishimura, K. Hirata, K. Kuwata, J. Phys. Chem., 95 (1991) 5359. 10. Y. Sueishi, M. Kasahara, Y. Kotake, Chem. Lett., (2000) 792. 11. K. Lingner, W. Saenger, Angew. Chem., 90 (1978) 738. 12. J.M. Maclennan, J. J. Stezowski, Biochem. Biophys. Res. Commun., 92 (1980) 926.
326
EPR in the 21' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR studies on a new phenyl t-butyl nitroxide biradical based on calix[4]arene Q. Wang, J.-S. Wang, Y. Li' and G.-S. Wu
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
A new paramagnetic calix[4]arene bearing two nitroxide radicals in one molecule was synthesized. At room temperature in toluene gives a typical spectrum of a nitroxide biradical with g = 2.0067 and a nitrogen coupling constant of 6.6G, which is ascribed to a strong spin-spin exchange interaction. At 77K a characteristic triplet ESR fine structure was observed. AMs = +2 half-field transitions were clearly detected at 16740 as multiple lines due to the electron-nitrogen coupling. Zero-field splitting parameters are estimated: Dlhc = 0.0175cm-', Elhc = 0.0008cm-'. These results are discussed in terms of conformational effects.
1. INTRODUCTION
Calixarenes and their derivatives have attracted more and more attentions because of their peculiar properties in host-guest chemistry and promising applications in analytic chemistry and biochemistry[ 11. Although many kinds of calixarene derivatives have been synthesized and studied, calixarene chemistry concerning free radicals is still an undeveloped field[2,3,4]. Up to now, only several paramagnetic calixarenes substituted with nitroxides have been prepared and it is revealed that this kind of species has potential applications in designing chemical sensors and magnet-functionalized devices[5,6]. However, in all of these compounds nitroxide radicals are introduced at the lower rim of the calixarenes. It is well known that the chemical modification at the lower rim of calixarenes is easily completed and can modulate the comformation effectively. If the nitroxide radicals are introduced at the upper rim of calixarenes, it would be expected that the variation of substituents at the lower rim would provide more possibilities to modify the conformation and thus to control the interaction between intramolecular radicals. In the present work, such a paramagnetic calixarene was synthesized and studied by ESR.
*
To whom correspondence should he addressed
327
EXPERIMENTAL The target compound was synthesized according to the scheme 1. Compound I was prepared according to the previous reference[7]. Br
I
I1 Scheme 1. Synthesis route for
111
@I) Compound I was first dilithiated through selective halogen-lithium exchange reaction by n-BuLi in anhydrous ether. Then the mixture was allowed to react with 2-methyl-2-nitrosopropane (MNP) at -78"C[8]. The solid thus obtained was recrystallized from benzene to give I1 as pure colorless plates, mp 230-232°C. Anal. Calcd for C40H4gN206Br2:C 59.1, H 5.95, N 3.44. Found: C 59.4, H 5.97, N 2.87. IR(F,,,,):-3400 and -3240cm-'(br, OH); 1365cm-'(s, C-N). 'H NMR, 6: 9.10 and 8.81(2H, OH); 7.32-7.18(8H, m, ArH); 4.33 2.93(20H, m, ArCH2Ar and OCHs); 1.07-0.97(18H, m, (CH3)3C). MS(ES1, m/z)[M-H]-: 809,811,813. W 21411t11,30011t11.
-
A benzene solution of I1 (20mM) was dealed with excess PbO2 for over lh. After filtration, the solvent was removed and some orange oil was obtained. Recrystallization from ether gave IIIas orange needles.MS(ES1, m/z)[M+e+2H]+: 810, 812,814. U V 240nm, 300nm, 380nm.
The ESR spectra were recorded on a Bruker ER2OOD-SRC spectrometer equipped with a IOOkHz field modulation. The magnetic fields were calibrated by using an ER 035M NMR gaussmeter. The microwave frequencies were measured with a Nanjing Sample Instrument SP 3382A frequency counter. The IR, NMR, and mass spectra were measured with a Perkin-Elmer System 2000 FTIR, a Bruker AC200P NMR and a PE API 3000 HPLC-MS spectrometer, respectively. X-ray analysis data were obtained on a Bruker P4 four-circle diffractometer.
328
3. KESULTS AND DISCUSSIONS
3.1. X-ray crystal structure of I1 An X-ray crystal structure analysis was carried out on a monoclinic single crystal (C2/c (No. 15)) ofII: a = 3.2255 (7) nm, b = 1.4394 (3) nm, c = 2.1753 (5) nm, p= 110.210 (9)", = 9.478 (3) and 2 = 8. According to X-Ray analysis I1 is presented in a flattened partial cone conformation(Figure 1, left), which is less common for calix[4]arenes in solid[l]. The two opposite phenyl rings bearing hydroxyamino groups are in an syn orientation. And each molecule exhibits two intramolecular hydrogen bonds 0-H---N(Figure 1, right). The average bond length of H---N is about 0.2nm and bond angle of 0-H---N is about 150".
Figure 1. The molecular structure for 11. 3.2. ESR spectra As Figure 2 shows, the typical spectrum of a nitroxide biradical was observed at room temnerature. Justifying from the different ESR linewidths, the spectrum can be regarded an overlap of two sets of signals. One is a wide triplet with nitrogen hyperfine coupling of AN = 13.8G at g = 2.0067. And another is a quintet with AN= 6.6G, which is attributed to the coupling of two nitrogen nuclei with the unpaired electron. This indicates that the spin-exchange coupling exists at J>>A"9]. The two types of spectra can be explained based on the various conformations of calix[4]arenes. As is well known, calix[4]arenes can exhibit four conformations in solution(cone, partial cone, 1,2-alternative and 1,3-alternative cone) due to the relatively free rotation of methylene bridge between two phenyl rings. In compound III,conformations can effectively change the distance between the two unpaired electrons in the molecule. For example, if the calix[4]arene molecules stay in partial cone conformation as the crystal structure of I1 shows(Figure 1, left), both of the two nitroxide groups lie at the upper rim of the calix[4]arene skeleton and they can come close. While if a molecule appears in a 1,2-altemative cone conformation, leaving the two radical parts at different sides of the
329
cavity formed by the four phenyl rings, the two unpaired electrons will be so far that it is difficult for them to interact with each other. In this state the spectrum will be similar to that of a monoradical. In the present system, it can be revealed that at room temperature nearly half molecules exist in the conformations, where the two nitroxide parts are so near that a Figure 2. ESR spectrum of m i n toluene at 298K. strong spin exchange interaction is ipduced, which results in the quintet observed. To be noted, no hyperfine coupling with the hydrogens of the phenyl rings was found. At low temperature shows ESR fine structures characteristic of triplet dinitroxides in frozen toluene matrix(Figure 3, marked by arrows). Half-field resonances corresponding to AMs = +2 forbidden transitions, which are the proof of a triplet, were clearly observed at 16746. The multiple lines arise from the electron-nitrogen coupling[lo], which indicates that the strong exchange interaction is maintained.
I
Figure 3. ESR spectra of
I
I
I
I
in toluene at 77K.
In AMs = +1 transitions, the lines in the middle of the spectrum is too complicated to be assigned exactly. This is caused by anisotropic coupling with nitrogen nuclei and broadening due to dipole-dipole interaction. On the basis of the resonance positions of the outermost signals, zero-field-splitting parameter is calculated as 0.0175cm-'. The parameter is roughly estimated as 0.0008cm-' from the positions of the peaks marked by
330
X, Y. 1 E/D I << 1/3 indicates that the biradical molecule has a axial symmetry approximately. According to the point dipole approximation DIG = 27812/(r/A)3[111, the average distance between the unpaired electrons of the two nitroxides in a molecule is determined as 0.526nm. Compared with 0.4261~11, which is showed in the molecular structure of II(Figure 1, right) as the distance between two hydroxyamino oxygens, it can be deduced that after oxidation the conformation of calix[4]arenes does not change largely. Although the elimination of hydrogen bonds makes rotations of nitroxides and phenyl rings more freely, the steric hindrance between the bulky tert-butyl groups help the unpaired electrons come close.
4. CONCLUSIONS In the present study, a new phenyl t-butyl nitroxide biradical based on calix[4]arene was successfully synthesized. The room temperature ESR spectra in solution reveal a strong intramolecular spin exchange interaction. AM, = f 2 half-field transitions corresponding to triplet species were observed at low temperature. Zero-field splitting parameters were also estimated. All results can be reasonably discussed in terms of conformation effects.
REFERENCES C. D. Gutsche, Calixarenes Revisited, Royal Society of Chemistry, Cambridge, 1998. P. Franchi , M. Lucarini , G. F. Pedulli Angew. Chem. Int. Ed., 39(2000) 263. A. Rajca, S. Rajca and J. Wongsriratanakul, J. Am. Chem. SOC.,121(1999) 6308. Q. Wang, Y. Li and G. S. Wu, Appl. Magn. Reson., 18(2000) 419. K. Araki, R. Nakamura, H. Otsuka et al.,J. Chem. SOC.,Chem. Commun., 20(1995) 2121. 6. G. Ulrich, P. Turek and R. Ziessel, Tetrahedron Letts., 37(1996) 8755. 7. C. D.Gusche, J. Org. Chem., 50(1985) 5795. 8. Q. Wang, Y. Li and G. S. Wu, Chem. J. Chin. Univ., 22(2001, supplement) 177. 9. H. Nishide, T. Kaneko, S . Toriu Bull. Chem. SOC.Jpn., 69(1996) 499. 10. G. Gagnaire, A. Jeunet and J. L. Pierre, Tetrahedron Letts., 32(1919) 2021. 11. W. B. Gleason and R. E. Barnertt, J. Am. Chem. SOC.,98(1976) 2701.
1. 2. 3. 4. 5.
Section 3 Chemical Reactions
EPR in the 2 1'Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
333
Time-resolved EPR studies of excited states: Some old and some new stories Noboru Hirota Professor Emeritus, Kyoto University, Kyoto, 606-8501 Japan
An overview of the application of the time-resolved EPR(TREPR) technique to the study of excited molecules is presented. The TREPR technique has been used very successfully to study short-lived triplet states of numerous molecules at low temperatures. Here after a brief introduction of the technique a few examples of such studies are discussed to show how TREPR provides rich information about electronic structures and dynamic properties of excited triplet states. The examples include elucidation of the electronic structures and dynamic properties of the lowest excited triplet states of some azaaromatic molecules, molecules related to excited state proton transfer and fullerenes. Exciting new advances have been made recently in the application of TRFiPR to the study of excited molecules. These include observation of excited triplet states in solution, detection of excited multiplet (quartet dimensional and quintet) states, and applications of Modem EPR techniques such nutation spectroscopy and W-band EPR. Some examples of such advances are also reviewed. 1. INTRODUCTION The time-resolved EPR(TREPR) spectroscopy has been very useful in chemistry, biology and physics. In Chemistry there are areas of research in which TREPR has been particularly valuable: chemically induced dynamic electron polarization (CIDEP) studies of short-lived radicals and radical pairs in photochemical reactions, and studies of short-lived excited states, excited triplet states in particular. In this review we are mainly concerned with the studies of excited states. Photoexcited triplet states of organic molecules play important roles in various areas of chemistry,particularly in photochemistry, because photochemical reactions often take place in these states. Even if we are interested in radicals and radical pairs in photochemical reactions, their precursors are often excited triplet states. Therefore it is important to know about excited triplet states. Usually molecules in the ground states are in the singlet states (SO). Upon photoexcitation molecules in the ground states are excited to excited singlet states, then
334
followed by intersystem crossing (ISC) to the lowest excited triplet (TI) states. Since the TI -'SO transition is spin forbidden, the lifetime of a TI state is often relatively long. The ordinary cw-EPR technique is adequate to study triplet states with lifetimes longer than about O.ls, and numerous investigations have been made since the pioneering work on the TI state of naphthalene by Hutchison and Mangum [13. There are, however, many molecules whose TI states are too short-lived to be studied by ordinary EPR, though these molecules are ofken interesting in view of their spectroscopic properties and reactivities. With advent of optically detected magnetic resonance (ODMR) [2-41 it became possible to study short-lived triplet states with lifetimes of an order of ms, if they are phosphorescent. However, triplet states of many interesting molecules eluded EPR detection because of their short lifetimes and nonphosphorescent character. The lifetimes of some of these molecules are often on the order of ps or even shorter, and it was necessary to fmd a different EPR technique to study these molecules. Population into triplet sublevels via ISC usually produces very large population differences among triplet sublevels. If one detects an EPR signal just after creation of a triplet state via ISC before averaging by spin-lattice relaxation, one can detect a strong EPR signal by taking advantage of a non Boltzmann distribution of spin sublevels (spin polarization). In 1976 Kim and Weissman showed that time-resolved EPR(TREPR) detection of short-lived triplet states could be made easily by combining direct EPR detection with laser excitation [5]. In such a TREPR detection the limit in the time resolution was extended to about 200 ns. More advanced pulsed EPR techniques were also used profitably to study the electronic structures and dynamics of short-lived triplet molecules [6]. Numerous studies on short-lived triplet states in low temperature rigid media have been made in the last twenty five years [7,8]. More recently TREPR studies have been extended to study excited triplet states in solution [9] well excited multiplet (quartet and quintet) states composed of excited triplet states and radicals [10,11]. These advances opened new possibilities in the studies of excited molecules by TREPR . In the present review we fxst give a short description of the experimental methods. Then we discuss some representative examples of TREPR studies on short-lived triplet states to show how TREPR studies can provide information about electronic and molecular structures and dynamic properties of excited molecules. Finally we touch upon some of recent advances in the studies of triplet states in solution and excited multiplet states. 2. EXPERIMENTALMETHODS
The simplest way to perform TREPR experiments is to use a cw-ERR spectrometer modified to make time-resolved EPR measurements [5,12]. This can be easily done by
replacing a lock-in amplifier with a boxcar integrator and a digital oscilloscope. An example of such a spectrometer system is shown in Figure 1. A time-resolved EPR spectrum can be obtained at a desired time after pulsed laser excitation by using the boxcar integrator and the time profile of the EPR signal can be recorded on the digital oscilloscope. The time resolution of such a system is usually about 20011s. A pulsed EPR spectrometer can also be used [6-8,121. Two different modes of operation are commonly used depending on the sample and experimental conditions; one is to take Fourier transform (FT) of the free induction decay signal after a d2 microwave pulse with a delay time after a laser pulse (ET-FID-EPR) and the other to take the FT of the electron spin echo signal after a series of d 2 - microwave pulses (FT-ESE-EPR). These modes of
IE2FYI
A
Signal Microwave Unit
Digital
50s Laser
cw Microwave
EPR signal Excimer Laser - - ' (XeCI. 308nm)
Boxcar gate ,
I
Figure 1. Schematic diagram of a cw-time resolved EPR spectrometer.
a
+ mw pulse
laser pulse
,1 2
delay dead time
b
5hL
Figure 2. Two types of pulsed EPR technique.
336
operation are schematically shown in Figure 2. More advanced 2 dimensional (2d) techniques can be used with advantages in some systems. A particularly useful 2d technique in the studies of multiplet excited states is the 2d nutation technique that utilizes a combination of nutation and refocusing pulses and show the echo signals functions of the nutation frequency and the applied field strength in two dimensions [131. 3. EXAMPLES OF TREPR STUDIES OF SHORT-LIVED TRPLET STATES
Short-lived triplet states are non- or very weakly phosphorescent even in rigid media at very low temperatures. This means that these molecules undergo very rapid T l- + So nonradiative transitions. It is interesting to understand why the nonradiative transitions are so effective in some triplet molecules. To answer this question one has to know their electronic and molecular structureswell and EPR is suited for this purpose. In the course of the last two decades we have investigated the short-lived triplet states of numerous molecules. The molecules studied include azaaromatics such pyridazine and phthalazine [14,15], aliphatic and aromatic carbonyls [16,171, conjugated enones and related molecules [181, molecules related to excited state proton transfer [19,20] and fullerenes [21,22]. In the following we discuss a few examples to show what sorts of information can be obtained from TREPR studies. 3.1. Azaaromatics: pyridazine and phthalazine [14,15] F’yridazine and phthalazine have quite different spectroscopicproperties compared with their isomers, pyrazine and quinoxaline whose triplet properties have been investigated in great detail by using various spectroscopic techniques including ODMR. Since pyridazine does not phosphoresce under any conditions, even the question whether the TI state is or n7c* in character was not known. Likewise the exact nature of the TI state of phthalazine was not known at all. Both pyridazine and phthalazine give rise to very good TREPR signals in rigid media at low temperatures. The pyridazine spectra in rigid media cover over a wide range of magnetic field spanning from 90mT to 5800 mT. The signals either absorptive or emissive reflecting the relative populations of spin sublevels. The TREPR spectra can be analyzed in terms of the usual spin Hamiltonian including the zero field splittings (ZFS: X, and Z) and hyperfiine splittings (Ax,Ay,A,) and taking account of the populating ratios into
spin sublevels (pX,py,p,), = @B*S- Xa2 -
- ZS2 + S.A.1
The decay rate constants of the spin sublevels can be obtained by analyzing the time profiles of EPR signals at canonical orientations. Single crystal experiments on
337
Table 1 Magnetic and dynamic properties of T1 Pyridazine ZFSIcm-' kits-' Di X 0.092 2 . 0 ~ 1 0 ~ 0.1 Y -0.162 1 . 7 ~ 1 0 ~ 1.0 Z 0.071 2 . 0 ~ 1 0 ~ 0.1
\
Y
X
X
z*J
k JI.7'
2,5-dichloropyridazine in the 1,4-dichlorobennzene host provide information about the in nitrogen hyperfine splittings and spin distributions.The data on pyridazine are Table 1 [14]. The large zero field splittings, the sublevel order X>z>Y, and the very large decay rate constants are most remarkable in this system. The hyperfine splittings due to nitrogen atoms are 2.7, 1.8 and 2.5 mT for A,, A,, and A,, respectively. These immediately tell that the TI state is nearly pure in character. The observed nitrogen hyperfine splittings are well reproduced by an calculation for nn* pyridazine. The calculation indicated that nearly % of the spin densities are located on the N=N group. It is believed that this electronic structure makes the potential for the out of plane vibration at the N=N position very shallow, producing a very large Franck-Condon factor for the nonradiative transition [23]. On the other hand, triplet properties of phthalazine are extremely dependent on the environment and temperature [l5]. The nitrogen hyperfine splittings were not observed in the benzoic acid and 1,4-dichlorobenzene hosts, but they were clearly resolved in biphenyl
4: durene
1
2
Figure 3. Sublevel energies and hction of the nz* character of phthalazine,
338
and durene. The order of the sublevels also changes from Z>Y>X in benzoic acid to DX>Y in durene. The analyses of the hyperfine splittings and the ZFS indicate that the TI state could be described a mixture of nx* and xx* states due to vibronic mixing and the extent of the mixing varies as shown in Figure 3. It was shown that in biphenyl the TI state is about 60% xx* and 40% nx* in character at 4.2K. The nn*- xx* vibronic mixing should accompany an out of plane distortion of the molecule. The occurrence of such a distortion was indeed detected by a time-resolved ENDOR work by Yamauchi et a1 [24]. 3.2. Molecules related to excited state proton transfer [19,201 Molecules undergo keto-enol isomerizasion accompanying excited proton transfer (ESPT) have been investigated extensively with various spectroscopic techniques. TREPR can also make unique contributions in this area of research. From the analysis of the spectra one can easily identify the transient triplet species produced by photoexcitation. In a molecule such as methylsalicilate it was shown that the transient triplet state is in the en01 form, indicating that the triplet state is produced from the en01 form of the singlet state produced via intersystem crossing [19]. In some cases one can not only clarify the nature of the triplet state, but also follow the dynamics of the proton transfer. Here we discuss the case of 2-(2’-hydmxyphenyl) benzoxazole (HBO) [20] as an example.
~s
1.0 ys
320
Figure 4. State energies and proton transfer scheme of HBO.
330
320
330
Figure 5 . Time dependence of the TREPR spectra Of C70. a) Observed, b) Simulated.
339
The enol form of HI30 undergoes ESPT and produces the keto form from which the TI state of the keto form is produced. Since the triplet state energies of the keto and enol isomers are nearly degenerate, proton transfers between the keto and enol forms in the triplet manifold take place (Figure 4). Since the TREPR spectra of the keto and en01 forms are very different, the TREPR spectrum of the triplet HBO after laser excitation changes as the enol form is produced from the keto form. By analyzing the time profiles of the both keto and enol K determined to be 0.8 x lo%-' and 0.5 x 106s-1, signals, the rate constants km and ~ E were respectively. 3.3. Excited triplet states of fullerenes [21,221 Fullerenes such as c 6 0 and C70 have highly symmetric structures O h for Cm and D5h for C70) in the ground state. It is interesting to see how these structures change upon excitation. TREPR can also give valuable information about the structures and dynamics of the TI states of fullerenes. Both CM and c 7 0 in low temperature rigid media give rise to strong TREPR signals, which immediately tell that in the T1 state they are distorted from the ground state geometries. Both spectra show considerable temperature dependences that are indicative of the dynamic nature of the distortion. Here we discuss the case of C70 [22]. is extremely The TREPR spectrum of c70 in methylcyclohexane (MCH) at dependent on the delay time after excitation as shown in Fig.5. The middle part of the spectrum loses intensity very quickly time goes on. At 2ps after excitation the central part of the spectrum completely disappears. This dramatic change of the spectrum could be explained in terms of the dynamic Jahn-Teller distortion by a scheme given in Figure 5 . When looked through the long (Z) axis C70 has a pentagonal shape in the D5h symmetry. It is considered that in the TI state this pentagon is distorted to have five equivalent structures shown in Figure 5. In the pseudo-rotation model C70 hops randomly among these equivalent structures averaging out the signals along the X and directions. The simulated spectra based on this pseudo-rotation model agree very well with those experimentally observed. From the comparison between the observed and simulated spectra the hopping rates were determined at various temperatures. It was estimated to be 3.6 x 106s% MCH at 77K. The activation energy was also determined to be 250 & 50 cm-'. 4. NEW ADVANCES IN THE TREPR STUDIES OF EXCITED STATES
The TREPR technique has been applied actively to study structures and dynamics of numerous interesting short-lived triplet molecules. The results of these studies were discussed in recent reviews [8,25] and interested readers are referred to these reviews for more details.
340
Here we discuss some new advances made by Yamauchi's group in Sendai: studies of excited triplet states in solution and multiplet excited states. 4.1. Excited triplet states in solution It been long considered that direct EPR observation of excited triplet states in solution is very difficult. In fact Weissman suggested in 1958 that large ZFS of triplet states make relaxation times too short to observe EPR signals [26]. Yamauchi and coworkers studied metallo-porphyrins [9], metallo-phtallocyanines [27] and subphtalocyanines [28] over wide temperature ranges from - 170 "C to room temperature . It was found that EPR signals could be observed even in room temperature solutions. Moreover, one can obtain some new information that cannot be obtained from low temperature studies, In Figure 6 TREPR spectra of zinc tetraphenylporphyrin (TPP) in toluene taken at various temperatures are shown. They show a gradual change from a typical rigid medium spectrum at -170 "C to a broad single peak at 20 "C. Similar spectra were obtained in other systems. These spectra were simulated by taking account of various mechanisms to average out the spectra. Comparisons between the observed and simulated spectra revealed details of the averaging processes and motions of the molecules in solution. It was concluded that the averaging among the Jahn-Teller split states takes place first, then followed by rotation in the molecular plane and finally out of plane rotation. Rate constants of individual averaging processes were estimated. Solution studies provide an opportunity to study both spin and reaction dynamics simultaneously [29]. From the time profiles of the signals both TI and triplet lifetimes can be
-170OC
-95 Abs.
+--/""-----
Table 2
- 90 - 80 - 50 20
Figure 6. Temperaturedependence of the EPR
.
TI of various TPP at different microwave frequencies Tllns X-band 0-band W-band ZnTPP 30 33 MgTPP 42 140 390 H2TPP 260 1200 45
341
Spectra of ZnTPP in toluene obtained. TI of various TPP complexes were determined at different microwave ffequencies given in Table 2. It is interesting to note that the fiequency dependence of T1 is strongly dependent of the system. This indicates that in addition to the usual mechanism of the spin-dipole relaxation another mechanism to be considered to explain the observed TI. 4.2. TREPR Studies of muitiplet excited states Radical triplet interaction is an important topic in CIDEP studies. The radical triplet pair mechanism (RTPM) is now well recognized an important CIDEP mechanism operating many photochemical systems [8, 251. When molecules such as metal tetraphenylporphyrin (TPP) axially coordinated with p-pyridylnitonyhitroxide(nit-p-py) and C ~ connected O with nitroxides (Figure 7) are excited, they form both excited doublet @I) and quartet (QI) states by the interaction between the radical (DO)and the excited triplet (TI) state shown by the scheme in Fig.7 [10,111. TREPR can detect signals coming fiom both the excited doublet and quartet states, but it is not so straightforward to identify the signals coming form the species with different spin multiplicities (Do, D1 and in Fig.7). Yamauchi and coworkers made use of advanced EPR techniques to solve this problem. The first was to make use of W-band EPR [30]. The g-values of the quartet and doublet signals are different, but the differencesare not large enough to separate the signals fiom DO,DI and Q1 states in the X-band. In the
J-=O
Figure 7. Molecules used to study multiplet excited states and the excitation scheme.
342
W-band the signals were well separated and identified clearly. The second was to apply the 2d nutation spectroscopy [31,32]. Since the nutation frequency varies depending on the spin multiplicity of the system, the signals coming from the species with different multiplicities could be clearly separated. The have also succeeded in detecting an excited quintet state in a hllerene connected to nitroxide radicals[33].
REFERENCES 1. C. A. Hutchison and B. W. Mangum, J. Chem. Phys., 29 (1958) 252,34 (1961) 908. 2 . M. Sharnoff, J. Chem. Phys., 46 (1967) 3263. 3. A. L. Kwiram, Chem. Phys. Lett., 1 (1967) 272. 4 . J. Schmidt and I. A. M. Hesselmann, M. S. de Groot and J. H. van der Waals, Chem. Phys. Lett., 1 (1967) 434. 5 . S. S. Kim and S. 1. Weissman, J. Magn. Reson. 24 (1976) 167. 6 . W. J. Buma, E. J. J. Groenen and J. Schmidt and R. de Beer, J. Chem. Phys., 91 (1989) 6549. 7 . N. Hirota and S. Yamauchi, chapter 13 in ‘Dynamics of Excited Molecules’ (K. Kuchitsu ed.) Elsevier, Amsterdam, 1994. 8. N. Hirota and S. Yamauchi, chapter 7 in ‘Dynamic Spin Chemistry’ (S. Nagakura, H. Hayashi, T. Azumi eds.) Kodansha, Tokyo, 1998. 9. J. Fujisawa, Y. Ohba and S. Yamauchi, J. Phys. Chem. A, 101 (1997) 434. 10. C. Corvaja, M. Maggini, M. Prato, G. Scorrano, M. J. Venzin, J. Am. Chem. SOC.,117 (1995) 8857. 11. K. Ishii, J. Fujisawa, Y. Ohba and S. Yamauchi, J. Am. Chem. 118 (1996), 13079. 12. L. Kevan and M. K. Bowman (eds) ‘Modem Pulsed and Continuous-Electron Spin Resonance’, John Wiley & Sons, New York, 1990. 13. R. Hanaishi, Y. Ohba, S. Yamauchi and M. Iwaizumi, a) J. Chem. Phys., 103 (1995) 4819 b) J. Magn. Reson. A, 116 (1995) 4819. 14. M. Treazima, S. Yamauchi and N. Hirota, J. Chem. Phys., 84 (1986) 3679. 15. M. Terazima, S. Yamauchi and N. Hirota, J. Chem. Phys., 83 (1985) 3234. 16. K. Tominaga, S. Yamauchi and N. Hirota, J. Chem. Phys., 94 (1990) 4425. 17. S. Yamauchi and N. Hirota, J. Chem. Phys., 86 (1987) 5963. 18. S. Yamauchi and N. Hirota, J. Phys. Chem., 92 (1988) 1346. 19. E. Hoshimoto, Y. Yamauchi, S. Nagaoka and N. Hirota, 95 (1991) 10029. 20. H. Nakmura, M. Terazima and N, Hirota, 93 (1993) 8952. 21. M. Terazima, N. Hirota, H. Shinohara and Y. Saito, Chem. Phys. Lett., 195 (1992) 333. 22. M. Terazima, K. Salcurada, N. Hirota, H. Shinohara and Y. Saito, J. Phys. Chem., 97
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(1993) 5447. 23. M. Terazima, S. Yamauchi, N.Hirota, 0. Kitao and H. Nakatsuji, Chem. Phys., 107 (1986) 81. 24. S. Ohkoshi, S. Yamauchi, Y. Ohba, and M. Iwaizumi, Chem. Phys. Lett., 224 (1994) 3 13. 25. H. Murai, S. Tero-Kubota, S. Yamauchi, Specialist Periodical Reports: Electron Spin Resonance, The Royal Society of Chemistry, 17 (2000) 130. 26. S. I. Weissman, J. Chem. Phys., 29 (1958) 1189. 27. I. S. M. Saiful, J. Fujisawa, N. Kobayashi, Y. Ohba and S. Yamauchi, Bull. Chem. SOC. Jpn., 72 (1999) 661. 28. S. Yamauchi, H. Takahashi, Y. Iwasaki, J. Fujisawa, Y. Ohba, A. Blank and H. Levanon, The 87* annual meeting Jpn. Chem. SOC.,Tokyo, March 28-30,2000. 29. J. Fujisawa, Y. Ohba and S. Yamauchi, Chem. Phys. Lett., 282 (1998) 181. 30. K. Ishii, J. Fujisawa, A. Adachi, S. Yamauchi and N. Kobayashi, J. Am. Chem. 120 (1998) 3152. 31. J. Fujisawa, K. Ishi, Y. Ohba, S. Yamauchi, M. Fuhs, K. Mobius, J. Phys. Chem. A, 103 (1999) 213. 32. N. Mizouchi, Y. Ohba and S. Yamauchi, J. Phys. Chem. A, 101 (1997) 5966. 33. K. Ishi, Y. Hirose andN. Kobayashi, J. Am. Chem. 120 (1998) 10551.
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EPR in the 21' Century A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Time-resolved EPR studies of the excited triplet states of p-methylcinnamic acid and its deprotonated anion Ichiro Yamamoto, Kanekazu Seki and Mikio Yagi* Department of Applied Chemistry, Graduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan
The effects of deprotonation on the excited states of trans-p-methylcinnamic acid have been studied through measurements of the time-resolved EPR and fluorescence in rigid organic glasses and in stretched poly(viny1 alcohol) films at 77 K. The deprotonation appears to have little effect on the zero-field splitting parameters. However, the triplet sublevel kinetics changes on deprotonation.
1. INTRODUCTION The esters of trans-p-methoxycinnamic acid are the most widely used UV absorbers in cosmetic sunscreens. To be effective as a UV protective compound in skin-care sunscreens, the UV absorber must be able to harmlessly transform the absorbed UV energy into vibrational energy. The deactivation mechanism of trans-cinnamic acids (CAs) and their esters have been an object of continuous interest for many years with particular interest not only in their application to cosmetic sunscreens but also in their photodimerization reactions [ 1,2]. However, the property of the lowest excited triplet (TI) states of CAs and their esters is poorly known. This is because of their weakly phosphorescent character and short T I lifetimes. We have applied the time-resolved EPR technique to study the TI properties of CA and its deprotonated anion [3,4]. The effects of deprotonation on the zero-field splitting (ZFS) parameters and relative populating rates of individual TI sublevels of CA were elucidated. However, we have not studied the lowest excited singlet (S1) states of CA and the absolute values of the triplet sublevel populating rates because of its nonfluorescent or very weakly fluorescent character. It is known that CA changes to fluorescent by the methylation at the benzene ring. In the present study, we have observed the time-resolved EPR and fluorescence spectra of trans-p-methylcinnamic acid (pMeCA) and its deprotonated anion (pMeCA-) in EtOH solutions at 77 K. The fluorescence lifetimes have also been observed to discuss the absolute values of the triplet sublevel populating rates because the computer simulation of the time-resolved EPR spectrum gives only the relative populating rates.
* Corresponding author. Fax: +8 1-45-339-3948. E-mail: [email protected]
M4
pMeCA Figure 1. Molecular structures and coordinate systems chosen for trans-p-methylcinnamic acid (pMeCA) and trans-p-methylcinnamate anion (pMeCA-).
2. EXPERIMENTAL pMeCA (Aldrich) was purified by recrystallization from EtOH. Solutions of pMeCA were prepared at a concentration of 1 x 10-3 mol dm-3. A poly(viny1 alcohol) (PVA) film was obtained by the same method as described previously [3]. The film was stretched about 300% in the direction with a Shibayama SS-60 film stretcher. The experimental setup for the time-resolved EPR measurements is the same as that reported previously [5]. A Lumonics HE-420 excimer laser (XeC1, 308 nm) was used as an exciting light pulse source with a repetition rate of 25 Hz. The sampling times were set at 0.30-0.94 p s after the laser pulse. The fluorescence measurements were carried out with a streak-camera detection system (Hamamatsu C4780). The excitations were carried out using a femtosecond Ti:sapphire regenerative amplifier laser system (Spectra-Physics Spitfire) with a repetition rate of 1 kHz. Samples were excited at 266 nm using the third harmonic of the laser.
3. RESULTS AND DISCUSSION 3.1. Zero-field splittings Assuming the isotropic g value, the spin Hamiltonian describing the magnetic field B is taken to be
sates in
external
= gpBB.S + S.D.S = ~,LQB.S = gpeB.S
+
2
-
2
-
(1/3) S2] +
-
(1)
Here, D is the ZFS tensor with principal values of and and and are the ZFS parameters. The other symbols have their usual meaning. Assuming the molecular planarity in y, of the ZFS tensor were taken to be as the rigid solutions at 77 K, the principal axes shown in Figure 1. The axis deviates an angle of from the axis. The ZFS parameters and are defined to be = -3212 and = - 4 1 2 . The steady-state and time-resolved EPR spectra of the T1 state of pMeCA were measured in EtOH solutions with various concentrations of H2S04 and NaOH at 77 K. The observed time-resolved EPR spectra do not depend on the sampling times between 0.1 and 2.0 ps after the laser pulse. As is clearly seen in Figures 2 and 3, the time-resolved EPR spectrum depends on the amount of NaOH. In NaOH-EtOH (0.5 wt% of NaOH) solution, the observed spectrum (Figure 3a) is ascribed to pMeCA- because the intensity of the emissive Bminsignal increases
346
Ern. I
I
100
200
I
t
I
300
400
500
100
300 BlmT
Figure 2. Time-resolved EPR spectra for the T I state of pMeCA (a) in EtOH and (b) in H2SO4-EtOH wt% of H2S04) at 77 K. (c) Computer-simulated time-resolved EPR spectra of pMeCA obtained by using the observed and values and = 0.05:0.95:0, (d) = 0.15:0.85:0 and = 0.25:0.75:0. (e)
Figure 3. Time-resolved EPR spectra for the TI state of pMeCA- (a) in NaOH-EtOH wt% of NaOH), (b) in PVA (Bils), (c) in PVA (Biin) at 77 K. (d) Computer-simulated time-resolved EPR spectra of pMeCA- obtained by using the = observed and values and 0.35:0.65:0.
200
400
500
BimT
with an increase in the amount of NaOH. In contrast, in EtOH and in H2SO4-EtOH wt% of H2SO4) (Figures 2a and 2b), the observed spectra are interpreted as due to pMeCA. This is in accord with the observed dissociation constant of pMeCA in the ground state, pK, = 5.88 in dioxane-water (1: 1 by volume) mixture at 30°C The EPR hMs = *1 transition signals for pMeCA and pMeCA- are too weak to be detected throughout the conventional EPR measurements under steady-state UV-irradiation of the sample solutions. On the other hand, they are strong throughout the time-resolved EPR measurements. Therefore, the assignment of the observed EPR signals was carried out for the time-resolved spectrum. Experiments involving the acid-base equilibria of organic molecules should be carried out in water or in alcoholic-aqueous solutions by changing the pH of the medium. Because of randomly oriented T I molecules, the measurements in such random rigid solutions do not permit one to determine the principal axes of the ZFS tensor relative to the molecular axes. It is difficult to find a host crystal suitable for the EPR measurements of organic acids in ionic
347
Table 1. Zero-field splitting parameters and relative populating rates, in the T1 states of ~rans-4-methvlcinnamicacid and trans-4-methvlcinnamate anion observed in EtOH at 77 K Molecule
X(cm-1)
(cm-1)
(cm-I)
(cm-1)
(cm-1)
p,
py
pMeCA
0.0780
-0.0109
-0.0675
0.1013
-0.0444
0.15
0.85
0
p
~0.0802 ~ -
-0.0125
-0.0677
0.1015
-0.0463
0.35
0.65
0
~
e
forms. To eliminate this difficulty, the host of our choice is a stretched PVA film in which the orientations of guest molecules have a tendency to be orthorhombic Figure 3 shows the time-resolved EPR spectra of the T I state of pMeCA- in stretched PVA films. According to the general relations concerning the orientation of guest molecules, the = A 1 transition signals is straightforward. The intensity of the Z assignment of the signals is weak and the intensity of the X signals is relatively enhanced when B is parallel to the stretched direction of a film s, as can be seen in Figure 3b. On the other hand, the intensity of the X signals is weak and the intensity of the Z signals is relatively enhanced when B is parallel to the normal of the film n, as shown in Figure 3c. As a result, all the observed = 1 transition signals can reasonably be assigned. In the same manner as for pMeCA-, all the observed time-resolved EPR signals were assigned for pMeCA. The observed ZFS parameters are listed in Table 1. We can see from this table that the values obtained are = 0.1 cm-1 in both pMeCA and pMeCA-. The T I lifetimes are about 40-50 ms. These values suggest that the Ti states possess mainly a character. It is known comprehensively that T, sublevels are the lowest in energy for 37171* states. As a result, the order of the T1 sublevels was determined to be T,, T, and T, from the top in both pMeCA and pMeCA-. Since the observed D values of pMeCA and pMeCA- are fairly smaller than those of toluene, ethylenic and carbonyl fragments, the two unpaired electrons localize neither on the toluene fragment, nor on the ethylenic one or on the carbonyl one. Table 1 shows that the ZFS parameters of pMeCA- are similar to those of pMeCA, reflecting the similar delocalization of the two unpaired electrons.
*
3.2. Relative populating rates The polarization pattern of the time-resolved EPR signals of pMeCA at the seven stationary fields is E, AEAIEAE from the low-field to the high-field (where E is emission and A is absorption of microwave), as shown in Figures 2a and 2b. The observed EPR spectrum with spin polarization was simulated in the same manner as that reported previously [5]. In the present simulation a Gaussian line width parameter of 3.0 mT and a microwave frequency of 9.19 GHz were used. With the aid of the computer simulation, the relative populating rates were estimated to be = 0.1 5:0.85:0, which indicates the preferential intersystem crossing (ISC) to Ty, the middle sublevel. Figure 3a shows the E, EEA/EAA(weak) polarized time-resolved EPR spectrum of pMeCA-. In the same manner as for pMeCA, the relative populating rates were estimated to be = 0.35:0.65:0 from the computer simulations, as shown in Figure 3d. The most notable observation is that the sublevel preferentially populated by ISC is Ty, in both pMeCA and pMeCA-. However, T,, the top sublevel, is also populated in the deprotonated anion.
348
At present we have no definitive explanation for the dominance of the T, sublevel populating rate in the T I sublevel populating processes. One possible explanation of the anisotropy of the ISC in pMeCA arises from a consideration of the presence of the carbonyl oxygen atom having a large spin-orbit coupling constant and the dominant role it plays in the radiationless decay processes. A large populating rate into the Tz sublevel has been known for aromatic carbonyls of the benzaldehyde type is along the C=O direction) [7]. In the present work, the principal axis y is close to the C=O direction in pMeCA, as shown in Figure 1. Therefore, the sublevel preferentially populated by ISC is expected to be T, for pMeCA as observed. The observed effect of deprotonation on the anisotropic populating rates of pMeCA is due to the effect of the change in the C=O structure of pMeCA as shown in Figure 1. pMeCA and pMeCA- are weakly phosphorescent and their steady-state EPR signals are weak. On the other hand, the time-resolved EPR signals are fairly strong. These facts show that the decays from the T I states are dominantly radiationless, although at present we have no data of the phosphorescence quantum yields. In the time-resolved EPR experiments, we have no quantitative explanation for the effect of deprotonation on the anisotropic ISC rates, because the simulation of the time-resolved EPR Therefore, the fluorescence lifetime spectrum only gives the value of measurements were carried out to discuss the absolute values of the ISC rates. The S1 lifetimes were obtained from the decay curves measured at the first peaks of fluorescence (at 323 nm for pMeCA and at 320 nm for pMeCA-). The lifetimes obtained are about 1 ns in both pMeCA and pMeCA- at 77 K. It should be noted that the S1 lifetime scarcely changes on deprotonation, although the anisotropy in ISC is sensitive to the deprotonation. These facts show that the deprotonation has little effect on the sum of the three ISC rate constants for the three T I sublevels, although these three rate constants change on deprotonation.
ACKNOWLEDGEMENTS The authors wish to thank Professor Jun Takeda and Professor Takao Sekiya for their help in measuring the fluorescence lifetimes. We also thank Mr. Hiroshi Saga for his help in the preliminary work. This work was supported by a Grant-in-Aid for Scientific Research No. 11640498 from the Ministry of Education, Science, Sports and Culture, Japan.
REFERENCES 1. G. J. Smith and I. J. Miller, J. Photochem. Photobiol. A, 118 (1998) 93. 2. S. D. M. Allen, M. J. Almond, J. Bruneel, A. Gilbert, P. Hollins and J. Mascetti, Spectrochim. Acta A, 56 (2000) 2423. 3. Y. Shioya and M. Yagi, J. Photochem. Photobiol. A, 86 (1995) 97. 4. S. Kakuho, K. Seki and M. Yagi, Chem. Phys. Lett., 277 (1997) 326. 5. Y. Shioya, K. Mikuni, J. Higuchi and M. Yagi, J. Phys. Chem., 98 (1994) 12521. 6. R. Murugesan, B. Rajasekar, T. L. Thanulingam and A. Shunmugasundaram, Proc. Indian Acad. Sci. Chem. Sci., 104 (1 992) 43 1. 7. E. T. Harrigan and N. Hirota, Mol. Phys., 3 1 (1976) 663.
EPR in the 21%Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
349
Quenching of singlet molecular oxygen ('Ag) by vitamins and polyphenols studied by time-resolved ESR Akio KawaPb, Takahito Fusea and Kazuhiko Shibuya" 'Department of Chemistry, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152-8551, Japan, ?REST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan. The quenching rate constants, k<s of singlet molecular oxygen ( 02('Ag) ) by several vitamins and polyphenols (PPL) were determined at room temperature by a time-resolved ESR (TRESR) method. The decays of 02('Ag) concentration were monitored by measuring chemically induced dynamic electron polarization (CIDEP) of nitroxide radical, 2,2,6,6(TEMPO). The CIDEP of TEMPO is created through the 0 2 ( '$)-TEMPO encounter process, which has been explained by the radical-triplet pair mechanism (RTPM) generally occurred in radical-excited molecule systems [ 1,2]. The kq values determined in the present study are analyzed on the basis of a charge transfer (CT) type quenching mechanism.
1. INTRODUCTION Singlet molecular oxygen, 02('Ag) is one of the active oxygens which have attracted much attention in the studies of photochemistry and biological chemistry [3]. There already exist many reports on the dynamics of this excited molecule utilizing time resolved spectroscopic methods such as detection, thermal lensing and laser flash photolysis utilizing photooxidation reactions [4]. Recently, we reported that intense CIDEP was created in the quenching of 02('Ag) by TEMPO in solution [2,5]. When CIDEP is created on a radical side, the ESR detection sensitivity is largely improved and the TR-ESR detection of short-lived radicals becomes possible. Spin polarization generated in the 02('Ag) -TEMPO system enables us to study O2('AS) dynamics by the TR-ESR method. Even for time-unresolved ESR detection, the signal intensity increases due to CIDEP, which has been used for a study of 02('Ag) diffusion in cotton [6].
350 The CIDEP mechanism in the 02(1Ag)-TEMP0 system has already been interpreted in terms of RTPM for a case of singlet excited state quenching [2,5,7]. Analyses of time profiles of the CIDEP have been done by using a Bloch equation and it is suggested that the decay rate of 02('Ag) is determined by CIDEP decay measurements. Although characteristic points of this recently-discovered TR-ESR method for the study of 02('Ag) dynamics have not been well understood yet, at least it is one of powerfiil methods to determine 02('Ag) lifetime. In this study, we adopted this method for determination of kq values by quenchers such as tocopherol and PPL's in solution at room temperature and the quenching mechanism was discussed on the basis of the kq values determined by the spin polarization probing technique.
2. EXPERIMENTAL TR-ESR signals were detected by an X-band ESR spectrometer (Brucker, ELEXIS 580E) combined with a boxcar integrator (Stanford, SR-250) for spectra or a digital oscilloscope (SONY/Techtroniks, TDS340) for time profiles. The excitation light was the third harmonics of a YAG laser (Continuum, Powerlight 8000, 355nm). The laser power was attenuated to be about 2-3 mJ/pulse at a sample cell in the microwave cavity for CIDEP The microwave measurements. The signals were collected at the repetition rate of 10 power was usually 5 mW for both measurements of spectra and time profiles. All the chemicals (Tokyo Kasei) were used as received. The concentration of TEMPO was 3 mM Sample solutions were ( M = mol dm-3). Solvents used were toluene and 1,cdioxane. air-saturated or degassed by bubbling of gas and were flowed through the sample cell. A sample cell is a quartz flat cell with 0.5 or (a) without 0.7 mm interior space. Experiments were carried out at room temperature (298 K). f _ _
3. RESULTS AND DISCUSSION 3.1 CIDEP in O~('A,)-TEMPO system
Figure 1 shows TR-ESR spectra of TEMPO with anthracene dissolved in toluene. The 355 nm laser excites anthracene to the SI state which undergoes intersystem crossing yielding the T1 state of anthracene. Without dissolved OZ(~C,-) (Figure la), net emission (Em) CIDEP of TEMPO appears in the spectra with sharp
h
Figure 1. TR-ESR spectra of TEMPO measured at 1.5-2.0~safter 355nm excitation.
351
triplet hyperfine structure due to the nitrogen atom. On the other hand, net absorption (Abs) CIDEP appears in the presence of the dissolved oxygen molecule (Figure lb) with each peak being broader due to dissolved 02(3Cg-). Figure 2 shows the time profile of net Abs CIDEP measured at the central peak in Figure lb.
9
.-
Z
.E
a
LU
0
The time profile is characterized by a fast 0 10 20 rise and a single exponential decay. The Time / decay time of 23.2 0.2 was determined Figure 2. Decay of CIDEP in the quenching by a-least-square fitting of the decay curve. of O2('A,) by TEMPO. This decay time is very close to the lifetime [4] in toluene measured by luminescence detection. of 02('Ag) (25 or 29 In the mixture solution of anthracene, TEMPO and dissolved 02(3Cg-), the following scheme is considered for deactivation of TI anthracene.
The denotes doublet state of TEMPO and all the others follow their standard notation. In the absence of 02(3Cg-), T1 anthracene is slowly quenched by TEMPO (eq. 1) at the estimated rate of 1x104 s-' and non-Boltzman distribution of TEMPO is created with electron spin enriched population through RTPM [8,9] as described previously. This condition gives the net Em signal in the TR-ESR spectrum. Under air-saturated condition, the TI quenching by dissolved 0 2 ( 3 C g - ) (eq.2) becomes important. The decay rate of TI anthracene by eq.2 is 1.1x lo7 s-' with 0 2 ( 3 C , - ) concentration of 2 . 0 lo5 ~ M and the rate constant of 5 . 3 lo9 ~ M's-' [4]. This rate is about a thousand times faster than quenching rate by TEMPO and, the TI anthracene is found to be dominantly quenched by 02(3Cg-). The process of eq.2 is known to yield 02('A,) very efficiently. This 02('Ag) decays mainly through eq.3 with minor contribution of decay channels, eqs.4 and 5. After 02('Ag) is produced, the encounter complex of TEMPO and 02('Ag) is formed with the doublet total spin state which undergoes internal conversion giving the 02(3Cg-) -TEMPO pairs. These pairs are prepared in doublet spin states due to spin conservation rule and subsequent time evolution of electron spin states
352
from doublet to quartet states with an electron dipolar-dipolar interaction of 02(3C,-) yields non-equilibrium distribution with p spin enriched population of TEMPO (eq.4), namely CIDEP of TEMPO [1,2,6]. Although the quenching rate of eq.4 is slow as described above, the enhancement factor of CIDEP creation in the 02('Ag)-TEMP0 collision process is very large due to strong electron dipolar-dipolar interaction of 02(3Cg-). The CIDEP signal induced by this process is, therefore, very large and net Abs signal appears in the TR-ESR spectrum (Figure lb). In general, the Bloch equation must be considered for analysis of CIDEP time profile. However, it is fortunate that a simple approximation is available for the Oz('Ag)TEMPO system. The spin-lattice relaxation time of TEMPO (<0.25ps) [lo] is much shorter than O2('Ag) lifetime (>20ps) and the CIDEP intensity at certain window time after laser excitation reflects the CIDEP intensity created during this time period [2]. Therefore, the time profile of CIDEP intensity itself corresponds to the time profile of the relative 02('Ag) concentration. This circumstance enables us to measure the 02('Ag) lifetime very simply by the TR-ESR method. The decay time of 02('Ag) observed here is slightly shorter than the reported values. This is mainly due to quenching of O2('Ag) by dissolved quenchers such as anthracene and TEMPO. The k i s by anthracene and TEMPO are 8 . 5 ~ 1 0and ~ 5 . 0 7 ~ 1 0M's-', ~ respectively [2]. The calculated quenching rates are 7 . 1 ~ 1 0and ~ 1 . 5 ~ 1 0s-'~for anthracene (8.4and TEMPO Adopting 29 for the lifetime of O2('Ag) without quencher, (3mM), respectively. calculated lifetime of 02('Ag) with these quenchers are 23.2 which is the same with observed lifetime of 02('Ag). 3.2 Determination of the kpvalues of 02('Ag) by vitamins and PPL's
Since the decay of 0z('Ag) can be observed by monitoring CIDEP of TEMPO, we carried
I \
0
a-tocopherol
5
10
15
I
20
Time / ps
Figure 3. CIDEP decays of TEMPO under various concentrations of a-tocopherol
0
1
2
3
4
[Q] I mM Figure 4. Stern-Volmer plot of CIDEP decay rates to determine k4 of O2('Ag).
5
out experiments to determine the OZ('Ag) quenching rate constant by Stern-Volmer analysis of the CIDEP decays. Figure 3 shows logarithm plots of CIDEP decays with various concentrations of a-tocopherol under 3 excitation of anthracene without bubbling. Decay rates of CIDEP increases with increasing the concentration of a-tocopherol, which suggests that the quenching of Oz('Ag) by a-tocopherol occurs to increase the decay rate. The Stern-Volmer plot of these decays is shown in Figure 4 where a good straight line is obtained. From this analysis, we determined the kq value by a-tocopherol as 1 . 0 ~ 1 W's-'. 0~ The kq value determined by the TR-ESR method is close to the value determined by other methods (2.2~10'M'i') [4], and we consider that this method is good enough to determine the kq value. We carried out similar quenching experiments for a number of quenchers including several PPL's which have been attracting much interest as important antioxidants in biological systems. The kq values determined by the present TR-ESR method are summarized in Table I. 3.3 Quenching mechanism of O,('Ad by PPL's and related phenol compounds
For a quenching mechanism of Oz('Ag), energy transfer (ET) and CT mechanisms could be considered. The ET is not the case since the energy of O,('Ag) is 0.98 eV which is much lower than the T1 energies of the quenchers examined here. According to the study by Thomas and Foote [l 11, quenching of Oz('Ag) by alkylphenols occurs through the CT mechanism.
I
I
02('Ag) quencher
Solvent
kq / M's-'
Em vs. SCE / V
Triethvlamine
Toluene
(1.5f0.4) X 10'
1.15
8-Carotene
I
I
I
a-Tocopherol 2,6-Di-tert-butylphenol
Toluene
(4.6f0.3) X lo5
1.48
Vanillic acid
1,4-Dioxane
(2.0f0.2) x lo5
1.62
trans-CCoumaric acid
1.4-Dioxane
( 5 . 4 f 1.4) X lo5
1.46
trans-Ferulic acid
I
1,4-Dioxane
I
(9.7f0.2)X 109[2] (1.0f0.04) X 10'
I
Benzene Toluene
(1.6f0.3)x lo6
I
I
Catechol (PPL)
Toluene
( 8 . 4 f 1.2) X lo5
1.29
Catechol PPL)
1.4-Dioxane
(1.6f0.07)X lo6
1.29
I Protocatechuic acid (PPL) I
1,4-Dioxane
Gallic acid (PPL)
1,4-Dioxane
(3.0f0.2)x lo5
Caffeic acid (PPL)
1,4-Dioxane
(1.6f0.2)x lo6
I
(9.3f0.8)x lo5
I
1.49 1.23
I
354 Gorman et al. also suggested the CT mechanism through exciplex formation for quenching by phenols including a-tocopherol [12]. It is well known that a-tocopherol is a very efficient antioxidant molecule in the cell membrane and it is reasonable that the measured kq value was relatively large among rate constants of the phenols examined here. 1 1.2 1.4 1.5 1.6 1.7 The kq values by PPL's and related phenol Eoxof quencher I V compounds studied here are relatively smaller Figure 5. Plot of kq values by alkyl- and polyand very close to those of alkylphenols such as phenols against Eoxvalues of quenchers. 2,6-di-tert-butylphenol. This seems to suggest that the quenching by PPL's occurs through the CT collision complex. Hence, we plot the kq values of PPL's and related phenol compounds against an oxidation potential (Eox) of quenchers as shown in Figure 5. The values adopted were measured in polar solvent such as acetonitrile and methanol while kq values determined in this work were measured in non-polar solvent. Therefore, the plot may include uncertainty of 0.1-0.3 eV. In this plot, kq values of alkylphenols reported by Thomas and Foote [l 11 are included. As is seen from the Figure, kq values depend on the Eoxof quencher. This means that the ( O:--QQSt) type CT collision complex may play an important role in the deactivation of Oz(lA,). Therefore, we conclude that Oz('Ag) is quenched by PPL's and related phenol compounds through the CT collision complex as is the same with the case of alkylphenols. '
I
'
"
'
'
I
'
REFERENCES 1. 2. 3. 4.
A.Kawai and K.Obi, J.Phys.Chem., 96(1992)52. M.Mitsui, K.Takeda, Y.Kobori, A.Kawai and K.Obi, Chem.Phys.Lett., 262( 1996)125. D.R.Keans, Chem.Rev., 71(1971)395. J.C.Scaiano, "Handbook of Organic Photochemistry," CRC Press, Inc., 1989. A.Kawai, M.Mitsui, Y.Kobori, and K.Obi : Appl. Magn. Reson., 12(1997) 405. 6. S.N.Batchelor, J.Phys.Chem.B, 103(1999)6700. 7. J.Fujisawa, Y.Ohba and S.Yamauchi, J.Phys.Chem.A, 101(1997)434. 8. C.Blattler, F.Jent and H.Pau1, Chem.Phys.Lett., 166(1990)375. 9. A.Kawai, T.Okutsu and K.Obi, J.Phys.Chem., 95(1991)9130. 10. G.H.Goudsmit, H.Pau1 and A.I.Shushin, J.Phys.Chem., 97(1993)13243. 11. M.J.Thomas and C.S.Foote, Photochem. Photobiol., 27( 1977)683. 12. A.A.Gorman, I.R.Gould, 1.Hamblettand M.C.Standen, J.Am.Chem.Soc., 106(1984)6956.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
355
A time-resolved EPR study of weakly coupled triplet-doublet pairs of copper(I1)-freebase porphyrin dimers M. Asano-Somedaa, A. Jinmon", Y. Kaizu", P. Ragognab, and A. van der Estb "Department of Chemistry, Tokyo Institute of Technology, 0-okayama, Meguro-ku, Tokyo 152-855 1, Japan bDepartment of Chemistry, Brock University, 500 Glenridge Ave., St. Catharines Ont., CanadaL2S 3A1 Time-resolved EPR spectra of copper@)-porphyrin-free base porphyrin dimers, in which the two halves are linked via an alkyl-chain or ap-biphenyl group, are presented together with a framework of spin-polarized spectra in weakly coupled triplet-doublet systems. these complexes, two pathways, i.e., intramolecular energy transfer ( EnT ) and intersystem crossing ( ISC ), are available to generate a weakly coupled triplet-doublet state, where the free base half is in the excited triplet and the copper half is the ground state doublet. The excitation wavelength dependence of the spectra reveals mechanisms, which can account for characteristic spin polarization patterns of the weakly coupled triplet-doublet pairs and their time developments. 1. INTRODUCTION
photodynamics of the triplet are changed as a result of the interaction the doublet. The electron spin polarization patterns of such pairs and their time developments are expected to reveal how the interaction controls the coupled extensive efforts have been devoted to studying strongly coupled triplet-doublet systems, relatively few examples have been 0 b w ~ e dfor the weakly coupled systems[ 11. Copper@) porphyrin-free base porphyrin dimers, in which the two
EnT \-\ r Z[ZT1..'So]
4[4Tq..'So]
cup
"J A PHz
?4[2So-3Ti]
excitation
excitation
2~2so..,sol
____._.___.______
PCU-PH~(ISO)
PCu@io)-.PH2
Figure 1. A schematic energy diagram of copper(II) porphyrin-free base porphyrin dimer,
moieties are linked via a spacer, provide a) Cu-C3-H2 an ideal system for studying triplet! T doublet pairs because the distance and orientation between the two halves can be designed and also there are two pathways to generate the final metastable tripletdoublet pair state. Figure 1 shows a schematic energy diagram of such dimers. The electronic absorption spectra in the visible region of the two moieties are overlapped only partially, and thus selective excitation is feasible. The excitation at 640 nm (or 630 nm) yields the photoexcited state of the dimer 2[2S,‘S,] ( see the right-hand side of the figure ), which is followed by ISC. Then the final excited state 2,4[2S0-’Tl],where Figure 2. Molecular structure of the dimers the free base is in the first excited triplet state and the copper is the ground state doublet, is produced. On the other hand, excitation with 540 nm leads to the copper porphyrin excited states( the left-hand side of the figure ). Since copper(II) has an unpaired electron, the ISC process within the copper porphyrin is quite fast and the lowest excited states, described as doublet(’T,) and quartetCT,) states, are generated immediately after the excitation. Intramolecular energy transfer ( EnT ) from the copper porphyrin produces the same final excited state, 2,4[2S0-’Tl] of the dimer as that of the free base ISC pathway. This situation allows us to investigate the role of the triplet-doublet coupling in relation with the photodynamics of the molecules. Although the strength of the coupling depends on the distance and orientation between the two spin centers, weakly-coupled systems can be established in the molecules which have rather a long spacer unit. Figure 2 shows the two specific dimers, which are studied here: One has an alkyl-chain and the other is a linear type dimer, where the center-to-center distance is ca. 17 A. In these two dimers, which consist of different monomer units, we have observed common features in their spin-polarization patterns and their time-dependent behaviors. Based on these results, we discuss the spin dynamics expected in weakly coupled tripletdoublet pairs.
2.MODEL 2.1 Spectra of coupled triplet-doublet systems Here, we will consider EPR spectra of triplet-doublet systems as a function of the coupling. In any coupled triplet-doublet system, the eigenstates form a six-level system and can be described using either a triplet-doublet-product or quartet-doublet basis. When there is no coupling between the triplet and doublet, the expected EPR spectrum is a superposition of those of triplet and doublet species. The top of Figure 3 shows a stick EPR spectrum of a triplet-doublet system without coupling and for a certain orientation. The splitting between the two triplet lines (labeled T) arises from dipolar coupling within the triplet, which we
357
assume much stronger than that between the triplet and doublet. When the coupling between the triplet and doublet is introduced, the triplet and doublet lines split into two and three lines, respectively, as shown in Figure 3. As the coupling increases, the splitting energies become larger and inequality of the transition intensities is prominent. In addition, a new transition line, which is forbidden without coupling, appears near the center of the resonance field. Finally, this new transition gets an appreciable intensity and is ascribed to the trip-doublet of the strong coupling case as in the bottom of the figure. The other three transitions are assigned to the trip-quartet state (Q)[2-41. 2.2 Transient behavior of a weakly coupled triplet-doublet system As described above, the spectrum of a weakly coupled triplet-doublet system, is expected to be very similar as those of original triplet and doublet. The coupling between the triplet and doublet may make the whole spectra slightly wider in the resonance field. On the other hand, weak coupling between the triplet and doublet may bring new features into their time-dependent behavior. Here, let us start with the influence of the coupling in terms of the wavefunctions. The eigenfunctions of the triplet-doublet system can be described by using two types of couplings, i.e., isotropic exchange coupling and dipolar coupling d, which is orientation dependent.
T
T
Figure 3. Stick EPR spectra of tripletdoublet pairs.
where, -2A( J tan28
+
d)
=
+D* - o D + ( J + l d \ 2 )
- 2 h ( J +:d)
,tan*
=
wT-D*-o,-(J+;d)
(2)
and are the resonance frequencies of the original triplet and doublet, respectively and and is a dipolar coupling within the triplet for a certain orientation. As can be seen above, the couplings and d mix the two Sz = 112 basis functions of the triplet-doublet product states, I T,, p> and I To with each other. The same holds for the Sz = -112, I To p> and I T., states. In the extreme of strong coupling, the above six states separate into doublet and quartet states, which can form another basis set of the system. It should be noted that the middle four state, Y, Y,, Y4,Y5have both of doublet and quartet characters whereas Y, and Y6,are of pure quartet character. The spin polarization patterns are determined by how the transient spin-states are initially populated and also by spin dynamics after the states are formed. Here, there are two possible mechanisms, which are likely to be characteristic in the weakly coupled triplet-doublet pairs but cannot operate in the absence of coupling. One is selective electronic relaxation from the middle four states to the ground state due to their doublet character and the other is spin-flipflop relaxation between the same Sz quantum-number states[2]. Both mechanisms should affect not only the spin-polarization patterns but also their time development. 3 EXPERIMENTAL RESULTS AND DISCUSSION
3.1 Cu-C3-H2 Figure 4 shows time-resolved EPR ( TREPR ) spectra of Cu-C,-H, in a toluene frozen glass at 80 K. The upper spectrum is taken with direct excitation of the free base half at 640 nm at early delay time, 0.8 ps, whereas the lower spectrum is after excitation of the copper porphyrin with 540 nm and its delay time is 10 ps. In both spectra, the main component is the weakly-coupled triplet-doublet pair state, 2,4[2S0-’Tl] (see Figure 1) in which a triplet excitation is localized on the free base half and the copper porphyrin is the ground state doublet as described in the Introduction section. The large difference in the spin-polarization patterns initially comes from the different pathways by which this final excited state is generated. However, the width of the spectra and the magnetic field positions corresponding to canonical orientations are the same, consistent with the assumption that we see the same excited state of the dimer in both cases. When the final excited state of the dimer is formed via ISC with hex = 640 nm ( top of Figure 4 ), the observed spectrum is quite similar to that of the free base monomer. This implies that the selection rules for the ISC process within the free base half is principally the same as the monomer, although the excited states of the dimer should be described as a coupled system with the copper doublet. However, the time dependence of the EPR signal is different from the monomer. After prompt formation of the triplet excitation in the free base, the EPR signals at the field positions for the X canonical orientation in the dimer decay much
n
kx=Wnm
t
quicker than those in the monomer. In contrast, the signal decays corresponding to the Z orientation are similar in the dimer and the monomer. On the other hand, the intramolecular EnT pathway gives a considerably different spin-polarization pattern as shown in the bottom of Figure 4. The spin-polarized spectra rise slowly compared with the response time of the spectrometer and energy transfer rate. 3.2 Cu-p-Bp-H, In this dimer, the two porphyrin
halves are rigidly linked via a biphenyl spacer and in a linear geometry. Due to a L slight difference of the peripheral substituents, the zero-field splitting Magnctic parameters of the corresponding monomer free base are different from those for Cu-C,-H,. However, apart Figure 4. TREPR spectra of Cu-C,-H, in from this difference and an additional toluene at 80 K. The top spectrum is obtained signal in the middle of the resonance with 640 nm excitation ( PH, selective, see field, the observed behavior in this dimer Figure 1) at 0.8 ps delay time whereas the is very similar to Cu-C,-H,, . With the bottom spectrum is taken with 540 nm ISC pathway, the spectral pattern is due excitation ( PCu selective) at 10 ps delay to spin-orbit ISC and has large intensities time. at X-canonical orientations, the decay of which is the fastest. When the intramolecular EnT leads to the final excited state, the characteristic polarization pattern is also observed with a slow rise. These observations confirm that the common features in TREPR signal behaviors, that is, i) fast decay in specific orientations in the ISC spectrum, ii) slow rise and characteristic polarization pattern via EnT pathway, are indeed of weakly coupled porphyrin triplet - copper doublet systems. I I
I
I
I I
Mechanisms With the direct excitation of the free base of the dimers, it is equivocal that the initial spin polarization of the final state 2*4[2S0-’T,] is due to SO-ISC within the free base moiety. The fast decay observed at the X-canonical orientations is ascribed to dynamics after the ISC. Two mechanisms are proposed as characteristic features of the triplet-doublet system in the previous section: selective depopulation and spin-flip-flop relaxation. Both of the mechanisms are consistent with the fast decay since these two decrease population differences between the Y, or Y6 state and the middle of four states in the case of the free base ISC, where the selection rule is Px>Py>Pz. This is because 1 To and 1 To p> states have the Xtriplet wavefunction character with B,/K and are populated dominantly with this orientation. On the other hand, the observed spin polarization via EnT pathway is generated after the
360
final excited state is formed. Under our experimental condition, the precursor copper quartet excited state is deduced to be in a thermal equilibrium[5-6] and thus energy transfer does not cause strong intensity of the initial spectra. Under the assumption that energy transfer occurs according to the spin-quantum numbers, either of spin-selective depopulation or spin-flip-flop relaxation increases spin population differences between the Yl or Y6state and the middle of four states in the six-level system. Thus spin-polarized spectra rise with this rate via the EnT pathway. While it is difficult to determine definitely which of the mechanisms is responsible at the moment, we note that the rise time for the ISC pathway and decay time for the EnT pathway are almost identical and obtained as 2ps and 1 ps for Cu-C,-H, and Cu-p-Bp-H,, respectively.
4.
This work is supported by the Natural Sciences and Engineering Research Council (NSERC) and by two Grants-In-Aid for Scientific Research from JSPS, No. 11694061( International Joint Research) and No. 13640554.
REFERENCES 1. M. Asano-Someda, A. van der Est, U. Kriiger, D. Stehlik, Y. Kaizu and H. Levanon, J. Phys. Chem. A, 103 (1999) 6704. 2. A. van der Est, M. Asano-Someda, P. Rogogna and Y. Kaizu, submitted. 3. K. Ishii, J. Fujisawa, Y. Ohba, S. Yamauchi, J. Am. Chem. SOC.,118 (1996) 13079. 4. C. Corvaja, M. Maggini, M. Prato, G. Scorrano, M. Venzin, J. Am. Chem. SOC.,117 (1995) 8857. 5 . W. A. J. A. van derPoel, A.M. Nuijs, J.H. van der Waals, J. Phys. Chem., 90 (1986) 1537. 6. N. van Dijk, M. Noort, J.H. van der Waals, Mol. Phys., 44 (1979) 891.
EPR in the 21* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
361
Pulsed-ESR investigations of the photo-excited triplet state of naphthalene Kunio Taguma" , Jun Yamauchi" and Masaaki Babab "Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan bFaculty of Integrated Human Studies, Kyoto University, Kyoto 606-8501, Japan
Electron spin echoes of photo-excited triplet states have been observed for naphthalene and naphthalene-d, molecules oriented in single crystals of durene at low temperature. In this study, three comparisons were made of the relaxation times (TI, T,) and echo decay curves at 6 K First, relaxation times were determined and compared at three magnetic field directions parallel to the molecular axes. Secondly, there were some differences between the echo decay curves at two resonance lines (low-field resonance and high-field resonance) of each molecular axis, in terms of modulation, number of decay components, and relaxation times. Thirdly, the characteristics of the echo decay for a naphthalene/durene system and those for a naphthalene-dddurene system were also compared. The origins of these differences were discussed.
1. INTRODUCTION The first ESR study of the photo-excited triplet states of aromatic molecules was reported on naphthalene in 1961 [l]. Since then, photo-excited triplet states have been studied for various aromatic molecules by ESR and optically-detected magnetic resonance (ODMR) [2, 31. Furthermore, pulsed-ESR techniques were applied to the studies of the photo-excited states of aromatic molecules. The first pulsed-ESR study was reported on quinoxaline-d, and naphthalene-d, at liquid helium temperature in 1975 ESEEM spectra of triplet excited states have been observed at room temperature, and hyperfine tensors have been determined for polyacene (anthracene, tetracene, and pentacene) in p-terphenyl single crystals [5-71. However, there is little information available about relaxation mechanisms and relaxation times. This paper reports on investigations about the spin echo decay on naphthalene and
362
naphthalene-d, in durene single crystal. By investigating relaxation times, we have obtained the key to understanding triplet spin dynamics, especially excited triplet state dynamics.
2.
EXPERIMENTAL
Naphthalene (or naphthalene-d,)-doped durene single crystal was prepared by the Bridgman method, described by McClure [8]. A mixture of guest and host molecules is sealed in a glass tube and degassed. Then, the mixture is melted and cooled very slowly. Concentrations of both guest molecules were 2% in mole ratio when melted, but, in the single crystal used in the measurements, the guest concentrations were unknown. It is assumed that the guest molecules lies in lattice sites of durene crystal where the molecular axes of the guest molecules nearly coincide with those of the host molecules (see Figure 1). The X-ray data of the durene single crystal [9] proves that there are two inequivalent sites in the unit cell. However, the Y axes of the durene molecules in both sites are almost perpendicular to the crystal cleavage plane (ab plane), and other axes are almost in the ab plane. Therefore, two resonance lines may be observed when magnetic field is applied perpendicularly to the ab plane, and four resonance lines are visible when it is applied parallel to the ab plane, in which case the two lines come from molecules of one site and two other lines come from those of another site. All measurements were done by X-band PULSE ESR spectrometer JEOL JES-PX1050 at 6K. A Hg-Xe UV lamp with filter (230-440nm) was used for continuous photo-excitation. A magnetic field was applied along the molecular axes of the guest molecules, the directions of which were determined by CW-ESR. Microwave power, frequency, and the time between the first and second pulses of the 3-pulse sequences were adjusted to maximize all spin echoes and obtain a better S/N ratio; therefore, these conditions were slightly different for every echo decay curves. The parameters used in this experiment are listed below. Microwave power : 110-180mW, Microwave frequency : 8.91-8.92GHz 2-pulse sequences : 90"(20ns)- - 180"(40ns) 3-pulse sequences : 90"(20ns) - - 90"(20ns) - T - 90"(20ns) :300 380ns)
-
Y
Figure 1.
Y
The molecular axes of the naphthalene and durene
Z axis is perpendicular to the molecular plane.
363
3.
RESULTS
3.1. Naphthalene/durene (NWD) system 3.1.1. Modulation For the 2-pulse echo decay, echo modulations were observed along all molecular axes, which were markedly stronger at low-field lines than at high-field lines (see Figure 2). From analyses of the modulation frequencies and the previous CW-ESR data [lo], these modulations are considered to be caused by protons at the a-position. From theoretical considerations, if the magnetic field is completely parallel to the guest molecular axes, only ,8 -proton modulations should appear. However, CY -proton modulations mainly appeared because there were crystal alignment errors, and, consequently, because hyperfine interactions of the a-proton influenced decay curves. This is supported by the fact that hyperfine interactions of the -proton are larger than those of the ,8-proton [lo]. 3.1.2. Echo decay curves and relaxation times The spin-lattice relaxation time (TI) and spin-spin relaxation time (TJ were determined for both low- and high-field resonance lines of all molecular axes by a single-exponential fitting of the echo decay curves. The relaxation times determined are listed in Table 1. It seemed that multiple decay components existed for several high-field decay curves (for example the highfield curves of Figure 2), so these curves were fitted by the sum of two single-exponential functions. The two numbers in parentheses indicate the two components of the decay curves in Table 1. For a comparison of the relaxation times, the underlined components used below because the relaxation times of the same order are considered to indicate the same c
-4.
0.6
0.4
I
I
I
I
0.7
fls
1
l tm.
1
1
I
0.75 0.4 fls
Figure 2.
I
0.8
1.2 fls
1.6
2
0.4
0.7 fls
X axis Y axis Z axis The 2-pulse echo decay curves of the naphthaleneldurene system. Uppers are the curves at the low-field lines, lowers at the high-field lines.
364
Table 1
The T, and T, of the two systems ; naphthalene/durene and naphthalene-dddurene Numbers in parentheses are the relaxation times when it is assumed that a decay curve consists of two decay components. ‘Low’ or ‘High’ means low-field resonance or high-field resonance. NaDhthalene/durene Navhthalene-dddurene Magnetic field TI (W T, TI T2 Direction Low High Low High High High H II 0.69 0.55 0.61 0.71* 2.8* 0.67 (0.05-0.06, (0.07,U) 0.74-0.75) * 2.3 11 1.1 55 0.36 H 1I y 0.61 (0.45,63) (Qa4.5) (0.20,2.0) 0.36 0.10 1.1 1.4 0.51 H 11 Z *not best-fitting values but approximate value
x
relaxation phenomena. Both TI and T, were significantly different when the magnetic field was applied parallel to the three molecular axes. The TI in the Z direction was the longest. On the other hand, the T, in the X direction was the longest. Another feature is the difference in both relaxation times between the low-field resonance and high-field resonance. The T, of the low field was longer than that of the high field in the X and Y directions, although it was shorter in the Z direction. The T, of the low field was longer than that of the high field in the Y and Z directions, while it was shorter in the X direction.
3.2. Naphthalene-dddurene (ND/D) system Echo decay curves were also obtained for the naphthalene-dddurene (ND/D) system except for the low-field resonance in the all magnetic field directions and for the high-field resonance in the Z direction (see Table 1).At the four resonance lines, the relaxation was too fast to record echo decay curves. The TI changed considerably, and the T, becomes shorter than in the naphthalene/durene (NH/D) system. The magnetic field direction dependence of the T, was the same as that of the NH/D system, i.e., the T, was the longest in the X direction. The relaxation times for this system are also presented in Table 1.
4. DISCUSSION Echo modulations at low-field lines were strong, whereas there were almost no modulations at high-field lines. One of the reasons for the difference is attributed to the
365
magnetic field magnitude. The influence of magnetic field fluctuation by nuclear spin precession on the electron spins becomes weaker as the magnetic field becomes stronger, the modulations become weaker. Not one but two (fast and slow) components evidently existed for the five echo decay curves, which are the 3-pulse echo decay curves at the high field in the Y direction for the NH/D system and the four 2-pulse echo decay curves at the high field in the X and Y directions for both systems (see Table 1). Although all echo decay curves are expected to contain two (or more) decay components, only the one (fast) component appeared in all the echo decay curves except the above-mentioned five. This was because their echo intensities were so weak that the echo itself could barely be seen when the slow decay began to appear. The T, of the ND/D system is quite different from that of the NH/D system, i.e., the deuteration of naphthalene molecules changes the T,. Since the major relaxation mechanism is the Raman process in this temperature region (6K) [ll], the vibrations of naphthalene molecules mainly affect the T,. By deuteration, the energy interval of C-H vibration becomes small, so the relaxation via the Raman process becomes faster for the ND/D system. Both components (the fast and the slow one) became faster, and the T, changed from 55ps to l l p s for the high-field resonance in the Y direction. It is predicted that the T, of the ND/D system includes two components, whereas that of the NH/D system only has a fast component (except for the high-field resonance in the Y direction). This accounts for the T, of the ND/D system being longer than the T, of the NH/D system in the X direction. The T, of the NH/D system is dependent on the magnetic field directions. The T, in the Z direction is the longest. Thermal fluctuation of the spin system by lattice vibrations, in this case, by the vibrations of naphthalene molecule, leads to spin-lattice relaxation and varies in the directions of the spin. Two spins are almost quantized along the applied magnetic field; thus, the T, differs when magnetic field is applied parallel to the three molecular axes. The T, of the high field in the X and Y directions and that of the low field in the Z direction are shorter than that of another resonance line. This is attributed to the wave functions between which the transitions occur. In the case of the naphthalene molecule, I O > e l + l > transition occurs at the high field in the X and Y directions and at the low field in the Z direction; then, in these transitions, the spin-lattice relaxation between 1+1>el-1>will occur as well as between I O > e l + l > . At the low field in the X and Y directions and at the high field in the Z direction, only the relaxation between 1-1>elO>occurs. Thus, the T, becomes shorter at the high field in the X and Y directions and at the low field in the Z direction. The T,, which is underlined in the both systems in Table 1, is almost the same, but by deuteration the T, component 4.5ps of the NH/D system changes to 2.0ps for the high field in the Y direction. Therefore, the underlined components are caused by spin-spin interactions, whereas the 4 . 5 ~ scomponent (and also the 2.0ps component) is caused by hyperfine interactions. The T,, which is underlined in the both systems, is the longest in the X direction.
366
This suggests that the T, is longer when the magnetic field is parallel to the long axis of the molecules (X axis) and shorter when parallel to the short axis (Y axis), and, consequently, that the length of the T, reflects the magnitude of the spin-spin interactions, i.e., the distance between the two spins. The T, in the Z direction of both systems includes the two components. The T, of the NH/D system differs between the low field and the high field. The characteristics of the T, for zero magnetic field wave functions should be included in this result but have not yet been extracted.
5. CONCLUSION
2-pulse and 3-pulse echo decay curves were obtained, and relaxation times were determined for a naphthaleneldurene system and for a naphthalene-dddurene system at 6K. Their features were compared and discussed in terms of magnetic field directions, resonance field, and deuterium effects. None of the echo intensities in this study is strong, and the relaxation is very fast. So we find better hosts in which these problems are improved and will perform more reliable analyses of echo decay, ESEEM, and temperature dependence of relaxation times in the better hosts.
REFERENCES
1. C.A.Hutchison Jr. and B.W.Mangum, J.Chem.Phys., 34 (1961) 908. 2. C.A.Hutchison Jr., et al., The Triplet State, edited by A.B.Zahlan, Cambridge Univ. Press, London, 1967, Section 2. 3. S.Geschwind, Electron Paramagnetic Resonance, edited by S.Geschwind, Plenum Press, New York, 1972, Chapter 5. B.J.Botter, et al., Mol.Phys., 30 (1975) 609. 5. Hsiang-Lin Yu, et al., J.Phys.Chem., 86 (1982) 4287. 6. Hsiang-Lin Yu, Tien-Sung Lin, and D.J.Sloop, J.Chem.Phys., 78 (1983) 2184. 7. D.J.Sloop, et al., J.Chem.Phys., 75 (1981) 3746. 8. D.S.McClure, J.Chem.Phys., 22 (1954) 1668. 9. J.M.Robertson, Proc.Roy.Soc., A141 (1933) 594. 10. N.Hirota, C.A.Hutchison Jr., and P.Palmer, J.Chem.Phys., 40 (1964) 3717. 11. J.P.Wolfe, Chem.Phys.Lett., 10 (1977) 212.
EPR in the 21%Century A Kawamori, J Yamauchi and H Ohta (Editors)
367
2002 Elsevier Science B.V. All rights reserved.
Light-induced ESR composite
studies
of
regioregular
K. Marumoto, N. Takeuchi and S. Kuroda Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Light-induced ESR (LESR) studies of regioregular poly(3-alkylthiophene) (PAT)-& composite using variable photoexcitation energy are reported. Two LESR signals with g value of = 2.002 and g2 = 1.999 are observed below 200 K, which come from positive and negative polarons on PAT and c 6 0 due to photoinduced electron transfer between PAT and c60, respectively. Microwave power saturation studies show a higher relaxation rate of the polaron spins of C6o than that of PAT. An excitation spectrum of the LESR signals shows a remarkable enhancement at around 1.8 eV and a monotonic increase above 2.5 eV up to 4.1 eV. The enhancement of the LESR signals due to the photoinduced electron transfer is consistent with the enhancement of the photoconductivity at around 1.8 eV where the optically forbidden transition of c 6 0 occurs.
1. INTRODUCTION Conducting nondegenerate conjugated polymers are quasi-one-dimensional electron systems, which show a wide variety of interesting physical properties such as electroluminescence, nonlinear optical effect, semiconductor-metal transition, etc. and have been investigated extensively. Photoinduced electron transfer in polymeric semiconductors is important in understanding the nature of excited states in these systems, and in producing efficient photovoltaic devices. Recently, composites of the conducting polymers poly(3-alkylthiophene) (PAT) and a high electron affinity species such as fullerene (C,) have been investigated, because highly efficient charge separation due to the photoinduced electron transfer occurs in the PAT-& composites, and the remarkable enhancement of the photoconductivity has been reported [1,2]. The photoinduced electron transfer between PAT and c 6 0 forms positive and negative polarons on PAT and 0 , respectively. Light-induced electron spin resonance (LESR) is a direct microscopic method for detecting the photogenerated polaron, as demonstrated in the studies of (PPV) composites using regiorandom PAT, derivatives [3-51 and oligothiophenes [6]. For the PAT-C~O the LESR spectra and microwave power dependence of the LESR intensity have been studied [7,8]. However, the PAT-C~O composites using regioregular PAT have not been investigated, where the regioregular PAT exhibits higher conductivity compared with the regiorandom PAT. Moreover, excitation spectra of the LESR signals of the PAT-& composites have not been reported so far. In this paper, we report on first LESR measurements of the regioregular PAT-C~O composite
368
Figure 1. Structural formula of the (a) poly(3-octylthiophene) (PAT8) and (b) fullerene (c60). using variable photoexcitation energy. Two LESR signals are observed below 200 K, which come from the positive and negative polarons on PAT and C ~ due O to the photoinduced electron transfer between PAT and C ~ Orespectively. , The microwave power saturation studies show higher relaxation rate of the polaron spins of C ~ than O that of PAT. The excitation spectrum of the LESR signals shows a remarkable enhancement due to the photoinduced electron transfer at around 1.8 eV, which is consistent with the enhancement of the photoconductivity at around 1.8 eV.
2. EXPERIMENTAL PROCEDURE Regioregular poly(3-octylthiophene) (PAT8) was used to prepare the PAT-C~O composite (Figure 1). The concentration of Cm to PAT8 is 5 mol%. Ultrasonic treatment of PAT-Cm toluene solution was carried out with an ultrasonic disintegrator for better uniform mixing. Cast films of the PAT-c60 composite were fabricated inside ESR sample tubes. ESR measurements were performed with a Bruker E500 X-band spectrometer with a microwave cavity with optical windows down to liquid He temperature using an Oxford ESR900 gas-flow cryostat. The absolute magnitude of g value was calibrated using diphenylpicrylhydrazyl (DPPH) as a standard. A JASCO SM-5 light source with a 300 W xenon lamp was used to provide excitation for 300-1100 nm (1.1-4.1 eV) at power levels up to 2 mW/cm2 with a spectral width of 10 nm. The light was delivered by an optical fiber to the quartz sample tube. For excitation spectrum, the light intensity was adjusted to give the same photon flux at each wavelength. The LESR spectrum was obtained by the subtraction between the dark ESR signal and that under illumination. The dark ESR signals after switching off light and warming the sample up to room temperature were confirmed to be completely the same with those before illumination at room temperature while measuring the temperature dependence, microwave power dependence, and excitation spectrum of the LESR signals.
3. RESULTS AND DISCUSSIONS Upper curves in Figure 2 show the observed first derivative ESR spectra of the composite under dark condition (dotted line) and 700 nm illumination (solid line) at 60 K with a microwave power of 0.06 mW. The LESR spectrum was obtained by the subtraction between the dark signal and that under illumination as shown by lower curve in Figure 2. Two LESR signals due to the photoinduced electron transfer between PAT and c 6 0 are observed.
369 6
I
I
LESR illurn. dark
-
-
.
+PAT
-
illurn. dark
I\ .a, = 1.999
2.002
V
5G
Figure 2. Upper curves: ESR spectra of the pA'-c60 composite under dark condition (dotted line) and 700 nm illumination (solid line) at 60 K. Lower curve: LESR spectrum of the PAT-60 composite obtained by subtracting the dark spectrum from that under 700 nm illumination at 60 K.
0
100 200 Temperature (K)
Figure 3. Temperature dependence of the LESR intensities of the PAT-& composite. The solid circles and open squares denote the signal intensities of PAT and Cm, respectively.
The obtained value of gl = 2.002 and g2 = 1.999 correspond to the positive and negative polarons on PAT and &, respectively, which are consistent with those of the regiorandom PAT-& composites reported [7,8]. The observed polaron spins in the dark condition are attributed to the ground state electron transfer in the PAT-C~Ocomposite [8]. The spin concentration in the dark condition is obtained as 1 spin per lx105 PAT-molecular unit. The LESR intensity is as about 10 times large as the dark ESR intensity. The dark ESR and LESR spectra are almost the same with each other and have asymmetric line shapes. The peak-to-peak linewidth (AHpp)of the ESR signals of PAT and Cm are about 3.1 G and 2.4 G, respectively. Figure 3 shows the temperature dependence of the peak-to-peak LESR intensities of the PAT-C~O composite. The solid circles and open squares denote the signal intensities of PAT (gl) and c 6 0 (gz), respectively. The data were recorded with a microwave power of 0.06 mW under 300 nm illumination. The LESR signals of both PAT and c 6 0 become undetectable above approximately 200 K due to the higher recombination rate, confirming that the LESR signals are transient in nature [3,5]. The decrease of the signals at low temperature is caused by saturation of the ESR signals due to the longer spin-lattice relaxation rate. The LESR signal of PAT tends to saturate at higher temperature than that of c60, indicating that the spin-lattice relaxation rate of the polaron spins of c 6 0 is higher than that of PAT, which is consistent with the results of the microwave power dependence of the LESR intensity as discussed below. Figure shows the microwave power dependence of the peak-to-peak LESR intensities of the PAT-C~O composite at 60 K under 700 nm illumination. The solid circles and open squares denote the signal intensities of PAT (gl) and c 6 0 (gz), respectively. In the case of low microwave power, the signal intensities of PAT are larger compared with that of c 6 0 . When
370
1200
Wavelength (nm) 600
300 I
1
Figure 4. Microwave power dependence of the peak-to-peak LESR intensities of the PAT-C~Ocomposite at 60 K under 700 nm illumination. The solid circles and open squares denote the signal intensities of PAT and C ~ Orespectively. ,
]
2 4 Photon Energy (ev)
Figure 5. Excitation spectrum of the LESR signals of the PAT-C~O composite at 60 K. The solid and dotted lines shows the absorption spectrum of PAT6-C60 composite (c60 : 10 mol%) and photocurrent spectrum of PAT18-C60 composite (c60 : 5 mol%) for the comparison, respectively.
the microwave intensity increases, however, the LESR intensity of PAT saturates, showing a maximum at around 0.6 mW, and then decreases above 2 mW. Finally, the signal of PAT becomes undetectable due to the saturation above 200 mW. On the other hand, the LESR intensity of C ~ increases O monotonically and does not saturate within the experimentally available microwave power range up to 200 mW. These features are similar to those of the regiorandom PAT-C~O composites [8] and the composites of PPV derivative and c 6 0 derivative [9]. The field for saturation maximum is related to the spin-lattice relaxation time by
where max is the excitation field amplitude at the sample where the maximum in the saturation curve occurs, T2 is the transverse relaxation time, and ye is the electronic magnetogyric ratio, respectively. The LESR signal of PAT can be saturated at around 0.6 mW (60 K), whereas that of Cm does not saturate under same conditions up to powers which are 100 times higher. Therefore, of the polaron spins of c 6 0 is approximately ten times shorter than that of PAT. Finally, we present first excitation spectrum of the LESR signals of the PAT-& composite, which provides important information concerning the mechanism of the charge separation. Figure 5 shows the variation of the normalized LESR intensity with the photon energy of the incident light for the PAT-& composite. The measurements were performed with a microwave power of 0.06 mW at 60 K. The data are plotted by using the peak-to-peak LESR intensities of the signals of PAT (gl). For the comparison, the previously reported absorption
371
spectrum (solid line) [lo] and photocurrent (P. C.) spectrum (dotted line) [ l ] of the PAT-C~O composites are shown together in Figure 5. The sample of each composite is poly(3-hexylthiophene) (PAT6)-C60 composite ( C ~ :O10 mol%) for the absorption spectrum and (PAT18)-C60 composite ((260 : 5 mol%) for the photocurrent spectrum, respectively. The absorption spectrum of the PAT-C~O composite ( C ~:O5 mol%) up composites is to 4 eV has not been reported so far. The photoconductivity of the PAT-C~O remarkably enhanced upon C60 doping due to the photoinduced electron transfer at around 1.8 eV where the optically forbidden transition of C60 + t l u ) occurs. The excitation spectrum of the LESR signals shows a remarkable enhancement at around 1.77 eV (700 nm) and a monotonic increase above 2.5 eV up to 4.1 eV. The enhancement of the LESR signals at around 1.8 eV is similar to the enhancement of the photocurrent spectrum on the whole, which is consistent with the photogeneration of the polarons due to the photoinduced electron transfer. However, the detailed shapes at around the peaks of the excitation spectra of the LESR signals and the photocurrents are somewhat different. This difference may be caused by the difference of the measurement conditions such as the effect of electric field, which may affect the charge separation processes. The effect of the electric field on the excitation spectrum of the LESR signals, as well as the C ~ O concentration dependence and PAT side-chain-length dependence of the charge separation are left open for further LESR studies. In summary, the regioregular PAT-C~Ocomposite was investigated by means of the light-induced ESR (LESR) method. Transient two LESR signals are observed, which come from the positive and negative polarons on PAT and C60 due to the photoinduced electron transfer between PAT and Cm, respectively. The microwave power saturation studies show higher relaxation rate of the polaron spins of C60 than that of PAT. The excitation spectrum of the LESR signals shows a remarkable enhancement at around 1.8 eV. The enhancement is similar to the enhancement of the photoconductivity at around 1.8 eV, which is consistent with the photogeneration of the polarons due to the photoinduced electron transfer.
ACKNOWLEDGMENTS This work is supported by NED0 International Joint Research Program, 99MB1 ‘Nonlinear Excitations in Molecular Electronic Materials: Detection, Control and Device Application’.
REFERENCES 1. K. Yoshino, X.H. Yin, S. Morita, T. Kawai and A.A. Zakhidov, Solid State Commun., 85
(1993) 85. 2. K. Yoshino, S. Morita, T. Kawai, H. Araki, X.H. Yin and A.A. Zakhidov, Synth. Met., 55-57 (1993) 2991. 3. S. Kuroda, K. Marumoto, H. Ito, N.C. Greenham, R.H. Friend, Y. Shimoi and S. Abe, Chem. Phys. Lett., 325 (2000) 183. 4. S. Kuroda, K. Marumoto, N.C. Greenham, R.H. Friend, Y. Shimoi and S. Abe, Synth. Met., 119 (2001) 655. 5. S. Kuroda, K. Marumoto, Y. Shimoi and S. Abe, Thin Solid Films, 393 (2001) 304. 6. K. Marumoto, N. Takeuchi, S. Kuroda, R. Azumi and M. Matsumoto, Synth. Met., 119 (2001) 549.
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7. L. Smilowitz, N.S. Sariciftci, R. Wu, C. Gettinger, A.J. Heeger and F. Wudl, Phys. Rev. B, 47 (1993) 13835. 8. S.B. Lee, A.A. Zakhidov, 1.1. Khairullin, V.Yu. Sokolov, P.K. Khabibullaev, K. Tada, K. Yoshimoto and K. Yoshino, Synth. Met., 77 (1996) 155. 9. V. Dyakonov, G. Zoriniants, M. Scharber, C.J. Brabec, R.A.J. Janssen, J.C. Hummelen and N.S. Sariciftci, Phys. Rev. B, 59 (1999) 8019. 10. S. Morita, A.A. Zakhidov and K. Yoshino, Solid State Commun., 82 (1992) 249.
EPR in the 2 1'Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
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ESR study of photodecomposition mechanism of a long-lived radical perfluoroESR spectrum of trifluoromethyl radical formed during solid-phase photodecomposition at 77 K in glassy matrix S.R. Allayarov and D. A. Gordon Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation Formation under some perfluorocompounds radiolisys of long-lived radicals (LR) incapable to mutual recombination in liquid was first discovered by our group. This work is concerned the [(CF~)~CF]~C*C~FS (LRl) formed during radiolysis or fluorination of hexafluoropropylene trimer (HFPT) by addition of F atom to double bond of HFPT. During the heating or UV irradiation of LR1 its decomposition takes place to give a molecule of perfluoroolefine and *CF3 radical initiating the polymerization of monomers but subjected to fast recombination in liquid. ESR study of mechanism of photolytic decomposition of LR1 and the analysis of spectrum of *CF3 radical formed during solid-phase photodecomposition of LR1 at 77 K has been performed. All = 25.15 mT, A 1 = 9.1 mT, and gll = 1.9996, g 1 = 2.0056 parameters were found by using computer-simulated ESR spectra. The further destiny of *CF3 radicals formed under the decomposition depends on the phase state of the matrix. About 90% of them recombine in liquid at 300 K. The rest attaches to the HFPT molecule giving another long-lived [(CF3)2CF]3C *radical. On photolysis at 77 K, a part of *CF3 radicals is stabilized in the glassy HFPT matrix. The *CF3 radicals formed during y-radiolysis of HFPT at 77 K are not stabilized. Quantum-chemical calculations showed that the mechanism of LR1 photodecomposition is different from the mechanism of thermal decomposition. That is the consequence of the fact that decay of photoexcited radical is predetermined by a radical structure and results in whereas the reaction of *CF3 detachment from CF-CF3 group being as twice as less endothermic than that from CF2-CF3 group (48-56 kJ/mole instead 98-104 kJ/mole) is to be dominant under thermolisys. The LR1 photodecomposition red cut-off has experimentally shown to be 320nm. In accordance with this the simulations manifest that LR1 electronic excitation of 4eV is sufficient for its decomposition, and, contrary to thermolisys, *CF3 radical detachment proceeds from CFz-CF3 group.
1. INTRODUCTION LR1 was obtained on radiolisys of hexafluoropropylene trimer (HFPT) [l]. LR1 has an alkyl nature [2] and is formed, as well as on fluorination of HFPT by addition of F atom to a double bond of HFPT molecule. During the heating or UV irradiation of LR1 its decomposition takes place. Earlier we assumed the mechanism of photodecomposition of LR1 is similar to the mechanism of thermal decomposition at 373 K [3] and occurs by removal of *CF3 group from perfluoroisopropyl fragment of LRl to give *CF3 radical and
374
molecule of perfluoroolefine. However, our last researches have shown the mechanisms of photo- and thermodecomposition may be different. The semiempirical quantum-chemical calculations. were carried out to check this hypothesis. The .CF3 radical probably exceeds all other low-molecular perfluoroalkyl radicals by the number of works devoted to ESR studies. In the ESR spectrum of the *CF3 radical in the liquid [4] where the dipole interactions are averaged and lines are narrowed, a quadruplet with splitting AH = 14.24 mT is detected. It is resulted from the interaction of an unpaired electron with three equivalent F nuclei. The ESR studies of the *CF3 radicals in the solid phase [5-71 exhibited complex multicomponent ESR spectra. The ESR spectrum of the *CF3 radicals chaotically oriented in the matrix of zeolite 13X was also studied However, it is difficult to interpret all lines of the multicomponent spectrum of the C F 3 radicals stabilized in polycrystalline or amorphous matrices due to the superposition of signals from other paramagnetic centers (PC). In this work, we attempted to obtain the spectrum of the *CF3 radicals in the solid matrix of the hexafluoropropylene trimer (HFPT) without admixtures of other PC during the photodecomposition of LR1.
2. EXPERIMENTAL Photolysis of samples at 300 K was performed by the full light from a high-pressure DRSh1000 mercury lamp in quartz cells 4-5 mm in diameter, which do not give ESR signals. ESR spectra were detected on a small-size PS 100.Kh radiospectrometer. ESR spectra of the radicals were automatically detected and simulated on a computer using the programs of the Scientific Technical Cooperative Center of Radiospectroscopic Instrument Production"Tsentrospektr" (Minsk). Quantum-chemical calculations were carried out by PM3 approach using the complex of MOPAC-93 program. 3. RESULTS AND DISCUSSIONS
The structure of LR1 obtained as a result of quantum-chemical calculations is given in Figure 1. Conformation of the radical is close to the determined experimentally by the ESR spectroscopy method where the spatial arrangement of p-atoms F was determined from ESR spectra on the basis of angular dependence aF@) =Q.p.cos28. The aF(P) - hyperfine interaction (HFI) constant with p - atoms F, Q - a constant, p - spin density of unpaired electron at - atom C, 8 - a corner between a projection of C-F bond and an axis of an orbit of unpaired electron. The absence of symmetry in LRl structure may be considered as a peculiarity. Similar bonds C(2)-C(1) (1.523A) and C(2)-C(7) (1.505 A), C(l)-C(5) (1.612 A) and C(1)-C(6) (1.627 A), C(7)-C(8) (1.609 A) and C(7)-C(9) (1.613 A) have different lengths due to steric interactions. Three bonds of atom C(2) lay in one plane: the departures of atoms C(1), C(2), C(3) and C(7) from the plane included these atoms does not exceed 0.015 A. At such structure of the radical (see Figure 1) there should be reduced contacts between volumetric trifluoromethyl groups. To avoid this is possible by a different turn of CF3-groups around of bonds C(2) - C(l) and C(2)-C(7). Really, the torsion angle C(3) C(2) C(l) F(18) is 141.9 whereas the similar torsion angle C(3) C(2) C(l) F(28) is 136.9 '. For the existence of steric
375
difficulties in the radical high values of a barrier of rotation (-105 kJ/mol) along the bonds C(2)-C( 1) and C(2)-C(7) argue also.
Fi191
p
plFilll
cis1
- F(221
I
Bll
Figure 1. Simulated structure of long-lived [ ( C F ~ ) ~ C F ] ~ C O C ~ F ~ radical.
Figure 2. Experimental (a-c) and simulated (d-f) ESR spectra of [ ( C F ~ ) ~ C F ] ~ C O C ~ F ~ radical in HPFT matrix before (a,d) and after UV irradiation at 77 K during 280 (b,e) and 800 min (c,Q and line-image assignment of ESR spectrum of oCF3 radical (0.
As a result of slowed down rotations along bond C(2)-C(3) of radical (a barrier of rotation along this bond 188 kJ/mol) also should become nonequivalence of F10 and F11 atoms of perfluoromethylene groups. On F atom (10) the constant of HFI is 4.57 mT whereas second F atom (I 1) lays close to nodal plane (0 = 69.5’) and is characterized by small value of splitting. Comparison of experimental data and results of quantum-chemical calculations has shown that the mechanism of photodecomposition of LR1 differs from the mechanism of its thermal decomposition. It has been experimentally determined that LR1 photodecay red cut-off is to be 320nm. Quantum-chemical calculations have shown that the energy difference between ground and exited states of LRl is 4ev. So, at an irradiation light with length of a wave less than 320 nm occurs transition of the radical in the excited state with the subsequent break of bond CFz-CF3 most lengthened in the ground state. At thermal decomposition of LR1 (heating up to 373 K) is more probable the break of bonds >CF-CF3 because the reaction of oCF3 detachment from CF-CF3 groups is as twice as less endothermic than that from CF2- CF3 group (42-54 kJ/mole instead 98-104 kJ/mole). Thus, the first is the main channel of LR1 thermal decomposition. Data obtained allow to assert, that selectivity of decomposition of LR1 at photolysis is connected to structural predetermination, whereas the most weak C(l)-C C(1)-C (6), C(7)-C(8) and C(7)-C(9) bonds are to be broken under thermolysis. Significant changes occur in the ESR spectrum during photolysis at 77 K of LR1 in the glass HFPT matrix (Figure 2). The disappearance of the spectrum of LR! is accompanied by the appearance of a complex ESR signal (see Figure 2,c) with a total splitting of 75.5 mT and characteristic four lines with a distance of 25.15 mT. In addition, two asymmetric lines are
376
detected in the central part of the spectrum. These two groups of lines have previously [l] been assigned presumably to different PC. However, the results of this work show that the dependence of all these lines on the UV irradiation time and kinetics of their appearance and decay upon heating coincide with each other and are parallel to the kinetics of changing the overall concentration of PC during the low-temperature (77 K) photolysis of the sample with LR1 followed by its heating. This suggests that all lines of the spectrum belong to the signal of the same PC. As shown by the analysis of the spectrum (see Figure 2,c), these PC are the CF3 radicals with axial symmetry. The line-image reconstruction of the spectrum of the *CF3 radical (see Figure 2,c) shows that all three F atoms of the *CF3 radical are equivalent. The following main values of the components of the g factor and components of the HFC tensor for the F atom were determined: All = 25.15 mT, A 1 = 9.1 mT, and gll= 1.9996, g 1 = 2.0056. Figure 2f presents the results of computer simulation of the ESR spectra of the *CF3 radicals in the glassy HFPT matrix at 77 K based on the experimental data. As known [7], the shape of the asymmetric ESR line at the Gaussian or Lorentzian line shape depends only on one anisotropy parameter d= (IHII-H~I)/DH~, where DHi is the individual linewidth between the points of maximum slope; HIIand H i are the magnetic field intensities corresponding to the limiting positions of the individual line for changing the angle between the Ho direction and symmetry axis from 0 to p/2. The spectra of the *CF3 radical (see Figure 2,c,f) allow the determination of four values of the d parameter: two from lateral (dl , p 9 ) and two from inner (d3,4=3) components of the Hi1 and H i components of quadruplets. According to earlier published data [7], at these d values analysis of experimental spectra is simple, and at d25 the HI]and H i values can be determined with a high accuracy (to 5%). The theoretical spectrum (see Figure 2,f) simulated taking into account the abovementioned parameters agrees well with the experimental spectrum (see Figure 2,c). Thus, simulation confirms that the central lines are, in fact, the components of the spectrum of the *CF3 radical. Only prolonged ( 4 0 0 min) photolysis allows the almost complete decomposition of LR1 to be achieved during solid-phase photolysis (see Figure 2,c). The ESR spectrum of the samples irradiated for a relatively short time (see Figure 2,b) is a superposition of the spectra of the *CF3 and LRl. The fraction of the *CF3 radicals can rather exactly be determined by computer simulation of experimental spectra. The ESR spectrum of LRl in the glassy HFPT matrix at 77 K exhibits a doublet with a splitting of 4.6 mT (see Figure 2,a). The g factor for LR1 is [3] 2.00286. Computer simulation of the spectrum of LR1 (see Figure 2,a) shows that the shape of its individual components is described by the Gaussian function and their width is -4.4 mT. Analysis of the ESR spectrum of a solution of LR1 in HFPT UV-irradiated at 77 K for 280 min showed that in this sample the ratio of concentrations of the radicals is [* CF3]:[LRl] =1:3 (see Figure 2 b,e). Thus, the simulation of the experimental spectra of the irradiated samples containing LRl and *CF3 allows the determination of the fractions of these radicals in the sample. Based on the interpretation of the ESR spectra of the *CF3 radical, which is the decomposition product, we can assume that the photodecomposition mechanism of LR1 includes the formation of the *CF3 radical. The ESR spectra of the *CF3 radicals similar to
377
that detected in the matrix of glassy HFPT were also observed in the g-irradiated at 77 K polycrystalline samples of aqueous solution of CF3COOH [6] and photolyzed at 77 K CF3COCF3 samples [5] adsorbed on zeolite I3X. The parameters of these spectra almost coincide with those obtained in this work and, therefore, the simulated spectrum of the *CF3 radicals in the HFPT matrix (see Figure 2,f) can be used for the identification of these radicals in other matrices. Russian Fund for Fundamental Research has supported this work. Cod N: 01-03-97006.
REFERENCES 1. Allayarov S.R., Asamov M.K., Barkalov I.M., Shvedova M.K. Izvestiya VUZov, Ser. Khimiya I Khim., Technologiya, 30 (1987) 98. 2. Gordon D.A., Allayarov S.R., Kuzina S.I., Barkalov I.M., Mikhailov A.I., Izvestiya AN SSSR, Ser. Khim., No. 10 (1989) 2203. 3. Scherer K.V., Ono T., Yamanouohi K., Femandez R., Henderson P., Goldwhite H., J. Amer. Chem. SOC.,107 (1985) 718. 4. R. W. Fessenden and R. H. Schuller, J. Chem. Phys., 43 (1965) 2704. 5. P. Svejda, J. Phys. Chem., 76 (1972) 2690. 6. K. Mach, Collection Czechoslov. Chem. Commun., 37 (1972) 923. 7. V. V. Voevodskii, in Fizika i khimiya elementarnykh khimicheskikh protsessov [Physics and Chemistry of Elementw Chemical Processes], Nauka, Moscow, 1969 (in Russian).
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EPR in the 2IstCentury A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
An ESR study and quantum-chemical calculations of alkyl radicals in the matrix of
polycrystalline n-alkane irradiated at 77 K. Effect of intermolecular interactions and carbon chain length on the radical formation S. R Allayarov, S.V. Konovalikhin and T.E. Chernysheva Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. The composition of alkyl radicals (AR) formed by y-radiolysis (T=77 K) of polycrystalline n-alkanes with different lengths of the carbon chain (C(5), C(7), C(10), C(l I), and C(18)) and their polymeric analog (polyethylene) was estimated from the ESR spectra. The ESR spectra of the irradiated n-alkanes are superpositions of the signals from the H~CC'HCHT and -CH2C'HCHr radicals, whose HFS constants with a and p protons well the equilibrium conformation independent of the chain length of the it-alkane molecule. A dependenceof the concentration of the radicals on the chain length of n-alkane was found. The absence of the -CH2C'H2radicals that may arise upon H atom elimination from the Me fragments of the n-alkane molecules is most likely related to the transfer of excitation energy from the Me group to the neighboring methylene fragment and the transformation of the -CH2C'H2 radicals into H&C'HCHT- radicals With account for this, the concentrations of the AR formed were suggested to be proportional to the number of H atoms at the correspondingC atom To reveal the reasons for the previously found absence of end radicals upon y-radiolysis of n-heptane polycrystals, we performed quantum-chemical calculations (SCF-MO, RHF, 6-3 1 G* basis set) of the n-heptane molecule and its four radicals. The energies of the crystal lattice were calculated by the atom-atom potential method. Comparisonof the experimental and calculateddata showed that the absence of the end radicals is not related to the intermolecular interaction in the crystals. The most probable reason for the selectiveradical formation upon radiolysis can be a transfer of the excitation energy within the n-heptane molecule occurring before the radical formation. 1.INTRODUCTION
Hyperhe structure (H9S) ofthe ESR spectrum of alkyl radicals(AR) in the solid phase reflects the interaction of an unpaired electron with the nearest environment only In these ESR spectra, the values of HFC constants with y-protons do not exceed 0.05 mT [11 which is much less than the half-width of the spectral components (-1 0 mT) [2] and a rule, they cannot be detected. Therefore, of ESR spectra of AR is the result of the interactions of an unpaired electron with (a- and fi-protons only As a result, only AR with the valence at the C atoms in positions 1,2, or 3 be distinguished in the solid phase from ESR spectra. Therefore, the composition of AR formed during y-radiolysis of n-alkanes unambiguously be determined from the ESR spectra of the latter only for molecules with a chain length of at most five C atoms. In the case of n-alkanes with a longer chain, the AR compositioncan be established indirectly. For this purpose, one should know how the carbon chain length of the n-alkane molecule affects the composition of radicals formed under its radiolysis.
379
Although numerous data have been obtained by the ESR study of irradiated solid organic substances [2], the quantitative compositionof AR and dependence on the chain length of the linear alkane are virtually unstudied. work is devoted to the ESR study of radicals formed under y-radiolysis of polycrystalline alkaneswith merent chain lengths. in this work, we compared the. ESR spectra with the results of quantum-chemical calculations of radicals to interpret the selectivity of radical formation during radiolysis of n-heptane polycrystals.In addition, we estimated the energies of the crystallinelattice of nonirradiated n-heptane and model crystals with the radicals.
2.EXPERIMENTAL Linear alkanes n-CsHlz, n-C7H16, n-CloHzz, and n-ClgH38 (content of the main substance 299.95%) and their high-mo-lecular analog, a low-pressure polyethylene (PE) powder, were
Used. Radiolysis of samples (@Coy-radiation, dose of irradiation power 28 5') was canied out in tubes of SK-4B glass in vacuo at 77 K. ESR spectra were recorded on a PS I00.Kh 3-cm radiospectrometer. Nonempirical quantum-chemical calculations of the n-C7H16 molecule (see Figure 1 a) were performed by the RHF/6-3IG* method, and the RI, Rz, R3, and & radicals (see Figure 1 b-e) were calculated in the UHF approximation in the 6-31G* basis set using the GAMESS program [8] with the complete optimization of geometry. It is that the quantum-chemically calculatedgeometry of radicalsis very sensitiveto the choice of the basis set. Comparison of the results obtained with the 3-21G, 4-3 IG, and 6-3 lG* basis sets showed that the structure of the ti-heptane molecule is well reproduced in all cases, and that of the radicals is reproduced only in the 6-3 1 G* basis set. Coincidence of the experimental and calculated data on the conformationof the radical served as a criterion for the validity of the structure found. spectra of radicals were simulated using the EPRTOOLS program (version 3, developed the Scientific Technical Cooperative Center for Radiospectroscopic Instrument-making "Tsentrospektr,"Minsk). 3.RESULTS AND DISCUSSION
The shape of ESR spectra of irradiated polycrystalline samples of linear alkanes depends, to a great extent, on the chain length of the alkane molecule. When the chain gradually elongates, the seven-component spectrum transforms into the Six-component spectrum. The ESR spectra of n-alkanes y-irradiated at 77 K are presentedin Figure 1 (spectra 3-8). The theoretical spectra of H3CC H ' CHZ- and -HKC.HCHr correspondingto the seven- and six-component spectra were obtained by the simulation of the experimental spectra of irradiated n-alkanes (see Figure 1, spectrum 2, Table 1). The H~CC'HCHTradical (R2) is formed when the H atom is eliminated fiom the second C atom of the n-alkane molecule. The HFC constants obtained for this radical agree well with the parameters of these radicals stabilized in single crystals of n-alkanes irradiated at 77 K [3]. As follows from Table 1, in the -CH?C'HCHr (R,) radical, all four Hp atoms are equivalent. The HFC constants with these protons are equal to -3.56 mT. The HFC constants
380
loll mT L 1
Figure 1 . Theoretical ESR spectra of H$.X'HCHZ- ( I ) and -CH2C'HCH2- radicals (2); experimental(3-8) and theoretical ( 3 ' 3 3 ESR spectra ofn-CjH1Z (3, 33, ~ z - C ~(4, H I43, ~ n-C~oHn ($59, n-CIIHZ4 (6(Ref 3), 67, n-ClsH3~ 73, and PE (8, 83 y-irradiated at 77 K with a of 30 kGy. close to this value have been obtained by examination of the spectra of the -CH2C'HCHr radicals stabilized in PE [MI. Examination of the ESR spectra of y-irradiated n-alkanes shows that they mainly represent a superposition signals from the R2 and %radicals. On going fiom one alkane to another, the main parameters of the spectrum of these radicals (HFC constants with and &protons) remain unchanged. However, the linewidths in the spectra of the R2 and Rm radicals can differ (Table 2). For example, in the theoretical spectrum of n-undecane Figure 1, spectrum 63, the lines of the R, radical are more narrow than those in spectra of other alkanes. This results in a substantial distinction of the spectrum of n-undecane from the spectra of other n-alkanes Figure 1). the observed distinction in the spectrum of n-undecane be explained by a decrease in the linewidths. We found that a specified quantitative correlation between the concentrations of the RZ and R, radicals was hifilled for y-radiolysis of polycrystalline n-alkanes. The ratio of the concentrations of these radicals in the theoretical spectra of ri-alkanes irradiated at 77 K are presented in Table 2. It follows from these data that the ratio of concentrations of the R2 and &radicals depends on the alkane chain length. In the case of n-pentane (see Figure 1, spectra 3 and 37, the sevencomponent ESR spectrumis mainly observed,which is attributed to the H3CC.H(CH2)2Me radical, whose concentrationis fivefold higher than that of the HC[CHzMe]2 radicals. An inverse ratio of concentrations of the R2 and R,,, radicals (5 : 14) is observed for the samples of y-in-admted n-octadecane. In the case of n-octadecane Figure 1, spectra and 73, the content of the R2 radicals (H,CC'HCH~(CH~)MM~) is almost threefold lower than that of the -CHZC'HCHT radicals in the spectra of irradiated n-CloH~and n-CilH24 contain signals &om the R2 and R, radicals with a close intensity.
381
Table 1 Parameters of the ESR spectra and equilibrium conformation of alkyl radicals formed under yradiolysis of n-alkanes 77 K Radical
HFC constantdmT
Angle Ides*
a,H amH wwl) amH'2) amH(3) m2w4)el ez H3CC'HCHr,R2 2.4 2.55 3.8 3.3 26 34 43&CAHCHr, 2.2 3.56 3.56 3.56 3.56 30 30 * The angle between the projection of the Cp-H bond of the methylene fragment of the radical and the axis of the orbital of an unpaired electron. the of the %, (see Figure 1, spectrum 2) and R2 radicals (see Figure 1, spectrum I ) substantially differ, the shape of the spectrum of irradiated n-alkane, being a superposition of signals from these radicals, is determined by the relative concentration of the latter. Thus, the main portion of radicals formed upon y-radiolysis ofpolycrystalline short-chain n-alkanes comprises R2 radicals whose free valence is localized at the second C atom. The R,,, radicals (-€H2C'HCH~) are mainly observed in the radiolysis of n-alkanes with a longer chain 77 K. With a gradual elongation of the chain of the it-alkane molecule, the &tion of the R,,, radicals increasesin the ESR spectrum, and the latter transforms from the seven-component into the six-component spectrum. These changes in the ESR spectra on going from high-molecular paraffin or PE to lowermolecularn-dodecane are explained by the transition of the radical from one conformation to another. However, our analysis showed that the observed changes were related to distinctions in the quantitative ratio between concentrations of the R2 and R,,, radicals rather than to the conformationalpeculiaritiesof the R,,, radicals. Let us consider the ratio of concentrationsof the alkyl radicals as a knction of the chain length of the molecule of irradiated n-alkanes. We suggest that the distribution of the alkyl radicals RY,R2, R3, ..., %formed by y-radiolysis of a polycrystalline n-alkane due to the elimination of the H atom from the first, second, third, and iath C atom correspondsto the equiprobableabstraction of the H atom from any C atom. In this case, the concentrations of the radicals with different structures should be proportional to the number of H atoms at the corresponding C atoms in the n-alkane molecule. For example, for n-pentane, the ratio of concentrations of the radicals should Table 2 Parameters of the theoretical spectra that most well describe the experimental ESR spectra of n-alkanes irradiated at 77 K Alkane R2 : h*/mT R2 n-C& 5:1 1.2 1.6 n47H16 5:3 1 .o 1.6 f~-CloHz 5:6 1 .o 1.8 11H24 5:7 1 .o 1 .o fsI$H38 5 : 14 1.2 1.6 PE 0:1 2.0 * h is the half-width of spectral lines.
382
d
e
Figure 2. Structures of the n-heptane molecule with numeration of the atoms.
and radicals RI (b), RZ(c), R;
(4,and R4 (e)
be the following:
The radical is not detected experimentally, and the ratio of concentrations of the observed radicalsR2 and should be 2 : 1. The experimental ratio of concentrationsof these radicals is 5 : 1 Table 2). three units, by which the concentration of the R2 pentyl radicals increases, are to the fraction of the radicals in Eq. (l), we may assume that during y-radiolysis of n-pentane at 77 K the RI radicals are transformed into Rz. Most likely, this transition occurs before the formation of the and is a result of the transfer of the excitation energy of the Me group to the adjacent CHZfiagment. The possibility of this energy transfer during radiolysis of the n-alkane molecule has been observed previously [101.Analysis of the ESR spectra of other n-alkanes irradiated at also indicates increase in the concentrations of the R2 radicals at the expense of the radicals. The ratio of the radical concentrations in the theoretical spectra (see Figure 1, spectra 3'- 77, which optimally describe the experimental ESR spectra of the corresponding irradiated n-alkanes (see Figure 1, spectra 3is presented in Table 2. The concentrationsof the alkyl radicals RI, ..., R,, (taking into account the transformation of the RI radicals into R2) are proportional to the number of H atoms at the C atoms in the n-alkane molecule. As follows from the data in Table 2, the concentrations of the alkyl radicals formed by y-radiolysis of polycrystalline n-alkanes whose molecules contain more than four C atoms are described by the ratio [H&C'HCH2]/[--CHzC'HCH~]= where is the number of C atoms in the n-alkane molecule. Thus, the observed deviation from the primary distribution of radicals proportional to the of H atoms at the atoms of the n-alkane molecule is most likely associated with the transfer of the excitation energy of the CH3 group to the adjacent methylene fiagment. The Quantum-chemical calculationsfor the R,, R2, R;, and R4 radicals Figure 1) showed that the total energies of all the radicals are almost the same: the highest difference between them does not exceed 2.3 moF' in the nonempirical calculation scheme and kcal mop' in the semiempirical Such low differences in energies do not allow one to speak with confidence about a higher stability of any of these radicals.
383
The calculation the electronic structure of the n-heptane molecule in the excited state showed that when the energy is absorbed, the excitation is mainly localized on the central C(3), C(4), and C(5) atoms Our experiments indicate that the radiation yield of the radicals is independentof the irradiation dose. we may assume the intramoleculartransfer ofthe excitation fiom the end atoms to the central atom during radiolysis. On the other hand, the excitation transfer is accompanied by the radical formation, and the rates of these processes comparable. Therefore, the indicated energy transfer fiom the end to central atoms "stops" precisely on the C(2) atom. the selectivity of radical fbrmation is due to the intramolecular transfer of the excitation energy the to central atoms; second, the intermolecular interaction has no efEa on the radical formation during irradiition of the n-heptane polycrystals; third, the PM3 method well reproduces the experimental data on the structure of molecules and radicals of n-alkanes and is appropriate for calculationsof the physicochemical properties of more complex (in composition and structure) ~ o l e c uand i ~ radical systems. Russian Fund for Fundamental Research has supported this work . Cod N 01-03-97006
REFERENCES 1. R. Fessenden and R. H. Schuler, Chem. Phys., 39 (1963) 2147 2 S. Ya. Pshezhetskii, A G. Kotov, V. Milinchuk, V. A. Roginskii, and V. I Tupikov, EPR svobodnykh radikalov v radiatsionnoi khimii P S R of Free Radicals in Radiation Chemistry], Khimiya, Moscow, 1972 (in Russian). 3. T. GiIlbro, P 0. Kinnel, A. Lund, J. Phys. Chem ,73 (1969) 4167 4. Yu. D. Tsvetkov, N. N. Bubnov, M A. Makul'skii, Yu. S Lazurkin, and V. V. Voevodskii, DOH. Nauk SSSR [Reports Acad. Sci. USSR], 122 (1958) (in Russian). Ohnishi, Bull. Chem. Jpn., (1962) 254. 6. B. Ran by and H Yoshida, Polym. Sci , 12C (1966) 263. 7 L. A. Blyumenfel'd, V. V. Voevadskii, and A G Semenov, Application of Electron Paramagnetic Resomce in Chemistry, Press of the Siberian Branch of the Academy of Sciencesof the USSR [U-vo SO AN SSSR], Novosibirsk, 1962 (in Russian) 8. S. Allayarov S. V. Konovalikhin, Izv. Akad. Nauk, Ser. Khim., No. (2000) 1038 [Russ. Chem Bull., Int 49 (2000) 1032 (Engl. Transl.)].
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
84
ESRENDOR study for new radical dianion species of 6-oxophenalenoxyl derivative Y. Morita," S. Nishida," J. Kawai," K. Fukui,b S. Nakazawa,bD. Shiomi,b K. Sato,b T. Takuib and K. Nakasuji" "Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka, 560-0043, Japan bDepartments of Chemistry and Materials Science, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka
8-85
Japan
A novel open-shell molecule, dipotassium
was
designed on the basis of the 6-oxophenalenoxyl system and generated by the chemical reduction of 6-hydroxyphenalenone derivative. The structure was unequivocally determined by ESR and ENDOR/TRIPLE spectroscopies, and DFT calculations, illustrating an unique spin density distribution depending on its redox states.
1. INTRODUCTION Extensive studies have been carried out on organic molecule-based ferromagnets and other molecular functionalities based on organic open-shell molecular systems. Understanding of the electronic structure of open-shell molecules is crucial for the design and preparation of such molecules and molecular assemblies. Phenalenyl is a well-known system as a highly symmetric odd-alternant hydrocarbon n-radical with high amphoteric redox ability and has been attracting great attention as a possible component for molecule-based conducting and magnetic materials [ 1,2]. We have recently reported on the design and synthesis of 2,5-di-tertbutyl-6-oxophenalenoxyl derivative 1 as a novel stable neutral radical based on the
phenalenyl skeleton [3]. Characterization of this neutral radical in terms of ESR and ENDOR/TRIPLE spectroscopies and MO calculation has illustrated that this system has an extremely spin-delocalized and highly spin-polarized nature which is similar to that of phenalenyl system. The topology of the spin density distribution in the 6-oxophenalenoxyl system, however, was contrasted with that of the phenalenyl system due to the change in the topological symmetry of the n-electron network. Thus, it is important to clarify the effect of heteroatomic perturbation on the electronic structure of the system under study. In this work, we will describe the redox properties and the spin structure of the radical dianion species of the 6-oxophenalenoxyl system.
2. RESULTS AND DISCUSSION
2.1. Synthesis and redox property In Scheme 1, a possible redox behavior of the 6-oxophenalenoxyl system is depicted. A neutral radical 1 is generated by the one-electron oxidation of the anion 3, while the radical dianion
seems to be generated by the one-electron reduction of 3. To investigate these
behaviors by the cyclic voltammetry (CV) method, we have selected the anion salt 3.(Et,N') considering its stability in solution. The neutral radical precursor 2 was prepared from
in seven
steps, which was transformed into 3.(Et4N+)by the treatment of one equivalent of NaOMe followed by Et4NC1 in THF-MeOH, exhibiting particularly high stability even in air
neutral radical
anion
radical dianion
Scheme 1 Possible redox states of the 6-oxophenalenoxyl system
386
atmosphere due to the negative-charge delocalization (Scheme 2). CV measurements in CH3CN at room temperature have given one reversible oxidation wave (-0.30 V) and one reversible reduction wave (-2.39 V), indicating the high stability of the neutral radical 1 and radical dianion species
[4]. These results have encouraged us to
generate 4"-by chemical reduction and to characterize its stability and electronic structure.
Scheme 2 Reagents: (a) cat conc H2SO4, excess t-BuOH, CF,COOH, 45 "C, 99%; (b) 10 equiv DMF-POCl3, (CHZCI)~, 90 "C, 99%; (c) 4.2 equiv LDA-~-BuCH~COOCH~P~, THF, 0 "C, 86%; (d) (i) 1.5 equiv Et,SiH, 4.5 equiv CF3COOH, CH2C12, rt; (ii) PdC, H2, EtOH, 85%; (e) (i) excess (COC1)2, reflux; (ii) 3.5 equiv AICI,, CH2C12,93%; (f) 3.3 equiv LiAIH4, THF, rt, (8) (i) 20 equiv LiI, HMPA, 170 "C; (ii) 2 M HCl aq, rt; (iii) reprecipitated from hexane-CH2C12, 62%; (h) (i) NaOMe, THF, rt; (ii) Et4NC1, THF, rt. -100%.
2.2. Generation and characterization of radical dianion species Generation of radical dianion 4"-.(2 K') was carried out by the treatment of the 6hydroxyphenalenone derivative 2
potassium mirror in a degassed diglyme (1.O x 10" M)
in a sealed tube, to afford green solution that showed well-resolved ESR signals (Figure 1A). The decrease in these signal intensities was not observed for a long time at room temperature in a sealed tube, being indicative of the high stability under such conditions. In 'H-ENDOR spectra measured in order to determine the hfcc's values, two sets of signals due to the protons on the phenalenyl skeleton were observed (Figure 1C). The observed ESR signals were fully simulated by the addition of another hfcc
0.025 mT) which is attributed to the
39
K nuclei, while signals due to 39K nuclei could not be observed clearly in the 'H-ENDOR
spectra (Figure lB, C). The relative signs of the hfcc's due to two kinds of protons were unequivocally found to be equal by invoking 'H-TRIPLE resonance measurements (Figure
387
S
A
. .
-
./1 1
1
'._
FieldhT
B
FieldmT
,
r----
t increase
t r
I
MHz
I
MHz
Figure 1 Observed hyperfine ESR spectrum (A), 'H-ENDOR (C), 'H-TRIPLE (D, pump frequency: 20.65 MHz) spectra in diglyme (1 lop3 M) at 290 K and simulated one (B); The microwave frequency used for ESR measurement was 9.6455240 Observed g-value is 2.0037. 1D). Assignments of two kinds of hfcc's of radical dianion 4"-.(2 K') were made with the help of the spin density distribution calculated in terms of a local spin density functional theory by using
method (Table 1 and
94 with
Figure 2). Agreement between the experimental and theoretical values is satisfactory. These results demonstrate that radical dianion
exhibits the remarkable change in the a-spin
density distribution as compared with the neutral radical 1. Table 1 Observed and calculated hfcc's for radical dianion 42A,/mTa 334 799 observed -0.434 -0.551 calculated -0.478 -0.508
39K *0.025 -
The observed hfcc's were determined by 'H-ENDOR spectra in diglyme at 290 K and simulation successfully reproduced the ESR hyperfine spectrum. Values in the lower line were calculated by the DFT calculations McConnell equation using the following parameters: A,, = pQ (Q = -2.7 mT).
a
388
A
B
Figure 2 The spin density distribution of neutral radical (A) and radical dianion (B) calculated in terms of a local spin density functional theory by using Gaussian 94 with uBLYPJ6-3 1G*//UBLYP/6-31G*. Vacant and filled circles denote negative and positive x-spin density, respectively.
3. CONCLUSION A novel open-shell molecule with delocalized negative charges, radical dianion 4"-.(2 K'),
was designed and successfully generated by the reduction of 2, demonstrating strikingly different n-spin density distribution from that of neutral radical
This result means that the
6-oxophenalenoxyl system is a peculiar but important chemical system in which the n-spin structure changes remarkably depending on its redox states. This study contributes not only to the molecular design for novel organic open-shell molecules, but also to the establishment of a novel approach for controlling magnetic properties by external stimulations. Consideration on the correlation between spin polarization and charge fluctuation by using VB methods will be reported in due course.
REFERENCES 1. K. Goto, T. Kubo, K. Yamamoto, K. Nakasuji, K. Sato, D. Shiomi, T. Takui, M. Kubota, T. Kobayashi, K. Yakushi, J. Ouyang, J.Am. Chem. SOC.,121 (1999) 1619. 2. X. Chi, M. E. Itkis, B. 0. Patrick, T. M. Barclay, R. W. Reed, R. T. Oakley, A. W. Cordes, R. C. Haddon, J.Am. Chem. SOC.,121 (1999) 10395. 3. Y. Morita, T. Ohba, N. Haneda, S. Maki, J. Kawai, K. Hatanaka, K. Sato, D. Shiomi,
T. Takui, K. Nakasuji, J. Am. Chem. Soc., 122 (2000) 4825. 4. CV was carried out by the following conditions: 10 mM in CH,CN with 0.1 M Bu,NClO, as supporting electrolyte at room temperature against Fc/Fc'; Au working electrode and Pt counter electrode; 0.20 Vls.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
389
Spin labeling study of polymer chain motion in PEG/PVP blend Shiming Chen", Guidong Jin", Zhenghua Pingb, Sizhao Jin' and Yimin Shena
" Center of Analysis and Measurement, Fudan University, Shanghai 200433,China Macromolecular Science Department, Fudan University, Shanghai 200433,China
' Structure Research Laboratory, University of Science and Technology of China, Hefei 230026, China The spin label ESR method was used to study the chain motion in the PEG/PVP blends under different temperatures. In the ESR spectrum, there were only one slow motion component in the low temperature or one fast motion component in high temperature for the PEG-labeled. While two components were detected in the blends, indicating phase separation in the blends. According to the ESR spectrum parameters, the correlation time of the samples was calculated. The correlation time of all samples is between 10-9-1011s, its value increases as the content of the PVP in the blend increases. 1. INTRODUCTION Polymer blending is an attractive technique of materials manufacturing as they provide a low-price way for the design and the fabrication of new materials whose properties generally combine those of the individual component polymers. The properties of the blends depend not only on those of the individual component but also on the miscibility of the blend. In many blends, a homogeneous phase was obtained because of the existence of favourable specific interactions between different polymer components. One of these favorable interactions is the hydrogen bonding, to which the miscibility of many polymer blends was attributed 11-31. The compatibility of poly(ethy1ene glycol) (PEG) and poly(vinylpyrro1idone) (PVP) in blends was shown, with FTIR spectroscopy studies, to be due to hydrogen-bonding interactions between the hydrogen atom of the PEG-terminal hydroxyl groups and the electronegative oxygen atoms in the carbonyl groups the comparatively longer PVP chains[4251.However, it is very difficult to evidence polymer miscibility on the molecular scale with the commonly- used investigation techniques. Spin label in Electron Spin Resonance (ESR) is an effective technique to investigate the structure and dynamic behavior of polymer chains in a complicated system [61. The information on polymer chain motions and intermolecular interactions can be obtained from the ESR spectra and the spectrum parameters ['-lo]. The spin-label method has been applied to the study of phase separation PEO/poly(methyl methacrylate) The spin-label method and the spin-probe techniques have
390
been combined together to study the phase structure in poly(viny1 methylether) /poly(styrene) blends"42'51. In this work, we present the application of spin-probe and spin-label methods to study molecular motions of polymer components in PEGPVP blends. 2. EXPERIMENT 2.1 Preparation of spin- labeled PEG Anionic living polymerization of ethylene oxide with sodium 4-oxy-2,2,6,6tetramethyl-piperidinyloxy (TEMPONa) as initiator was used to attach 4-oxyfree radical to the end of PEG . The synthesis of spin-labeled PEG was quantitative, with all chain ends hnctionalized by TEMPO (Scheme 1).
Scheme 1 Synthesis of spin-labeled PEG Molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC) on a Waters Instrument equipped with two Waters Styragel columns (pore size:10' and lo4 A) in series. The other parameters were: THF eluent at lml/min, 38"C, polystyrene with M,= 3750 as molecular weight standard. 2.2 Preparation of polymer blends The polymer blend samples called EV1, EV5, EVlO were prepared by mixing spin-labeled PEG and PVP ( M ~ 3 6 0 0 0 0 )with a weight ratio of 1:1, 1 5 , l:lO, respectively. Sample films was prepared by casting solutions of 5% (wt./vol.) of polymers in tetrahydrofuran (THF) onto glass Petri dishes and by drying the cast films at 323 K in vacuum for 2 days. 2.3 ESR experiment ESR spectra were recorded on a Bruker ER 200 D-SRC spectrometer with ER 4 111VT temperature controlled device under the measurement condition of X-band, microwave power 20 mW, modulation amplitude 0.lmT and modulation frequency 1OOKHZ. 3. RESULTS AND DISCUSSION 3.1 ESR spectra at different temperatures ESR spectra of the spin-labeled pure PEG and its three blends were measured at
39 1
different temperature. Selected ESR spectra are shown in Figure 1. The observed temperature dependence of the spectra was due to the changes in the rotation rate of nitroxide radical, which is characterized by the correlation time ( T c). The two motional regimes usually detected in ESR experiments correspond to the slow motional spectra, with a correlation time in the range of 10-7-10-9~, and the fast motional spectra, with a correlation time in the range of IO-'-lO-" s[16'. The spectra of pure spin-labeled PEG in Figure l(a) are characteristic of a label in one type of environment. The spectra can be divided into two types. At low temperatures (slow motion region) i.e. below 280 K, only the slow component was detected in the spectra. At 120 K, the spectrum approached to the rigid spectrum, in which there is a separation between the two outer-most peaks (2Azz') of about 70 G. As the temperature increases, the spectrum narrows and the outer peak shifts inwards. Above 320 K, the value of 2Azz' reached 30.8 G, the mobility of nitroxide radicals in pure PEG reached motionally the narrowed regime. Figure l(b) shows the ESR spectra of the EV5 sample. Two significant differences are detected in the spectra of the blend compared with those of the SLPEG:
390K
--------. 4 30 K
320K
280K 240K
120K
420K 400K 370K 310K
370K
300K 290K
380K
2 76 53 K
320K
120K
300K
(C)
- - - - - - -
& (d )
Figure 1. X-band spectra as a hnction of temperature for (a) spin-label PEG, (b) EVl,(c)EV5 and (d) EV10.
392 (1) The spectra of the blend are broader than that of the pure SLPEG above 280 K. In general, the value of ~ A z zis' a measure of the mobility of the nitroxide radical. The width of 2Azz' in the blend shows that the motion of nitroxide attached to the PEG chain in the blend was hindered by the presence of PVP. (2) The two spectral components of the blends, indicating different rates of motion, were detected in the temperature range 275-370 K. As the temperature increases, the intensity of the fast motion component increases, and that of the slow motion component decreases. When the temperature reaches 380 K, the slow component in the spectrum disappears. The same results were detected in the EV1 and EVlO samples. But the temperatures where the fast component appears and the slow component disappears are different. In EV1 (Figure l(b)), the fast component appears at 270 K and the slow component disappears at 355 K. In EVIO, the fast component appears at 3 13 K, and the slow component disappears at 396 K. These two components in the ESR spectra indicate that the nitroxide radical in the blends was located in two different molecular environments, i.e. there was phase separation in the blends"731X1. The fast and slow components could be attributed to the radicals in the high-segment-mobility and rigid regions respectively. The molecular mobility of the labeled- PEG chain in the PEG-rich regions would be higher than that in the PVP-rich regions, since the glass-rubber transition temperature of PVP (Tg=446 K) is higher than that of PEG (Tg=208K). To confirm the phase separation in the blends, we annealed the EV5 sample at 450 K for 30 min and measured the ESR spectrum at the ambient temperature. In contrast to the spectrum of the sample stored at the ambient temperature, the intensity of the slow component of the annealed sample was enhanced. It seems that after molecular mixing at 450 K, part of the labeled-PEG chains was transferred into the PVP-rich region. Thus, a thermal annealing could affect the composition of the blend microphase.
Correlation time Figure 2 shows the temperature dependence of the correlation time for different samples; the correlation time is obtained from the temperature dependence of the ESR spectra, by using Kivelson's theory with the assumption of isotropic molecular motion
3.2
U61.
Here hl, ho,and h-1 are respectively the peak heights of low field, center field and high field, and A Hppis the peak width of the center field.
393
-19
-v-
$ -
-21
-
v .
. -22
-23
-
*Be
m
-24
EVIO
v-v
P o
0
m
.
.
,
.
,
2.2
2.3
'
,
2.4
'
,
2.5
.
,
.
2.6
,
2.7
'
2.8
I
.
I
2.9
3.0
'
I
3.1
.
1
32
Figure 2. Plot of In -i versus 1/T of spin labeled PEGPVP. The correlation time increases with increasing PVP content. Figure 2 could be divided into three regions. In the A region, the correlation time of the blends decreases with increasing temperature because of the faster motion of nitroxide radicals. A sharp jump of the correlation time was found in the B region, at which the correlation time increases as temperature increases. In Figure 2, we can find the process of molecular mixing of the two polymer chains. In the A region, the PEG chain movement was restricted by that of PVP because of weak hydrogen bond; the molecular motion increases as the temperature increases. In the B region, the PVF-rich phase begins to melt, the intermolecular interactions become stronger and stronger with increasing temperature, because of the chain entanglements. In the C region, the blends become compatible. This interpretation agrees with the fact that the values of 2Azz' for the blends at higher temperatures are broader than that for the spin-labeled PEG. 4. CONCLUSION In this work, end-labeled polyethylene glycol was prepared to form blend system with polyvinyl pyrrolidone in different ratios. The spin label ESR method was used to study the chain motion in the PEGRVP blends under different temperatures. In the ESR spectrum, there were only one slow component in the low temperature and one fast component in high temperature for the PEG-labeled. While two components were detected in the blends, indicating phase separation in the blends. The fast and slow components were respectively attributed to the SLPEG trapped in the PEG-rich and
394
PVP-rich region in the blends. According to ESR spectrum parameters, the correlation time of the samples was calculated. The correlation time is between 10-9-10-'1s, its value increases as the content of the PVP in the blend increases. ACKNOWLEDGMENT This investigation was supported by National Natural Science Foundation of China (NNSFC 29974006). REFERENCE 1. Watanabe T, He Y, Asakawa N, et al. Polymer International, 50 (4) (2001) 463-468. 2. Chen H L, Wang S F, Lin T L. Macromolecules, 3 1 ( 2 5 ) (1998) 8924-8930. 3. Li L, Chan C M, Weng L T. Polymer, 39 (1 1) (1998) 2355-2360. 4. Fel'dshtein M M, Lebedeva T L, Shandryuk G A, et al. Vysokomol Soedin, 41 (8) (1999) 1316-1330. 5. Lebedeva T L, Igonin V E, Feldstein M M, Plate N A. Proc Int Symp Controlled Release Bioact Mater, 24 (1997) 447-448. 6. Veksli .Z.Andreis .M.Rakvin .B.,Progress. In polymer science. 25 949-986. 7. Boyer R F, Keinath S E. Eds. Molecular Motion In Polymers by ESR; Harwood Academic,New York, 1980. 8. Brown I M. Macromolecules,1980,14 (1980) 80 1. 9. Noel C , Laupretre F, Friedrich C , Leonard C, Halary J L, Monnerie L. Macromolecules, 19 ( 1986) 20 1. 10. Perkan 0, Kaptan Y, Demir Y,Winnik M A. J Colloid Interface Sci, 11 (1986) 269. 11. Shimada S, Keiichi Kasima, Hisatsugu Kashiwabara. Macromolecules, 23 (1990) 3769. 12. Shimada S. Polymer Journal,l996,28(8) (1996) 647. 13. Shimada S, Isogai 0. Polymer Journal, 1996,28(8) (1996) 655 14. Cameron G G, Qureshi M Y, Tavern S C. Polymer International, 47(1) (1998) 15. Muller G, Stadler R, Schlick S. Macromolecules, 27 (6) (1994) 1555-1561. 16. Kivelson D. J Chem Phys ,33 (1960) 1107. 17. Schlick S, Harvey R D, Aloson-Amigo M G, Klempner D. Macromolecules, 22 (1989) 822. 18. Harvey R D, Schlick S. Polymer, 30 (1989) 104. 19. Bullock A T,Gameron G G,Miles S. Polymer, 23 (1982) 156. 20. Braun D,Tormala P,Weber G. Polymer, 19 (1978) 598.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
395
EPR and W-VIS studies on the influence of solute-solvent interactions on the self-redox reaction of Nicola D. Yordanov and Kalina Ranguelova EPR Laboratory, Institute of Catalysis, Bulgarian Academy of Sciences, 1113Sofia, Bulgaria The present communication describes the influence of non-polar, polar and coordinating solvents, temperature of solution and disulfide concentration on the self-redox reaction taking place between two molecules of complexes, {Cun[(R0)2PS2]2, where R = Me, Et, i-Pr}. The studies are performed by EPR and spectroscopy. It is found that the EPR parameters and the position of the charge-transfer band at 420 nm of Cun[(R0)2PS2]2 are not influenced by non-polar solvents. However, this is not the case with the intensity of the spectra since both the EPR intensity and the molar absorptivity are strongly dependent on the type of the used solvents, size and shape of remote ligand substituents, time after dissolution and quantity of added disulfide of dithiophosphate, (RO)2P(S)S-S(S)P(RO)2. In coordinating solvents the g-values increase whereas copper hyperfine splitting decreases and a hypsocromic shift is observed in the absorption band as compared to non-polar solvents. The results are ex lained with the formation of adducts with an axial or equatorial coordination between Cu [(R0)2PS2]2 and the coordinating solvents. A decreasing intensity of the EPR and electronic absorption spectra of CuE[(R0)2PS2]2 is observed 24 h after preparation of solutions in two of them OMSO and pyridine). A transitory increase in intensities is recorded in these solvents after the addition of the corresponding disulfide of dithiophosphate, but the initial complex is destroyed as a result of adduct formation. The observed effects are discussed in terms of specific solute-solute-solvent interactions governing the self-redox reaction.
f
1. INTRODUCTION
Metal salts or esters of diorganoderivatives of dithiophosphoric acid have found wide application as analytical reagents for extraction and spectrophotometric determination of copper(II). In addition they are recommended as antioxidants, antiwear additives to motor oils and polymeric materials, and also insecticides, fungicides, pesticides, flotation agents, etc. These are the reasons for studying them in order to get deeper insight into their structure, properties and reactivity. These problems are often discussed in the literature, but regarding their interaction with copper(II) ions there is a significant controversy up to now. 2. MECHANISM OF THE SELF-REDOX REACTION
The reduction of Cu(II) ions by a variety of thiols in water solutions to give Cu(I) and disulfides reported for the first time 100 years ago. From that time until recently the
396
following reaction Scheme 1 has been commonly accepted on the basis of the final reaction products:
Scheme 1 RSCu'
+ Cuz+ + RS'
-
-
+
Cu' + RS' Cu'(SR)
2RS' RS-RS in toto: 4RS' + 2Cu2+ 2Cu'(SR)
-
+ RS - SR
According to this scheme electron transfer from RS to Cu2+takes place and a free radical recombination yields the final disulfide. However, in a number of observations was found that such reactions proceed through a short-lived color not typical of the reactants themselves, which disappears with a characteristic time between milliseconds and hours followed by immediate precipitation of a pale-yellow solid phase found by EPR spectroscopy to be diamagnetic. This color is independent of the copper(II)/RS- ratio and the shape and size of remote ligand substituents. The precipitate is soluble in organic solvents and each recrystallization is connected with decreasing of S, C, H and increasing of Cu content in it [l].This phenomenon could be explained taking into account the high solubility of the disulfide and the low solubility of the complex of copper (I) in the used solvents. The self-redox reaction is typical for many sulfur-containing copper complexes as dithiophosphinates, xanthates, carboxylates, etc [11. Recent studies in our laboratory on the formation of Cun(R2-dtp)2 in alcohoVwater solutions using stopped-flow technique [2,3] have shown that the absorption band of the intermediate species at 420 nm fully coincides with that of the bis(chelate)copper(II) complex dissolved in organic solvents. On the other hand the absorbance reaches maximum value at the molar ratio Cu(II)/dtp = 1:2 and within few seconds decays through a second-order reaction in respect to its concentration. The data obtained for the rate constants in water-alcohol solutions prove that the more bulky the ligand substituent, the lower is the rate constant, thus confirming the idea of an association process between two absorbing molecules. The rate constant dependence on the remote ligand substituents follows the order (1): Me > Et > Pr > i-Pr > Bu > s-Bu > t-Bu
(1)
which suggests that the stereochemistry of the molecules is an important factor in the reaction course. On the basis of these and other results [4] a different mechanism was suggested involving self-association between Cu and S atoms from one molecule and S and Cu from another molecule (Scheme 2). In this case the isolated molecules are usually paramagnetic and exchange reaction between them, called self-association, is connected with decreasing covalence of the Cu-S bonds as well as expansion and rhombic distortion in the equatorial plane. When the ligand substituents are not bulky, sulfur atoms could approach enough close to each other to form an S-S bond between the ligands of two neighbouring Cun(R2-dtp)2 molecules. This yields a disulfide from the ligand and a reduction of Cu(I1) to Cu(I). Furthermore, two copper(1) dimmers form tetramers. The self-redox reaction could be expressed with the equilibrium reaction (Scheme 3). When these complexes are dissolved,
397
Scheme 2
the equilibrium should be set up. For Cun(R2dtp)2 complexes with bulky substituents, close approach of the molecules is prevented and the monomeric structure is favoured.
Scheme 3 2RS- + Cu2+4 Cun(SR)2 2Cun(SR)2 +m [Cun(SR)2.. ..... Cun(SR)2]
4
2Cur(SR) + RS-SR [C~'(dtp)4]+ 2ds
4Cun(dtp)2
All studies reported up to now confirm this mechanism of the self-redox reaction and strongly suggest that it can be affected by the following factors [5] which are discussed in the present paper. 3. EFFECT OF COMPLEXES SOLUTESOLVENT INTERACTIONS AND TEMPERATURE ON THE SOLUTION SPECTRAL PROPERTIES
80 G
P
I
Figurel. Room temperature EPR spectrum of Cun(R2- dtp)z complexes.
398
The self-redox reaction was investigated in two groups of solvents - polar and nonpolar and with different ligand substituents @PryEt and Me). DMFA, DMSO and pyridine were used as polar solvents and CHCl3, C6HsCH3, c6&,Cc4, C7H16, C6H14 as nonpolar. The solution EPR spectrum of Cun(R2-dtp)2 at 293 K has the typical features of the chromofore CuS4 (Fig. 1). It is characterized with a hyperfine splitting due to the interaction of copper(I1) unpaired electron with 63p65Cu nuclei (1=3/2). In addition a superhyperfine (shf) splitting appears with the intensity ratio 1:2:1, due to the interaction of copper(I1) unpaired electron with two equivalent "P nuclei from the ligands which is unambiguous evidence for the presence of the CuE(R2-dtp)2. This splitting could not be observed in some polar solvents due to line broadening.
Scheme 4
I
The effect is connected with the fact that Cu(II) chelate complexes react with some basic molecules. The products of this D-A interaction are 1:l or 1:2 adducts with axial or equatorial coordination. (Scheme 4). ( In our case adduct is a molecular (D-A) complex formed between two valently saturated molecules, one of which is a metal chelate.) The influence of the axial adducts on the EPR spectra is: (i) shifting the EPR spectra to lower magnetic field; (Table 1). (ii) increasing the g-values and decreasing of The reason is axial followed by equatorial coordination of the polar solvent molecules to the metal ion in the chelate complex which changes its electronic structure lowering the covalent character of Cu-S bonds and causes deformation in the plane xy thus facilitating its destruction.
Table 1.EPR parameters of the Cu(R2-dtp)z in the used solvents. Solvent I g, [+0.001] Ah', 2.067 2.057 DMFA 2.053 74.2 solvents
I
I
399
Our investigations on the effect of solvents and time after dissolution on the EPR and electronic spectra of Cun(R2-dtp)2 show that the EPR intensity of Cun(R2-dtp)2 in the used polar and non-polar solvents one hour after dissolution is different. It becomes 2-3 times higher on keeping the complexes for 24 h at room temperature due to the shifting of equilibrium (3) towards Cun(R2-dtp)2 and remains constant in non-polar solvents and DMFA for 60 days period of monitoring (Table 2). The highest EPR-intensity of Cun(R2-dtp)2 obtained in DMFA compared to the other solvents (Mn2+/Mg0was used as a standard in all measurements) could be explained with the formation of Cun(R2-dtp)2 adducts with axial DMFA coordination. From the used non-polar solvents maximum value of the intensity is obtained in CHCl3 due to the formation of D-A complex and H-bonds, characterized by ENDOR spectroscopy, between Cun(R2-dtp)2 and CHCl3 which facilitates the reverse reaction shifting equilibrium (3) towards Cun(R2-dtp)2. It is well known that CC4 forms stronger D-A complexes with Cun-complexes but obviously it is not enough and most probably H-bonds are much more important for improving the reverse reaction. The complexes exhibit lower EPR intensity in hexane and heptane which have weaker effect on the self-redox process because these solvents are not capable for specific interactions with the solute. In the polar solvents DMSO and Pyridine the EPR intensity gradually decreases and the EPR signal disappears several days after preparation of their solutions because the obtained axial adducts in these solvents are precursors to the equatorial. After that the complexes are destroyed (Scheme 4). The obtained EPR intensities are in concert with the
Table 2. Relations of the EPR - intensity (a.u.) of Cun(i-Pr2 - dtp)z in different solvents versus intensity of Mn2+/Mg0(standard) and values of the molar absorptivities [m-’.cm-’]
DMSO Pyridine
I
11,2.10’ 7,2.103
I
10,5.10’ 5,9.103
9,6.10’ 4,5.103
22,O.lO” 9,6.103
400
electronic spectral changes: (i) the dependence of the absorbance at 420 nm vs. concentration is not linear when the electronic spectra are recorded 1h after preparation of solutions and Beer's law is not obeyed. (ii) after keeping Cuu(R2-dtp)2 solutions for 24 h at room temperature the dependence A 4 2 0 /c becomes linear, Beer's law is satisfied and the molar absorptivity values are higher than those obtained 1h after dissolution. At least 24 h after preparation the EPR spectra of Cun(R2-dtp)2 solutions were recorded at different temperatures in the interval 20 - 50 OC. The obtained spectra show increased signal intensity within 30 - 40% in non-polar solvents and almost 50% in DMFA. This effect could be also explained with the shifting of equilibrium (3) towards Cun(R2-dtp)2. After heating to 50°C and then cooling the solution to room temperature the increased EPR-intensity was kept constant for 4-5 days. Heating the solutions in DMSO and Pyridine decreases the intensity of the EPR spectra with 10% in DMSO and 30% in Pyridine. The explanation is that the process of adduct formation with these solvents is accelerated at higher temperature and contributes to the destruction of the complexes. After cooling the solutions to room temperature the previous intensities of the EPR signals are not obtained and they continue to decrease. Studies were also carried out after addition of small portions of the appropriate disulfide dissolved in the same solvent to the solution of Cuu(R2-dtp)z. The results show a significant increase of the EPR-intensity of Cuu(R2-dtp)2 (Table2). The obtained EPR intensities remain constant in DMFA but they gradually decrease in DMSO and Pyridine because of destruction of the complexes. On the other hand the results show that the more the solution is diluted the larger should be the quantity of added disulfide to obtain increased EPR intensity. By comparing the results of EPR and UV-VIS investigations the maximum molar absorptivity of Cuu(R2-dtp)2 band at 420 nm was found to be 2 . 9 ~ 1 0M-' ~ cm-' (for i-Pr substituent in DMFA). 4. CONCLUSION
The reported data show that the proceeding of self-redox reaction is influenced by many factors. The EPR intensity of Cuu(R2-dtp)2 strongly depends on the nature of the solvent, time after dissolution, temperature, and quantity of the added disulfide. Polar solvents form adducts with Cuu(R2-dtp)2 which could be axially (DMFA) or equatorially (DMSO and Pyridine) coordinated towards the plane of the chromophore CuS4. After keeping the solutions for several hours, heating them to 50°C and/or adding of disulfide, the molar absorptivity was found to increase up to 2 . 9 ~ 1 0M-' ~ cm-' for Cun(i-Pr2-dtp)2 in DMFA which is the highest reported value in the literature up to now.
REFERENCES 1. N.D.Yordanov, Transition Met. Chem,22 (1997) 200. 2. N.D.Yordanov, LAntov and G.Grampp, Inorg.Chim.Acta, 272 (1998) 291. 3. N.D.Yordanov, K.Ranguelova and G.Grampp, Inorg.Chem.Commun., 3 (2000) 383. 4. N.D.Yordanov, V.Alexiev, J.Macicek, T.Glowiak and D.R.Russe1, Transition Met. Chem., 8 (1983) 257. 5. N.D.Yordanov, KRanguelova and LGadjanova, Transition Met.Chem. in press (2001).
Section 4 Environmental Sciences
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EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
403
In and ex EPR spectroscopy and imaging of endogenously produced nitric oxide under physiological and pathophysiological conditions Tetsuhiko Yoshimura* and Naoki Kato Laboratory of Applied Biomedicinal Chemistry, Institute for Life Support Technology, Yamagata Public Corporation for the Development of Industry Matsuei, Yamagata 990-2473, Japan
It is now widely known that nitric oxide (NO) is an ubiquitous messenger molecule in physiological and pathophysiological processes. Although NO is an uncharged free radical, direct electron paramagnetic resonance (EPR) detection of endogenous NO in biological samples seems almost impossible because of its low level (less than and short half life (3-5 s for NO from endothelial cells) These difficulties can be overcome by applying a spin-trapping technique. Iron complexes with dithiocarbamate derivatives (FeDTCs) are noted among the spin-trapping reagents for NO because NO has a high affinity for the iron complexes and resultant stable nitrosyl iron complexes exhibit intense signal both at room temperature and at low temperature, enabling and determination of endogenous NO. We have been studying the roles of endogenously produced NO by employing EPR spectroscopy and Fe-DTCs as NO trap. Here, we will present our researches concerning the EPR detection and imaging of endogenously produced NO under physiological and pathophysiological conditions.
1. INTRODUCTION
NO is a small uncharged free radical containing one unpaired electron. NO exists in space as an interstellar molecule [ 11, and in the atmosphere of Venus and Mars [2,3]. On our earth, NO has been recognized as an atmospheric pollutant and a potential health hazard. In Japan, the air pollution by nitrogen oxides @Ox) has become a subject of discussion since 1970s. Although relatively high concentration of NO has been detected in urban atmosphere, we do not yet have the detailed knowledge on the biological effects of atmospheric NO. However, in 1987 it was reported that NO is identical with endothelium derived relaxing factor (EDRF), which is biosynthesized in the living body [4-61. It is now widely known that NO is an ubiquitous messenger molecule in the cardiovascular, nervous, and immune systems [7-91. In addition, L-arginine-derived NO has been found in a wide variety
404
of organisms ranging from mammals to invertebrates, bacteria, and plants. To elucidate a variety of biological actions of NO, we must have information concerning the quantities and distributions of NO in cells, tissues and organs. Because NO formed has a low level (less than yM) and a short half life (3-5 s for NO from endothelial cells), it is difficult to determine the NO. As one of the method to overcome these difficulties, spin-trapping technique has been applied to and NO determinations by electron paramagnetic resonance (EPR) spectrometry. Iron complexes with dithiocarbamate derivatives (Fe-DTCs) are noted among the spin-trapping reagents for NO because NO has a high affinity for the iron complexes. Resultant nitrosyl iron complexes are highly stable and exhibit intense three-line signal at room temperature and axial signal at low temperature, enabling and determination of endogenous NO. Therefore, iron dithiocarbamates and their NO complexes can be effectively utilized as an NO trapping reagent and a spin probe or an imaging reagent, respectively. We have been developing the quantification method of NO employing an EPR spectrometry combined with spin-trapping technique in which utilizes Fe-DTCs as NO trap [lo]. Further, we have been studying the roles of endogenously produced NO under physiological and pathophysiological conditions through the detection of NO in cultured cells, resected organs and tissues, and living small animals [ll-261. In this paper, several examples on ex and EPR detection and imaging of endogenously produced NO are presented.
2. BACKGROUND OF NO DETECTION BY EPR SPECTROMETRY The elucidation of physiological and pathological roles of NO can lead to the developments of biomedical research reagents and clinical drugs and the research results can be utilized in clinical or therapeutic trials. Successful examples are a clinical therapy with sildenafil for patients with an erectile dysfunction and an inhaled NO for infants with primary pulmonary hypertension. To clarify the roles of NO, NO detection and quantification in biological systems are required. Newly developed analytical techniques have revealed numerous roles of NO in various biological specimens [27].Determination methods of endogenous NO are classified roughly into five techniques: chemiluminescence technique using ozone or luminol, spectrophotometry using Griess reagent or hemoglobin, fluorometry using diaminonaphthalene (DAN) or diaminofluorescein (DAF), amperometry using microelectrode, and EPR spectrometry combined with spin-trapping technique. Among them, three techniques of chemiluminescence technique, amperometry, and EPR spectrometry have been reported to be feasible to detect NO in vivo. So far, EPR spectrometry alone has been employed to obtain images of endogenous NO [lo]. Typical NO trapping agents reported are as follows: hemoglobin derivatives such as deoxy hemoglobin and carbonyl hemoglobin; iron dithiocarbamate complexes; nitronyl
nitroxide such as -yloxyl-3-oxide (PTIO); NO cheletropic trap (NOCT) [27]. At present, NO trapping reagent applicable to measurements are limited to iron dithiocarbamates [lo]. Three dithiocarbamates (Figure 1) are frequently used in NO researches: diethyldithiocarbamate (DETC); N-methyl-D-glucamine dithiocarbamate (MGD); (dithiocarboxy)sarcosine ( DTCS). Fe-DTC + NO
+ NO-Fe-DTC
(1)
Ferric or ferrous dithiocarbamates react with NO to form an NO complex. The reaction of ferric dithiocarbamate with NO occurs through novel reductive nitrosylation. Resultant NO complex is highly stable and exhibits characteristic intense EPR spectra, both at room temperature and at low temperature. Recently, we reported the second-order rate constant of this reaction [23,26]. The value, 5 x lo8 M%', clearly indicates that this reaction proceeds very rapid. The differences in water solubility among the Fe-DTC complexes allow selective usages as NO traps [lo]. Three ligands in Figure 1 have high water solubility. On the other hand, the solubilities of iron complexes are different each other. Fe-DETC is essentially insoluble in water, but it is lipid soluble. In contrast, Fe-DTCS and Fe-MGD are soluble in water, but they have low lipid-solubility. As a result, Fe-DETC has an advantage for the detection of intracellular NO, while water soluble complexes are effective in the detection of extracellular NO. Therefore, we have employed the Fe-DETC as an NO trap for tissues and organs and the Fe-DTCS for extracellular NO and imaging of NO. NO is biosynthesized by three distinct isoforms of NO synthase [7,8,9]. Neuronal NOS (nNOS) and endothelial NOS (eNOS) are constitutively expressed in neuronal and endothelial cells, respectively. Hence, both enzymes are called as cNOS. On the other hand, inducible NOS @NOS)is expressed in macrophages and microglias through stimulation with cytokines and endotoxins. NO levels produced from cNOS are sub-yM at cellular level, while NO levels from iNOS are 1 to 10 pM. Thus, iNOS is high output NOS. Reported values of half life of NO under physiological conditions range from 0.1 to 5 seconds.
IN& -sbrcoo-
S
S C2H5
I
CH,
DETC
S OH
MGD
OH
CH3
OH
DTCS
Figure 1. Dithiocarbamate derivatives used as NO trapping reagents
406
In NO researches, NOS inhibitor has been utilized for judging whether the observed events are NOS dependent or independent. If biological phenomenon is reduced or eliminated on the administration or addition of NOS inhibitor, this phenomenon is NOS dependent. Here, we used three kinds of NOS inhibitor; N-nitro-L-arginine methyl ester (LNAME), N-monomethyl-L-arginine (L-NMMA), and 7-nitroindazole (7-NI).
3. EX VZVO NO DETECTION IN KAINIC-ACID INDUCED SEIZURE IN MOUCE
Kainic acid was originally isolated from the seaweed Digenea simplex, which grows near Japan and Taiwan; known as Kaininso in Japan. Kainic acid is a structural analog of the excitatory amino acid glutamate that binds and activates ionotrophic glutamate receptors. Kainic acid-induced limbic seizures have been extensively used as a model for studying human temporal lobe epilepsy [28]. Recently, the close relationships between kainic acidinduced nitric oxide (NO) and seizures has been suggested [29]. In the present study, to elucidate the involvement of kainic acid-induced NO with seizure, we attempted to determine NO level in the right temporal cortex region by EPR spectrometry [30]. In the control group, no spike discharges were observed throughout the experiments in the electroencephalogram at mouse temporal lobe. After treatments of kainate 30 mgkg, epileptic discharges began to emerge and they were observed frequently 60 minutes after it. Thus, kainate administration to animals effectively induced an epileptic seizure. Kainate Dose (mg/kg) -o-
control
+ 10
I
0
I
I
60 90 Time after kainate injection (min)
I
120
Figure 2. Sequential changes of NO signal intensity in the temporal cortex of mouse brain after kainate injection (n = 5, mean k SE)
407
Kainic acid was administered to a mouse intraperitonealy. Then, intraperitoneal administration of DETC and subcutaneous administration of iron-citrate were performed. Thirty minutes after the administration, we observed X-band EPR signals in the brain tissues at room temperature. Since it has been known that this signal height is proportional to the amount of NO generation, the signal height was measured as an index of NO generation. Moreover, we can convert the signal height into NO concentration by using a standard NO complex. Figure 2 shows sequential changes of NO signal intensity in the temporal cortex of mouse brain after kainate injection (n = 5 , mean SE). After kainate injection, NO quantities gradually increased and reached a maximum at 90 minutes, and then decreased. As shown in Figure 2, NO quantities increased dose-dependently. Maximum signal intensity corresponds to about 4 nmol/g-tissue/30 min-accumulation of NO concentrations. However, preadministration of NO synthase inhibitor (L-NAME and 7-NI) reduced NO to control level at 90 minutes after kainate injection,. This suggests that the NO production induced after kainate administration is truly derived from NO synthase. On the other hand, the severity of kainate-induced seizure increased dose-dependently. Furthermore, the seizure severity was aggravated by the preadministration of NO synthase inhibitor. In summary, kainic acid-induced NO could be measured by using EPR NO trapping technique. NO level was increased dose-dependently in kainic acid doses of 10 to 40 mg/kg, accompanying the development of seizure severity and the increase in mortality. Preadministration of NOS inhibitors suppressed the increase in NO production, but promoted the severity of kainic acid-induced seizure. These results suggest that the enhanced NO production on kainic acid treatment contributes to the suppression of seizure severity.
4. IN WVO DETECTION 4.1. EPR-CT system developed in our institute Here, in vivo EPR detection and imaging methods will be briefly presented with emphasis on the EPR-CT system developed in our institute. Since at conventional X band (9 - 10 GHz) frequencies, water-rich samples have a high dielectric loss which reduces the factor of the resonant cavity [31,32], we utilized 700 MHz microwaves to lower the dielectric loss of water. A loop-gap resonator was used because common cavity resonators have poor filling factors and have an inhomogeneous microwave field inside the resonator [33]. The resonator has the dimensions of 10 mm in axial length and 41 mm in inner diameter, which can accept the head of a rat or the whole body of a mouse. We utilized an air-core magnet to rapidly scan the magnetic field; a pair of magnetic field gradient coils for the X-, Y-, and Zaxes were attached to the surface of the main magnet [34,35]. Three-dimensional EPR images are constructed on the basis of 3D zeugmatography. We collected many EPR spectra by changing the direction of magnetic field gradients under
408
computer control. The EPR spectral data were deconvoluted by a fast Fourier transform method with low pass filtering. Then, EPR images are reconstructed from the deconvoluted data by a filtered back projection. We have limitations in the attempt of current EPR detection and imaging. First, the sensitivity of low frequency spectrometer is much lower than the X-band instrument because the sensitivity is proportional to the square of the frequency. Therefore, to obtain data, we utilized the pathological models to produce high levels of NO and the NO donating reagent to produce much NO Second, the resolution of EPR image is affected by several factors such as linewidth and field gradient. Thus, broad linewidth of NO complexes results in low resolution of image. vivo NO detection in experimental meningitis in rat In various phases during bacterial meningitis, the involvement of excessive NO synthesis has been reported [36,37]. Experimental bacterial meningitis was induced in rat by administration of LPS and IFN-y, where LPS is a bacterial endotoxin and IFN-y is a cytokine. First, we used Fe-DETC complex as an NO trapping reagent and measured X-band EPR spectra of brain tissues at 77 K [21]. Sequential changes of signal intensity were measured in the rat brain tissues during experimental meningitis. The signal intensity reached a maximum at 8 hours after the injection, and then decreased. Then, EPR measurements were performed on the head region at 8 hours after the injection and a weak triplet signal was observed (Figure 3). This signal disappeared after the pretreatment with NO synthase inhibitor, L-NMMA. These results showed that in vivo EPR signal originated from NO synthase induced by experimental meningitis. Furthermore, expression of iNOS gene was also confirmed with RT-PCR technique. 4.2.
a
1 mT
Figure 3. In vivo EPR spectra in the rat brain during experimental meningitis. (a) EPR spectrum recorded at 8 hours after the injection of LPS and IFN-y. (b) EPR spectrum after the injection of LPS and IFN-y and then of L-NMMA.
Figure 4. The structure of isosorbide dinitrate (ISDN)
409
vivo EPR detection and imaging of NO produced from isosorbide dinitrate More than 90 of information from the external environment is visual. An appeal to the eye is more effective than that to the ear. Therefore, seeing is believing. We performed in vivo EPR detection and imaging of NO produced from isosorbide dinitrate (ISDN, Figure 4) [17]. In the ISDN, NO is released from two nitrate ester groups in living body. ISDN is a long-acting nitrovasodilator while glyceryl trinitrate is a quick acting drug. Both drugs have been used as therapeutic agents for the angina pectoris for many years [27]. First, we administered iron-DTCS complex to mouse, subcutaneously. 30 Minutes later, S.C. administration of I4N- or "N-ISDN was performed. Then, in vivo EPR spectra were measured with our EPR-CT system. vivo spectrum of upper abdomen of mouse had a triplet in the treatment of I4N-ISDN and a doublet in that of "N-ISDN. The latter had a higher signal-to-noise ratio than the former. EPR-CT images from the upper abdomen of I4N- and 'jN-ISDN -administered mouse was obtained, respectively. The spatial resolution of images improved from 5.7 mm in the treatment of I4N-ISDNto 3.9 mm in the treatment of IjN-ISDN. The outline of a slice image obtained with I4N-ISDN corresponds to liver alone, while that obtained with "N-ISDN corresponds to liver and kidney, suggesting that ISDN is metabolized primarily in the liver. These results clearly demonstrated that the I5N substitution of 14Nin ISDN provides a high quality EPR image of NO in a living mouse.
4.3.
In conclusion, L-arginine-derived NO is an ubiquitous messenger molecule in cardiovascular, nervous, and immune systems. Therefore, in vitro, ex vivo, and vivo detection and imaging of NO are required elucidating the biological roles of NO. EPR NO trapping technique using iron-dithiocarbamates as NO trap is considered to be most effective for detection and imaging of endogenously produced NO. At present, vivo EPR detection and imaging are not applicable to physiological levels of NO because of limitations of currently available EPR instrumentations. However, this method has the potential to contribute to diagnosis of pathophysiological conditions involving the overproduction of NO. I believe that further developments of EPR instrumentations and NO traps with novel functions will open up new applications. We would like to acknowledge the contribution of our coworkers whose names appear in referenced papers.
REFERENCES 1.
H. S. Lisa and B. E. Turner, Astrophys. J., 224 (1978) L73-L76.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
A. J. Watson, T. M. Donahue, and D. H. Stedman, Geophys. Res. Lett., 6 (1979) 743746. W. L. Chameides, J. C. G. Walker, and A. F. Nagy, Nature, 280 (1979) 280, 820-822. R. M. J. Palmer, A. G. Femge, and S. Moncada, Nature, 327 (1987) 524-526. L. J. Ignarro, G. M. Buga, K. S. Wood, R. E. Byrns, and G. Chaudhuri, Proc. Natl. Acad.Sci. USA., 84 (1987) 9265-9269. R. F. Furchgott, Angew. Chem. Int. Ed., 38 (1999) 1870-1880. S. Moncada, R. M. J. Palmer, and E. A. Higgs, Pharmacol. Rev., 43 (1991) 109-142. P. L. Feldman, 0. W. Grifith, and D. J. Stuehr, Chem. Eng. News., 71 (1993) 26-38. J. F. Kenvin, Jr., J. R. Lancaster, Jr., and P. L. Feldman, J. Med. Chem., 38 (1995) 43434362. T. Nagano and T. Yoshimura, Chem. Rev., 102 (2001) in press. T. Yoshimura, S. Fujii, H. Yokoyama, and H. Kamada, Chem. Lett., (1995) 309-310. T. Yoshimura, H. Yokoyama, S. Fujii, F. Takayama, K. Oikawa, and H. Kamada, Nature Biotechnol., 14 (1996) 992-995. S. Fujii, T. Yoshimura, and H. Kamada, Chem. Lett., (1996) 785-786. S. Fujii, G. Miyakoda, M. Chihiro, T. Yoshimura, and H. Kamada, Chem. Lett., (1996) 1055-1056. H. Yokoyama, S. Fujii, T. Yoshimura, H. Ohya-Nishiguchi, and H. Kamada, Magn. Res. Imag., 15 (1997) 249-253. Y. Suzuki, S. Fujii, T. Tominaga, T. Yoshimoto, T. Yoshimura, and H. Kamada, Biochim. Biophys. Acta, 1335 (1997) 242-245. S. Fujii, Y. Suzuki, T. Yoshimura, and H. Kamada, Am. J. Physiol., 274 (1998) G857G862. Y. Suzuki, S. Fujii, Y. Numagami, T. Tominaga, T. Yoshimoto, and T. Yoshimura, Free Radic. Res., 28 (1998) 293-299. K. Inoue, T. Akaike, Y. Miyamoto, M. Otagiri, S. Suzuki, T. Yoshimura, and H. Maeda, J. Biol. Chem, 274 (1999) 27069-27075. T. Ueno, Y. Suzuki, S. Fujii, A. F. Vanin, and T. Yoshimura, Free Radic. Res., 31 (1999) 525-534. Y. Suzuki, S. Fujii, T. Tominaga, T. Yoshimoto, S. Fujii, T. Akaike, H. Maeda, and T. Yoshimura, J. Cereb. Blood Flow Metab., 19 (1999) 1175-1178. M. Okuyama, S. Yamaguchi, M. Yamaoka, J. Nitoube, S. Fujii, T. Yoshimura, and H. Tomoike, Arterio. Throm. Vasc. Biol., 20 (2000) 1506-1511. S. Fujii, K. Kobayashi, S. Tagawa, and T. Yoshimura, J. Chem. SOC.Dalton Trans. (2000) 33 10-3315. H. Endoh, S. Fujii, Y. Suzuki, S. Sato, T. Kayama, Y. Kotake, and T. Yoshimura, Free Radic. Res., in press. T. Yoshimura, H. Yokoyama, and S. Fujii, J. Magn. Reson. Anal., 3 (1997) 125-140. S. Fujii and T. Yoshimura, Coodin. Chem. Rev., 198 (2000) 89-99.
41 1 27. M. Feelisch and J. S. Stamler (eds.), Methods in Nitric Oxide Research, John Wiley & Sons, Chichester 1996. 28. G. Sperk, Prog. Neurobiol. 42 (1994) 1-32. 29. E. Przegalinski, L. Baran, and J. Siwanowicz, 170 (1994) 74-76. 30. N. Kato and T. Yoshimura, to be published. 3 1. K. Ohno, Magn. Reson. Rev. 11 (1987) 275-3 10. 32. G. R. Eaton, S. S. Eaton, K. Ohno, EPR Imaging and in vivo EPR; CRC Press: Boca Raton, FL, 1991. 33. M. Ono, T. Ogata, K. -C. Hsieh, M. Suzuki, E. Yoshida, and H. Kamada, Chem. Lett., (1986) 491-494. 34. H. Yokoyama, Y. Lin. 0. Itoh, Y. Ueda, A. Nakajima, T. Ogata, T. Sato, H. Ohya Nishiguchi, and H. Kamada, Free Radic. Biol. Med., 27 (1999) 442-448. 35. K. Oikawa, T. Ogata, H. Togashi, H. Yokoyama, H. Ohya-Nishiguchi, and H. Kamada, Anal. Sci., 11 (1995) 885-888. 36. A. R. Tunkel and W. M. Scheld,Annu. Rev. Med., 44 (1993) 103-120. 37. K. M. K. Boje, Brain Res., 720 (1996) 75-83.
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EPR in the 21" Century A Kawarnori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Molecular-electronic mechanism of the toxicity of Dioxin ability of some natural structures to concurrently interact to inhibit its activity Nguyen Van Tri", Pham The Vung", Dinh Pham Thaia, Ha Van Maob and Dinh Ngoc Lamb
" Institute of Engineering Physics, Hanoi University of Technology, P.CHT, C2-101, Dai hoc Bach khoa, 1 Dai Co Viet, Hanoi, Vietnam Hanoi Cancer Research Center, 1 Tran Thanh Tong, Hanoi, Vietnam Numerous particular ESR line groups from Dioxin (2,3,7,8-TCDD) in biological fat and oil, and from its interaction effects in human blood and liver have been revealed and interpreted on the basis of the exchange combinations of the x-electrons of the Dioxin molecule and their specific superexchange complexes with the structures of the nuclei of the respective proteins. On the other hand, the results achieved in long-term experimental clinical studies suggest that some special biological natural compounds from traditional sources are able to inhibit the toxicity of Dioxin. On the basis of the ESR and also the HPLC experimental results, we have revealed some new active natural complexes present in special vegetable products. These complexes are able to bind strongly to Dioxin molecules, thereby inhibiting the toxicity of Dioxin, and its carcinogenic activity. 1. INTRODUCTION AND EXPERIMENTAL
Polychlorinated dibenzo-p-dioxins have generated considerable concern because of their resistance to biological and chemical degradation, ubiquity, fat solubility, long biological half-life and extremely high toxicity. Numerous results of experiments with dioxins in animals (especially on the liver) suggested that dioxins can induce various biological effects. They can adhere strongly to the body, have high affinity binding to protein structure, and deeply perturb genetic control centers in cell nucleus. The experiments also verified of of Dioxin toxicity is selectively dependent on the number and the positions of the substituted chlorine atoms. Especially, the prototypical and most potent member, of maximum toxicity, of these compounds is (TCDD, Dioxin) [ 1-51, On the other hand, the results achieved in long-term experimental and clinical studies [6-81 suggest that some special biological natural compounds from traditional sources have of Such a compound called "Gacavit" is presented in this paper, a Vietnamese plant drug prepared from Momordica Cochinchinensis Spreng [9]. The nutritive effect of this compound is well known and has been used by the population for centuries. It is rich in p-carotene and other carotenoids, oleic, stearic, palmitic, linoleic acids and various microelements [lo]. A primitive chemicopharmaceutical analysis showed that the Gacavit samples contain 0.165% p-carotene.
413 The complicated relationships mentioned above can only be clearly interptreted on the basis of the molecular electronic dynamics of the interaction of dioxin with the biological structures. Keynote problems include the mechanism of the apperance of the respective unpaired electrons - the prerequisite of the interactions - and the nature of the particular binding complexes between dioxin and the biological structure. In this paper, ESR is used as an especially effective means to reveal and explain the nature as well as the molecular electronic mechanism of the interactions. In order to clearly observe the respective effects, the experimental samples have been treated with Dioxin at the dose of about lo3 ppt. The Dioxin-treated samples and the original samples have been studied in situ and in vivo. The ESR measurements were performed on an ERS-220 standard electron resonance spectrometer system with the sensitivity of 10'' Spin/G. The sample temperature was controled automatically at diffrent values from 77 K to room temperature. Some results will be briefly reported as follows.
2. MOLECULAR ELECTRONIC MECHANISM OF THE DIOXIN ACTIVITY Typical ESR spectra from Dioxin [ l l ] are given in Fig.1. These spectra are exactly evidenced by a spin Hamiltonian for the exchange coupled combinations of the electrons of the Dioxin molecule. These electrons may be considered as quasi-free electrons moving in an open resonant cavity constituted by the short-range order structure [12] of the Dioxin molecule. The calculation on the measured data suggests that the line pair B,,B2 originated from the triplet state (S=2/2) combined with the two quinone electron spins corresponding to the Dewar structure of each aromatic ring of the Dioxin molecule (Fig.2). The four line group Dl,DZ,D3,D4 is the signal of the quintuplet state (S=4/2) of a super-exchange combination of the four quinone electron spins on the whole Dioxin molecule (Fig.3). This combination creates very strong electron wave bundles pointing out perpendicularly to the Dioxin molecule plane. But the ground state of the combination is singlet (S = 0). Thus, in the ground state, Dioxin molecules can easily move and worm their way deeply into biological structures. Here, they may be promoted to the quintuplet state - an excited state - and become able to interact strongly with partners. The spectrum in F i g . 1 ~shows clearly a particular binding complex between the Dioxin molecule and the heme nucleus of hemoglobin (Fig.4). A similar result was also obtained from the interaction effect of Dioxin with the Mn-bearing porphyrinoprotein in human liver [I 11. These experimental revelations suggest that the nature of the specific binding complexes between Dioxin and the functional nucleus structures of partners is just a super-exchange interaction through a combination electron spins delocalized between the two molecular systems. This interaction can develop and burrow very deeply into the nuclear framework of the nucleus structure of partners and form a super-stable complex. These results also coincide with the suggestion of M.S. Denison from many biochemical studies on dioxin toxicity [l] and especially, with a well known elementary hypothesis about the molecular-electronic mechanism of the carcinogenic activity [13-141. After this hypothesis, in a carcinogenic aromatic hydrocarbon there must be a "K activity zone" of n electrons induced from the oxidation by epoxy bridges, or N atoms, etc. When such a hydrocarbon meets an adequate partner, the odd electrons from the K activity zones will be promoted to sufficiently high energy, for them to interact strongly with an important functional structure in the cells and form there a particular binding complex.
414
Figure 2. Two triplet coupled pairs of the (1-4) and (6-9) n-electron spins in the Y B (Dewar)-configuration.
, I
,--103-4NI-HFS (Hyperfine ~ t r u c t u r e )
Figure 1. ESR spectra (first derivative) of Dioxin (2,3,7,8-TCDD): a) powder, b) in biological fat, oil and human blood plasma, c) in-a particular binding complex with human Hemoglobin.
Figure 4. Model of the particular binding complex of the Dioxin molecule to the heme nucleus structure in human Hemoglobin. This model is elaborated on the basis of the recorded ESR spectra from the interaction effect of Dioxin in human Hemoglobin (Fig. lc) : The HFS including nine lines with the relative intensities of 1,4,10,16,19,16,10,4,1 shows exchange coupling interaction between the n-electron spin combination of the whole Dioxin molecule and the nuclear spins (I=l) of the four Nitrogen atoms of the heme nucleus in Hemoglobin.
Figure 3. Super-exchange combination ofthe (1-4-6-9) n-electron spins on the whole Dioxin molecule in the (Dewar)-configuration.
415
3. ABILITY OF A NATURAL STRUCTURE TO CONCURRENTLY INTERACT TO INHIBIT THE DIOXIN ACTIVITY The typical ESR spectra of Gacavit recorded at different temperatures are shown in Fig.5 [ 151. At room temperature, the B and D* signals appear very weakly, but as the temperature falls they become stronger. It is especially notable that the positions of the lines and D* coincide respectively with the positions the lines and 4 in the spectrum Dioxin (Fig.l), and correspond to a higher spin number at the lower temperature. From the measured data it can be seen that the line group Ro in Fig.5 is the hyperfine structure (HFS) signal of a stable radical H2C. at the end of a branching chain. The signal groups BlB2, D* must derive from a specific nucleus structure of the "dibenzo" type including two aromatic rings combined with each other by the epoxy bridges similar to the dibenzo structure of dioxin. These suggest that a new structure, called "X-carotene", persists in Gacavit. According to the remarkable spin number relations
1 2
n [XI = (-n[B] + n[D*]}(at 20°C) and
n[Ro]
=
= n [D*] (at -160°C)
2n[X]
it may be imagined that the ground organization of X-carotene must include a dibenzodioxygen structure - the nucleus structure - and two non-aromatic branching chains ended by the stable free radicals R'. It is possible, X-carotene appears as a result of the epoxyreconstruction into a new marcromolecule from the two halves subdivided from an initial pcarotene molecule. In this normal structure X-carotene contains no unpaired electrons, it gives no ESR signal. However, in its own tautomeric structure, every X-carotene molecule shows four unpaired electrons in the aromatic nucleus rings and two at the ends of both branching chains. Thus, the line group Bl,B2 is the signal of the triplet state of the combination of the two odd 7t electrons in the every aromatic ring, and the line D* is the strongest of the four line signal coming from the quintuplet state of the combination of the four odd x electrons delocalized strongly on the aromatic nucleus rings of the tautomeric structure of X-carotene. It is clear that, there is a very interesting coincidence between the ESR-revelation of this new structure "X-carotene" and the corresponding peak reported directly by the HLPC study on Gacavit. This result is in conformity with the well known general rule that the natural tautomeric abundance generally occupies 7.5 % or more depending on the ambient condition [16-171.
Figure ESR spectra from "Gacavit" : a) An overview spectrum measured at 20"C, b) The overview spectrum measured at - 160°C, c) A separately described spectrum of the radical Re (C0-2H) measured at 20"C, c') Second derivative corresponding to c).
416
The measured exchange integral of the D*-system values J - 250 The minus sign of J shows that the ground state of the combination of four unpaired x electrons in the nucleus structure of X-carotene is a this is Thus, concerning the energy relation to the surrounding the X-carotene structure becomes with the high wall I J I . This structure, of course, is able to attract and bind strongly a suitable interaction partner. This micro-quantitative X-carotene complex in Gacavit may be in agreement with the result of some work maintaining that natural carotene is not only a source of A-vitamin but also has its own specific activity [18]. The ESR spectrum obtained from the Gacavit sample treated with Dioxin is given in Fig.6 [15]. This spectrum provides characteristic information. The radical R* shows no HFS with its own H protons. It can be seen that the odd electron of this radical is hybridized after the exterior orientation to exchange with an another odd electron complex, i.e. with a Dioxin molecule. On the other hand, the line D3 of Dioxin becomes very strong and shows an equivalent hyperfine interaction with eight protons. This important event suggests that the wave fimction of the combination of the four unpaired electrons of the Dioxin molecule has burrowed deeply into the nucleus structure of X-carotene. In this structure combined by two aromatic rings, eight H atoms are present. The measured HFS constant with a very characteristic value a = 21.1 G shows that the average density of the unpaired electrons distributes in the ratio: 1 electron spin of Dioxin / 1 H proton of X-carotene. Thus, it can be seen that The specific binding complex [X-carotene - Dioxin] is illustrated as in Fig.7. The measured values of the exchange integrals : J [Hemoglobin - Dioxin] = + 118.4 kK,
Figure 6. ESR spectra from the interaction effect of Dioxin in Gacavit : a',b') First derivative, and a",b") Second derivative.
Figure 7. A model of the particular binding complex [X-carotene - 2DioxinI corresponding to the ESR spectra in Fig.5 and Fig.6 : a) Conformational molecular structure, b) Block symbolic scheme of the exchange interaction of molecules. : Epoxy bridge 'CV: Unpaired electron
417 and J [X-carotene - 2DioxinI = -130.1 show that in the ground state, the electron combination in the first complex is able to transfer easily and act strongly, and in the second complex, the electron combination is confined to a deep well, moves with difficulty, and therefore, it is almost inactive. The measured values of the spin numbers of the specific binding complexes show that the probability for creating the second complex is time higher than the one of the first complex. Thus, if there is a continuous and a long concurrence and competition between the two interactions, the second interaction can predominate over the first one. Eventually, almost all the molecules may be trapped by X-carotene, and the concentration of free Dioxin will no longer be enough to provide the toxic effect. In other words, the X-structure can interact concurently to eliminate the activity of Dioxin. Obviously, this ability depends on other factors, such as the concentration of X-structure present in the sample, its tautomeric probability, the life times of the Re and the D* states, etc. This research was supported by the National Basis Research Program in Natural Sciences, the "10-80" National Committee and the Vietnamese General Union of Medicine. The authors would like to thank Doctor Elizabeth Duel1 in the Laboratory of Professor John Voorhees, University of Michigan, U.S.A., for the very helpful HPLC data on Gacavit, Professor Keith S. Henley of the Medical Center, Doctor William R. Dunham, Distinguished Research Scientist of the Biophysics Research Division, University of Michigan, U.S.A., and Professor Jim Simpson of the Chemistry Department, University of Otago, New Zealand, for their very useful suggestions as well as their read and careful correction of the manuscript.
REFERENCES 1. M.S. Denison et al, DIOXIN '90, Bayreuth, Vol. 4 (1990), 95. 2. 0. Hutzinger, Dioxins and Furans in the Environment: An overview, Oesterr. Gesellschaft fuer Natur- und Umweltschutz, Wien, Heft 18 (1984) 9 1. 3. 0. Wassermann, Toxikologie von Dioxinen und verwandten Verbindungen und des Herbizids, Oesterr. Gesellschaft fuer Natur- und Umweltschutz, Wien, Heft 18 (1984) 121. 4. T. Weidenbach et al, Dioxin - die chemische Zeitbombe, Kiepenheuer-Witsh, Koeln, 1984. 5 . D. Neubert, DIOXIN '90, Bayreuth, Vol. 4 (1990), 117. 6. H.V. Mao et al, 10th World Congress of Gastroenterology, Los Angeles, USA (1 994) 1737. 7. T.V. Bao, H.V. Mao, J. Gastroenterology and Hepatology, Vol. 8, No. 5 (1993) A42. 8. T.V. Bao, H.V. Mao, 2nd International Symposium on Herbicides in war, Hanoi 1993, 434. 9. D.T. Loi, Vietnamese Medicinal Plants and Remedies, 5th Edition, Science & Technology Publishing House, Hanoi, 1986. 10. N.V. Dan, Natunvissenschaften, Heft I (1959), 18. 11. N.V. Tri et al, Proceedings of the Fourth National Conference on Physics (1 994), 542. 12. N.V. Tri, Habilitation Dissertation, TU Ilmenau, Germany 1990. 13. A. and B. Pullman, Nature, Vol. 196 (1962), 228. 14. P.L. Grover et al, Biochem. Pharmacol. Vol. 21 (1972), 2713. J. Caldwell, Xenobiotica, Vol. 9 (1979), 63. 15. N.V. Tri et al, J. Medicine, Vol. 171, No. 5 (1993), 89. 16. J.E. Spice, Chemische Bindung und Struktur, Geest & Portig K.-G., Leipzig, 1971. 17. A.T. Pilipenko et al, Spravocnik PO Elementarnoj Chimii, Kiev Naukova Dumka, 1985. 18. O.E. Privalo et al, Vitaminy v Kormlenii Sel'skochozjajstvennych zivotnych, Kiev, 1983.
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Section 5 Biology and Life Sciences
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EPR in the 21" Century A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
42 1
Kinetic EPR study on reactions of vitamin E radicals Zhihua Chen, Bo Zhou, Huihe Zhu, Long-Min Wu, Li Yang and Zhong-Li Liu* National Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, Gansu 730000, China The role of vitamin E (a-tocopherol) in lipid peroxidation is discussed on the basis of kinetic studies on the reactions of vitamin E and vitamin E radical in micelles, erythocytes and low-density lipoprotein by using stopped-flow electron paramagnetic resonance (EPR) and other methods.
1. INTRODUCTION Free radical mediated lipid peroxidation and DNA damage has been suggested to be associated with a wide variety of chronic health problems, such as cancer, atherosclerosis and aging [11. Inhibition of lipid peroxidation by supplementation of antioxidants has become an attractive therapeutic strategy to prevent and possibly to treat these diseases [2]. In the antioxidant family vitamin E plays a central role. It is well-known that a-tocopherol (TOH), the principal component and the most active form of vitamin E, is the major endogenous lipid-soluble chain-breaking antioxidant in human plasma [3] and low-density lipoprotein (LDL) [4]. It could effectively trap the lipid peroxyl radical (LOO') to inhibit the free radical initiated lipid peroxidation (eqn. 1). The antioxidant efficiency of vitamin E can be enhanced by another co-existed antioxidant (AH, such as vitamin C, ubiquinol-10 and green tea polyphenols) if the
latter could reduce a-tocopheroxyl radical (TO', vitamin E radical) to regenerate vitamin E (eqn. 2). On the other hand, Bowry and co-workers [5] have reported that a-tocopherol might become a prooxidant the so-called tocopherol-mediated peroxidation (TMP) to oxidize polyunsaturated lipids (LH) in LDL particles (eqn. 3). The overall picture of the role of vitamin E in lipid peroxidation is depicted in Scheme 1. An issue of ongoing debate is whether, and under what conditions, vitamin E inhibits or ~~
~~~~~~~
* Author for correspondence, Fax: +86-931-8625657;E-mail: [email protected]
422
promotes the lipid peroxidation. This article summarizes our recent kinetic and mechanistic studies on this issue. TOH
+
LOO'
-
TO'
TO'
+
AH
-
TOH
+
A'
TO'
+
LH
TOH
+
L'
-
--
-TO.
,.
+
(1)
LOOH
TOH
T0H7
(3)
r
AH+LJoo.l .-
A-
Scheme 1. The role of vitamin E in lipid peroxidation
2. INHIBITION OF LIPID PEROXIDATION BY VITAMIN E The inhibition of lipid peroxidation by a-tocopherol has been extensively studied in the past two decades by Ingold [3], Barclay [6], Niki [7] and others. The rate constant of reaction (l), kid, has been determined to be ranging from 3.2 x lo6 M-' s-l in chlorobenzene [3] to 5.8 x lo3 M-' s-l in dilinoleoyl phosphatidylcholine (DLPC) membrane [6], depending remarkably on the microenvironment of the reaction medium. We have pointed out that the predominant factors which control the antioxidant activity in membrane mimetic systems are the inter- and/or intra-membrane diffusion rate of the antioxidant and the electrostatic interaction between the antioxidant and the surface charge of the membrane [8,9]. Generally, the antioxidation reaction of a-tocopherol conducted in micelles and bilayer membranes follows the same classical rate law of free radical chain reactions as that conducted in homogeneous solutions (6,101. That is, a-tochopherol should decay linearly with time. However, we found recently that the linear decay of a-tocopherol no longer exist when the rate of initiation was very slow and/or the concentration of a-tocopherol was very high. Representative results are shown in Figures 1 and 2. It reveals clearly that appreciable deviation from the linear decay of a-tocopherol appeared when the concentration of the initiator,
423
dihydrochloride (AAPH), was lower than 2.5 mM and when the initial concentration of a-tocopherol was higher than 7.5 pM when the reaction was conducted in sodium dodecyl sulphate (SDS) micelles at 37 'C. The reason for this deviation will be discussed in the next section.
-.-.-.-.-.-.-.-. 40
16
f
-
9-
2 v
P
P t
8
4
0
I
. 400
t / min Figure 1. The consumption of a-tocopherol (TOW
t 1 min Figure 2. The consumption of a-tocopherol (TOH)
during the peroxidation of Iinoleic acid in SDS during the peroxidation of Iinoleic acid in SDS micelles. The peroxidation was initiated by AAPH and inhibited by TOH. The initial Concentration of
micelles. The peroxidation was initiated by 0.63 mM of AAPH and inhibited by TOH. The initial
TOH was 15.0 pM and the initial concentrations of
concentrations of TOH were: (a) 2.5 pM, (b) 5.0
AAPH were: (a) 6.3 mM, (b) 5.0 mM, (c) 3.8 mM,
p M (c) 7.5 pM, (d) 10.0 p M (e) 12.5 pM, (f) 15.0 pM.
(d) 2.5 m M (e) 1.25 pM, (0 0.63 mM.
3. a-TOCOPHENOL-MEDIATED PEROXIDATION Although a-tocopherol behaves as an antioxidant under normal experimental conditions in low-density lipoprotein (LDL) [ll-131, Bowry and Stocker reported that a-tocopherol might become a prooxidant to accelerate the peroxidation of LDL the so-called tocopherol-mediated peroxidation (TMP, eqn. 3) when the initiating radical flux was low and the concentration of a-tocopherol was high [14]. As shown in Figures 1 and 2 that the decay of a-tocopheroxyl radical in SDS micelles is decreased when the concentration of AAPH is low and the concentration of TOH is high. Therefore, it is reasonable to assume the co-existence of the antioxidative reaction (1) and prooxidative reaction (3) in SDS micelles. Computer simulation of the experimental data from Figures 1 and 2 by taking into account of the both reactions gave rate constants kih = 3.6 lo4 M-' s-l and kTMp = 0.46 M-l s-l for reactions (1) and (3) respectively. We found that although a-tocopheroxyl radical decays rapidly in homogeneous
424
solutions due to the fast self-recombination reaction ( 2 kt = 2 x lo3 M-l s-' at [15]), it decays very slowly in micelles (e.g., Figure 3). Therefore, the reaction kinetics of TMP (reaction 3 ) can also be directly determined via stopped-flow EPR by rapidly mixing the micellar solution of a-tocopheroxyl radical with the micellar solution of polyunsaturated lipids situ and stopping the flow (e.g., Figure 4). As shown in Figure 4A a-tocopheroxyl radical decayed very slow in the absence of linoleic acid, while it gave a fast and exponential decay in the presence of large excess of linoleic acid. Plot of the pseudo-first-order decay rate with the initial concentration of linoleic acid gave a straight line (Figure 4B) from which the second-order rate constant, kTMp for reaction 3 was obtained as 0.44 M-l s-', in good agreement with the result obtained indirectly by computer simulation mentioned above.
Figure 3. EPR spectra of a-tocopheroxyl radicals Figure 4. (A) Decay of a-tocopheroxyl radical in recorded in 0.2 M SDS micelles at pH 7.4 and 37 0.2 M SDS micelles at pH 7.4 and room "C. The radicals were generated by temperature under a, Intrinsic decay; b, in the oxidizing a-tocopherol with Pb02. presence of 18.75 m M linoleic acid (B) Plot of the pseudo first-order decay rate with the concentration of linoleic acid.
4. a-TOCOPHEROL REGENERATION REACTIONS The antioxidant synergism of vitamin E and vitamin C has been extensively
425
studied and proved to be due to the reduction of a-tocopheroxyl radical by vitamin c that regenerates vitamin E [16,171. We found recently that polyphenolic constituents extracted from green tea (Figure 5) are very good antioxidants against lipid peroxidation in homogeneous solutions [18], in micelles [10,19], in low density lipoproteins [13], in erythrocytes [20] and in red blood cell membranes [21]. Stopped-flow EPR showed that the decay of a-tocopheroxyl radical was greatly increased by reaction with green tea polyphenols as exemplified in Figure 6. Plot of the pseudo-first-order rate constants with the initial concentration of the green tea polyphenols gave straight lines (Figure 7) from which the rate constants for reaction 2, kRm, were obtained. It is found that the rate constants correlated linearly with the oxidation potential of these green tea polyphenols, implying that the rate-determining step is the electron transfer between a-tocopheroxyl radical (TO
and the green tea polyphenol (GOH) as shown in eqn.
Since the radical cation of GOH must be very acidic and very easy to deprotonate in the neutral medium and transfer the proton to the anion of a-tocopherol, the overall reaction is the same as eqn. (2). Rate constants and the equilibrium constants for reaction (4) are listed in Table 1.
- kREG
TO’
+
GOH
k - ~ G
TO-
+
GOH”
Figure 5. Molecular structures of green tea polyphenols When vitamin C was added together with green tea polyphenols and a-tocopherol the antioxidant efficiency of the latter two antioxidants was much more enhanced. The inhibition time of the mixed antioxidants was 200 % longer than the sum of the inhibition times when the three antioxidants were used individually (e.g., Figure 8).
426
Examination of the decay kinetics of the antioxidants revealed that a-tocopherol was regenerated by green tea polypenols and the latter was regenerated by vitamin C as depicted in Scheme 1.
1
50
100
150
200
40
250
[GOH] / 10' M
t (set)
Figure 6. Decay of a-tocopheroxyl radical in 0.2 M Figure 7. Plot of the pseudo-first-order rate SDS micelles at pH 7.4 and room temperature constants versus the initial concentration of GOH. under air. a. Self decay; b. in the presence of 0.1 A. EGCG; b. ECG; c. EGC; d. EC. mM of ECG.
GOH
I
EC EGC ECG EGCG
I
kmG
k-mG
(10' M'' s-') 0.45
( M-' s-') 4.6 1.8 1.7 1.3
1.11 1.31 1.91
I
K
I
10.2 66.8 71.3 143.9
1
E,b (V vs.-SCE) 0.33 0.29 0.27 0.23
1
427
50
-50
0
50
100 I50 200 250
t
3M
Figure 8. Formation of lipid hydroperoxides (LOOH) during the AAPH-initiated peroxidation of linoleic acid in SDS micelles and its inhibition by mixed antioxidants. a. Uninhibited reaction; Inhibited by: b. VC; c.TOH; d. TOH+VC+GA, e. TOH+VC+EC; f. TOH+VC+ECG g. TOH+VC+EGC; h. TOH+ VC+EGCG.
min
Aqueous phase
GO'
vc -*
Scheme 2. Regeneration of vitamin E by green tea polyphenol and vitamin C in membranes. 5. CONCLUSION
The antioxidation reaction of a-tocopherol is always accompanied by tocopherol-mediated peroxidation (TMP) in its reaction against lipid peroxidation. The rate constant for the antioxidation reaction, ki*, is over four orders magnitude than kTm. Therefore, the TMP is only observable when the initiating radical flux is very low andlor the concentration of a-tocopherol is very high. On the other hand, green tea polyphenols and vitamin C can regenerate vitamin E and greatly enhance the antioxidant efficiency of the latter.
Acknowledgements We thank the National Natural Science Foundation of China for financial support (Grant No. 29832040). We are also grateful to the Organizing Committee of 3rd Asia-Pacific EPRESR Symposium for financial support for attending the symposium.
428
REFERENCES 1. L. J. Marnett, Carcinogenesis, 21 (2000) 361. 2. C. A. Rice-Evans and A. T. Diplock, Biol. Free Radical Med., 15 (1993) 77. 3. G. W. Burton and K. U. Ingold, ACC.Chem. Res., 19 (1986) 194. 4. H. Esterbauer and P. Ramos, Rev. Physiol. Biochem. Pharmacol., 127 (1995) 30. 5. V. W. Bowry and K. U. Ingold, ACC.Chem. Res., 32 (1999) 27. 6. L. R. C. Barclay, Can. J. Chem., 71 (1993) 1. 7. E. Niki, T. Saito, A. Kawakami, Y. Kamiya, J. Biol. Chem., 259 (1984) 4177. 8. Z. L. Liu, in Bioradicals Detected by ESR Spectroscopy, H. Ohya-Nishiguchi and L. Parker (eds.), Birkhauser Verlag, Basel, 1995, pp. 259. 9. Z. L. Liu, Z. H. Han, K. Z. Yu, Y. L. Zhang and Y. C. Liu,. Org J. Phys. Chem., 5 (1992) 33. 10. Z. H. Chen, B. Zhou, L. Yang, L. M. Wu and Z. L. Liu, J. Chem. SOC.Perkin Trans. 2, (2001) 1835. 11. N. Noguchi, N. Gotoh and E. Niki, Biochim. Biophys. Acta, 1168 (1993) 348. 12. Z. Q. Liu, W. Yu and Z. L. Liu, Chem. Phys. Lipids, 103 (1999) 125. 13. Z. Q. Liu, L. P. Ma, B. Zhou, L. Yang and Z. L. Liu, Chem. Phys. Lipids, 106 (2000) 53. 14. V. W. Bowry and R. Stocker, J. Am. Chem. SOC.,115 (1993) 6029. 15. T. Doba, G. Burton, K. U. Ingold and M. Matsuo, J. Chem. SOC. Chem. Commun, (1984) 461. 16. J. E. Packer, T. F. Slater and R. L. Willson, Nature, 278 (1979) 737. 17. Y . C. Liu, Z . L. Liu and Z. X. Han, Rev. Chem. Intermed., 10 (1988) 269 and references cited therein. 18. Z. S. Jia, B. Zhou, L. Yang, L. M. Wu and Z. L. Liu, J. Chem. SOC.Perkin Trans. 2, (1998) 911. 19. B. Zhou, Z. S. Jia, Z. H. Chen, L. Yang, L. M. Wu and Z. L. Liu, J. Chem. SOC. Perkin Trans. 2, (2000) 785. 20. L. Ma, Z. Liu, B. Zhou, L. Yang and Z. L. Liu, Chin. Sci. Bull., 45 (2000) 2052. 21. L. Ma, Y. Cai, L. Yang and Z. L. Liu, Chin. J. Org. Chem., 21 (2001) 518.
EPR in the 21” Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
429
ESR investigation on ROS initiated by visible light in PSII particles of high plants Yang Liu”” Jian SunaxbKe Liu” Qiyuan Zhang” and Tingyun Kuangb ahstitUte of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China bInstitute of Botany, The Chinese Academy of Sciences, Beijing 100093, China Experimental evidence of the reactive oxygen species (ROS) generation in photosystem 2 (PS particles separated from spinach upon an irradiation of visible light have been directly obtained by Electron Spin Resonance technique in combination with several new phosphorylated spin traps, such as (DEPDMPO), (DEPMPO) and 5-diethoxy Further ESR observations indicate that not only the plastoquinones located at QA and QB of PS I1 is the primary site responsible for superoxide formation, but also the generation of superoxide anion radicals is positive correlated to the components of oxygen in the air, its spin state and proton concentration in the solution. Meanwhile, in order to theoretically confirm the new mechanism of O i generation therein we also examined the reaction by means of calculations and ESR detection in a self-constructed artificial system. Another research interest was to experimentally determine which reactive species was mainly responsible for the photo-induced inhibition and destruction among various ROS. Accordingly, we have mainly probed the photo-damage effects of superoxide and hydroxyl radicals that can be responsible for the inactivation of water splitting complex in PS particles, the degeneration D1 and D2 proteins and the pigments photobleaching during irradiation of visible light. 1. INTRODUCTION
Photosystem (PS 11) is a large membrane complex that performs the unique chemistry of water splitting. It catalyzes the light-driven electron transfer from water to plastoquinone to give rise to the generation of molecular 0 2 . The chemical reactions involved are intrinsically dangerous by the reason that oxygen can readily form reactive oxygen species (ROS) that can attack proteins and other components of PS 11. Singlet oxygen is formed by the reaction between P680 triplet state and oxygen when PSII is subjected to acceptor side induced photodamage[ 1-41. Singlet oxygen has been suggested to be responsible for the chlorophyll bleaching and protein degradation and crosslinking[5-81. Despite the arguments for singlet oxygen as a main damaging source of PSII, other ROS may be involved. By means of cytochrome c reduction[9], voltammetric detection[lO] and spin - trapping EPR [ 11-12] superoxide was found to be actively produced by PS 11fragments Corresponding author. Tel. ++86-10-62571074; Fax. ++86-10-62559373; E-mail. [email protected]
430
under strong illumination. Although superoxide dismutase has been detected in the PSII fragments preparation indicating a physiological demand for disproportion in PSII, the exact role of superoxide in PSII reaction center remains to be established since inconsistent phenomena were found that SOD alone could slow down the photodamage in chloroplast[ 131 and PSII fragments preparation[ 141, but neither in PSII core nor in PSII RC preparation[ 141. Hydrogen peroxide also is one of ROS generated by PS I1 under illumination. Several researches have shown the light-induced H202 production at donor side of PSII when the function of the water-splitting was disturbed[ 15-17]. Besides, large amounts of hydrogen peroxide can be able to arise from the disproportioning of superoxide catalyzed by SOD in chloroplast. Under physiological conditions, hydrogen peroxide can further produce hydroxyl radical by the transition metal-catalyzed reduction[ 181. hydroxyl radical had been detected in the illuminated PS 11[191, however, like the situation of superoxide, the exact destructive role of hydroxyl radical in PS 11is not clear enough. PS RC complex prepared by the method of Nanba and Satoh (1987) is composed of D1, D2 and cytochrome and only contains three electron transfer intermediates: primary donor Tyr, P680 and primary acceptor Pheophytin. It is proved to be an ideal material for studying many basic phenomenon of photosynthesis. In this study we just experimentally and theoretically investigated formation of ROS, especially for superoxide anion radicals, and the corresponding photooxidative damage in PS I1 particles.
2. MATERIALS AND METHODS 2.1. Isolation of PSII and Its Reaction Center Complex PSII fragments were prepared from market spinach by the procedure of Kuwabara and Murata (1982)[20]. PSII RC complex were obtained from PSII fragments by a method based on that of Nanba and Satoh (1987)[21] and Chapman et al. (1988)[22] as modified by Miyao (1994)[23]. PS I1 RC preparation was further concentrated by the centrifugal filter device (YM-30) from Millipore.
2.2. Spin Trapping-ESR Measurements For ESR measurement, the PSII RC complex (10 pg Chl /mL) was suspended in 1mM DM, lOmh4 NaC1, 2mM DETAPAC, 0.lmM ubiquinone, 50mM Mes-NaOH (pH 5.0) and spin trap DEPMPO(25 mM) or DMPO(75 mM). ESR spectra were recorded on a Bruker ESP 300 instrument operating at X-band. Photoinhibitory illumination was performed by continuous He-Ne laser (25mW, 663nm) that could give strong irradiance without heating the sample. The ESR spectra were recorded simultaneously with the illumination at ambient temperature (20°C). The instrument settings used were as follows: modulation amplitude, 2.5G; time constant, 0.3 S; modulation frequency, 100 kHz; microwave power, 1OmW; microwave frequency, 9.76 GHz.
2.3. Photo-Damaging Measurements To monitor the pigments bleaching, PS I1 RC complex (10 pg Chl /mL) was suspended in ImM DM, l O m M NaCI, 2mM DETAPAC and 50mM Mes-NaOH (pH 5.0). The sample was placed into a 10x10 quartz cell. Photoinhibitory illumination was performed as same as that for ESR measurement. Absorbance spectra were recorded by a PC2000-UV-VIS fiber optic spectrometer (Ocean Optics, Inc., Dunedin, FL) simultaneously with the illumination. Parts of sample were reserved at certain illumination time for SDS-PAGE. SDS-PAGE was carried out in a 13.75% gel containing 6 M urea.
43 1
2.4. Theoretical Calculations: Gibbs free energy is theoretically expressed by =
+
+
-
-
Here,
was obtained by means of geometric optimization; Zero point vibration energy heat content and entropy S were obtained with frequency calculations. Gaussian 98 was utilized throughout the present work. Marcus cross electron transfer theory[24] provided a good way to calculate the electron transfer reaction between semiquinone anion radicals and oxygen molecular by the formula of
k,, = (kl,kzXl&1'2W12 where k,,and kIzare the rate constant of two self-exchange reactions; K12is the equilibrium constant;fand W,,are often close to unity[25-281. Rate constants of two electron self-exchange reactions, k,,and kz2,can be obtained by [24]
k,, = K
exp(-AG*/RT)
where K is the transmission coefficient or averaged transition probability for electron transfer per passage of the system through the intersection region; 2 is effective collision frequency for the reaction in solutions; AG* is the free energy of activation.
3. RESULTS AND DISCUSSION 3.1 Evidence in Oz-' Production Spin trapping-ESR evidence of superoxide radicals ( 0 2 was obtained by ESR measurements in PSII particles, as exhibited in the upside of Figurel-B. The ESR signal can be obviously enhanced by tetracyanoethylene (TCNE) that acts as an inhibitor of superoxide and inhibited by dismutase (SOD). on 0 2 . formation Another observation on the reduction of cytocrome c upon the irradiation of PS particles further supported the ESR evidence. Similarly, decreasing in the rate of Cyt c reduction induced by SOD and its increasing by TCNE could be also observable. When using other new phosphorylated spin traps, such as pyrroline-N-oxide (DEPMPO) and 5-diethoxy
4
o o - l . , 5
.
, 10
, 15
.
,
.
25
Illumination Time(min)
Figure 1 Production of of Cyt c(B).
02-'
in PSII indicated by spin trapping-ESR(A) and reduction
432
Tablel. ESR hyperfine splitting constants for spin adducts of DMPO, DEPMPO, DEPDMPO and DEPPEPO from the photo-irradiated PS 11particles spin T~~~~
DMPO
Oxidative Radicals
aN (mT)
ESR hyperfine splitting constants aH'? (mT) aHY (mT)
a~(mT)
1.53
1.53
1.43
1.17
0.125
____ ____
'OH
1.40
I .30
0.027(3H)
4.74
02.'
1.32
1.03
0.09(1 H), 0.043(6H)
4.85
'OH
1.46
___.
0;'
1.34
.__.
'OH
1.40
1.37
0;
1.36
1.14
'OH
_.__
-i
DEPMPO
DEPDMPO
DEPPEPO
____ ____ ____ ____
4.68 5.05 5.11 5.28
(DEPPEP0)[29] to trap ROS in PSII, the signals were more persists comparing with DMPO, and when using (DEPDMP0)[30] the ESR picture became more simplified. ESR parameters of the corresponding phosphorylated spin adducts were listed in Table 1.
3.2. Oxygen and Oz-' Formation There are two possibilities for the 02-' generation inside PSII. One pathway is the oxidation of water, which only occurred in donor side of the photoinduced primary ET process; another is the reduction of oxygen that can happen in the acceptor side of the ET chain. Correlation between concentration of superoxide spin adduct and oxygen components in PSII particle of spinach under strong illumination was illustrated in Figure 2-A, which clearly indicated that the 0 2 - ' was generated from the consumption of oxygen The further observation in Figure2-B indicated that the generation of superoxide was
Figure 2. Oxygen and Formation. A, Reaction system: PSWDEPMPO. the samples were saturated by pure oxygen (I), by air(II) and by high purity Ar(1II) respectively; B, Reaction system: PSII/DEPMPO in H20 buffer(I), in D20 buffer(1I) and in H20 buffer but containing histidine.
433
Figure 4. ESR evidence of the photo-initiated 0; generation in the UQOreconstituted PSII reaction center. a. PSII RC/DEPMPO in aerobic; b. U Q n S I I RC/ DEPMPO in anaerobic; c. UQdPSII RC/DEPMPO in aerobic.
Figure 3. Inhibiting effects by TEMED and DCMU on the electron transfer (ET) activity and OF formation.
increased under D20 environment that prolonged the half-life of singlet oxygen and was decreased when histidine existed as a scavenger of singlet oxygen. It is highly possible that the superoxide generated in phototsystem particle originates from singlet oxygen.
3.3 Quinone and Oz-' Formation The inhibiting effects of two ET inhibitors, TEMED and DCMU, on the formation of have been examined in Figure 3. There was no observable inhibiting effects on superoxide radical formation by TEMED, which might indicate that the formation of 02-' not directly linked with donor side of the ET chain. As contrast, only DCMU, an electron transfer inhibitor between two plastoquinones, QA and QB,could exclude about 50% superoxide production. Therefore, the generation of superoxide could be located at plastoquinone site. Further evidences have been presented with the detection of superoxide formation by the artificial quinone reconstituted PS I1 reaction center preparation. Several lines of ESR spectrum (in Figure 4-c) have indicated that the semiplastoquinone which located at the QA site of PS directly participated in superoxide formation. 3.4. pH Effects on Oz-' Generation As the results shown in Figure 5, superoxide production by PS has been found to be proton dependent, which indicated that the protonation of semiqinone was the key step for the generation of 0; in PS 11fragments.
-
*
100-
Y
.
-*
80-
-2
60-
2
(0-
5
20-
0-
Figure 5 Effects of different pH value on the generation of superoxide in PSII particles. The PSII particles were suspended either in Mes-NaOH (pH6.0, pH6.5) or in Hepes-NaOH (pH7.0, pH7.5, pH8.0) during the mearurement. , 6.0
6.5
7.0
7.5
8.0
434
3.5. Theoretical Calculations on Semiquinone Induced Oz-' Formation and Their Chemical Mimicry Table 2. Ab calculations on the rate constants in reaction 1 and 2 km
Reaction Model
Q
a 1
BQ
2.23~10-l~
2
2.88
1
UQo
C
2 . 2 5 1~0-14
1 . 7 910-14 ~
2.38
1.87
1.58xIO-~~
1.40~10-~3
2
b
18.0
1.54~10-13
16.0
15.9
Reaction models for the theoretical calculation are listed as following
+ 302-~ Q + '02-Q Q H + 302-QH+ QH + IO~-QH+ QH' + 3 0 2 - ~ Q H + 302-Q Q
+
0;
Reaction- 1
+
O$
Reaction-2
+ +
+
0;
Reaction-3
+
02"
Reaction-4
02H
Reaction-5
OzH
Reaction-6
As shown in Table 2 , rate constants of cross ET processes in reactions 1 and 2 were calculated via the rate of the corresponding self-exchange reactions and the equilibrium Table 3. Ab Q
BQ
UQo
calculations on the thermodynamic constants AGO and Keq in reaction 1-6. AGO(kJ/mol) b
Keq b
Reaction Model
a
1 2
209.4
209.2
210.5
2x10-37
42.34
43.30
45.56
3.8~10-8 2.6~10-8 1.6~10-8
3
783.6
766.2
766.1
4 5
616.7
609.6
609.4
42.59
41.25
39.50
3.4~10~8 6.0~10-8 1.2~10-8
6
-124.4
-124.6
-126.4
6.1~102'
1.4~1022 6.8~1022
1
203.3
202.7
202.8
2x10-36
3x10-36
2
36.28
36.86
36.90
4.4~10-7
3.5~10-7 3.5~10-7
3 4
696.6
697.1
687.6
529.65
521.1
521.7
56.57
55.19
50.79
-110.4
-110.7
-115.2
5 6 h:
C
a
1x10-'0
2x10-37
2xIOlO
C
1x1037
3x10-36
1.0~10-9
2.2~1019 2.4~1019 1.5~1020
1 .
A
Photoinhibimn lime(min)
0, incibation time(min)
Figure 6. Damage of water splitting complex induced by photoinhibition(A) and exogenous superoxide radicals. constants Kq according to Marcus theory. We use both benzoqinone and ubiquinone-0 as the model compounds of plastoquinone in PSII of high plants. Obviously, the singlet oxygen can kinetically benefits the reactions of superoxide generation. 0 Data of the thermodynamic calculations on AG and Keq of 6 models are listed Table 3. As the results, we find that only the reaction 6 can be thermodynamically performed for both quinines. Fortunately, The theoretical result on reaction model is totally in accordance with the experimental estimation as described before. The existences of protonated quinone and singlet oxygen are key factors for the generation of superoxide anion radicals. In addition, could also be generated in an irradiated solution composed by TPP, UQO and DEPMPO. The reaction system was an artificial model of theoretical estimation on the formation. Singlet oxygen came from irradiation of TPP and protonated semiquinone from the irradiation of UQ0 in the protonated solvent.
3.6. Photo-damageInduced by ROS The inactivation of water splitting complex in PSII particles can be comparatively obtained by photoinhibition and exogenous superoxide as indicated in A and B of Figure 6. The exogenous superoxide anion radicals came from redox couple of XOROD. To make sense of the other destructive roles of superoxide and its derivatives generated by the illuminated PS RC complex, the pigments photobleaching inside PSII RC complex under photoinhibitory illumination were also monitored (Figure not shown). The results indicated that the existence of ubiquinone could inhibit the formation of singlet oxygen and the hydrogen peroxide or hydroxyl radical might play an important role in the further destruction of chlorophyll in presence of SOD. In contrast, a similar situation in the protein damage of D1 and D2 of PSII RC can be observed by means of SDS-PAGE method. 4. ACKNOWLEDGEMENTS The study has been financially supported in part by the State Key Plan for National Natural Science (G1998010100) and in part by the National Natural Science Foundation of China (No. 39890390 and 39870208)
436
REFERENCE 1. B. Andersson and J. Barber, In Baker N R (ed): Photosynthesis and the Environment, pp.
101, Kluwer Academic Publishers, Netherlands, 1996. 2. J. R. Durrant, L. B. Giorgi, J. Barber, D. R. Klug, G. Porter, Biochim. Biophys. Acta, 1017 (1990) 167. 3. E. Hideg, C. Spetea, I. Vass, Photosynth. Res., 39 (1994) 191. 4. A. Telfer, S. Dhami, D. Phillips, J. Barber, Biochemistry, 33(1994) 14469. 5. I. Setlik, I. Suleyman, S. I. Allakhverdiev, L. Nedbal, E. Setlikova, V. V. Klimov Photosynth. Res., 23 (1990) 39. 6. D. Kirilovsky, J. M. Ducruet, A. L. Etienne, Biochim. Biophys. Acta, 1020 (1990) 87. 7. I. Vass, S. Styring, T. Hundal, A. Koivuniemi, E. M. Aro, B. Andersson, Proc. Natl. Acad. Sci. USA, 89 (1992) 1408. 8. N. P. Mishra and D. F. Ghanotakis, Biochim. Biophys. Acta, 1187 (1994) 296. 9. Ananyev G, Renger G, Wacker U, and Klimov V V, Photosynth. Res., 41 (1994) 327. 10. R. E. Cleland and S. C. Grace, FEBS Letters, 457 (1999) 348. 11. F. Navari-Izzo, C. Pinzino, M. F. Quartacci, C. L. M. Sgherri, Free Radical Res, 3 l(Supp1.) (1999) 3. 12. K. Liu, J. Sun, Y. Liu, Q. Y. Zhang, T. Y. Kuang. Prog. Biochem. Biophys., 28 (2001) 372. 13. B. Barenyi and G. H. Kame, Planta, 163 (1985) 218. 14. M. Miyao, Biochemistry, 33 (1994) 9722. 15. T. Wydzynski, J. Angstrom and T. Vdnngdrd, Biochim. Biophys. Acta, 973 (1989) 23. 16. G. M. Ananyev, T. Wydrzynski, G. Renger, V. V. Klimov, Biochim. Biophys. Acta, 1100 (1992) 303. 17. V. V. Klimov, G. M. Ananyev, 0. M. Zastryzhnaya, T. Wydryzynski, G. Renger, Photosynth. Res., 38 (1993) 409. 18. B. Halliwell and J. M. C. Gutteridge (eds): Free Radicals in Biology and Medicine, pp. 1, Clarendon Press, Oxford, 1985. 19. 8. Hideg and C. Spetea, Biochim. Biophys. Acta, 1186 (1994) 143. 20. T. Kuwabara and N. Murata, Plant Cell Physiol., 23 (1982) 533. 21.0. Nanba and K. Satoh, Proc. Natl. Acad. Sci. USA, 84 (1987) 109. 22. D. J. Chapman, K. Gounaris, J. Barber, Biochim. Biophys. Acta, 933 (1988) 423. 23. M. Miyao, Biochemistry, 33 (1994) 9722. 24. R.A. Marcus and N. Sutin, Biochim. Biophys. Acta, 8 11 (1985) 265. 25. M. Chou, C, Creutz, N, Sutin, J. Am. Chem. SOC.,99 (1977) 5615. 26. M.J. Weaver and E.L. Yee, Inorg. Chem., 19 (1980) 1936. 27. Jr. K. W. Frese, J. Phys. Chem., 85(1981) 3911. 28. J.T. Hupp and M.J. Weaver, Inorg. Chem., 22 (1983) 2557. 29. Y. K. Xu, J. Sun, K. Liu, Z.W. Chen, Y. Liu, Chem. J. Chin. Univ., 23, (2002)in press. 30. Y. K. Xu, Z.W. Chen, J. Sun, K. Liu, Y. Liu, Chem. J. Chin. Univ., 22, (2001) 1732.
EPR in the 21’ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
437
EPR and theoretical investigations of [NiFe] hydrogenase: Insight into the mechanism of biological hydrogen conversion W. Lubitz3b,M. Brechtb, S. Foersterb,M. Steinb,Y. Higuchi‘, T. Buhrked and B. Friedrichd aMax-Planck-Institut
Strahlenchemie, Stiftstr. 34-36,45470 MiilheindRuhr, Germany
bMax-Volmer-Laboratoriumf%rBiophytsikalischeChemie, Technische Universitsit Berlin, Str. d. 17. Juni 135, 10623 Berlin, Germany ‘Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto, Japan dInstitut Biologiehlikrobiologie, Humboldt-Universitatzu Berlin, Chausseestr. 1 17, 10 1 15 Berlin, Germany* Advanced EPR techniques are applied to study the paramagnetic intermediates in the enzymatic cycle of [NiFe] hydrogenase. The g-tensor magnitudes and orientations were obtained for Ni-A/B, Ni-C and Ni-L by experiments performed on hydrogenase single crystals. Pulse ENDOR and ESEEM experiments aided in the determination of electronnuclear hyperfiie coupling constants of the magnetic nuclei in these species. Density functional theory calculations were performed on a large geometry-optimized model cluster for the active center and gave magnetic resonance parameters in good agreement with experimental data. Based on the experimental and theoretical data, the structure of the intermediates has been deduced and a reaction mechanism proposed for this enzyme.
1. INTRODUCTION Hydrogenases catalyze the reversible heterolytic cleavage of molecular hydrogen: H 2 e Hc+ H-. The most commonly found enzymes [l] contain two major subunits with a hetero-dinuclear NiFe center the active site in the large subunit and three iron-sulfur centers electron transfer components in the small subunit. The structure of the active center and its unusual ligation sphere is shown schematically in Figure 1 . The Ni is coordinated by four cysteines, two of them act bridging ligands between Ni and Fe. The six-coordinated Fe, in addition, carries three small inorganic diatomic ligands. Another bridging ligand X (0 or S) is present in the oxidized NiFe cluster and is removed in the reduced state. Although the geometrical structure of the [NiFe] hydrogenase is known from X-ray crystallography for the oxidized [2,3] and reduced forms [4], details of the related electronic structures are still not fully understood *
Supported by Deutsche Forschungsgemeinschq7(Sfb 498, TP
and C2).
43
CYs
-
Figure 1. Schematic structure of the active center of [NiFe] hydrogenase from [2]. 2 CN and 1 CO ligand at the Fe were determined by FTIR [6]. In the oxidized enzyme X is O(S), in the reduced enzyme no bridging ligand is detected in the X-ray structure M.
I)\ Fe ICN CO cys-s / \;/ C N ‘ Cys-S
-Ni -X-
CYs
The [NiFe] hydrogenase cycles through various states in the catalytic process Ni-AA3 -+Ni-Si -+ Ni-C -+ Ni-R
(1)
The isolated’ oxidized forms Ni-A and Ni-B and the reduced form Ni-C are paramagnetic and can thus be studied by EPR techniques. The Ni-C state is believed to be directly involved in the catalytic turnover and to carry the substrate hydrogen [5]. This state is light-sensitive and can be converted reversibly at low temperatures into the paramagnetic Ni-L state which is spectroscopically different from Ni-C. Although much work has been devoted to the investigation of the [NiFe] hydrogenases [5] many open questions remain. These concern the oxidation states of the Ni and Fe in all intermediate steps of the reaction cycle, the electron spin and charge density distribution of the NiFe center, the binding site of the substrate hydrogen and, finally, the detailed mechanism of the hydrogen conversion. In this paper we show a way how such questions can be answered by a combined approach of advanced EPR techniques [7] and density kctional theory (DFT) calculations. 2. MATERIALS AND METHODS Experiments have been performed on the “standard”[NiFe] hydrogenases of Desulfovibrio vulgaris Miyazaki F (DvH) and the regulatory hydrogenase from Ralstonia eutropha (RRH) which exhibit very similar Ni-C and Ni-L EPR characteristics in their reduced states. The isolation and purification of the hydrogenases have been described earlier [8,9]. In single crystals of DvH [3] the Ni-AA3 well the Ni-C and Ni-L states were generated and angular dependent EPR experiments were performed described [lo-121. X-band pulse EPR and ESEEM measurements were done on a Bruker ESP 380 E spectrometer equipped with a Bruker dielectric ring cavity (ESP 380-1052 DLQ-H) an Oxford CF 935 liquid helium cryostat. Pulse ENDOR experiments were carried out on the same instrument by use of a Bruker ESP 360 D-P ENDOR system. For data analysis of samples in frozen solutions and single crystals simulation and fit programs were used that were previously described [13]. DFT calculations were performed by using the ADF program (SCM, Vrije Universiteit, Amsterdam).
439
3. RESULTS AND DISCUSSION 3.1. Determination of g-tensors of Ni-C and Ni-L The determination of the complete g-tensors for the oxidized states Ni-A and Ni-B by Xband EPR spectroscopy in oxidized single crystals of DvH and the assignment to the molecular structure of the complex has been described by us earlier [10,11]. Here we focus on the reduced active state Ni-C and the light-induced paramagnetic state Ni-L of DvH. The X-band EPR spectra obtained in frozen solutions and the related principal values gl, g2 and g3 of the rhombic-g-tensor are shown in Figure 2. g-value 24
2.3
22
2.1
20
19
I
I
I
I
I
I
exp
I
the0
9 -
1
2
2.144
2.097
3 2.010 2.001 9 -
1 2
3
Figure 2. Left: EPR-spectra in frozen solutions of reduced (Ni-C) and illuminated (Ni-L) [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F (DvH). Experimental conditions: X-band 9.6 GHz, T = 50K, microwave power 1 mW, field modulation 100 kHz, modulation amplitude 1 mT. Right: g-tensor principal values of the [NiFe] hydrogenase determined by simulations of the EPR spectra and DFT calculations (see text). These values have been used starting parameters.in the analysis of the angular dependent EPR spectra of these species in DvH single crystals (Figure 3). Both states Ni-C and Ni-L are present in the crystals (space groups P212121, 4 protein molecules/unit cell). A simultaneous fit of the effective gz values of all four sites yielded the g-tensor magnitude and orientation. In a next step, an assignment of the g-tensor axes to a particular site (one of the four possible sites in the unit cell) had to be made. This became possible due to additional structural information derived from ENDOR and ESEEM spectroscopy (see below). In Figure 4 our final assignment of the g-tensor axe$ in the molecule is depicted. Note that in case of Ni-C the g3 axis (smallest g-value) - which is close to points to the empty sixth coordination site of the Ni, whereas the largest gl value is directed to the (open) Ni-Fe bridge. From the g-tensor magnitude and orientation a formal Ni(1II) with a d, ground state seems to be likely. For the Ni-L state significantly different g-values are obtained indicating a different ground state - although the direction of the tensor axes is similar to those of the Ni-C state. The question of the correct formal oxidation states of the paramagnetic Ni centers and the electronic ground states can be solved by a comparison of the data with those obtained from density functional calculations.
-
440 g-value 2.3
2.2
2.1
2.0
I
I
a 1900
1800
1600
1400
1300 1zoo
1100
iaoo
800 700
600
500
.loo 200
100
00
300
320
€3,
340
360
[mTl
Figure 3. Left: Angular dependence of EPR transitions of single crystals of the reduced DvH. Rotation of the sample in an arbitrary plane about an angle perpendicular to the applied magnetic field, Bo. The signal at g = 2.00 results from reduced methyl viologen (for details see reference 12). Experimental conditions: X-band 9.6 GHz, T = 50 K, microwave power 1 mW, field modulation 100 MIz, modulation amplitude 1 mT. Right: Angular dependence of g’ values. The dots represent experimental values derived !&omthe spectra. The curves show the theoretical resonance positions calculated by a simultaneous fit of all four sites. 3.2. DFT calculations of the g-tensors of Ni-C and Ni-L DFT calculations were performed on a large (41 atoms) model cluster of the active site of DvH. Based on the X ray coordinates [2-41 a 111 geometry optimization has been performed with the BP86 functional. For the Ni-C and Ni-L states various oxidation states of the Ni and Fe were investigated, calculations were also performed with different bridging ligands X (Figure 1). For these models the charge and spin density distributions were calculated [14] well the g-tensor magnitudes and orientations using a relativistic Hamiltonian [14-161. The g-tensor principal values of Ni-C could be reproduced well with a Ni(II1) oxidation state and a bridge X = H-. This shows that the bridge in the reduced state is occupied by a hydride which results from the heterolytic splitting of hydrogen by the enzyme. This is not detectable by Xray structure analysis. Best agreement for Ni-L was obtained a formal NiO) state and an empty bridging position. This shows that upon illumination the Ni-C state looses a proton from the bridge which leads to a conversion Ni(II1)H+ Ni(I) In all states the iron remains in a low spin Fe(I1) state which is caused by the three strong inorganic diatomics. The Fe is thus
441
Figure 4. Stereo views of g-tensor orientations of Ni-C and Ni-L derived from analysis of the single crystal EPR-spectra of the reduced (Ni-C) and illuminated (Ni-L) enzyme (top). Bottom: g-tensor orientation of the respective states obtained from DFT calculations on geometry-optimized structures. not redox active and seems to be only indirectly involved in the catalytic process by providing a coordination site for the bridging ligand. The calculated g-tensor principal values of Ni-C and NI-L are given in Figure 2 (table), the g-tensor orientations are shown in Figure 4; they compare well with the experimental ones. A population analysis of the DFT calculations supports a predominant d, ground state for Ni-C, whereas for Ni-L a larger contribution of a dx2-p state is obtained. In Ni-L 75% of the spin is localized at the nickel, whereas in Ni-C it is 50%. The remaining spin density is distributed over the sulfurs of the cysteine ligands with the largest part on one of the sulfur bridges (see Figure 5, bottom). There is virtually no spin at the iron.
-
-
Further corroborationof the proposed structural model of the Ni-C and Ni-L states, which is deduced here from the g-tensor magnitudes and orientations, can be obtained from experiments that are able to directly address the interaction of the electron spin with the magnetic nuclei. Consequently, a series of ENDOR and ESEEM measurements were carried out on the Ni-C and Ni-L states. Some of the experiments are briefly described in the following section.
3.3. ENDOR spectroscopy of Ni-C and Ni-L The following ENDOR studies were done on the Ni-C and Ni-L signals of the RH of eutropha (RRH) since this enzyme shows no spin-spin coupling between the Ni and the proximal 4Fe4S center observed in DvH [17]. Figure5 shows a comparison of the orientation-selected 'H ENDOR spectra obtained at the smallest g value (g3) of RRH for both states. The spectra indicate a significant change of the hyperfine couplings (hfcs) of all protons. The assignment indicated by dotted lines is the result of an extensive orientationselection ENDOR study (not shown) that aimed to measure all the major 'H hfcs of both states [17]. The measured and assigned hfcs yield information about the spin density
442
Figure 5. Top: 'H Pulse-ENDOR spectra of Ni-C and Ni-L recorded at g3, the special ENDOR-pulse sequence [7] used is also shown (top). Exp. cond.: 7tm = 96 ns, ntr = 16 ns, = 8 ps, = 400 ns, rep. rate 100 Hz. Bottom: Distribution of unpaired spin density obtained from DFT calculations. Contour plots at a value of 0.003 e/(a.~)~.Table: total atomic spin populations obtained from a Mulliken analysis.
lMHzl Figure 6. High frequency part of 'H Pulse-ENDOR spectra of Ni-C in H2O (H2 activation), upper trace, and DzO (Dz activation) lower trace. Both spectra were recorded at g3. The shaded region in the upper spectrum indicates the contribution of the "exchangeable" proton to the spectrum.
443
distribution in the cluster. Clearly, the extent of spin delocalization onto the cysteines is smaller in Ni-L than in Ni-C. Figure 5, bottom, also shows the spin density distribution for both states calculated by DFT methods. A direct detection of the substrate hydrogen (hydride) in the bridge of Ni-C should be possible by replacing this nucleus with deuterium. Consequently, the oxidized enzyme in D20 buffer has been activated with D2. In Fig. 6 the 'H ENDOR spectrum of this sample is compared with the one activated with H2 in H20. The broad resonance between 20 and 21 MHz is reduced for DdD20 showing that at least one strongly coupled hydrogen has been exchanged (hfc = 11 MHz at g3). An analysis of further orientation-selected ENDOR experiments at different field positions suggested an anisotropic hfc tensor of (+20, -6, -14) MHz. Note that the DFT calculation yields (+19, -7.5, -1 1.5) MHz for the bridging hydrogen [18]. The exchanged hydrogen in Ni-C should also be detectable in the 'H ENDOR spectrum. However, such experiments are difficult to perform at X-band frequencies owing to the small 'H Larmor frequency and the reduced hfcs. An alternative approach to the problem is to perform ESEEM experiments, as shown below. 3.4. ESEEM spectroscopy of Ni-C and Ni-L In Figure7 a 4-Pulse ESEEM experiment (HYSCORE) [7] on RRH activated by D2 in D20 is shown, taken at the g3 edge of the EPR spectrum. In the Ni-C state a 'H hfc of = 1.7
Figure 7. Left: HYSCORE (4-Pulse ESEEM) spectra of Ni-C and Ni-L recorded at g3 in the 2H regime show the removal of the hfc of the "exchangeable"deuteron from the bridging position after illumination. The 'H Larmor frequency (- 2.2 MHz) is indicated by arrows. Experimental conditions: x = 16, z = 120 rep. rate 100 Hz, AT1 = AT2 = 24 ns. Right: Structural model for the conversion f'rom Ni-C to Ni-L.
444
\ proton transfer channel Figure 8. Model for the heterolytic splitting of Hz by [NiFe] hydrogenase. The activation of the oxidized enzyme is believed to remove the (oxygenic) bridging ligand via a protonation step (shown here as water) [181. MHz is clearly visible which corresponds to the exchangeable 'H hfcs of 11 MHz observed by 'H ENDOR along this g direction (Figure 6). Upon illumination of the sample at 77 K the 2Hhf splitting is removed indicating that the respective deuteron is photodissociated from the complex as depicted in Figure 7 (right). This shows that it is the single exchangeable hydrogen in Ni-C that is photolabile and lost upon conversion to Ni-L. It is interesting to note that in Ni-L a 2H hf coupling can still be detected but it is about one order of magnitude smaller than in Ni-C. Annealing of the Ni-L sample at 200 K in the dark fully restores the Ni-C signal showing that the process is reversible. Orientation-selected 2HESEEM and 'HENDOR experiments finally allowed the determination of the full hf tensor of the bridging photolabile hydrogenic species [17]. In agreement with the EPR and DFT results the hydride is located in the bridge between Ni and Fe with a distance of 1.8 +O.l to both metals which agrees with the DFT calculations.
-
4. CONCLUSIONS In the turnover of the [NiFe] hydrogenase, the H2 entering the oxidized enzyme is obviously heterolytically cleaved. H- remains in the complex whereas leaves via a proton transfer channel (see Figure 8). The EPR, ENDOR and ESEEM experiments described here together with DFT calculations showed that the reduced Ni-C state is carrying the hydride (H-) in the bridging position between Ni and Fe. Ni-C can be formally described by a Ni(II1) species with a dZ , ground state. The bridging hydrogen species is lost upon illumination and transfered to a nearby proton acceptor. This leaves the complex in a formal Ni(1) oxidation state with an empty bridge (Ni-L). This process is reversible. Based on the structure of these intermediates, insight into the reaction cycle of the [NiFe] hydrogenase is obtained [ 15-181.
REFERENCES 1. P.M. Vignais, B. Billoud, J. Meyer, Microbiol. Rev. FEMS, 25 (2001) 455.
2. A. Volbeda, M.H. Charon, C. Piras, E.C. Hatchikian, M. Frey, J.C. Fontecilla-Camps, Nature, 373 (1995) 580. 3. Y. Higuchi, T. Yagi,N. Yasuoka, Structure, 5 (1997) 1671. 4. Y. Higuchi, H. Ogata, K. Miki, N. Yasuoka, T. Yagi, Struct. Fold. Des., 5 (1999) 549. 5. R. Cammack, R.L. Robson, M. Frey (eds.), "Hydrogen as a Fuel", Taylor and Francis, London, 2001. 6. R.P. Happe, W. Roseboom, A.J.Pierik, S.P.J. Albracht, K.A. Bagley,Nature, 385 (1997) 126. 7. A. Schweiger, G. Jeschke, "Principles of Pulse Electron Paramagnetic Resonance", Oxford University Press, 2001. 8. T. Yagi, K. Kimura, H. Daidoji, F. Sakai, S. Tamura, H. Inokuchi, J. Biochem. (Tokyo), 79 (1976) 661. 9. A.J. Pierik, M. Schmelz, 0. Lenz, B. Friedrich, S.P.J. Albracht, FEBS Lett., 438 (1998) 231. 10. C. GeBner, 0. Trofanchuk, K. Kawagoe, Y. Higuchi, N. Yasuoka, W. Lubitz, Chem. Phys. Lett., 256 (1996) 518. 11.0. Trofanchuk, M. Stein, C. GeBner, F. Lendzian, Y. Higuchi, W. Lubitz, J. Biol. Inorg. Chem., 5 (2000) 5 . 12. S. Foerster, M. Brecht, M. Stein, Y. Higuchi, W. Lubitz, in preparation. 13. C.GeBner, PhD thesis, Technische Universitlit Berlin, 1996. 14. M. Stein, W. Lubitz, Phys. Chem. Chem. Phys. 3 (2001) 2668. 15. M. Stein, E. van Lenthe, E.J. Baerends, W. Lubitz, J. Am. Chem. SOC.123 (2001) 5839. 16. M. Stein, PhD thesis, Technische Universitlit Berlin, 2001. 17. M. Brecht, PhD thesis, Technische Universitlit Berlin, 2001. 18. M. Stein, W. Lubitz, Phys. Chem. Chem. Phys., 23 (2001) 51 15.
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EPR in the 21RCentury A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
EPR studies on free radical generation by the reaction of methylglyoxal with amino acids and protein Hyung-Soon Yim", Cheolju Leea, P. Boon Chockb, Moon B.
and Sa-Ouk Kanga
aLaboratory of Biophysics, School of Biological Sciences, and Institute of Microbiology, Seoul National University, Seoul 151-742, Korea bLaboratory of Biochemistry, NHLBI, National Institutes of Health, Bethesda, Maryland 20892, U. S. A.
We studied the reaction between a three-carbon a-dicarbonyl compound, methylglyoxal, and amino acids. This reaction generated yellow fluorescent products as formed in some glycated proteins. In addition, three types of free radical species were also produced, and their structures were determined by EPR spectroscopy. These free radicals are the cross-linked radical cation and the methylglyoxal radical anion as the counterion. Time course studies suggest that the cross-linked radical cation is a precursor of yellow fluorescent glycation end products. Glycation of bovine serum albumin by methylglyoxal generated the protein-bound free radical, probably the cation radical of the cross-linked Schiff base in the reaction of methylglyoxal with N"-acetyl-L-lysine. The glycated bovine serum albumin showed increased electrophoretic mobility suggesting that the basic residues, such as lysine, were modified by methylglyoxal. The glycated protein catalyzed the oxidation of ascorbate in the presence of oxygen, whereas the protein free radical signal disappeared. These results indicate that glycation of protein generates active centers for catalyzing one-electron oxidation-reduction reactions. This active center, which exhibits enzyme-like characteristic, was suggested to be the cross-linked S c M b a d t h e cross-linked Schiff base radical cation of the protein.
1. INTRODUCTION Glycation reaction (nonenzymatic glycosylation; Maillard reaction), which produces brown fluorescent compounds, is a chance event that may occur when a protein is in solution with a reducing sugar, such as glucose. In this reaction, free amino groups of protein react slowly with the carbonyl groups of reducing sugars to yield Schiff-base intermediates, which undergo Amadori rearrangement to stable ketoamine derivatives. These Amadori products subsequently degrade into a-dicarbonyl compounds, deoxyglucosones. Schiff bases can also be fragmented to glyoxal). These compounds are more reactive than the parent sugars with respect to their ability to react with amino groups of proteins. Thus, the a-dicarbonyl compounds or a-ketoaldehydes are mainly responsible for forming inter- and intramolecular cross-links of proteins, known as advanced glycation end products (AGES)' The AGES, which are irreversibly formed, accumulate with aging, atherosclerosis, and diabetes mellitus,
447
especially associated with long-lived proteins such as collagens, lens crystallins, and nerve proteins (4-8). The a-dicarbonyl compounds are produced in a variety of ways. Fenton reaction-mediated oxidation of sugars, lipids, and proteins produces various a-dicarbonyl compounds. Accordingly, the transition metal ion-catalyzed oxidation of glucose is suggested to be a more important factor in glycation than the formation of the Amadori product of glucose itself (911). The a-ketoaldehydes, such as methylglyoxal, are also found as a normal metabolite in mammals and microorganisms. The methylglyoxal is formed by the non-enzymatic or enzymatic elimination of phosphate from triose phosphate and by the oxidation of hydroxyacetone and aminoacetone (12-14). The increased formation of methylglyoxal was observed in hyperglycemia associated with diabetes mellitus (15, 16). In addition, it was shown that methylglyoxal-modified albumin underwent receptor-mediated endocytosis by macrophage, which may suggest the involvement of methylglyoxal in pathophysiology (17). it WAS suggested that cellular oxidant stress or free radicals are generated by AGEs themselves (18, 19) or as a consequence of the AGEs interactionwith their receptors (20,21). Several AGEs were identified from the products formed during the reaction of methylglyoxal with model compounds and proteins. These species include N"(carboxyethy1)lysine (22), imidazolone compounds (23), and imidazolium cross-link species, methylglyoxal-lysine dimer (24-26). In addition to these AGEs, several investigations have also shown by electron paramagnetic resonance (EPR) spectroscopy that unidentified protein free radicals were produced during the reaction of methylglyoxal with proteins, such as bovine serum albumin (BSA) and casein (27,28). In this report, the free radical is assigned to be the radical cation of the cross-linked Schiff base on the basis of the detailed analysis of EPR spectra observed from the reaction mixture containing methylglyoxal and alanine. We also suggest that the cation radical sites in the cross-linked proteins could serve as reactive centers for one-electron oxidation-reduction with appropriate substrates. These reactions will produce free radicals for a long duration and contribute to accelerating oxidative modification of the biological macromolecules (29). And we use methylglyoxal-modified bovine serum albumin (MG-BSA) as a model protein to study the free radical generating properties of glycated proteins and their capacity to catalyze oxidative modifications of macromolecules (30).
2. EXPERIMENTAL PROCEDURES 2.1. Materials Methylglyoxal, diethylenetriaminepentaacetic acid (DTPA), N"-acetyl-L-lysine, N"-acetylL-arginine, BSA, Cu,Zn-superoxide dismutase from bovine erythrocyte, catalase from bovine liver, and cytochrome c from bovine heart were obtained from Sigma. Cu,Zn-superoxide dismutase from bovine erythrocytes was obtained from Boehringer Mannheim, nitro blue tetrazolium (NBT) was from Calbiochem, and Chelex 100 resin (sodium form) was from BioRad. Stable isotope-enriched alanines ("N-, l-13C-, 2-13C-, and 3-13C-labeled), D4-alanines, and D2O were purchased from Cambridge Isotopes. Ascorbic acid was purchased from Merck. Commercially supplied BSA was purified by gel filtration chromatography on a column of Superdex 200 HR10/30 (10 x 300 mm; Vo:8.2ml; Amersham Pharmacia Biotech). The eluent was 20 mM sodium phosphate buffer (pH 7.4) containing 0.15 M sodium chloride, and the
448
flow rate was 0.4 ml/min. The eluted sample was desalted and lyophilized to dryness. The catalase was also purified by gel filtration chromatography (GFC) to remove contaminating superoxide dismutase. Commercial catalase dissolved in Tris glycine buffer (pH 8.0) containing 1 mM EDTA and 0.3 M NaCl was fractionated using the same column. The buffers used for the reaction of BSA with methylglyoxal were treated with Chelex 100 resin (BioRad) to remove traces of transition metal ions.
2.2. Modification of BSA with methylglyoxal BSA was reacted with methylglyoxal in 0.1 M phosphate buffer (pH 7.4). Unless otherwise indicated, the concentrations of BSA and methylglyoxal were 20 mg/ml(O.3 mM) and 30 mM, respectively. After modification, samples were repeatedly filtered though PM-10 ultrafiltration membrane (Amicon) using 20 mM phosphate buffer (pH 7.4) and further desalted with Fast Desalting Column H10/10 (Amersham Pharmacia Biotech).
2.3. Characterization of modified protein The effect of methylglyoxal modification on the net charge, adduct formation, and oligomerization of BSA was investigated by polyacrylamide gel electrophoresis (PAGE) with/without urea, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), isoelectric focusing, cation exchange chromatography, and gel filtration chromatography. PAGE was performed on 9% polyacrylamide gel with or without 6 M urea, and SDS-PAGE was on 5 2 0 % linear gradient polyacrylamide gel. Isoelectric focusing was carried out at 4 "C under constant power of 7 watts, using a polyacrylamide gel (5% T, 3% C) containing 2% Bio-Lyte (Bio-Rad), 5% glycerol, and 6 M urea at a pH range between 3.8 and 9.4. GFC was performed using a Superdex 200 HR10/30 on fast protein liquid chromatography system (Amersham Pharmacia Biotech) with 20 mM phosphate buffer (pH 7.4) containing 0.15 M NaCl as eluent. In ion exchange chromatography, a Protein Pak SP 5PW column (Waters) was used on a Waters Delta Prep 4000 system. The mobile phase was 50 mM acetate buffer (pH 4.5) at a flow rate of 5 ml/min. Proteins were eluted isocratically for 15 min, and then the eluent was shifted linearly to 50 mM acetate buffer containing 1 M NaCl for 10 min.
2.4. Degradation of ascorbate by MG-BSA Ascorbate was incubated with MG-BSA in 20 mM phosphate buffer (pH 7.4) at 30 "C. The concentration of the unreacted ascorbate in the reaction mixture was determined by the high performance liquid chromatography. An aliquot of the sample solution was loaded onto an ODS Hypersil column (4.6 x 100 mm) (Hewlett-Packard) in a Waters HPLC 600s equipped with a Hewlett-Packard 1100 UV detector. Mobile phase was 0.04% trifluoroacetic acid, and the eluent was monitored at 254 nm. 2.5. EPR spectroscopy EPR spectra were recorded on a Bruker ESP EPR spectrometer. For some analysis, samples were frozen at 77 K using liquid nitrogen. The operating conditions were as follows: microwave frequency, 9.44 GHz; microwave power, 10 milliwatts; sweep width, 100 G; conversion, 40.96 ms. Modulation amplitude was set to 4.00 G at 77 K or 1.47 G at room temperature.
449
3.
3.1. EPR spectra of the cross-linked free radical cation The first derivative EPR spectrum shown in Figure (upper spectrum) was obtained with the anaerobic reaction mixture of methylglyoxal and natural-abundance L-alanine in carbonate buffer at pH 9.5. A similar spectrum also appeared in phosphate buffer at pH 7.5 with a much slower rate and a weaker signal amplitude. For the assignment of hyperfine coupling constants (hfc) and structural identification of this radical, similar experiments were carried out in various isotope-enriched reaction mixtures. The upper spectrum in Figure lB, obtained from the reaction mixture prepared with ~-[N]alanine,exhibits a different hyperfine splitting pattern compared with the spectrum in Fig. This alteration is entirely caused by changes of nitrogen nuclear spins, N (I = 1/2) in place of 14N (I = l), and their nuclear moments, p(1sN)/p('4N) = 1.40. With this information, the experimental spectra in Fig. 1 (A and B, lower spectra) were simulated. MO+Ala
I
3440
,
,
,
I
,
3480
.
.
I
Figure 1. EPR spectra obtained from the reaction mixture containing methylglyoxal (0.2 M) and various isotope-enriched L-alanines (0.2 M) in carbonate buffer (0.5 M) at pH 9.5 (upper spectra) and simulated spectra (lower spectra). A, methylglyoxal and naturalabundance L-alanine; B, methylglyoxal and L[lSN]alanine;C, methylglyoxal and ~-[2-'~C]alanine.
,
3520
Gauss
From the spectrum in Fig. 1C and the spectra obtained with the reaction mixture containing L-[2-13C]alanine,~-[l-'~C]alanine,and ~-[3-'~C]alanine in place of natural-abundance alanine (29), respectively, they exhibited extra hyperfine interactions due to C (I = 1/2) nuclei. Although we have obtained a large number of hfc constants from this cross-linked radical, exact structural assignment for this radical is difficult because of asymmetry in hfc constants. Two equivocal structures are shown in Figure 2 (structures a-I and a-2). In the case of structure a-1, the asymmetric nature of the spin distribution may be caused by the effect of the methyl group of methylglyoxal on the singly occupied molecular orbital, most likely the orbital, which includes two N=C bonds. On the basis of experimental hfc constants, structure a-1' is assigned to the conformations of two alkyl groups with respect to the p-orbitals of the two nitrogen atoms. In this conformation, carboxyl, and methyl carbons will have one small and one large I3C hfc constant in each carbon group because of the different dihedral angles to the p-orbitals of the nitrogen atoms (one carbon is close to I% and the other is
450
closer to 0') and the cos20 dependence of these hfc constants. The assignment of C hfc constants to individual carbons will be as follows: for the N-1 side, 8.52 G for 1-C, 4.10 G for 2-C, and 0.3 G for 3-C, and for the N-2 side, 0.3 G for 1-C, 0.2 G for 2-C, and 3.0 G for 3-C. This assignment gives a ratio of 3.7 for the total carbon spin densities between the N-1 and N2 sides. This value is closest, among several possible assignments, to the value of 3.2, the ratio of spin densities on N-1 and N-2 atoms (NA). The other possible structure of this radical is shown in Figure 2 (structure a-2). A protonation of a nitrogen in the cross-linked Schiff base will produce a triene-type compound, which may lose an electron to form the crosslinked radical cation. In this structure, the observed large C hfc constants will originate entirely from one alanine molecule in the cross-linked radical. In addition, we expect to detect two sets of methyl hydrogen hfc constants if the radical has this structure, in contrast to the experimental observation of only one set of methyl hydrogen hfc constants. It may be possible, however, that AH(3) of one set is smaller than the line width, which may arise from the canceling effects of spin delocalization (hyperconjugation) and spin polarization in the spin transfer to the s-orbitals of the methyl hydrogens from the delocalized x-center. Although we prefer structure a-1 as the structure of the cross-linked radical, structure a-2 cannot be ruled out at this time. It is certain, however, that the radical formed due to the cross-linking reaction contains two amino acids and one methylglyoxal. To find whether the Schiff base is the precursor of this radical, the base was reduced with NaCNBH3, which is known to reduce Schiff bases selectively and to inhibit the subsequent reactions. When NaCNBH3 (1.0 M) was added to the reaction mixture, the EPR signal of the cross-linked radical and the yellow color were not detected. The effect of NaCNBH3 may indicate that methylglyoxal dialkylimine, -02C(CH3)is the intermediate for the formation of this cross-linked radical. nac
H\oC/C"
-%c (rpHc N-'\N
CWR)
H
-i;$a-(dNp n' (+')
a21
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'
Figure 2. Chemical structures of the cross-linked radical cation and the radical anion observed in this reaction and identified by EPR.
3.2. Observation of the enediol radical anion of methylglyoxal The EPR parameters obtained from the EPR spectra using isotope-enriched 2,3,3,3-D4alanine are identical to those of the cis-form of the methylglyoxal radical anion (see Figure 2, structure b) (29). This result suggests that the cross-linked radical observed in this reaction is a cation formed with the methylglyoxal radical anion being the radical counterion.
3.3. Modification of BSA by methylglyoxal BSA was incubated with methylglyoxal in 0.1 M Chelex-treated phosphate buffer (pH 7.4) at 37 "C for 5 days. Further incubation had little effect on the modification of BSA as indicated by PAGE analysis. The native PAGE of the reaction products (30) demonstrated that the electrophoretic mobility of MG-BSA increases with increasing concentration of
45 1
methylglyoxal used for the glycation. This result indicates the progressive loss of the positive charge in the MG-BSA during the glycation reaction. When a high concentration of methylglyoxal (30 mM or higher) was used for glycation, additional bands appeared with lower electrophoretic mobility. The species with lower mobility may represent the higher aggregates of MG-BSA. At 600 mM methylglyoxal, the aggregate formed precipitates (30). The result obtained from the denaturing PAGE (6 M urea; data not shown) also supports this conclusion. The identical results were also obtained with the reaction products prepared anaerobically. Isoelectric focusing revealed similar results, suggesting that glycation of BSA by methylglyoxal introduced net negative charges into MG-BSA adducts, although at a higher concentration of methylglyoxal (30 mM or higher) some species possessing net positive charges were also formed (data not shown). In accordance with the above observation, the SDS-PAGE data (30) indicate the presence of additional protein bands for samples obtained with higher concentration of methylglyoxal at the positions corresponding to the molecular mass of the dimer and tetramer of BSA. Chromatographic analysis also gave similar results. Cation exchange chromatography showed that, as BSA incubation with methylglyoxal proceeded, its net charge becomes more negative. Gel filtration chromatographic analysis of MG-BSA gave peaks that correspond to the dimer and tetramer. The basic lysines and arginines are known to be the residues modified and responsible for forming Schiff bases for intra- and intermolecular cross-links during the glycation reaction. Thus, the net charges of MG-BSA adducts may be shifted to more negative values. Previous investigations of the reaction of methylglyoxal with protein suggested that, in addition to the modification of protein, the treatment of methylglyoxal fragmented the protein (17). In contrast, we did not detect any protein fragments during the modification of BSA by methylglyoxal. The difference may have arisen from the fact that we treated all the solutions with Chelexresin to remove traces of transition metal ions, such as ironand copper, known to cleave protein in the presence of oxygen. Thus, the characteristics of the glycated protein observed in this study are caused mostly by the covalent binding of the methylglyoxal with few effects from protein fragmentation or reaction products of the fragments. 3.4. Cross-linking residues and free radical formation We examined the residues responsible for cross-linking by investigating various substituted amino acids for their ability to undergo the glycationreaction and form the crosslinked radical cation. The EPR spectrum was obtained from the reaction mixture containing 0.2 M N"-acetyl-Llysine and 0.2 M methylglyoxal in carbonate buffer at pH 9.5 (30). This spectrum was compared with the obtained from alanine-methylglyoxal system, which was identified as the radical cation of the cross-linked Schiff base, the methylglyoxal dialkylimine radical cation (29). Thus, &-amino groups of two N"-acetyl-L-lysines form a cross-linked Schiff base with a methylglyoxal, which lose one electron to form the cross-linked radical cation. With other N"-blocked amino acids, including N"-acetyl-L-arginine, no detectable EPR signal was observed (30). This result is in agreement with previous reports showing that hP-acetyl-L-arginine did not produce a Schiff base (23). Methylglyoxal is also known to react with arginine and cysteine residues in proteins, yielding stable imidazolone and hemithioacetal compounds, respectively (23). Several investigators have shown, however, that lysine residues are the main cross-linking sites in MG-treated proteins by forming imidazolium cross-links between two lysines (24-26). These observations together with our results showing that the cross-linked radical cations were
452
detected only with N"-acetyl-L-lysines suggest that the lysine residues are likely the main cross-linking sites for forming protein free radicals in MG-BSA adducts.
3.5. Effect of reducing ascorbate on the protein free radical The effect of the reducing agent ascorbate on the protein free radical of MG-BSA (30). Addition of ascorbate to MG-BSA, which originally displayed spectrum A, quenched the EPR signal as shown in spectrum B. This result indicates that ascorbate reduces the protein-radical cation of MG-BSA to the non-radical species. This reduction of radical cation was accompanied by the degradation of ascorbate (30). Under anaerobic conditions, a small portion of incubated ascorbate was consumed, which reached a plateau with time when 0.1 mM ascorbate was consumed. The controlled experiments showed that unmodified BSA failed to degrade ascorbate, and metal chelator, DTPA, had no inhibitory effect on the ascorbate degradation. The latter experiment indicates that adventitious metal ions were not the cause of the observed ascorbate consumption. These results suggest that the ascorbate was directly oxidized by the protein free radical cation in MG-BSA according to Reaction 2, and the concentration of the protein free radical cation in MG-BSA used in this experiment is approximately 0.1 mM. In the presence of oxygen, however, the oxidation of ascorbate continued to reach far beyond 1 molar ratio of the degraded ascorbate to MG-BSA (30). The degradation of ascorbate increased with increasing MG-BSA concentration. The initial rates obtained from these degradation data showed that the oxidation of ascorbate proceeded linearly with respect to the MG-BSA concentration. The straight line obtained from the concentration-dependent plot, however, intercepts at 0.02 mM/h, indicating that the initial rate observed for the ascorbate degradation contained a small MG-BSA independent term. The oxidation of ascorbate as a function of time obtained by using various concentrations of ascorbate and a fixed concentration of MG-BSA (7 mg/ml; 0.1 mM BSA) (30). The initial rates of the ascorbate oxidation determined from the degradation curves increased as a saturation function with respect to the ascorbate concentration. The double-reciprocal plot of the initial rates yielded a K,of 1 mM for ascorbate and a k,, of 3.3h if the MG-BSA used contained0.l mM protein free radical cation (30). Furthermore, superoxide dismutase, but not catalase, exerts partial inhibition on the degradation of ascorbate (approximately 20% inhibition by 0.4 pM Cu, Zn-superoxide dismutase in the reaction between 0.4 mM ascorbate and 2 mg/ml MG-BSA at pH 7.4). The fact that this inhibition is only partial indicates that 0 2 , but not is directly involved in this catalytic reaction. The partial inhibition, however, suggests that superoxide radical anions are produced during this reaction, and they play a role in ascorbate degradation, probably via the superoxide-scavenging reaction by ascorbate. Together these results indicate that MG-BSA behaves as an enzyme, which has an ability to catalyze the oxidationof ascorbate in the presence of oxygen to produce superoxide radical anion and semi-dehydroascorbate radical. This reaction is initiated by the protein-radical cation of MG-BSA. 4. DISCUSSION
Free amino groups in protein react with the carbonyl groups of reducing sugars or ketoaldehyde, which has been implicated as the onset of glycation. Previous investigations have shown that free radicals were produced in the reaction of methylglyoxal with proteins
(27, 28). We also detected protein free radical from the reaction between methylglyoxal and BSA. The results obtained in this study are summarized in the reaction scheme shown in Figure 3. The structure of the free radical of the MG-BSA is most likely to be the radical cation of the cross-linked Schiff base (Figure 3, species 0)on the basis of our previous results obtained with alanine a model system (29). When methylglyoxal was reacted with various N"-acetyl substituted amino acids, the free radical signal was observed only with fl-acetyl-L lysine. In addition, previous investigations have shown that lysine residues are the main crosslinking sites in MG-treated proteins (24-26). These results together suggest that methylglyoxal cross-links inter- or intramolecular lysine residues of the protein to form cross-linked Schiff bases (Figure 3, species A and B). This cross-linked Schiff base of MG-BSA can donate an electron to methylglyoxal to produce the radical cation of the cross-linked Schiff base (29). During these processes, the Schiff bases or the protein free radicals may be oxidized to form N"-(carboxyethy1)lysine (22) or matured to imidazolium cross-links (24-26) and other products such as imidazolysine (23). In the presence of electron-donating ascorbate, however, the radical cation of the crosslinked Schiff base accepts electron from ascorbate to produce the cross-linked Schiff base and semi-dehydroascorbate radical (see Figure 9) (30). These reactions can proceed even in the absence of oxygen. Moreover, in the presence of oxygen, MG-BSA behaves as an enzyme, which is capable of catalyzing the oxidation of ascorbate (K, = 1mM). We do not know at this time the exact mechanism for this catalytic reaction. However, the reaction catalyzed by MG-BSA in the presence of oxygen is similar to the transition metal ion-catalyzed oxidation of ascorbate. In a transition metal ion-catalyzed oxidation system such as Fe3+/ascorbate/Oz, the reaction is initiated via the one-electron reduction of Fe3+by ascorbate, whereas in the MG-BSA/ascorbate/Oz system, it is initiated via the one-electron reduction of the protein free radical cation by ascorbate. In both reactions, superoxide radical anions are generated by the oxidation of ascorbate.
-
CEL+
q o
- -"* - --
I+N
NHz
I+N
+
H/Gc,
1
+ N p NH2
0
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;C-C
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intermolecular
.-.eSr/yKF
HzN
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Imdazolium
;C-<
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B
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++ ~oSS4,"ks
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02 MG
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Figure 3. Proposed reaction scheme for glycation of protein by methylglyoxal and reaction of glycat ed protein. CEL, N"- (carboxyethy1)lysine.
454
The AGES formed are heterogeneous species, which may have a variety of chemical structures. There is no direct evidence so far that the MG-BSA radical cation or similar radical species are formed We showed, however, in this model study that one type of reactive structure (a cross-linked Schiff base/the radical cation of the Schiff base) has the enzyme-like characteristic for catalyzing the one-electron oxidation-reduction reaction yielding free radicals including superoxide radical anions. Similar types of reactive centers, if formed may exert significant effects on their biological environment by generating free radicals for a long duration.
REFERENCES 1. T.M. Reynolds, Adv. Food Res., 12 (1963) 1. 2. H. Kato, D.B. Shin and F. Hayase, Agric. Biol. Chem., 51 (1987) 2009. 3. RJ. Wells-Knecht, D.V. Zuzak, J.E. Litchfield, S.R. Thorpe and J.W. Baynes, Biochemistry, 34 (1995) 3702. 4. E. Gordillo, A. Ayala, J. Bautista and A. Machado, J. Biol. Chem., 264 (1989) 17024. 5. V.M. Monnier, R.R. Kohn and A. Cerami, Proc. Natl. Acad. Sci. U. S. A., 81 (1984) 583. 6. A. Elgawish, M. Glomb, M. Freidlander and V.M. Monnier, J. Biol. Chem., 271 (1996) 12964. 7. V.M. Monnier and A. Cerami, Science, 211 (1983) 491. 8. H. Vlassara, M. Brownlee and A. Cerami, Diabetes, 32 (1983) 670. 9. S.P. Wolffand R.T. Dean, Biochem. J., 249 (1988) 618. 10. S.P. Wolff and R.T. Dean, Biochem. J., 245 (1987) 243. 11. Z.Y. Jiang, A.C.S. Woollard and S.P. Wolff, FEBS Lett., 268 (1990) 69. 12. Y. Inoue and A. Kimura, Adv. Microbiol. Physiol., 37 (1995) 177. 13. P.J. Thornalley, S. Wolff, J. Crabbe and A. Stern, Biochim. Biophys. Acta, 797 (1984) 276. 14. S.A. Phillips and P.J. Thornalley, Eur. J. Biochem., 212 (1993) 101. 15. S.A. Phillips, D. Mirrlees and P.J. Thornalley, Biochem. Pharmacol., 46 (1993) 805. 16. A.C. McLellan, P.J. Thornalley, J. Benn and P.H. Sonksen, Clin. Sci. (London), 87 (1994) 21. 17. M.E. Westwood, A.C. McLellan and P.J. Thornalley, J. Biol. Chem., 269 (1994) 32293. 18. C.J. Mullarkey, D. Edelstein and M. Brownlee, Biochem. Biophys. Res. Commun., 173 (1990) 932. 19. T. Sakurai and S. Tsuchiya, FEBS Lett., 236 (1988) 406. Yan, A.M. Schmidt, G.M. Anderson, J. Zhang, J. Brett, Y.S. Zou, D. Pinsky and D. 20. Stern, J. Biol. Chem., 269 (1994) 9889. 21. H.M. Lander, J.M. Tauras, J.S. Ogiste, 0. Hori, R.A. Moss and A.M. Schmidt, J. Biol. Chem., 272 (1997) 17810. 22. M.U. Ahmed, E. Brinkmann Frye, T.P. Degenhardt, S.R. Thorpe and J.W. Baynes, Biochem. J., 324 (1997) 565. 23. T.W.C. Lo, M.E. Westwood, A.C. McLellan, T. Selwood and P.J. Thornalley, J. Biol. Chem., 269 (1994) 32299. 24. E. Brinkmann, K.J. Wells-Knecht, S.R. Thorpe and J.W. Baynes, J. Chem. SOC.Perkin Trans. 11, (1995) 1.
25. K.J. Wells-Knecht, E. Brinkmann, M.C. Wells-Knecht, J.E. Litchfield, M.U. Ahmed, S . Reddy, D.V. Zyzak, S.R. Thorpe and J.W. Baynes, Nephrol. Dial. Transplant. 11, Suppl., 5 (1996) 41. 26. R.H. Nagaraj, I.N. Shipanova and F.M. Faust, J. Biol. Chem., 271 (1996) 19338. 27. P.R.C. Gascoyne, Int. J. Quantum. Chem. Symp., 7 (1980) 93. 28. J.A. McLaughlin, R. Pethig and A. Szent-Gyorgyi, Proc. Natl. Acad. Sci. U. S. A., 77 (1980) 499. 29. H.-S. Yim, S.-0. Kang, Y.C. Hah, P.B. Chock, P. B. and M.B. Yim, J. Biol. Chem., 270 (1995) 28228. 30. C. Lee, M.B. Yim, P. B. Chock, H.-S. Yim and S.-0. Kang, J. Biol. Chem., 273 (1998) 25272.
456
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
EPR monitoring on the quality of life N. D. Yordanov*, G. Petkova, I. Naidenova EPR Laboratory, Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
The advantages of EPR spectrometry in the practical applications directed to the monitoring of the quality of life is demonstrated on the basis of some recent results of selective quantitative estimations of: (i) Soot and polycyclic aromatic hydrocarbons in aerosols of the urban and in-door air as well as in home dust; (ii) Nitrates and nitrites in vegetables, fruits; meat products, milk and cheese.
1. INTRODUCTION
One of the main reasons not to exploit up to now in practical aspects the advantages of EPR spectroscopy - its high selectivity and sensitivity, is the expensive, heavy and sophisticately operating equipment. This situation has been dramatically changed in the last 10 years because completely new generation of small, portable, inexpensive and fully computer controlled EPR spectrometers became commercially available. This turning point in the instrumentation is expected to facilitate the wide applications of EPR spectrometry in different areas of life. In the present paper, we report our recent results obtained in a connected with our life quality demonstrating the advantages of EPR spectrometry in the field of practical applications. *E-mail: [email protected] Or [email protected]
457
2. SELECTIVE ESTIMATIONS OF SOOT AND POLYCYCLIC AROMATIC HYDROCARBONS AEROSOLS OF THE URBAN AND IN DOOR AIR.
The urban aerosols, as typically, contain three components: (i) mineral dust coming from the soil; (ii) soot (elemental carbon, EC) originating from different burning processes but mainly from exhaust gases of diesel engines; (iii) polycyclic aromatic hydrocarbons (PAH) or organic carbon (OC) adsorbed on the surface of soot aerosol particles. The last two components reached a significant level as a result of human activities. The soot and especially PAH are known to be responsible for a number of diseases as cancer, rheumatoid arthritus and the process of aging [l]. Thus the control of their quantity is essencial and several different analytical methods are used (see for example [2]). The applicability of EPR has also been examined [3 - 61 based on the fact that soot is paramagnetic [7]. The EPR spectrum on Figure 1 demonstrates the EPR detectable content of aerosol particles collected on a paper filter by passing a given volume of air through it The broad singlet signal with g = 2.0 and AH = 400 G corresponds to Fe3+and Mn2+ ions present in the mineral dust whereas the narrow sharp singlet line shown in a rectangle in Figure1 corresponds to the soot. Its EPR parameters are g = 2.0028, AH = 12 G. In this way recording the content of air aerosol particles collected on a paper filter gives a direct information about the soot content. Using this procedure we have performed an extended six months experiment on EPR monitoring of the content of soot in the air of Sofia, Bulgaria [4]. The data of two stations, one situated in the central part and the other in a suburb of the town, were used. Whereas the area in which the first station is positioned is characterized by an intensive automotive traffic, the second station is situated in a very silent place. Samples were
I
I
25100
4000
M a g n e t i c field
Figure 1 . Typical EPR spectrum of aerosol particles. T= 300K.
45 8
collected twice a day in the winter period October - March, characterized by a cold weather, sometimes with temperature inversion processes. The results obtained [4], have shown that air pollution in the central part of Sofia is at lower level compared to the suburb. The explanation of this finding is that independently of the intensive traffic, there is a central heating in downtown part, whereas every family house, or flat in the suburb buildings use their own heating facilities working with coal, wood, pertol, etc. It is worth to note that these data, as well as the quantitative estimations peformed on few samples of the air in Athens, Greece, are comparable to those obtained by other techniques [S, 91. Further investigations on soot in aerosols in the urban air were connected with studies on their vertical distribution because of the effect of temperature inversion during the winter season. For this purpose samples were collected from two stations - one situated on the ground level and the other 20 m over it. The studies carried out for an eigth months period (October - March) have shown that the soot content in all samples of the upper station is higher than that of the ground level. This finding may be explained by the fact that the air movement in 20 m altitude is higher than near to the ground. In order to find by EPR the content of diamagnetic PAH, adsorbed on the soot, they were transformed into paramagnetic cation-radicals [6]. For this purpose, after recording the EPR spectrum of the soot, the filter with the collected aerosol particles and PAH adsorbed over the soot, was dipped in a given quantity of toluene. An aliquot part of the toluene solution of desorbed PAH was transferred into a standard quartz EPR sample tube containing a pellet of silicdalumina catalyst [6]. The EPR spectrum was recorded one hour after mixing the toluene extract with the catalyst, following the procedire in [lo]. The detected singlet EPR signal (g= 2.0030, AH = 10 G) was used for quantitative purpose. After PAH desorption the filter was dried and its EPR spectrum was recorded again in respect to soot. The differences between the soot quantity before and after the desorption of PAH was attributed to the presence of tar or oil soluble soot particles in the air aerosols. Data obtained by the analysis of four samples collected: 1 m away from a motonvay; in a silent street situated at 500 m distance from roads with intensive automotive traffic; in office in which smoking is forbidden; and in a cafeteria in which smoking is allowed, are given in Figure 2. For easy visual comparison only the EPR responses (in arbitrary units) are presented. It follows from these data that the EPR signals vary broadly depending on the spots from which samples were collected. However, it is clear that total content of soot (EC + OC), recorded before treating with toluene, as well as that of insoluble particles (EC), obtained after PAH desorption follow the order (1): motonvay >urban air > cafeteria > office (1) This result could be explained having in mind the common air polution which may be expected to follow the same order of decreasing - first of all, in-door pollution with soot will be on lower level than that of open-air (motonvay and urban air) pollution. In addition, soot pollution of motonvay from one group and the cafeteria from the other one
45 9
Figure 2 . EPR response of (EC+OC), OC and EC found for different pollution sourses
could be expected to be higher than urban and office air respectively. In respect to PAH content Fig. 2 shows the following order of decreasing: cafeteria = motonvay > urban air > office (2) The highest concentration of PAH in cafeteria could be explained by the fact that smoking is permitted in it. These data are in concert with literature data that PAH are among the main components of the cigarette smoke. PAH concentration in motonvay air is similar. At the same time PAH content in the urban air is ca. 50% lower and the lowest concentration of PAH, as may be expected, is found in the office air. On the other hand, the EPR spectrum of soot recorded directly on the filter (EC + OC) may not be expected as only due to insoluble in toluene particles (graphite carbon). This is because tar or toluene soluble paramagnetic particles may also be present on the filter Such paramagnetic particles may be expected in samples collected near to the motonvay due to exhaust gases of diesel engines as well as in cafeteria due to cigarette smoke. The data of Fig. 2 confirm these considerations.
3. SELECTIVE ESTIMATION OF SOOT IN HOME DUST.
The aerosol particles in open air are extremely mobile and their concentration depends on many factors as time of the day and night, climatic conditions, wind direction,
460 raining, etc. However, aerosol particles precipitate in closed room and thus they are accumulated. Having in mind this fact and also that people spend most of the time in closed rooms, we have extended our studies to the direction of selective estimations of soot in the home dust [ 111. The samples were collected once monthly, in a period of seven months (November - May) in eight homes situated in different areas of Sofia, Bulgaria, seven of them situated in downtown area and one in a Sofia suburb. Standard vacuum cleaners every time equipped with a new paper filter were used. Samples were collected from hrniture only. Only the quantity of soot was estimated because during the one month period the quantities of PAH could change due to desorption. The first step in the processing of all samples with home dust was the separation of ferromagnetic particles which interfere the EPR estimations. After that 100 mg sample was transferred to a standard quartz EPR sample tube for recording the soot content. The results of these studies are shown in Table 1. As seen from this table for the period of investigations the quantity of soot in homes # 2 - 8 remains approximately equal, whereas in home # 1 it is ca. twice or three times as higher as the others. Houses, or flats, numbered 2 - 8 are situated in downtown area where central heating is used whereas house # I is situated in a Sofia suburb where local heating facilities are in operation. This fact explains why soot content of the home dust of this house is higher compared to the others. Also, the highest content of soot was detected in the period December - May, which is characterised by lower temperatures and more intensive heating. To obtain reproducible data all spectra were recorded in a 100 G magnetic field sweep simultaneously with a reference standard of MnZ'/MgO symmetrically inserted into the sample under investigation [12]. On the other hand, in order to obtain quantitative results the EPR parameters - peak-to-peak intensity (I) and line width (AH) of the first derivative spectra of the filter with soot and/or cation-radicals of PAH were compared with a reference sample of known quantity of charcoal (Wst in mg), obtained by sucrose pyrolyzed at 550 "C [4]. The EPR spectrum of the pyrolyzed sucrose was recorded together with the reference standard of Mn2+/Mg0. The weight of the unknown sample was obtained as: W, = W d x ( ~ x ) z ~(AH,)' Lt ] (3) Table 1 Soot content (mg/g) estimated by EPR in home-dust
~
May
1 1 4 1 3 1 - 1 - 1
-
I
2
1
-
1
2
46 1
The advantage of EPR in the described type of estimations is that the procedure is shortened and simplified. Moreover, if only soot is monitored, the filter remains unchanged after the measurement, because EPR is a nondestructive method. Thus sample may be kept as a document for use in other estimations.
SELECTIVE QUANTITATIVE ESTIMATIONS OF NITRATES AND NITRITES VEGETABLES, FRUITS, MEAT PRODUCTS, MILK AND CHEESE 4.
The content of nitrate and nitrite ions in foodstuffs is of significant importance for the human health. The presence of nitrates in foods is mainly due to plants, taking nitrogen from the soil in its ionic forms. The use of nitrogen-containing fertilisers in the intensive way of plants growing increases nitrate concentration in the soil and therefore nitrate content in the plants becomes over the normal level. The presence of nitrates in foodstuffs may be considered hazardous because during storage of food prior to ingestion or during digestion in human body they can be reduced to nitrites which may react with naturally present amines to form carcinogenic nitrosamines [13 - 151. Usually nitrite level is much lower in vegetables than nitrate’s. However, small amounts of nitrites are added to meat products to prevent them against bacteria as and to enhance flavour [ 16 - IS]. In view of this several different methods have been developed for determination of nitrate and nitrite content in foods, for example, colorimetry, h.p.l.c., g.c., ionchromatography, polarography, potentiometry with ion selective electrodes, enzymatic, spectrophotometry. Up to now the EPR method has only been applied for determination of ham and cheese nitrate and nitrite content by their reaction with phenothiazine or giving stable free radicals [ 19, 201. In previous EPR studies from this laboratory [21 - 251 we have described for the first time trapping of nitric oxide and dioxide on several transition metal complexes. It was found [ 2 1, 241 that the EPR silent tris(diorganodithiocarbamato)iron(III), Fe(dtc)3, yields the EPR active Fe(NO)(dtc)z in stoichiometric reactions with both N204 and NO (4): NO2 (NO) + Fe(dtc)3 -----> FeNO(dtc)z +.. . (4) (In addition a selective reaction of Cu(dtc)2 was found with N204 and a spectrophotometric
method for N204 determination in the working atmosphere was published [24] on the basis of this reaction.) The number of papers on nitric oxide determination mainly in the living organism was growing during the following 15 years using its interaction with Fe(dtc)3 to give Fe(NO)(dtc)2 which was determined by EPR (see for example [26 - 281). In this paper we describe the application of EPR for consecutive determination of nitrate and nitrite ions in vegetables, h i t s , meat products (sausages, ham), milk and milk products (yogurt, cheese) using the selective reactions of nitrite and nitrate ions with the EPR silent Fe(dtc)s for stoichiometric generation of the EPR active Fe(NO)(dtc), complex. The applicability of the proposed method for qualitative and quantitative EPR estimation
462
of nitrate and nitrite content in foodstuffs is based on reactions (5) and (6) including Quantitative liberation of NO gas in the reaction of nitrite ion with acidic solution of FeS04 NO2- + nFeS04 + mH2S04 -----> NO +. , , In aerobic systems NO gives NO2 (or N204). Both NO and NO2 are hrther trapped by Fe(dtc), to give Fe(No)(dt~)~ according to reaction (4). The nitrate present in the acidic solution of FeS04 quantitatively yields Fe(NO)SO4 in solution according to reaction (6): NO3-+ nFeS04 + mH2S04 -----> Fe(NO)S04 + . . . Further addition of Nadtc to this solution gives the EPR active Fe(NO)(dtc)l which may be separated by extraction. Using reactions and (6) we have first prepared a calibration graph [29] The preparation of calibration graph as well as the estimations were performed using the reference standard containing Mn2'/Mg0 [ 121. Some numerical results on the nitrate and nitrite content in vegetables and h i t s from the local market are given in Table 2. We have also performed studies on some home-grown vegetables without use of any mineral fertilizers. The results are presented in Table 3 [29]. The results show that the level of nitrates and nitrites is much lower (5 - 10 times) in vegetables grown in soil with organic fertilizers (home-grown). Table 2 . Concentration of nitrate and nitrite in vegetables and fruits obtained in local market.
I
I
suinach radishes
I
860
737
I
74
5.9
I
463
Table 3 . Comparative data of the concentration of nitrate and nitrite in vegetables and fruits obtained in local market and home-grown.
nitrate
nitrite [mg/kg]
I 451.65 I
beet root
reen onions
29.37 13.28 11.19 10.09 17.65
123.07 39.90 30.07 38.09 40.17
1
0.45
0.14
1.54
0.48 0.66
Table 4. Content of nitrate and nitrite ions in mg/kg found in different sausages
I
Sausage
Nitrate
Nitrite
Soft salami Flat sausages Drv salami - I Dry salami Dry salami - I11 Paste Sausages(smoked) Ham - I Ham - I1
1080 2120
580 50
I
I 80 10 10 170 750
1310 2250 2280
310
Table 5 . Distribution of nitrate and nitrite content in milk.
I -
--
I
I
464
Table 6. Distribution of nitrate and nitrite content in milk products. Sample
White cheese from cow White cheese from shee Yellow cheese I Yellow cheese I1 Yellow cheese (smoked)
I
Nitrate [mgikg]
I
Nitrite [mgkg]
I
140 3 90
Further we have extended these studies on meat products (ham, sausages, fillet, paste, soft and dry salami), milk and milk products (yoghourt, white, yellow and smoked chese) All products were obtained from the local market. The results are given in Tables 4 - 6. As seen from table 4 most of the studied dry salami contain relatively small quantity of nitrate in comparision with other meat products. The explanation of this finding is that recently other ingredients are used instead of nitrate ions. On the other hand all kinds of ham investigated contain nitrates on the upper level of the commonly accepted interval (0.5 -2.2 g/kg). Whereas nitrates are added to meat products in their preparation, their content in milk and milk products is only expected as transferred by the animals from grass The results obtained are given on Table 3 . Increased content of nitrates in milk products may be expected to depend on the milk used for their production. As typical 3 - 5 1 of milk is used in the preparation of 1 kg of cheese. The data about the nitrate content shown in Table 6 are in concert with theseproportions. The increased content of nitrate ions in smoked cheese may be attributed to the additional processing of the cheese.
5. CONCLUSIONS
The present data about the selective EPR estimations of soot, PAH, nitrate and nitrite ions clearly demonstrate the high capability of EPR in several practical fields. It may be expected that the EPR advantages like extremely high selectivity and sensitivity will be in the focus of many chalenging studies in the future. In view of this it may be predicted that such studies will be among the main directions of the EPR activity in the next decade.
465
REFERENCES 1. W. Douglas, D. W. Dockery, A. PopeIII, X. Xu, J. D. Spengler, J. H. Ware, M. E. Fey, B. G. Fems, F. E. Speizer, J. Medicine, 329 (1993) 1753 2. K. Spurny, Analytical Chemistry of Aerosols, Lewis Publ., Boca Raton, FL., USA, 1999. 3. S. Dzuba, S. G. Puskin, Yu. N. Tsvetkov, Doklady USSR, 299 (1988) 1150. 4. N. D. Yordanov, B. Veleva, R. Christov, Appl. Mag. Res., 10 (1996) 439. 5. N. D. Yordanov, in Analytical Chemisrty of aerosols (K.R. Spurny, Ed.), Lewis Publ., 1999, p. 197. 6. N . D. Yordanov, S. Lubenova, S. Sokolova, Atmospheric Environment, 35 (2001) 827. 7. D. Austen, D. Ingram, Trans. Faraday SOC.,54 (1958) 400 8. B. M. Ddyk, Urban Aerosol particles of Santiago, Chile: organic content and molecular Characterization, Atm. Environ. 34 (2000) 1167. 9. C. Venkatamataran, Friedlander, Size distrubutions of PAHs and EC 2. Ambient Measurements and Effects of Atmospheric Processes, Environ. Sci.Techno1, 28 (1994) 563. 10. D. Burns, M. Salem, R. Baxter, B. Flockhart, Analyt. Chim. Acta, 183 (1986) 281 11. N D. Yordanov, I. Naidenova, to be published.. 12. N . D. Yordanov, S. Lubenova, Analyt. Chim. Acta, 403 (2000). 13. B. Schuster, K. Lee, J. Food Sci. 52 (1987) 1632. 14. H. Mills, A. Barker, D. Maynard, J. Amer. Hort. Sci., 101 (1976) 202. 15. C. Walters, Oncology, 37(1980) 289. 16. J Siciliano, S. Kmlick, E. Heisler, J. Agr. Food Chem., 23 (1975) 461 17. R. Shirly, Bioscience, 25 (1975) 789. 18. E. Heisler, J. Siciliano, S. Kmlick, J. Agr. Food Chem., 22 (1974) 1029. 19. E. Tsang, D. Burns, B. Flockhart, Analyt. Proc. (London), 28 (1991) 10. 20. E. Tsang, D. Burns, B. Flockhart, Talanta, 39 (1992) 1561. 21. N. D. Yordanov, V. Terziev, B. G. Zheliazkowa, Inorg. Chim. Acta, 58 (1982) 213 22. N. D. Yordanov, V. Iliev, D. Shopov, A. Jezierski, B. Jezowska-Trzebiatowsaka, Inorg. Chim. Acta, 60 (1982) 9. 23. N. D. Yordanov, V. Iliev, D. Shopov, Inorg. Chim. Acta, 60 (1982) 17. 24. B. G. Zheliazkova, P. B. Vardev, N. D. Yordanov, Talanta, 30 (1983) 185. 25. N. D. Yordanov in Recent development in molecular spectroscopy (B. Jordanov, N. Kirov, P. Simova, eds.), World Scientific, Singapore, 1989, p.514. 26. P. Mordvintchev, R. Mulsch, R. Busse, Analyt. Biochem., 199 (1991) 142. 27. D. J. Singel, J. R. Lancaster, Methods Nitric Oxide Res., (1996), 341, Wiley,
Chichester, UK. 28. T. Yoshimura, Analyt.Sci., (1997), 13 (Suppl., Asianalysis IV), 451. 29. N. D. Yordanov, E. Novakova, S. Lubenova, Analyt. Chim. Acta, 437 (2001) 13 1 30. N. D. Yordanov, G. Petkova, Talanta, accepted 2002.
EPR in the 21” Century A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
466
EPR studies of manganese spin centers in the even-number oxidation states of water oxidizing complex in photosystem I1 S. Araoa, S. Yamadaa, A. Kawamoria,J. -R. She$, N. Ionnidisc and
Petrouleas‘
aFaculty of Science, Kwansei Gakuin University, Gakuen 2-1, Sanda, 669-1337, Japan bRIKEN Harima Institute, Mikazuki-cho, Sayo-gun Hyogo 679-5148, Japan ‘Institute of Molecular Science, NCSR Democritos, 15310 Aghia Paraskevi, Attikis, Athens, Greece
To investigate the position of the spin center of the manganese clusters in water oxidizing complex in photosystem I1 (PS 11), SO-, S2-, and S-2-states EPR signals were investigated of the preparations of each &-state. The distances between the spin centers in the SO- and S-2- state multiline and YDradical have been determined by simulation of the time profiles of pulsed ELDOR in the biochemically reduced spinach PS 11. These are 34.0 2 0.5 A and 33.0 1.0 A, for the SO- and S-2-state, respectively. The distance for the &-state was obtained to be 27.0 0.5 A. in the 200 K illuminated PS 11. The distances in the SO- and Sz-states were same as those of a cyanobacterial photosystem I1 prepared by flash light illumination. 1. INTRODUCTION Photosystem I1 (PS 11) in plants, algae, and prokaryotic organisms catalyzes the light-driven electron transfer from water to plastoquinone. The oxygen-evolving center containing a tetra-nuclear Mn-cluster located at the lumenal side of PS I1 protein complexes, carries out photosynthetic water oxidation. Oxidized equivalents are accumulated in the Mn-cluster by the successive absorption of four photons by the PS I1 reaction center and are used for water oxidation. Upon illumination, the PS I1 reaction center P680 is oxidized, and immediately reduced by a nearby tyrosyl residue, This reaction is repeated four times and finally oxidizes two water molecules to oxygen which cycles through the five oxidation states So-S4 of OEC. The one-electron oxidation steps SO S1, S1 S2, S3, S3 S4 oxidize the water, while 0 2 is evolved during the step of s4
so.
To investigate the position of the spin center of the manganese cluster in OEC in PS 11, we generated the three even number oxidation states by chemical reduction of spinach and flash light illumination of cyanobacterial PS I1 from Synechococcus Vulcanus [l]. EPR measurements were carried out for these samples. The hyperfine structures of manganese, so-called multiline in these samples, were observed by CW method to investigate the state of the PS I1 sample and its saturation behavior. Pulsed EPR technique, PELDOR (Pulsed ELectron electron Double Resonance), was applied to determine the distances of the
467
manganese spin center from the YD radical [2] to investigate the structure and magnetic property of the cluster. 2. EXPERIMENTAL
Samples The oxygen-evolving PS I1 membranes were prepared from market spinach using the method of Kuwabara and Murata [3]. The membranes were stored at liquid N2 until use. All steps of preparation were performed under dim green light. For So-state samples, the membranes of spinach PS 11, after the dark-incubation for 2 h on ice at the concentration 12 mg of Chl/ml, were incubated during various time between 10 and 60 min on ice with 100 i M NH20H and 3 % v/v methanol. The obtained states, after incubations, were mixtures of the S-I-,SO-,and S1-states, and the best yield for the So-state was found to be 22 % for the incubation time of 20 min with NH20H. For cyanobacterial PS 11, with 3 % methanol added at the concentration of 2-3 mg of ChVmg, about 60 % of the So-state was obtained by 3-flash illuminations. For the S2-state samples, illumination of cyanobacterial and spinach PS I1 was performed at 200 K for 5 and 10 minutes, respectively, and 1-flash illumination was given to only cyanobacterial PS 11. For S-2-state sample, the membrane concentrated to 6 mg of ChVmg was prepared by reduction of spinach PS with NO gas at 243 K [4].
EPR measurements CW (continuous wave) EPR measurement was carried out on an X-band EPR spectrometer ESP300 (Bruker) spectrometer equipped with a rectangular cavity. The PELDOR measurements were performed on a pulsed EPR spectrometer ESP-380 (Bruker) using a three pulses sequence. Mmicrowave (m.w.), magnetic field amplitude, HI, and pulse width in the three pulses were adjusted to provide the spin rotation angles of go", 180" and 180", respectively. The first and third pulses of frequency 01, separated by a time interval z, form the primary ESE (Electron Spin Echo) of one of the spin species. The second pulse of frequency separated from the first one by the time interval changes orientations of the other radical spin. If the magnetic dipole interaction between the pair-wise distributed radicals is noticeable, flip of one of the spins changes the local magnetic field at its partner in the pair. As a result, the magnetization after the third pulse cannot be completely refocused at the time 2r and the amplitude of the ESE signal exhibits a dependence on the second pulse position (i.e., r'). The following expression describes this dependence [21:
Do is the dipole interaction constant between the two spins, in which r is the distance between the radical pair. P is the angle between the external magnetic field and the vector r joining
468
(1) is averaged over angles b to fit the observed time profile. The the two spins. constant DOwas estimated and the distance r was determined.
3. RESULTS AND DISCUSIONS CW EPR signals observed in SO-and Sz-state samples show hyperfine structures arising from the Mn-cluster, the so-called multiline signal. They are similar in line shape but can be distinguished. Figure 1 shows the microwave power dependencies for SO-and SZstate observed at K and 6 K, respectively. The difference in temperatures of saturation characteristics was used to discriminate the different Si-states. Other differences between SO-and Sz-state multiline signals were revealed in the overall line widths and the number of the hyperfine lines. The yields of the Sz-state were estimated to be almost 100 % with 3 % methanol added. EPR signal consisted of about 16 almost isotropic hfs lines over the field 1800 G with a dark-stable YDradical signal centered. The So-state EPR signal shows about 19 lines over 2400 G magnetic field range. For the &-state, the EPR signal was very similar to that for the So-state in the number of hfs lines and field range. However the line width of each component was much sharper than that for the So-state. The distances between the spin center of the Mn-cluster on even-number oxidation Si-state and the stable tyrosyl radical, YD, were determined by using PELDOR measurement and simulation of the observed spectra. For the Sz-state, two samples of cyanobacterial and spinach PS were used for this experiment. In figure 2, we show the experimental (circles) and simulated (lines) data. Resulting from this measurement, the distances between YD and Sz-state multiline signal were 27.0 0.5 A in both PS n preparations. In figure 3 and we show the results of SO-,and S-2-state samples in spinach PS 11, respectively. By simulations the distances were assigned to 34.0 2 0.5 A for in &-state, and 33.0 1.0 A for the &-state. The distances obtained for SO-and Sz-states from the YD radical differ by about 7 A. These results show that the spin center in the Mn-cluster moves
So-state
S,-state
r
I
26.5A
ipinach PSI1
27.
~
-- - - __ - 27.5A
ynechococcus Vulcanus PSI1 200
Figure 1. Microwave power dependence of the SO-,Sz-state EPR signal
400
600
800
Figure 2. PELDOR Signal for YD-SZ Multiline in spinach and cyanobacterial PS n
469
0
200
400
600
1200
800
.
1
Figure 3. Pulsed ELDOR Signal for YD-SOMultiline (in spinach PS
,
.
,
.
,
2&
.
I
.
loo0
Figure 4. Pulsed Signal for YD-S-~ Multiline (in spinach PS
when the oxidation advances to the S2-state. In Fig. 5 this result is illustrated based on the recently investigated crystal structure with 3.8 A resolution [5]. the magnetic properties and the accurate structure have not yet been elucidated, the movement of the “spin center” occurring during manganese oxidation will provide valuable information to the mechanism of water oxidation. It is concluded from these results (see figure 5), that the position of the spin center in the &-state moves from that in the So-state, and that in the reduced S-2-stateis almost same as in the So-state. The oxidized Mn-atom in the &state is closer to YD than that in the SO- and S.2-state. In cyanobacterial PS 11, 3-flash illumination produced not only the So-state but also %state multiline signals. It will be necessary to generate higher concentration of pure Si-state for further studies.
34.0 4 0.5 A for So-state 33.0 5 1.0 A for &-state
/
- Membrane Plane 27.0 0.2 A for S2-state Figure 5. The distances between YD and each spin center of the Mn-cluster in Si-state.
470
REFERENCES 1. J. -R. Shen and N. Kamiya, Biochemistry, 39 (2000) 14739-14744. 2. A.V. Astashkin, H. Hara and A. Kawamori, J. Chem. Phys. 108 (1998) 3805-3812. 3. T. Kuwabara and N. Murata, Plant Cell Physiol., 23 (1982) 533-539. 4. N. Ioannidis, J. Sarrou, G . Schansker and Petrouleas, Biochemistry, 37 (1998) 16445-16451. 5. A. Zouni, H. -T. Witt, J. Kern, P. Fromme, N. Krauss, W. Saengert and P. Orth, Nature, 409 (2001) 739-743.
EPR in the 21' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
471
Magnetic resonance studies on ascorbate binding to albumin E. Lozinsky", A. Novoselsky", A. I. Shamesb,R. Glaser", G. I. Likhtenshtein" and D. Meyersteina'c b
Departments of hemistry and Physics, Ben-Gurion University the Negev, P. 0. Box 653, 84105 Be'er-Sheva, Israel The ollege of Judea and Samaria, Ariel, Israel
The fluorescence intensity of the fluorophore in dansyl-piperidine nitroxide is intramolecularly quenched by the nitroxyl fragment. Therefore, the oxidation of ascorbic acid by the Fluorophore-Nitroxide (EN) probe can be monitored by two independent methods: EPR and steady-state fluorescence. The adsorption of both ascorbate and FN to Bovine Serum Albumin (BSA) influences the rate of this reaction. The spatial distribution of the ascorbate and molecules on the BSA surface changes their ability to interact with each other. The effect of BSA on this reaction rate strongly depends on pH and ionic strength since these factors affect the ascorbate binding to the protein. The binding constant of ascorbate to BSA was calculated from kinetic studies. The adsorption of ascorbate to BSA was also confirmed by 'H Nh4R experiments via measurement of the transverse relaxation time, Tz, of ascorbate protons in BSA solutions. Ascorbate binding to BSA resulted in the appearance of a short TZcomponent in the magnetization decay. The ratio between the short and long components of the magnetization decay reflects the ratio between bound and free ascorbate.
1. INTRODUCTION
Albumin is one of the most important components of mammalian plasma: it influences the osmotic pressure, transport, distribution and metabolism of many endogenous and exogenous species. Albumin accounts for approximately 60% of the total protein in blood plasma and its concentration is -0.6 mh4 (40 g/l) El]. Therefore, BSA solutions can serve as a model for mammalian plasma. The function of albumin is related to its very high affinity toward many organic and inorganic compounds (e.g. fatty acids, amino acids, steroids, metal ions, numerous drugs and dyes. Albumin has numerous and varied binding sites which enable the binding of many molecules [2-31. Ascorbate, known to be an important antioxidant [4], is expected to have a very high affinity to albumin since it is a small negatively charged molecule. The antioxidant power of ascorbate might be affected by its adsorption to albumin versus the unbound state in aqueous medium where it is free to rotate. Thus, the investigation of
472
ascorbate binding to albumin is important for understanding ascorbate’s function. While the literature contains data on ascorbate binding, the reported results on its binding to albumin are contradictory [5-61. These contradictions are probably due to the difference in the conditions under which the measurements were made. In the present study, the factors affecting the binding of ascorbate to albumin were investigated using two different methods. Both methods are based on magnetic resonance spectroscopy: the first method is direct, while the second one utilizes a paramagnetic compound which reacts with ascorbic acid.
2. MATERIALS AND METHODS
2.1. Materials Ascorbic acid, amino-TEMPO and Bovine Serum Albumin (fatty acid free) powders were purchased from Sigma hemical 0. The FN probe was synthesized according to the procedure described by Likhtenstein and coworkers [7]. 2.2.1. Electon Paramagnetic Resonance (EPR) EPR spectra were recorded on a Bruker EMX-220 digital X-band spectrometer equipped with a Bruker EP 4241VT temperature control system. All spectra were recorded with the following parameters: 9.40 GHz microwave frequency, 20.12 mW non-saturated microwave power, 100 KHz field modulation of 1 G amplitude. Kinetics were measured by recording in the Time Scan mode with a modulation amplitude of 5 and 1 G for immobilized and free probe, respectively.
2.2.3. Nuclear Magnetic Resonance (NMR) lH spectra (5mm sample tubes, 298 K) and the measurment of lH T2 relaxation times were performed at 500.1 on a Bruker DMX-500 Fourier transform spectrometer. D 2 0 was used as an internal lock, and the residual H 2 0 solvent was used as an internal reference ( 6 4.8). ~ Standard Bruker microprograms were utilized for the PMG ( arrPurcell-Meiboom-Gill) technique [8]. The error of the T2 measurement was smaller than +lo%.
3. RESULTS AND DISCUSSION Direct evidence for the binding of ascorbate to BSA was obtained by the measurement of the NMR transverse relaxation time, T2, for methylol (&OH) protons in free ascorbate in D2O and in ascorbate solutions containing BSA. The amount of free versus bound ascorbate determined by TZ measurements is reported in Table 1 . The decay of the transverse magnetization of these protons in pure D20 fits a single exponential, with T2 = 765 ms. However, in the presence of BSA, the decay can be fitted only by the sum of two exponents. Binding of the ascorbate molecule to the massive protein causes a drastic decrease in the T, of the ascorbate protons. This shortening of ascorbate methylol proton T2 values is due to their interactions with protein nuclei, e.g. spin-spin relaxation. The values of T, for the two
473
components (bound and free) are independent of the concentration, while their ratio varies. The ratio between these two components is the ratio between free and bound ascorbate. The contribution of the viscosity to the shortening of T2 can be neglected since the presence of does not affect the long Tz value of acetone which is not adsorbed to Furthermore, in the containing solutions, the decay of the transverse magnetization for acetone can always be fitted by a single exponent. These results suggest that the nature of the interaction between ascorbate and is electrostatic, and binding is expected to depend on pH. In solutions containing 0.05 mM the largest contribution of the short component of T2 was observed at pH 5.0 (80%) vs. 32% at pH 3.8 and 45% at pH 7.0. the first pK, of ascorbate is -Y_ + H,O+ pK, = 3.8, K, + BSA AK -BSA,
A H 2 + H20
-
and as is expected to be bound stronger to i.e. Kl>K2, one expects an increase in binding when the pH is raised from 3.8 to 5.0. The observed decrease at pH 7.0 is attributed to the isoelectric point of albumin at pH 5.2 [9]. The binding of to the negatively charged at pH although naturally positively charged substituents are still present on at this pH, is clearly weaker. Hence, ionic strength should be another factor affecting the binding process. To demonstrate this factor, measurements of TZof the ascorbate protons in the presence of at different ionic strengths were performed. It was observed that at low ionic strength (determined by the ionization of ascorbate itself), the decay of transverse magnetization is fitted by two-exponents. High ionic strength is achieved by addition of the 0.012 c(FN)= 0.1 mM strong electrolyte NaCl to the /0 pH 7.4 solution. Addition of a large amount "0.15 O h 0 of NaCl to a mixture of and 7 resulted in the disappearance of 3 the short component of T2, while the o/ long component of T2 retained its value, i.e. all the was released e from % h
Q)
/
0.000 (
'
1
I
.
,
.
,
.
0.4
Ascorbate concentration (mM)
,
Figure 1. Dependence of the observed rate on ascorbate concentration, T = 298K. Squares - EPR measurements, circles - fluorescence measurements. Phosphate buffer pH 7.4, ionic strength, 0.15.
474
Thus, NMR experiments provide direct evidence that ascorbate is electrostatically bound to BSA and its binding is dependent upon both pH and ionic strength. Since the ratio between long and short components reflects the ratio between free and bound ascorbate, the binding constant can be therefore be calculated. Although this method is direct and reliable, it suffers a serious drawback the method is time-consuming. Each Tz measurement takes about 3 hours and mathemathical operations must be done. Therefore, in addition to the method based on NMR measurement, another technique is proposed. Ascorbate is known to reduce nitroxide radicals and in biological systems it is a reducing agent for piperidine nitroxide. The proposed method is based on use of piperidine nitroxide covalently linked to a dansyl moiety. Highly fluorescent dansyl has a very low fluorescence intensity when it is ligated to piperidine nitroxide because nitroxides are strong quenchers of the fluorescence. Therefore, EPR signal decay of dansyl-piperidine caused by ascorbic acid, is accompanied by the increase in the fluorescence intensity. Table 1. Percentage of bound and free components of ascorbate ( 5 mM) in BSA containing solutions and in the absence of BSA. pD = 3.8 [BSAI, mM omp.1 (765 ms), % omp.2 (90 ms), %
0.001 87 13
0 100 0
0.01 85 15
0.05 68 32
0.25 10 90
Thus, the reaction of dansyl-piperidine can be followed by two techniques: EPR and fluorescence. When excess ascorbate is used, the reaction kinetics obey a pseudo-first-order rate law (Figure 1). The addition of BSA to the reaction mixture significantly accelerated the reaction rate. This could be due to the binding of both ascorbate and dansyl-piperidine to BSA. While the binding of ascorbate to BSA was proven by NMR, the binding of dansylpiperidine nitroxide is observed from its EPR spectrum. Broad peaks attributed to the probe incorporated into BSA matrix are clearly seen [9]. The reaction between the FN probe and AH- was investigated by two indeDendent techniaues: EPR and 0.030 steady-state fluorescence. The observed rate was found to . . . . . 0 _... 0.025..--0____.. increase with BSA concentration _.. 3 _.(Figure 2). 3 0.020-
--
-
/
-
-
Y
5 9
p 6
,o.’. _ _ _ - - _ _ _------Q __._-p”o-___ - - - - --
0.015-
0.010-
a,
. 0.005.
&,
,
....... ...-~-
~
*
... .-.----~---
,-----..
&‘/&/&------a ___---
~
.---u
-_____.._.-
_.--
f&-ly----
0.000.,
. , . , . , . , . , .
Figure 2. Dependence of the observed rate on BSA concentration at different ascorbate concentrations: circles0.6 mM, diamonds-0.4 mM, triangles-0.2 mM, squares-0.1
475
At low concentrations of BSA, the effect of albumin on the rate constant is insignificant. While at higher concentrations the reaction is accelerated, not all the FN is reduced although [AIp]>[lWl. After the reaction is completed, the EPR spectrum of BSA bound dansyl-piperidine nitroxide can be observed. The intensity of this EPR signal increases with BSA concentration. In addition, at higher ionic strength, the effect becomes stronger. At 1 mM BSA (and above 1 mM) and physiological ionic strength (0.15), the reaction is very low and the EPR spectrum shows complete binding of dansyl-piperidine to BSA. Apparently, when BSA is present at low concentrations, molecules of and ascorbate bind to the same BSA molecule. The sites of the primary binding are close, and therefore, the probability of collision increases and the reaction proceeds faster. At high concentrations of BSA, the number of available sites on BSA for dansyl-piperidine and is large. Some of the dansylpiperidine and molecules are located at remote sites of the BSA or are even bound to different BSA molecules. Thus, they are spatially separated from each other and the reaction is slowed down. This result suggests that three types of probe are present in the system: 1 . not adsorbed (free FN in the solution); 2. loosely bound FN to the BSA (this fraction is in fast equilibrium with the free FN, and therefore their rates of reaction with ascorbate are equal); 3. tightly bound FN (FN reacts slowly with the ascorbate either inherently or due to low exchange rate with the other types of the probe). The observed rate is a sum of rates of the at least, four reactions involving: free probe and free ascorbate, free probe and bound ascorbate, bound probe and bound ascorbate and bound probe and free ascorbate. Since the EPR signals of free and loosely bound probe decay with the same observed rate, it can be concluded that a fast exchange exists between bound and free probes and hx>> kobs. Therefore, the observed rate is independent of the binding of the probe to BSA: kobs = akeee + pkbo,,,,d, where k,, is the observed rate of the reaction with free ascorbate and kbo,,,,d is the observed rate of the reaction with bound ascorbate. and p denote the contribution of kh, and kbound to kobs. Following the study of the ascorbate binding to BSA, the contribution of kbound to kobs was examined as a function of pH and ionic strength on the binding process. At p = 0.15, kpp values were calculated to be 14, 87, 42 M-' s-' at pH 4.0, 5.0 and 7.4, respectively. In the absence of BSA, the kinetics of the reaction between the probe and ascorbate was only slightly affected by changes in pH. Since ascorbate binding decreases with increasing the contribution of the kbound to kobs was also found to decrease (Figure 3). The apparent rate constant kaPpwas found to be 82.1 (at p = 0.0015); 41.5 (at = 0.048); 18.0 (at = 0.15) and 8.0 (at p = 0.8) M-' At p 0.8, the effect of albumin was completely was completely k& = kee,. Thus, it can be concluded that dansyl-piperidine is a convenient tool for the determination of unknown ascorbate concentrations when pH and p are known. For the determination of ascorbate in a biological system, one should use a calibration curve obtained under physiological conditions at pH 7.4 and p 0.15. Moreover, under each condition, the binding constant of ascorbate to BSA can be easily calculated. Since the dependence of k& on the ascorbate concentration is linear (see Figure 3), it can now be concluded that at [BSA] = constant, the a / p ratio is constant, i.e the alp ratio is independent of the A H concentration. As [ M I 0> [BSA], this observation shows that each molecule of albumin has a large number of available sites at which might
476
be adsorbed, in accord with earlier reports Therefore, [BSAIo might be considered as a constant. Thus, the ratio between free and adsorbed AH.BSA is constant:
K[BSA] =
[AH-aBSA] [AH1
0 10
/
Figure 3. Dependence of observed rate on ascorbate concentration in the presence of 0.1 mM BSA.
Therefore, the slope of a plot of [ A H - BSA]/[AH-] versus [BSA] yields the equilibrium constant K, which was found to be 3.0*105, 2.3*104 and l.2*103 M' at 0.0015, 0.048 and 0.15, respectively. The ratio of bound and free ascorbate is expressed as a l p and can readily be calculated. k,, =7 M' s" is derived from the kinetic measurements performed in buffer in the absence of BSA, while kbo,,,,d is obtained under conditions of complete binding of ascorbate to BSA. It was experimentally found that at pH 7.4 and = 0.0015, the ascorbate binding is maximal and kapp='kbo,,,,d = 82 M-' sd. Thus, k, = 82a + 78. Since a + p = 1, these parameters are easily extracted for each kpp.
REFERENCES 1. B. Halliwell, Medical Pharmacology, 37 (1988) 569-571. 2. X. M. He, D. . arter, Nature, 358 (1992) 209-214. 3. D. . arter, X. M. He, S. H. Munson, P. D. Twigg, K. M. Gernert, B. Broom, T. Y. Miller, Science, 4 (1989) 1195-1198. 4. B. Frey, L. England, B. N. Ames, Proc. Natl. Acad. Sci. USA, 86 (1989) 6377-6381. 5. V. , Okore, Arzneim.-Forsch. Drug Res, 44 (1994) 671-673. 6. K. R. Dhariwal, W. O.Hartzell, M. Levine, Am. J. lin. Nutr. 54 (1991) 712-716. 7. G. I. Likhtenshtein, V. R. Bogatyrenko, A. V. Kulikov, K. Hideg, 0. H. Hankovsky, N. V. Lukianov, A. I. Kotelnikov, B. S. Tanaschelchuk, Dokl. Akad. Nauk SSSR, 253 (1980) 481-484. 8. T. . Farrar and E. D. Becker Academic press, New York. 1971. 9. E. Lozinsky, A. Novoselsky, A. I. Shames, 0. Saphier, G.I. Likhtenshtein, D. Meyerstein, Biochim. Biophys. Acta, 1526 (2001) 53-60.
EPR in the 21" Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
477
Electron magnetic resonance study on the effect of radioactive radiation on the photosynthesis of chlorophyll in lipid bilayers Y. S. Kang, a D. K. LEE, a S. M. Parkb and K. W. Seoa aDepartment of Chemistry, Pukyong National University, Pusan 608-737, Korea* bCooperative Laboratory of Pukyong National University, Pusan 608-737, Korea
Photoinduced
electron
transfer from chlorophyll a through the interface of (DPPC) headgroup of the lipid bilayers was studied with electron magnetic resonance (EMR). The photoproduced radicals were indentified with electron spin resonance (ESR) and radical yields of chlorophyll a were determined by double integration ESR spectra. The formation of vesicles was identified by measuring h,,, value change from diethyl ether solution to vesicles solutions indirectly, and observed directly with SEM and TEM images. When the systems were y-irradiated with 100 Gy at room temperature, the photoyields were decreased about 30%. This was identified with the destruction and decomposition of chlorophyll a and vesicles molecules.
1. INTRODUCTION Molecular assemblies such as micelles and vesicles may be used as model systems for the storage of light energy [ 1,2]. These self-forming molecular assemblies compartmentalize the electron donors and acceptors relative to the solvent, typically water. Although the structures of vesicles and micelles are not as complex as natural membranes, photochemical studies of chlorophyll in such organized assemblies have proved to be relevant to the determination of fundamental properties of chlorophyll in photosynthetic system. The objective of this research is to study on the effect of radioactive y-ray on photosynthesis. Charge separation may be partially controlled by various factors such as the vesicle surface charge, headgroup variation, and alkyl chain length variation [3-71. The addition of slightly water soluble, surface active compounds such as alcohols and cholesterol modify the assembly interface. It has been shown that charge separation may be partially controlled by the addition of such intercalating agents [8,9]. In the current investigation, we have studied the photoionization of chlorophyll a in vesicles with or without a radiation of radioactive y-ray.
2. EXPERIMENT Chlorophyll a was extracted from fresh spinach leaves by the conventional method. Its purity was determined to be 96% from its extinction coefficient in diethyl ether at 660 nm vs
478
the literature value of 8.6 x M" cm-l [lo]. DPPC was purchased from Sigma Chemical Co. and were used without further purification. Buffer solutions were prepared with sodium phosphate, sodium pyrophosphate and sodium ethylene diaminetetraacetate (EDTA) from Aldrich Chemicals.
2.1. Sample preparation and Y-irradiation DPPC vesicle solutions of chlorophyll a were prepared by the method developed by Huang [ l l ] and modified by Norris et al. [12]. After the chloroform solutions of DPPC containing chlorophyll a were evaporated, the resulting film was sonicated in aqueous buffer solutions with a Fisher Model 300 sonic dismembrator operated at 30 W with a 4 mm 0.d. microtip for 1 h at 55 'C (above the liquid crystal gel phase transition temperature). Phosphate buffer solutions contained 0.1 M sodium phosphate, 0.1 M sodium pyrophosphate, and 1 mM EDTA in triply distilled water and were adjusted to pH 7.0 with sulfuric acid. Tris-HC1 buffer in triply solutions were prepared by dissolving 0.5 M distilled water and were adjusted to pH 7.0 with hydrochloric acid. The formation of DPPC vesicles containing chlorophyll a was identified by measuring value change from diethyl ether solution to vesicles solutions indirectly. Optical absorption spectra were measured in 1 cm path length quartz cell with a Varian CARY 1C UV-vis spectrophotometer at room temperature. The morphologic analysis of vesicles was carried out by scanning electron microscopy (SEM), with a Hitachi S-4200 FE-SEM, after coating the sample with Pt in vacuum chamber and transmission electron microscopy (TEM), with Jeol JEM-2010, after staining of the sample with uranyl acetate (2%), respectively. DPPC vesicle solution of chlorophyll a was y-irradiated at room temperature with a dose of 100 Gy in 6oCoGammacell-220 from Korea Atomic Energy Research Institute (KAERI). 2.2. Electron spin resonance experiments Photoirradiation at 77 K was performed with 300-W Cermax xenon lamp with a power supply form ILC Technology. The light was passed through a 10-cm water filter and a Corning 5030 band pass filter for blue light irradiation (300 nm < hlrr< 558). ESR spectra were recorded at X-band on JEOL JEX-FX 2000-300. Mn2+in MgO was used as a magnetic field marker. The efficiency of photosynthesis in model system was determined by measuring the amount of chlorophyll radical yields which were obtained from integration of spectra.
400
500
600
Wavelength (nm)
Figure I . Optical absorption spectra of DPPC vesicle and chlorophyll a in diethyl ether and DPPC vesicles.
479
RESULTS AND DISCUSSION
Optical absorption spectra of DPPC vesicle and chlorophyll a in diethyl ether and DPPC vesicles is shown in Figure 1. There is no optical absorption band in DPPC vesicle solution. Some shift of the absorption bands of chlorophyll in the vesicle solution from the bands in the organic solvents might be caused by different environmental interactions of chlorophyll a [13]. In DPPC vesicle solutions of chlorophyll a an absorption band at 685 nm has been assigned to aggregated chlorophyll a [141. The formation of hydrated chlorophyll polymer has been reported to give an absorption band at 740 nm [15]. Since there are no absorption bands at 740 nm or 685 nm in our preparations, we conclude that chlorophyll is solubilized in its monomeric form in our samples. Chlorophyll a solubilized into a phospholipid vesicles, causes a 10 nm shift to the red region compared to diethyl ethyl solution in which hmax= 660 nm. This red shift is still persisted in DPPC vesicle solution with hmax= 670 nm [16]. This red shift indicates the chlorin ring being located in a polar environment near the surfactant headgroup region and possibly exposed to the aqueous environment.
Figure 2. Scanning electron micrographs of DPPC vesicles (left) and DPPC vesicles containing chlorophyll a (right). The size of vesicles is 80 -1 80 nm.
15
[Chloraphyll]l[DPPC]
Figure 3. Dependence of the normalized photoyield of chlorophyll a upon the molar ratio of DPPC in DPPC frozen vesicles. Photoirradiation was carried out with bluelight for 20 min at 77K.
45
Irradiation Time (mm)
Figure 4. Dependence of the normalized photoyield of chlorophyll a upon the bluelight irradiation in DPPC frozen vesicles at 77K. [Chlorophyll a]/[DPPC] molar ratio is 0.03.
480
Scanning electron micrographs of DPPC vesicles (left, ~15,000)and DPPC vesicle containing chlorophyll a (right, ~100,000)were represented in Figure 2. The size of vesicles is 80 - 180 nm. The vesicles were formed almost homogenously in both DPPC vesicles and DPPC vesicles containing chlorophyll a.
B
A
C
Figure 5 . Transmission electron micrographs of DPPC vesicles containing chlorophyll a (A) and DPPC vesicles containing chlorophyll a y-irradiated with 100 Gy at room temperature (B and C). The samples were stained with uranyl acetate. 0 8 ,
(u
Y)
400
Wavelength (nrn)
Figure 6. Optical absorption spectra of chlorophyll a in DPPC vesicles (solid line) and chlorophyll a in DPPC vesicles y-irradiated with 100 Gy at room temperature (dot line). The photosynthesis of chlorophylls in vesicles was studied with ESR. No radical formation was observed in DPPC vesicle solutions not containing chlorophyll after blue-light irradiation at 77K. After blue-light irradiation of chlorophyll a in DPPC vesicle solutions the ESR singlet was observed. This ESR singlet did not decay within several days at 77K. The photoproduced cation radical of chlorophyll a was identified as g = 2.0026 and broad singlet in frozen state at 77K. Dependence of the normalized photoyield of chlorophyll a upon the molar ratio of DPPC in DPPC frozen vesicles was shown in Figure 3. Photoirradiation was carried out with blue-light for 20 min at 77K. The normalized photoyield increases linearly with the Chlorophyll dDPPC molar ratio up to 0.03 and then is constant. Dependence of the
48 1
normalized photoyield of chlorophyll a upon the blue-light irradiation in DPPC frozen vesicles at 77K was shown in Figure 4. The normalized photoyield increases rapidly with blue-light irradiation up to 30 min and then increases slowly after 30 min. Transmission electron micrographs of DPPC vesicles containing chlorophyll a (A) and DPPC vesicles containing chlorophyll a y-irradiated with 100 Gy at room temperature (B and C) were shown in Figure 5 . TEM images show that the round shaped DPPC vesicle before yirradiation (Figure 5 A) is destructed by y-irradiation (Figure 5 B and C). Optical absorption spectra of chlorophyll a in DPPC vesicles (solid line) and chlorophyll a in DPPC vesicles y-irradiated with 100 Gy at room temperature (dot line) were shown in Figure 6. The optical absorption bands of chlorophyll a in DPPC vesicles in both before and after y-irradiating show no significantly change. This result can be interpreted as the chorin ring of chlorophyll a molecular is not almost decomposed by y-irradiation with 100 Gy at room temperature. First derivative X-band ESR spectra of chlorophyll a in DPPC vesicles (left) and chlorophyll a in DPPC vesicles (right) y-irradiated with 100 Gy at room temperature was shown in Figure 7. The spectra were recorded at 77K after 90 min blue-light irradiation. The 2+ . sharp lines on both sides of the spectra are form Mn in MgO used as a magnetic field marker. The efficiency of photosynthesis in model system was decreased about 30% by yirradiation with 100 Gy at room temperature. This is interpreted as with the destruction of DPPC vesicle system the photoinduced cation radicals of chlorophyll a were decayed due to back electron transfer to water. And the decomposed radical was disappeared at room temperature as increased temperature from 77K.
1
I ' g=20026
Figure 7. First derivative X-band ESR spectra of chlorophyll a in DPPC vesicles (left) and chlorophyll a in DPPC vesicles (right) y-irradiated with 100 Gy at room temperature. The spectra were recorded at 77K after 90 min blue-light irradiation. The sharp lines on both sides of the spectra are form M? in MgO used as a magnetic field marker.
CONCLUSION The formation of DPPC vesicles containing chlorophyll a was identified by a 10 nm shift to the red region compared to diethyl ethyl solution in which hmax= 660 nm indirectly. From
482
SEM images we could confirm that the vesicles were formed almost homogenously in both DPPC vesicles and DPPC vesicles containing chlorophyll a with a size of 80 - 180 nm. After blue-light irradiation of chlorophyll a in DPPC vesicle solutions, the ESR singlet was observed. The photoproduced cation radical of chlorophyll a was identified as g = 2.0026 and broad singlet in frozen state at 77K. From TEM images we could observe that the round shaped DPPC vesicle is destructed by y-irradiation. Due to the decay of the photoinduced cation radicals by back electron transfer to water, the efficiency of photosynthesis in model system was decreased about 30% by y-irradiation with 100 Gy at room temperature. In the furthermore future study, the destruction of systems and decomposition of chlorophyll and vesicle molecules will be studied and the degree of destruction and decomposition versus dosage of y-radiation will be studied. Acknowledgments This project was supported by Ministry of Science and Technology (MOST) as a part of the Nuclear R&D Program.
REFERENCES 1. 2. 3. 4. 5. 6.
J.H. Fendler, Acc. Chem. Res., 13 (1980) 7. J.K. Hurley and G. Tollin, Sol. Energy, 28 (1982) 197. P.A. Narayana, S.S.W. Li and L. Kevan, J. Am. Chem. SOC.,104 (1982) 6502. E.-M. Rivara, P. Baglioni and L. Kevan, J. Phys. Chem., 93 (1998) 2612. M.P. Lanot and L. Kevan, J. Phys. Chem., 93 (1989) 998. T. Hiff and L. Kevan, J. Phys. Chem., 93 (1989) 2069. 7. P.J. Bratt, Y.S. Kang, L. Kevan, H. Nakamura and T. Matsuo, J. Phys. Chem., 95 (1991) 6399. 8. P. Baglioni and L. Kevan, J. Phys. Chem., 91 (1987) 2016. 9. I. Hiromitsu and L Kevan, J. Am. Chem. SOC.,109 (1987) 4501. 10. H.H. Strain and W.A. Svec, The Chlorophylls, Academic Press, New York, 1966. 11. G.H. Huang, Biochemistry, 8 (1969) 344. 12. W. Oettmeier, J.R. Norris and J.J. Katz, Naturforsch, 31 (1976) 163. 13. G. R. Seely and R. G. Jensen, Spectrochim. Acta, 21 (1965) 1835. 14. A. G. Lee, Biochemistry 14 (1975) 4397. 15. R. G. Brown and E. H. Evans, Photochem. Photobiol., 32 (1980) 103. 16. T. Trosper, D. Raveed and B. Ke, Biochem. Biophys. Acta, 223 (1970) 463.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
483
Effects of tannin compounds on metal ion-hydrogenperoxide systems* A. Nakajima,a Y. Ueda) N. Endoh,c K. Tajima,c and K. Makinod a Department of Chemistry, Miyazaki Medical College, Kiyotake, Miyazaki Japan Department of Psychiatry, Miyazaki Medical College, Kiyotake, Miyazaki Japan C Department of Applied Biology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto Japan d International Innovation Center, Kyoto University, Yoshida, Sakyo-ku, Kyoto Japan
When (DMPO) and hydrogen peroxide (H202) were added into a uranyl solution, ESR signal of DMPO-OH was observed. No DMPO-OH signal was observed in U022+-DMPO solution without H202. The ESR signal intensity of DMPO-OH in U022+-DMPO-H202 solution gradually increased with time. Based on these results, it may be proposed the following mechanism for DMPO-OH formation; (a) reduction of U022+ to U02+, (b) disproportionation of U02+ to U022+ and UO2+, (c) Fenton-like reaction, and (d) spin-trapping reaction. Persimmon tannin affected peculiarly on the DMPO-OH formation in U022+-DMPO-H202 solution compared with those in other metal solutions. The addition of constituent compounds of PT, especially epigallocatechin (EGC), enhanced the DMPO-OH formation by their reduction abilities. Thus, the behavior of PT in the U022+-DMPO-H202 solution reflected the metal-reducing effect of these components of PT.
1. INTRODUCTION Polyphenol compounds, such as tannins, are well known to act as anti-oxidants and free radical scavengers.1-3 In metal ion-H~O2solution, these compounds affected the generation of hydroxy radical by the chelation with metal ion as well the direct scavenging of hydroxy radical.4.5 The authors previously investigated the interaction between these metal ions and tannins, and found that persimmon tannin has high affinities for metal ions, such as
* This work was supported by the Grant-in-Aid for ScientificResearch, the Ministry of Education, Culture, Sports, Science and Technology of Japan, and the REIMEI Research Resources of Japan Atomic Energy Research Institute.
484
uranyl, ferric and vanadyl ion^.^-^ Recently, Hamilton et a1 reported that hydroxy radical was generated a U022+-DMPO-H202 solution.10 It is, therefore, very interesting to analyze the rise and fall of hydroxy radical in U 0 2 2 + - D M P O - H 2 0 2 - ~ nsolution for elucidation of not only the radical scavenging effect of tannins but also the interaction between uranyl ion and tannins. In this paper, the mechanism of hydroxy radical generation in the solution and the effects of tannin compounds on the system were examined in comparison with those in Fe2+-DMPO-H202and V02+-DMPO-H202 solutions.
2.1 Materials Purified persimmon tannin powder used throughout this study was obtained from Tomiyama Shoten Ltd., Kyoto, uranyl nitrate hexahydrate, UO2(NO3)2 6H20, from Merck Chemical Industries, Ltd., 5,SDirnethyl- 1-pyrroline-N-oxide(DMPO), from DOJINDO Ltd., Kumamoto, Japan, and other chemicals used in this study, from Nacarai Tesque, Inc., Kyoto and Wako Pure Chemical Industries, Ltd., Osaka.
2.2. Electron paramagnetic resonance measurements Uranyl nitrate (final concentration 1 mM), DMPO (final concentration 100 mM), H202 (final concentration 100 mM), and tannins (final concentration0 - 0.625 mg/ml) were mixed. Then, the mixture was sucked up into a capillary tube and quickly measured ESR spectrum using X-band ESR spectrometer (JEOL JES TE-100) under the conditions of microwave frequency, 9.44 magnetic field, 334.5 mT; field amplitude, 5 mT; field modulation, 100 kHz; modulation width, 0.079 mT; microwave power 2 mW, and the response time, 0.1 sec, and sweep time 1 min.
3. RESULTS AND DISCUSSION
3.1. Reaction mechanism of hydroxy radical formation in U0z2+-DMPO-H202 solution When 5,5-dimethyl-1-pyrroline-N-oxide(DMPO) and hydrogen peroxide (H202) were added to a uranyl solution, ESR signal with four lines (intensity ratio, 1:2:2:1) was appeared. From its g-value, g = 2.0066, and hyperfine constants, = 1.49 mT, aH = 1.49 mT ,the signal was identified to be that of DMPO-OH (Fig. la). The addition of methanol and dimethyl sulfoxide (DMSO) into the system decreased DMPO-OH signal and DMPO-CH3, and DMPO-CH20H radicals were appeared, respectively (Fig. l b and c). It is, therefore, confirmed that the DMPO-OH was resulted by the trapping of hydroxy radical as reported by Hamilton et a l l 0 A signal of DMPO-OCH3 was also observed in U022+-DMPO-H202 solution with 20 % of methanol (asterisks in Fig. lc). These results suggested that the nucleophilic addition of water molecule was also occurred in these solutions, being similar Fe3+-DMPO-H202, and Cu2+-DMPO-H202 solution^.^>^ However, no DMPO-OH
485
signal was observed in U022+-DMPO solution without H202. The ESR signal intensity of DMPO-OH in V02+-DMPO-H202 solution decreased with time, and that in Fe2+-DMPOH202 solution increased rapidly in the initial stage and then decreased in a manner similar to that in V02+-DMPO-H202 solution (Fig. 2), as reported by Mizuta et a112 and Sakurai et al.13 On the other hand, DMPO-OH signal intensity in U022+-DMPO-H202 solution gradually increased with time, which is quite different from other metal ions. When hydrogen peroxide was added to the uranyl solution, U02(H202)2 (or UO4.2H2O) was precipitated,14 which should retard the DMPO-OH formation. Hamilton et al. suggested that three reaction steps were assumed for hydroxy radical generation in U022+-DMPOH202 solution,1° namely reduction of U022+ to U02+, disproportionation of the resulting U02+ to UO2+ and U022+, and generation of hydroxy radical through the Fenton-like reaction. As no signal of DMF'O-OH or DMPOX was observed in the U022+-DMPO solution without H202, the possibility of the oxidation of DMPO by metal ion, such as Fe3+,11was omitted. Thus, H202 should be only an agent for the reduction of U022+ to U02+. It is therefore possible to propose the following reaction scheme for DMPO-OH formation; (a) reduction of U022+ to U02+ ( 2U022+ + H202 2U02+ + 2 H+ + 0 2 7 ), (b) disproportionation of U02+ to U022+ and U02+ ( 2 U02+ + H20 U02+ + U022+ + 20H- ), (c) Fenton-like reaction ( UO2+ + H202 U022+ + OH + H+ ), (d) spintrapping reaction ( DMPO + * OH DMPO-OH ). +
+
+
+
332
334 336 Magnetic field
338
Figure 1. ESR spectra of free radicals in U0z2+-DMPOH202 solutions. (a) UOz2+ (1 mM), DMPO (100 mM), H202 (100 mM) water were mixed for 1 h. (b) (a) in 20 % MeOH, (c) (a) in 20 % DMSO. Asterisk indicates the signal of DMPO-OCH3.
3.2. Effect of tannins on the DMPO-OH in U022+-DMPO-H202 solution When persimmon tannin (PT) was added to U022+-DMF'O-H202solution, the DMPOOH signal rapidly increased in first few minutes, followed by a slow increase and slow
486
decrease (Fig. 2). On the other hand, PT caused the rapid decrease of DMPO-OH signal in V02+-DMPO-H202 solution. In Fe2+-DMPO-H202solution, PT gave extremely high DMPO-OH signal intensity within first few minutes, and then the signal came back to that of control with time. Matsuo and Itoh reported that PT consists of catechin, catechin-3gallate, gallocatechin and gallocatechin gallate at the ratio of 1:1:2:2 (chemical formula weight 2248).15 It contains two 1,2-dihydroxyphenyl(catechol) groups and seven 1,2,3trihydroxyphenyl (pyrogallol) ones, acting as hard bases, in a chemical formula unit. As V@+, Fe3+, and UOz2+ions are classified into hard acids, they form stable complexes with both hard bases.16.17 Thus, PT couples strongly with VO2+ ion, which suppress the Fenton-like reaction. Ferrous (Fe2+)ion, being an intermediate acid, could not combine very strongly with PT, so that the apparent effect of PT was not observed in Fe2+-DMPO-H202 solution, except an initially high DMPO-OH signal. As PT has a high reduction ability,*y9 it should reduce Fe3+ produced by air oxidation to Fez+, which leads to the high DMPO-OH signal. On the other hand, the behavior of the DMPO-OH signal in U022+-DMPO-H202PT solution was quite peculiar compared with those of other metal solutions. Thus, the effect of PT on the DMPO-OH signal in U022+-DMPO-H202 solution was examined carefully in comparison with its constituent compounds, such as gallic acid (GA), catechin (CA), epicatechin (EC), epigallocatechin FGC), and epigallocatechin gallate (EGCG).
b
l4
20
I
1-
8 0
2
4 E
10
0 0
20
40
60
Reaction time (min) Figure 2. Effect of persimmon tannin (PT) on DMPO-OH signal in metalDMPO-H202 solutions. Metal ion (1 mM), DMPO (100 mM), PT (0.625 mg /ml) and H202 (100 mM) were mixed. 0 : U(VI), 0 : U(VI)-PT, : Fe(II), A : Fe(I1)-PT, : V(IV), : V(1V)-PT.
20
0
40
60
Reaction time (min) Figure 3. Effect of components compounds on DMPO-OH signal in U022+-DMPO-H202 solutions. U022+ (1 mM), DMPO (100 mM), tannins (0.153 mg /ml) and H202 ( 100 mM) were mixed. 0 : control, 0 : PT, A : GA, : CA, H : EC, : EGC, 0 : EGCG.
+
487
As shown in Fig. 3, in GA, CA and EC solutions, DMPO-OH signal increased gradually in similar manner that of control. In EGCG solution, the signal increased slightly rapidly in the first stage, and then gradually increased up to that in EC solution. In EGC solution, the signal increased very rapidly up to six-times of that in control, and then gradually decreased. As a whole, the DMPO-OH signals were enhanced by the addition of these compounds. The order of magnitude of the enhancement effect was EGC >> EGCG > PT, EC > CA, GA. Both the free radical-scavenging effect and the metal-chelation effect5 of these compounds were omitted in the present discussion, because they reveal as the suppression of DMPO-OH formation. As tannin compounds have high abilities to reduce metal ions,*>9these compounds could reduce U022+ to U02+, which precedes the formation of hydroxy radical through processes (b) and (c) in the previous section. According to this line of reasoning, EGC has the hghest reducing ability. Summarizing these results, the anomalous behavior of PT in the U022+-DMPO-H202 solution reflected the metal-reducing effect of EGC group in PT.
1. K. Kondo, M. Kurihara, N. Miyata, T. Suzuki, and M. Toyoda, Free Radical Biol. Med., 27 (1999) 855. 2. H. Yoshioka, H. Kurosalu, and H. Yoshioka, J. Radioanal. Nucl. Chem., 239 (1999) 217. 3. M. Noferi, E. Masson, A. Merlin, A. Pizzi, and X. Deglise, J. Appl. Polymer Sci., 63 (1997) 475. 4. M. Kashima, K. Saitoh, Y. Higashi, Y. Tsujimoto, and M. Yamazaki, Magnetic Resonance in Medicine, 10 (1999) 89. 5. H. Yoshioka, Y. Senba, K. Saito, T. Kimura, and F. Hayakawa, Biosci. Biotechnol. Biochem., 65 (2001) 1697. 6. T. Sakaguch and A, Nakajima, Sep. Sci. Technol., 29 (1994) 205. 7. A. Nakajima and T. Sakaguchi, J. Radioanal. Nucl. Chem., 242 (1999) 623. 8. A. Nakajima and T. Sakaguchi, J. Chem. Technol. Biotechnol., 75 (2000) 977. 9. A. Nakajima, unpublished results. 10. M. M. Hamilton, J. W. Ejnik, and A. J. Carmichael, J. Chem. SOC.Perkin Trans. (1997) 2491. 11 K. Makino, T. Hagiwara, A. Hagi, M. Hishi, and A. Murakami, Biochem. Biophys. Res. Commun., 172 (1990) 1073. 12. Y. Mizuta, T. Masumizu, M. Kohno, A. Mori, andL. Packer, Biochem. Mol. Biol. Int., 43 (1997) 1107. 13. H. Sakurai, M. Nakai, T. Miki, K. Tsuchiya, J. Takada, andR. Matsushita, Biochem. Biophys. Res. Commun., 189 (1992) 1090. 14. J. J. Katz, G. T. Seaborg, and L. R. Morss, The Chemistry of the Actinide Elements, Chapmann and Hall Ltd., New York (1986) Vol. 1, Part 1. 15. T. Matsuo and S. Itoh, Agric. Biol. Chem., 42 (1978) 1637. 16. R. G. Pearson, J. Am. Chem. SOC.,85 (1963) 3532. 17. R. D. Hancock and A. E. Martell, Chem. Rev., 89 (1989) 1875.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
488
The [2Fe-2S] cluster in sulredoxin from the thermoacidophilic archaeon strain 7, a novel water-soluble Rieske protein* Toshio Iwasakia, Asako Kounosua and Sergei A. Dikanovhc aDepartment of Biochemistry and Molecular Biology, Nippon Medical School, 1-1-5 Sendagi, Bunkyo-ku, Tokyo 113-8602, Japan bIllinois EPR Research Center and Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A. CInstitute of Chemical Kinetics and Combustion, Novosibirsk 630090, Russia The [2Fe-2S] cluster surrounding in reduced sulredoxin from the thermoacidophilic archaeon strain 7 was examined by one- and two-dimensional electron spin echo envelope modulation (ESEEM) spectroscopy. ESEEM spectra revealed two coordinated nitrogens assigned to two histidine ligands and the lines from several nonexchangeable protons. No strongly coupled protons involved in hydrogen bonds near the reduced Rieske center were detected. These results are discussed in light of the structural, redox, and spectroscopic characteristics of this ubiquitous electron transfer protein family. INTRODUCTION
The eponymous Rieske iron-sulfur protein (ISP) is an intrinsic constituent of complexes from mitochondria, chloroplasts, and bacteria, which acts to transfer reducing equivalent from a quinol bound at the Q,-site of cytochrome (cyt) b to cyt c, off[l-3]. The Rieske [2Fe-2S] cluster has a high midpoint potential of +150--k490 mV [3,4] and characteristic optical and EPR spectra that are distinctively different from those of the conventional cluster bound to four cysteine residues. The Rieske center comprises the asymmetric iron-sulfur core environment, with the Sy atom of the two cysteine residues coordinated to one iron site and with the Nfiatom of the two histidine residues coordinated to the other iron site [5,6]. Recent crystal structures [7-lo], along with others evidence [l l-131, indicated a conformational movement of the extrinsic domain of the Rieske subunit that bridges long electron transfer distances between the electron-donating and electron-accepting cofactors in the complex, apparently in association with the occupancy of the Q,-site. *This investigation was supported in part by Grants-in-aid from the Ministry of Education, Science, Sports and Culture of Japan (no. 11 169237 to T.I.) and by a grant of Cooperative Research under the Japan4.S. Cooperative Science Program from JSPS (BSAR-507 to T.I.) and NSF (INT-9910113 to S.A.D.). S.A.D. thanks the Illinois EPR Research Center (NIH grant RR01811) for assistance.
489
The [2Fe-2S] clusters in several ferredoxins involved in bacterial oxygenase systems also have properties analogous to those of the respiratory Rieske iron-sulfur proteins, and are called the "Rieske-type" ferredoxins [2,14,15]. Their redox properties are considerably different from those of the ISPs, despite the common structures of the cluster and ligands [16]. These bacterial Rieske-type ferredoxins have midpoint redox potentil below 0 mV, which are probably independent of pH, while the ISPs involved in the bcllbdcomplexes have midpoint potential well above 0 mV, which decreases upon deprotonation due to a pK on the oxidized form [3]. This has a special significance in mechanism, because the pK,,, on ISP allows it to act as a H-carrier (instead of electron carrier generally assumed), and thereby transfer both an electron and a proton away from the catalytic site. Archaeal "sulredoxin" is a novel water-soluble and high-potential Rieske-type [2Fe-2S] protein isolated from the hyperthermoacidophile Sulfolobus strain 7 (formerly Sulfolobus sp. strain 7; optimal growth conditions, pH 2.5-3 and 80 "C) [17]. It possesses = +188 mV; pK,,,I of -6.2, tightly linked ionization affecting the redox properties (&(low pK,,,2 of -8.6), which is similar to those found for the ISPs in the bcllbdcomplexes [IS]. Deprotonation of one of the two putative coordinated histidine imidazoles in sulredoxin, having the pKaloX2 of -8.6, causes a decrease in the midpoint redox potential, the change in the optical and circular dichroism spectra, and the appearance of a new Raman transition at 278 cm-1, without major structural rearrangement of the overall protein conformation [ 181. The redox potential of each particular cluster is determined by its protein surroundings and the factors that influence its value need to be considered individually for each protein at the atomic level. A variety of factors have been proposed to control the redox potential of iron-sulfur cluster including hydrogen bonding, solvent accessibility, ligand residue arrangement, the proximity and orientation of proton-based dipoles and charged residues. EPR spectroscopy plays an important role in the characterization of iron-sulfur clusters, providing direct data about their type, oxidation state, and nearest surroundings. In this work, we have applied ESEEM spectroscopy to examine the Rieske [2Fe-2S] cluster in archaeal sulredoxin, particularly to probe histidine coordination and hydrogen-deuterium exchange around the cluster. We then investigated the possible structural reorganization accompanying the reduction of the Rieske center, based on the comparison of EPR spectra of the [2Fe-2S] center reduced chemically at room temperature and radiolytically at 77 K.
2. RESULTS AND DISCUSSION
2.1. Structural features The structural gene sak coding for an archaeal sulredoxin from strain 7 has been cloned and sequenced (to be published). The deduced amino acid sequence does not contain potential signal sequence or transmembrane region at the N-terminus. The cluster binding site of sulredoxin involves total of four cysteine and two histidine residues arranged in the same motif as observed for regular Rieske iron-sulfur proteins found as a part of cyt bc complex. The sequence feature at the cluster-binding site of sulredoxin is in line with the redox properties of the protein [18]. Thus, archaeal sulredoxin can be regarded as a new hyperthermostable water-soluble redox module of the cyt bcj-associated high-potential Rieske protein as they share the same redox site cysteine and histidine motif.
490
0.00
2.S0
5.00
7.50
10.00
MHz
Figure 1. Superimposed plot of a set of three pulse ESEEM spectra of the sulredoxin stored 24 hours in excess D,O as a modulus of the Fourier transformation along T axis. Parameters: magnetic field 362.7 mT, microwave frequency 9.697 GHz; the initial is 88 ns in the nearest trace, increased by 16 ns in successive traces. F2:[MHz] I
FI:[MHz]
Figure 2. HYSCORE spectrum of reduced Rieske cluster in the sulredoxin recorded at g, = 1.91 where the EPR intensity is maximum. The time ‘c between first and second pulses 176 ns; magnetic field 362.3 mT, and microwave frequency 9.688 GHz. 2.2. EPR spectrum In all previously reported cases, the reduced Rieske [2Fe-2S] clusters exhibit a characteristic g tensor (gl = 2.01-2.02, g2 = 1.90-1.92, g3 = 1.76-1.8) with larger anisotropy and considerably lower g, =: 1.91 than those of the plant ferredoxin-type cluster (g, =: 2.042.05, g2 = 1.96, g3 = 1.87-1.88) or adrenal ferredoxin (gl =: 2.025, gy= 1.932), both with g,
49 1
= 1.96 [2]. Dithionite-reduced sulredoxin shows an EPR spectrum in the form of anisotropic line with a rhombic g tensor having principal values of gl = 2.01, g2 = 1.91, g3 = 1.79 (g, 1.90). They are typical of Rieske-type clusters [17] (Fig. 3A).
=
2.3. Nitrogen ESEEM Figure 1 shows a stacked plot of three-pulse ESEEM spectra of dithionite-reduced sulredoxin in the region 0-8 MHz. Two peaks from the spectra with maxima at 6.3 and 7.5 MHz are the characteristic indication of the Rieske type cluster reported in all previous ESEEM studies. They belong to double-quantum transitions (dq), vdq+,of two directly coordinated histidine nitrogens [ 15,191. In accordance with the properties of nitrogen nuclear frequencies, the dq transitions from the opposite electron spin manifold, (vdq.),have smaller values, with a frequency difference less than 4v,. The assignment of these transitions from three-pulse spectra is not straightforward. The direct way to determine the paired (vdq+,vdqJtransitions is using HYSCORE spectra. These transitions produce easily recognizable cross-peaks with coordinates corresponding to frequencies (vdq+)and (vdq.) in the (+-) quadrant of the HYSCORE spectra with the frequencies 7.5, 3.4 MHz and 6.3, 2.8 MHz (Fig. 2). The hyperfine coupling and quadrupole coupling parameter K = K2 (3 + q2)(K = e*qQ/h and qasymmetry parameter) can be estimated from the following expression vdq+= 2 [
.t A12)’
+ K’ 1’”
The application of Eq. (1) to two pairs of dq-frequencies gives the values A, = 5.0 MHz and A,= 3.6 MHz that is in a good agreement with the values 4.6-5.5 MHz and 3.6-4.5 MHz reported for the two nitrogens coordinated with the iron-sulfur cluster of other Rieske centers [2]. The K’ parameter constrains the quadrupole coupling constants for coordinated nitrogens to K, = 0.55.tO.04 MHz and K, = 0.65k0.04 MHz. These constrains follow the noted tendency for K of a coordinated imine nitrogen to decrease from that of an uncoordinated imidazole residue. They also agree well with the K values 0.65 and 0.57 MHz from the nitrogen of the histidine ligands in phthalate dioxygenase [19]. Thus, our ESEEM data confirm the coordination of the [2Fe-2S] cluster in the sulredoxin with two histidine nitrogens. These nitrogens belong to His-44 and His-46 because they are only histidine residues present in the sulredoxin sequence. The comparison of the nitrogen ESEEM spectra of sulredoxin with the available spectra of respiratory Rieske proteins and Rieske-type ferredoxins shows their individual character for each protein. Comparative analysis of these spectra provides us with information about either differences resulting from variation of the ligand geometry or from other groups in the local environment that influence the electronic structure of the cluster and affect the hyperfine interaction with nitrogen.
2.4. Proton and deuterium ESEEM HYSCORE spectra of the Rieske center in the sulredoxin resolve cross-peaks from three groups of nonexchangeable protons with couplings around 2-5 MHz. These couplings, however, were less then the maximum couplings 8-10 MHz observed for [2Fe-2S] cluster with four cysteine ligands in ferredoxin [20]. We tentatively assign these lines to the pprotons of cysteine ligands, because recent NMR studies of Rieske type proteins report the
492
paramagnetic shifts of these protons, which are several times larger then the protons shifts of imidazole residues [2]. This observation opens the way for the determination of cysteine ligand conformation in the sulredoxin using the approach developed on [2Fe-2S] cluster in ferredoxin [20]. This has not been studied previously by magnetic resonance. Figure 1 shows three-pulse ESEEM spectrum of the sulredoxin stored 24 hours in excess D,O. The line at -2.2 MHz is from deuterium nuclei that have replaced exchangeable protons ( i e . protons of hydrogen bonds, if present, and solvent molecules). However the intensity of this line is relatively low, especially taking into account its partial overlap with nitrogen lines. The ratio of two- and three-pulse echo envelopes of the sulredoxin prepared in D,O and H,O only shows the line at the Zeeman frequency of deuterium from weakly coupled nuclei. Also, exchangeable protons with significant coupling in HYSCORE spectra were absent. This indicates a low accessibility of solvent to the cluster and the absence of the strongly coupled protons involved in hydrogen bonds with sulfur atoms of the cluster. 2.5. Radiolytic reduction of the Rieske center in sulredoxin The traditional technique for the preparation of the reduced iron-sulfur cluster is chemical reduction in solution at room temperature by dithionite or ascorbate, followed by freezing. Another method of cluster reduction is low temperature radiolytic reduction. Since molecular motion is limited at 77 K, the cluster produced by cryogenic reduction are trapped in a constrained non-equilibrium state with ligand coordination similar to that of the initial oxidation state [21-221. Radiolytic reduction of the Rieske center in sulredoxin at 77 K, brought no significant change in the EPR spectrum as compared with the chemically reduced form (Fig. 3). The same results have been reported for monooxygenase [22]. It led to the conclusion that reduction produces negligible effects on the ligand field of the Fe” ion, which brings in the major contribution to the g-tensor anisotropy. The redox-linked deprotonation of the Rieske proteins occurs only in the oxidized state and the reduced state generated at room temperature is always protonated. Therefore, if radiolytic reduction produces the Rieske cluster with the ionizable group(s) responsible for the redox-linked ionization still in the deprotonated state, the EPR characteristics of this state might be indistinguishable from the characteristics of protonated state. On the other hand,
*
I
2.01
330
1.91
360 Magnetic Field [mT]
Sdx
390
Figure 3. X-band EPR spectra of dithionite-reduced (A) and the radiolytically-reduced sulredoxin at 77 K (B). For radiolytically reduced sample, only the g, and g, features are shown because of masking of the g, signal by strong radical signals also formed during the irradiation.
493
the interaction of the Rieske cluster with occupants of the Q,-site in the cyt complex realized over hydrogen bond formation with the ITH fragment of one histidine ligand produces noticeable changes in the EPR spectra [2,12,23]. This implies that one would expect to see a difference if the reduced form is deprotonated, and that Rieske center reduction may initiate its reversed protonation upon strong irradiation of the frozen aqueous system even at 77 K. In this case, significantly higher pH (well above pK,,,,) or lower temperatures might be required for the stabilization of the deprotonated reduced state.
REFERENCES 1. B.L. Trumpower and R.B. Gennis, Annu. Rev. Biochem., 63 (1994) 675. 2. T.A. Link, Adv. Inorg. Chem., 47 (1999) 83. 3. E.A. Berry, M. Guergova-Kuras, L.-S. Huang and A.R. Crofts, Annu. Rev. Biochem., 69 (2000) 1005. 4. M. Brugna, W. Nitschke, M. Asso, B. Guigliarelli, D. Lemesle-Meunier and C. Schmidt, J. Biol. Chem., 274 (1999) 16766. 5. S. Iwata, M. Saynovits, T.A. Link and H. Michel, Structure, 4 (1996) 567. 6. C.J. Carrell, H. Zhang, W.A. Cramer and J.L. Smith, Structure, 5 (1997) 1613. 7. D. Xia, C.-A. Yu, H. Kim, J.-Z. Xia, A.M. Kachurin, L. Zhang, L. Yu and J. Deisenhofer, Science, 277 (1997) 60. 8. Z. Zhang, L.-S. Huang, V.M. Shulmeister, Y.-I. Chi, K.K. Kim, L.-W. Hung, A.R. Crofts, E.A. Berry and S.-H. Kim, Nature, 392 (1998) 677. 9. S. Iwata, J.W. Lee, K. Okada, J.K. Lee, M. Iwata, B. Rasmussen, T.A. Link, S. Ramaswamy and B.K. Jap, Science, 281 (1998) 64. 10. H. Kim, D. Xia, C.-A. Yu, J.-Z. Xia, A.M. Kachurin, L. Zhang, L. Yu and J. Deisenhofer, Proc. Natl. Acad. Sci. U.S.A., 95 (1998) 8026. 11. H. Tian, S. White, L. Yu and C.-A. Yu, J. Biol. Chem., 274 (1999) 7146. 12. M. Brugna, S. Rodgers, A. Schricker, G. Montoya, M. Kazmeier, W. Nitschke and I. Sinning, Proc. Natl. Acad. Sci. U.S.A., 97 (2000) 2069. 13. E. Darrouzet, M. Valkova-Valchanova, C.C. Moser, P.L. Dutton and F. Daldal, Proc. Natl. Acad. Sci. U.S.A., 97 (2000) 4567. 14. B. Kauppi, K. Lee, E. Carredano, R.E. Parales, D.T. Gibson, H. Eklund and S. Ramaswamy, Structure, 6 (1 998) 57 1. 15. S.A. Dikanov, L. Xun, A.B. Karpiel, A.M. Tyryshkin and M.K. Bowman, J. Am. Chem. Soc., 118 (1996) 8408. 16. C.L. Colbert, M.M.-J. Couture, L.D. Eltis and J. Bolin, Structure, 8 (2000) 1267. 17. T. Iwasaki, T. Isogai, T. Iizuka and T. Oshima, J. Bacteriol., 177 (1995) 2576. 18. T. Iwasaki, T. Imai, A. Urushiyama and T. Oshima, J. Biol. Chem., 271 (1996) 27659. 19. R.J. Gurbiel, C.J. Batie, M. Sivaraja, A.E. True, J.A. Fee, B.M. Hoffman and D.P. Ballou, Biochemistry, 28 (1989) 4861. 20. S.A. Dikanov and M.K. Bowman, J. Biol. Inorg. Chem., 3 (1998) 18. 21. R. Davydov, S. Kuprin, A. Graslund and A. Ehrenberg, J. Am. Chem. Soc., 116 (1994) 11120. 22. S.A. Dikanov, R.M. Davydov, L. Xun and M.K. Bowman, J. Magn. Reson. Ser. B, I12 (1996) 289. 23. M. Guergova-Kuras, R. Kuras, N. Ugulava, I. Hadad and A.R. Crofts, Biochemistry, 39 (2000) 7436.
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EPR in the 21' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
EPR and saturation recovery investigations of spin probes in dispersions of hydrogenated castor oil Kouichi Nakagawa Research Center, Fukushima Medical University, 1 Hikarigaoka, Fukushima, 960-1295, Japan Rotational correlation times and electron spin-lattice relaxation times (T,J of spin probes in 10 wt % dispersions of poly(oxyethy1ene) hydrogenated castor oil (HCO) were investigated using continuous wave (CW) and saturation recovery (SR) electron paramagnetic resonance (EPR) spectroscopy. The TI,, and activation energy (EJ were obtained in the system for the first time. The E, calculated from the correlation times in the HCO vesicle phase was -6.8 [kcal/mol] for TEMPO and DTBN. TI, was measured using SR as a function of temperature (22-50 "C) for spin probes in the aqueous and vesicle phases. TI, is about the same for TEMPO in the vesicle, TEMPO in the aqueous phase, and DTBN in the aqueous phase. T,, was shorter for DTBN in the vesicles than in the aqueous phase. Shorter TI, of DTBN in the vesicle phase can be attributed to location of DTBN within the vesicle. For both TEMPO and DTBN, there was no significant difference between T,, obtained in H,O and D,O solutions. 1. INTRODUCTION Nonionic surface active agents have been of great interest in the fields of cosmetics well as pharmaceuticals (1). In drug encapsulation, vesicle formation of the surfactant is necessary. In addition, nontoxic and naturally degradable substances are desired. Recently, a poly(oxyethy1ene) hydrogenated castor oil (HCO) was found to be a natural nonionic surfactant that forms vesicles (2, 3). Initial investigation of HCO vesicle formation in the aqueous dispersions was made by Horiuchi and co-workers (3). A stable mutilamellar vesicle with an average diameter of -500 nm was observed. Additional physical characterization of the vesicle for the nonionic amphiphilic compound HCO could lead to detailed understanding of the bilayer structure as well its physical behavior. Electron paramagnetic resonance (EPR) techniques in conjunction with spin probe methods are useful for characterization of micellar and bilayer structure well (4-5). Spin probe methodology is a powerful technique for studying molecular dynamics in biological and physical sciences on timescales from nanoseconds to milliseconds (4). The lineshape of the EPR signal can be analyzed to determine the rotational correlation time and electron spin-lattice relaxation times (TI,) provide insight concerning processes by which spin probes exchange energy with the surrounding membrane (6). Measurement of TI, is useful for monitoring Heisenberg exchange between oxygen and spin labels in biological systems (7-10).
E-mail: [email protected]
The effective Ti, mechanism leads to a description of membrane fluidity, including translational diffusion. Furthermore, theoretical understanding of the longitudinal relaxation mechanism is still controversial (10-12). In the present study, rotational correlation times and longitudinal relaxation times (T,J of various spin probes in aqueous solution containing a dispersion of poly(oxyethy1ene) hydrogenated castor oil were investigated for the first time. The behavior of spin probes in both the aqueous and vesicle phase a function of temperature was studied. The correlation times of spin probes are also discussed in relation to Tie. 2. MATERIALS AND METHODS 2.1. Samples Poly(oxyethy1ene) hydrogenated castor oil (termed HCO) was donated by Nikko Chemicals Co. Ltd. (Tokyo, Japan) and used as received. The HCO had about 10 moles of oxyethylene moieties per mole of oil. The chemical structures of HCO and the spin probes used in this study are depicted in Figure 1. The spin probes, di-tert-butyl nitroxide 0 o.(cn,cn,o).H (DTBN), 2, 2, 6, 6-tretramethylpiperidine-1~2~o-(cn2CHzo) o-(cn2cn20)p oxyl (TEMPO), 4-hydroxy-2, 2, 6, C H ~ O ( C H ~ C H ~ O o.(cti,cn20),n (TEMPOL), 4cI n , . o - ~ c H , c H , o ) , - P I 1-oxyl Oxyethylene Group = I + m + n + x + y + z (TEMPONE), and 3-(aminomethyl)-proxy1 - 10 (CTPO) were obtained from Aldrich Chemical Fi ure 1. Molecular structure of Co. and used as received. poTy(oxyethy1ene) HCO. A weighed amount of HCO was dissolved in a few milliliters of chloroform (13). The spin probe was dissolved in -0.3 milliliters of chloroform and mixed with the HCO solution. After evaporation of the chloroform on a rotary evaporator, a 10 wt % dispersion of HCO/spin probe in distilled water was prepared. The final concentration of the spin probe was approximately 10 micro molar for CW EPR and 100 micro molar for saturation recovery. 2.2. Deoxygenation For CW EPR, the sample solutions were degassed about 15 minutes in an AtmosBag (Aldrich, USA) and the solutions were put into capillaries (I.D., 0.9 mm; O.D., 1.4 mm; Nippon Rikagaku Kikai Co. Ltd., Japan). The sample capillary was inserted in a 3 mm EPR tube (JEOL Datum) in the AtmosBag and taped around the tube cap. Degassing for SR measurements was carried out using TPX gas-permeable plastic capillaries (7-10, 14). We used Teflon tubes to achieve degassing in the following manner. Two Teflon tubes were inserted side-by-side in a 4 mm EPR tube. One Teflon tube (I.D., 0.96 mm; O.D., 1.56 mm) contained the sample solution. Nitrogen was passed through the second Teflon tube (I.D., 1.5mm; O.D., 2 mm) to purge oxygen from the solution. The degree of degassing of an aqueous CTPO (0.2 mM) solution after 90 minutes was estimated to be -98% using the equation provided by Hyde and co-workers (14). 2.3. CW EPR Measurements EPR measurements were made with a 9 GHz JEOL FE 1XG spectrometer with a TE,,, cylindrical cavity. Sample temperature was controlled by nitrogen gas flow through the
)
~
-
~
496
Dewar using JEOL ES-DVT system. EPR signals were digitized using a Scientific Software Service System (Illinois, USA). The microwave frequency was measured using an EMC-14 X-band microwave frequency counter (Echo ElectronicsCo., Ltd., Japan). 2.4. Rotational Correlation Time Various methods have been developed for determining the correlation time for molecular motion based on changes in the amplitude, position, and widths of EPR lines (15, 16). Rotational correlation time (zR) of the order of 10"' sec can be estimated from spectra of a nitroxyl spin probe using equation (1) (17-20),
where p, is the electron Bohr magneton, R is Plank's constant, I is peak amplitude and the subscripts, +1, 0, -1, are nuclear quantum numbers for I4N. is the peak-to-peak line width of the centerline. Values of g,,, = (1/3)(gn + g, +gJ and b = (2/3)[AA,- 0.5(An +A,)] were calculated from the parameters for an immobilized spin probe. Calculations of for TEMPO and DTBN were made using the magnetic principal values (21). The values of g's and A's were assumed to be appropriate. The parameters I+l, I,, I+ and AH,,, obtained from the experimental spectra were used to calculate zR.
2.5. Saturation Recovery (SR) Measurements Electron spin-lattice relaxation time (TI,) was measured on a home-built SR spectrometer at the University of Denver (22). A 5-loop-4-gap resonator (LGR, Medical Advances model XP-0201 (Milwaukee, Wisconsin)) was used (23). The klystron was locked via an AFC circuit to a high-Q cylindrical external reference cavity. The EPR signal was amplified by a lownoise GaAsFET amplifier and detected by use of a double-balanced mixer. No magnetic field modulation was used. Sample temperature was controlled using a Varian V-6040 with nitrogen gas passed over the LGR. Sample temperature was monitored with a thermocouple positioned immediately above the resonator. The magnetic field for recording the recovery signal was set on the high-field nitrogen hyperfine line for the probe in the vesicle or aqueous phase. Artifacts were removed by subtracting an instrumental background response that was measured with the magnetic field set 100 G higher than for the signal. The signal was amplified and then digitized with an EG&G 9825 in a Pentium PC. Usually -3 lo5 recovery signals of about 1000 digitized points were averaged. The digitized signal was fitted to a single exponential. The data were recorded with pump times of 5 ps, which are long relative to the recovery time constants and relative to the tumbling correlation times. Under these conditions the contribution to the recovery due to spectral diffusion is minimized and the time constant is assigned as TI,. 3. RESULTS AND DISCUSSION
3.1. CW EPR of Spin Probes CW EPR spectra of spin probes DTBN and TEMPO in aqueous HCO dispersions consist of two overlapping triplets with slightly different g and hyperfine values presented in
491
Figure 2. The contributions from the two triplets are best resolved for the high-field hyperfine line. EPR lines from the probes in the vesicle phase are broader than those from the aqueous phase. Molecular motions of the spin probes in the vesicle are somewhat restricted. In addition, the EPR spectrum obtained from DTBN/HCO/H,O shows relatively sharp EPR lines. The linewidth difference between the aqueous and vesicle phase is more significant than the one for TEMPO. Relative partitioning of spin probes between the aqueous and vesicle phases was studied a function of temperature. Focusing on the high-field lines, the relative amplitude of the signal from a probe in the vesicle increases as temperature increases. The EPR signals from the vesicle phase becomes dominant above -40 "C. In order to verify the partitioning of spin probes, the amplitudes of the aqueous peaks were analyzed indicated in Figure 2. Linewidth change at half-height in the aqueous phase was minimal throughout the temperature studied. The normalized intensities of the aqueous peaks are plotted in Figure 3. The magnitude of the intensity reduction for DTBN was more significant than that of TEMPO. The results suggest that the temperature dependence of DTBN partitioning is more efficient in comparison with TEMPO. When TEMPONE and TEMPOL were examined in dispersions similar to those examined with DTBN and TEMPO, there were no distinguishable vesicle peaks for TEMPONE and TEMPOL at any temperature studied. The EPR hyperfine splitting suggests that both TEMPONE and TEMPOL might remain in the aqueous phase or might have very small gvalue and hyperfine differences in the two phases. In order to analyze the molecular motion of spin probes in the vesicle, the rotational correlation time (zd was calculated using eq. (1). EPR spectra of the vesicle phase were obtained by subtracting two spectra at the same experimental conditions, but with different partitioning of the probe between the vesicle and aqueous phases. The subtraction eliminated the aqueous peak. Based on the EPR spectrum of the vesicle phase, was obtained. In addition, an Arrhenius analysis of the rotational correlation time in the vesicle phase gave the activation energy. The activation energies obtained for TEMPO and DTBN are 6.6 0.4 and 7.1 0.4 [kcaVmol], respectively. Both activation energies in this phase are similar. The calculated for TEMPONE in TEMPONE/HCO/H,O is approximately 5 x lo-" s and is independent of temperature in the 20-50 "C range. The estimated from the spectra for TEMPO and DTBN in the aqueous phase was close to this value. Similar values were obtained for TEMPOL over the same temperature range. The correlation times of TEMPOL and TEMPONE in water were one order of magnitude shorter than for TEMPO in the vesicle phase.
~~~~
1:: P
0 5
10
20
40
50
Temperature ("C)
Figure 2. CW EPR s ectrum of DTBN obtained in aqueous gspersion of HCO.
Figure 3. Normalized intensity of aqueous phase for DTBN and TEMPO in dispersion of HCO.
498
3.2. Spin Lattice Relaxation Time (Tie) of Spin Probes Direct observation of the interaction between spin probes and lattice (membrane) was accomplished. TI, for the probes in the aqueous and vesicle phases was measured at the magnetic field that corresponds to the maximum intensity in the first-integral spectrum for each of the components of the high-field hyperfine line. The SR signal from the vesicle peak in DTBN/HCO/H,O system at 22 "C is presented in Figure 4. A single exponential fit to the experimental data is shown in the dotted line. At this temperature the signal-to-noise ratio of the SR signal from the vesicle phase is poorer than that for the aqueous phase because of weaker EPR intensity. The value of T,, for TEMPONE in H,O is very similar to that reported previously (10). With reference to the values obtained, TI, of DTBN in the vesicle phase is close to that reported for DTBN in paraffin oil. T,, of TEMPO in the HCO vesicle is slightly shorter the one for TEMPO in paraffin oil. TI, of the probes in the vesicle and aqueous phases showed little temperature dependence in the range examined (Figure 5). The relaxation times for TEMPO and DTBN in the aqueous phase are similar and consistent with a shorter in the phase. The TI, of TEMPO is similar in the aqueous and vesicle phases. The TI, of DTBN in vesicle phase is slightly shorter than the one for aqueous phase. The difference in TI, obtained from direct SR measurements can be due to the interaction between the probe and the environment. Thus, a shorter TI, for DTBN in the vesicle phase implies that DTBN is located within the vesicle. In addition, shortening of TI of DTBN towards the terminal methyl group was shown by NMR (24). The relative resistance might account for the results obtained. On the other hand, the and longer TI, in the vesicle suggest that TEMPO may locate slightly different region from DTBN.
=
ps
Time I p s
Figure 4. Saturation recovery signal from DTBN in H,O dispersion of HCO. The dotted line indicates a single exponential fit to obtain T,,.
Temperature /"C
Figure 5. Plot of TI for DTBN (aqueous: 0 , vesicle: H) and YEMPO (aqueous: 0, vesicle: +) in H,O dispersion of HCO as a function temperature.
Furthermore, we made measurements in D,O dispersions. The T,, values of DTBN and TEMPO for the aqueous and.vesicle phase are similar to the case of H,O. The present SR measurements on T,, did not show clear evidence of dehydration of the membrane based on the present results regarding the dispersion of HCO in H,O or D,O. It may be due partly to location of the spin probes not near the oxyethylene moiety.
Acknowledgment. Author thanks Profs. Sandra S. Eaton and Gareth R. Eaton for the use of saturation recovery apparatus, and fruitful discussion concerning the results.
499
REFERENCES 1. Y. Okahata, S. Tanamachi, M. Nagai, and Y. Kumitake, J. Colloid Interface Sci., 82 (1981) 401. 2. M. Tanaka, H. Fukuda, and T. Horiuchi, J. Am. Oil Chem. SOC.,67 (1990) 55. 3. T. Honuchi and K. Tajima, Yukagaku, 41 (1992) 1197. 4. L. J. Berliner (ed.), Spin Labeling, Theory and Applications, Academic Press, New York, 1976. 5. W. K. Subczynski, A. H. Lewis, R. N. McElhaney, R. S. Hodges, J. S. Hyde, and A. Kusumi, Biochemistry, 37 (1998) 3156. 6. G. R. Eaton and S. S. Eaton, In Biological Magnetic Resonance Vol. 19, G. R. Eaton, S. S. Eaton, and L. J. Berliner (eds.), Chapter 2, Plenum Press, New York, 2000. 7. W. K. Subczynski and J. S. Hyde, Biochim. Biophys. Acta, 643 (1981) 283. 8. W. K. Subczynski, J. S. Hyde, and A. Kusumi, Biochemistry, 30 (1991) 8578. 9. A. Kusumi, W. K. Subczynski, and J. S. Hyde, Proc. Natl. Acad. Sci. USA, 70 (1982) 1854. 10. J. S. Hyde and J. B. Feix, In Biological Magnetic Resonance Vol. 8, L. J. Berliner and J. Reuben, J. (eds.), Chapter 6, Plenum Press, New York, 1989. 11. B. H. Robinson, D. A. Haas, and C. Mailer, Science, 263 (1994) 490. 12. B. H. Robinson, A. W. Reese, E. Gibbons, and C. Mailer, J. Phys. Chem. B, 103 (1999) 5881. 13. R. R. C. New, (ed.), In Liposomes, A Practical Approach, Chapter 2, Oxford University Press, Oxford, 1990. 14. C-S. Lai, L. E. Hopwood, J. S. Hyde, and S. Lukiewicz, Proc. Natl. Acad. Sci. USA, 79 (1982) 1166. 15. D. Kivelson, J. Chem. Phys., 33 (1960) 1094. 16. R. Wilson and D. Kivelson, J. Chem. Phys., 44 (1966) 154. 17. N. D. Chasteen, and M. W. Hanna, J. Phys. Chem., 76 (1976) 3951. 18. A. L. Buchachenko, A. L. Kovarskii, A. M. Vasserman, In Advances in Polymer Science, Z . A. Rogovin (ed.), p. 26. Wiley, New York, 1974. 19. D. J. Schneider and J. H. Freed, Biol. Magn. Reson., 8 (1989) 1. 20. K. Nakagawa and K. Tajima, Langmuir, 14 (1998) 6409. 21. L. J. Berliner (ed.), Spin Labeling, Theory and Applications, Appendix 11, p. 565. Academic Press, New York, 1976. 22. R. W. Quine, S. S. Eaton, and G. R. Eaton, Rev. Sci. Instrum., 63 (1992) 4251. 23. G. A. Rinard, R. W. Quine, S. S. Eaton, G. R. Eaton, and W. Froncisz, J. Magn. Reson. A, 108 (1994) 71. 24. J. A. Dix, D. Kivelson, and J. M. Diamond, J. Membr. Biol., 40 (1978) 3 15.
Section 6 Medical Sciences
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EPR in the 21'' Century A Kawarnori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
503
Advances in the spin labeling method Lawrence J. Berliner Dept. Chemistry & Biochemistry, University of Denver 2190 E. Iliff Avenue, Denver, CO 80208, U.S.A. 1. INTRODUCTION
The spin labeling technique, coined by Harden McConnell in the early 1960s, was modeled after the "reporter group" approach of placing a spectroscopic probe in a biological system such as an enzyme or a protein in order to monitor spectroscopic variables to which the biosystem alone is transparent. This might be a fluorophore, NMR isotope such as 'F or I3C, resonance Raman probe, a stable paramagnetic nitroxyl molecule, etc. The spin label technique has flourished since then with several texts devoted specifically to the subject. Spin labels have now enjoyed a history of close to twenty-five years in demonstrating the applicability of paramagnetic nitroxides to biochemical problems of structure and function in enzymes, membranes, cells and animals (1-4). These have been employed traditionally as reporter groups; that is, the nitroxide spin label serves as a physical probe that reports aspects of its structure and environment at a localized molecular site. It has been quoted many times that this reporter group must "report the news" not "make the news". That is, it is important to insure that the sometimes bulky nitroxide spin label, does not perturb the macromolecular system under study. Although the method is technically not restricted to just nitroxyl compounds (most authors use the term nitroxide for these structures) it is fair to say that more than 99.5% of all papers using ESR reporter group techniques employ nitroxyl (nitroxide) spin labels. These molecules are classified as spin when a covalent linkage to the biological system is employed (i.e. with proteins, polymers and nucleic acids) while a spin probe involves noncovalent interactions (cells, membranes, liquid crystals and some polymer systems). 2. WHAT THE SPECIFIC BIOLOGICAL SYSTEMS/APPLICATIONS OF SPIN LABELS?
The answer to this question is literally everything large and small: proteins and enzymes, lipids and membranes, nucleic acids, pharmaceuticals, drug-receptor interactions, cells and cell membranes, polymers, animals. A number of examples are presented here.
5 04
2.1. Stoichiometry and specificity A major obstacle for reporter group (labeling) studies is obtaining a unique, specific covalent modification of a protein or enzyme that yields a 1:l nitroxyl label to macromolecule complex. That is, the labeling must be to one amino acid and not, e.g., 0.3 here, 0.4 there, and 0.3 spread over several others. Otherwise the spectral results will be ambiguous and any eventual distance measurement will weight the “closest” spins inordinately. The most ideal candidate is the thiol side chain (-SH) of the amino acid cysteine (Cys). However, it is frequently rare to find free cysteines in proteins since they are mostly always coupled to another cysteine in a disulfide (cystine) bridge. On the other hand, most thiol (cysteine) reagents are very specific, particularly the methanethiomethane sulfonate analogs. The technique coined as “site-directed spin labeling” (SDSL) offers unique local probing of the environment at a specific amino acid residue in a protein or protein complex of interest (2). It has been especially powerful in discerning membrane bound vs. exposed segments, distinguishing aspects of secondary structure (alpha- vs. beta vs. random structure), identifying helix-helix interfaces and facilitating intramolecular distance measurement s within a protein. The details of the method involve site directed ‘cysteine scanning’ or specific incorporation of cysteine residues at targeted regions of interest on the protein molecule. The technology allows one to conformationally examine several different positions in a protein representing a significant fraction of the amino acid sequence. The Cys residue is subsequently spin labeled with specific, reversible covalent probes which are analogs of methanemethylthiol thiosulfonate, R-S-SOZ-CH~, where R is a pyrrolinyl, pyrrolidinyl or piperidinyl nitroxyl group. The second critical advance involved utilizing molecular biological techniques to incorporate cysteine groups at different selected positions in a protein. Hubbell first demonstrated this methodology with membrane-bound protein bacteriorhodopsin (5). 2.2. The reporter group approach -who makes the news? While spectroscopists are ‘purists’ in that they observe natural spins (nuclei) without altering the physical or chemical environment, spin labels and fluorescent probes are by necessity exogenously introduce, sometimes bulky structures. Consequently, spin labeling is constantly accused of making the news instead of reporting the news. In fact, the fluorescence community occasionally accuses the EPR community, while neglecting the typically bi- and tricyclic aromatic molecules employed in luminescence studies. When substituting a nitroxyl side chain in a specific position in a protein, some type of structural perturbation may be produced, but the evidence continually shows that the majority, including those at internal positions in a protein, do not result in debilitating structure/function changes while the thermal stability of the protein may be altered somewhat. The fact is that one must slightly perturb or interact with the macromolecular environment in order to report meaningful information.
2.3. Site directed spin labeling (SDSL) - how is it done? Briefly, you must specifically incorporate one (or occasionally two) thiol groups in a protein sequence and subsequently covalently label it with a uniquely specific reactive, reversible spin label. Berliner and coworkers (6) introduced the MTSL label and demonstrated its efficacy with a highly reactive thiol protease, papain. This was followed by the groundbreaking work of Hubbell and coworkers mentioned earlier (5). The following
505
protocols and parameters, taken in part from the chapter of Feix and Klug (7), summarizes the SDSL paradigm and what we can learn. 2.3.3. The SDSL paradigm
no
by 2.3.4. SDSL parameters 0 2
-
-
The real success of this method has been the possibility of describing protein structure almost entirely from the ESR results. That is, one can determine local rigidity and proximity to other structural elements, accessibility to solvent or the nonpolar interior and quantitative conformational movements form double labeling studies. In fact, the trends and cycles in increasing and decreasing exposure (accessibility) can trace an alpha-helical backbone or beta-sheet structure. By utilizing MTSL labels of varying tether length and rigidity one can confirm these results unambiguously. 2.4. Spin labeled DNA - structure, dynamics and DNA sequence analysis The major challenge in probing DNA structure and conformation was introducing the probe at sensitive sites (ie., purine or pyrimidine bases) which sense local structure, global motion, DNA wobbling, etc. A selected sample of a range of modified pyrimidine bases are shown in Figure 1. These labels offer properties from very rigid, restricted tumbling volumes to very flexible (8). Figure 2 depicts a motional model of the various features Thai contribute to the EPR spectrum. Needless to say, the plethora of modified basis, the methodology to incorporate them into various polynucleotides, etc., is a great feat.
506
UUMHT
y;-%y-. R t*?. yij+o DCAVAP I
O l NI
DCAT
Jy;-y. I DCAP
Figure 1. A selection of structures of spin labeled pyrimidine bases that can be incorporated into nucleic acid structures (adapted from reference 8 with permission).
Figure 2. The nitroxide (reporter group) monitors DNA dynamic processes according to the degree of coupling to each mode. The nitroxide ring is couple the base motion through a tether linkage. The labeled bas is couple to another base which together experience base pair motion. This base pair is couple to the collective bending and twisting of all base pairs in the helix. Collective motions are also coupled to the global tumbling (adapted from reference 8 with permission).
507
2.5. pH Sensitive Spin Labels EPR has the advantage of being a noninvasive, nondestructive technique. Recently, Volodarsky and Khramtsov have synthesized and tested imidazoline nitroxides which can be protonated (9). These labels have been tested over an array of pH ranges, each one shown in Figure 3 acting as a pH range probe. Where the results have been unusually impressive are with in vivo studies where the stomach pH of a laboratory animal was determined noninvasively. In addition, one could covalently attach a pH sensitive imidazoline spin label on a protein, perhaps based on a dithio label or a methylthiolsulfonate analog (ie a pH sensitive MTSL). Consequently, the local pH near the labeling site and/or the environment of the labeled protein could be determined.
16.2
8
z
15.4
S
0
14.6
0
2
4
8
1
0
1
2
1
4
Figure 3. Imidazoline nitroxide protonation and a plot of pH induced changes in the hyperfine splitting for eight different imidazoline nitroxides (adapted from reference 9 with permission). 3. SPIN LABELS AND PHARMACOLOGY - IN VIVO ESR/EPR
There has been a plethora of studies on the metabolic fate of nitroxyl compounds in vivo, which in all cases lead to the reduced hydroxylamine product. Various research groups have taken advantage on the pharmacokinetics to learn something about the redox state of the suborganelles where the nitroxide localizes. This has been particularly instructive with the use of EPR imaging or localized spectroscopy. However, this is not technically the case of a ‘spin labeled’ animal since the nitroxyl is simply circulating throughout the tissue and is not a new covalent product. On the other hand some intriguing results have come form the study of nitrosobenzene and its analogs with living mice and rats. 3.1. Nitrosobenzene Nitrosobenzene is a potent, toxic mutagen. Hence, we want to avoid exposure to even trace levels. One should note however, as shown in Figure 4 that there are various oxidation
508
states of nitrogen substituted aromatics going from aniline through to nitrobenzene and that there are hepatic and intestinal enzymes capable of converting anilines and nitrobenzenes to the nitroso form. Our studies focused on the radical products from nitrosobenzene both vitro and vivo.
Aniline
phenylhydroxylarnine Nitrosobenzene
Nitrobenzene
Figure 4. Oxidation states of nitrogen substituted aromatics Typical X-band spectra of nitrosobenzene in either hemolyzed blood or in NADH or vitamin C are shown in Figure 5. This spectrum was simulated as the phenylhydronitroxide radical from a one-electron reduction of nitrosobenzene. This spectrum will persist only if continuous, stoichiometric oxygen is supplied (lo). The overall scheme depicting this chemistry is shown in Figure 6. Although most biological redox chemistry is two electron, this radical can be produced from either the one electron "back oxidation" of the hydroxylamine by oxygen (yielding also superoxide) or as a result of the disproportionation between nitrosobenzene and
Phen) Ihydronitrode radical
Figure 6. Reduction pathway of nitrosobenzene Figure 5. X-band EPR (A) Nitrosobenzene and excess NADH (also found with red blood cell hemolysates and 3 mM nitrosobenzene); (B) Sample A lus yeast glucose oxidase which depletes a\ oxy en (C) Sample A with continuous oxygen. EPR conditions were microwave power, 5 mW; modulation amplitude, 0.1 auss; time constant, 0.1 sec; scan rate, 5 8 gauss/min. From reference 10 with permission.
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3.1.1. Pathway of nitrosobenzene in-vivo A 15-20 g mouse was injected with an ethanol or liposome suspension of nitrosobenzene and the L-band in vivo EPR spectrum measured after 20min (Figure 7). As the control, the animal was injected with the (liposome suspension or ethanol) vehicle alone (1 1). Note the broad three line spectral component in Figure 7a, which is an immobilized nitroxyl radical, but a slowly tumbling phenylhydronitroxyl radical, such as seen in Figure 5. This spectrum remained persistent for many hours (in fact this broad-line EPR spectrum remained days after death!). If buttocks muscle tissue was excised from an anesthetized mouse within 5 minutes after injection, on observed a multiline phenylhydronitroxide radical spectral component which disappeared with time, leading to the broad three-line nitroxyl component shown in Figure 6. Most likely, the early multiline spectral component appeared since hemolysis occurs during tissue excision. The precise chemical nature of this adduct was suggested most convincingly by observing this excised muscle tissue at high microwave power and comparing it with the radical adduct obtained from mixing nitrosobenzene with oleic acid or any other fatty acid (Figure 7). The product in muscle tissue was spectrally identical with that from a pure oleic acid sample. The mechanism of this adduct formation is shown in Figure 8, which is commonly called an “ene-addition” or pseudo Diels-Alder reaction (Sullivan. 1966), resulting in a migration of the double bond, then formation of an hydroxylamine intermediate which is easily oxidized to the nitroxyl adduct (nitroxide radical I).
10 G
Figure 6. (a) L-Band EPR spectra of an anesthetized (25g) mouse at room temperature. (a) 20 min. after an ip injection of O.lml of 50mM nitrosobenzene (in a liposome suspension); (b) control injection without nitrosobenzene. Spectrometer conditions: frequency, 1.128GHz; microwave power, 10mW; modulation, 1.9G; applied magnetic field, 390G; sweep width, 100G; sweep rate, 25G/min.; time constant, 1.O. From ref. 11 with permission.
Figure 7. X-Band EPR spectra of excised muscle tissue after in-vivo i.m. administration of 0. lml 50mM nitrosobenzene (in ethanol) in the mouse buttocks: (a) 20 min. or (b) 80 min. after injection, and (c) 200 min. after injection. The muscle tissue was removed within 5 minutes after injection. Spectrometer conditions were: microwave power, 10mW; kequency, 9.25GHz; modulation, 0.5G; applied magnetic field, 3400G; sweep width, 100G; sweep rate, 25G/min.; time constant, 0.128s. From ref. 11 with permission. In summary, nitrosobenzene form persistent radical adducts with unsaturated fatty acids in fat tissue or membranes. It is considerably less accessible to bioreduction, yielding a strong, stable nitroxyl EPR spectrum. Recall that there are metabolic pathways for converting anilino- and nitroaromatics to the nitroso analog (see Figure hence one may not be able to avoid nitroso compounds that may further lead to stable nitroxyl radical adducts in fatty tissue. Lastly, these adducts have been shown to catalyze lipid peroxidation (12).
Ph-N=O
i
or 0,-.+
H H N Ph'
nitroxide radical (1) Figure 8. Pseudo Diels-Alder "ene-addition" reaction scheme
511
Both the nitroxyl radical and its hydroxylamine are potentially capable of reducing trace femc to ferrous ion with possible initiation of lipid peroxidation (13). 3.2. Nifedipine: a pharmaceutical that forms nitrosoaromatic intermediates Nifedipene dimethylester) is a very popular slow calcium channel blocker or calcium ion antagonist. It lowers blood pressure by inhibiting the transmembrane influx of Ca(I1) into cardiac and smooth muscle and by inhibiting calcium ion flux across the cell membrane (14). Additionally, nifedipene can be converted by high power illumination, or by prolonged normal room light, to its nitroso-fonn under (15-17) as shown in Figure 9. Extensive exposure to the sun may result in severe adverse skin inflammation (18).
Yifedipine
COOCHj
H3C00C
-
H3C00C
/
COOCHj )OCH3
,
H3C' I
H3C
CH3 33
NitrosoNifedipine
H
Figure 9. Nifedipene photochemically converted to its nitroso-form. A 100 mM nifedipine solution (in DMSO), illuminated under 15 W visible light for hr. was injected as a 100 pL bolus i.m. in a 15g anesthetized mouse. The animal was then placed in an L-band EPR loop gap resonator 15 minutes later (17). A broad three-line spectrum was observed, which persisted for more than 1 hr. shown in Figure 10. Excised tissue observed at X-band 15 min after injection yielded'an even stronger signal, especially in the liver (Figure 10).
512
gauss
Figure 10. L-band in-vivo EPR spectra of an anesthetized (15g) mouse at room temperature 15 min after an im injection of 100 mM illuminated nifedipine (100 pL in DMSO). Spectrometer conditions were similar to those noted in Figure X-band ex-vivo EPR spectra of excised liver tissue and buttocks muscle. Spectrometer conditions were similar to those noted in Figure 6. Nifedipine solutions were illuminated by placing solutions in a 5-mL (8 mm i.d.) glass test tube on a 15 W common household fluorescent tube for 24 hrs. Adapted from ref. 11 with permission. Since the liver contains nitroreductases, conversion of nifedipine to its nitroso analog could potentially occur Figures 11A and B compare X-band EPR spectra of freshly excised livers treated with illuminated and non-illuminated nifedipine, respectively. the two spectra are superimposable, suggesting that non-illuminated nifedipine was converted to its nitroso analog by hepatic nitroreductases. The relative level of radical adduct was 10% in the non illuminated nifedipine case, however, this is still a quite
513
respectable level (17). Nifedipine analogs which are meta-substituted (i.e. nimodipine, nicardipine) do not show radical adducts when illuminated. They appear to be less susceptible to conversion to the nitrosoforms (15).
Figure 11. X-band EPR spectra of liver tissue exposed to (a) non-illuminated and (b) illuminated nifedipine. Nifedipine solutions were freshly prepared and soaked with liver tissue for 5 min at room temperature in the dark. Illuminated nifedipine was prepared as described in Figure 10. Adapted from ref. 11 with permission. Epidemiological studies showed that patients have a 40-60% higher susceptibility to myocardial infarction with nifedipine than with other classes of slow calcium channel blockers (19,20). Furthermore, as mentioned earlier, nitroxyl radical adducts of unsaturated fatty acids can support lipid peroxidation (12). Both the radical and the hydroxylamine are potentially capable of reducing trace Fe(II1) to Fe(I1) with possible initiation of lipid peroxidation (1 3). Considering the many controversies over free radicals, reactive oxygen species, and lipid peroxidation in heart disease, the radical adducts found with nifedipine are worth further study. 4. SYNOPSIS
The spin label technique offers some unique advantages over other biophysical methods of probing conformation in biological systems. There are no real molecular weight limits; optical transparency or physical homogeneity of the sample is not required; sensitivity is much higher than, e.g., NMR, intramolecular distances can be assessed and the tools of molecular biology now allow one to virtually label any position in a protein or enzyme. This technology leaves great promise for future studies of proteins and enzymes.
5 14
REFERENCES 1. L.J. Berliner (ed.), Spin Labeling: Theory and Applications, Academic Press, New York, 1976. 2. L.J. Berliner (ed.), Spin Labeling 11: Theory and Applications, Academic Press, New York, 1979. 3. L.J. Berliner and J. Reuben (eds.), Spin Labeling: Theory and Applications, Biological Magnetic Resonance, Volume 8, Plenum., New York, 1989. L.J. Berliner and J. Reuben (eds.), Spin Labeling: The Next Millenium, Biological Magnetic Resonance, Volume 14, Plenum., New York, 1998. 5. C. Altenbach, T. Marti, H.G. Khorana and W.L. Hubbell. Science, 24 (1990) 1088. 6. L.J. Berliner, J. Grunwald, H.O. Hankovszky and K. Hideg, Anal. Biochem., 119 (1982) 450 7. J. B. Feix and C.S. Klug, Biol. Magn. Reson., 14 (1998) 251. 8. R.S. Keyes and A.M. Bobst, Biological Magnetic Resonance 14 (1998), 283. 9. V.V. Khramtsov and L.B. Volodarsky, Biological Magnetic Resonance 14 (1998), 109. 10. H. Fujii, and L.J. Berliner, Free Radical Research Commun. 21 (1994) 235. 11. H. Fujii, B. Zhao, J. Koscielniak and L.J. Berliner, Magn. Reson. Med., 3 1 (1994) 77. 12. L. J. Sammartano and D. Malejka-Giganti,Chem. Biol. Interact., 77 (1991) 63. 13. G. Minotti and S. D. Aust,. J. Biol. Chem., 262 (1987) 1098. 14. K. A. Lamping and G. J. Gross, J. Cardiovasc. Pharmacol., 7 (1985) 158. 15. V. Misik, A. Stasko D. Gergel and K. Ondrias, Molec. Pharmacol., 40 (1991) 435. 16. A. Stasko, V. Brezova, S. Biskupic, K. Ondrias and V. Misik, Free Rad. Biol. Med., 17 (1994) 545. 17. H. Fujii and L.J. Berliner, Magn. Reson. Med., 42 (1999) 691. 18. S. E. Thomas and M. L. Wood, Brit. Med. J. 292, (1986) 992. 19. B.M. Psaty, S.R. Heckbert, T.D. Koepsell, D.S. Siscovick, T.E. Raghunathan, N.S. Weiss, F.R. Rosendaal, R. N. Lemaitre, N.L. Smith, and P.W. Wahl, J. h e r . Med. Assoc., 274 (1995) 620. 20. B.M. Psaty, N.L. Smith, D.S. Siscovick, T.D. Koepsell, N.S. Weiss, S.R. Heckbert, R.N. Lemaitre, E.H. Wagner, and C.D. Furberg, J. Amer. Med. Assoc., 277 (1995) 739.
EPR in the 2 Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
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Recent progress and future prospects of free radical imaging by PEDRI David J. Luriea, Margaret A. Fostera, Wiwat Youngdeea, Valery V. Khramtsovb, Igor Grigor’evc aDepartment of Bio-Medical Physics and Bio-Engineering, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, UK* bInstitute of Chemical Kinetics and Combustion, Institutskaja 3, Novosibirsk 630090, Russia ‘Institute of Organic Chemistry, Lavrent’eva 9, Novosibirsk 630090, Russia PEDRI is a double-resonance method for imaging free radicals which relies on the Overhauser effect. A proton M R image of the sample is recorded while an EPR resonance of the free radical is irradiated. In this way high resolution images of the free radical distribution can be obtained. Field-cycled PEDRI improves the sensitivity and reduces the RF power deposition by switching the magnetic field during the pulse sequence. Applications of PEDRI and Field-cycled PEDRI include the pharmacokinetics of exogenous free radical contrast agents as well as the determination of local oxygen concentration and pH via their effect on exogenous agents. The detection and imaging of endogenous free radicals is more challenging, but may become feasible by virtue of improvements in techniques.
1. INTRODUCTION PEDRI (Proton-electron double-resonance imaging) is a technique for imaging free radicals in animals or biological samples which is based on the Overhauser effect [l].It offers high sensitivity and high spatial resolution which is independent of the linewidth of the paramagnetic sample under study. In addition, it can image free radicals in large animals, potentially even in humans.
1.1. The Overhauser effect In the Overhauser effect (also known as dynamic nuclear polarization, DNP), the signal of a solution is observed (often the proton NMR signal in water) while an EPR resonance of the free radical under study is irradiated. Provided there is efficient coupling (usually dipole-dipole) between the solute’s unpaired electrons and the solvent protons, a transfer of polarization from the unpaired electrons to the protons can occur, resulting in a change in amplitude of the observed NMR signal. This work was funded by the UK EPSRC and by INTAS (project 99-1086). WY received a studentship from the Thai government. We are grateful to Klaes Golman of Nycomed Innovation, Malmo, Sweden, for the kind gift of TAM radical.
516
e T E P R RF
EPR_I--~
RF NMA
Signal 0
0.02
0.04
0.06
0.08
+
-Av-
n
U
Time (s)
Figure 1. NMR FIDs obtained at field strength of 10 mT. Sample was 2 mM aqueous solution of TEMPOL. (a) Without EPR irradiation; (b) with 1.6-watt EPR irradiation at 288 MHz; (c) with 10-watt EPR irradiation at 288 MHz.
Figure 2. Typical PEDRI pulse sequence. In this case a gradient-echo spin-warp pulse sequence is shown. It is preceded by a period of EPR irradiation, lasting approximately 3 times the NMR TI (typically, TEpR= 500 ms).
The enhancement factor is defined as the ratio ZdZo, where and are the measured NMR signals with and without EPR irradiation. The enhancement factor is given by:
where “/s and ‘/I are respectively the electron and proton gyromagnetic ratios, the absolute value of their ratio being equal to 658. p is the coupling factor (0 I p I l), which depends on the extent and nature of the coupling between the unpaired electrons and the protons. For dipole-dipole interactions, p = %. is the leakage factor (0 I f I l), which accounts for the fraction of nuclear relaxation caused by the presence in solution of the paramagnetic solute. In a concentrated free radical solution, with few competing relaxation mechanisms, will approach unity. The saturation factor, s (0 s 5 l), measures the amount of saturation of the EPR resonance. It depends on the unpaired electron’s relaxation times and on the strength (power) of the EPR irradiation. Under conditions of complete saturation (high irradiation power, or narrow EPR linewidth), s = 1. Finally, n is the number of lines in the free radical’s EPR spectrum. With stable nitroxide radicals, n = 3. Taking all the factors in equation (1) into account, the maximum achievable enhancement signal undergoes a phase reversal factor is = -329. The minus sign means that the upon EPR irradiation. With a nitroxide free radical, the presence of the hyperfine triplet reduces this to -1 10. In practice, however, complete saturation is not achieved. This, together with the relatively low concentration of free radicals in many experiments (f. 1) means that the observed enhancement factor is often no more than -10. Figure 1 shows NMR FID signals obtained from a nitroxide free radical solution at a field strength of 10 mT, without EPR irradiation and with irradiation at two different power levels. With 1.6-watt irradiation a reversal in phase of the NMR signal occurs, together with a 25% reduction in amplitude. This represents an enhancement factor of With an irradiation at 10-watt incident power an enhancement factor of -8 is obtained.
517
R F EPR
Y
.v E switched
Signal 1
Time (s)
Figure 3. NMR FIDs obtained 10 mT, showing build-up of Overhauser enhancement after switching on 288 MHz EPR irradiation. Sample was 2 mM TEMPOL solution. Solid line was calculated on the basis of a TI value
Polarization
I -
A
. . u n
Evolution
Detection
Figure 4. Field-cycled PEDRI pulse sequence. EPR irradiation is applied during the evolution period at field strength BoE.
of 500 ms.
2. TECHNIQUES 2.1. PEDRI PEDRI is essentially the imaging version of the DNP experiment [2]. Instead of just measuring the NMR signals, they are used to generate an image, by using pulsed magnetic field gradients in the usual way. EPR irradiation causes an enhancement in the NMR signal from parts of the sample containing free radicals, and these regions have different intensity in the final image, showing the distribution of the free radical. A typical PEDRI pulse sequence is shown in Figure 2. Sometimes an interleaved PEDRI pulse sequence is used to obtain images with and without EPR irradiation simultaneously: each line of k-space is recorded twice, once with and once without EPR irradiation. Subtraction of the with-EPR and withoutEPR data sets yields a 'difference' image showing only the distribution of the free radical under study. As seen in Figure 3, after switching on the EPR irradiation the DNP enhancement builds up with a time constant equal to the longitudinal NMR relaxation time TI. Therefore the duration TEPR of the EPR irradiation in a PEDRI experiment should be at least 3 x TI in order to achieve over 95% of the available enhancement.
2.2. Field-Cycled PEDRI (FC-PEDRI) PEDRI is normally implemented at low field (S20 mT) so that the EPR irradiation can penetrate into the sample and not cause excessive heating. Despite the use of low field strengths, excessive specific absorption rate ( S A R ) in biological samples may still be problematic. In principle extremely low field strengths mT) could be used, in an attempt to reduce the EPR irradiation frequency and hence lower the SAR. However, the use of such of the NMR low field strengths would severely compromise the signal-to-noise ratio experiment (despite the DNP enhancement). What is needed is to irradiate the EPR at very
518
low magnetic field (hence low frequency and low SAR) and to observe the NMR signals at a much higher field (to preserve SNR). This conflict can be resolved by FC-PEDRI [3]. In an FC-PEDRI experiment the field strength Bo is switched between three levels during the pulse sequence, as shown in Figure 4. The pulse se uence commences with the . 3-5 mT) for the polarization period at BZ. The field is then reduced to BoB (typically evolution period, during which the EPR irradiation is applied at low frequency (-100 MHz), with correspondingly good penetration and low power deposition (SAR). The Overhauser enhancement occurs during the evolution period, affecting the proton magnetization. Next, the field is ramped up to the detection value, BoD, for the application of the NMR detection pulse(s) and imaging gradients. The SNR of the experiment is increased by virtue of the higher value of BoD.To ensure that as little enhancement as possible is lost, the time to ramp the field from BoE to BoDshould be shorter than the T I relaxation time of the sample. We have constructed an FC-PEDRI imager with a large field-cycling magnet [4] which makes use of the field-compensation method of field cycling. A human whole-body sized ferrite permanent magnet provides the vertically-oriented detection field (BoD) of 59 mT. A resistive magnet is mounted coaxially within the bore of the permanent magnet. When energised, it partially cancels the field from the permanent magnet, so that field cycling can be accomplished by controlling the current in the resistive magnet coil. With this system the magnetic field can be ramped from 5 mT to 59 mT (or vice-versa) in 40 ms. Provided coil noise is dominant over sample-induced noise, it can be shown that the and therefore the sensitivity and image quality, is proportional to the detection magnetic field strength BoD. For experiments with small animals (e.g. rats), this will hold true up to at least 0.5 tesla. This means that as high a detection field as possible should be used. Recent theoretical and experimental studies have shown that the optimum value of the evolution field BoE lies in the range 3 - 6mT, depending on the sample size, the applied EPR irradiation power and on the EPR linewidth of the free radical under study [ 5 ] . 2.3. Field-Cycled DNP spectroscopy In studies of free radicals in biological samples it is often useful to obtain information on the EPR spectrum of the paramagnetic species under study. In an FC-PEDRI system, this can be achieved by field-cycled DNP (FC-DNP) spectroscopy [6]. Figure 5 shows the FC-DNP pulse sequence. In order to obtain an Overhauser-detected EPR spectrum the pulse sequence is repeated a number of times, always with the same EPR irradiation frequency, but each time incrementing the value of the evolution field strength BoE. After each evolution period the magnetic field is ramped to BoD and an NMR detection pulse is applied to generate an FID, which is measured. When an EPR resonance is encountered the Overhauser effect causes an enhancement of the NMR signal and its amplitude is altered. A plot of NMR signal amplitude versus BoE shows the positions of the EPR resonances. The relative amplitudes of the peaks provide information on the EPR line intensities, as well as on the electron-proton coupling.
2.4. Rapid FC-PEDRI Even with field-cycling, it is still possible that excessive non-resonant power deposition may result from the EPR irradiation. This might occur, for example, when large animals are imaged (e.g. rabbits), because the SAR increases quadratically with the size of the conductive sample under study. In ‘conventional’ PEDRI or FC-PEDRI one period of EPR irradiation is applied per phase-encoding step (in other words, one EPR irradiation period per line of kspace). Another way of reducing the total energy deposited per is to reduce the number
519
4
RFEPR
Repeated M times
b
\EJ Bo” -------------I l I \
Field RF RF NMR
I
I
R F NMR
(0
Signal Evolution
Detection
I I I
._ 0
1
......
e
I
V
i
a
i
I 1
c I I
I Signal
I I
Figure 5. FC-DNP pulse sequence. It is repeated a number of times, incrementing B$ in order to obtain a DNP-detected EPR spectrum.
--
A
V V
I I
Figure 6. Hybrid FISP FC-PEDRI pulse sequence for rapid imaging of free radicals.
of EPR irradiation periods necessary to record a free radical image. An extreme example of this was described by Puwanich et al. [7], where only one period of EPR irradiation is applied (at field BoE), followed by rapid acquisition of all the image data at field strength BoD. This ‘snapshot FC-PEDRI’ method was successful, but suffered from some image blurring and relatively low sensitivity, due to the decay of the Overhauser-enhanced proton magnetization during the data acquisition period. We have recently developed a new pulse sequence called Hybrid FISP FC-PEDRI [8]. The method lies somewhere between conventional FC-PEDRI (one EPR irradiation per line of kspace) and snapshot FC-PEDRI (one EPR irradiation per image). In the hybrid FISP FCPEDRI method k-space is subdivided into M segments, each of N lines, with M x N = n, the image matrix size. A period of EPR irradiation precedes the rapid collection of M lines of kspace. So, for example, with n = 64 we might use a pulse sequence with M = 2 and N = 32, i.e. 2 periods of EPR irradiation, each followed by the rapid application of 32 NMR detection pulses to acquire 32 lines of k-space. Figure 6 shows the pulse sequence. Experimentally, we have demonstrated that image quality is only slightly degraded relative to the conventional method when 4 periods of EPR irradiation are used per image.
3. APPLICATIONS 3.1. Imaging exogenous free radicals Perhaps the most straightforward application of PEDRI or FC-PEDRI is the imaging of exogenous free radical ‘contrast agents’ injected into living animals. A variety of experiments of this sort have been carried out. The first example, in 1990, involved the injection of a solution of Fremy’s salt into the peritoneal cavity of rats [9]. PEDRI images were recorded at a field strength of 6.8 mT with an EPR frequency of 197 MHz. Although Fremy’s salt has the benefit of narrow EPR lines which are easy to saturate, its toxicity is too high for it to be injected intravenously. However, it was found that the nitroxide free radical proxy1 carboxylic acid (PCA) could be administered in this way, allowing PEDRI images of PCA in the rat to be obtained [lo]. These experiments were carried out at 10 mT with EPR
520
Figure 7. Projective coronal FC-PEDRI images of prone, anaesthetized adult Sprague-Dawley rat. Image on left was obtained before injection of radical. Image on right was obtained 1 min after injection of Nycomed OX063 TAM radical (0.5 mmol/kg). Sequence paramaters were: B$ = 4.25 mT, BOD= 59 mT, TR = 1200 ms, TEPR = 400 ms, FOV 120 mm, EPR irradiation at 120.7 MHz, 20-W incident power. irradiation at 237 MHz. Time series of PEDRI images allowed the pharmacokinetics of PCA in the rat to be studied, in order to elucidate its excretion mechanism fkom the kidney [l 11. While nitroxide fkee radicals such as PCA are non-toxic, they are not ideal for use with Overhauser detection methods because of their relatively wide EPR line-widths. Broader l i e s mean that more irradiation power must be used to achieve a given enhancement. the applied power will be limited by considerations, this means that the observed enhancement is reduced. Another disadvantage is the presence of the triplet of hyperfine lines in the nitroxide’s EPR spectrum, further reducing the enhancement. An ideal ‘contrast agent’ for PEDRI would have a single, narrow EPR line. The triaryl-methyl (TAM) compounds developed by Nycomed Innovations (Malmo, Sweden) fulfil this characteristic [12]. It has been used in experiments on anaesthetized rats at 9.5 mT with EPR irradiation at 263 MHz [131 and in mice at 20 mT with EPR irradiation at 564 MHz [141. TAM has also been used in FC-PEDRI experiments: Figure 7 shows FC-PEDRI images fkom our laboratory, of an anaeasthetized rat which had been given a dose of TAM radical through a jugular cannula. The image collected before injection of TAM shows very little contrast, but the later image shows clearly the distribution of the compound in the animal’s heart, lungs, blood vessels and kidneys.
3.2. Oximetry Images of the distribution of exogenous fkee radicals in the body can provide pharmacokinetic data by measuring the time course of the agent through different organs. In addition, it is possible to obtain information on other useful parameters fkom PEDRI or FCPEDRI images. One of these parameters is the concentration of dissolved oxygen. Because molecular oxygen is paramagnetic, it will broaden the EPR linewidth of a fi-ee radical contrast agent present in solution. In an Overhauser experiment the presence of oxygen in solution will reduce the observed enhancement, because the oxygen-broadened EPR lines will exhibit a lower degree of saturation at a given EPR irradiation power. Therefore, regions of the sample or animal with high oxygen concentration will exhibit lower enhancement values than regions with low oxygen concentration. The enhancement will also be influenced by the concentration of the fkee radical agent itself, and the main difficulty of oximetry by PEDRI is to separate out
52 1
Evolution Field (mT)
Figure 8. FC-DNP spectra of 15 ml 2.5 mM HMI solutions. Top: at pH 6.6, bottom: at pH 2.3. TR = 1200 TEPR = 400 17-W EPR irradiation at 121 MHz.
Evolution Field (mT)
Figure 9. FC-DNP spectrum of anaesth-etized 225-gram rat following gavage of 3 ml of 5 mM HMI into its stomach. TR = 1200 ms, TEPR = 400 ms, 20-W EPR irradiation at 120.7 MHz.
these effects. Golman et have suggested collecting PEDRI images at two different EPR irradiation power levels, together with an image obtained without EPR irradiation. An image collected with high EPR irradiation power, close to saturation of the electron spins, will be less affected by oxygen-induced changes in EPR linewidth than will an image obtained at a lower irradiation power. Pixel-by-pixel analysis of the data from the three images allows the calculation of separate radical concentration and oxygen concentration images.
3.3. pH measurement pH is another important physiological parameter which can affect the EPR spectrum of certain types of stable free radicals, in particular the imidazoline and imidazolidine nitroxides The hyperfine splitting in the EPR spectrum of these agents is affected by the local pH, and EPR spectroscopy and EPR imaging [18] studies on these agents have been carried out in recent years. We have recently conducted and FC-DNP and FC-PEDRI experiments using (HMI). The a variety of such agents, including range of pH which can be sensed using this agent is approximately 2 pH units, centred on its pK value of (the pK is the pH value at which half of the free radical molecules are (top) and protonated). Figure 8 shows FC-DNP spectra of aqueous solutions of HMI at pH pH 2.3, representing the two extremes of the range. At near-neutral pH this agent is unprotonated, and the splitting of the high and low-field lines is mT. At low pH the free radical is completely protonated and the splitting is reduced to mT. Anaesthetized adult Sprague-Dawley rats (body weight g) were given a gavage (intubation into the stomach) of a neutral-pH solution of HMI. Stomach acid caused a lowering of pH which was detectable by FC-DNP as a splitting of mT (Figure FCPEDRI was also used to confirm the position of the HMI solution in the animal’s stomach, as shown in Figure
522
Figure 10. Coronal projective FC-PEDRI images of supine, anaesthetized rat in vivo, following gavage of 3 ml of 5 mM HMI into its stomach. B," = 5.5 mT, FOV = 120 mm, TR = 1200 ms, TEPR = 400 ms. Lefk image obtained without EPR irradiation. Middle: image obtained with 20-W EPR irradiation at 120.7 M E . Right: 'difference' image, showing the HMI solution in the animal's stomach.
4. FUTURE PROSPECTS 4.1. Techniques The main impetus for the further development of PEDRI must lie in attempts to increase the sensitivity, to enable lower concentrations of free radicals to be detected and imaged. As mentioned above, in an FC-PEDRI experiment the SNR, and hence the sensitivity, is Hence, one way to determined largely by the strength of the detection magnetic field improve the sensitivity is to build an imager with a higher value of BoD. In our laboratory we are doing just this, and construction of a new instrument with a detection field of 0.5 tesla is underway. As with our existing imager [4], the new instrument will use a double magnet system. This time, however, the 0.5 tesla detection field will be generated by a whole-body sized superconducting magnet. Again, a resistive field-offset coil will be used, mounted coaxially inside the superconducting magnet. In order to avoid eddy-currents in the metallic structure of the superconducting magnet, the field-offset coil will employ an active shield, in a manner exactly analogous to that used for several years in actively-shielded gradient coils. Due to the high power-density of the resistive offset coil, the inner bore of our new imager (inside the shield and offset coils and the gradient and shim coils) will be only 12 cm, suitable for imaging rats and mice. Nevertheless, the instrument should exhibit an order of magnitude greater sensitivity than our existing 59-mT FC-PEDRI imager, and so should be able to detect free radicals in vivo at sub-micromolar concentrations. 4.2. Applications As with EPR imaging, the greatest challenge to PEDRI and FC-PEDRI is the imaging of naturally-occurring fiee radicals. Endogenous ftee radicals are involved in normal metabolism, and it is widely believed that changes in their concentration can occur a result of many types of disease. However, naturally-occurring radicals such as hydroxyl (.OH) or nitric oxide (.NO) are very short-lived in vivo, and normally exist at very low concentrations. Nevertheless, spin-trapping may be used to stabilize these moieties and, for example, spintrapped nitric oxide has already been imaged by EPR in mice administered with lipopolysaccharide to stimulate a septic-shock reaction [191. The improved sensitivity of our new imager mentioned in the previous section should allow similar studies to be achieved
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using FC-PEDRI, with the benefit of the improved spatial resolution afforded by this method. In fact, the use of an exogenous spin-trap may not even be necessary, as *NO produced in the body can form long-lived paramagnetic compounds by complexing with haem proteins, and it has already been shown that these are detectable by FC-DNP and FC-PEDRI [20].
5. CONCLUSIONS It is encouraging to see that the number of research groups working on PEDRI and FCPEDRI is gradually increasing. One obstacle to the increase of activity in this area has been the non-availability of a commercial PEDRI or FC-PEDRI imaging system (although a FCPEDRI system was briefly available from the manufacturer Philips). However, virtually any resistive-magnet MRI system could be used for PEDRI, with the addition of relativelystraightforward hardware [ 141. Overhauser-based free radical imaging methods will always be complementary to the more conventional EPR imaging techniques (PEDRI cannot, for example, detect unpaired electrons in solid materials such as oxygen-sensitive lithium pthalocyanine crystds). On the other hand, PEDRI and FC-PEDRI do have the advantage that the spatial resolution is entirely determined by the NMR resonance, and is therefore independent of the EPR linewidth. Furthermore, PEDRI by definition produces a proton M R image of the sample or animal under study, and this can be very useful in determining the underlying anatomy in experiments. With improvements in technology, it is likely that the applications described above, namely pharmacokinetic, oximetric and pH-sensitive imaging, will become more widespread and these, together with the detection of spin-trapped endogenous radicals, will be of genuine use in biomedical research.
REFERENCES 1. D.J. Lurie, in: “In Vivo EPR (ESR): Theory and Applications”, Biological Magnetic Resonance, Vol. 18, ed: L.J. Berliner, Kluwer / Plenum, New York., In Press (2001). 2. D.J. Lurie, D.M. Bussell, L.H. Bell and J.R. Mallard, J. Magn. Reson., 76 (1988) 366. 3. D.J. Lurie, J.M.S. Hutchison, L.H. Bell, I. Nicholson, D.M. Bussell and J.R. Mallard, J. Magn. Reson., 84 (1989) 431. 4. D.J. Lurie, M.A. Foster, D. Yeung and J.M.S. Hutchison, Phys. Med. Biol., 43 (1998) 1877. 5 . W. Youngdee, P. Planincic and D.J. Lurie, Phys. Med. Biol., 46 (2001) 2531. 6. D.J. Lurie, I. Nicholson and J.R. Mallard, J. Magn. Reson., 95 (1991) 405. 7. P. Puwanich, D.J. Lurie and M.A. Foster, Phys. Med. Biol., 44 (1999) 2867. 8. W. Youngdee, D.J. Lurie and M.A. Foster, Proc. ISMRM 9th Scientific Meeting, Glasgow, (2001) 940. 9. D. Grucker, Magn. Reson. Med., 14 (1990) 140. 10. D.J. Lurie, I. Nicholson, M.A Foster. and J.R. Mallard, A333 (1990) 453. 11. I. Seimenis, M.A. Foster, D.J. Lurie, J.M.S. Hutchison, P.H. Whiting and S. Payne, Magn. Reson. Med., 37 (1997) 552. 12. J.H. Ardenkjaer-Larsen, I. Laursen, I. Leunbach, G. Ehnholm, L.G. Wistrand, J.S. Petersson and K. Golman, J. Magn. Reson., 133 (1998) 1.
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13. K. Golman, I. Leunbach, J.H. Ardenkjaer-Larsen, G.J. Enholm, L.-G. Wistrand, J.S. Peterson, A. Jiirvi and S. Vahasalo, Acta Radiologica, 39 (1998) 10. 14. D.J. Lurie, H. Li, S. Petryakov and J.L. Zweier, Magn. Reson. Med., In Press (2001). 15. Golman, J.S. Peterson, J.-H. Ardenkjaer-Larsen, I. Leunbach, L.-G. Wistrand, G. Ehnholm and K. Liu, J. Magn. Reson. Imaging, 12 (2000) 929. 16. V.V Khramtsov, LA. Grigor’ev, M.A. Foster, D.J. Lurie and I. Nicholson, Cell. Mol. Biol., 46 (2000) 1361. 17. B. Gallez, K. Mader and H.M. Swartz, Magn. Reson. Med., 36, (1996) 694. 18. A. Sotgiu, K. Mader, G. Placidi, S. Colacicchi, C.L. Ursini M. Alecci, Phys. Med. Biol., 43 (1998) 1921. 19. T. Yoshimura, H. Yokoyama, S. Fujii, F. Takayama, K. Oikawa, H. Kamada, Nature Biotech., 14 (1996) 992. 20. A. Mulsch, D.J. Lurie, I. Seimenis, B. Fichtlscherer and M.A. Foster, Free Radical Biology and Medicine, 27 (1999) 636.
EPR in the 21" Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
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Electron paramagnetic resonance in medicine R.Saifoutdinov Chair of Therapy of Medical Academy, 420012, Mushtary str.,l 1, Kazan, Republic of Tatarstan, Russia
The own results on electron paramagnetic resonance (EPR) investigation of biological fluids from the organism of normal individuals and patients with different diseases have been presented in this lecture. The EPR spectra of blood and blood components, gastric contents, synovial fluid and some other tissues and biological fluids have been considered. A qualitative and quantitative analysis of paramagnetic centers in the organism of volunteers and patients suffering from various diseases is presented in this lecture. Much attention is given to the use of the EPR method diagnostics, investigation of metabolism, free-radical reactions in the development of different pathologies.
1. PREFACE Pathologic states are caused by not only disorders of lipid, hydrocarbon, protein and mineral metabolism. Alterations in redox processes are also of importance. Paramagnetic centers present in many biological tissues and fluids can serve as indicators of these changes owing to the presence of free radicals and cations of transition microelements. These centers present in different organic and inorganic biomolecules. Electronic paramagnetic resonance spectroscopy presents an objective and accurate method of recording paramagnetic centers widely accepted in chemistry, biochemistry, biology and medicine [11. The EPR method was discovered in 1944 by Evgeny Konstantinovich Zavoisky in the city of Kazan. In the middle fifties, nearly simultaneously in the United States and the Soviet Union, EPR spectroscopy was first used for the investigation of tissues and fluids in human and animals. In that time, owing to low sensitivity of EPR spectrometers, the method of tissue lyophilic drying was used. The development of low-temperature registration of biological tissues in the early seventies extended the scope of application of EPR spectroscopy in biochemistry and medicine. By the present time a high sensitivity and precision of this method has been achieved, which allows investigation of even native water-containing tissues. 2. CONTENTS OF STOMACH AND DISEASES ONE
In the stomach contents the following paramagnetic centers have been found: a sixcomponent Mn2+EPR signal, a Hem-NO signal with line half-width 7.5 mT and maximal at
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g = 2.1,2.07,2.007 and 1.98 with a triplet structure and centers at g = 2.007 and splitting of 1.5-1.6 mT [2]. The application of EPR spectroscopy Hem-NO studies has been considered in a review [3]. The Hem-NO signal was fixed in tumor cells and in the blood of animals exposed to nitrites and other toxic compounds [4]. This signal was also present in the spectra tumors of human liver, large intestine and stomach. It is most often found in necrotic area of tumor 151. The nitrosyl complexes can form cytochromes, peroxydases, catalases and other enzymes. The signal shape is specific for each type of hemoproteins which allows them to be distinguished. For instance, a Hem-NO signal was recorded in the EPR spectrum of human hemoglobin [6]. In the stomach contents, in the presence of hem-containing groups (hemoglobin, myoglobin, catalase, cytochromes, Hem-NO can form nitrates or nitrites, nitrate- and nitrite-reducing enzymes or microorganisms containing the above enzymes. Hem-NO also appears at pH above 3.0, under reduced acidity, which promotes the development of microflora. Maximum Hem-ON concentration is recorded in the sample taken from the empty stomach. Thus, it is likely that nitrate- and nitrite-reducing microorganisms act as NO-transporters to the human stomach. There is a close relation between the Hem-NO level and the content of microorganisms in the stomach. From the empty stomach portion the following bacteria were sown out in concentrations ranging from lo4 to lo7 cells in 1 ml: Thus, Hem-NO appears in the stomach contents owing to the reproduction of nitrate- and nitrite-reducing microorganisms which form NO bound to hemcontainin groups [2]. The Mn$+ signal having a characteristic six-component superfine structure can be detected in the spectrum of stomach contents only in the presence of duodenal-gastric reflux [7]. The development of gastric and duodenal chronic diseases is accompanied by some changes in the host paramagnetic centers content. In the stomach contents of patients with gastritis, gastric and duodenal ulcer EPR Hem-NO signal was observed. Its concentration in healthy individuals is higher than in patients with chronic gastritis and peptic ulcer (71 and 66%, respectively) [8]. Hem-NO exerts an activating effect on guanylate cyclase thus increasing tissue c-GMP and stimulating cell proliferation. This supports the gastric wall integrity and prevents ulceration [9]. C-GMP inhibits thrombocyte aggregation and produces a vasodilatory effect. The above diseases are characterized by a reduced blood flow and the appearance of thrombocyte aggregations. Certain amount of carcinogenic nitro- and nitroso-compounds are brought into the gastro-intestinal tract with food. The activity of nitrate- and nitritereducing microorganisms is most likely to be compensatory reaction preventing the development of oncologic diseases. Malignant tumors are often met in patients with chronic gastritis and peptic ulcers. This suggests Hem-NO to be a compulsory component of gastric contents. Hem-NO is supposed to be involved in the development of chronic gastritis and peptic ulcer by the following pathogenic mechanism. Changes in the stomach microflora, in particular, a decrease in the content of nitrate- and nitrite-containing microorganisms leads
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to a drop of the Hem-NO level [lo]. This inhibits cell proliferation, decreases mucous wall restoration, strengthens thrombocyte aggregation and reduces vascular tonus regulation.
3. SYNOVIAL FLUID AND DISEASES OF JOINTS Synovial fluid resulted mainly from plasma dialysis acts as synovia. Only small and, on rare occasions, slightly larger size molecules penetrate synovial fluid due to gel-filtration through sinovial cells. The synovial fluid viscosity is ensured by hyaluronic acid. The major synovial fluid properties are dependent on various pathological conditions. This allows differentiation of dystrophic and pathologic diseases of joints. Investigation of the synovial fluid composition provides an informative method in the diagnostics of various diseases of joints. In the synovial fluid of patients with rheumatoid arthritis and deforming osteoarthrosis paramagnetic centers of three es were observed. They correspond to the following EPR signals: g = 4.3 (fe3'), 2.05 (Cu ) and free radical with g 2.0024-2.0029, AH 0.6-0.8 mT of unidentified nature [l 11. It is suggested that the g = 4.3 signal is caused by transferrin Fe3+since the g factor value and form (characteristic splitting in the upper maximum and a jag on the descending line) are coincident with the Fe3+ spectrum. The g = 2.05 signal is most likely to correspond-to Cu2' atoms incorporated into a protein of ceruloplasmin type. This suggestion was confirmed using an EPR spectrum of rheumatoid synovial fluid (1 ml) after addition of 0.2 ml of 0.01M FeS04 solution. It turned out that the amplitude of the EPR signal at g = 4.3 having a characteristic spectral form of three valence iron on transferrin had increased more than ten-fold, whereas that of the signal at g = 2.05 decreased 4 times. This shows that Fe2+is oxidized to Fe3+by Cu2' of the rheumatoid synovial fluid. Besides, this is also indicative of the presence in the fluid of a considerable amount of protein of transferrin type unbound to iron. Quite a different picture is observed in the rheumatoid synovial fluid (1 ml) drawn from deforming osteoarthrosis patients. The addition of 0.2 ml of 0.01M FeS04 solution did not cause such an intensive rise of the Fe3+level. This indicates either poor ferroxidase activity of the fluid or the presence of a decreased number of transferrin molecules, or saturation with Fe3+[ 111. In rheumatoid arthritis patients the synovial fluid Fe3+content is four times lower than in deforming osteoarthrosis patients. The Cu" content inversely changes in these diseases [121. Table 1 Synovial fluid paramagnetic centers content in rheumatoid arthritis (RA) and deforming osteoarthrosis (DOA) patients (M*m) RA DOA Indexes Fe", (mcmol/l) 4.5k0.38 13.5k0.86 cu2+,( l o 5 M) 2.24k0.11 1.20+0.10 FR, (10-6M) 0.5k0.03 0.4k0.03
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The analysis of synovial fluid of 18 rheumatoid arthritis patients was carried out on admission and after 3 weeks of stationary treatment. The positive dynamics in the patient state was characterized by a statistically reliable decrease in the Cu2+level and an increase in the Fe3+signal amplitude in EPR spectra [121. 4. FECES The EPR spectra of normal human feces display the following signals: at g = 6.0 corresponding to Fe3+of MetHb and its derivatives, at g = 4.3 arising from low-spin Fe3+, Cu2+ with characteristic SFS, a six-component spectrum of Mn", Hem-NO, and free radicals [7]. To elucidate the source of paramagnetic centers in feces the large intestine microorganisms were isolated. In feces anaerobes of the Bacteroides genus as well E.coli and Citobacter aerobes were mainly found. In the EPR spectra of anaerobes the Mn2+signal is fixed whereas Fe3+signal is absent. In anaerobes, on the contrary, the Mn2' content is very low (Mn2' is observed only under maximum amplification of the spectrometer), the Fe3+content being high. Colon bacillus (E.coli) contains significant amount of Fe in both heme and non-heme forms. Recombinant human microsomal heme oxygenase-2 contained in coIi was studied using EPR spectroscopy. At pH = 7 the ferric heme is six coordinate high-spin and six coordinate low-spin at pH > 7 (p& = 8.5). The reaction with hydrogen peroxide converts the heme of the heme oxygenase-2 fragment complex into a verdoheme-like product. The oxidation of 3-chloroperbenzoic acid leads to an oxofenyl derivative. As the spectroscopic properties of heme oxygenase-1 and heme oxygenase-2 are similar their catalytical mechanism seems to be identical [131. Thus, in the EPR spectrum of feces main contribution to the Mn" signal is made by anaerobes, whereas aerobes are responsible for the Fe3+ signal. It should be noted that anaerobes show higher FR concentrations [ 141. In feces Hem-NO is formed of nitrates, nitrites and heme brought to the intestine with food, water, bile and saliva. In the intestine there are nitrate- and nitrite-reducing microorganisms. In diarrhea clearly defined signals at g = 6.0 and Hem-NO appear in the EPR spectrum the Cu2' and Mn2+contents are decreased. This seems to be related to changes of the intestinal microflora and increased water content in feces [12].
5. PERITONITIS
Peritonitis is peritonium inflammation, the morbidity and death rate of which remain very high. In surgery, there is no consensus of opinion on the mechanism of pathologic reactions and the cause of endogenous intoxication in peritonitis. The EPR method was used to study paramagnetic centers in blood, urine, peritoneal exudate, intestinal contents of 61 patients with general purulent peritonitis. The vast majority of these was represented by men (71%). In these patients, peritonitis was caused by
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surgical invasion and traumas of peritoneal cavity organs. In 16% of the patients an intermediate, severe and extremely severe course of peritonitis was observed [151. For EPR examination, the patients' blood was taken from the subclavian vein and centrifuged (2000 rev/min, +4OC). Peritoneal exudate was withdrawn from drainages installed during surgery in the subdiaphragmatic and subhepatic spaces and in small pelvis. The intestinal content was taken by means of stable vasointestinal probe of Miller-Ebbot type. Blood, peritoneal exudate, gastrointestinal tract contents were taken three times every 48 hours. Blood and urine nitrate levels were determined by the photoelectrocolorimetry technique [161. In postoperational peritonic patients the samples of blood, peritoneal exudate and intestinal contents were taken every 24, 48 and 72 hours. It was found, on average: (26%), (23%), (12%), (14%), (12%). Simultaneously, the nitrate-reducing capability of peritoneal and intestinal microorganisms was investigated by EPR spectroscopy. Most gastro-intestinal microflora are able to reduce nitrates to nitrites. This process is initiated by a nitrate reductase enzyme inhabiting these microorganisms. Nitrites, in turn, react with blood hemoglobin to form methemoglobin. The study of 9 species of the above bacteria has shown 7 of these (80%) to exhibit nitrate-reducing capability [151. As shown by EPR spectroscopy, erythrocyte methemoglobin content in peritonic patients depends on the duration and severity of disease. 20 volunteers served as donors (methemoglobin concentration is 0.6kO.1 dl). The highest methemoglobin concentration is observed on the 10-1lth day after the onset of disease. In peritonic patients with extremely severe course of disease the methemoglobin level is increased with the very first days and further reaches the highest values [161. In donor plasma no methemoglobin is fixed. In blood plasma of peritonic patients the methemoglobin level depends on the severity of state. The presence of methemoglobin in patients plasma may be due to its absorption from the abdominal cavity by lymphatic vias. A linear correlation (r = +0.4) between methemoglobin levels in plasma and peritoneal exudate is observed [17]. The highest plasma methemoglobin concentration in peritonic patients appears on the 2 - 5 ~ day and later it is decreased. This may be explained by the fact that during this time purulent exudate character, its biochemical composition, concentration and microflora virulence point out to a clearly-expressed peritonitis. Methemoglobin and Hem-NO have been detected in peritoneal exudate by the EPR method. In the course of treatment and abdominal cavity assanation, MetHb inestinal content is decreasing independently of the severity and duration of peritonitis. The highest peritoneal exudate MetHb level is observed in peritonic patients in extremely severe state. This is explained by exudate character and contamination degree. A Hem-NO signal, detectable in the peritoneal exudate and intestinal content EPR spectrum, corresponds to the hemoglobin-nitric monoxidecomplex. Hemoglobin oxidation to Hem-NO occurs in the following manner. Interacting with blood, nitrites find access into erythrocytes and get involved in the reaction with hemoglobin. In the results redox reactions desoxihemoglobin is oxidized to methemoglobin, the nitrite ions being reduced to nitric oxide. Hb2++NO, +2H -4MtHb +NO + H,O
Nitric oxide reacts with the reduced hemoglobin to form stable Hem-NO complexes [16]. The Hem-NO signal was detected in peritoneal exudate EPR spectrum in 20% of patients. In 75% of the patients the hem-NO signal was detected in the exudate displaying a clearlyexpressed hemorrhagic component. In patients with extremely severe peritonitis the HemNO signal appears in the spectrum late in the disease [181. On the 2-3d day after the onset of disease, the intestinal methemoglobin concentration in peritonic patients does not differ much depending on the severity of illness and decreased by the 4-5th day. This due to intestinal probe evacuation of congested intestinal contents, detoxication measures and correcting therapy [191. In patients with severe and extremely severe peritonitis the methemoglobin level increases proportionally with time. This is related to intestinal trophic and motor changes, which amplify fermentation, ammonia formation, intestine mucus necrotic changes accompanied by hemorrhage, growth of microflora showing nitrate reductase capability. In peritonitis the Hem-NO signal detected in the intestinal EPR spectrum does not change significantly in the first 1-3 days independently of the severity of illness. On the 4-5th day the Hem-NO level decreases in patients with intermediate and severe peritonitis and increases in patients in the extremely heavy state. On the 6'h or 7'h day the patients with severe peritonitis show a tendency towards decreasing the intestinal Hem-NO level. The same is observed for next 2-3 days. In patients with extremely severe peritonitis, the intestinal Hem-NO concentration is increasing in the period from 6 to 11 days since the onset of disease and the tendency towards decreasing is observed only on the 12" day [19]. Comparison of time-dependent peritoneal exudate methemoglobin level to the intestinal one allows a conclusion that the former exhibits a tendency to decrease, whereas the latter is increasing. These suggests that the paretically affected intestine and its contents, and not the peritoneal exudate, for a long time remains the main source of intoxication [20]. In peritonic patients the paretically affected intestine provides favorable conditions for extensive growth of microorganisms and alteration of necrotic cells due to harmful agents and disorder in blood circulation. The formation of methemoglobin enhances tissue hypoxia. The iron-containing enzymes present in tissues are bound to nitric oxide and become switched out of the respiratory system. This intensifies disorder in intracellular metabolism and may lead to the formation of new low molecular nitrogen compounds, including nitric oxide. Nitric oxide contributes to microcirculation disorders and thrombosis followed by trophic alterations in tissues. Nitric oxide acts as a secondary mediator in intracellular signaling system owing to its effect on Ca-mobilizing cell system. This property of subcellular structures may be of special value in the development of pathologic processes occurring on the background of ischemia and tissue hypoxia. This also results in increased NO binding by heme and non-heme iron with the formation of methemoglobin and Hem-NO. In nitrite methemoglobinemia-induced tissue hypoxia, not only globulin transport h c t i o n s are broken, but cellular iron-containing enzymes are blocked. According to modern concepts, lipopolysaccharides of bacterial origin interact with neutrophils and macrophagues thus inducing accumulation of free radical oxygen forms. Free radical-mediated oxidative processes facilitate the formation of nitric oxide and ammonia conversion to nitrites and nitrates. Nitric oxide is the intermediate of these processes.
1
Microflora of the host organism plays an important role in nitrate metabolism. This is mainly due to nitrate reductase activity and the ability to synthesize nitrites and nitrates from ammonia. Investigation of methemoglobin and Hem-NO in biological fluids of patients with peritonitis reveals wide possibilities of this method in clearing up pathogenic aspects in the course of the disease at a molecular level. These results suggest the study of paramagnetic centers using EPR spectroscopy to be of value as supplementary diagnostic tests as well as criteria in assessment of the severity of clinical course of many diseases and the efficiency of treatment. Summing up the above said the EPR method is suggested to extend our knowledge of the role of transition microelements in vital activities of man, to provide an understanding of thin mechanisms of pathogenic processes of various diseases and to offers a great promise for new therapeutic and prophylactic measures. This all demonstrates the unlimited possibilities of the use of EPR spectroscopy in biochemistry and medicine.
REFERENCES 1. D.J.E. Ingram (ed.), Biological and Biochemical Applications of Electron Spin Resonance, Adam Hilder LTD, London, 1969. 2. R.G. Saifoutdinov (ed.), Paramagnetic Centers in human biological fluids and their diagnostic and pathogenetic role in some internal diseases. Paramagnitnye tsentry biologicheskikh zhidkostei cheloveka i ikh diagnosticheskaya i patogeneticheskaya rol' pri nekotorykh zabolevaniyakh vnutrennikh organov, Thesis (Doct. biolog. sci.) Tomsk, 1989 (Russ.). 3. G. Bemski, Mol. Biol. Rep., 24 (1997) 263. 4. A.F. Vanin, L.V. Vakhnina, A.G. Chetverikov, Biophysics, 15 (1970) 1044. 5. M.C. Symons, I.J. Rowland, N. Deighton, K. Shorrock, K.P. West, Free Radic. Res., 21 (1994) 197. 6. H. Rein, 0. Ristau, W. Sheler, FEBS Letts, 24 (1972) 24. 7. K.R. Sedov, R.G. Saifoutdinov (eds.), Electron Paramagnetic Resonance in internal diseases clinics, Irkutskii dom pechati, Irkutsk, 1993 (Russ.). 8. R.G. Saifoutdinov, EPRNewslette, 4 (1992) 3. 9. Wennmalm, B. Lame, A.S. Petersson, Analytical Biochem., 187 (1990) 359. 10. V.P. Kryshen, M.F. Nesterova, Vracheb. Delo, 9 (1982) 16 (Russ.). 11. R.G. Saifoutdinov, E.V. Popova, Yu.A. Goryaev, EPR Newsletter, 3 (1991) 6. 12. R.G.Saifoutdinov, L.I.Larina, T.I.Vakul'skaya, M.G.Voronkov (eds.), Electron Paramagnetic Resonance in Biochemistry and Medicine, Kluwer Academic/Plenum Publishers, New York, 2001. 13. Ishikawa, N. Takeuchi, S. Takahashi, K.M. Matera, M. Sato, S. Shibahara, D.L. Rousseau, M. Ikeda-Saito, T. Yoshida, J. Biol. Chem. 270 (1995) 6345. 14. R.G. Saifoutdinov, International Conference on Bioradicals Detected by ESR Spectroscopy; Yamagata, 1994. 15. L.A. Sadokhina (ed.), A study of NO endogenous synthesis markers in diffuse abscess peritonitis, Thesis Ph.D. Irkutsk, Medical University, 1998 (Russ.).
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16. R.G. Saifoutdinov, L.A. Sadochina, E.G. Grigorjev, Russian journal of gastroenterology, hepatology and coloproctology, 6 (1996) 60 (Russ.). 17. R.G. Saifoutdinov, L.A. Sadokhina, Russian journal of gastroenterology, 4 (1997) 131 (Russ.). 18. R.G. Saifoutdinov, L.A. Sadochina, E.G. Grigorjev Investigated of paramagnetic centres of the intestinal content by the method of ESR at the patients with the peritonitis. The ninth annual conference of the international society for Environmental Epidemiology; 1997 August 17-20; Taipei. Academia Sinica International Center, Taiwan, Republic of China, 1997. 19. R.G. Saifoutdinov, L.A. Sadochina, E.G. Grigorjev, J. Inorganic Biochem., 1997. 20. R.G. Saifoutdinov, L.A. Sadokhina, E.G. Grigorjev, An EPR study of MetHb intestinalcontent in peritonitic patients. International Workshop Fal'ka; 1996 June 20; SanktPeterburg. Sankt-Peterburg, (1996) 72.
EPR in the 2 1* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
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Development of in vivo ESWspin probe technique for oxidative injuries H. Utsumi, J.-Y. Hana, and K. Takeshitab Graduate School of Pharmaceutical Sciences, Kyushu University, Higashi-ku, Fukuoka 8 12-8582, Japan Present address; aFaculty of Pharmaceutical Sciences, Seoul National University, Seoul, Korea and bBioregulation Research Group, National Institute of Radiological Sciences, Chiba 263-8555, Japan
Free radicals are reportedly involved in various oxidative diseases, but the kind, amount, and origin of radical generation have yet to be clarified. Recent development of an free radical ESR spectrometer enables us non-invasive and direct measurement of reactions in living organism. This non-invasive method can be utilized widely for animal disease model to investigate the mechanism of oxidative injuries and the effect of antioxidant drugs. In the present paper, DEP-induced lung injury is precisely introduced to demonstrate ESWspin probe technique for non-invasive evaluation of ROS generation.
1. INTRODUCTION Free radicals such as reactive oxygen species (ROS) and Reactive nitrogen species (RNS) are believed to be very essential and functional compounds in various biological systems [ 13. Free radicals are also reportedly involved in various diseases involving, but the kind, amount, and origin of radical generation have yet to be clarified. Non-invasive measurement of free radical reactions is very important to understand the role of free radicals in diseases and to evaluate antioxidant activity, since oxygen concentration in tissues is much lower than that in experiments and there are many substances and reaction pathways, which influence radical reactions.
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Recently, ESR spectroscopy has developed and enabled non-invasive measurement of radicals in whole animals [2-61. Figure I demonstrates our ESR-CT system. This non-invasive method can be utilized widely for animal disease model to investigate the mechanism of oxidative injuries and the effect of antioxidant drugs. As shown in Figure 1 b, combination of the ESR measurement with MRI makes possible to visualize radical distribution and degree of free radical reaction in living organism. The sensitivity of an ESR spectrometer is unfortunately insufficient to detect trace amount of endogenous radicals. Thus, we utilize nitroxyl radicals as a probe for free radical reaction and reactive oxygen generation. The nitroxyl probe is reported to react with OH radical, causing loss of the paramagnetism of the probe [7-91. We have used this character of nitroxyl radicals to analyze free radical reactions and reactive oxygen species (ROS) generation in experimental animal disease model, including hyperoxia [ 10,1I], muscular ischemia-reperfusion [ 121, streptozotocin-induced diabetes [ 131, liver damage induced by iron-overload [ 141, diesel-exhaust-particles (DEP) induced lung injuries [ 151, etc. In the present paper, DEP-induced lung injury is precisely introduced to demonstrate ESWspin probe technique for non-invasive evaluation of ROS generation. DEP are one of the main air pollutants in urban areas, and they are increasing in quantity because of the rising number of diesel-engine-powered cars. DEP are composed of carbon nuclei, many adsorbed organic compounds, and trace heavy metals, including iron and copper [16]. DEP have been reported to reach the alveoli pulmonis easily during inhalation and to cause pulmonary tumors, fibrosis, and edematous change in experimental animals [17, 181. Thus, air pollution by DEP is a cause for serious concern about our health, and the toxicological mechanism of DEP should be clarified as soon as possible to reduce the risk of DEP injuries. In order to provide the first direct evidence of ROS generation in the lung after exposure to DEP and to clarify the mechanism of DEP toxicity, combined technique of ESR spectroscopy with a nitroxyl probe was applied to mice treated with DEP.
Figure 1.
ESR-CT system (a) and combined system of ESRI with MRI (b)
2. EXPERIMENTS Experimental procedure is demonstrated in Figure 2. As a spin probe, 4- trimethyl(CAT-I) was used for this experiment because of its membrane-impermeability. DEP were the kind gift of Dr. M. Sagai (National Institute for Environmental Studies, Japan). DEP were suspended in 50 mM phosphate-buffered saline (pH 7.4, PBS) containing 0.05 % Tween 80 and sonicated for 3 min using a bath-type ultrasonic disrupter. Mice (ddY, female, 2.5 weeks old) were administered with SO pL of DEP suspension through an intratracheal cannula into the lungs of mice under anesthesia with 4% halothane. Vehicle-treated animals received the same volume of PBS containing 0.05 % Tween 80. One day after DEP and vehicle challenges, mice were anesthetized with intramuscular injection of 1.8 g / k g b.w. of urethane (Aldrich Chemical Co., Inc.). After 100 pL of a sterilized solution of 10 mM CAT-1 (Molecular Probes Inc.) in PBS was administered through an intratracheal
cannula into the lungs of the mice under anesthesia with urethane. L-band ESR spectra were immediately recorded at mid-thorax of the mice with a JEOL JES PE-1X spectrometer equipped with a L-band unit and a loop-gap resonator (33 mm i.d., 5 mm long). The microwave frequency was 1.1 GHz, and the power was 5.0 mW. The amplitude of the 100-kHz field modulation was 0.16 mT. The external magnetic field was swept between 40 and 45 mT at a scan rate of 5 mT/min. Experimental Method
(1.8mglgb.w.: i.m.)
14 V
InlmlrBCheal administration
cimrn
CH.
CAT-1
Figure 2. Experimental procedure for DEP-induced lung injuries
536
3. RESULTS An aqueous solution of CAT-1 was intratracheally administered into the lung of mice treated with DEP one day before ESR measurement, and then L-band ESR spectra of CAT-I were recorded at the mid-thorax of living mice. Figure 3 shows a typical L-band ESR spectrum of CAT- 1 recorded at the mid-thorax of a mouse treated with 0.05 mg of DEP. The spectra consist of narrow triplet lines with 1.67 mT of hyperfine splitting constant, indicating that CAT-I is freely tumbling in a hydrophilic atmosphere in the lung. The ESR signal decreased gradually with time in the living mice. A semi logarithmic plot of the signal intensity against time showed a linear relation (Figure indicating the first-order reaction kinetics for the signal reduction of nitroxyl probe in the lung. The signal decay in a DEP-treated mouse was clearly faster than that in the vehicle-treated one, while DEP themselves induced no decay of the CAT-1 signal (data not shown). The enhancement of the signal decay in the mice occurred in a DEP dose-dependent manner, and a significant difference (p<0.005) was observed between vehicle-treated and 0.05 mg DEP-treated mice (Table 1). The enhanced signal decay was observed in the DEP-treated mice in more than 6 individual experiments. To examine whether or not the enhanced signal decay in DEP-treated mice reflects the generation of ROS, the same dose of -OH scavengers as CAT-1 was administered together with CAT- I . Dimethylthiourea, dimethyl sulfoxide, and mannitol all decreased the enhanced signal decay to the levels observed in vehicle-treated cases (Figure 5). The effect of mannitol was in a dose-dependent manner, and 1 pmol of mannitol completely suppressed These results suggest that the enhanced signal decay in the DEP-treated mice (Figure the enhancement of signal decay by DEP results from the interaction of CAT-I with -OH generated in the lungs of DEP-treated mice.
Figure 3. Typical ESR spectrum of CAT-I in mouse after intra-tracheal injection
537
Since DEP contain trace heavy metals such as iron, .OH may be generated through the Fenton reaction, if H202 is produced in the lungs treated with DEP. To confirm the contribution of the Fenton's reaction to *OHgeneration in the lung of mice exposed to DEP, we examined the effect of catalase or the iron-chelator desferrioxamine administered to the lungs of DEP-treated mice. Administration of desferrioxamine together with DEP decreased the decay rate in a dose-dependent manner, while desferrioxamine itself had no effect on the signal decay rate in control mice (data not shown). 2.5
1.5
0
Figure
Table 1 .
5
10
15
20 min
Semi-logarithmic plot of the signal intensity of CAT-I against time after intra-tracheal injection into mouse
Effect of DEP on the in
Signal Decay of CAT- 1 at Mouse Thorax
Dose of DEP (mg/mouse)
Signal Decay Rate (/min)
0 0.025
0.031 & 0.003 0.038 ? 0.005 0.046 k 0.005"
(7)
The values in parentheses are numbers of mice. Mice for 0 mg DEP/mouse treatment received vehicle solution. Signal decay rates are presented as mean ? SD, and the symbol * indicates significant difference compared to vehicle-treated mice (p<0.005), as determined by Student's t-test.
538 I
I
DEP
Vehicle
-
DMTU
DEP
Vehicle Mannitol 0
DMSO
0.5
1.0
Figure 5. Effect of OH radical scavengers on the enhanced signal decay of CAT-1
In the presence of SOD 8'
I
In the absence of SOD 8' 1-
6'
n
6'
T
4'
4'
2'
2'
0' V
DEP
0' Catalase(mg/mouse)
DEP, SOD 0.025
0.05
Figure 6. Effect of Catalase on OH radical generation in DEP-treated mice lung Simultaneous administration of catalase with CAT-1 suppressed the enhanced signal decay caused by DEP treatment in a dose-dependent manner, and 300 U of catalase decreased it to
the level of the vehicle-treated group (Figure 6). These results strongly suggest that *OH is derived from H202 through an iron-catalyzed reaction in the lungs of mice after the intratracheal administration of DEP. SOD administration was reported to reduce the incidence of various pulmonary injuries caused by oxidative stress [ 191, although in experiments, the dismutation of superoxide by SOD enhanced .OH generation through a
effect of SOD on *OH generation, SOD was Fenton-like reaction. To examine the simultaneously administered into the lungs of DEP-treated mice together with CAT- 1. The administration of SOD (0.1 mg/mouse) further accelerated the enhanced signal decay significantly in DEP-treated mice, although SOD had no effect in vehicle-treated mice (Figure The acceleration of the enhanced signal decay by SOD in DEP-treated mice was suppressed by simultaneous administration of catalase with SOD in a dose-dependent manner, and 600 U of catalase decreased the enhanced signal decay in DEP-treated mice completely to the control level (Figure 6). It is noteworthy that half of that dose, 300 U of catalase, which showed insufficient inhibition to the enhanced decay in the presence of SOD, was enough to decrease the enhanced signal decay to control levels in the absence of SOD. The histopathological analysis of lungs demonstrated the infiltration of neutrophiles into the alveolar air space one day after the instillation of 0.05 mg DEP (our unpublished data). Stimulated neutrophiles should contribute to the production of superoxide in the lungs of DEP-treated mice, and the resulting ROS could interact with CAT-I on the outside of cells, causing the enhanced signal decay of CAT-1. Further experimentation using the ESR technique should be useful to clarify the SOD effects on lung injuries.
DISCUSSION *OH generation in the lung of In this paper, we provided the direct evidence of living mice after trans-tracheal administration of DEP, using L-band ESR spectroscopy and a membrane-impermeable nitroxyl probe, CAT-I . The treatment of DEP enhanced the signal decay of CAT-I at mid-thorax on the next day after DEP treatment. CAT-I is positively at pH 7.4) [20]. We charged and has a very low n-octanol/water partition coefficient previously observed that CAT-I was not reduced in sacrificed mouse lungs where membrane-permeable nitroxyl radicals were readily reduced [20]. The ESR signal of CAT-I decreased gradually in control mice due to the various mechanisms of the living system, but the transfer of CAT-1 from alveolar space to blood vessel was also very slow in comparison with those of non-charged nitroxyl probes (our unpublished data). These facts indicate that most of the CAT-I remains in alveolar space and intercellular fluid during ESR measurement. The enhanced signal decay of CAT-I observed in the group of DEP-treated mice was completely suppressed by administration of *OH scavengers (mannitol, dimethyl sulfoxide, or dimethylthiourea), catalase, or iron-chelator desferrioxamine to the level for vehicle-treated mice. These observations strongly suggest that the reaction of CAT-1 with .OH causes the
540
enhanced signal decay in DEP-treated mice and that *OH is generated in the mouse lung through a metal-catalyzed reaction of H202. The involvement of the H202-relating mechanism in the enhanced signal decay was also supported by the experiment of SOD administration. One micro mol of mannitol completely suppressed the enhanced signal decay of CAT-I in the experiment using 1 pmol of CAT-I. These facts indicate the occurrence of competition between CAT-1 and mannitol for the reaction with *OH,and confirm again that the enhanced signal decay by DEP results from the interaction of CAT-1 with *OHgenerated in the lungs of DEP-treated mice, although the amount of *OHgenerated is difficult to estimate exactly with the present ESWspin-probe technique. In conclusion, the present paper provides the first direct evidence of *OH generation through a Fenton-like reaction in the lungs of mice one day after exposure to DEP, using L-band ESR spectroscopy and a membrane-impermeable nitroxyl probe. This technique is non-invasive and enables one to make a time-resolved analysis of free radical reactions with individual animals.
ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan, and Takeda Science Foundation. We thank Dr. M. Sagai and Dr. T. Ichinose for the donation of DEP, technical instruction in the method of DEP administration and helpful discussion.
REFERENCES
1. B. Halliwell (ed), Free Radicals in Biology and Medicine. Oxford University Press, London, 1985. 2. W.K. Subczynski, S. Lukiewicz, and J. Hyde. Magn. Reson. Med., 3 (1986) 747. 3. L.J. Berliner, H. Fuji, X. Wan, and S. Lukiewicz. Magn. Reson. Med., 4 (1987) 380. 4. S. Ishida, H. Kumashiro, N. Tsuchihashi, T. Ogata, M. Ono, and H. Kamada. Phys. Med. Biol., 34 (1989) 1317. 5. H. Utsumi, E. Muto, S. Masuda, and A. Hamada. Biochem. Biophys. Res. Commun., 172 ( I 990) 1342.
54 1
6. GR. Eaton, S.S. Eaton, and K. Ohno (eds), EPR Imaging and in vivo EPR. Boca Raton: CRC press, 1991. 7. S. Nigam, K.D. Asmus, and R.L. Willson. Physical Org. Chem., 85 (1976) 2324. 8. R.LWillson. Int. J. Radiation Biol., 21 (1972) 401. 9. A.D. Asmus, S. Nigam, and R.L. Willson. Int. J. Radiation Biol., 29 (1976) 21 1 . 10. Y. Miura, H. Utsumi, and A. Hamada. Biochem. Biophys. Res. Commun., 182 (1992) 1108.
11. Y. Miura, A. Hamada, and H. Utsumi. Free Rad. Res. Commun. 22 (1995) 209. 12. H. Utsumi, K. Takeshita, Y. Miura, S. Masuda, and A. Hamada. Free Rad. Res. Commun., 19 (1993) S219. 13. T. Sano, F. Umeda, T. Hashimoto, H. Nawata, and H. Utsumi. Diabetologia, 41 (1998)
1355. 14. N. Phumala, T. Ide, and H. Utsumi, H. Free Radic. Biol. Med. 26 (1999) 1209. 15. J.Y. Han, K. Takeshita, and H. Utsumi. Free Radic. Biol. Med. 30 (2001) 516. 16. N. Ishinishi, A. Koizumi, R.O. McClellan, and W. Stoeber (eds). Carcinogenicity and Mutagenecity of diesel engine exhaust. Elsevier Science. Pub]. Amsterdam, 1986. 17. T. Ichinose, A. Fumyama, and M. Sagai. Toxicology, 99 (1995) 153. 18. M. Sagai,A. Fumyama, andT. Ichinose. Free Radic. Biol. Med., 21 (1996) 199. 19. B. Omar, S.C. Flores, and J.M. McCord. Adv. Pharmacol., 23 (1992) 109. 20. K. Takeshita, A. Hamada, and H. Utsumi. Free Radic. Biol. Med., 26 (2000) 95 1.
542
EPR in the 2 1st Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
vivo ESR Studies on Spin Clearance in GPxl-Transgenic Mice K. Murakami', 0. Mirochnitchenko2,and H. Utsumi' Laboratory of Bio-function Analysis, Graduate School of Pharmaceutical Science, Kyushu University, Fukuoka, 812-8582, Japan. Department of Biochemistry, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Piscataway, New Jersey, 08854-5635, USA.
Glutathione Peroxidase (GPx) is considered as the major enzyme responsible for removing H202. Overexpression of this enzyme was reported to induce protection activity in the cell against oxidative damage. Non-invasive measurement of in vivo free radical reactions with transgenic mice overexpressing GPxl (GPxl-Tg) should clarify the role of this endogenous GPxl in animal under normal and stressed conditions. In this study, in vivo free radical reactions were evaluated as spin clearance of three PROXYL derivatives, carbamoyl- (CmP), carboxy- (CxP) and acetoxy-methyl-carboxy- (AMC-PROXYL) in GPxl -Tg without oxidative stress. The GPx activity was higher in various tissues in GPxl-Tg, especially in brain than that of wild mice (WT). In ESR study, there were no difference in the spin clearance rates of CmP and CxP either at the upper abdomen or the head between GPxl-Tg and WT. On the other hand, the spin clearance of AMC-PROXYL was significantly smaller at the head of GPxl-Tg than that of WT. In addition, AMC-PROXYL's signal decay rate was correlated with GPx activity. These data suggest that overexpression of GPxl modulates redox status in the parenchymal cells of the brain even in normal condition.
1. INTRODUCTION
Small amount of reactive oxygen species (ROS) are physiologically generated in our body. There exist several non-enzymatic and enzymatic antioxidant defense systems to protect fiom ROS, including superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase and glutathione (GSH). To keep the homeostasis in living organism, it is necessary to maintain a balance between oxidants and the intracellular levels of these antioxidant contents. Cytosolic GPx (GPxl ;ECl.ll.1.19) catalyzes the reduction of hydroperoxides using GSH, thereby protecting cells against oxidative damages. There are five GPx isoenzymes found in mammals [l] and their expression is ubiquitous, whereas the level of each isoform is inherent depending on the tissue type. Especially, GPxl reduce fatty acid hydroperoxides and H20z
into alcohols and water, respectively. Although GPx shares the substrate HzO2 with catalase, it alone can react efficiently with lipid and other organic hydroperoxides so that it would be the major protector against oxidative stress. Recently, overexpression of GPxl in mice (GPxl-Tg) was found to be resistant to ischemia-reperfusion injuries in brain, kidney and heart [2-31, but the function of endogenous GPxl on free radical reactions have not clarified yet. To evaluate the biological significance of endogenous GPxl, non-invasive measurements of free radical reactions should be clarified with GPxl-Tg. Several nitroxyl spin probes are used for ESR measurement to evaluate free radical reactions. CxP and CmP hardly pass through the plasma membrane, thus they may not accumulate in cells. On the other hand, AMC-PROXYL, which is permeable through plasma membrane and blood-brain-barrier, is hydrolyzed by intracellular esterase into anionic CxP and then accumulates in cells. The aim of the present study was to evaluate whether endogenous GPxl modulate the intracellular redox status or not, using three different nitroxyl spin probes in GPxl-Tg.
2. MATERIALS AND METHODS
2.1 Transgenic mice Heterozygous transgenic mice 6months old in a C57BL/6xCBA/J background overexpressing human GPxl, which was developed by Mirochnitchenko’s laboratory and wild type (WT : C57BL/6xCBA/J) were used in this experiment. Transgenic progeny were identified by polymerase chain reaction (PCR) amplification of tail DNA. WT were used as controls of GPxl-Tg. 2.2 GPx activity GPx activity was measured by couple enzymatic assay with t-butyl-hydroperoxide as substrate. In brief, the tissue was homogenized in ice-cold 0.1M sodium phosphate buffer, pH 7.5, and centrifuged. GPx activity was assayed in the supernatants, and calculated using extinction coefficient of 6.2~10”per pM per cm for NADPH at 340 nm. Specific activity was expressed as nmol of NADPH oxidized per min at 37°C per mg of protein. Protein concentration was determined by the method of Lowry. Bovine serum albumin was used as a protein standard. 2.3 Lipid Peroxidation Lipid peroxidation was estimated from the concentration of thiobarbituric reactive substances (TBARs). TBARs in the tissue were measured spectrophotometrically as follows. The tissue was homogenized (l:lO, W/V) in 1.15 % KCl and 5 mM BHT. A 0.5 mL homogenate was mixed to give a final concentration of 2.2 % TCA, 0.5 mM EDTA and 0.8 % SDS, and then reacted with 0.2 % thiobarbituric acid (TBA) in a boiling water bath for lhr. After cooling, the chromogen was extracted in n-butanol. The concentration of TBARs was
544 calculated from the difference between the absorption values of the butanol-extracted at 532 nm and 520 nm. 1, 1, 3, 3,-Tetraethoxypropane was used as the standard. 2.4 Zn vivo ESR measurement After anesthesia by an intraperitoneal injection of pentobarbital (50 pg/g b.w.), either CmP, CxP or AMC-PROXYL (0.125 mmoVg b.w.) was injected into the tail vein of mice. Immediately after injection of the nitroxyl spin probes, ESR spectra were measured at the upper abdomen and the head of the mice with a JEOL L-band ESR spectrometer equipped with a loop-gap resonator (diameter 33 mm and length 5 mm). The microwave frequency was about 1.1 GHz and the power was 1.0 mW. The amplitude of the 100 field modulation was 0.125 mT for the upper abdomen and 0.1 mT for the head. The external magnetic field was swept at a scan rate of 5 mT/min. The initial signal decay kinetic constant was obtained from the slope of the semi-logarithmic plot of signal intensity vs. time after injection.
RESULTS 3.1 Characterization of GPx1-Tg The specific activity of GPx in the brain of GPxl-Tg was found to increase approximately 3 times than that of WT (Tablel), but other tissue showed relative low GPx activities in GPxl-Tg rather than that in WT. 3.2 Lipid Peroxidation Figure 1 shows the degree of lipid peroxidation in the tissues of GPxl-Tg and WT. Overexpression of GPxl showed no significant influence on the level of the lipid peroxidation in every tissue even in the brain. 3.3
vivo ESR measurement Intravenous injection of nitroxyl spin probes in mice produced triplet lines in the vivo ESR measurement both at the head and the upper abdomen. Fig.2A and 2B show the typical ESR spectra and signal decay constants of AMC-PROXYL. There were no significant differences of the signal decay constants of CmP between GPxl-Tg and WT (Fig.3A and 3B). The decay rate of CxP at head was almost the same between GPxl-Tg and WT (Fig.3C and
Table 1 The GPx activity in tissue of WT and GPx 1-Tg mice Mice Brain Liver Kidney WT 27.87*3.042(10) 776.4*63.39(9) 822.7*113.5(9) GPxl-Tg 85.78*9.062*(10) 606.1*54.34(9) 454.9*73.07(8) The samples were assayed spectrophotometrically for GPx activity as described in the text. Its activity was defined as nmol of NADPH oxidized per min per mg protein. Values are meanf S.E.(n). *statistically significant difference in the activity when compared to WT; p
545
0
80
3, Po
60 40
20
0
Brain
Liver
Kidney
Fig. 1 Effect of the overexpression of GPxl on lipid peroxidation in tissues of WT and GPxl-Tg mice. Specific activities are shown as nmol/g tissue, and the values represent S.E.(n). 3D). However, the signal decay constant of AMC-PROXYL in head was significantly lower in GPxl-Tg than that in WT (Fig.3E and 3F). Figure 4 4 C, and E show the signal decay curves of CmP, CxP and AMC-PROXYL in the upper abdomen of the WT and GPxl-Tg. There were no significant differences of the signal decay constants of every spin probes between GPxl-Tg and WT (Fig.4B, D and F). In addition, as shown in Fig. 5, AMC-PROXYL's signal decay rate was tended to correlate with GPx activity.
4. DISCUSSION
The in vivo signal decay of nitroxyl spin probe has been reported to be associated to in vivo reducing capacity and in vivo free radical reactions. The findings of this study that the signal decay of membrane-permeable nitroxyl spin probe, AMC-PROXYL, was significantly lower in head of GPxl-Tg and tended to correlate with GPx activity may provide hrther support that GPxl modulate the redox status in parenchymal cells of the brain under
Fig.2A. Typical ESR spectra of AMC-PROXYL at head of WT
min 10.0 15.0
Fig.2B. ESR signal decay curves of AMC-PROXYL of WT( and GPx 1-Tg(0 )
0.125rnrnoVkg b.w. CrnP
(B)
0.125rnrnoVkg b.w. CxP
0.125
vo.125
-
0.125rnrnoVkg b.w. AMC-PROXYL
F)O.IZ
0.1
0.1
0.1
5 0.05
0.075
0.075
0.05
0.05
0.025
0.025
0.025
.‘0.075
WT
0
I I
M
0
GPXl -Tg
Fig.3 ESR signal decay curves an-^ signal cay rate of three ---troxyl spin prc ..:s at head of WT and GPxl-Tg mice. ***statisticallydifferent at the p<0.05 level according to Student’s t-test. The values represent the meanf S.E.(n). physiological condition. In fact, despite an ability of GPx is well known to remove HzOz and hydroperoxides, the exact role of this enzyme under physiological and oxidative stressed conditions is still unclear. Several reports support the notion that GPx play a major role in the protection against oxidative damage under physiological condition [4-61, but on the other hand, there are many 0.125mmol/kg b.w. CmP
0.125mmol/kg b.w. CxP
._ 6 1.0
0.125mmollkg b.w. AMC-PROXYL
1.o
1.o
0.0 0.0
5.0
min 10.0 15.0
0.0 0.0
min 10.0 15.0
5.0
min o-oO.O
5.0
10.0 15.0
(F) 0.125
(D) 0.125
0.1 0.075
0.075
0.05
0.05
0.025
0.025 0
WT
GPxl-Tg
WT
GPxl-Tg
The values represent the mean
“,k WT
GPxl-Tg
standard errots (n).
Fig.4 ESR signal decay curves and signal decay rate of three nitroxyl spin probes at upper abdomen of WT and GPxl-Tg mice. The values represent the mean+-S.E.(n).
547
0.1
R = -0.775 I
I
100
150
GPx(nmo1lminlmg p)
Fig.5 The correlation between GPx activity and AMC-PROXYL's signal decay rate at head of WT(0) and GPx 1-Tg(*) reports that suggest a protective role for this enzyme only in oxidative stressed condition [7-91. Furthermore, despite GPx catalyzes the reduction of hydroperosides using GSH as a substrate, there were no relation between GPx activity and oxidative form of glutathione (GSSG) and total amount of GSH. Further studies would be needed to clarify the role of this endogenous GPxl by comparing under stressed condition or injecting the specific inhibitor of GPx using GPxl-Tg and its littermates, non-Tg. References 1. J. B. Ham, C. Bladier, P. Griffiths, M. Kelner, R. D. O'Shea, N. S. Cheung, R. T. Bronson, M. J. Silvestra, S. Wild, S. S. Zheng, P. M. Beart, P. J. Hertzog and I. Kola, J.Bio1. Chem., 273 (1998) 22528. 2. T. Yoshida, M. Watanabe, D. T. Engelman, R. M. Engelman, J. A. Scheley, N. Maulik, Y. Ho, T. D. Oberley and D. K. Das, J. Mol. Cell. Cardiol., 28 (1996) 1759. 3 . 0 . Mirochnitchenko, 0. Prokopenko, U. Palnitkar, I. Kister, W. S. Powell and M. Inouye, Circ. Res., 87 (2000) 289. P. Nicholls, Biochem. Biophys. Acta, 279 (1972) 306. 5. D.P. Jones, L. Eklow, H. Thor and S. Orrenius, Arch. Biochem. Biophys., 210 (1981) 505. 6. N. Makino, Y. Mochizuki, S. Bannai and Y. Sugita, J. Biol. Chem., 269 (1994) 1020. 7. N. Gin, Z. L. Chen, W. R. Younker and M. Schiedt, J. Toxicol. Appl. Pharmacol. 71 (1983) 132. 8. M. J. Kelner and R. Bagnell, J. Biol. Chem. 265 (1990) 10872. 9. I. Rahman, G. D. Massaro and D. Massaro, Free Radic. Biol. Med. 12 (1992) 323.
548
EPR in the 2 1%'Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Non-invasive analysis of stress-induced gastric ulcer in rats K. Yasukawa and H. Utsumi Laboratory of Bio-function Analysis, Graduate School of Pharmaceutical Science, Kyusyu University, Fukuoka, 812-8582, Japan
A combined technique of in vivo electron spin resonance (ESR) spectroscopy with a nitroxyl probe provides us non-invasive and real-time information about in vivo free radical reactions relating to diseases. Water immersion restraint (WIR) is widely used as an animal model of stress-induced gastric ulcer. To investigate the relation of in vivo free radical reactions with gastric ulcer formation, the combined technique was applied to WIK model rats. Both oral and intravenous administration of nitroxyl probe, 3-carbamoyl-2, 2, 5, (carbamoyl-PROXYL), gave a typical triplet line having hfs of 1.65mT, and the signal intensity of the oral-administered nitroxyl probe decreased gradually in WIR-treated rat, while that did not decay in non-treated one. The decay rate was significantly enhanced by WIR treatment of 2hr, and the enhancement of the signal decay continued till 6 hr. The area of gastric lesion was slightly increased WIR treatment of 2hr and significantly of 6hr till 24hr. These results suggest that the enhancement of signal decay was occurred before the progression of lesion formation. The oral treatment of the spin-probe suppressed the lesion formation to approximately half of vehicle-treated group, indicating that the enhancement of signal decay reflects the in vivo free radical reactions needed for the gastric ulcer formation in WIR-stressed rat.
1. INTRODUCTION It has been reported the relation of gastric ulcer with stress such as trauma, serious burn, and organ transplantation. Several ulcer animal models, including water immersion restraint (WIR), have been developed in order to investigate the mechanism of ulcer formation and to assess the effect of anti-gastric-ulcer medicines. Reactive oxygen species (ROS) are thought to take parts in pathogenesis of gastric ulcer induced by stress [1-31. In some ulcer models, it was reported the inhibition of lesion formation by either OH radical scavengers or iron chelator [2] and the enhancement of lipid peroxidation [3]. However, all these reports are of indirect evidences for in vivo free radical reaction. We have examined in vivo free radical reactions in various physiological conditions [4] and experimental disease models such as streptzotocin-induced diabetes [5], iron-overload [6],
549 and lung injury by diesel exhaust particles [7] using ESR spectroscopy and a nitroxyl probe [4-71. Recently, we developed an 300 MHz ESR-CT system for large-scale animals such as rats and rabbits, and reported the suppression of free radical reactions of streptzotocin-induced diabetic rats by vitamin E and insulin In the present study, the non-invasive measurement with 300 MHz-ESR spectrometer and nitroxyl probe was applied to WIR model rats in order to investigate the relation of free radical reactions with gastric ulcer formation.
2. METHODS Male Sprague-Dawley rats (age 5 weeks, weight 120-150 g) were fasted for 24 hr but allowed free access to water until lhr before the experiment. WIR was carried out by restraint in plastic case (height 10 cm, diameter 6 cm) and immersion up to the rat’s xiphoid process in water bath 2°C). Lesion formation in gastric mucosa was evaluated by measuring the area of lesions (mm2) under a dissecting microscope after the fixing of gastric wall using 1 % formaldehyde. ESR measurement was performed with control and WIR-treated rats under urethane anesthesia (2 g/kg w.t., i.m.). A spin-probe, 3-carbamoyl-2,2,5, oxyl (carbamoyl-PROXYL) was orally (1.5 mL of 3 mM) or intravenously (lpl/g w.t. of 200 mM) administered to rats, and then the ESR spectrum was obtained at the gastric region with a 300 MHz-ESR spectrometer (JES-CM-3L, JEOL, Japan). The microwave power was 1.19 mW. The amplitude of the 100 kHz field modulation was 0.1 mT. The external magnetic field was swept at a scan rate of 1.25 mT/min.
3. RESULTS AND DISCUSSION
The gastric lesion was slightly formed by WIR treatment of 2hr. The significant lesion was observed in WIR treatment of 6hr and the increment of lesion area continued till 24hr. Both oral and intravenous administration of nitroxyl probe, carbamoyl-PROXYL, gave a typical triplet line having hfs of 1.65 mT, indicating that the spin probe presents in water-phase. The ESR signal intensity of carbamoyl-PROXYL was gradually decreased. As shown in Figure la, the signal intensity of the oral-administered nitroxyl probe decreased gradually in WIR-treated rat, while that did not decay in non-treated one. On the other hand, the signal decay of the probe injected intravenously did not obey first order kinetics, and no difference was observed between control and WIR-treated group (Figure lb). The decay rate was significantly enhanced by WIR-treatment of 2hr. The enhancement of the signal decay continued till 6 hr and then diminished in the group of 24hr treatment. These results suggest that the enhancement of signal decay was occurred before the progression of lesion formation. When carbamoyl-PROXYL was orally administered immediately before and during WIR treatment at intervals of 2 hr, the lesion area was suppressed to half of the level in vehicle group (Figure 2). These findings suggest that the reaction of the spin-probe that caused the
h .4
- Q
8
-
%
Non-Stress
. I
3
WIR 6hr
. I
v rn
3 6
200
;
Vehicle
carbamoyl-PROXYL
Figure 2. The effect of oral administration of carbamoyl-PROXYL. (1 0 0 molhat ~ in a time) on lesion formation in 6hr WIR-treated rats. Carbamoyl-PROXYL was administrated to rats immediately before and 2hr and 4hr after the initiation of WIR treatment (total 3 times; n=3). Vehicle was administrated to rats instead of carbamoyl-PROXYL (control; n=4). Each value represents meanX3.E. of 3-4 rats. *p < 0.05.
55 1
enhanced signal decay involves the gastric lesion formation in WIR-treated rats and that the enhanced signal decay of the spin-probe reflects free radical reactions needed for the lesion formation in WIR-treated rats.
4. CONCLUSION To investigate the relation of free radical reactions with gastric ulcer formation, the combined technique was applied to WIR model rats. The significant enhancement of the signal decay was observed with the nitroxyl probe administered orally to WIR-treated rats but not with that administered intravenously. The free radical reaction resulted in the enhanced signal decay in stomach may cause the lesion formation in WIR-treated rats.
REFERENCES 1. 2.
3. 4. 5. 6. 7.
K. Nishida, Y. Ohta, T. Kobayashi, I. Ishiguro. Digestion, 58 (1997) 340. D. Das, D. Bandyopadhyay, M. Bhattacharjee, RK. Banerjee. Free Radic. Biol. Med. 23 (1997) 8. M. Yoshida, T. Kitahara, G. Wakabayashi, H. Tashiro, H. Ono, Y. Otani, M. Shimazu, T. Kubota, K. Kumai, M. Kitajima. Dig. Dis. Sci. 40 (1995) 1306. K. Takeshita, A. Hamada. H. Utsumi. Free Radic. Biol. Med. 26 (1 999) 95 1. T. Sano, F. Umeda, T. Hashimoto, H. Nawata, H. Utsumi. Diabetologia 41 (1998) 1355. N. Phumala, T. Ide, H. Utsumi. Free Radic. Biol. Med. 26 (1999) 1209. J. Han, K. Takeshita, H. Utsumi. Free Radic. Biol. Med. 30 (2001) 516.
552
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Nitric oxide production and inducible nitric oxide synthase induction in iron-loaded rats* T. Kawabatat, A. I. Hida, M. Fujisawa, M. Kamekawa and S. Okada
Department of Pathological Research, Okayama University Graduate School of Medicine and Dentistry, 2-5- 1 Shikata-cho, Okayama 700-8558 Japan We investigated effects of iron on iNOS induction and NO production in We prepared chronic iron-loaded rats with diets supplemented with carbonyl iron and induced NO in vivo by an i.p. injection of lipopolysaccharide (LPS, 5.0 mg/kg body wt.). On immunohistochemical study, iNOS was induced in hepatic Kupffer cells, splenic and lung alveolar macrophages, monocytoid cells in pulmonary vessels at 6 hours after LPS injection i n iron-loaded and control rats i n the same manner, but i n iron-loaded rats many Kupffer cells showed both iNOS and iron deposition. On western blotting, iNOS was induced in liver, spleen and lung at 6 hours after LPS injection and iNOS induction was more enhanced in iron-loaded liver than the control. To assess NO production, nitrosyl hemoglobin (NO-Hb) in blood was measured by EPR, and nitrite and nitrate (NOx) in serum by Griess method. NO-Hb and NOx increased rapidly after LPS injection and showed a peak at 6 hours and decreased until 48 hours. NO-Hb and NOx productions were not significantly different between iron-loaded and control rats. Chronic iron-loading enhanced iNOS induction in the hepatic Kupffer cells compared with the control, but did not change NO concentration of blood because the concentration means total NO production not only from the Kupffer cells, but also from the other all organs.
1. INTRODUCTION Iron-overload in human is a well known pathological condition and iron plays a crucial role on the pathological mechanism of the disease through generation of free radicals and reactive oxygen species [ 1,2]. The relationship between iron and nitric oxide is also reported. Iron had effects on iNOS induction in culture cells, but the results are contradictory depending on cell types and iron complexes. Excess iron may disturb NO metabolism because iron interacts with NO very rapidly. To elucidate effects of iron as an NO modulator, we prepared chronic iron-loaded rats with diets supplemented with carbonyl iron and induced NO in by an i.p. injection of LPS. We studied iNOS induction by immunohisochemistry and western blotting and assessed NO production by NO-Hb in blood and NOx in serum. 'This study was supported by a grant-in-aid of Ministry of Education, Culture, Sports, Science and Technology of Japan. %mail: [email protected]
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2. MATERIALS AND METHODS
2.1. Animal Male Wistar rats (4-weeks-old) were fed with 2.5 % (w/w) carbonyl iron containing Oriental Yeast rat standard diet (Tokyo, Japan) for four weeks and with standard diet for the next week [3]. The control rats were fed with the standard diet for five weeks. LPS was injected i.p. at the dose of 5.0 mg LPS/kg body weight. Rats were anesthetized with ether and the blood was taken from the vena cava inferior. The rats were killed by bleeding from abdominal aorta and several organs were resected for the morphological and biochemical studies. 2.2. iNOS induction Some parts of organs were fixed in 20 and the usual paraffin blocks were made. Inducible nitric oxide synthase (iNOS) was demonstrated with a mouse anti-rat iNOS monoclonal antibody (Santa Cruz, CA) and the following Perl’s iron staining was done. For western blotting the other parts were homogenized in 10 volumes of RIPA buffer with proteinase inhibitors and centrifuged at 15,000 rpm with Hitachi Himac CFI 5 (Tokyo, Japan). The supernatant (100 p g protein) mixed with sample buffer was applied to 7.5 % SDS-PAGE and transferred to nitrocellulose membrane. For the detection we used Western Blot Chemiluminescence Reagent Plus (NEN Life science Products, MA). 2.3. NO-Hb assay Blood was frozen rapidly in nitrogen-filled EPR tube by liquid nitrogen and EPR spectra were recorded with a JEOL ESR spectrometer JES-1XG (Tokyo, Japan) on the following condition: scan field, 320.0 50.0 mT; modulation, I00 kHz; scan time, 2.0 min; microwave power, 20.0 mW, time constant, 0.01 s. Data were treated with EPR software packages (ESR Data Analyzer, Radical Research, Tokyo, Japan). Spin concentration was standardized with CuSOq and spin Hamiltonian parameters were determined with i n MgO (JEOL, Tokyo, Japan). 2.4. Assay of Nitrite and Nitrate (NOx) Serum was separated from the blood and the contents of NOx were measured with Griess method Assay Kit-C, Wako Pure Chemical Industries, Kumamoto, Japan).
3. RESULTS The liver and spleen were loaded with much iron, which was demonstrated by non-heme iron assay of some organs, and iron staining also showed much iron deposition i n the liver and spleen. We demonstrated iNOS protein and iron deposition i n organs 6 hours after LPS injection with histochemical methods. Both iNOS and iron were stained on the same section. Kupffer cells in the liver were positive for both iNOS and iron with a high frequency. To quantify the amount of iNOS protein, we did western blot analysis of the liver, spleen and lung. Iron-loaded rats had a tendency to increase of iNOS in the three organs and the liver showed the clear increase compared with control rats. After LPS injection we detected typical EPR spectra of NO-Hb in blood, the signal of which was detected around g=2.0 from 6 hours to 18 hours after LPS injection and showed triplets due to nitrogen nuclear spin (Figure 1 ) . NO-Hb of blood after LPS injection was recorded at the indicated times and the concentration was calculated by computer-aided double integration comparing with Cu(I1) standard. NO-Hb showed a peak
6 hours after LPS injection and rapidly decreased to an original level until 24 hours. We did not observe a significant difference between iron-loaded and control rats. We also measured the final metabolites (nitrite + nitrate, NOx) in serum of iron-loaded and control rats after LPS injection. Blood was taken at the indicated time with the same methods as NO-Hb measurement and the total amount of NOx i n sera was measured with Griess method. NOx showed the similar time-course with NO-Hb and no significant difference between iron-loaded and control rats.
iron
ready for iNOS production
4
NO
48h
NO
NO
intensive ]NOS induction ;
by Kuptter cells
I+----+
20 mT
Figure 1 . Typical EPR spectra of NO-Hb after LPS injection. Typical EPR signal of NO-Hb was detected at 6 hours after LPS injection.
Figure 2. Scheme to summarize our results. Kupffer cells may be ready to gnerate NO after phagocytosis of iron.
4. SUMMARY (I) In iron-loaded rats Kupffer cells i n the liver often had both iNOS protein and iron depo-
sition.
(2) Iron-loading enhanced iNOS induction i n the liver after LPS injection, but not in other organs. (3) NO-Hb concentration in blood and NOx i n serum after LPS injection was not different between iron-loaded and control rats. Chronic iron-loading makes Kupffer cells in the liver ready to induce iNOS protein intensively and LPS stimulus lets the Kupffer cells generate NO more intensively than the control. Since the fraction of NO generated by Kupffer cells is much less than those of other cells, total NO concentration in blood may not show a significant difference between iron-loaded and control rats. We summarized our conclusion in Figure 2.
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REFERENCES 1. T. Kawabata, Y. Ma, I. Yamadori and S. Okada. Carcinogencsi5 I8 ( 1997) 1389. 2. S. Okada, Pathol. I n t . 46 ( 1 996) 3 1 1. 3. T. Kawabata, T. Ogino and M . Awai, Biochem. Biophys. Acta 1004 ( I 989) 89.
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EPR in the 21' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Collapse of redox state by glutamate transporter inhibition in the rat's hippocampus Y. Uedaa*, Y. Hayashia, A. Nakajimab, H. Yokoyama', Y. Mitsuyamaa, H. Ohya-Nishiguchi', H. Kamada' aDepartment of Psychiatry, Miyazaki Medical College Kihara, Kiyotake, Miyazaki, Japan bDepartment of Chemistry, Miyazaki Medical College, 5200 Kihara, Kiyotake, Miyazaki, Japan 'Institute for Life Support Technology, Yamagata Public Corporation for Development of Industry, 2-2-1 Matsuei, Yamagata Japan Perfusion of acid (PDC) through in vivo microdialysis probe into the ventral hippocampus resulted in the generation of lipid radical and decreased antioxidant. These results indicate directly that glutamate re-uptake inhibition resulted in the collapse of redox state, which leads to not only the enhancement of excitatory neurotransmission, but also to the neuronal necrosis through lipid peroxidation in the hippocampus. 1. INTRODUCTION Glutamate is one of the amino acids, which plays important role for neurotoxicity in the brain. Continuous increased change in the glutamate level of rat hippocampus with epileptogenesis accounts for the enhancement of excitatory neurotransmission efficacy in the model of epilepsy. It has been already proved that glutamate neurotoxicity is derived from transient and repetitive glutamate receptor activation and the following free radical generation at every seizure onset. Recent studies have revealed that the enhancement of glutamatergic excitatory neurotransmission efficacy in the epileptogenesis is modified by not only the activation of NMDA-R, but also the decreased expression or the dysfunctions of EAATs (excitatory amino acids transporters) (1) (2). We have also explored the sequential changes in EAATs
*Corresponding author; Yuto Ueda, M.D. Tel; (+8 Fax; (f8 med. ac.jp
e-mail address; usan@,postl .mivazaki-
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and AMPA/NMDA receptors expression in the hippocampus during the acquisition of epileptogenesis. Three recently cloned rat glutamate transporter subtypes, viz. EAACl (neuronal), GLT 1 and GLAST (glial), possess a sensing-property, undergoing opposite functional changes in response to oxidation or reduction of reactive sulphydryls present in their structure. In particular, thiol oxidation and disulphide reduction result, respectively, in reduced and increased uptake capacity by a preparation of partially purified brain transporters (3) (4) (4) The SH-based redox modulatory site in glutamate transporters is targeted by endogenous oxidants (viz. free radicals) and might constitute an important physiological or pathophysiological regulatory mechanism of glutamate uptake It seems to be not enough only to discuss the glutamate transporters function from the results of biochemical evidences. It will be necessary to evaluate redox state affecting EAATs. We have already developed the methodology to detect free radical generations and to evaluate the anti-oxidant ability using microdialysis. In this study, we wonder whether continuous increase in the hippocampal glutamate level induced by glutamate transporter inhibitor (L-transacid; PDC) changes the redox state and causes the neurotoxicity in the hippocampus. To ascertain this question, microdialysis was applied to EPR spectroscopy to examine the changes in redox state during the perfusion of L-trans PDC.
2. MATERIALS AND METHODS Twelve male Wistar rats weighing 200-250 g at the time of surgery were anesthetized with sodium pentobarbital (37.5 mg/kg, i.p.). In this experiment, stereotaxic coordinates were determined according to the rat brain atlas of Pellegrino (6). The incisor bar was set on the intraaural line. Each rat was also stereotaxically implanted with a 22-G guide cannula, at coordinates 5.6 mm posterior to and 5.0 mm to the right of the bregma. The 22G guide cannula was firmly anchored to the skull with miniature screws and dental cement. The microdialysis probe tip used here was covered with a 5.0 mm length of permeable hollow fiber (11 ym thickness, 0.2 mm outer diameter, molecular weight range 7000-8000, Cuprophan, Nikkiso, Japan). The dialysis cannula was connected to a microinfusion pump (101 model, Kd Scientific, USA) for continuous perfusion at the rate of 2.0 yllmin of spin probe as mentioned followings. In this experiment, to detect the changes in EPR intensities at a high-time resolving power, the perfusate was directly led by polyethylene tube (inner diameter, 0.12mm; BAS) to quartz duct (inner diameter, 4mm; LST-SHS, Labotec, Japan) placed in the resonator of EPR spectrometer. If in the case that the flow rate of microinfusion pump was set at 2pl/min, the necessary time from liquid switch to the resonator was 14 min, and that from the probe inserted in the brain to resonator was 8min. In this experiment to estimate [he changes in hippocampal redox condition, spin trapping experiment and evaluation of antioxidant ability were performed during glutamate re-uptake inhibition.
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2.1. Spin Trapping Experiment The dialysis probe connected to a micro-infusion pump for continuous perfusion, which was filled with artificial cerebrospinal fluid (aCSF) containing 150 mM a-(4pyridyl- 1-oxide)-N-tert-butylnitrone (POBN), was acutely inserted through in guide cannula. Following a 2-hr stabilization period, vivo microdialysis procedures to evaluate the intensity of the extracellular spin adduct of POBN were performed before and after 60 min-perfusion of 1mM PDC containing aCSF. Switch of the contents of perfusion was liquid switch (typellO, CMA). EPR measurements were conducted under the following conditions: microwave power, 4 mW, microwave frequency, 9.42GHz; static magnetic field, 335.36; modulation width, 1.25G; sweep width, 11G; sweep speed = 2G/min; gain = 2500 x. 2. 2. Evaluation of Anti-Oxidant Ability Anti-oxidant ability under the freely moving state was examined basically in the same way described at elsewhere, utilizing the principle that exogenously applied nitroxide radicals are reduced and lose their paramagnetism in biological system, of which the decay rate (i.e. half-life) of ESR signal intensities would reflect a biological system’s reducing ability (7) (8). (carbamoyl-PROXYL) was used here for exogenous nitroxide radical, because of its water soluble character. Following a 2-hr stabilization period, 6 mM carbamoyl-PROXYL Ringer’s solution was perfused for 60min. And then, 1mM PDC aCSF solution was perfused for 60 min. All of the switching the contents of perfusate were performed using the liquid switch. As control, aCSF not containing PDC was perfused. EPR measurements were conducted under the following conditions: microwave power, 4 mW; microwave frequency, 9.42GHz; static magnetic field, 32946; modulation width, IG; sweep width, 8.5G; sweep speed = 3.2G/min; gain = 40 x.
3. RESULTS 3.1. Spin Trapping Perfusion from every rats of PDC group showed POBN spin adduct; its hyperfine coupling constant (hfc) were aN=15.7G and aH=2.5G if in the case of lOmT sweep width. From the reason that hfc of spin adduct shown here was identical to that of sextet-POBN adduct of linoleic acid (polyunsaturated lipid acid) oxided by lipoxidase, free radicals detected here following glutamate re-uptake inhibition for 60 min was assigned as lipid radical (see Figure. 1). Sequential changes in the signalhoke ratio (SNR) of the low field EPR signal intensity of lipid radical adducted by POBN between 3336+/-1IG were observed at every 5.5 min intervals. Three SNR (dB) was considered as POBN spin adduct of lipid radical. We could observed spin adduct from 20 min to 50 min after PDC perfusion,
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of which SNR (dB) was reached at maximum (mean value; 6 dB) 30 min after PDC perfusion in PDC group. No spin adduct was observed in control group. Additional data revealed POBN spin adduct in polyethylene tube was stable for 1 hr at least.
A B
C c
332
I
333
334
335
336
337
338 (mT)
Figure. 1 There was no spin adduct before PDC perfusion (A). Perfusion from each rat of PDC group showed POBN spin adduct; its hyperfine coupling constant were aN=15.7G and aH=2.5G, when sweep width was set lOmT (B). From the reason that hfc of spin adduct shown here was identical to that of sextet-POBN adduct of linoleic acid (polyunsaturated lipid acid) oxidized by lipoxidase (C), free radical detected was assigned lipid radical. 3.2. Anti-Oxidant Ability Sequential changes in the low field EPR signal intensity of carbamoyl-PROXYL between 3294+1-8.56 were observed at every 2.67 min intervals. The EPR spectra of the dialysate samples detected at 2-min intervals were characterized by a successive decrease in amplitude without change in linewidth compared with the first spectrum in both groups. The decrease in amplitude reflects the decrease in the amount of paramagnetic species because of no changes in the signal linewidth. When plotted on a semilogarithmic scale, the signal intensity at the lowest component in the scan field (A41=+1) decayed in a linear and highly reproducible fashion, which indicated that the signal intensity decayed exponentially. From the curve of the exponential decay, half-life is available for the comparison of eliminating speed of exogenous free radical (i.e. nitroxide radical). Half-life of the decay of EPR signal intensity of carbamoyl-PROXYL is determined by two factors: 1) excretion of the applied carbamoyl-PROXYL from the hippocampal extracellular space to the extrabrain space by successive dialysis with aCSF solution, and 2) reduction of carbamoylPROXYL by the reducing agents (i.e., antioxidant) in the brain (7). The former factor, excretion by dialysis, equally applies to both groups, but the later depends on the reducing
5 60
ability in the extracellular space. Thus, half-life is available to analyze antioxidant ability in semi-quantitatively (7) (8) (9). The median half-life of the EPR signal of carbamoyl-PROXYL obtained from the perfused area in the ventral hippocampus in PDC groups was 27.419 +/-3.299min (mean+/SEM), and that in the control group was 16.582+/-4.001min. The median half-life for the PDC group was significantly longer than that for the control group (Mann-Whitney U-test, p < 0.01), that indicates the decrease in the anti-oxidant ability of PDC group. When the carbamoyl-PROXYL solution was mixed with 1mM PDC and the EPR measurement was repeated every 20 min for 120 min, no changes in the signal intensities of carbamoylPROXYL were observed. The result indicates the absence of an interaction between PDC and carbamoyl-PROXYL. 4. DISCUSSION
Regulation of extracellular glutamate level by glutamate transporters plays a role in certain aspects of various neuronal diseases. Regulation of extracellular glutamate level would be deeply associated with enhancement of excitatory neurotransmission efficacy and neurodegeneration throughout NMDA-R activation in the hippocampus. Enhancement of excitatory neurotransmission efficacy, the one of major role of NMDA-R activation, has been already proved by many experiments under the condition of and NMDA-R activation resulted in Ca++-influxto postsynapse followed by glutamate release at every times of seizure were thought to be important process of enhancement of excitatory neurotransmission efficacy. With epileptogenesis, NMDA-R is thought to be important in enhancement of excitatory neurotransmission efficacy and neurodegeneration in the hippocampus. Neurotoxic effects of glutamate may derive from an increased formation of hydroxyl radical resulting from excessive activation of NMDA receptors and downstream enzymes such as NOS and PLA2 (10). Not only enhancement of excitatory neurotransmission efficacy, Ca*-influx throughout NMDA-R is deeply related with activation of NOS (11) (12) and phospholipase A2 (11) and the following free radical generation, viz nitric oxide and superoxide anion. Highly reaction between nitric oxide and superoxide anion resulted in peroxynitrite anion, which provide hydroxyl radical (13) (14). Hydroxyl radical easily abstract hydrogen from polyunsaturated lipid acid constructing neuronal membrane and initiates lipid peroxidation leading to necrosis. Lipid radical detected here is biochemical marker of lipid peroxidation. In this study, we successed to detect the lipid radical generation (viz. oxidation) and the changes in the anti-oxidant ability in the hippocampus at a high-time resolving power, and presented the collapse of redox condition during glutamate re-uptake inhibition. Almost membrane protein is modulated their functions with synapse environment, such as redox state. The SH-based redox modulatory site in glutamate transporters would be targeted by collapsed
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redox condition important physiological or pathophysiological regulatory mechanism of glutamate uptake Oxidation decrease glutamate uptake by glutamate transporters and alkalosis and reduction increase it. Ow study showed direct EPR evidence that glutamate re-uptake inhibition resulted in the generation of lipid radicals and reduced antioxidant ability in the hippocampus, viz. the collapse of redox condition. These results indicates directly that glutamate re-uptake inhibition resulted in the collapse of redox state, which leads to not only the enhancement of excitatory neurotransmission, but also to the neuronal necrosis through lipid peroxidation in the hippocampus.
REFERENCES 1. B. S. Meldrum, M. T. Akbar, A. G. Chapman, Epilepsy Res, 36 (1999) 189-204. 2. A. G. Chapman, J Nutr, 130 (2000) 1043s-1045s. J Biol Chem. 1995 Apr 28;, 270 (1995) 9890-9895. 3. D. Trotti 4. D. Trotti, S. Nussberger, A. Volterra, M. A. Hediger, Eur J Neurosci, 9 (1997) 22072212. 5. D. Trotti, N. C. Danbolt, A. Volterra, Trends Pharmacol Sci, 19 (1998) 328-334. 6. J. L. Pellegrino, A. S. Pellegrino, A. J. A. Cushman, brain"(P1enum Press, New York and London, 1986). 7. Y. Ueda, H. Yokoyama, H. Ohya-Nishiguchi, H. Kamada, Magn Reson Med, 40 (1998) 491-493. Free Radic Biol Med, 27 (1999) 442-448. 8. H. Yokoyama J. Tokumaru Neurochem Res, 25 (2000) 1107-1111. 9. 10. E. Lancelot, M. L. Revaud, R. G. Boulu, M. Plotkine, J. Callebert, Brain Res, 809 (1998) 294-296. 11. S. Samanta, M. S. Perkinton, M. Morgan, R. J. Williams, J Neurochem, 70 (1998) 2082-2090. 12. J. B. Schulz J Neurosci, 15 (1995) 8419-8429. 13. J. S. Beckman, J Dev Physiol, 15 (1991) 53-59. 14. M. Lafon-Cazal, M. Culcasi, F. Gaven, S. Pietri, J. Bockaert, Neuropharmacology, 32 (1993) 1259-1266.
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EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Non-invasive assessment of oxidative stress in the brain of small animal models by using in vivo electron spin resonance (ESR) imaging system Masaichi-Chang-il Leea*,Hirofumi Shojia, Hiroyuki Miyazakia, Fumihiko Yoshinoa, Kohki Nakazonoa, Kazunori Anzaib, and Toshihiko Ozawab aDepartmentof Pharmacology and ESR Laboratories, Kanagawa Dental College, 82 Inaoka-cho, Yokosuka, Kanagawa 238-8580, Japan. bDepartmentof Bioregulation Research, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage, Chiba, 263-8555, Japan.
We have developed in vivo ESR imaging system with high-quality three dimensional (3D) images using nitroxyl spin probe, blood brain barrier (BBB)-permeable, 3-methoxycarbonyl(MC-PROXYL) in living small animals. We performed the ESR imaging experiments in the brain of small animal models after injection of nitroxyl spin probe. In the 2D, 3D ESR imaging experiments it was clearly observed that MC-PROXYL was well distributed in the brain of head region. These results suggested that in vivo ESR imaging system using MC-PROXYL would be useful for the non-invasive assessment of oxidative stress in the brain of small animal models. We also confirmed the high oxidative stress in the isolated brain of spontaneously hypertensive rat (SHR) compared normal rat by using ESR imaging system.
1. INTRODUCTION In recent years, free radicals have been implicated in the pathogenesis of a wide variety of disorders. ESR techniques is only specific to free radicals and provide direct evidence for the presence of free radical in biological models. We have made to develop the application of in vitro ESR spin trap technique for the detection of free radical in biological system [1-31. ESR imaging techniques have enabled the spatial mapping of unpaired electrons of free radicals in isolated organs and other vivo or in vivo biological systems [4-61. A stable nitroxyl radical *Correspondenceto Author: Tel +81-468-22-8836/Fax +81-468-22-8868;E-mail; [email protected]. This work was supported in 12877300 (E.K.), 10357021 (T.O.) and 12771116 from the Japanese Ministry of Education, Science, and Culture and by a grant Research Fund of JEOL Ltd., Tokyo, Japan. This part of work was performed in Kanagawa Dental College, Research Center of Advanced Technology for Craniomandibular Function and also supported by grants-in-aid for Bioventure Research from the Japanese Ministry of Education, Science, and Culture.
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such as
(carbamoyl-PROXYL) or (hydroxy-TEMPO) is often used for the ESR operating in the lower frequency microwave bands (< 1 GHz), where the dielectric losses of biological systems are lower, as a spin probe. A nitroxyl radical has been reported to lose its paramagnetism when exposed to a reducing agent in biological systems The changes of the physiological redox state in response to oxidative stress caused by aging, ischemia-reperfusion, hyperoxia and hypoxia impact the decay rates of the nitroxyl radicals Kuppusamy et al. developed three-dimensional ESR imaging for biological organs; our ESR imaging procedure is also based on reconstruction methods of improved NMR imaging We have reported the development of ESR imaging system enabling distribution images of a novel blood brain barrier (BBB)-permeable nitroxyl spin probe, (MC-PROXYL). This present study demonstrated that MC-PROXYL is useful spin probe for the assessment of oxidative stress in the brain of small animal models by using ESR imaging system.
2. MATERIALSAND METHODS 2.1. ESR imaging processing & instrument setting Figure illustrates a block diagram of the ESR imaging instrument of our institute. We had been developed ESR-CT imaging system, after we purchased from ESR imaging system constructed in the JEOL ESR application laboratory was used. This system consists of a commercially available electromagnet (modified JEOL, Tokyo, Japan), a pair of field scan coils, power supplies, a personal computer, and a GHz microwave unit containing a four-window loop-gap resonator. Instrument settings for ESR detection of MC- and carbamoyl-PROXYLwere follows: microwave power, mW, magnetic field, mT; field modulation width, mT; receiver gain, time constant, sec; gradient step, and field intensity, 0.9 mT. The system is provided with four different coil sets, three for the gradients and one for rapid scanning. A setting of mT/cm produced a field gradient with the maximum strength. The gradient field was controlled by a current stabilizer, which is controlled by personal computer (Hewlett-Packard). The ESR images were constructed on the basis of Lauterbur's method known as a zeugmatography; we applied linear magnetic field gradients along the x-, y-, and z-axes produced by magnetic field gradient coils (Fig. For the imaging, projections alternating between gradient and nongradient were acquired in 55 sec. Each projection required pixels of acquisition data for imaging. The ESR absorption spectra were obtained by integrating the derivative spectrum with the recorded gradient. The second signal in each absorption spectrum was separated from the triplet signal of the nitroxyl radicals tested. Each signal data was convoluted with Shepp's filter function into the Fourier domain before performing the inverse Fourier transformation to the spatial domain. The imaging pictures of x pixels were obtained from correct projections for gradient step at in the spatial domain. For the imaging, the scanning plane was assigned an arbitrary set of two axes in x-, y-
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Figure 1.
diagram of
imaging system.
and z. Rotation of the plane was controlled by another axis. The imaging picture was reconstructed from planes of imaging in different directions. Total acquisition time for this set of projections x was min for slice pictures of imaging. The distribution of a cubic matrix of x x pixels was reconstructed (computer software, IRIS Explorer, was from Numerical Algorithms Group Ltd., Oxford, UK) by interpolation of the spin probe intensity. 2.2. Spin probe Carbamoyl-PROXYL was purchased from Sigma (St. Louis, MO). MC-PROXYL was synthesized from (carboxy-PROXYL; Tokyo Kasei, Tokyo, Japan) and diazomethane (Wako Chemical, Osaka, Japan) by the The purified MC-PROXYL characterized by using method as described previously [ [‘HINMR and FT-IR
23. Animal preparation for the studies Male Wistar rat (MWR) and spontaneously hypertensive rat (SHR) were used for ESR imaging of the isolated brain. The rats were anesthetized with mg/kg pentobarbital m a g of mM MC- or (Dinabot, Osaka, Japan) by i.p. injection, then were received carbamoyl-PROXYL solution via tail vein; 30 sec after the treatment, brain was isolated. In most experiments, the brain was restrained in a four-window loop-gap resonator just after the isolation, with its bregma aligned to the center of the resonator; then the ESR measurement was performed. For vivo ESR imaging, ICR mice were used. The mice were anesthetized with m a g
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pentobarbital by injection. With this dose, anesthesia could be maintained for about 2.5 hr. Respiration rate was also monitored in selected mice by the observation of changes in the microwave reflections. After the treatment with nitroxyl compounds tested (140 mM, 10 mVkg) tail vein, each animal was automatically restrained (computed) in the ESR unit, with its possible interaural line (vertical zero plane) aligned to the center of the resonator.
4. RESULTS AND DISCUSSION In the experiments with rat brain (held in this resonator) isolated 30 sec after treatment with MC- or carbamoyl-PROXYL, we firstly assessed the distribution of the nitroxyl compounds tested from the 2D projection of the coronal section (y-z plane) (Fig. 2). These results shows that the images of MC-PROXYL, but not carbamoyl-PROXYL, is well distributed in the brain; the images distributed were decayed in a time-dependent fashion (Fig. 2). MC-PROXYL was more lipophilic (partition coefficient, 8.7) than carbamoylPROXYL and other spin probes, implying the good permeability to the BBB of this compound [15, 161. We also obtained the data of slice pattern of ESR-CT images of MCPROXYL distribution at head region living mice (Fig. 3). These data suggests that quantitative ESR imaging analysis using MC-PROXYL would be useful for the non-invasive assessment of oxidative stress in the brain. Furthermore, we confirmed the high oxidative
Figure 2. An example of 2D ESR projection images on MC-PROXYL (A) and carbamoyl-PROXYL (B) distributions in y-z plane for isolated rat brain.
Figure 3. A typical slice pattern of ESR-CT images obtained at 3 min after the treatment with MC-PROXYL in x-y (a), z-x (b) and y-z plane (c) in head region of living mouse, and superimposed by photograph of the mouse studied.
566
stress in the isolated brain of SHR compared as normal rat by using ESR imaging system (data not shown). In all of these experiments, the higher MC-PROXYL distribution was demonstrated possibly in the transverse sinus and/or the septa1 and thalamic regions, however the resolution in ESR images as shown Fig. 2 or 3 were not sufficient to resolve exact anatomical location of MC-PROXYL. The broad application of the ESR imaging technique to obtain high-quality images of lossy biological samples has been limited by several factors, including gradient design and accuracy, sensitivity, and speed of acquisition. The problems facing a successful and useful biological ESR imaging experiment include the rare occurrence of sufficient concentration of endogenous free radicals, the lack of availability of ideal stable spin probes, short relaxation times, slow data acquisition, and drifts in microwave frequency as well as in the magnitude of the static and gradient fields. However, with further advances in ESR imaging instrumentation for applications and the development of optimized nontoxic spin probes, this technology holds great promise for non-invasive measurement and imaging of oxidative stress in the small animal models vivo.
REFERENCES 1. C. Lee and E. Okabe, Jpn. J. Pharmacol., 67, (1995) 21-28. 2. C. Lee, X. Liu, and J. L. Zweier, J. Biol. Chem. 275, (2000) 9369-9376. 3. C. Lee, K. Miura, X. Liu, and J. L. Zweier, J. Biol. Chem. 275, (2000) 38965-38972. 4. J. L. Berliner and H. Fujii, Science, 227, (1985) 517-519. 5. T. Yoshimura, H. Yokoyama, Fujii, F. Takayama, K. Oikawa, and H. Kamada, Nat. Biotechnol., 14 (1996) 992-994. 6. P. Kuppusamy, M. Chzhan, K. Vij, M. Shteynbuk, D. J. Lefer, E. Giannella, and J. L. Zweier, Proc. Natl. Acad. Sci. USA, 91 (1994) 3388-3392. 7. G Bacic, M. J. Nilges, R. L. Magin, T. Walczak, and H. M. Swartz, Magn. Reson. Med., 10 (1989) 266-72. 8. H. Utsumi, E. Muto, S. Masuda, and A. Hamada, Biochem. Biophys. Res. Commun., 172, (1990) 1342-1348. 9. F. Gomi, H. Utsumi, A. Hamada, and M. Matsuo, Life Sci., 52 (1993) 2027-2033. 10.Y. Miura, H. Utsumi, and A. Hamada, Biochem. Biophys. Res. Commun.,l82, (1992) 1108-1114. 11. Y. Miura and T. Ozawa, Free Radic Biol Med, 28 (2000) 854-859. 12. P. Kuppusamy, P. Wang, J. L. Zweier, M. C. Krishna, J. B. Mitchell, L. Ma, C. E. Trimble and C. J. Hsia, Biochemistry 35 (1996) 7051-7057. 13. P. Kuppusamy, M. Chzhan, and J. L. Zweier, J. Magn. Reson. B, 106 (1995) 122-130. 14. P. C. Lauterbur, 289, (1980) 483-487. 15. Y. Miura, K. Anzai, S. Takahashi, and T. Ozawa, FEBS Lett., 419 (1997) 99-102. 16. H. Sano, M. Naruse, K. Matsumoto, T. Oi, and H. Utsumi, Free Radic Biol Med, 28 (2000) 959-969.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Possible production of hydroxyl free radical in the gastric legion of nitroso carcinogen-administrated rats T. Mikuni",', T. Moriib, H. Najimab, M. Eharab and M. Tatsuta" Department of Gastrointestinal Oncology", Department of Clinical Laboratory Medicineb, Osaka Medical Center for Cancer and Cardiovascular Diseases, Osaka, 537-85 11 Japan We have examined the mechanism of l-metyl-3-nitro-lnitrosoguanidine(MNNG)-induced gastric cancer with respect to the production of hydroxyl free radical ('OH). In this work we showed that 'OH was produced with (MNG) by the nucleophilic attack of H202 derived from the XO system on the nitroso group of MNNG via the formation of the intermediate peroxynitric acid (ONOOH) which splits into hydroxyl free radical ('OH) and nitrogen dioxide using ESR spin trapping technique, suggesting that the administration of MNNG to the gastric legion with the high XO activity results in the 'OH production.
1. INTRODUCTION
MNNG (Figure 1A) induces gastric cancer in rats within 1 year after the administration as drinking water( 1). Some investigators have suggested that the methylation of nucleic acids or protein by MNNG may be responsible for carcinogenesis(2, 3), but this does not completely explain the carcinogenic
567
568
N
N-NO,
I
II
CH3-N-C -NH,
A MNNG
N-NO, H II CH3-N-C -NH, 0 MNG
Figure 1 The chemical structures of MNNG(A) and MNG(I3).
mechanism(4). We have already showed that butylated hydroxytoluene (BHT) inhibited the development of MNNG-induced gastric cancer (5) and scavenged free radicals derived from the reaction of MNNG and H2O2(6). Xanthine oxidase (XO) induces the production of HzO2 or 0 2 ' - from molecular oxygen, depending on the overall level of the enzymic reduction(7). In the inflammation legions of the gastric tract where cancers may be induced, Hz02 as well as 0 2 " is particularly well formed by XO (8, 9), suggesting that the administration of MNNG to the gastric legion with the high XO activity results in the cancer production. In the present work we examined that 'OH was produced via the intermediate ONOOH by addition of MNNG to the XO-hypoxanthine system (the MNNGXO-HX system) using ESR spin trapping technique.
2. METHODS 'OH Production was examined in the MNNG-XO-HX-DMPO system containing 50 mM sodium-phosphate bufer (pH 7.4), 1.25 mM MNNG, 0.03mM DTPA, 0.15 mM HX, 2.9 mu/ml XO, and 300 mM DMPO. Catalase or SOD was added to the MNNG-XO-HX-DMPO system both at concentrations of 500
569
C
D
H
- 1
Figure 2 ESR spectra of the MNNG-XO-HX-DMPO system. A) the MNNGXO-HX-DMPO system, B) the MNNG-XO-HX-DMPO system with addition of ethanol, C) the MNNG-XO-HX-DMPO system with addition of catalase, D) the MNNG-XO-HX-DMPO system with addition of SOD, E) the MNNG-XO-HXDMPO system after bubbled with Ar gas, F) the MNNG-HX-DMPO system in the absence of XO, G)the MNNG-XO-DMPO system in the absence of the substrate HX, H) the XO-HX-DMPO system in the absence of MNNG, I) the XO-HX-DMPO system in the absence of MNNG and with addition of of SOD.
units /ml. NO2 and MNG in the MNNG-XO-HX system were detected by the specrtrophotometry with a slight modification of the diazotization procedure and by HPLC, respectively.
3. 3.1. 'OH production from HzOz The ESR spectrum of the MNNG-XO-HX-DMPO system consisted of a 1:2:2:1 quartet for the DMPO-(' OH) and did not show the signal of
570
DMPO-('OOH) (Figure 2A). On addition of ethanol to the MNNG-XO-HXDMPO system, a signal corresponding to a-hydroxyethyl radical adduct was detected (Figure 2B), confirming the .OH production in the MNNG-XO-HXDMPO system. Catalase extremely decreased the intensity of 'OH signal (Figure 2C), but SOD did not show any effect on the 'OH production (Figure 2D), indicating that 'OH was produced from H202. The production of 'OH was inhibited by replacement of air gas to argon (Figure 2E) and further no signal was detected in the systems either in the absences of XO (the MNNG-HXDMPO system, Figure 2F) or in the absence of HX (the MNNG-XO-DMPO, Figure 2G), showing that 'OH was produced from the oxygen product derived from the XO-HX system. The XO-HX-DMPO system in the absence of MNNG showed the signal of DMPO-('OOH) (Figure 2H) and on addition of SOD to the XO-HX-DMPO system, the whole signals disappeared (Figure 21), indicating the production of 'OH in the presence of MNNG in the XO-HX-DMPO system. These results indicate that 'OH was produced by the reaction of MNNG and H202 derived from the XO-HX system.
3.2. NOiproduction from the XO-HX system with ad' ition of INNG Since the intermediate ONOOH rapidly decays to form 'OH and NO,, which reacts immediately with water to produce nitrite and nitrate ions"'], the Table 1 production of NO, was examined by the detection of nitrite ion shows that on addition of MNNG to the XO-HX system, nitrite ion was produced, suggesting the production of ONOOH in the MNNG-XO-HX system.
Table 1 NO2-production from the XO-HX system with addition of MNNG
System
MNNG-XO-HX
xo-Hx
NO,- (lO-sM) 1.75
0.08 (n=4)
0
(n=4)
A
MNG
.1
J 5
Figure 3
10 inin
HPLC of the MNNG-XO-HX system (A) and MNG solution (B).
3.3. Identification of MNG As shown in Figure 3A and 3B, MNG was identified in the MNNG-XO-HX system but not in the MNNG solution by HPLC, indicating that MNG was produced by the reaction of MNNG with the product from the XO-HX system.
4. DISCUSSION We indicate that 'OH was produced via the intermediate ONOOH formed by the nucleophilic attack of H202 derived from the XO-HX system on the nitrso nitrogen of MNNG when MNNG was added to the XO-HX system as described in the following reactions.
572
HX + 0,
+ 2H,O
-
&uric
acid + 2H,O,
CH3N(NO)C(NH2)NN02 + H202 ONOOH
-
CH3NHC(NHz) NN02 + ONOOH
'OH + NO2
2N02+HzO-N0;
+ N03- +2H+ CH3N(NO)C(NH2)"02 : MNNG CH3NHC(NH2)"02 : MNG
Therefore we suggest that 'OH is produced by the reaction of MNNG with H,O,derived from the XO-HX system in the inflammatory legion of the gastric tract where reduction activity of XO is extremely high, resulting in the gastric cancer production'"].
REFERENCES 1. T. Sugimura and S. Fujimura. Nature, 9216 (1967) 943. 2. D. R. McCalla. Biochim. Biophys. Acta, 155 (1968) 943. 3. M. Nagao, T. Yokoshima, and et a1.H. Hosoi and T. Sugimura. Biochim.Biophys. Acta, 192 (1969) 191 4. P. Kleihues, S. Bamborschke and G. Doerjer , Carcinogenesis. 1 (1980) 111. 5. M. Tatsuta, M. Mikuni and H. Taniguchi. Int. J. Cancer, 32 (1983) 253. 6. T. Mikuni, M. Tatsuta and M. Kamachi. J. Natl. Cancer Inst., 79 (1987) 281. 7. I. Fridovich. J. Biol. Chem., 245 (1970) 4053. 8. A. Russo, G. Maconi, P. Spinelll, G. D. et al.Felice, S. Andreola, F.Ravagnami, D. Settesoldi, D. Furrai, C. Lompamdo, and L. Bettaria. J. Gastroenterol., 96 (2001) 1402. 9. T. Otamiri, R. Sjdahl, Dig. Dis. 9 (1991) 141. 10. L. R. Mahoney, J. Am. Chem. Soc., 92 (1970) 5262. 11.T. Mikuni, M. Tatsuta. Free Radic. Res. (in press).
Section 7 Geology
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EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
575
EPR and optical absorption spectroscopy on minerals B.J. Reddy", Jun Yamauchib, Y.P. Reddya, A.V. Chandrasekhar" and R.V.S.S.N. Ravikumara
" Department of Physics, Sri Venkateswara University, Tirupati - 517502, India b
Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, 606-8501, JAPAN In solid state spectroscopy, the study of natural minerals and their synthetic analogues grown in the laboratory plays an important role. In the last three decades a number of investigations have been carried out in this direction using primarily EPR and optical absorption spectroscopy as tools. An attempt at the systematic presentation of the results has been made in the present paper. Geo-chemically, Mn2+and Fe3+occur in many major mineral groups. Estimating the exact percentage of these impurity ions in minerals is useful to grade the minerals for industrial applications. EPR technique is quite useful to assess these impurities even if they are present as minor impurities/traces. EPR technique is also quite effective in the understanding of the properties of these minerals. Other ions Cr3+, Co*+, Ni", Cu2+and Ti3+ are also present as minor impurities though in restricted groups of minerals. These ions are also used as EPR probes. Optical absorption studies of many minerals have been carried out in our laboratories and EPR investigations of suitable minerals have also been undertaken. The optical and EPR results have been correlated to understand the nature of the transition metal environment and bonding characteristics. The details are presented in the paper.
1. INTRODUCTION Minerals are naturally occurring solid compounds having definite composition and physical properties. Minerals are highly complicated inorganic substances which cannot be understood even when its complete chemical analysis is available. a result of the synthesis of large single crystals, minerals have become available for use as lasers, piezo crystals and probing devices in scintillators, ferromagnetics, ferroelastics and acousto-optical instrumentation. In the case of grown crystals, transition metal ions are deliberately doped into the particular host lattice and its study is also complicated [l]. This complexity of behavior in minerals and their synthetic analogues gives scope to employ different techniques such as optical absorption and Electron Paramagnetic Resonance to understand their behaviour. Electronic spectra of transition metal ions in these minerals and synthetically grown analogues can be studied both by optical and EPR spectra. But in optical spectroscopy, transitions proceed between orbital levels, whereas in EPR spectroscopy they take place between spin sub levels emerging in the external magnetic field following splitting of the ground orbital state of the atom. Thus EPR spectroscopy is a natural sequel to optical
576
spectroscopy. In detailed nature of the ground state comes under consideration since only the spin degeneracy of the lower orbital state is more sensitive in these studies. Optical spectroscopy supports this analyses and also gives information of the higher energy states. Thus optical absorption not only supplements but also complements the results
PI.
Free transition metal ions have unfilled d-shells and d-d transitions are forbidden. However, when the transition metal ions are embedded in crystal lattice, the degeneracy of the d-levels is removed and they split into various energies. The study of the nature of splittings and position of the absorptions in and optical spectra of the transition metal ions in solid state reveal the site symmetry of the transition metal ion and the nature of the surrounding ligands, and it also gives the oxidation state of the transition metal ion [3].
2. RESULTS AND DISCUSSION The spectrum of titanite (powdered sample) at (9.2GHz) displayed a broad line at the centre. It is primarily due to the zero nuclear spin isotopes. The measured g-factors at 335.9 and 341.4 mT are g = 1.957 and g = 1.926 indicating the ground state as 'B2. The central eight-line transition is su erposed on Ti spectrum and the components (four lines on either side) are attributed to VO'. Ti3' in titanite is in tetragonally distorted octahedral site 141. The spectra of powdered anatase show a large number of resonances centered around g = 2 and g = 4.The signal at about g = 2 is due to Ti3'. The additional structure around g = 2 and 4 is attributed to Fe3' impurity. The Fe3+impurity in different lattices give rise to number of resonances and this explains present structure. Both these ions are in tetragonally distorted octahedral sites [5]. The spectra of the polycrystalline sample lamprophyllite are recorded at RT and LNT. The broad unresolved spectrum is due to Ti3+and its g value is around 2.0. The broadness (the line width is 56.6 mT) arises due to the dipolar-dipolar interaction of Ti3+ions. The g-value, a little less than 2.0, is typically due to Ti3' ions and agrees well with the reported values for Ti3'. At liquid nitrogen temperature the signal intensity increased in conformity with the Curie law, proportional to 1/T and the line width increased up to 66.5 mT. In addition to the main peak, two weak absorptions are observed at the low field side. The g values for these two signals are g = 2.648 and 3.473. The weak absorptions observed in the range of g = 2 and 4 are ascribable to Fe3+ and such observations have also been reported earlier. The unsymmetrical nature of spectrum, therefore, is attributed to the presence of the minor Fe3' impurity. Ti3' ion in solids is characterized by three broad bands around 7000, 2000 and 18000 cm-'. Three bands of titanate at 7140, 13700 and 16130 cm areassigned to the transitions 'Bzg+ 2 2B2,+2B1, and 'B2,+ 3A1, respectively. In the case of anatase also the three bands, observed at 6945, 12050 and 18180 cm-', are attributed to Ti ion. In addition to them, a set of bands observed in the region 250- 700 nm are explained as due to the presence of Fe3+. observations also support the same. The optical absorption spectrum of lamprophyllite also consists of similar nature. From the observed band positions, the crystal field (Dq) and tetragonal field (Ds and Dt) parameters are evaluated. Dq=877,Ds=-1426 and Dt = 1525 cm-'. The magnitude of Dt indicates the strength of the tetragonal distortion. This is more in lamprophyllite compared to other samples [6]. Vanadium ion, v4' (d') generally occurs in minerals and crystals as V02' because of the more stable nature of the vanadyl ion. The optical and spectra of VO" are expected to be
-'
'.+
577
similar in nature to those of Ti3’ (d’) complexes. However, because of the presence of nuclear spin I= 7/2, the EPR spectrum in tetragonal symmetry (C4v) will give rise to characteristic sixteen-line pattern. The EPR spectrum of polycrystalline sample of wavellite with the sixteen-line pattern indicates the presence of VO” ion as impurity in the mineral. The spin-Hamiltonian parameters evaluated are g = g, = 1.933, 1.970 and A = A, = 19.0 mT, A1 = 6.2 mT. The results and analysis suggest tetragonal site for VO” impurity. To be intune with the structure of wavellite, it is suggested that V02+ substitutes the Al-OH group which has shortest bond [7]. The optical absorption spectrum of wavellite consists of four ligand field bands observed at 8930, 12500, 16950 and 22220 cm-’ and are assigned from 2A1 to ’B2, ’B1, ‘A’, and ’A1 transitions which are attributed to VO” in octahedral coordination with CzV symmetry (rhombic distortion). The other bands observed at 37040, 41670 and 46510 cm-’ on higher energy side have the characteristic of metal-ligand charge transfer transitions. Using EPR results and optical absorption energies (8930 and 16950 cm- ), a spin-orbit splitting constant has been evaluated as 145 cm-’. Considering the values of g , g A , Al, and the spin-orbit splitting constant, the Fermi contact term (K) and dipolar hyperfine splitting constant (P) are evaluated as K = 97.72 and P = 13.7 mT. Mild rhombic (C,) distortion is observed in optical absorption but this is too small to split the EPR lines. Cadmium ammonium phosphate hexahydrate (CAPH) is analogues to the naturally occurring mineral struvite (MgNH4P04.6H20). EPR spectrum of vanadyl doped CAPH is characteristic of VO” ion. The following crystal field and spin Hamiltonian parameters are evaluated: Dq = 1600, Ds = -3275 and Dt = 488 cm-’, g = 1.931, gL= 1.993, A = 1 8 3 ~ 1 0 ‘ ~ and A1 = 72 x cm-’. The optical absorption spectrum of vanadyl doped CAPH exhibits three characteristic bands at 815, 625 and 375 nm. Since only three bands are observed the distortion is attributed only to tetragonal but not any other lower symmetry. This supports the EPR data also. The lower values of p*2 = 0.64 and E,*’ = 0.44 calculated form the optical and EPR data indicate lesser covalency in-plane bonding and out of plane x-bonding compared to those in the magnesium struvite and zinc struvite [8]. Turquoise [CuAl6(PO4) (0H)g. 4H20] is sky-blue coloured rounded pebble of turqouise from Nishapur, Iran. In the EPR spectrum, gl = 2.313 2 0.005, gz = 2.119 2 0.005 and g3 = 2.034 2 0.005 and the eak to peak line-width of the middle component is 83 G. The gvalues obtained for CU’ in-turquoise are attributed to octahedral symmetry with a small orthorhombic distortion and assignments of the Cu’+ site to an elongated rhombic octahedron are in conformity with the structure analysis that two water molecules are much farther than the four OH ions surroundings. The temperature dependence of the g-factor indicates that the Cu’+ ion is in rhombic field with D2h symmetry. A small blue shift of 368 cm-’ is observed at 80 K for the transition ’4 ’Blg The optical absorption spectrum exhibits two broad bands at 14970 and 18354 cm-’, and is attributed to rhombic (Dzh) site of Cu2+[9]. Libethenite Cu~(P04)(0H)is light green coloured mineral procured from Southern Shaba, Zaire. In this case, a powdered sample is used for spectroscopic studies because the mineral is not transparent. Two low field lines gives g-values: g = 2.278 2 0.005 and = 2.102 2 0.005. As the mineral contains very high percentage of copper, the hyperfine lines due to Cu” could not be resolved. The resonances due to PO.: radical are also identified. However these resonances disappear when the sample is heated to 400°C. The optical absorption spectrum exhibits three broad bands at 8545, 10525 and 12190 cm-’, which are characteristic Cu” ion in tetragonally elongated octahedral site symmetry in the mineral [lo].
578
Smithsonite ZnC03 EPR spectrum of USA sample (Figure 1) exhibits a broad peak with a sextet pattern of sharp lines. The line marked with an arrow is due to the internal standard DPPH (g=2.0036). The most intense absorption is attributable to the copper ions and the line becomes rather broad due to the dipolar-dipolar interactions. In the present case an axial treatment (g, and would be appropriate. From the spectrum one obtains gll=2.337 and =2.087, giving average g, =2.170. In the axial component the hyperfine interaction of the copper nuclei is clear, although not well resolved, implying the hyperhe splitting of 10.0 mT The sharp six line pattern observed in this sample can be assigned to Mn2+ hyperfine components and has a g-value (2.006) close to that of DPPH. The hyperfine coupling constant is 8.52 mT. These values are typical for a Mn2+ion impurity in the lattice. The room temperature spectrum of UK sample is shown in Figure 2a. The strong line, almost at the center of the spectrum, is due to DPPH. The six line pattern having a g value close to DPPH and a hyperfine spacing of 8.6 mT can be attributed to Mn2+impurity. The low intensity lines between the main sextet components are due to the forbidden transitions of Mn2+which are also clearly seen in the spectrum obtained at 77 K (Figure 2b). The broad peak apart from the central absorption at g = 2.0 may be due to Fe3+impurities (g=2.962). At 77 K this peak disappeared, probably due to the variation of relaxation times. The optical absorption spectra of the two samples of smithsonite are depicted in Figure 3. The spectrum of the USA sample (marked as 'a') exhibits three prominent absorption bands, of which the bands at 1140 nm (8770cm-') and 800 nm (12497 cm-') are broad and intense, while the band at 620 nm (16125 cm-') is very weak. The broad band at 800 nm (12497 cm-') is characteristic of octahedrally bound Cu2+.The spectrum of UK sample (marked as 'b') also exhibits three bands, of which the band at 810 nm (12342 cm-') is very broad and intense. The bands at 1090 (9172 cm-') and 630 nm (15869 cm-') are weak. In both samples, these three bands are assigned to the 2Blg 2Alg,2Blg 2B2, and 2Blg 2E,. Based on these assignments the octahedral crystal field and tetragonal field parameters are evaluated as : USA sample : Dq = 1250, Ds = 1770 and Dt = 338 cm'', UK sample : Dq = 1235, Ds = 1813 and Dt = 384 cm-' ~ 1 1121. , EPR spectrum of uvarovite garnet in polycrystalline form is recorded at 11.67 K and is depicted in Figure 4. The EPR spectrum is characteristic of Cr3+ion with S = 3/2. Cr3' is a d3 system and being a Kramer's ion each of the levels 1-.1/2> and 1&3/2>will be degenerate in the absence of external magnetic field and the separation due to spin-spin interaction between them is 2D. The g value is almost isotropic and is equal to 1.975. This is attributed Cr3+ion. The separations between the parallel components give an idea about the zero-field parameter. The part of the Hamiltonian containing the zero-field parameter D is D[S?-S(S+l)/3]. The lowest line corresponds to the transition 1-1/2> to 1-3/2> and the highest line corresponds to the transition 1+1/2> to 1+3/2>. The separation between these extreme components is 4D and hence, the powder spectrum gives the value of D directly. In the present investigation D is 75 mT. EPR spectrum also exhibits characteristic six line hyperfine structure and is attributed to Mn2+ion in the mineral as traces. The calculated g and A-values are 2.065 and 9.6mT. The lines in between the hyperhe structure are due to spin-forbidden transitions. The optical absorption (300K) spectrum exhibits bands at 14323,15392,16638cm-' and are assigned to the transitions from 4A2 to 'E, 9 1 and 4T2 respectively. The bands observed at 14323 and 16638 are split into two components each at 90K. These splittings are attributed to trigonal distortion [13]. The separation of bands observed at 14322 and 14381 cm-' is direct measure of h = 59 cm-'. The band ascribed to 4T2 is also split into two components (15823 and 16845 cm-') which is equal to v/2. Here is trigonal splitting parameter and the value is -+
-+
-+
579
I
--I-
% 1
I
Figure 1. EPR spectrum of smithsopite from USA at RT
Figure 2. EPR spectra of smithsonite from UK at RT'and LNT
k
E
n
810
LLJ
V 2
m
I
0.4-
:
v)
I
I
I
1000
400
1200
WAVELENGTH
Figure 3. Optical absorption spectra of smithsonite a :USA Sample b : UK Sample
1
1400
580
Figure 4.
Spectrum of uvarovite garnet at 11.67 K.
2044 cm-'. The crystal field parameter is evaluated as Dq = 1700 cm-'. Optical absorption spectrum of uvarovite garnet is characteristic of Cr3+ ion with mild trigonally distorted octahedral site symmetry. By correlating and optical parameters bonding parameter is evaluated as 0.62. This value of indicates covalent nature of Cr3+ions in the mineral. The spectrum of Co2+ ion is observed only at low temperature because the spin lattice relaxation time is extremely short for octahedral coordination of Co2+ion. At higher temperature the spectra become broader probably due to short relaxation time characteristic to high spin state of Cozc ion. The polycrystalline spectrum of Co'+-doped ZAPH (zinc ammonium phosphate hexahydrate) recorded at 11.5 K (Figure 5) exhibits resonances at g11=2.79, g~=5.72. The effective g value is geff=(81 1t2gJ3 = 4.74. The polycrystalline spectrum of Co2+doped CAPH (Figure 6 ) recorded at 11.5 K exhibits resonances at 811 = 8.04, = 3.25 and geff=4.85. These spectra are characteristic of distorted octahedral site for the high spin state S=3/2 for both ZAPH and CAPH. The position of the optical absorption bands assignments and analyses indicate that the Co2+ion in both the crystals exhibit near octahedral symmetry [14, 151. The nature of the distortion, if any could be ascertained in these cases, since the bands do not exhibit any splitting even at LNT. Therefore it is concluded that the distortion in Coz+ ion is negligibly small even at LNT and the symmetry is near octahedral. The results of spectra recorded at 11.52 K show as explained above that the site symmetry is distorted octahedral in both the
581
crystals. It is quite interesting to note that g1 lg1 in CAPH. The g values of ZAPH exhibiting indicate the presence of dynamic Jahn-Teller effect in which the axial ligands are slightly compressed whereas in the case of CAPH the EPR spectrum exhibits normal behavior. One can assume that, in case of CAPH, the Coz+impurity might have entered the lattice in place of cadmium and hence the g-values agree well with literature data for Coz+ion surrounded by six water molecules. However in case of ZAPH, the Co2+might have entered interstitially. Polycrystalline EPR spectrum of Ni" doped CAPH at 11.5 K is shown in Figure 7. The EPR line profile of polycrystalline samples permits the approximate determination of zero
I
Figure 6.
Figure 5. EPR spectrum of Co2+in ZAPH at 11.5 K.
i 1
I
4M mT spectrum of Co2+
inCAPHat 11.5 K.
i
0
I
100
1
200
Figure 7. EPR spectrum of Ni'+ in CAPH at 20 K.
I
I
400
mT
582
field splitting parameters D and E, responsible for the fine structure and splitting of the EPR spectrum. The assymetric signal appears close to zero magnetic field in the 9.032 GHz band as a result of zero field splittings of Ni2+complexes of S=l. One can evaluate the zero field splitting parameter D from exact formula H = [H,(H,-D)]'" where H, = hv/gg, Y is the microwave resonance frequency, h is the Planck's constant and fi is Bohr magneton [16]. In the present investigation the evaluated g value is 2.272 and D is 0.30 cm-'. These values are in good agreement with those reported for usual high-spin Ni2' complexes. The Ni" (S = 1) ion in distorted octahedral fields is generally characterized by a high D (zero field splitting). Optical absorption spectra (RT and LNT) exhibits near octahedral symmetry [17]. The asymmetric nature of the EPR spectrum at 11.5 K exhibits typical distorted octahedral site symmetry of Ni2' ion in CAPH crystal. Manganese doped Cadmium Maleate Dihydrate (CMDH) crystals are investigated. Single crystal EPR spectrum exhibits five sets of resonances with six lines in each set. This spectrum is characteristic of a system with S = 512 and I = 512. The polycrystalline powder spectrum also exhibits the characteristic pattern and near the low field weak lines are observed. This weak feature has g value very close to 2.0032 in one site for Mn2+ and g value 2.0370 due to second site of Mn2+in the host. The value of hyperfine splitting is 9.4 mT. The absorption spectrum (RT) reveals the existence of three bands at 535,440 and 405 nm. At LNT, the band at 535 nm shows small shift towards longer wavelength side and is observed at 540 nm. The band at 440 nm splits into two components (450 and 460 nm). The band at 405nm neither shows any shift nor splittings. This indicates that Mn2+enters into the host lattice interstitially and exhibits slightly distorted octahedral symmetry [181. Talc [Mg3(Si04010) (OH)2], collected from Kurnool district of Andhra Pradesh, India. Talc is an important mineral which is having commercial value in the market and it is used in making the talcum powders, paints, soaps, pastes, dusting agents, pesticides, insulating materials, ceramics. Small amounts of iron, manganese or aluminum may substitute for magnesium. EPR spectra of three grades of talc exhibits large number of lines ranging from 10 to 550 mT. In addition, a strong sextet is visible around g = 2.002. This sextet is due to presence of Mn" impurity and hyperfine splitting (A) is 9.0 mT. Fe3+impurity is located in different sites. The samples are heated at different temperatures 300, 600 and 850 "C for 12 hours and EPR spectra are studied. The spectrum of the sample heated to 300 "C remains the same when compared to the one recorded before heating. However, one can note a sudden increase of iron resonances and a decrease of Mn2+resonances for a sample heated to 600 "C. Further on heating the sample to 850°C only two lines having g = 4.26 and 2.00 are observed [19]. From the study of the optical absorption spectra of three talc samples, the observed band features are attributed to the presence of Fe3+impurity. The bands at 22720,20613 and 17090 cm-' are common for all the three samples and remaining bands are also characteristic of Fe3+ impurity in octahedral cite symmetry. The abundance of iron is in the ratio of 1:10:20 in talc, soapstone and pesticide grades respectively. Since the content of iron is maximum in pesticide grade the absorption edge starts from 22750 cm-' whereas in the case of soapstone absorption edge is shifted to the higher energy position and located at 27500 cm-'. However, in the case of talc, instead of edge absorption, a sharp band attributable to 4T1,(P) could be observed at 29403 cm-' because of very low content of Fe3+ion. The small amounts of iron substitution for magnesium in the crystal structure are confirmed and Fe3+ is situated in an octahedral coordination.
3. CONCLUSIONS spectra of minerals show numerous signals due to the presence of more than one metal ion in each of the samples. A broad spectrum due to Ti3' could be seen around g=2.00 in the Ti-bearing minerals and is due to tetragonally distorted octahedral site. An eight line transition superposed on Ti spectrum is due to V02' in titanite, whereas anatase shows a large number of resonances centered around, g=2 and g=4 attributable to Fe3+ impurity. Similarly lamprophyllite also shows two weak absorptions in the range of g=2 and 4 and are ascribed to Fe3+.Characteristic three broad bands due to Ti3+areobserved in all three samples. The magnitude of Dt indicates strong tetragonal distortion for Ti3' ion in lamprophyllite compared to other samples. spectrum of wavellite indicates the presence of V02+ ion as impurity and is The supported by the presence of four bands in the optical absorption spectrum, which is the characteristic of V02+ in octahedral coordination with C2, symmetry. In the case of vanadyl spectrum is observed and optical spectrum also displays doped CAPH a characteristic three bands due to tetragonally distorted octahedral site (C,) for V02+ ion in the sample (CAPH). and optical absorption spectra of four copper minerals are presented here. The spectra studied at different temperatures show three principal g-values and the temperature dependant nature of leads to the conclusion that Cu2+in turquoise is in an elongated rhombically (D2h) distorted octahedral site. Another phosphate mineral libethenite containing high percentage of copper shows no hyperfine splittings but two low field lines appear whose g values are similar to those generally observed for Cu2+in minerals. The resonances due to radical could be identified. In the case of smithsonite minerals of different origins (USA & UK) intense broad absorption is attributed to the copper ion. An additional sextet observed is characteristic of Mn2' impurity in these minerals. The UK sample exhibits a peak due to Fe3+close to g=2.00 in addition to the broad copper signal. In the optical absorption spectra of both the samples three prominent bands due to Cu2+areobserved. The spectrum of uvariovite garnet at low temperature is a characteristic of Cr3+ion. Appearance of an additional six line hyperfine structure is due to Mn2+ion in the mineral as traces. The splitting of the sharp bands in optical spectrum suggests trigonal distortion for Cr3+ ion. The spectra of Co2+ doped ZAPH and CAPH are characteristic of distorted octahedral site for the high spin state S=3/2. The positions of the absorption bands and their assignments indicate that Coz+ ion in both the crystals exhibits near octahedral symmetry. From the study of low temperature spectra it is suggested that Ni2+ doped CAPH exhibits distorted octahedral site symmetry. The spectrum of Mn" doped CMDH exhibits five sets of resonances with six lines in each site and is characteristic of a system with S= 512 and I = ?hfor Mn2+. The industrially important talc minerals could be graded by optical and spectroscopy. The is an effective probe in identifying the Mn2' and Fe3+ions as impurities. The optical spectrum is concentration dependent and is clearly reflected by edge absorption due to Fe3+ ion for the grades of talc, pesticide and soapstone. But in the case of talc, instead of edge absorption a sharp band is observed due to lower concentration of Fe3+ than in the other two samples.
5 84
Acknowledgments One of the authors (BJR) is thankful to Kyoto University, Kyoto, Japan for the award of visiting Professorship. Authors are thankful to CSIR, UGC, New Delhi for providing financial assistance.
REFERENCES 1.
A.S. Marfunin, Physics of Minerals and Inorganic Materials, Springer-Verlag, Moscow, 1979. 2. A.S. Marfunin, Spectroscopy, Luminescence and Radiation Centers in Minerals, Springer-Verlag, Moscow, 1975. 3. C.J. Ballhausen, Introduction to Ligand Field Theory, McGraw Hill Book Inc., New York, 1962. 4. B.J. Reddy, K.B.N. Sarma and S.V.J. Lakshman, Proc. Ind. Nat. Sci. Acad., 4 8 4 (1982) 636. 5. B.J. Reddy, S. Vedanand and R. Ramasubba Reddy, Ed. M.P.Saksena et al., National Institute of Science Communications, CSIR, New Delhi (1997) 124. 6. B.J. Reddy, Jun Yamauchi, Y.P.Reddy, M.Venkataramanaiah, R.V.S.S.N. Ravikumar and A V . Chandrasekhar, N. Jb. Miner. Mh., Inpress (2001). 7. S. Lakshmi Reddy, B.J. Reddy and P.S. Rao, Spectrochimica Acta, 49A (1993) 599. 8. R.V.S.S.N.Ravikumar, N. Madhu, B.J. Reddy, Y.P. Reddy and P.S. Rao, Physica Scripta, 55, (1997) 637. 9. K.B.N. Sarma, L.R. Moorthy, B.J. Reddy and S. Vedanand, Phys. Letters A, 132 (1988) 293. 10. S.N. Reddy, R.V.S.S.N. Ravikumar, B.J. Reddy and P.S. Rao, Ferroelectrics, 166 (1995) 55. 11. L.G. Berry, B. Mason and R.V. Dietrich, Mineralogy, CBS Publishers, Delhi (1985) 331. 12. A.V. Milovsky, O.V. Kononov, Mineralogy, Mir Publishers, Moscow, (1985) 218. 13. S.V.J. Lakshman and B.J. Reddy, Physica 71 (1974) 197. 14. R.V.S.S.N. Ravikumar, S.N. Rao, B.J. Reddy and Y.P. Reddy, Ferroelectrics, 189 (1996)139. 15. A.V. Chandrasekhar, R.V.S.S.N. Ravikumar, M. Venkataramanaiah, B.J. Reddy, Y.P. Reddy and Jun Yamauchi, National Seminar on Recent Trends in Crystal Growth Processess and Applications, Nehru Memorial College, Puthanampatti, Tiruchirapalli, March 9-10, 2001. 16. P.B. Sczaniecki and J. Lesiak, J. Mag. Res., 46 (1982) 185. 17. R.VS.S.N. Ravikumar, M. Venkataramaiah, B.J.Reddy and Y.P. Reddy, Asian J. Phys., 9 (2000) 391. 18. S.N. Rao, Y.P. Reddy and P.S. Rao, Solid State Comm., 82 (1992) 419. 19. S. Vadanand, P. Sambasiva Rao and B.J. Reddy, Radiation Effects and Defects in Solids, 127 (1993)169.
EPR in the 2 151 Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
585
Thermoluminescence and ESR centers of Fluorapatite crystal from Brazil Henrique K.de FranGaf, Luciana R. P. Kassab and Sonia H. Tatumi Faculdade de Tecnologia-SP, PGa. Cel. Fernando Prestes, 30, cep.: 01124-060, SLo Paulo, Brazil ESR and TL measurements were performed in fluorapatite crystal. The ESR signals were identified as g= 2.052520.0002 related to center situated in F site, g=2.0065-~0.0002and g=2.0098+0.0002 associated with CO; centers, and g=2.002220.0002 and g=1.9980+0.0002 due to C O i related to F center. TL has been investigated in order to observe the thermal stability of the TL centers. Regeneration and additive doses methods were applied to 275°C TL peak and ESR signal g=2.0098 and they supplied similar equivalent doses of about Q=31+2 Gy.
1. INTRODUCTION Apatite Cas(PO.&(OH, F, C1) can be found in nature in igneous, sedimentary and metamorphic rocks. Many authors studied the TL and P n L emission of apatite and observed anomalous fading in its emission [l], [2], [3], [4] and Ishii et a1 [6] reported a study about ESR, TL and Fission Track (FT) of the fluorapatite. They used the hole center that substituted F (g=2.0027 and g=2.0528) to evaluate an equivalent dose (Q) of the sample. In the present work we investigated TL properties above RT in fluorapatite mineral sample to observe the stability of the TL emission. We have characterized the glow peaks through second order kinetics based on the monoernergetic trap model [7], determining the TL glow peak parameters (activation energy, E and frequency factor, and the mean life of the TL traps. Regeneration and additive dose methods were applied in order to observe the TL growth curves of the peaks. Furthermore, we tried to identify the ESR centers and also to analyze theirs growths with y-ray irradiation to determine the equivalent dose (Q) of the sample.
2. MATERIALS AND METHODS Apatite crystals from the State of Minas Gerais, Brazil were identified as fluorapatite with X-ray diffraction analysis; the crystal has a green coloration and was collected from a metamorphic formation. Quantitative analysis using electronic micro-probe was made in 40 parts of the same sample and we found the following average values: 1.220.2 wt% of Ce2O3, 0.5820.07 wt% of Laz03,6.420.9 wt% of F, 0.1120.02 wt% of C1, 36.420.8 wt% of P205 and wt% of CaO. For TL and ESR measurements the crystals were mechanically and gently crushed and treated with HCl 10% during minutes and then sieved to separate grains with 75-49pm diameter. Irradiation was performed with a %o source (300 Ci). TL reading was carried out in the Daybreak Automated TUOSL System, Model 1100. Heating rate of lO"C/s was used for TL measurements; the optical filter employed was Schott BG-39.
ESR powder spectra were obtained at RT by means of an homodyne X-band Brucker spectrometer, with a 1G of magnetic field modulation and 20mW microwave power. 3. RESULTS AND DISCUSSION
Figure l a shows a TL glow curve of the y-rays irradiated sample (100Gy), with peaks at 19OoC, 275OC and another broad one at a higher temperature. Figure l a also shows the theoretical TL glow curves, that were calculated using first and second order kinetic theory [7]. The best fittings were obtained using second order kinetic theory and the following values for activation energy and frequency factor: E=1.44eV, ~ = l x l Os-l ' ~ for the 275OC peak and E=1.8leV, s = lx1014s-l for the 35OoCpeak. Subtracting the theoretical curves from the experimental ones, it was observed another peak at a higher temperature about of 450OC. Vaz [4] found in his work a peak at the same temperature and Wintle [l]already mentioned the peak at We observed a decrease of about 20% in the intensity of the higher temperature peak at 35OoC, after 6 months of storage at RT and in dark room, as is shown in Figure lb. Additive dose and regeneration methods for evaluation of the Paleodose (P) and Q values were performed to this sample. We observed a supralinear growth in the low dose region and a single saturating exponential behavior at the high doses region.
1 axperlmentalTL Glow Curve rum of t h e o r e t ~curves l -lheore(kal curve (T=275%. E.1 A4 eV. s=lx10'3 5.1) ( ~ = 3 6 5 o cE=I.~V . ev, s = i x i 0 1 4 s - ~ ) sublracted CUNe 0
6x10'1
1
3
,
--after
.
, . , . ,
, .
6 montr
,
1
-.-
100
150
200
250
300
Temperature
350
(OC)
400
450
im
$50
2m
250
350
450
Temperature (OC)
Figure la) Experimental and theoretical TL glow curves, b) TL glow curve of fluorapatite showing the anomalous fading at RT.
587
associate -4th F c d e r
t
I
I
I
206
I
i 1
g-value
Figure 2.: ESR powder spectra of the fluorapatite measured at RT, for natural and y-rays irradiated samples.
Figure 2 shows a typical ESR powder spectra with two hole centers: 0 center situated in F site with g = 2.052520.0002 and CO; centers with axial symmetry related to g = 2.0065~0.0002 and g=2.0098+0.0002. It was also observed an electron center with g=2.002220.0002 and g = 1.9980=0.0002, due to COY associated with F center, was also observed. The dose dependence curves of the centers were analysed and it was observed the saturation of the signals after 100Gy, approximately, however the signals started to increase again when a high dose of irradiation was applied, of about lo4 Gy. Experimental points could be fitted using a single saturating exponential function; three equivalent doses were determined of Q = 2621 Gy, Q = 3222 Gy and Q = 2121 Gy, for g = 2.0065, g = 2.0098 and g=2.0525, respectively. The signals associated to COY centre were not used because their growths could not be fitted by linear or single saturating exponential functions.
4. CONCLUSIONS
In the present work we obtained, using the signal g=2.0098 and the TL peak of 275'C, similar values for the equivalent dose 3122Gy. TL fading at RT in natural and irradiated sam les were investigated. After six months decay of 20% was observed in 35OoCpeak. The 275 C eak showed weaker fading. The estimated mean life of the 275OC peak was about 6.5~10 years, considering RT = 25OC, E=1.44eV, ~ = l x l O s-l ' ~ and second order .kinetics theory. As a metamorphic formation generally has very old ages, we supposed that the TL and ESR results must be corrected taking in account the fading and the low mean life value of the 275°C TL peak. In the studied sample Ce and La were found as impurity that replace Ca in fluorapatite crystal lattice. These rare earth elements are well-known
luminescence centers that can be used in TL dosimeters and laser crystal. We guess that they may be the impurities related to TL emission.
ACKNOWLEDGMENTS The authors wish to thank FAPESP and Organizing Committee of APES'Ol for providing financial support.
REFERENCES 1. A.G. Wintle, Nature, 245 (1973) 143. 2. LK. Bailiff, Nature, 264, (1976) 531. 3. S.R.Sutton and D.W. Zimermann, Ancient TL, no.3,(1978)10-12. 4. E.Vaz, PACT, 6, (1982) 340. 5. G. Kitis ,P. Bousbouras ,C. Antypas and S. Charalambous, Nuclear Tracks and Radiation Measurements, 18,1/2,(1991)61. 6. Ishii, M. Ikeya, M. Kasuya and M. Furusawa, Nuclear Tracks and Radiation Measurements, 18, M, (1991) 189. 7. G. F. J. Garlick and A. F. Gibson, Proceedings of the Physical Society; A60 ( 1948) 574.
EPR in the 21* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
589
Spectral studies of divalent copper in antlerite mineral
',
R. Rama Subba Reddy S.Lakshmi Reddy 2*, G.Siva Reddy I & B.J.Reddy Dept. of Chemistry, S.V.U.P.G.Centre, Cuddapah 516 001, India
* Dept. of Physics S.V.D.College, Cuddapah 516 003, India
Dept. of Physics S.V.University, Tirupathi 517 502, India.
Electron paramagnetic resonance (EPR) spectra on antlerite mineral are recorded both at RT & LNT. Optical asorption spectra are recorded from 200 - 2500 nm at RT. The chemical analysis of antlerite reveals that the concentration of CuO in the mineral is 11.94 wt%. Optical absorption spectrum of antlerite shows four bands, which are mainly due to Cu(I1). EPR and optical results of the antlerite sample confirmed that Cu(I1) is in rhombically distorted (D2h) octahedral symmetry. Spin-Hamiltonian parameters are evaluated as gl= 2.36, g2 = 2.16 and g3 = 2.07, A1 = 1 6 0 ~ 1 0 ~ ~ c m ~=' ,4A0z~ 1 0cm-'and -~ A3 = 60~10-~cm-'.The NIR spectra of antlerite are attributed to water fundamentals.
1. INTRODUCTION
Most of the copper minerals are coloured. The geochemical behavior of these minerals depends on the oxidation states of copper present in the minerals. In brochantite group of (OH) ~ 41 copper is in +2 minerals in both brochantite [CuqSO4 (OH) 61 and antlerite [ C U ~ S O oxidation state. Antlerite shows a closed crystallographic and optical similarity to brochantite [l]. The crystal structure of antlerite has been reported [2] and it's lattice parameters are a=8.224(2), b= 6.623(1) and c=l 1.987(3) A.U. Its space group is orthorhombic P,; The structure indicates that copper atoms are in two sites. Copper atom is surrounded by four oxygen ligands of hydroxyl group in equitorial plane (Cu-0 = 1.9 A.U.) and oxygen ligands of sulphate group in spatial axis (Cu-0 =2.54 A.U.) in distorted octahedral coordination. Using X-ray absorption spectroscopy, oxidation state variations in copper minerals were studied [3]. Literature survey reveals that no attempt has been made on EPR and optical absorption studies in antlerite. Divalent copper being d9 system is prone to Jahn-Teller distortion and EPR study of this ion will give us valuable information about the type of distortion. The optical absorption spectral studies of transition metal compounds provide information on excited state energies, symmetrical and structural distortion. Therefore in this paper the authors report the results of spectral studies by EPR and optical absorption techniques of antlerite.
* E-mail address: [email protected]
590
2. EXPERIMENTAL The compositional analysis of antlerite from Czechoslovakia reveals that Cu0=66.64 wt%[4]. Antlerite originated from Chuquicamate, Chile denoted by Dr Michel Delien5 Head, Department of Mineralogy, Institute Royal des Sciences Naturelles, De Belgiqui, Bruxelles is used in the present work. It is dark greenish blue in colour and contains 11.94 wt% of CuO. The copper content is determined by standard spectrophotometric method [5]. EPR spectra of anlterite mineral in the powder form are recorded both at room temperature (RT) and liquid nitrogen temperature (LNT) on a Varian E-112 EPR spectrometer operating at X-band frequencies (u=9.07 GHz) having 100 kHz field modulation and phase sensitive detection to obtain first derivative spectra. The optical absorption spectra of the sample are recorded at room temperature on Cary 5E UV-Vis-NIR spectrophotometer in mull form in the region 200-2500 nm. 3. THEORY
Cu(I1) has an electronic configuration (Ar) 3d9, where Ar stands for closed argon shell. In an octahedral crystal field, the corresponding ground state electronic configuration is hp6 e: and yields 2Egterm. The excited electronic configuration t22 :e corresponds to 2T2gterm. Thus only one single electron transition 2E+2T2, is expected in an octahedral crystal field. Normally the ground 2Eg state splits due to Jahn-Teller effect and hence lowering of symmetry is expected for Cu(I1) ion. This state splits into 2Blg(d,2-,2)and 2Al,(d:)states in tetragonal symmetry and the excited term *T2, also splits into 2B2g(dxy)and 2Eg(dx,,dyz)levels. In rhombic field 'E, ground state splits into 2AIg(d,2-,2)and 2A2g(d,2)whereas 2T2gsplits into 2Blg(d,y), 2B2g(dxZ)and 2B3g(dyz)states. Thus three bands are expected for tetragonal (C4") symmetry and four bands are expected for rhombic (4Jsymmetry [6]. 4. RESULTS AND ANALYSIS 4.1. EPR studies The EPR spectrum of antlerite sample recorded both at RT and LNT is shown in Fig.1 and Fig.2. The spectrum of antlerite at LNT reveals three sets of resolved four lines in low, mid and high fields corresponding to gl, g2 and g3 respectively. From the peak positions and their separations the spectroscopic splittingfactor (g value) evalued as gl = 2.36, g2 = 2.16 and g3 = 2.07. The hyperfine structure constants (A values) are determined using LNT EPR spectrum for antlerite as A, = 1 6 0 ~ 1 cm-', 0 ~ A2 = 4 0 ~ 1 0cm-' . ~ and A3 = 6 0 ~ 1 0 cm-I. ' ~ The calculated g values provide valuable information on the electronic ground state of the ion. If for g values, gl >g2 >g3 the quantity R = (g2-g3)/(gl-g2) is greater than unity, the ground state is 2Al(d,2) and if it is less than unity it is 2Al(d,2-,2) [7,8]. In the present study the value of R=0.45 which is less than unity and thus 2Al(d,2-y2) is the ground state for Cu(I1) ion in antlerite. Using the EPR results of antlerite and assignin the value of free ion dipolar term P = 0.036 cm-' and ge = 2.0023 the covalency parameter ( a ) has been calculated by making use of the relevant formula [9]
591
Figurel. EPR Spectrum of antlerite at R T ( ~ 9 . 0 7GHz)
Figure 2. EPR Spectrum of antlerite at LNT(u=9.07 GHz) a2 = 716 {A3-A1)/p-(ge-gl)+ 1 1114(ge-g3)-6/14(g,-g2)} The value of
is found to be 0.76.
4.2. Optical Absorption Studies Fig.3 illustrates the optical absorption spectrum of antlerite at room temperature. The antlerite spectrum observed here has bands at 49260,44445, 16390, 10990,9435, 8475 cm-' in the UV-Vis region. For Cu(I1) in rhombic symmetry the general ordering of the energy
592
(
-
411.0':'
f
I
IW
Figure 3. Optical absorption spectrum of antlerite at RT
levels is as follows [6] Accordingly the optical absorption bands observed for Cu(II) in antlerite at 16390, 10990,9435 and 8475 cm-' are attributed to the transitions from 2A~g(dx2-y2) to 'A2,JdZ2), respectively. These observations are in tune with those reported earlier [lo] and the bands accordingly are ascribed to Cu(I1) in octahedral coordination with rhombic distortion ( Q h ) symmetry. The bands observed at 49260, 44445 cm-I on higher energy side may have the characteristic of metal-ligand charge transfer (CT) transitions. The band headed data and their assignments of antlerite mineral presented in Table-1.
Table 1. Band headed data and assignments for Cu(I1)in antlerite mineral at room temperature. Transitions Antlerite [CU~SO~(OH)~] Band Positions
3 'A2g(dz2) 2 Ag(dx2-,2) ~ ?k(d*Y) B2g(dxz) 2 B3g(dyZ) CT CT
Cm-' 16390 10990 9435 8475 49260 44445
nm 610 910 1060 1180 203 207
593
4.3. NIR Studies The NIR spectrum of antlerite is shown in Fig.4. In the NIR region serval bands are observed in the sample. These bands are due to overtones and combination tones of water fundamentals. These are assigned as follows. Water has CzV symmetry and gives three fimdamental modes. They are u1,v2 and u3. vl is the symmetric OH stretch, v2 is H-0-H bending mode and u3 is the asymmetric OH stretch .In vapour phase these modes occur at 3652(u1), 1595(v2) and 3156(v3) cm-', where as in solid they appear at 3220, 1620 and 3400 cm-' [ll]. The shifting of vl and v3 towards the low frequency side and u2 toward higher frequency is due to of hydrogen bonding [12]. Accordingly the band observed at 5680 cm'l is identified as u3 mode of H20 molecule. The prominent band observed at 5800 cm-' with shoulder on either side is attributed to first overtone of fundamental stretching combined with CEO-H bend. The band observed at 4265 cm-' is due to the fundamental bending mode combined with the lowest frequency of OH stretching fundamental. In general two characteristic bands appear around 7000 cm'l due to 2V3 and 5200 crn-' due to (v2+ u3) when water is present in mineral. When the bands are broad, it indicates that water molecules are relatively disordered and when the bands are sharp it indicates that water molecules are located in well defined ordered sites [12]. Accordingly one set of sharp bands around 5245 cm-' and another set around 5430 cm-' is attributed to water due to the first over tone of the hndamental OH stretching mode. The unassigned bands might be due to metal-oxygen absorptions.
r----------
Figure
NIR Spectrum of antlerite.
--
5 94
CONCLUSION The quartet structure of the gl, g2 and g3 components of the EPR spectrum of antlerite suggests rhombically distorted nature of Cu(I1) ion with 2Al,(dx2-y2) as the ground state. Even at low temperate no changes are observed in g values in the EPR spectrum. The bonding parameter a2(0.76) indicates predominent covalency for the in plane (T bonding. [ 131. Optical absorption spectrum of the sample reveal characteristic features of rhombically distorted octahedral Cu(I1). The NIR results in the antlerite mineral sample is due to water fundamentals. Thus the results of the optical and EPR investigations reported in the present paper conclusively prove that the site symmetry of Cu(I1) ion in antlerite mineral is rhombically distorted octahedron with 'A,,(d,2-,2) as the ground state. ACKNOWLEDGEMENTS
The authors wish to express their sincere thanks to Dr Michel Deliens, Head, Department of Mineralogy. Institut Royal des Sciences Naturelles, De Belgiqui, Bruxelles for providing antlerite mineral sample as a gift. The authors are also greatful to UGC, New Delhi for providing financial assistance to R.R.Reddy and S.L. Reddy. The authors also acknowledge the continued support extended by A.G.Reddy, Principal and correspondent of S.V.D.College, Cuddapah, India.
REFERNCES 1. Danas's System of Mineralogy, gth edition Wiley Eastern Ltd. New Delhi (1989)pp.756. 2. W.Hawthrone, C.Franck, A.Groat Lee,K.Raymond, Can.Min.27 (1989) 205. 3. RAD,Pattrick,CMB.Henderson, J.Phys.Chem.Solids, 53 (1992) 1185. 4. Redkosik, Thomas Povandra Pavel, Cas.Mineral.Geol.27 (1982) 79. 5. J.Basett,R.C.Denry,G.H.Geffery and J.Mendham, A Vogel's Text Book of quantitative Inorganic Analyisis, ELBS Edn., (1979). 6. M.A.Hitachmann,T.D.Waite, Inorg.Chem.,lS (1976) 2150. 7. (1970). 8. & (1971). 9. J.Phys.Condens.Matter.2 (1990) 5595. 10. S.Lakshmi Reddy,K.Ramesh & B.J.Reddy, 31d Asia Pacific Physics Conference, 2 (1988) ,994 Hong Kong, China. 11. K.Nakamoto, Infra-red spectra of Inorganic and Coordination Compounds Wiley, (New York) 1970. 12. G.R.Hunt& J.W.Saliesbury, Mod.Geol.l (1970) 283. 13. D.Kivelson & R.Neiman, J, Chem.Phys.35 (1961) 149.
EPR in the 2 1* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Paramagnetic criterions of prognosis for oil and gas rocks content
K. Kudaibayev', S z S . Szamarov2,B.K. Kuspanova2, A.S. Kalauova2, R.N.Nasirov2 'Joint Venture KazakhOil-TELF, 46501 5, Tengiz, Republic of Kazakhstan *Atyrau Institute of oil and gas, 465002, Atyrau, Republic of Kazakhstan
As have been indicated by VJ.Vemadsky deposits of hydrocarbon are described by halo dispersing both hydrocarbon and nomhydrocarbon components around them. There are anomalous content such chemical elements as thcrium, uranium, radium, potassium, nickel, vanadium, cobalt, manganese, iron, titanium under deposit and in wateFoil contact zone in rocks, which contacting with hydrocarbon during a long time [ 11 At the last years have beet taken valuable information durng searching investigations of hydrocarbon deposits by means the separate forms such transition metals as uranium, manganese and iron [2-41. Uranium may be in six-valent and four-valent state under conditions of the earth's crust. Under natural conditions six valence uranium is hydrolyzed easily and form complex divalent cation UO? which play an important role thanks to it's mobility during migration and concentration of uranium. In reduction condition six-valent uranium turn into immobile fourvalent state and is concentrated on reduction barriers. Geo-chemical effects was received during detailed study distribution of uranium in terrigenous mesozoic deposits of West Siberian oil and gas province. Results of uranium distribution into cut of rocks Penomay's are shown that uranium concentration five times as much in water-saturated sandstone than in oil-saturated one. In works [5-61 was worked out the method of reconnaissance the oil and gas deposits, which named as powder ferrometric. This method is founded on investigation of authigenic iron-contenting chemical compounds by magnetic method, particular by measuring magnetic receptivity of powder's samples of rocks. Theoretically this method is founded on the phase equilibrium oxide and protoxide (Fe2') forms of iron, which under reduction conditions thanks to migrating hydrocarbon from deposits, is moved in the last side. In works [6,7] is suggested the method of oil and gas content on distribution of Mn (TI) in over-salt terrigenous rocks in Caspian Hollow. Concentration of ions Mn (11) adjoining to oil horizontal layers is caused by chemical impact of organic agents on the reductionoxidation equilibrium in system:
Mn4'
Mn3+
Mn2'
There is not date about correlation of content between general marganese and manganese (11) in oil rocks of Pricaspian region in literature.
596 Table 1. The results of the determination of manganese by different methods
Deposit, number well
Kamyshitovoye South, 3
Depth, m
Content of general manganese,
Content Mn2' git by EPR method after treatment rock by
407-412
1500 (1360)
495,O
1370
110 (100)
absence
120
Kemercol, 9
350-355 1420-1425 1080-1085
39 200
35,l 193,2
36 193
Koja South,3 Kemercol,20 Zaburunje Moldobec, 16
1255-1260 1120-1125 875-880 5 12-515
150 (200) 114 69 (90) 98
738 absence 23 77,2
125 100 66 96
Koja South, 6
In this work brings the results of investigation the distribution bivalent manganese by method of electrical paramagnetic resonarce (EPR) and general manganese by method of roentgen-fluorescence analysis (RFA) in terrigenous rocks Southwester and in coakmining deposits of Northen part of Pricaspian Hollow. As shown from analysis EPR spectrums, from big amount of elements (S, Sr, Fq Ca, Ti, Mn, Si, K) determined by roentgerrfluorescence spectrometer VRA-30, ions Fe(II1) and Mn(I1) are recorded. Iron ions gives a wide single line without spit structure and such spectra. are uniform from a structural viewpoint. The spectrum of Mn is more various. Some rocks was subjected to treatment by concentrated hydrochloric acid for corroboration of existence more highly degree of oxidation and was determined the depth of reduce of manganese in studied rocks. As obvious from the table (1) after the treatment of rocks by concentrated hydrochloric acid, the content of di-valent manganese is comparable with the general content of manganese which was determined by RFA-and chemical methods. In figure la shows EPR- spectrum of di-valent manganese from cut of well 1 deposit of Zaburunje, which is consist of 8 line of super-thin structure (STS), conditioned by interaction between the unpaired electron with magnetic moment of manganese nucleus (nuclear spin I=5/2). The intense line with g-factor nearly 2 in the center of spectrum is caused by aluminum silicate and organic components of rocks. As shown fromthe figure, the content of Mn2+is very low. After the treatment this rock by concentrated hydrochloric acid, the content Mn in it is sharply increasing (see figure 1B). Analogous case is obsemd to some extent for rocks, which was received from wells of other deposits. Besides manganese in di-valent form, there is manganese in higher degree of oxidation in oil deposits, and we can suppose that there is an oxidation-reduction equilibrium between Mn2+ and manganese in higher oxidation state.
597 Figure 2a shows the spectrum EPR of Mn-ion from some tens wells of Astrakhan deposits. There are following descriptions: g=2,003, constant hyperfine coupling (HFC) have been measured between first and second lines, a=87,5 mT; lowintensive prohibited transitions are recorded between lines of basic sextet, which connected with HFC by nucleus SsMn. These descriptions are typical for Mn2' in high spin condition ds with values of thin structure D and E, which are less comparatively with HFC. D is the measure of axial distortion of cubic symmetry and E-is the measure of rhombic distortion. How well see from spectrum, values D and E less comparatively with We can suppose, that relatively visible splitting of fifth and sixth lines connected with considerable value of D. Figure 2b shows the spectrum EPR of rock from Volodar deposit (well 2, depth 5593,6 5600,8 m), which completely coincide with spectrum of dolomite fran reservoir of Becbulat deposit (content of dolomite according to date of RFA is 98%) (fig.3). In EPR spectrum the lines are fixed to 2 ions Mn2+,g-factor ions of Mn2' are equal: 2,0100 and 2,0028; ~ " ~ mT 9 5 (between third and fourth lines). The lines 1 and 2 at strong field part of spectrum belonging to Mn2+, which replace ions Mn2' and Ca2' accordingly in crystalline lattice of dolomite. Belonging such spectrum to dolomite was cdlaborated by IR-spectroscopy. Figure 4 shows IR-spectrum of dolomite rock of Volodar deposit, which was studied by EPR method. The lines of standard sample and investigated sample have been compared and the lines of absorption 729;853;880 and 1450 sm-' have been fixed, which are typical for mineral dolomite. a
I 1 2
Figurel. EPR spectra of Mn2+are registered in the rock of well No.1 Zaburunje deposit (800-805 m): a- until treatment with HCI; b- after treatment with HCI.
Figure 2. EPR spectra of the two valency
manganese (Mn") from samples: a- the rock (calcite) is taken from well No. 1 (42 18-4225 m) Astrakhan oil field; b- calcite- dolomite of rock well No.2 (5822-5824 m) Volodar oil field; c-dolomite of rock well No.2 (5593,6-5800,8 m) Volodar oil field.
598
I
'
'!
1
Figure 3. EPR spectra of Mn2+ in the
dolomite (98%). The core (sample) is taken from well No.1 (3825-3832 m), high stratum Becbulat deposit; 11112 is defined at strong field.
v,
Figure 4. The infrared (IR) spectra of dolomite. The core is taken from well No.2 (5593,6-5600,s m) Volodar oil
field. By means of intensity the lines 1 and 2 the distribution ofdi-valent manganese in dolomite have been studied and ratio of these lines by formulaa=25 11/12 have been calculated. There are two groups of dolomites. The first group is formal from the limestone as result o f the interaction: 2Ca (HC03)2 + MgS04 = CaS04 + CaMg (CO3)2 + 2C02 + 2H20
(1)
in freshwaterlseawater mixing zone. They are characterized bya<5. As shown from table 2, we can choose two groups of dolomites in oil's deposits of Pricaspian Hollow. Irregular distribution of di-valent manganese is getting possible to use it as criterion of oik gas-contenting of investigated cuts. The example of dismemberment of the investigating part of cut by means of change the content of di-valent manganese may be the cut of EastMoldabec deposit The results shows that the main part of terrigenous deposits is characterized by low content Mn (11). The extreme content Mn (11) on the some depth is picked out clearly against this background. The supposed interval of oipgas-content by extreme significance Mn (11) is good agree with gewhemical methods of investigation of wells. The most investigated cuts of prospecting wells, which was drilled on the salt dome structures (Ural-Emba, Kotartas, Krikmiltic, Tagan south) and UrakVolga (Zaburunje, Sazankurak) have the coincidence of the higher concentration Mn (11) with proved oikgascontent at the same intervals of the cuts. The same conformity was formed for Astrakhan and Volodar coal-mining deposits (fig.5 and 6). Results of investigations shown, that the greater part of Devonian deposits is characterked by content Mn(II), compose on an average from 0.6 to 2.5 in relative units. On this background is separated legibly the external content of Mn (11) in particular depths. Proposed intervals of oil and gas bearing by extremal content Mn (11) are in good keeping with dates of geophysical method of investigations (GMI) of wells.
599
Figure 5. The change of Mn2' and FR contents of Devonian deposits of Volodar oil field, well No 2.
-Mn2+
___
Figure 6. The change of Mn2' and FR
contents of Devonian deposits of Astrakhan oil field, well No 1.
FR
In the capacity criterions of prognosis oil and gas bearing rocks-layers may be used also free radicals of the organic source, which are presented in layers together with ions Mn The formed inter-communication of high content Mn (11) with oil-content deposits is the effective addition to exposure the new and omitted oil deposits by known geologicalgeophysical methods, especially during the prediction oil-content collectors in the process of drilling the wells.
Deposit, number well Ongar, 6 Imashev, 1 1 1
Depth, m 680-685 2698-2703 3593-3596 380 1-3804
Age Jurassic
3,65
Triassic Permian Permian
2,77 3,87 3,75
Becbulat, 1 1
3825-3832 (low) 3925-3832 (top)
Early- Permian Early- Permian
6,39 8,3 1
Volodar, 2 2 2
5822-5824 5898-5902 5593,6-5600,8
Devonian Devonian Devonian
73 18,O 924
600
REFERENCES 1. Nontraditional methods of geechemical investigations of oil and gas (collected scientific articles), Moscow, geo-informsystem, (1989) 171. 2. I.M. Stolbov, LA. Fomin, T.H. Badretdinov, N.F. Stolbova Geochemistry of uranium of hydrocarbon deposits. Nontraditional methods of geochemical investigations of oil and gas. M., VNII geo-informsystem, (1989) 61. 3. R.N. Nasirov, Paramagnetic properties of oil and gas rocks Caspian region, Moscow, Nedra, (1993) 128. 4. O.V. Zvereva, Prognostication of oil-gas content cut by means of powder ferrometric method. Nontraditional methods of geechemical investigations of oil and gas, M., VNII geo-informsystem, (1989) 73. 5. R.N. Nasirov J.0il Industry (Russia), V.12. (1995) 46. 6. R.N. Nasirov, V.E. Tavrizov, S.P. Solodovnikov, V.V. Strelchenko,. Patent (Russia), No.206819, B.No.29 (1996).
Section 8 Dosimetry
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EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
603
EPR dose reconstruction in teeth: Fundamentals, applications, problems and perspectives A.A. Romanyukhaa and D.A. Schauerb aNational Institute of Standards and Technology, Gaithersburg, MD 20899-8460, USA bUniformed Services University of the Health Sciences, Bethesda, MD 208 14-4799, USA Fundamentals, applications, problems and perspectives of EPR dose reconstruction in tooth enamel are presented. EPR dose reconstruction results are reviewed for the Chernobyl accident, Russian nuclear workers and the population residing in the vicinity of the Mayak nuclear plant, and two Soviet nuclear test sites: Semipalatinsk (Kazakhstan) and Totskoye (Russia). Unanswered questions in EPR dose reconstruction are formulated. Possible new directions and perspectives of EPR retrospective dosimetry are also considered.
1. INTRODUCTION The use of EPR to detect radiation-induced radicals in biological materials dates back to the mid 1950s [l]. However, its potential for radiation dosimetry of bone and tooth enamel was not explored until the mid 1960s [2]. Early investigations were mainly devoted to studying the parameters and properties of the EPR signals in dental tissues and bone. In the following two decades the technique was applied to bones [3] and teeth [4] from individuals exposed to the Hiroshima A-bomb detonation. The latter paper represents the first attempt at applying the method and the beginning of the second stage of EPR dose reconstruction development. Strong motivation for practical applications and further development resulted from the Chernobyl accident in 1986. At this time EPR dose reconstruction became an element of broad epidemiological studies on the health effects of radiation. This was followed by several EPR dose reconstruction studies with teeth samples prepared from the general population and nuclear workers of the Southern Urals region of Russia. These groups were overexposed from radioactive contamination and occupational exposure from the Mayak nuclear weapons facility. The scale of EPR measurements with tooth enamel required in epidemiological studies is quite large. During the second stage EPR measurements and sample selection and collection played an important role. Another important development of EPR dosimetry was the conduct of several international intercomparisons of the results of blind EPR dose reconstruction [5,6]. Such intercomparisons allowed selection of the best dose evaluation procedures and the identification of weaknesses in applied techniques. In 2000, further efforts were initiated to regulate practical applications of EPR retrospective dosimetry. For example, the International Commission on Radiation Units and Measurements (ICRU) prepared a report on the Retrospective Assessment of Exposures to Ionizing Radiation [7], including EPR dose reconstruction in tooth enamel and the International Atomic Energy Agency (IAEA)
604
started preparation of its own report on EPR Tooth Biodosimetry [S]. There are a number of reviews on the different technical aspects of EPR dose reconstruction with tooth enamel, (see, [9-151). Therefore, we are not going to duplicate them by considering technical details of the procedure. The aims of this paper are: 1) to formulate some useful conclusions gained from the most recent results of the practical application of EPR dose reconstruction; and 2) to discuss some problems of the method and the ways to solve them. Because it is simply impossible to discuss examples of applications and problems without a general description of the method itself some fundamental information and components of EPR dose reconstruction in tooth enamel will be provided.
2. FUNDAMENTALS OF EPR DOSE RECONSTRUCTION IN TOOTH ENAMEL EPR dosimetry with teeth is based on the measurement of radiation-induced radicals in hydroxyapatite CalO(PO&(OH)2, which is the mineral component of teeth and bones. Hydroxyapatite has a hexagonal crystalline structure = b = 9.432 A, = 6.881 A) and constitutes 95% to 97% by weight of tooth enamel, 70% to 75% by weight of dentin and 60% to 70% by weight of compact bone. All calcified tissues (tooth enamel, dentin and bone) are composed of hydroxyapatite crystallites bounded by an aqueous-organic matrix. The sizes of these crystallites are varied in different calcified tissues from approximately 200 - 400 A in dentin and bone to 7000 - 8000 A for tooth enamel. The aqueous-organic matrix is typically about 60% water and 40% organic component (mostly protein and fat). Conventionally two main components can be selected in the X-band EPR spectrum of irradiated dental tissues (Fig. 1). The so-called native or background signal is centered at a g-value of 2.005. The origin of this signal is not known exactly. However, it is widely believed that the main contribution of the native signal is due to the organic radicals in the aqueous-organic gel; this signal can be essentially reduced by chemical treatment of the tooth enamel [ 161. Improvements to this method were recently demonstrated [ 10,17, 181. It has been shown [ 14,17,19]) that the amplitude of the native signal depends very strongly on the tooth enamel grain size, increasing considerably with decreasing grain size. Thus, on one hand the native signal can be reduced by different types of chemical treatment of tooth enamel, on the other hand it rises with decreasing grain size. More recently, a strong correlation between surface water content and native signal intensity in tooth enamel has been reported [20]. This finding has connected grain size with higher surface water content in tooth enamel samples with smaller grains. The other prominent component in the tooth enamel spectrum is radiation-induced and can be used for retrospective dosimetry. It has an anisotropic g-factor shape (gl=2.0018, g11=1.997). EPR and ENDOR investigations of carbonated hydroxyapatite [ 131 have shown that the radiation-induced signal is a composite (i.e., it consists of contributions from several paramagnetic species differing in location (hydroxyl (A-site), phosphate (B-site) or surface site location) and molecular structure (CO3-, C033s,COz-, 0-,03-).However, the most important radiation-induced radical is COzlocated in both the A and B-sites of hydroxyapatite lattice and on the surface. The concentration of radiation-induced radicals and hence the intensity of the EPR signal increases proportionally with the absorbed dose from about 10 mGy to above 10 kGy [21], a significant range for the reconstruction of accidental doses. Therefore, in order to calibrate EPR dose response (peak-to-peak amplitude or intensity of radiation-induced
605
J
100-
.-
+ 3
.-5j
50 -
0-
(I)
LT -50-
-1ooJ 2 02
2 01
2 00
199
g-factor
Figure 1. EPR spectrum of irradiated (300 mGy) tooth enamel and its components. signal (Fig. I)) in dose units one simply needs to re-irradiate a tooth enamel sample with known doses. A linear back extrapolation is then used to determine the unknown dose. The variation of the radiation sensitivity of tooth enamel (EPR radiation response per unit mass) is not considerable (10% maximum according to [22]) and maybe caused by variation of the surface water content in different samples [20]. The later circumstance simplifies the procedure of dose reconstruction to multiplication of the EPR dose response by an experimentally determined calibration factor. As evidenced by the latest international intercomparison [6], doses in the range 100200 mGy can be reconstructed with precision and reliability. Under conditions of the best case scenario (availability of 100 mg chemically purified tooth enamel) and use of the latest generation EPR spectrometer, Bruker ELEXSYS, the threshold of radiation dose detection can be lowered to 29 mGy [23]. However, in practice the accuracy and precession of the EPR dose reconstruction can be quite different from the above estimations due to the following factors: 1) small amount of the tooth enamel available for EPR measurements, 2) unknown spectrum of accidental exposure, 3) effects of shielding, and 4) unknown additional components of exposure.
3. APPLICATIONS OF EPR DOSE RECONSTRUCTION IN TOOTH ENAMEL Table 1 summarizes some EPR dose reconstruction studies and gives information on numbers and values of reconstructed doses. The most extensive study was conducted for survivors of the Chernobyl nuclear accident. The huge volume of measurements aided in making the procedure of dose reconstruction almost routine. In addition, it provided dosimetric information on differences for back (molars and premolars) and front teeth (incisors and canines), which comes from the sun light exposure [24]. Two other essential
606
Table. 1 Population studies with EPR dose reconstruction in tooth enamel. Description of group
Survivors of A-bombingof Hiroshima, Japan Mayak nuclear workers, Russia Techa riverside population, Russia Eye-witnesses of Totskoye nuclear test, Russia GoiAnia radiation accident victims, Brazil Personnel of atomic submarines, Russia Chernobyl clean up workers, Ukraine, Russia Population of the areas contaminated by Chernobyl fallouts, Russia Background population, Russia Population of the areas nearby Semipalatinsk Nuclear Test Site, Kazakhstan
Year of exposure
Number of doses
Values of reconstructed doses, Gy
Reference
1945 10,100 1948-1961 -100 1948-1958 -100
0.3-4.0 0.2-6.0 0.1-10
[4,191 [ 16,25,26] [27,281
1954
10
0.1-0.4
~ 9 1
1987
6
1-12
[301
1960-1999 40 1986 660 122
0 - 3.0 0 - 2.0 Gy 0 - 0.8 Gy
[ 12,31,321
1986
0 - 0.3 Gy 0 - 0.1 Gy
[12,33] [24,33]
0 - 3.0 Gy
[34,35]
2500 136
1949-1962 26
[I21
studies are connected with the first Soviet nuclear weapons plant Mayak. The first study was EPR dose reconstruction with teeth from Mayak nuclear workers [16,25,26]. Generally it showed good agreement between EPR derived doses and personal dose monitoring data. The existence of consistent and in many cases reliable dosimetric information for Mayak nuclear workers made the results of the independent EPR dose reconstruction study more valuable. It also established an important bridge between doses measured by individual dosimeters and dose reconstruction results. The Techa River population is the second cohort involving the Mayak nuclear plant. This population was exposed as a result of radioactive waste releases into the river during the early 1950s [27,28]. From 1949 to 1956 about 7.6.107 m3 of liquid radioactive waste with a total activity of l O I 7 Bq (2.7.106 Ci) was released into the Techa River. This cohort was highly overexposed by combined external and internal (from 90Srdeposited in the mineral tissues) exposure and the exposure duration was protracted (tens of years). Therefore, the dose reconstruction was complicated by effects of metabolic and tissue development processes. The later plus limited penetration depth at internal beta-exposure (90Sr is a pure betaemitter) makes it necessary to introduce corrections to the results of EPR dose reconstruction that account for the real time of dose accumulation and dose attenuation inside teeth [28]. In order to account for dose attenuation a simple dosimetric tooth model incorporating 90Sr as two concentric cylinders: the inner cylinder composed of dentin, and the outer cylindrical shell composed of enamel was developed. Based on extensive Monte Carlo calculations distributions of absorbed dose in dentin and enamel for teeth of different
607 20 -
? 1000
$? 15U
B
.
P
10-
0
5 I
n
a
%
a 100
-
5-
s 3: 3
I,
0
.t
gm
10
g Y
1930
1935
1940
1945
1950
1955
1
10
Effective year of dose accumulation
Figure 2. EPR reconstructed doses in tooth enamel (solid squares, solid line, left axis) versus onset time of dose accumulation in tooth enamel. Dotted line and open circles show the sequence of radioactive releases (right axis) into Techa River. sizes and size correction factors were obtained [36] Knowledge of the radiation exposure regime could be obtained from EPR dose reconstruction of internal exposure. Some radionuclides, like 90Sr, are chemically equivalent to calcium and accumulate in teeth. If radionuclide intake occurred at the time of tooth formation the 90Sr concentration, and hence, accumulated dose could be very hi h Thus, EPR dose reconstruction can be used as an extremely sensitive indicator of "Sr releases. EPR dose reconstruction with teeth from Techa riverside residents revealed very high doses (up to 15 Gy) absorbed in tooth enamel of the individuals born in 1945-1949 (Fig. 2). Reconstructed doses for tooth donors of other ages were a factor of 50 lower [28]. This finding demonstrates the ability of EPR dose reconstruction in teeth collected from donors of different ages to determine both the doses and the regime of radionuclide intake. According to the results of EPR dose reconstruction (Fig. 2), high 90Sr intake started in 1950 (which is in an excellent agreement with the official data of radioactive releases) and ended in 1956. The more extended period of 90Srintake compared with the actual period of radioactive release can be explained by a time lag due to hydrological, metabolic and other transport processes, which mediate the internal dose accumulation in humans. The existence of the special age group, which has a high sensitivity to the 90Srintake, promises to provide important dosimetric information on exposure of the population to global radioactive fallout. The dose reconstruction study with teeth from residents near the Semipalatinsk nuclear test site (456 nuclear explosions in the period between 1949 and 1989) promises to provide insights in to the reliability of theoretical models for dose calculations [34,35]. Such models are based on bomb characteristics (explosive power, location and altitude of detonation), the speed and trajectories of individual fallout plumes at different altitudes, wind and
608
precipitation patterns, and measurements of radionuclides remaining in the soil at different times after detonation. They have been widely used to reconstruct fallout exposures from tests carried out by the United States, the former Soviet Union, and other countries but these models have never been verified independently, There are several important aspects, which should be considered in the use of EPR dose reconstruction, i.e., Choice of specific groups for teeth collection; Use of other, supplementary dose reconstruction methods; Conversion of dose absorbed in tooth enamel to effective dose; Typically, it is impossible to collect teeth and carry out dose reconstruction for every person of the group of interest; moreover it may be unnecessary. The main mission of EPR dose reconstruction is to validate models, which predicts accidental dose for exposed individuals depending on their activities during exposure. The most important consideration is to select subgroups, which are different from each other with respect to dose levels, and conditions of exposure. Then EPR dose reconstruction should be carried out with a representative set of teeth from every selected subgroup. This approach has been implemented, for example, at dose reconstruction studies of Russian nuclear workers, where subgroups of the nuclear workers were selected based on the radiation conditions of their work places [26]. There is one peculiarity of the EPR dose reconstruction, which makes this task of dose validation non-trivial. As a result it gives total life-accumulated dose, which besides accidental (or occupational) contribution contains radiation background and medical contributions. Moreover, accidental doses are comparable or even less than the background component. For example, this was the case for the EPR dose reconstruction for the population exposed to Chernobyl accident. In order to determine the radiation background contribution, teeth from a control or “unexposed” group, which have the same life and work conditions as the exposed population, were studied with EPR. Such control groups were selected in the framework of Chernobyl and Southern Urals studies (see Table 1) and showed background dose rates between 1 and 2 mGy per year for tooth enamel samples prepared from molars and premolars. Separation of the external and internal dose contributions for the Techa riverside residents can be made based on the independent assessment of 90Sr distribution in teeth. Recently a new method of photostimulable phosphor imaging was suggested for 90Sr mapping in tooth tissues [37]. This new method can be used as an individual indicator of radionuclide intake. Its advantages are high sensitivity (0.02 Bq/(g.mm2) of 90Sr), small detectable cross sectional area of dental tissue (dentin) contaminated with 90Sr (from 0.01 mm’), non-destructive method of analysis, and simplicity of use. The combined application of this method with EPR tooth biodosimetry can provide more accurate dose reconstruction and, perhaps, lead to more effective radiation risk assessments. It is quite possible that photostimulable phosphor imaging can be also used for EPR dating studies to determine internal dose contribution. As a result of EPR dose reconstruction measurements the dose absorbed in tooth enamel is obtained, whereas for epidemiological purpose the effective or organ doses are required. Translation of the dose absorbed in tooth enamel requires complete knowledge of the dose distribution throughout the body. Typically it is solved through Monte Carlo calculations of radiation energy absorption by a mathematical model of the human body called a phantom. Organs are represented by simplified models, which have density, shape and sizes close to the actual ones. Until recently there was no phantom for teeth and calculations have not
609
included the doses absorbed in tooth tissues at different geometries and types of exposure. Recently several phantoms for different types of the teeth have been suggested [38]. Monte Car10 calculations provided quantitative relations between tooth enamel dose and organ doses for external photon exposure of different energies. The later put EPR retrospective dosimetry on the same level of suitability as routine occupational dose monitoring.
4. PROBLEMS OF EPR DOSE RECONSTRUCTION IN TOOTH ENAMEL As previously mentioned, the radiation-induced signal in tooth enamel (and bone) has a composite nature. The most important radiation-induced radical is COY, located both in the A and B-sites of the hydroxyapatite lattice and on the surface. According to 1R spectroscopy data the total carbonate content in a tooth enamel sample prepared from US teeth is about 4%, about 39.7% substitute phosphate, 23.3% substitute hydroxide and 36.9% located on the surface (so-called labial carbonate) [39]. Thus, tooth enamel and other bioapatites can be considered as a natural mixture of A and B types of hydroxyapatite. Therefore, important information on the nature of radiation-induced signal in tooth enamel can be obtained by comparison of EPR properties of tooth enamel and pure synthetic hydroxyapatites with different locations of carbonate. EPR investigations with synthetic carbonated hydroxyapatites of A and B types show that both are very similar to each other and to stable radiation induced signals in tooth enamel. The stable signal from COY species with an axial symmetry at g=2.0028 and 1.9973 has been found in irradiated A-type hydroxyapatite [40]. Schramm and Rossi [41] found the most intensive and stable signal in the EPR spectrum of B-type hydroxyapatite belongs to COY species with g=2.0025 and 1.9971. Effects of heating at 400 “C on the EPR spectrum in irradiated tooth enamel were reported in [42]. It was found that after heating the radiation induced signal in tooth is split into at least two types of orthorombic COz- signals, one of which does not exhibit anisotropy in blocks and therefore has been assigned to the surface COz-. The second orthorombic ion gives rise to an anisotropic block spectrum and can be linked to a position inside the apatite structure (e.g. OH- or PO2-). This could mean that at least three C02- species are located in the OH- and PO.: sites of hydroxyapatite and at its surface contributing to the radiation induced signal in tooth enamel. In [40] the dose dependencies of the EPR peak-to-peak amplitude for tooth enamel and two A-type carbonated apatites with 2.5% and 4.7% of carbonate respectively were compared. The latter comparison showed that these dependencies are very different depending on the actual carbonate content and its location. According to this paper tooth enamel (which has about 4% of carbonate located in three different positions) has the highest radiation sensitivity (EPR dose response per dose unit) and saturation dose. A-type carbonated apatite with 4.7% of carbonate is second, after tooth enamel dose saturation and radiation sensitivity and A-apatite with 2.5% of carbonate has the lowest parameters. Thus, the radiation-induced signal in tooth enamel (or any other bioapatite) may consist of several (at least three) C02- species, which are located in different sites. They may also different dose Taking into account the known difference in the dose dependencies of these three components of radiation-induced signal one can question linearity of dose dependence of the radiation dose response in bioapatites. The primary problem of EPR dose reconstruction is the “intrinsic” signal or pre-dose. This “initial” or “predose” EPR signal in enamel is phenomenologically identical to the dosimetric signal but is not caused by exposure to ionizing radiation, and its intensity can
610
be considered as a bias of the individual evaluation method. As seen in Table 2, most participants who achieved consistent results used pre-dose subtraction to correct their results of dose reconstruction. In some respect pre-dose subtraction can be considered as a way to correct deviation of the dose dependence in low dose range from linearity. Table 2 Selected results of the second intercomparison. This table was prepared based on data published in [6] and [43]. Applied dose, 99 146 mGY Lab ID Measured dose, mGy 2 88 +21 121 f155 4 10 k100 200 +loo 7 50 k228 151 +87 9 58 It146 153 k274 10 115 f 8 2 178 k59 14 97 +16 142 +27 15 100 f 3 5 40 f 3 5 16 130 +26 73 f 2 5 19 110 +20 185 k20
326
329 f145 360 *I00 510 +90 286 k273 270 +50 214 +37 310 f 3 9 284 +29 300 +20
409
399 +I40 2101100 504 +274 375 h199 394 +61 327 f 5 0 510 +64 423 +32 405 +25
815
708 +I6 600 *I00 940 f126 717 h146 662 +85 608 +93 790 f 9 9 866 +47 715 f 2 0
Pre-dose subtraction
150 mGy 100 mGy 57 mGy 60 mGy 60 mGy
Thus two appearances of the second intercomparison - underestimation at reconstruction of “high” doses Gy) and necessity to subtract some predose at reconstruction of low doses (
5. CONCLUSIONS The most recent international EPR intercomparison [6] and the blind comparison of EPR reconstructed doses and individual dose monitoring data of Russian nuclear workers [ 16,
61 1
251 have demonstrated that doses in the range 100-1000 mGy can be reconstructed with precision and reliability. Moreover occupational exposure results of EPR dose reconstruction are consistent with individual dosimeter data. These findings have made the method of retrospective dose assessment a valuable tool for epidemiological radiation studies. In the case of internal exposure, EPR tooth dosimetry is able to provide crucial information on the regime of radioactive intake (281. In combination with digital radiography of teeth [37] EPR may be able to separate doses of internal and external exposure, which will lead to more effective radiation risk assessment. In spite of the above achievements there are some unsolved problems of the method. Most important is the effect of the composite nature of the radiation induced EPR signal on the results of dose reconstruction and possible non-linearity of EPR dose response in low dose region (<200 mGy). Efforts should be undertaken in order to better understand the structure of the radiation-induced signal in tooth enamel and its dose dependence.
REFERENCES W. Gordy, W.B. Ard and H. Shields, Proc. Natl. Acad. Sci., 41 (1955) 983. H.M. Swartz, Radiat. Res., 24 (1965) 579. S. Mascarenhas, A. Hasegams and K. Takeshita, Bull. Am. Phys. SOC.,18 (1973) 579. M. Ikeya, J. Miyajima and S. Okajima, Jpn. J. Appl. Phys., 23 (1984) 697. V. Chumak and 25 others, Appl. Radiat. Isot., 47 (1996) 1281. A. Wieser and 29 others, Radiat. Meas., 32 (2000) 549. Retrospective Assessment of Exposures to Ionising Radiation. ICRU Report 68, To be published as a separate issue of the Journal of the ICRU, Vol. 2, No. 2 (2002). 8. EPR Tooth Biodosimetry Report. International Atomic Energy Agency Vienna, Austria, (in preparation). 9. A.A. Romanyukha and D. Regulla, Appl. Radiat. Isot., 47 (1996) 1293. 10. A.A. Romanyukha, M.F. Desrosiers and D.F. Regulla, Appl. Radiat. Isot. 52 (2000) 1265. 11. A.I. Ivannikov, V.G. Skvortsov, V.F. Stepanenko, A.F. Tsyb, L.G. Khamidova, D.D. Tikunov, Appl. Radiat. Isot., 52 (2000) 1291. 12. V.G. Skvortsov, A.I. Ivannikov, V.F. Stepanenko, A.F. Tsyb, L.G. Khamidova, A.E. Kondrashov, D.D. Tikunov, Appl. Radiat. Isot., 52 (2000) 1275. 13. F. Callens, G. Vanhaelewyn, P. Matthys, and E. Boesman, Appl. Magn. Reson., 14 (1998) 235. 14. E. H. Haskell, R. B. Hayes, G. H. Kenner, S. V. Sholom and V. V. Chumak, Radiat. Res., 148 (1997) S51. 15. M. Desrosiers and D.A. Schauer, Nucl. Instrum. Meth. B, 184, (2001) 219. 16. A.A. Romanyukha, D. Regulla, E. Vasilenko and A. Wieser, Appl. Radiat. Isot., 45, (1994) 1195. 17. P. Fattibene, D. Aragno and S. Onori, Health Phys., 75 (1998) 500. 18. A.I. Ivannikov, D.D. Tikunov, V.G. Skvortsov, V.F. Stepanenko, V.V. Khomichyonok, L.G. Khamidova, D.D. Skripnik, L.L. Bozadjiev and M. Hoshi, Appl. Radiat. Isot., 55 (2001) 701. 19. N. Nakamura, C. Miyazawa, M. Akiyama, S. Sawada and A.A. Awa, Int. J. Radiat. Biol., 73 (1998) 619. 20. A.A. Romanyukha, F.C. Eichmiller, D.V. Ivanov, Z. Lin, R.B. Hayes and B.M
1. 2. 3. 4. 5. 6. 7.
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Coursey, to be submitted to Journal of Applied Physics. 21. M. Ikeya, New Application of Electron Spin Resonance - Dating, Dosimetry and Microscopy. Word Scientific, Singapore, 1993. 22. A. Wieser, N. El-Faramawy and R. Meckbach, Appl. Radiat. Isot., 54 (2001) 793. 23. A.A. Romanyukha, V. Nagy, 0. Sleptchonok, M.F. Desrosiers, J. Jiang and A. Heiss, Health Phys., 80 (2001) 71. 24. A.I. Ivannikov, V.G. Skvortzov, V.F. Stepanenko, D.D. Tikunov, I.M. Fedosov, A.A. Romanyukha and A. Wieser, Radiat. Prot. Dosim., 71 (1997) 175. 25. A.A. Romanyukha, E.A. Ignatiev, E.K. Vasilenko, E.G. Drozhko, A. Wieser, P. Jacob, I.B. Keriim-Markus, E.D. Kleschenko, N. Nakamura and C. Miyazawa, Health Phys., 78 (2000) 15. 26. A. Wieser, A. Romanyukha, S. Onori, E. Vasilenko, S. Bayankin, P. Fattibene, E. Ignatiev, V. Knyazev and P. Jacob, To be submitted to Health Phys. 27. A.A. Romanyukha, M.O. Degteva, V.P. Kozheurov, A. Wieser, E.A. Ignatiev, M.I. Vorobiova and P. Jacob, Radiation and Environmental Biophysics, 35 (1996) 305. 28. A.A. Romanyukha, S. Seltzer, M. Desrosiers, E.A. Ignatiev, D.V. Ivanov, S. Bayankin, M.O. Degteva, F.C. Eichmiller, A. Wieser and P. Jacob, Health Phys., 81 (2001) 554. 29. A.A. Romanyukha, E.A. Ignatiev, D.V. Ivanov, and A.G. Vasilyev, Radiat. Prot. Dosim., 86 (1999) 53. 30. A.M. Rossi, C.C. Wafcheck, E.F. De Jesus and F. Pelegrini, Appl. Radiat. Isot., 52 (2000) 1297. 31. V. Chumak, S. Sholom and L. Pasalskaya, Radiat. Prot. Dos., 84 (1999) 515. 32. V. Chumak, I. Likhtarev, S. Sholom, R. Meckbach and V. Krjuchkov, Radiat. Prot. Dos., 77 (1998) 91. 33. V. Stepanenko, V. Skvortsov, A. Tsyb, A. Ivannikov, A. Kondrashov, D. Tikunov, E. Iaskova, V. Shakhtarin, D. Petin, E. Parshkov, L. Chernichenko, V. Snykov, M. Orlov, Yu. Gavrihn, V. Khrousch and S. Shinkarev, Radiat. Prot. Dosim., 77 (1998) 101. 34. A.I. Ivannikov, Zh. Zhumadilov, B.I. Gusev, Ch. Miyazawa, L. Liao, V.G. Skvortsov, V.F. Stepanenko, J. Takada and M. Hoshi, Health Phys., (2001) in press. 35. A. Romanyukha, M. Desrosiers, 0. Sleptchonok, C. Land, N. Luckyanov and B.I. Gusev, Radiat. Prot. Dosim., (2001) in press. 36. S.M. Seltzer, A.A. Romanyukha and V. Nagy, Radiat. Prot. Dosim., 93, (2001) 245. 37. A.A. Romanyukha, M.G. Mitch, Z. Lin, V. Nagy and B.M. Coursey, Radiat. Res., in press. 38. F. Takahashi, Y. Yamaguchi, M. Iwasaki, C. Miyazawa and T. Hamada, Radiat. Prot. Dosim., 95 (2001) 101. 39. N. Eidelman, private communication. 40. L.M. Oliveira, A.M. Rossi and R.T. Lopes, Appl. Radiat. Isot., 52 (2000) 1093. 41. D.U. Schramm and A.M. Rossi, Phys. Chem. Chem. Phys., 2 (2000) 1339. 42. F. Callens, P. Moens, R. Verbeeck, Calcif. Tissue Int., 56 (1995) 543. 43. A. Wieser, private communication. 44. G. Vanhaelewyn, S. Amira, R. Debuyst, F. Callens, T. Glorieux, G. Leloup and H. Thierens, Radiat. Meas., 33 (2001) 417. 45. M. Jonas and R. Grun, Radiat. Meas., 27 (1997) 49. 46. W.J. Rink, Radiat. Meas., 27 (1997) 975.
EPR in the 21" Century A Kawamori, 1 Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
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ESR dating applications in archaeology and earth sciences Rainer G R m Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia
'
Electron spin resonance (henceforth ESR) dating was first applied by M. Ikeya (1975) who dated a speleothem from Akiyoshi Cave, Japan. Since then, a wide range of materials were dated in applications in geology, geography and archaeology. This paper will give a brief introduction into geochronology, explain the basis for ESR dating and illustrate the specific strength of ESR dating in range of applications. ESR has made aparticular impact through thedating of tooth enamel from archaeological sites atid human fossils documenting the emergence of modern humans in Israel at about 100,000 years ago and their arrival in Australia at about 60,000 years ago. In Earth Sciences, ESR can be used to assess cooling and denudation rates. These new applications allow the investigation of a wide range of subjects relating to palaeothermometry as well as recent geodynamics.
EPR in the 21’‘ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
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ESR and NMR dosimetry 0. Baffa”, A. Kinoshita”’*,F. Chen Abregoa*b*C and N. A. Silva”
”Faculdade de Filosofia CiCncias e Letras de RibeirCo Preto, University of 14040-901 Ribeirfio Preto, SP, Brazil,
Paulo,
’Departamento de Fisica, Universidad de Panama, El Cangrejo, Panama, ‘Departamento de Salud Radiologica, Caja de Seguro Social, Panama. Ionizing radiation can create stable free radicals in solids that can be quantified, in a non-destructive way, by Electron Spin Resonance (ESR) leading to a dosimetric technique. Ionizing radiation can change the valence of iron ions and the viscosity of certain polymers leading to changes in the relaxation times of protons, allowing the use of nuclear magnetic resonance imaging (MRI) to image three-dimensional dose distribution. Both techniques are expected to improve the quality of medical care that rely on the use of ionizing radiation.
INTRODUCTION The use of magnetic resonance techniques such as ESR and NMR for dosimetry is relatively new and could be classified as a kind of chemical dosimetry since what is measured are stable free radicals, that ultimately are the result of some broken chemical bonds. However the complexity involved in the interpretation and measurement justifies treatment separate from the conventional chemical dosimetry such as Fricke dosimetry. Until very recently, this subject was restricted to the application of Electron Spin Resonance (ESR) to measure of electron spin concentration as a function of the dose absorbed in samples that have been irradiated. Another way to assess the effects of ionizing radiation on matter is by measuring the changes in relaxation times of a nuclear spin system due to the paramagnetism produced by the radiation of molecules or ions. With the advent of magnetic resonance imaging scanners the effect of the radiation on the relaxation properties of the material can be exploited to produce images of a phantom contrasted by the relaxation time that give a quick information about the spatial deposition of the dose in the phantom. Examples and applications of these approaches will be given.
2. APPLICATIONS In this section an overview of ESR dosimetry will be given. This technique has * Permanent address:Universidade de Marilia - UNIMAR - 17525-902 Marilia, SP, Brazil
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been applied to dose assessment for a wide range of materials such as minerals, fossils, amino acids, cloth, etc. We will group the studies by the type of material utilized, since their ESR signals are similar. The method consists of measuring the concentration of spins in a given sample that has been exposed to radiation. The material is calibrated by additional irradiation with known doses in the laboratory, to assess the sensitivity to a specific kind of radiation. In this kind of measurement precision and sensitivity of the ESR spectrometer are of paramount importance. From the spin concentration data a fitting can be made to derive the relevant quantities such as sensitivity (slope of the curve) and the previous dose or total dose (TD) the sample received.
2.1 Hydroxyapatite Ionizing radiation can produce stable paramagnetic centers in the mineral matrix (Hydroxyapatite - HA) of bones and teeth that can be used for dosimetric purposes. It can change in bones due to growth or demineralization but, supposedly, it does not change appreciably in tooth enamel. Therefore tooth enamel is the most appropriated material for dosimetry applications but in some cases when it is not available, bone can give us useful information. The main radical used for ESR dosimetry in bone and in tooth enamel is the axial COY but the isotropic COY, orthorhombic COY and COT and CO2- radicals are also present [l]. Several authors [2-41 pointed out the importance of ESR dosimetry in bone and/or tooth enamel using the axial COY radical as a probe for the case of personal accident dosimetry or for the determination of the radiation exposure history to the exposure of low level radiation. Recently, the ESR dosimetry was also used for evaluation of dose deposited in bone by radioisoto es lS3Sm-EDTMP (samarium acid) and 9SrC12 (strontium chloride). These bone-seeking radiopharmaceuticals are used in systemic radiotherapy for pain palliation in bone metastases cases [5]. The knowledge of the dose deposited in bone is important in order to avoid side effects to the bone marrow. Others works analysed the dose delivered by Ho-EDTMP [6] and 90Sr [7]. Another application of ESR bone dosimetry is the estimation of the dose in radiation processed food where chicken, fish, etc, are irradiated for microorganisms control [8]. ESR dosimetry in tooth enamel is a subject of several works. ESR spectra of irradiated tooth enamel presents two components: a background signal (g = 2.0045) related to organic radicals and a dosimetric signal, with axial symmetry characterized by g spectroscopy factor g l = 2.0018 and g// = 1.9975. The background or native signal is not useful for dosimetry applications and in low doses causes problems for dose assessment. To optimize the dosimetry it is necessary to separate the dosimetric and background signal. Several authors contributed in this way. Ignatiev et al. [9] proposed the selective saturation method, based on the difference in the microwave power dependence of the background and radiation induced signal. Romanyukha et al. [ 101 reported the elimination of the native signal and attributed the ESR signal obtained of non irradiated sample to the natural background radiation. We are using software to simulate the known dosimetric signal where appropriate Hamiltonian parameters can be entered until good fitting is found. Figure 1 shows these components separated by this simulation. Using this method, the amplitude signal in g l is related to the radical concentration. The calibration curve is obtained using a set of samples irradiated with several doses. In Figure 2 an example of
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this curve is demonstrated. We are also studying the use of higher microwave frequency (24 GHz) for better separation of dosimetric and background signal. Our results on K-Band experiments show improvements in signalhoise ratio, better resolution on g-factors and the use of lesser samples of tooth enamel to record a spectrum. The smaller sample mass is a dosimetry where an desirable factor in dosimetry with enamel when one thinks in enamel sample has to be excised [ll]. The ESR dosimetry of bones and teeth has been useful also for dating purposes [12-141 of human or animal remains. Figure 3 shows a typical ESR first derivative spectrum of a piece of human skull that was buried in the soil for approximately 5 thousand years. The signal is composite showing the super-position of the free radical signal produced in the mineral part and the Mn2+sextet due to the absorption of manganese from the soil. The signal is shown in Figure 10 for the same sample before and after irradiation. Figure shows the growth curve of the signal intensity I as a function of the dose. Extrapolation to zero ESR signal intensity gives the total dose (TD) the sample had received. If the annual dose rate is known where the sample was found the age of the sample can be determined. -Simulated signal ... -. ..Experimental curve Background signal I
I
91
.
g//
Magnetic Field (mT)
Magnetic Field (mT)
Figure 1. ESR signal of enamel irradiated with 1Gy of 6oCoy rays showing the superposition of the background and dosimetric signal. (left). Separation of the components by simulation of the dosimetric signal (right).
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0
2
4
6
8
10
Dose
Figure 2. ESR signal intensity versus Dose for bovine tooth enamel. Linear regression gives the dosimetric parameters.
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Magnetic Field (mT)
Figure 3. ESR signal of a fossil piece of fossil skull. The signal produced by radiation fiom the environment is superimposed to the one due to Mn2+from the soil contamination.
Signal
r
=
-40 4
Magnetic Fidd (A.U.)
10 Dose (Gy)
Figure 4. The ESR signal before (a) and after (b) 6oCoirradiation. The calibration curve is shown on the right with the an archeological dose of (19+2) Gy.
2.2 Alanine Alanine is one of the twenty amino acids existing in nature its formula being CH3CH(NH2)COOH. Exists in two isomeric form: L- and D- alanine, and a racemic mixture of them result in the DL-alanine form. The L- and DL- are the most commonly forms used for dosimetry. The effects of ionizing radiation on proteins and amino acids were first studied by Gordy et al. [15]. Further studies by this group [16] found that the free radical formed is of the form CH3CHR, where R is a group that has no detectable magnetic influence. The hydrogen ions of the methyl group CH3 have an isotropic hyperfine interaction of 26 G and that of the CH group is isotropic with 20 G amplitude and anisotropic with 7 G amplitude. Bradshaw et. al. [17] reported the first work showing that alanine has suitability for dosimetry purpose. Later, the technical aspects and progresses of ESR dosimetry with alanine were reviewed by Regulla and Deffner [MI. Today, the AlanineESR dosimetry system has gained wide acceptance in many irradiation laboratories and has been adopted as a standard dosimetry method for measurement of high radiation dose in industrial radiation processing applications [19-25]. Additionally, several laboratories around the world have been working with the objective to apply this dosimetric system to the therapy dose level. Among the several applications are: electron dosimetry [26], radiotherapy dosimetry [27-301, brachytherapy dosimetry [31], therapeutic proton beams dosimetry [32, 331 and blood irradiation [34, 351. Efforts have been made to increase the sensitivity of the technique decreasing the detection limit. With a spectrum fitting method developed to minimize baseline distortion, Sharpe et. al. [36] reported a detectable dose around 5 Gy with 0.05 Gy of standard deviation. In the work of Schaeken and Scalliet [37] a considerably broad variation in the background signal (dose equivalent: 0.44 Gy to 13.2 Gy) from non-irradiated dosimeters among different manufacturers was found. Juncheng and Zaiyong [38] studied the effect of microwave power and modulation amplitude in the signal amplitude of the irradiated alanine and with optimum parameters, a background signal between 0.1 to 0.2 Gy could be detected and their dosimetric system was recommended to detect doses above 5 Gy. With a mathematical method based on the Fast Fourier Transform (FFT), and used for low frequency background and high frequency noise suppression of the alanine/ESR signals, Ruckerbauer et. al. [39] reported an average background signal equivalent to (0.05k0.02) Gy from non-irradiated samples. Metha and Girzikowsky [40] suggested that the alanineESR dosimetry system has the potential for radiotherapy applications and could replace the LiF-TL dosimetry system for intercomparison amongst SSDLs. Using a complex method based on optimized ESR parameters, complex spectral accumulation, subtraction techniques, and multiple peaks measurement, Haskell et. al. [41] achieved uncertainties of k5 mGy with high quality alanine dosimeters; the uncertainty attained by this method makes possible the use of AlanineESR dosimetry for radiotherapy applications. With a special combination of several methodologies already described in [28, 29, 36, 39, 421, Hayes et. al. [43] attained an accuracy of 10 mGy for low doses and 1% for high doses, during blind tests for reconstruction of unknown doses. With a VARIAN E-4 ESR Spectrometer operating in X-Band (-- 9.5 GHz) and equipped with a rectangular resonance cavity model E-231 operating in the mode, our research group has implemented this dosimetric technique since 1997. Optimizing some ESR parameter like microwave power and modulation amplitude and using D L alanine/paraffin (80%/20%)pellets, we attained a minimum detectable dose of 1 Gy. With
619
this system, some works have been done. Using an anthropomorphic phantom and dosimeters with 240 mg mass and 4.7 mm diameter and 12 mm length, an experimental determination of isodose curve for a normal radiotherapy treatment [44] was made. Using smaller dosimeters: 60 mg mass, 4.7 mm diameter and 3 mm length, determination of the transit dose in 19'Ir HDR brachytherapy [45] was also peformed. Some others applications have been done in radiation quality control such as: chicken and blood irradiation [35]. We also have studied the suitability of other ESR signal detection scheme for dosimetry. Preliminary results have showed that the second harmonic directly detected overmodulated signal from irradiated alanine presents some advantages over the traditional first harmonic one in the low dose range. Among them are: more easy localization of the signal because the position of the line it is defined by a peak rather than a zero crossing (see Figure 5), better spectral resolution, and lesser perturbation of the baseline distortion [46]. Another interest of our group is the development of the K-Band ESR dosimetry. We made reliminary tests with miniature solid state DL-alanine dosimeters irradiated with 30 Gy O ' Co gamma radiation. The dosimeters have dimensions: 1.5 mm diameter and 2.5 mm length with approximately content of 2 mg DL-alanine. The K-Band EPR spectrum is showed in Figure 6. With this initial result, we think to apply this miniature dosimeters to the dosimetry of small radiation field (like used in stereotactic radiosurgery), where the dosimeter size is very critical.
- 180 -._
4. .
D L-alan ine
microwave power: 5 0 m W modulation amplitude: 1 m T linewidth: 1.3 m T (1 h)
4-
E &
.-
Lo
-4-
-
-
Figure 5. First (lh) and second (2h-dotted line) harmonics ESR signals from DL-alanine irradiated with 3 kGy 6oCogamma radiation. Both spectrum were recorded in a X-Band ESR spectrometer. The trace line correspond to the 2h signal.
620
840
845
850
855
860
M a g n e t i c field ( m T )
Figure 6. K-Band ESR spectrum of irradiated (60Cogamma rays) and non-irradiated (dotted line) DL-alanine miniature dosimeters.
NMR Dosimetry Chemical dosimetry has been well studied for many years [47]. The Fricke dosimeter is one of the most commonly used dosimeters. The method is based on the fact that ferrous ions are oxidized to ferric ions (Fe2' + Fe3') upon irradiation. By measuring the concentration of ferric ions, with a colorimetric method, before and after irradiation one can then estimate the radiation dose received by the Fricke solution. Since ferrous and ferric ions are paramagnetic they both affect the proton (NMR) relaxation rates but with significantly different efficacies. Based on this fact one can monitor the change in the concentration of ferric ions in the Fricke solution using magnetic resonance imaging (MRI). This was demonstrated in 1984 by Gore et al. where it was found a linear dependence of the proton relaxation rate with the absorbed dose in a conventional Fricke solution [47] and that the changes in relaxation produced by conversion of ferrous ion to ferric ion could be detected and used to estimate radiation dose. Another result was the existence of a variation in the image intensity for Fricke solutions that had received different radiation doses. Gore et al. [49] also suggested the use of ferrous ions incorporated in a gel that contained a large water fraction to produce MRI images of dose distribution. Several authors also contributed to this are and in the last years the dosimetry using polymer gels have been intensively studied [.SO-591.
62 1
4 6 8 1 0 1 2 Distance to the radiation axis (cm)
Figure 7. Brightness of MRI at 5 cm below the irradiation surface measured along the distance perpendicular One of the problems that still need more attention is diffusion of the species (Fe3', micropolimers, chelated ions) created by radiation from the sites of high density to the low density positions. This problem hampers the possibility of having a latent threedimensional imaging of the dose. To tackle this problem we propose a phantom with small cells that can block the diffusion from one position to another. The dosimeter gel was preparation was similar to previous works and and irradiation was done using a Co teletherapy unit (Gammatron S.80) that a specific dose distribution was produced within the gel volume. Magnetic resonance images were acquired on a 1.5 T clinical MRI system (Magnetom- Vision Siemens). Imaging parameters were and TR= 500 ms and the slice thickness of 5 mm. The image intensity distribution was analyzed in different times to verify the existence of diffusion. Showing no diffusion effect on the profiles in the irradiated volume, although autoxidation of the ferrous ion was observed.
3. DISCUSSION AND CONCLUSIONS
In summary it can be seen that ESR NMR spectroscopy can contribute in several ways to improve the dosimetry of ionizing radiation. Efforts should be directed in the future to improve the sensitivity and precision of these techniques when it comes to dosimetry. Alanine is a tissue equivalent dosimeter and enamel is a always present dosimeter in case of accidents. A lower detection limit for both materials is desirable to better ascertain the effects of ionizing radiation in several situations. The possibility of performing a tri-dimensional image of dose distribution in phantom to verify complex radiotheraphy planning in a straightfoxward way is also desirable and a demand, as conformational radiotherapy becomes more used.
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REFERENCES 1. F. J. Callens, R. M. H. Verbeeck, P. F.A. Matthyns, L. C. Martens, and E. R. Boesman, Calcif. Tissue Int., 41 (1987) 124. 2. J. M. Brady, N. 0. Aarestad and H. M. Swartz H.M., Health Phys., 15 (1968) 43. 3. M. Ikeya, Magn. Res. Review, 13 (1988) 91. 4. A. A. Romanyukha, E. A. Ignatiev, M. 0. Degteva, V. P. Kozheurov, A. Wieser, P. Jacob, Nature, 381 (1996) 199. 5. A. Kinoshita, J. F. H. N. Braga, C. F. 0. Graeff, 0. Baffa, Appl. Radiat. Isot., 54 (2001) 269. 6. M. F. Desrosiers, M. J. Avila, D. A. Schauer, B. M. Coursey, N. J. Parks, Appl. Radiat. Isot., 44 (1993) 459. 7. E. A. Ignatiev, N. M. Lyubashevskii, E. A. Shishkina, A. A. Romanyukha, Appl. Radiat. Isot., 51 (1999) 151. 8. M. F. Desrosiers, G. L. Wilson, C. R. Hunter, D. R. Hutton, Appl. Radiat. Isot., 42 (1991) 613. 9. E. A, Ignatiev, A. A, Romanyukha, A. A. Koshta, A. Wieser, Appl. Radiat. Isot. 47 (1996) 333. 10. A. A. Romanyukha, V. Nagy, 0. Sleptchnok, M. F. Desrosiers, J. Jiang, A. Heiss, Health Phys., 80 (2001) 71. 11. A. Kinoshita, C. F. 0. Graeff, 0. Baffa, Abstracts APES’O1 (2001). 12.0. BaEa and S. Mascarenhas, IONICS-Tokyo, 139 (1985) 43. 13. S. Mascarenhas, 0. Baffa Filho, and M. Ikeya, Am. J. Phys. Anthropol., 59 (1982) 413. 14. M. Ikeya, 0. Baffa Filho and S. Mascarenhas, J. Spelol. SOC.Japan, 9 (1984) 58. 15. W. Gordy, W. B. Ard, and H. Shields, Phys. Duke University, North Carolina, 1955. 16. I. Miyagawa, and W. Gordy, J. Chem. Phys., 32 (1960) 255. 17. W. W. Bradshaw, D. G. Cadena Jr., G. W. Crawford, H. A. W. Spetzler, Radiation Research, 17 (1962) 11. 18. D. F. Regulla, and U. Deffner, Int. J. Appi. Radiat. & Isot., (GB), 33 (1982) 1101. 19. D. Regulla, A. Bartolotta, U. Deffner, S. Onori, M. Pantaloni, and A. Wieser, Appl. Radiat. Isot., 44 (1993) 23. 20. J. M. Arber, and P. H. G. Sharpe, Appl. Radiat. Isot., 44 (1993) 19. 21. W. L. McLaughlin, and M. F. Desrosiers, Radiat. Phys. Chem., 46 (1995) 1163. 22. K. Mehta, Appl. Radiat. Isot. 47 (1996) 1155. 23. G. Juncheng, Appl. Radiat. Isot. 47 (1996) 1161. 24. T. Kojima, H. Tachibana, N. Haneda, I. Kawashima, and P. H. G. Sharpe, Radiat. Physics and Chem., 54 (1999) 619. 25. A. C. Alexandre, 0. Baffa, 0. R. Nascimento, Appl. Radiat. Isot., 43 (1992) 1407. 26. S. Chu, A. Wieser, H. Feist, D. F. Regulla, Appl. Radiat. Isot., 40 (1989) 993. 27. H. P. Nette, S. Onori, P. Fattibene, D. Regulla, A. Wieser, Appl. Radiat. Isot., 44 (1993) 7. 28. A. Bartolotta, P. Fattibene, S. Onori, M. Pantaloni, E. Petetti, Appl. Radiat. Isot., 44 (1993) 13. 29. A. Wieser, C. Lettau, U. Fill, D. F. Regulla, Appl. Radiat. Isot., 44 (1993) 59. 30. R. Kudynski, J. Kudynska, H. A. Buckmaster, Appl. Radiat. Isot., 44 (1993) 903.
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31. C. De Angelis, S. Onori, E. Petetti, A. Piermattei, L. Azario, Phys. Med. Biol., 44 (1999) 1181. 32. K. Gall, M. Desrosiers, D. Bensen, C. Serago, Appl. Radiat. Isot., 47 (1996) 1197. 33. S. Onori, F. D’Errico, C. De Angelis, E. Egger, P. Fattibene, and I. Janovsky, Appl. Radiat. Isot., 47 (1996) 1201. 34. C. Fainstein, E. Winkler, and M. Saravi, Appl. Radiat. Isot., 52 (2000) 1195. 35. F. Chen, D. T. Covas, 0. Baffa, Appl. Radiat. Isot., 55 (2001) 13. 36. P. H. G. Sharpe, K Rajendran, J. P. Sephton, Appl. Radiat. Isot., 47 (1996) 1171. 37. B. Schaeken, and P. Scalliet, Appl. Radiat. Isot., 47 (1996) 1177. 38. G. Juncheng, and Z. Zaiyong, Appl. Radiat. Isot., 47 (1996) 1193. 39. F. Ruckerbauer, M. Sprunck, D. F. Regulla, Appl. Radiat. Isot., 47 (1996) 1263. 40. K. Mehta, and R. Girzikowsky, Appl. Radiat. Isot., 47 (1996) 1189. 41. E. H. Haskell, R. B. Hayes, and G. H. Kenner, Rad. Prot. Dos., 77 (1998) 43. 42. R. B. Hayes, E. H. Haskell, A, A. Romanyukha, and G. H. Kenner, Meas. Sci. Technol., 9 (1998) 1994. 43. R. B. Hayes, E. H. Haskell, A. Wieser, A. A. Romanyukha, B. L. Hardy, and J. K. Barrus, Nuclear Instruments and Methods in Physics Research A, 440 (2000) 453. 44. F. Chen, 0. Baffa, C. F. 0. Graeff, Proceedings of the First Iberolatinoamerican and Caribbean Congress on Medical Physics, Mexico City, Mexico, 1998. 45. C. S. G u m a n Calcina, F. Chen, A. De Almeida, 0. Baffa, Proceedings of the Second Iberolatinoamerican and Caribbean Congress on Medical Physics, Caracas, Venezuela, 2001. 46. F. Chen, C. S. Guzmfin Calcina, C. F. 0. Graeff, 0. Baffa, Proceedings of the Sixth Brazilian Congress on Medical Physics, Rio de Janeiro, Brazil, 2001. 47. H. Fricke, and E. J. Hart, Chemical Dosimetry: Radiation Dosimetry, Vol.11, ed. Attix, F. H. and Roesch, W. C., New York: Academic, 1966. 48. J. C. Gore, Y. S. Kang, and R. J. Schulz, Phys. Med. Biol., 29 (1984) 1189. 49. J. C. Gore, Y. S. Kang, and R. J. Schulz, Magn. Reson. Imag., 2 (1984) 244. 50. M. J. Maryanski, J. C. Gore, R. P. Kennan, and R. J. Schulz, Magn. Reson. Imag., 11 (1993) 253. 51. M. J. Maryanski, R. J. Schuluz, G. S. Ibbott, J. C. Gatenby, J. Xie, D. Horton, and J. C. Gore, Phys. Med. Biol., 39 (1994) 1437. 52. P. J. Harris, A. Piercy, and C. Baldock, Phys. Med. Biol., 41 (1996) 1745. 53. M. J. Maryanski, G. S. Ibbott, P. Eastman, R. J. Schulz, and J. C. Gore, Med. Phys., 23 (1996) 699. 54. J. C. Gore, M. Ranade, M. J. Maryanski, and R. J. Schulz, Phys. Med. Biol., 41 (1996) 2695. 55. J. M. Maryanski, Y. Z. Zastavker, and J. C. Gore, Phys. Med. Biol., 41 (1996) 2705. 56. M. J. Maryanski, C. Audet, and J. C. Gore, Phys. Med. Biol., 42 (1997) 303. 57. C. Baldock, R. P. Burford, N. Billingham, G. S. Wagner, S. Patval, R. D. Badawi, and S. F. Keevil, Phys. Med. Biol., 43 (1998) 695. 58. M. Oldham, I. Baustert, C. Lord, T. A. D. Smith, M. McJury, A. P. Warrington, M. 0. Leach, and S. Webb, Phys. Med. Biol., 43 (1998) 1113. 59. E. Pappas, T. Maris, A. Angelopoulos, M. Paparigopoulou, L. Sakelliou, P. Sandilos, S. Voyiatzi, and L. Vlachos, Phys. Med. Biol., 44 (1999) 2677.
624
EPR in the 21‘ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
K-band ESR spectra of irradiated tooth enamel A. Kinoshita, C. F. 0. Graeff and 0. Baffa Departamento de Fisica e Matemgtica, FFCLRP - Universidade de Brazil
Paulo -
Paulo -
Tooth enamel has been widely used for ESR retrospective dosimetry in radiation accidents. The ESR signal of irradiated tooth enamel is characterized by the superimposition of two components: the dosimetric signal, ascribed to created in the mineral part or hydroxyapatite with axial symmetry (gl = 2,0018 and gll= 1.9975) and a wider, and a sometimes large, native signal (g = 2.0045) ascribed to organic radicals. In X-Band (9.5 GHz) this overlapping represents a limiting factor to improve the sensitivity of this technique. The use of less sample mass and higher sensitivity are desirable goals for low dose detection in tooth enamel dosimetry. In this work we studied the use of K-Band (24 GHz) for dosimetry applications. We found that with fewer sample mass (6 fold smaller than used in X Band) it is possible to record a spectrum with good signalhoise (WN). Better resolution in g factor was also verified. Improvements in sensitivity of a 2.5 fold in S/N ratio and lo3 fold increase in amplitude/mass in comparison with X-Band were obtained.
1. INTRODUCTION Tooth enamel has been intensively investigated as a dosimeter for retrospective dosimetry. It is a very sensitive dosimeter due to its high degree of crystalinity. When irradiated, stable free radicals are generated in its mineral part, hydroxyapatite. The COZ’ radical is characterized by its axial symmetry (gl = 2.0018 and g// = 1.9975). This dosimetric signal is superimposed by a wider, and sometimes, large native signal. At low doses, this component causes problems for assessing the dose. Literature lists works using chemical etchings in sample preparation for reducing the native signal [l], selective saturation of the signals, with spectra taking at different microwave power [2] and use of software to simulate one of the components for signal separation by subtraction. According the resonance equation: hv= gf3H
(1)
the use of high frequencies can improve S/N because a large energy quantum is used to produce the resonance phenomenon.
625
In addition, the sensitivity of ESR spectrometer is dependent on several parameters such as: sample volume, temperature, filling factor of the cavity, microwave power, but the cavity microwave frequency (m) [3]. Sensitivity will increase with increasing dimensions are inversely proportional to m. Thus, improvements in sensitivity can be lost by reducing mass. The use of K-Band (24 GHz) is an intermediate option between the common X-Band ( 9.5 GHz) and Q-Band (35 GHz) frequencies where we can obtain higher sensitivity than X-Band without large mass losses as in Q-Band.
2. MATERIALS AND METHODS
2.1. Sample preparation Enamel and dentine of bovine tooth was separated in a chemical treatment with saturated solution of NaOH (30%) in ultra-sound bath during several hours, followed by a fast etching with HCl (10%) and distillated water. The NaOH soaking dissolve the dentine. The enamel was dried and grounded in fine powder to avoid imprecision on determination of amplitude signal due to angular dependence of the ESR signal. The crushing is made carefully in an agate mortar because physical stress can generate radicals similar to that produced by ionizing radiation [4, 51. 2.2. Irradiation About 150 mg of this powder were placed into small plastic tubes for irradiation. The irradiation was performed by a radiotherapy gamma source (Gammatron-S 80 by Siemens) in air, at room temperature, using 0.4 g/mm2 thick Lucite build-up cap over the samples. The doses were: 0 , 5 , 20, 50, 100, 150 Gy 2.3. ESR spectrum The K-Band spectrometer is composed by a 12 inch electromagnet, a magnetic field controller, a lock-in amplifier, a Bruker microwave bridge (Y = 24 GHz), a microwave digital frequency counter and a cylindrical cavity. These parts of the spectrometer are controlled by a microcomputer via GPIB card. The data acquisition is made by software written in a HP-WE platform. We also used a computer interfaced Varian E-4 spectrometer operating at X-Band (v = 9 GHz) for comparisons of sensitivity.
2.4. Microwave Power To increase the sensitivity it is important to work at the highest possible microwave power. However care must be taken to avoid saturation. So, before any systematic measurement is started a saturation curve has to be taken in order to know the maximum working microwave power. We used 10 mg of enamel irradiated with 'dose of 150 Gy for evaluation of the best microwave power.
626
3. RESULTS Figure 1is a comparison of spectra of tooth enamel irradiated with a dose of 3 Gy obtained using X-Band ( 9.5 GHz) and K-Band (24 GHz). We used de following parameters to record spectrum in X-Band (Fig. la): Sample mass of 63.3 mg, microwave Power : 20 mW, Central magnetic field : 338 mT, Microwave Frequency : 9.483 GHz, Modulation amplitude : 0.25 mT, Time constant : 500 ms, Scan range : 10 mT, Number of scans : 10/2 minutes. At K-Band, the parameters were: Sample mass : 9.8 mg, Microwave Power : 1mW, Central magnetic field : 852 mT, Microwave Frequency : 23.845 GHz, Modulation amplitude : 0.50 mT, Time constant : 500 ms, Scan range : 10 mT, Number of scans : 10/2 minutes Figure 2 is the signal intensity versus microwave power showing saturation at high powers and Figure 3 is the signal amplitude versus dose.
4. DISCUSSION AND CONCLUSION The initial results are promising showing that with a small quantity of enamel sample (6 fold smaller than used in X-Band) it is possible to record a spectrum using K-
1
Magnetic Field (mT)
Magnetic Field (mT)
Figure 1. ESR spectrum of tooth enamel irradiated with dose of 3 Gy using (A) KBand and (B) X-Band.
627
,,i ,,?
,.
; 0,4
0,6
0,8
Microwave power (mW)"*
Figure 2. Signal Intensity versus Microwave Power
I
'
I
'
I
'
'
'
1
'
I
"
"
'
Dose (Gy)
Figure 3. Signal Intensity versus dose.
band with good S/N ratio. In dosimetry with enamel, a smaller quantity of sample is a desirable factor when one thinks in dosimetry where an enamel sample has to be excised. Better resolution in g factor was also verified. Improvements in sensitivity by a 2.5 fold increase in S/N ratio and lo3 fold increase in amplitude/mass in comparison with band was found indicating possible improvements in sensitivity and minimum detectable dose with this technique. The use of a secondary standard can be useful for minimize the scatter of the data in intensity-versus-dose curve.
Acknowlegements The authors are grateful to L Aziani, C. Brunelo and E. Navas for technical support. Work partially funded by the Brazilian Agencies: FAPESP, CNPq, CAPES.
REFERENCES 1. A. A. Romanyukha, D. F. Regulla, Appl. Radiat. Isot., 47 (1996) 1293. 2. E. A. Ignatiev, A. A. Romanyukha, A. A. Koshta, A. Wieser, Appl. Radiat. Isot., 47(1996) 333. 3. C. P. Poole, Electron Spin Resonance, 2"d ed, Dover Publications, 1996, USA. M. F. Desrosier, A. A. Romanyukha, Biomarkers: Medical and Workplace Applications, (1998) 53. 5. V. Polyakov, E. Haskell, G. Kenner, G. Huett, R. Hayes, Radiation Measurements, 24(1995) 249.
628
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Assessment of contribution of confounding factors to cumulative dose determined by EPR of enamel S. V. Sholom, V. V. Chumak and E.V. Bakhanova Scientific Center for Radiation Medicine, Melnikova str., 53, Kiev, 04050, Ukraine X-ray and UV components of cumulative dose of enamel have been studied. It was shown that the first component may introduce an additional error to accidental dose estimation as much as few hundreds mGy. The second component is affecting both lingual and buccal parts of front teeth, no way of adequate account for this component is known presently.
1. INTRODUCTION Retrospective EPR dosimetry with tooth enamel is generally accepted as a technique of dose reconstruction for people exposed due to nuclear and technogenic accidents which are associated with ionizing radiation. In this technique, the cumulative dose of enamel is determined through measurement of some stable paramagnetic centers. Threshold values of reconstructed dose in EPR dosimetry with teeth today are lower than 100 mGy [l]. Such sensitivity covers the entire range of doses being of practical interest for dosimetric support of radiation effect epidemiological studies. The main interest in this regard has the accidental dose D,,, which, in general case, is related to cumulative dose D,, via equation:
where Dbg, Dx.rayand D,, are the doses from background environmental exposure, possible xray diagnostic procedures and ultraviolet solar exposure respectively. The first component may be determined by multiplying of enamel age by environmental dose rate. The second component depends on type and number of life time x-ray diagnostics. The last component is mainly manifested in frontal teeth. That is why such teeth are usually excluded from dosimetric consideration with loss of almost 40% of available dosimetric material in that way. Despite of its importance, the problem of non-accidental components of dose is not enough investigated and elucidated in literature. Some aspects of this problem were the subject of the present paper.
2. X-RAY COMPONENT OF CUMULATIVE DOSE 2.1. Materials and Methods In the previous study [2], teeth of different types were exposed in different geometries using x-ray facilities that are most common in Ukraine, and then '37Cs-equivalent
629
doses were reconstructed separately for lingual and buccal parts of each tooth. In such a way the empirical values of doses per single x-ray examination were obtained. In the present study, whole teeth were exposed inside the human phantom (Alderson type). Two of the most common types of dental diagnostics were simulated: so-called local and panoramic x-ray pictures. Only few neighboring teeth are exposed in the first case while practically all teeth are affected by x-ray radiation in the second case. Simulation of the first type of picture was done with the x-ray dental devices MINIDENT 55 (Slovakia) and 5D1 (Russia), the x-ray machines RUM 20M (Russia) and TUR (DDR) were used for the second type of diagnostics. 6 teeth were put each time into left and right sides of the phantom's lower jaw - see Fig. 1, b. It should be noted that for all devices x-ray radiation was incident to the phantom's left cheek. This geometry corresponded to diagnostics of the tooth number 2 in case of the local picture simulation (teeth number 1 and 3 were in the main radiation beam, teeth number to 6 were exposed by scattered radiation only) and teeth 4 to 6 in case of the panoramic picture. The total number of pictures was 50 for local and 40 for panoramic picture machines. The parameters of devices were the same as for routine dental x-ray diagnostics. The profiles of corresponding x-ray doses were obtained experimentally (using EPR) and simulated theoretically (using Monte-Carlo method, MCNP4B code). Before EPR
Figure 1. Geometry of simulation of x-ray dental diagnostics: a - head of human phantom used for laboratory x-ray exposure of teeth, b - jaw-level slice of head with 6 teeth inside. All teeth are located in the position of molars and numbered 1 through 6.
630
measurements each tooth was cut into 4-5 layers in direction parallel to buccal and lingual surfaces using a low-speed saw. The separation and EPR measurement of enamel as well as cumulative dose estimation were conducted according to SCRM technique [3]. Monte-Carlo simulation was applied to the Kramer’s mathematical phantom [4], which was slightly modified by adding the teeth model, with the following approaches: - accounting the contribution to dose from both incident and scattered radiation, - check of possible influence of energy spectrum, - test of effect of incident radiation angle variation, - calculation of attenuation due to additional filters used in some x-ray apparatuses.
2.2. Results and Discussion Typical dose profiles for the nearest tooth relative to x-ray tube (number 2 according to Figure 1) are shown in Fig. 2 for both local and panoramic x-ray diagnostics. The profiles 1 to 4 correspond to MINIDENT 55, 5D1, RUM 20 and TUR respectively. It is seen that dose of 2-mm buccal layer was in the ranges of 17-65 mGy for local and 5-9 mGy for panoramic diagnostics. The corresponding values for lingual 2-mm layer fell to intervals 5-9 and 1-1.5 mGy. Doses of teeth from opposite side (numbers 4 to 6 in Fig. 1) were much lower and don’t exceed a value of 2 mGy (MINIDENT 55). The example of calculated dose profiles is shown in Fig. 3. These profiles correspond to a local picture produced on ideal monochromatic 50-kV x-ray source: plot a and b show the dose change in the tooth number 2 and 4 respectively. The similar dependencies were calculated for other energies in the range 10-60 keV. In order to transform dependencies shown in Fig. 3 to dose profile of some certain x-ray source, one should know the energy spectrum of this source. SucL Spectia may be found in the literature (see e.g. [5]), and corresponding profiles of x-ray doses may be calculated (see Fig. 4 for example). It is clear that direct quantitative comparison of experimental and calculated profiles is hampered due to lack of unique calibration for different x-ray devices. This is a reason why some relative units, e.g. ratio RBLof doses in first 2-mm buccal and last 2-mm lingual parts, are more preferable. Monte-Carlo method gives possibility to estimate how this ratio will
:1
8o
6
1
T
Y b
a
20
f 0
2
4
6
8
1
Distance from buccal surface, mm
0
I
0
2
4
6
8
1
0
Distancefrom bum1 surface, mm
Figure 2. Dose profile for single x-ray exposure for the teeth number 2 (see Figure 1). a - local dental diagnostics, b - panoramic dental diagnostics. Profiles 1-4 correspond to following x-ray machines: 1 - MINIDENT 55, 2 - 5D1, 3 - RUM 20,4 - TUR.
63 1
04 2
Distance
4
6
8
2
10
4
Distance
buccal surface, mm
6
8
10
buccal surface, mm
Figure 3. Profiles of dose normalized on air kerma for teeth number 2 (plot a) and 4 (plot b) (see Figure 1) exposed to monoenergetic beam (E=50 keV, local diagnostics). change if exposure parameters (spectrum of x-ray tube, presence of low-energy filter, x-ray incident angle etc) vary. Influence of above parameters was estimated and obtained results are summarized in Table 1. Data are presented as the ranges of possible values for RBL. It is seen from this data that variability of studied ratio is mainly determined by variability of x-ray source spectrum and covers range 6 to 21. Corresponding experimental values fell into the range 5-9, which could be considered as acceptable coincidence. Taking into account that x-ray exposures usually take place several times over life for UkrainiaE population, one may expect that significant addition21 uncertainty (up to several tens of mGy) could be introduced into accidental dose value even if only buccal part of teeth is used for dose reconstruction. The correction procedure for x-ray component account has been developed using dose profiles similar to shown in Figs. 2 and 4. According to this procedure, x-ray contribution to lingual dose is estimated using the difference of buccal and Table 1. Ranges of possible variation of RBL (see above for explanation) due to variability in x-ray device parameters (MC simulation) Parameter of Studied range Range of RBL x-ray device i 4
6
Distance from buccal sufiace. mm
Figure 4. Dose profiles for tooth number 2 (see Figure 1) calculated for 60 (curve 1) and 50 (curve 2 ) kV x-ray tube voltages. Both curves are normalized on dose in 0-2 mm tooth layer.
x-ray tube voltage Thickness of A1 filter*
45-70 kV
6-2 1
0-4 mm
8-13
Angle of x-ray incidence*
0-30"
13-17
*These parameter were studied at the 50 kV x-ray tube voltage
632
lingual values. Overall uncertainty of accidental dose reconstruction using such procedure could be characterized by 60-80 mGy. 3. UV COMPONENT OF CUMULATIVE DOSE This component of cumulative dose has been studied on teeth from few liquidators each of which supplied at least two teeth, one being front (incisor, canine) and second being lateral (molar, premolar) tooth. Total number of studied cases was 3. Cumulative doses of enamel for these individuals were determined using routine SCRM technique [3] separately for lingual and buccal halves of corresponding teeth. Reconstructed doses are presented in Table 2. It is seen that lingual and buccal doses of lateral teeth were the same (within experimental error), therefore the mean values of two halves were considered as the "true" doses for corresponding persons. Doses of front teeth then were compared with the corresponding true values. It was found that uncertainties due to UV dose may be as large as few Gy even if only lingual parts of front teeth are used for dose reconstruction. This result is in agreement with data, obtained for the Japanese population [6]. No correlation of UV dose gradient with absolute value of UV contribution to lingual and buccal doses was detected, which makes unlikely the perspective of front teeth use for the retrospective dosimetry. Table 2. Doses of liquidators' teeth reconstructed separately for lingual and buccal halves of front and lateral teeth Person number
Type of tooth
1
Incisor
Molar
2
Incisor
Molar
3
Incisor
Molar
Characteristics Cumulative dose, of aliquot mGY Lingual
728
Buccal
1429
Lingual
492
Buccal
499
Lingual
605
Buccal
815
Lingual
402
Buccal
417
Lingual
183
Buccal
625
Lingual
101
Buccal
109
Deviation of lingual dose for front tooth from mean value for corresponding lateral tooth, mGy/%
233147
196148
78174
633
ACKNOWLEDGEMENTS This work was supported by the common US-Ukrainian project: Case-control leukemia study among Chernobyl liquidators.
REFERENCE 1. A. Wieser, K. Mehta, S. Amira, D. Aragno, S. Bercea, A. Brik, A. Bugai, F. Callens, V. Chumak, B. Ciesielski, R. Debuyst, S . Dubovsky, O.G. Duliu, P. Fattibene, E.H. Haskell, R.B. Hayes, E.A. Ignatiev, A. Ivannikov, V. Kirillov, E. Kleschenko, N. Nakamura, M. Nather, Nowak, S . Onori, B. Pass, S. Pivovarov, A. Romanyukha, 0. Scherbina, A.I. Shames, S. Sholom, V. Skvortsov, V. Stepanenko, D.D. Tikounov, S. Toyoda, Radiat. Meas., 32 (2000) 549. 2. S. Sholom, V. Chumak, and Ju. Pavlenko The lRPA Regional symposium on Radiation Protection. Prague. (1997) 571. 3. V. Chumak, S. Sholom and L. Pasalkaya, Radiat. Prot. Dosim., 84, (1999) 515. 4. R.Kramer and G. Drexler, Radiat. Prot. Dosim., 3 (1982) 13. 5 . Radiation Spectra of X-ray apparatuses (in Russian: Spectry Izlucheniya Rentgenovskikh Ustanovok), Energoizdat, Moscow, 1990. 6. N. Nakamura, J. F. Katanic and C. Miyazawa, J. Radiat. Res., 39 (1998) 185.
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EPR in the 2 1" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Retrospective EPR-dosimetry in Semipalatinsk nuclear test site region. S. Pivovarov, A. Rukhin, T. Seredavina, A. Zhdanov Institute of Nuclear Physics, National Nuclear Centre of Kazakhstan, Almaty, RK The peculiarities of retrospective EPR dosimetry for reconstruction of doses of the population and environmental objects of Semipalatinsk nuclear test site (SNTS) are considered. Influencing of tooth enamel pretreatment on an accuracy of a dosimetric signal evaluation is shown. The reconstructed doses for a number of the inhabitants of the inspected villages from SNTS region are adduced. The technique of an assessment of doses of SNTS soils by EPR method is offered. The preliminary estimation of doses along South-East trace is held. The data obtained show, that for a correct assessment of dose loads of the population it is needed using complex of EPR, cytogenetics and traditional computational methods.
1. INTRODUCTION AND METHODOLOGY There is a large number of places with an unfavorable radioecological situation now in Kazakhstan. First of all, it is a huge territory of former Semipalatinsk nuclear test site (SNTS), where during forty years (1949-1989) about 456 nuclear and thermonuclear explosions, including 30 surface and 86 atmospheric ones, have been fulfilled. Besides, a considerable number of special tests without nuclear explosion, but with radioisotopes exhaust were conducted. Radioactive clouds of 55 surface and atmospheric explosions and gas fraction of some underground explosions have left the test site outline. The following 4 explosions resulted in the largest contamination: 29.08.1949 of 22 Kt power, 24.09.1951 of 38 Kt power, 12.08.1953 of 400 Kt power and 24.08.1956 of 27 Kt power. Last atmospheric explosion was conducted in 24.12.1962, last tests of nuclear devices at SNTS were conducted in a tunnel in 4.10.1989 and in a well in 19.10.1989; no more nuclear explosions there were performed [l]. The tests have resulted in severe radioactive contamination of vast territory in North- Eastern Kazakhstan and in irradiation of local population. Outside the polygon about 30 underground nuclear explosions were conducted in various places of Kazakhstan. Beside this, the uranium mining and refining industry worked intensively. Additionally, new sources of radioactive contamination related to concentration of natural radionuclides, particularly 226Raand 228Ra,due to intensive oil production and burning of large amounts of coal has appeared last years. For example, the dose rate from some parts of the oil-extracting equipment reaches 3010-~ mGy/s what is comparable to the most contaminated places in the center of SNTS [2]. Therefore, the problem of development and application of the retrospective dosimetry methods is rather urgent for Kazakhstan. At the same time, the complexity of this problem is obvious also, since we deal here with the very different types of radiation and a variety of possible forms and ways of irradiation.
1.1. Advantages and problems of EPR dosimetry applications There are possible aspects among the problems of retrospective dosimetry in the Semipalatinsk polygon region. The first, and the most important, is a determination of the dose loads on the population of neighboring to SNTS settlements, the second one is determination of doses got by environment both in the polygon and outside its borders. The detailed study of possible ways for solving these problems has shown that the most perspective and fruitful method here is EPR dosimetry [3], however, its data should be supplemented by results of other methods. The EPR dosimetry technique based on tooth enamel is absolute in the sense that it registers only a radiating signal, which one accumulates during whole life after child teeth changing. The functionability of the method was demonstrated by special international experiment conducted under aegis of IAEA in 1999-2000 One of the main advantages of the method is a possibility to verify in other labs the data obtained once in a lab. But the method requires advanced and rather expensive equipment. The technique of sample preparation is complex enough. The main difficulty of the method is the necessity to extract a tooth for the analysis what is not always possible for obvious reasons. Besides, a problem on a degree of internal irradiation effects fixation is still open. Large success was reached in other technique of retrospective dosimetry, founded on the analysis of chromosome aberrations in blood cells. Alongside with a widely known J - method fixing unstable aberrations, there was designed and developed FISH method that enables to register stable long-lived violations in the chromosome set. An advantage of this method is the equally successful fixation of effects both external and internal irradiations, shortcomings are the technique complexity and poor efficiency, dependence of data on staff proficiency and possibility of aberrations appearance caused not by irradiation, but by some chemical combinations and other factors. To the present time practically all available data regarding dose loads on the Semipalatinsk region population were obtained using the traditional computational method only, which are based on radiometric data. A shortcoming of the method is a possibility of considerable errors in a general absorbed dose assessment due to incorrectnesses in external irradiation effects accounting in absence of a personal dosimeter and difficulties at calculation of the contribution from short-living radionuclides, which have already decayed and for which both initial composition and the distribution cannot be practically restored. These factors explain an observed now very considerable divergence in dose load assessment by various authors, which vary within the limits from 0,lGy up to 2,5-3,5 Gy. 2.
AND DISCUSSION
2.1. Dosimetry of population Our experience on examination of the SNTS region population demonstrates, that the most complicated problem for EPR- dosimetry is obtaining of teeth with sufficient amount of unimpaired enamel. Since a tooth can be extracted according to medical indications only and public dental service in this region is poor, the obtained tooth samples appear, as a rule, hardly injured, so, the output of good quality enamel is essentially reduced what reduces the accuracy and sensitivity of EPR dosimetry. In this connection much attention is paid to a preliminary preparation of tooth enamel. In it was marked, that the process of enamel preparation could introduce essential errors.
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Figure 1. Intensity of radiating EPR signal of one child- donor after preliminary treatment: 1. Mechanical removing of dentine. 2. Treatment in aceticum acid during 3 hrs. 3. Treatment in lactic acid during 3 hrs. 4. Treatment in dry white wine during hrs. This stimulated us to run direct experiments on the specially taken unirradiated in an initial state samples of teeth from children and adults. EPR spectra were registered at EPR300E spectrometer at X-band, their processing was done with the help of ESP300E and WINEPR software. source produced by irradiation of pure The additional irradiation was conducted on the metallic cobalt with neutrons from the WWR-K nuclear reactor. The source dose rate was calibrated by IAEA IDAServce using a set of alanine EPR dosimeters. Selected unimpaired teeth samples with a neglectively small initial radiating signal after dezinfection in formalinum were irradiated with large enough doses of about 5 Gy to ensure registration of both radiating signal and its changes after the consequent treatment. The irradiated teeth were divided into two halves, one of which was treated with acid or alkaline reactants before enamel separation, from the other half the enamel was obtained by a simple mechanical way without any reactants using. Solutions of acetic and lactic acids, dry white wine, food vinegar, alkali and hydrazine were used for processing. The duration of processing was varied from 3 to 48 hours. It became obvious from the experiments, that the chemical treatment notably influences on a registered radiating signal. At acidizing the decrease of a radiating signal amplitude from 12 up to 40 % was observed as contrasted with enamel obtained by mechanical allocation only. The experiments conducted on two samples of milk tooth of the same child are the most indicative in this respect. In this case, there is no reason to presuppose a difference in initial radiation sensitivity what allowed us to compare quantitively results of various variants of processing (Fig. 1). These data indicate that when reconstruct an absorbed dose by comparison EPR signal in enamel with the prepared before calibration curve an error can appear in a case of acidizing of the obtained enamel. Taking into account the fact, that in a food of locals a considerable place is given to acidmilk products, it is reasonable to expect, that the reconstruction of a total absorbed dose by EPR signal of tooth enamel for the inhabitants of Semipalatinsk region could give incorrect results. Thus, the dose estimation on EPR signal intensity by comparison with the standard - irradiated samples could give some lower value, and an additional irradiation using might, vice-versa, overestimate it.
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Table 1. Some data on EPR - doses of people from Sq*alvillage. Age of tooth Weigh of Number of EPR Linear regression EPR dose, donor, sample irradiations sensitivity analysis correlation Gy years powder, coefficient 1 40 0,356 0,9999 0,12 70 3 0,9997 0,54 3 0,517 91 2 72 0,9997 0,12 3 53 79 3 0,434 4 37 102 3 0,113 0,9997 1,21 0,402 0,9996 0,29 5 39 78 4 We have conducted reconstruction of doses for several dozens of permanent residents in the vicinity of SNTS (from Sarjal, Kurchatov, Mayskoye, etc.). Some data are given in Tablel. All these data were obtained with some extra irradiation, fig. 2-3 illustrate that. Allowing, that this information is confidential and taking into account all said above, it can be considered as preliminary only, and we do not present here the names of teeth donors. It is possible only to mark, that in some cases the considerable doses were registered, but nevertheless the doses much higher then which were mentioned by some authors [6], were not revealed. Similar data were obtained on the basis of analysis of the donor chromosome aberrations. The determination of a true absorbed dose for the population of Semipalatinsk region can be additionally complicated by effects of internal irradiation. So, according to [5], contamination of the polygon with Pu, Am and Sr radioisotopes, including laces accessible to the population and herding of domestic animals, varies within the limits 10-' lo4 Bq/Kg. Therefore, the total absorbed dose reconstruction for the population of Semipalatinsk region can not be correctly calculated only on the basis of EPR data. It is also necessary to consider in this respect the biodosimetry data based on chromosome aberrations analysis in blood cells, and methods based on the data of conventional radiometry as well as the analysis and generalization of available archive materials. Only simultaneous application of all these three essentially different approaches with corresponding processing and generalization of these data can ensure, in a certain extent, obtaining of really reliable estimations of true dose loads for both the population of Semipalatinsk region and other places of Kazakhstan with unfavorable radioecological situation.
U
0
MAGNETIC FIELD, G
Figure 2. EPR- spectra of irradiated TE for the person #1 (see table 1). Side lines are due to Mn2+standard.
1
2
3
DOSE, Gy Figure 3. Example of dose reconstruction by additional irradiation (#l, table 1)
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2.2. Dosimetry of environmental objects Other problem is the retrospective dosimetry of soils and environment in the region. At first a heightened radiating EPR signal in granite of a bridge in Hiroshima was observed by M.Ikeya et al. [3]. The similar signal was registered also in soils at the center of SNTS [7]. EPR spectra of different types of soil samples give a broad intensive line of ferromagnetic resonance (FMR) due to a natural iron admixture in the test site soil. Magnetic separation was conducted several times in order to lower the influence of FMR on EPR signal. More narrow EPR signals with a complicate shape at 8-2 were observed along with FMR line. One can choose several different signals as "dosimetric", it requires their separate analysis. One of these signals (g-2.006) legibly discharging from a general spectrum at a low microwave power is, apparently, the most convenient for retrospective dosimetry [8] due to its pure radiation origin. This was proved by additional irradiation, and the linearity of dose dependences in rather wide limits was shown. The parameters of this signal, including a course of annealing curves, have allowed to identify it with a signal from El'-center, which one assign to an electron localized in oxygen vacancy of cryptocrystalline quartz [3,8]. As the soil consists of a set of minerals, the efficiency of a radiating signal displaying in different minerals was studied at irradiation them by identical doses. These researches have allowed to select as the sensor of a dose a flint - mineral formation, which one is present at SNTS soils in enough plenty. The main complexity at usage of minerals for the purposes of retrospective dosimetry is the presence in them of a large relict signal conditioned by natural irradiation for geological age. According [3], it is very desirable to find the gear removing this relict signal, i.e., using a [3] terminology, setter of the sensor on a zero It seems, we managed to offer such gear for a flint. It is well-known, that at mincing a large chunk of the irradiated mineral the radiating signal starts to decrease at a size of fragments less some critical (- 0,2 mm), and at further mincing it decreasing down to zero. At the same time, this size is still great enough, so that pieces of the given fraction can be esteemed as a solid, in which one at repeated irradiation the same radiating defects giving the same EPR signal will be stored. Thus, it is possible to suspect, that during splitting of fragments of soil, for example, by wind erosion, the relict signal will be eliminated, then this grain-size fraction can be used for definition of doses obtained by soil of polygon only for last some hundreds years, i.e. during nuclear-weapon tests. The conducted researches of accumulation of a radiating signal in the given fraction at additional irradiation by gamma rays '%o have shown, that ordinary exponent here takes place, and at doses more than 15 - 20 KGy the saturation starts, so that for the higher doses this natural sensor is unsuitable. This technique also with usage of additional irradiation and subsequent extrapolation was applied by us for an estimation of dose loads of South- East track soils on SNTS. The results are adduced in a Fig. 3 (continuous line is a simply cubic spline on points, i.e. it is not a function and it has no functional load). It is visible, that there are considerable oscillations of a signal intensity (and, hence, of the summary dose) along the track, the manifestative anomaly with a hill up is watched on the 30* kilometer, it is interesting to note, that the same hill in this place is watched and on the radionuclides contents [5]. Now there is an information, that in this place the specific test was provided which was resulting in considerable sputtering of radioactive materials. 'I.
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DOSIMETRIC
t
40
100
DISTANCE, km Figure 4. Estimated integral doses of soils along the South- East SNTS trace of radioactive clouds. In frame : EPR-spectra of a sample of soil. We regard necessary to mark, that the results of doses estimation, shown in a Fig. 4, should be considered also preliminary, because here even in the greater degree than for tooth enamel, the same errors could appear, since instead of a straight line in this case for an extrapolation the exponent had been used. This problem demands further detail researches. Nevertheless, now there are no doubts, that the method of EPR dosimetry is rather fruitful and the further development and application of it in a complex with other methods will allow to evaluate really correctly an actual damage brought to the population and the nature both on Semipalatinsk region by perennial intensive nuclear-weapon tests, and on other similar places. Authors express the great thanks to the People and Government of Japan for the granted complex of EPR equipment, with help of which all the experimental data were obtained, and to professor M. Ikeya for the stimulating discussions and very useful consultations.
3. REFJ3RENCES 1. Semipalatinsk Nuclear Test Site. 10 years after closing, NNC RK, Kurchatov (2001) 28. 2. K. Kadyrzhanov, S. Lukashenko, SNTS - Kazakhstan’s problem or national prosperity, Abstr. Int. Conf. XXI century without nuclear weapon, Almaty, 29-30 August (2001) 25. 3. M. Ikeya., Applications of ESR: Dating, Dosimetry and Microscopy, World. Sci., Singapoor (1993) 499. 4. A. Wieser, K. Mehta, et al. The Second Intern. Intercomparison on EPR tooth dosimetry. Radiation Measurements, 32 (2000) 549-557. 5. K. Kadyrzhanov, Ch. Rofer, S. Khajekber, et al. A systematization of the radionuclide contamination of the SNTS by the nuclear tests specific features. Proc. of the Int. Conf. “Radioactivity at nuclear explosions” Sankt- Petersburg. HydrometeoIzdat (2000) 465-471. 6. B. Moskevich, B. Atchebarov. Nuclear polygon in Semipalatinsk and Kazakhstan‘s population health. Abstr. Conf. <
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EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Determination of total ionizing radiation doze on animals from west Kazakhstan by EPR method R.N.Nasirovaa B.K.Kuspanovaa, K.Kudaibayevb, M.B.Kilibayevb "Atyrau Institute of Oil and Gas, 465002, Atyrau, Republic of Kazakhstan bJoint Venture Kazakhoil-Emba, 465002, Atyrau .Republic of Kazakhstan
A wide application in industry and medicine of ionizing radiation sources, and a possibility of accidents at nuclear power stations led to a development of dosimetry techniques, allowing one to determine the dose of various samples. Tooth and bone EPR dosimetry is now one of the most powerful methods of retrospective and accident dosimetry [1,2]. Tooth enamel EPR dosimetry is of a significant practical interest due to the possibility of use of human and animal teeth as individual dosimeters. Electron paramagnetic resonance was applied for a long time as a tool to study radiation effects. Under ionizing irradiation unpaired electrons in materials are produced, the concentration being dependent on the absorbed dose, therefore the latter can be determined from EPR spectrum of the sample. Radiation induced center' lifetime in solid specimens (minerals, mollusk shells, bones, teeth, ets.) is long enough (ca. lo9 years for tooth enamel, for example). It allows one to determine with a high accuracy the total dose absorbed by the sample using the signal intensity of radiation induced unpaired electrons. The following advantages are pertinent to EPR in comparison with other dosimetry methods (e.g. thermoluminescence, or spectrophotometry): 1)the possibility of nondestructive analysis; 2) quantitative dose evaluation; 3) high sensitivity and large dynamic range of dose evaluation. The most important is retrospective dosimetry for dose evaluation of inhabitants of radionuclide soiled regions. In such cases any available data concerning the extent of radiation effect on inhabitants are necessary to determine urgent medical and organizing activity aimed at elimination of the pathogenic irradiation effect. In this work the total ionizing irradiation doses were determined for animals of regions near Azgyrskij and Taisoiganskij nuclear experimental ranges (West Kazakhstan). The animal teeth study looks like a very convenient way of the local irradiation evaluation due to radioactive soils because of low contribution of natural background irradiation (cosmic rays, ets.) during the period of animal's life.
1. EXPERIMENTAL The total accumulated (to the investigation momentum) irradiation dose (initial dose) was determined with the use of additional artificial irradiation: 1) the radiation induced EPR signal intensity was measured (at the initial dose - before artificial irradiation); 2) a number of additional artificial sample irradiations were performed along with the intensity of radiation induced EPR signal measurement after each irradiation; 3) the initial dose (Do) was
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determined on the base of linear extrapolation of the dose dependence of radiation signal to zero value of signal. EPR spectra registration and analysis were performed using EPR dosimetry technique based on the spectra registration at the rapid passage conditions (second harmonic phase quadrature detection of absorption) [3,4]. This technique is characterized by considerably higher sensitivity in the low dose range (10-100 cGy) which results in more reliability as compared with the standard EPR dosimetry procedure [1,2] based on the first derivative spectra registration. The spectra were recorded with X-band Bruker ESP300 EPR spectrometer at 77K as described in [3,4]. Microwave power was 5 mW. Polarizing magnetic field modulation fre uency was 100 kHz, modulation amplitude was 3.2 G. The rectangular microwave cavity of L o 2 mode was used. The following animal teeth were studied: 1)two teeth of a camel (near Taisoigan region) enamel from the front surface of tooth (1)- sample No. la, enamel from the back surface of tooth (1) - sample No.lb, enamel from tooth (2) - sample No. 2; 2) a cow tooth enamel (Taisoigan) - sample No. 3; 3) a cow tooth enamel (Azgyr) - sample No. 4. Tooth enamel was carefully extracted to eliminate dentine and impurities. Samples were prepared as cylinders of 5 mm diameter and 5 mm height made of small pieces (ca. 0.3 mm) of enamel embedded in polystyrene matrix. Enamel mass in each sample was 151mg. The total studied samples ages (including a period of their life and a time lapse after death) were for cows not more than 15 years, for camel - not more than 45 years. The sample irradiation was performed with a standard %o y-ray source. The dose control was made with thermoluminescent detectors.
Fig.1 shows EPR spectra of sample No. la (front surface enamel of camel tooth) at various additional y-irradiation doses. It can be seen that even without artificial irradiation the spectrum has a significant radiation component, thus the sample possesses apparent initial dose. Fig. 2 represents the dependence on artificial %-irradiationdose of the radiation signal intensity evaluated on the base of spectra analysis of samples No. la, No.1 b, and No.2 (as described elsewhere [4]):
Ira&') (D) = Lq(') (D) - Lq(2) (D=O),
(1)
where Ira&') - is the signal intensity at the position "1"(the radiation signal center), lew(') experimental spectrum intensity at the position "l",LqQ(D=O) -experimental spectrum intensity without additional I-irradiation at the position "2" symmetrical to the position "1" with respect to background signal line center (corresponding to the maximum of the background signal). Initial doses evaluated from these dependencies are: Do 3 Gy (sample No.la), DO 3.3 Gy (sample No.lb) and DO 3 Gy (sample No.2).
642
3420
3440
3460
Mu((nslic field.
EPR spectra at rapid passage conditions (second harmonic phase quadrature detection of absorption) of the camel enamel (sample No. la) at several additional 5-radiation dose (N - is the position of the background signal maximum).
Fig.1.
Additional y-radiation dmc. c C y / Fig.2. Dependence of radiation induced signal intensity of camel tooth enamel samples on additional &radiation dose: "*" - sample No. la (Do 3 Gy); "0" sample No.lb, (DO 3.3 Gy); "+" - sample No.2 (DO 3 Gy).
---
Fig. 3 shows the dose response radiation signal intensity evaluated by spectra analysis for cow tooth enamel samples No.3, and No.4. Initial doses determined from these dependences are Do 0.5 Gy (sample No. 3) and Do 0.25 Gy (sample No.4). 80
Fig.3. Dependence of radiation induced signal intensity of cow tooth enamel samples on additional y-radiation dose: "*" - sample No.3 (DO 0.25 Gy); "0" sample No.4 (Do 0.5 Gy).
;2 B '
z -
-
1 : $20
. I
-
0
loo
500
800
Mdilional r-radislnon dose. COY
The results obtained allow us to conclude the following:
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1) the initial accumulated doses of all the studied animals exceed the natural radiation background dose values (considering normal background intensity to be ca. 1-1.5 mGy/year); the camel's dose (- 3 Gy) exceeds significantlybackground dose; 2) a comparison of dose for frontal and back surface of camel tooth gives evidence that high energy %rays were dominant in irradiation; 3) a good agreement of the results obtained for two teeth of one camel is an additional proof of the reliability of EPR dosimetry technique and the method perspectiveness for retrospective evaluation of accumulated irradiation dose.
REFERENCES 1. M. Ikeya, New Applications of Electron Spin Resonance: Dating, Dosimetry and Microscopy, Singapore (1993) 500. 2. J.E. Aldrich and A Pass, Determining Radiation Exposure from Nuclear Accidents and Atomic Tests using Dental Enamel.- Health Physics, v. 54 (1988) 469. 3. V.E. Galtsev, E.V. Galtseva, O.Ya. Grinberg, Ya.S. Eebedev, Human Tooth EPR Dosimetry with Enhanced Sensitivity., J. Radioanal.Nucl.Chem., Letters, v.186 (1994) 35. 4. V.E. Galtsev, E.V. Galtseva, O.Ya. Grinberg, Ya.S. Lebedev (1994) Improvement of Sensitivity of ESR Dosimetry of Tooth Enamel., Doklady Biophysics, v. 334-336 (1994) 9, (Translated from Doklady Akademii Nayk, v. 334 No. 5 (1994) 649.
Section 9 Cross-Disciplinary and Methodology
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EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
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Pulsed ESR Double Resonance (PELDOR) Spectroscopy : Application to spin-labeled peptides Yuri D.Tsvetkov, Alexander D. Milov. Institute of Chemical Kinetics and Combustion Russian Academy of Sciences, Siberian Branch, Novosibirsk, 630090 Russia The applicability of a new pulsed ESR technique (PELDOR) for obtaining the weak dipoledipole interactions between spin labels is demonstrated, from which the distances up to several tens angstrom can be derived. The results of the spin labeled trichogin GA analogues studies are discussed. The intramolecular distances between spin labels are estimated for a series of frozen solutions of double-labeled trichogin GA analogues. The distances for 27 or 3 1 ~ helical conformations of the trichogin peptides are obtained. It is shown that the particular peptide chain conformation and its fraction depend on peptide structure and the polar properties of a solvent. By analyzing of the intermolecular dipole-dipole interactions between spin labels the aggregates of these peptides have been revealed in a series of frozen weakly polar solutions. It is shown that the number of peptide molecules in aggregates tends to four. The 310 helical conformation is detected for the trichogin molecules inside the aggregates. An aggregate model of four flo-helicaltrichogin molecules has been proposed. 1. INTRODUCTION High sensitivity of pulsed ESR spectroscopy in weak d-d (dipole-dipole) interactions measurements provided new applications of ESR in chemistry and biology. Structural properties of peptides are important for the biophysical behavior in complex peptide membrane systems. A useful and promising method to study these properties is spin labeling of the peptide chain, which allows one to exploit of the ESR techni ue.' Aggregation of membrane-active peptides is of great interest and can be studied by ESR. Trichogins, peptaibol antibiotics, are linear peptides containing a large amount of aaminoisobutyric acid (Aib) residues. They are thought to act through channel formation in cell membranes, leading to leakage of cytoplasmic compounds or dissipation of the membrane potential, and as a result to cell death. Trichogin GA IV displays a number of unique characteristics. Surprisingly, despite of its short main-chain length, it exhibits membrane-modifying pro erties on liposomes comparable to those shown by the long-chain members of the family! Conformational studies in organic solution 3 p suggested that trichogin GA IV has a right-handed a-helical structure and that the peptide helix has an amphophilic character. Recently these results were confirmed using a spin labeling technique?p6 So far, only conventional CW ESR technique for studying spin labeled peptaibols has been applied. In this review the results of PELDOR applications to the problem of conformation and aggregation of spin-labeled trichogin peptides are collected and discussed. This started in Novosibirsk in 1998 in collaboration with University of Padova (Laboratory of Prof. C.Toniolo) and Leiden University (Dr. J. Raap).
9
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2. EXPERIMENTAL In trichogin GA IV analogues, which have been studied in this work, Aib groups in peptide chain positions 1, 4, 8 are substituted by spin label TOAC groups and C terminal leucinol (1,2 amino alcohol) is substituted by Leu-OMe group (Table 1).It is also possible to change n-Octanoyl N-group to Fmoc. We shall use below the definitions TOAC and FTOAC with corresponding numbers to denote the trichogin peptides studied, as it is shown in Table 1. In some experiments we used non-labeled trichogin peptide, Tric-OMe. All these peptides have been synthesized in Prof. C. Toniolo laborator in Padova. The synthesis and characterization of these compounds is described elsewhere." Stable nitroxyl radical 2,2,6,6-tetramethyl-lpiperidinyloxy-4-one (TEMPONE) was used as a standard stable nitroxide radical. The high purity solvents methanol, ethanol, chloroform, toluene, decaline, dichloroethane, tetrachloromethane, further purification. The PELDOR experirnentsl6 are usually performed using conventional ESE spectrometers (working frequency VA) with an additional device for microwave pulses at frequency VB. The pulse pattern is shown in Figure 1. The quality factors of the spectrometer resonators were Q,=220 and Qb=150. The isolation between resonators at VA and at ~rg(at frequencies VAZ9.4 GHz and VA-VB=~OO MHz) was about 20 dB. Durations of the first and the second pulse were 40 and 70 ns, respectively. Duration of the pumping pulse was about 40 ns. CW ESR spectra of spin labeled peptides were recorded on an ESP 380 Bruker X-band spectrometer at a modulation frequency of 100 kHz and its amplitude of 0.1 mT. In the CW ESR and PELDOR experiments at 77 K the samples were placed inside the nitrogen-cooled finger of a Dewar ,The number of spin labels in the sample or concentration was determined by comparing the double integrals of the CW ESR spectra of the sample with that of CuC12.2H20 crystals containing a known number of paramagnetic centers. It should be noted that all the results were obtained for glassy state peptides at 77 K. 3. CONFORMATIONAL PROPERTIES OF DOUBLE-LABELED PEPTIDES IN POLAR SOLUTIONS In complex spin systems such as double-labeled peptides two types of d-d interactions (inter- and intra-molecular d-d ) exist. The corresponding PELDOR decays will be denoted Table 1. Spin-labeled and N-terminal substituted trichogin GA (Tric-OMe), structures and its notations used in this paper. (Example: FTOAC1,4 is Fmoc substituted Tric-OMe with Aib substituted in positions 1,4 by TOAC )
TOAC
Fmoc
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Figure 1. a) Sequence of 3d2 and mw pulses at VA separated by time interval produce ESE signal at time V(T=O).Then pumping pulse at % applied at time T , the amplitude of ESE signal changes up to value; b) ESE signal decay when time interval changes V(t,T=O), - dead time in ESE two pulse method; t 2. .. t i - starting fixed time intervals in experiments, - example of
as inter and V(T)intra. If it is assumed that cou lings are independent, then the total PELDOR decay can be represented as the product: 12, l? V(T) =V(Ointra V(T)inter
(1)
This gives the possibility to separate the influence of inter- and intra-molecular d-d couplings on the relaxation in complex systems taking into consideration that intermolecular part is usually approximately or precisely exponential. For the case of uniformly distributed spins PELDOR decay in the a sample with spin concentration C V ( T )= V(0)exp
- -y
' v b N T )= V(0)exp[-2pbAw,,,T]
wherepb is the degree of ESR spectrum excitation at . This type of PELDOR decay was observed for single labeled peptides FTOAC1, FTOAC8 and it is typical for many nitroxyl radicals in frozen glassy solution^.^"^. For the doublelabeled peptide FTOAC1,8 the PELDOR signal decay is much more complicated. Figure 2 shows the dependencies of the PELDOR decay for peptide FI'OAC 1,s with various concentrations of spin labels. At least three types of decay phenomena are observed. A fast decay during the first 50 ns, subsequent weak damping oscillations of PELDOR signal and a slow decay at T>50 ns. The last depends on the concentration of the peptide and should be attributed to intermolecular coupling of spin labels, i.e. the dipolar spin coupling between labels belonging to different biradical molecules. The first two characteristics must originate from the coupling inside the biradical peptide. The contributions of both intra- and inter-molecular couplings of spin labels to the
650
]
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0.90
1:-' . -.
-100
0
100
200
300
400
500
600
C .
-1
700
Figure 2. PELDOR signal decay at 77 K of double-labeled mOAC1,8 in CHCIDMSO (7:3) at different mean concentrations of spins in a 3sample. 1- 1.3 x 10'' cmS3;2 - 7.6 x 10'' 1.2 x 1019cm-'.
PELDOR signal decay can be separated using eqs. 1, 2 as follows. Let Vl(T) and V2(T) be PELDOR signals corresponding to concentrations and C2 of biradicals, respectively. Taking logarithm of eq 1and using eq. 2 in the exponential form V = exp [-C'T)] we can obtain system of two equations:
Figure 3. PELDOR signal decay of FTOACl,8 caused by intramolecular d-d interaction between spin labels. Curve 1 (dots) is obtained from experimental data given in Figure using eq.4. Curve 2 shows calculation results for peptide with fixed distance between unpaired electrons of 19.7 A. Curve 3 presents PELDOR signal for uniform distribution over distance in the range 18.7-20.7 8, ' b e relation between the experimental signal amplitude at T=O and the fitting curve indicates a 25% fraction of such peptides. Unfitted initial PELDOR signal decay at short time (T<70 ns) caused by the rest of the molecules.
The relaxation curves for the FTOAC1.8 in CHClnMSO, shown in Figure 2, can be used to extract the effect of intramoleculard-d interaction. As follows from eq. 3: C1
ln(V(T)intm) = In(Vx(T))- - Iln(VdT) - W2(T))I
c1- c2
(4)
The factor in square brackets is proportional to the contribution caused by interparticle interactions. Figure 3 shows the dependence of the PELDOR amplitude on obtained by subtracting the intermolecular contribution from the experimental curve 1 (VI)in Figure 2. This is the mean value obtained for two estimations of intermolecular contribution when respectively curve 2 and curve 3 in Figure 2 were used as V2. The concentration dependent factor in eq .4 was estimated each time from experimental data. The dependence of V(T)intra versus obtained this way is given by dots in Figure 3 and shows an oscillating decay. The existence PELDOR decay oscillations indicates that a conformation of the peptide molecule is resent whose spin labels are far removed from each other with a small spread in that distance9 2.The oscillation frequency for this case is : R3
65 1
The modulation amplitude decreases with time T so fast that it seems reasonable that the width of the distance distribution is not negligible. This is illustrated by the curve 2 in Figure 3, which was calculated assuming a single conformation without any spread in the spin-tospin distance. In this simulation we used p b value as the parameter to obtain the same amplitude of modulation as the experimental one. It is clearly seen that the signal intensity, modulation amplitude or modulation decay are quite distinct between the experimental data and the calculated curve. In our model calculations we could achieve the same picture of modulation decrease using a narrow uniform distribution for the inter-spin distance in the range between 18.7 and 20.7 A (curve 3 in Figure 3). The possible reason of the narrow distance distribution could be the existence of some different conformations of the TOAC spin label, differently oriented in the space.' The fraction of the biradicals FTOAC1,8, which produce the modulation effects, is roughly estimated as about 25% follows from the relation between the p b value used for this simulation and its estimation from experimental data (pb=l-V(T-~)-O.l7, see eq.7). The rest of the molecules (about 75%) have longer distances between the radical fragments with broad distribution over this parameter and a fluently decaying PELDOR signal (this part of the signal is not fitted in Figure 3). In order to follow the conformational (or R-distance) changes of peptide due to substitution of N-terminal group together with the changes of glassy solution properties we studied double labeled peptides TOAC1,8 and FTOAC1,8 in several different polar solvents: methanol, ethanol, TFE and CHCldDMSO mixtures of different ratio. All measurements have been done in 2 x lo3M solutions at 77 K. After the V(7)intersubtraction from the general PELDOR decay by the same procedure as described above, the part of V(7)inm for these measurements is shown in Figure 4. In these data only TOAC1,8 manifests oscillations in the PELDOR signal decay together with its amplitude damping. For the FTOAC1,8 peptide the oscillations of the signal decay
......'*.....*.......... 0
I00
200
400
500
0,4 8 ,
mo
Figure 4. signal decay due to htramolecular d-d interactions double-labeled peptides:l-TOAC1,B in TFE, 2-FTOAC1,8 TFE; 3-TOAC1,8 CHClflMSO(1:l); 4FTOAC1,8 CHClflMSO(73); 5-TOAC1,8 CHCInMSO('I3); 6-TOAC1,8 in ethanol; ETOAC1,I in ethanol; 8-TOAC1,8 in methanol.
I
I
0
100
.
I
200
.
1
300
.
1
400
.
1
SO0
.
I
.
600
I
4
-
3
-
1 2
.
.
700
Figure 5. signal decay for glassy peptide solutions at 77 K: 1 and 2 peptides FToAc4 and TOACl a chloroform-toluene mixture, respectively; 3 and 4 - peptides FTOAC4 and TOACl a mixture, respectively; 5-double-labled peptide ETOAC1,8 methanol.
-
652
are observed only for the CHCLDMSO solution. This molecular pattern do not change when we changed the ratio CHClflMSO from 7/3 to 1/1 by volume. In this case we still have glassy specimens of FTOAC1,S at 77 K. The other solvents containing FTOAC 1,s do not exhibit oscillations in the range 100-700 ns. As before, from the oscillation period the distance R between spin labels has been determined and from the initial modulation amplitude the relative fraction of peptides, which have this distance estimated. All these data are collected in the Table 2. The distance R obtained from the PELDOR decay should correspond to the distance between labels at the particular peptide chain conformation. In order to calculate this parameter for different molecular conformations we use the known structure parameters of these peptides obtained by X-ray ana1y~is.l~ When the backbone torsion angles of peptide FTOAC1,S were adjusted to generally accepted values for secondary structures like 310-, 27- and 25-helix, the distances between the spin lables could be calculated. The calculated ~alues'~'~**~ are : 10.6-12.04 for , 13.9814.08 for 310,22.0 for 27 and 28.0 for 25 conformers (in A). It is evident from the calculated and measured R values that TOAC1,S and FTOAC1,S in all frozen solutions, except in TFE, take a ribbon like conformation 27-helix type. In the case of the TFE matrix TOAC1,S adopts 310 conformation, while FTOAC1,S has no fixed distance conformers except in CHClflMSO solutions. PELDOR data for TOAC1,8 and FTOAC1,8 evidently show that the conformation type strongly depends upon the properties of solvents and the structures of peptide. It is not clear at the moment if one should attribute these effects to conformational properties of the particular peptide at low temperatures, or to the structure and polarity of these solvents which form the glassy matrix. It is evident, however, that the comparison between the data obtained by different methods will be possible only for the same experimental conditions (temperature, matrix, etc.) 4. AGGREGATION PROPERTIES OF TRICHOGIN PEPTIDES IN WEEKLY
POLAR GLASSY In this part the PELDOR method has been employed to investigate the self-assembling (aggregation) of the spin labeled trichogin GA analogues. It is generally assumed that the membrane modifying properties of this peptides are due to formation of amphiphilic helix bundles with polar groups pointing to the inside of the bundle and hydrophobic groups projecting towards the hydrophobic membrane
4.1. Aggregation of peptides: detection and investigation. As the first step of our investigation of spin labels d-d couplings we have measurements of frozen solutions of FTOAC4 and TOACl in chloroform-toluene (7:3). This solvent mixture is appropriate because it contains components of low polarity, the peptides are readily solubilized in a transparent glass upon freezing to 77 K. An additional solvent system, chloroform-toluene-ethanol (3.5: 1.5:5), was also examined in order to reveal the effect of a polar solvent on the intermolecular interaction between peptides. We present also PELDOR data on the double labeled peptide FTOAC1,S in frozen methanol solution, which will be used as the reference for the determination of experimental parameters in the analysis of the PELDOR results for peptide FTOAC4 and TOAC1. Curves 1 and 2 in Figure illustrate the PELDOR signal decays, for two single labeled peptides FTOAC4 and TOACl in glassy chloroform-toluene. A comparison between this results and phase relaxation data (exponential decay ) for polar solvents7 reveals
Table 2. PELDOR results of distance determination (in A) and corresponding fraction of double labeled trichogins (in %) adopted fixed distance conformation in different glassy matrix at 77 K
Peptide
CHCLDMSO
Glassy matrix CF3CH20H CH30H
CH3CH20H
lTOAC1,8
19.7 25%
*
*
*
TOAC1,8
19.7 30%
15.3 15%
19.7 36%
21.8 31%
-
two striking features. The first peculiarity is a fast decrease in the amplitude of V(T) at the initial time region up to T 150 ns. A fast decrease of the in this region indicates the existence of compact groups of spin labels in the system under study instead of a uniform spatial distribution of A strong d-d coupling of spin labels in a group leads to a fast dephasing at short times. This type of dependence was repeatedly observed for glassy solutions of biradicals'2r21and double labeled peptides? Averaging due to a random orientation of group provides a fast decay of V(T)at time T T* corresponding to the value of the mean d-d coupling of spins and effective distance R,ff :
and function V(T)tends to its limit V, at T 2 P : = (l-pby-'
pi
l-(N-l)pb
(7)
Equation 7 makes it possible to estimate the number of spin labels in the group, N. As for double labeled peptides, this systems represent a simple case of groups consisting of two spins which is exemplified by curve 5 in Figure 5, obtained for a frozen solution of double labeled peptide FTOAC1,8 in methanol. By comparing curves 1, 2 and 5 in Figure 5 one see that, within the initial time region, the depth of the fast PELDOR signal decay of peptides R O A C 4 and TOACl is much greater than that of peptide FTOAC1,8 whose structure contains only two spin labels. According to eq 7, greater depth for peptides FTOAC4 and TOAC1, relative to the corresponding value for the biradical, indicates that these compounds form aggregates which contain more than two peptide molecules. The second remarkable feature is the observation of a slower decrease in the time region at b 1 5 0 ns, which is accompanied by signal oscillations. The slow decrease of is related to the d-d coupling of spin labels belonging to different aggregates. Since this coupling is almost independent of the interaction between spin labels within the aggregate, the total decay of the PELDOR signal can be considered as the product of two time dependencies: Y = Vmter Yhm. The V(T) oscillations observed for curves 1 and 2 in Figure 5 are related to the interaction of labels within aggregates (Vbtra)and are of the same origin as the previously observed oscillations for biradicals and double labeled peptides in frozen polar glassy solutions. Oscillations observed for the experimental curves 1 and 2 in Figure 5 indicate that the aggregates of peptides FI'OAC4 and TOACl have fragments with a fixed structure in which the distances between spin labels have a minor spread. These distances can be estimated from the period of oscillations.
654
Addition of ethanol to the mixture of low polarity (chloroform-toluene) sharply changes the behavior of the PELDOR signal decay. Thus, experimental curves 3 and 4 in Figure 5 were obtained from solutions of peptides FTOAC4 and TOACl in a frozen mixture of chloroform-toluene-ethanol. As compared with curves 1 and 2, the signal decay now has no fast decay component at short T and no oscillations. Curves 3 and 4 in Figure 5 can be described by a simple exponential decay, typical for a random distribution of spin labels in the bulk. Transitions from dependencies 1 and 2 in Figure 5 to 3 and 4 after addition of ethanol to the chloroform-toluene mixture strongly indicate dissociation of peptide aggregates into their monomeric constituents. The number of peptide molecules in the aggregate can be estimated from curves 1and 2 in Figure 5 on the basis of eq. 7. For that it is necessary to obtain the limiting value of PELDOR decay function V,. It is difficult to make a reliable determination of V, directly from the experimental decay (Figure 5), because is the product of V(T)intraand V(T)htep However, at short T values the contribution to the total decay from V(T)hteris relatively small and we can assume that V(T)-V(T)hha.This means that the depth of the initial fast decay of the experimental function at short T corresponds to the V, value with an uncertainty determined by the oscillation amplitude. Therefore the V, value was taken from the mean at T = 150 ns for curves 1 and 2 (Figure 5). The fast decay is by then over and the slow decay of V(T)hteris not significant, due to weak interaction between spin labels in different aggregates. This estimate gives the mean value of V,=0.51?0.01 for both peptides. In order to determine thepb value for them, we used the function of the double labeled peptide FTOAC1,8 (curve 5 in Fig. 5) and foundpb = 0.2. By substitution of V,=O.Sl?O.Ol and pb=0.2?0.01 into eq we obtain N=4?0.3 for both peptides. Thus, the quantitative estimate shows that in a chloroform-toluene mixture peptides FTOAC4 and TOACl form aggregates consisting of four molecules. Note that the assumption we have made above about the absence of a spread in N needs additional verification. If varies for different aggregates, on1 a mean effective value can be determined using the method that has been developed. 21, It is possible to estimate a mean value roughly for distances between the labels within the aggregate from the time T* of the fast decay function (see eq 6). This time corresponds to the mean value of d-d interaction. For T* =loo-150 ns this will give re^ =30-36 A. This value gave us the upper limit for the aggregate dimension. But more correct distance values between spin labels in the aggregates can be calculated from the frequency of oscillations of the PELDOR signal. The experimental values of oscillation frequencies are 2.51 x lo7radh for peptide FTOAC4 and 1.85 x lo’ rad/s for peptide TOAC1. Using eq.5 for the oscillation frequency, we obtain a distance of 23.5 A for FTOAC4 and 26.0 A for TOAC1. The error in R does not exceed 1.5% estimated from the measurement of oscillation frequencies. Although the position of the spin label in the primary structures of peptides FTOAC4 and TOACl is different, the difference between the observed distances of the respective peptides is not large. mentioned above, oscillations indicate the existence of rigid fragments in the aggregates without rather small spread in distances between spin labels. Not all the possible distances between spin labels are manifested in our experiments. We are likely to observe oscillations due to pairs of spin labels with relatively small distances, because these pairs are located in rigid fragments of the aggregate. Therefore, it should be particularly emphasized that the values of distances obtained from oscillations can only characterize the size of the rigid part of the aggregate structure.
4.2. Aggregation effect as general property of peptides in weakly polar glassy solutions.. As described in previous part, the oscillation frequency and amplitude of PELDOR signal
655
decay depend upon the position of spin labels in the peptide structure and the difference in the structure of terminal peptide groups which could affect the structure of aggregates. These preliminary observations require additional investigation. To start this program we have studied the d-d relaxation for the group of trichogin peptides TOAC and FTOAC types (Table 1) single labeled in positions 1, 4 and 8 and double-labeled peptide FTOAC1,8. before, the last one was used in test measurements of experimental parameters based on the study of the intramolecular d-d interaction of its two spin labels. To elucidate the influence of solvent properties, the solutions of peptide TOAC4 were also studied in different glassy mixtures at 77 K (see Table 3.). To avoid the aggregation, we have studied solutions of FTOAC1,8 in polar solvent ethanol. The results of all this investigations are collected in Table 3. Of interest is the existence of two oscillation frequencies in the case of TOAC4 peptide presented in Table 3. This can indicate either the existence of two fixed distances inside the aggregate or the possible existence of the two different types of aggregates with different distances between spin labels. The values of the effective number of spin labels in aggregates (Table 3) vary from 3.1 to 4.3 depending on the peptide structure and solvent composition.
4.3. Conformational structure of trichogin inside the aggregates The data on magnetic d-d relaxation obtained by PELDOR in frozen glassy solutions of single-labeled peptides indicate that the aggregates include peptide chains with a fixed structure where the intermolecular distances between spin labels show minor spreading. This observation allows one to assume a fairly ordered spatial structure for the peptide building blocks of the tetrameric peptide cluster. Therefore, the problem is of interest to experimentally determine the nature of the secondary structure adopted by the peptide constituents that form aggregates. Table 3. Experimental frequencies of oscillations, distances between spin labels R, V, and for different spin labeled peptides.
Peptide
Solvent (volume mixtures)
FTOAC1.8
Etanol
Frequency, MHz R (4
Vp22%
0.17
2
values
656
I " " " " " ' " 1 .o
0.9
g
l a 0.8
5 '
1:10
4
13 0.1
2-
6:l
0
100
200
300
400
500
600
Figure 6. Dependence of V/Knkron T for the frozen glassy solutions of the mixtures of peptides TOACl,8 and Tric-OMe at different ratios of peptides: 1 - TOAC1,8 in chloroform-toluene; 2 TOAC1,8/Tric-OMe(6:l) chloroform-toluene; 3 - TOAC1,8/Tric-OMe (1:3) in chloroform-toluene; 4 TOACl,8/Tric-OMe(1:lO) chloroform-toluene; 5 - TOAC1,8 in
We have studied the intramolecular d-d couplings of spin labels in double-labeled peptide TOAC1,8 in a frozen glassy solution, containing an excess of unlabeled peptide Tric-OMe (Table 1) under conditions that aggregates are formed." For comparison, in this part the data on magnetic phase relaxation are also given for the double spin-labeled peptide TOAC1,8 in a polar solution of TFE. In this case, according to the previous part, no aggregation of peptides is observed. The following results have been obtained. As in the previous part, by subtracting the lnVhterdependence from the experimental ln(V) curve we found the V/Knterratio close to and sufficient to estimate the oscillation frequencies as well as the Vp values. Curves 1-5 in Figure 6 are the dependencies of V/Knter on T at 77 K, obtained by the method described above, for different ratios of double labeled and unlabeled peptides in chloroform-toluene (curves 1-4) as well as for peptide TOAC1,8 in TFE (curve 5). In general, Figure 6 shows the features characteristic for spin aggregates: a fast decrease in the PELDOR signal at T400ns with subsequent passage beyond the limiting V, value, accompanied in some cases by rapidly attenuating oscillations. By comparing curves 1 and 5 in Figure 6, it is seen that the V, value for peptide TOAC1,8 in the aggregated form (curve 1)is smaller than that for the same peptide in the monomeric form (curve 5). The V, value observed for curve 5 corresponds to two spin labels per monomeric peptide. According to our data presented in the previous parts, the number of molecules in the aggregate is about four. Therefore, the number of spins per aggregate for curve 1should be about eight. Whenpb is constant, according to relation 7, the V, value will decrease with increasing number of spin labels, N, in the aggregate. This corresponds qualitatively to the change of the experimental V, value (Figure 6, curves 1-4). Figure 6 shows that dilution of double labeled peptide TOAC1,8 by unlabeled peptide Tric-OMe leads to an increase in the V, value and the appearance of oscillations in the dependence of V/Vinter on time T. The oscillation period determined for curve 4 recorded for the peptides TOAC 1,8/Tric-OMe mixture (at a 1:lO ratio) in chloroform-toluene is 75k5 ns. In particular, using eq. 5 and the observed oscillation period 7525 ns, we get the intramolecular distance between spin labels of peptide TOAC1,8 in the aggregate, i.e. R = 15.7 A. This distance most closely corresponds to a 310-helical structure according the data presented in the previous part.
657
mentioned in the previous part, for the frozen TFE solution of peptide TOAC1,8, the observed amplitude of oscillations is much smaller than that expected from theoretical calculations for a pair of spins at a fixed distance’. A similar behavior is shown by peptide molecules included in aggregates (Figure 6, curve 4). This is indicative for the fact that the aggregated labeled peptides do not all have the 310-helicalstructure. The relationship between the depth of the fast decay and the oscillation amplitude makes it possible to estimate the fraction of aggregated peptides with an 3lo-helical conformation. This have been done for TOAC1,8 in chloroform-toluene” to be 0.2. From our data there is no direct evidence for the conformation of the remaining fraction of aggregated peptides. In other types of frozen solutions, TOAC1,8 may adopt different conformations depending upon the nature of solvent. Indeed, the intramolecular distance between spin labels may vary from 15.3 A (for TFE) to 21.8 A for ethanol (Table 2). A somewhat different situation was observed in MeOWEtOH glass at 77 K. By analyzing the CW ESR spectra at half-field (g- 4.0), it was reported that trichogin GA exist in a mixed a,/3lo-helical conformation in equilibrium with unfolded conformers?. The same type of mixed helical conformation was found in the cr stal state by X-ray diffraction analysis for trichogin GA IV24and the TOAC4,8 analog.’It may be concluded that short peptides as trichogin exhibit conformational flexibility depending upon temperature and the nature and organizationof surrounding molecules into the matrix (glass, crystalline,etc.).
4.4 Molecular model of aggregated trichogin peptides. The values of distances found for aggregates of two different trichogin analogues, one labeled at the first and the other at the fourth position of the pe tide chain, establish a specific set of constraints that may be used to build a molecular model! X-ray diffraction anal sis of TOAC4,8 trichogin revealed two independent molecules in the P21 asymmetric The N-terminal region of each molecule folds in a 310-helicalconformation, while the central and C-terminal regions are mainly a-helical. From CW ESR studies of three different double labeled trichogin analogues it was concluded that the overall secondary structure of these lipopeptaibol analogues in solution remains essentially unchanged.’’ An aggregate model was constructed from four 310-helices by adjusting the helical axes in pairs with the polar sides pointing to the center of the tetrameric peptide cluster’. After steps of energy minimization a model was proposed with the following average inter-residue distances: TOAC1...TOAC1: 26 A and TOACC.. TOAC4: 22 A. These values are consistent with the distances obtained from the PELDOR experiments: 26.0 A (peptide TOAC1) and 23.5 A (peptide FTOAC4) and in agreement with 310 conformation state. The exterior of the aggregate appears to be highly hydrophobic. The interior of the peptide cluster leaves room for several solvent molecules. The aggregation properties of trichogin peptides in weakly polar solwents discovered in9-” were confiied lately by CW ESR spectroscopy in spin-labeled peptides liquid solutions26.
nit."^'
5. The results given in this paper show the applicability and potentialities of the method of pulsed electron-electron double resonance (PELDOR) in electron spin echo in combination with the method of spin labels for studying the structure of macromolecules in a solid phase. compared with CW ESR, the PELDOR technique extends the range of measured distances between spin labels up to several tens of angstrom. This provides valuable information on both the structure of studied molecules and the structure of their aggregates in different systems.
658
ACKNOWLEDGEMENTS We are extremely grateful to Prof. C.Toniolo and his collaborators Dr. F.Formaggio and Dr. M.Crisma (Biopolymer Research Centre CNR, Department of Organic Chemistry, University of Padova, Italy) for their of the spin labeled peptides and the participation in the part of this research.The authors express their gratitude to Dr. Jan Raap, Dr. A.Maryasov and Dr. R.Samoilova for their participation in these investigations, many stimulating discussions and help. We are also grateful to Prof. G.Millhauser and Dr. M.Bowman for extremely helpful discussions of these results and fruitful comments. We are grateful to Elsevier Science, Springer Verlag and American Chemical Society for permission to reprint figures from our publications. This work was supported by The Netherlands Organization for Scientific Research (NWO) project 047.009.018, US Civilian Research and Development Foundation for the Independent States of the Former Soviet Union (CRDF), grant RCI-2056 and by the Russian Basic Research Foundation, grants 95-03-10770,9943-33149,OO-15-97321.
REFJSRENCES 1. P. Hanson, G. Millhauser, F. Formaggio, M. Crisma, C. Toniolo, J. Am. Chem. SOC.118(1992), 2170. Cafiso, Biophys. (1991), 389. 2. J. Arsher, S.F. Ellena, 114 (1992), 2170. 3. C. Auvin-Guette, Rebuffat, Y. Prigent, B. J. Bodo, Am. Chem. 4. C. Toniolo, E. Benedetti, Trends Biochem. Sci. 16 (1991), 350. 5. S.M. Miick, G.V. Martinetz, W. R. Fiori, A. P. Todd, Millhauser, G. L. Nature 359 (1991), 653. 6. W. R. Friori, S. M. Miick, G. L. Millhauser, Biochemistry 32 (1993), 11957. 7. A. D. Milov, A. G. Maryasov, Yu. D. Tsvetkov, J. Raap, Chem. Phys. Lett. 303 (1999), 135. 8. A. D. Milov, AG. Maryasov, R. I. Samoilova, Yu. D. Tsvetkov, J. Raap, V. Monaco, F. Formaggio, M. Crisma, C. Toniolo, Dokl. Akad. Nauk. 370 (ZOOO), 265. 9. A. D. Milov, Yu. D. Tsvetkov, Formaggio, M. Crisma, C. Toniolo, J. Raap, J. Am Chem. SOC.122 (2000). 3843. 10. A. D. Milov, Yu. D. Tsvetkov, J. Raap Appl. Magn. Reson. 19 (2000), 215. 11. A. D. Milov, D. Tsvetkov, F. Formaggio, M. Crisma, C. Toniolo, J. Raap, J Am. Chem. 123 (2001), 3784. 12. A. D. Milov, A. G. Maryasov, Yu. D. Tsvetkov, Appl. Magn. Reson., 15 (1998), 107. 13. A. G. Maryasov, Yu. D. Tsvetkov, J. Raap, Appl. Map. Reson. 14 (1998), 101. 14. A. G. Maryasov, Yu. D. Tsvetkov Appl. Magn. Reson. 18 (2000), 583. 15. P. Hanson, G. Martinez, G. Millhauzer, F. Formaggio, N. Crisma, C. Toniolo, Vita, S. J Am. Chem. SOC.118 (1996), 271. 16. Yu. D. Tsvetkov, In: Pulsed ESR: A New Field of Applications, Keijzers C., Reijerse E., Schmidt J. Amsterdam: North Holland, (1989), 206. 17. K. M. Salikhov, Yu. Tsvetkov, In Time Domain Electron Spin Resonance, L. Kevan, R. Schwartz, Wiley: New York, (1979), 232. 18. K. M. Salikhov, A. G. Semenov, Yu. D. Tsvetkov, Electron Spin Echo and Its Applications; Nauka: Novosibirsk, (1976) (in Russian). 19. V. Monaco, F. Formaggio, M. Crisma, C. Toniolo, P. Hanson, G. Millhauser, C. Gegrge, J. Deschamps, J. Flippen-Anderson,Bioorg. Med.Chem. 7 (1999), 119 20. D. Anderson, P. Hanson, J. McNulty, G. Millhauser, V. Monaco, F. Formaggio, M. Crisma, C. J. Am. Toniolo, Chem. SOC.121 (1999), 6919 21. A B. Ponomarev,A. Milov, Yu. D. Tsvetkov, Fiz, 7 (1988), 1673. 22. A. B. Ponomarev, A. Milov, TYu. svetkov, Khim. Fiz, 9 (1990), 498. 23. A. D. Milov, A. D. Ponomarev, Yu. D. Tsvetkov, Chem. Phys. Lett., 67 (1984). 24. C. Toniolo, C. Peggion, M. Crisma, F. Formaggio, X. Shui, D. S. Eggleston, Nature Struct. Biol. 1 (1994). 908. 25. M. Criima, V. Monaco, F. Formaggio, C. Toniolo, C. George, J. L. Flippen-Anderson,Lett. Pept. Sci. 213 (1997), 213. 26. AD. Mi1ov;Yu.D. Tsvetkov, F. Formaggio, M. Crisma, C. Toniolo, G. Millhauser, J. Raap, J. Phys. Chem. 105 (2001), 11206.
EPR in the Zlst Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
659
The carotenoid triplet state in Rhodobacter sphaeroides reaction centers. An EPR magnetophotoselection study. Igor V. Borovykha,kina B. Kleninab,Ivan I. Proskuryakovb ,Peter Gasta, Arnold J. a
HOW'
Department of Biophysics, Huygens Laboratory, P.O. Box 9504,2300 RA Leiden, The Netherlands Pushchino, 142290, Russia Institute of Basic Biological Problems
The triplet state of the carotenoid spheroidene, a pigment cofactor of reaction centers from the photosynthetic bacterium Rhodobacter sphaeroides 2.4.1., was studied with time-resolved directdetection EPR under polarized light excitation (magnetophotoselection). Four types of magnetophotoselection experiments, differing by excitation wavelength and temperature, enabled determination of the orientations of the optical transition moments of the reaction center spheroidene and the primary donor relative to the principal triplet axes of these molecules. Making use of the projections of the primary donor principal triplet axes onto the X-ray structural coordinate system determined earlier, we were able to calculate for the first time the orientation of the spheroidene optical transition moment and its principal triplet axes relative to the RC molecular frame.
INTRODUCTION Carotenoids (Car) are universally found in living organisms. In photosynthesis carotenoids fulfill light-harvesting and protective roles [l-31 and are located both in the antennae systems and in the reaction centers (RCs). In this work we present a direct detection EPR (DDEPR) magnetophotoselection (MPS) study of RC preparations of Rhodobacter (Rb.) sphaeroides that contain the carotenoid spheroidene. Population of the 'Car state by direct excitation of carotenoid and intersystem crossing proved successful mostly when strong intermolecular interactions are present, and 'Car population proceeds by a very fast process of singlet excitation fission into two carotenoid triplets [4-61. Another way to populate carotenoid triplets in solution is to use triplet sensitizers, such as naphthalene, anthracene, or chlorophyll [7]. The triplet state of a RC-bound carotenoid is populated in a third way. When functional electron transfer in RCs is blocked, the triplet state of the so-called primary electron donor (BChl dimer, P) is populated by the recombination of photoinduced radicals (radical pair mechanism) with unique To spin polarization [8,9]. The triplet state of the carotenoid is formed by quenching of the primary donor triplet state (T-T energy transfer). Note that because of conservation of spin angular momentum, the spin polarization of 'Car is the same as that of 'P, i.e. only the To level is populated. However, the polarization pattern, observed in the 'Car EPR spectrum, is inverted because the sign of the zero field splitting (ZFS) constant D (negative) in 'Car is opposite to that of 'P [lo]. The quantum yield of 'P varies with temperature, increasing from ca. 0.14 at 300 K to about 1 below 77 K [11,12]. On the contrary, the yield of 'Car is low at T < 30 K, and increases above this temperature. This behavior was explained by the presence of an intermediary thermoactivated stage of the T-T energy transfer from to Car [13], which is most probably the population of the triplet state of the monomeric bacteriochlorophyll located between P and Car [14,15]. Thus, the current scheme of the 'Car population looks as follows: 'P'War
+
'[P'WICar
-
'[P'WICar
+
'PQCar
+
'PQ'Car
--+
'Pacar, T > 30 K
660
where Q, stands for the active bacteriopheophytin molecule of the RC, and the intermediary monomeric BChl triplet is not shown. By varying the temperature, the triplet states 'P and 'Car can be populated almost selectively, and studied separately. This makes RCs of Rb. sphueroides 2.4.1 a very attractive object for optical [16,17] and EPR [4,18,19] studies of the carotenoid triplet state (for reviews see [20-221). Much effort was spent to determine the orientation of the principal triplet axes of the carotenoid spheroidene in the RCs of Rb. sphueroides 2.4.1. utilizing the technique of MPS [23]. In these studies conventional cw EPR and, to improve sensitivity, excitation with amplitudemodulated light were used. the 'Car state is essentially short-lived, kinetic parameters had to be introduced into the spectral simulation procedure, decreasing the precision of the information obtained from MPS. Earlier we have demonstrated the feasibility of MPS measurements with direct-detection EPR (DDEPR) [24]. The advantage of this approach is its time resolution, high enough to measure transient signals before they have relaxed appreciably. In the present work this technique is applied to the study of the spheroidene triplet state in Rb. sphaeroides 2.4.1 RCs. By combining our results with crystallographic data from the literature we were able to determine for the first time the orientation of the optical transition moment and the principal triplet magnetic axes of the carotenoid relative to the molecular frame of the RC.
2. EXPERIMENTAL SECTION 2.1. Biochemical procedures. Cells of Rb. sphueroides 2.4.1. were grown photosynthetically without oxygen. Under such conditions RCs are known to accumulate almost exclusively (>95%) spheroidene [13,25]. RCs were isolated as described by Frank et ul. [26] and frozen under light in the presence of 10 mM sodium ascorbate to pre-reduce the primary electron acceptor QA.This procedure created the conditions for 'P formation. Typical EPR samples contained 60-70% (v/v) glycerol and were prepared in 3 mm i.d. quartz tubes. Final optical density of the samples was 10-30 cm-' in the primary donor absorption band. The samples were degassed by three freeze-pump-thaw cycles and sealed under vacuum. This procedure decreased the probability of cracks formation when cooling the samples. For optical measurements the samples were diluted to an optical density of 2 per cm and placed in 2.5 mm (inner size) cuvets. Glycerol was added to obtain a clear glass at low temperature.
2.2. Instrumental. As an excitation light source a Continuum Surelite I pumped OPO laser was used with flash duration of cu. 4 ns. The incident energy was attenuated to cu. 0.1 ml m-*at the sample inside the EPR cavity to avoid light saturation of the magnetophotoselection effects. Samples were excited into the near-infrared absorption band of the primary donor, at 896 (bandwidth ca. 5 nm), which corresponds to the transition of P at low temperature for RCs from Rb. sphaeroides, and into the corresponding to the central peak of the carotenoid absorption band at 472 nm (bandwidth cu. 1 vibronic structure of the spheroidene Sz + So optical transition. The technique of magnetophotoselection measurements with DDEPR was described earlier [24,27]. Note that in the DDEPR experiment the signal appears in direct absorption and emission mode. Control experiments demonstrated the absence of significant relaxation effects. The overall time resolution of our setup was about 50 ns. The temperature in the Oxford Instruments helium gas-flow cryostat was regulated with a home-built temperature controller, which could be set with 1 K accuracy, and stabilized the temperature to 0.2 K. Room and low-temperature absorption spectroscopy was performed with a single-beam spectrophotometer described earlier [28]. The spectral resolution was 0.5 nm. Measurements at cryogenic temperatures were performed with an Oxford Instruments helium flow cryostat.
23. Simulationprocedures The spectral simulations were done as described previously [24]. Two spectra, obtained with the
66 1
B , mT
B , rnT 31 0
350
31 0
330
350
Direct-detection EPR suectra of the state measured under excitation l i d t Dolarked parallc (a& and Figure = 896 nm (Type 1 experiment) Boxcar gate 1.3 p,DAF = 0.2 p. perpendicular@,f) to the magnetic field. T = 10 K, &,,, (a,b) -RCs of Rb. sphaeroides 2.4.1.; (e,f) -additionally purified QA-depletedRCs of Rb. sphaeroides (c,d) -spectral RCs [29] could simulations performed with the following parameters: D = 0.0188 cm-', E = 0.0033 cm-I, sphaeroides be used for relating the optical transition moment vectors and magnetic axes systems to the molecular frames. g, = g, = 2.002, & = 2.000, AHx = AH, = AH, = 1.3 mT. Values of the best-fit angular parameters 6, y are given in Table 1. I
-
excitation light polarization plane parallel and perpendicular to the magnetic field were simulated with the same set of parameters, providing the projections of the optical transition moment of the excited molecule with respect to the principal magnetic axes of the triplet-bearing molecule. This orientation is not determined from MPS in a unique way, because MPS is insensitive to the signs of the projections. Thus all possible sign combinations had to be tested. The resulting vectors of the optical transition moments were then recalculated to the crystallographic coordinate system, using the data of the singlecrystal 'P study [29]. In this work the orientation of the 'P principal magnetic axes relative to the X-ray coordinate system was determined, so that any vector specified within the 'P magnetic system, as is the case for the MPS measurements, could be overlaid on the molecular structure. In the experiments described below, orientations of the primary donor optical transition moment, QY, and that of the carotenoid, D, were obtained both in the 'P and in the 'Car principal magnetic axes systems. This enabled calculation of the 'Car magnetic axes relative to the molecular frame of the carotenoid. RasMol version 2.7.1. was used in calculations involving structural data files and for creating Figure 4.
3. 3.1. Magnetophotoselection data and orientation of optical transition moments in the principal triplet axes systems. Taking advantage of the marked difference in the P and Car absorption spectra, so that those molecules can be excited almost selectively, and of the possibility to control population of the 'P or 'Car states by changing the temperature [13,17], four types of MPS experiments were designed: Type 1. Direct excitation of P and detection of the 'P signal (a, = 896 nm, T = 10 K) enabled determination of the orientation relative to the 'P principal axes. Type 2. Direct excitation of P and detection of the 'Car signal (axc = 896 nm, T = 100 K) enabled determination of the Qy orientation relative to the 'Car principal axes.
662
Type 3. Indirect excitation of P, preceded by S-S energy transfer from the excited Car, and detection of the 3Car signal (Lxc = 472 nm, T = 100 K) enabled determination of the D orientation relative to the 'Car principal axes. Type 4. Indirect excitation of P, preceded by S-S energy transfer from the excited Car, and detection of the 'P signal (kxc = 472 nm, T = 10 K) enabled determination of the D orientation relative to the 3P principal axes. Figure la,b shows DDEPR spectra of the primary donor triplet state in chemically reduced reaction centers from Rb. sphaeroides 2.4.1. The spectra were obtained in a Type 1 experiment with two different polarizations of the excitation light, (a) parallel to the magnetic field, and (b) perpendicular to the magnetic field. In the following these spectra will be shortly referred to as the Ellband spectra, respectively. The best fits of the experimental data are shown in Figure lc,d. They were calculated for the orientation of Qy relative to the 'P principal magnetic axes (x, y, z -axes of 'P zero-field splitting tensor) determined by the angles 6 = 90" * lo", y = 90" 2 10"(6 and y are the spherical coordinates of [24]; the angles in the first octant are given). These value are slightly different from the values of 6 = 80 5", y = 70 * 5" obtained earlier [24] for the orientation of the optical transition moment of primary donor relative to 'P magnetic axes in the RCs from carotenoidless mutant of Rb. sphaeroides R26. However small the difference is, it is of a special importance because of the fact that the single-crystal EPR study of 'P, which relate the principal magnetic axes of the primary donor triplet to the X-ray structure of the RC, is available only for the R26 mutant strain of Rb. sphaeroides [29]. When using this information for transforming vectors determined in the 3P principal coordinate system of Rb. sphaeroides 2.4.1. into the crystallographic axes system, one has to be certain that the properties of P are of the R26 mutant. The X-ray structures of P in the both RCs are almost identical [30,31]. Excitation of QA-depletedreaction centers at 850 nm resulted in a 'P MPS spectra (Figure 2a,b, dots) different from that obtained earlier for excitation at 900 nm with a similar preparation [24]. The most prominent features are the apparent wide 'wings' in the EllBospectrum, which cannot be reproduced by our usual spectral simulation procedure (Figure 2, dashed lines). When we introduced normal distributions of the D and E parameters, the spectra of Figure 2 could be simulated quite well (Figure 2, solid lines). Therefore, it is realistic to assume that preparation inhomogeneity affects the shape of the 'P spectra and the angular parameters obtained from MPS. In fact, the above mentioned wings could not be detected in a R26 RC preparation that had
0. mT
8.mT 310
350
310
350
Figure 2. Dots - EPR spectra of the 'P state detected the QA-depletedRCs of Rb. sphaeroides R26 under excitation at 850 contrast to Fig. 1, the reaction center preparation was not subjected to additional purification. Other experimental and other parameters. conditions are Fig. 1. Dashed lines -spectral simulations with D = 0.0200 cm-', E = O.OO40 in Fig. 1. Solid lines - spectral simulations for normal distributions of the ZFS parameters, Do = 0.0200 cm-', AD = 0.0020 cm-', = AE = 0.0030 cm-'. Other parameters in Fig. 1.
663
b
d
f
B. m T 290
310
330
350
370
290
310
330
350
370
3. EPR spectra obtained in Type 2 -Type 4 experiments (dots) and their simulations (solid lines). (a,b) -T = 100 K, (qd) -T = 100 K, Lxc = 896 (e,f) -T = 10 K, 5 , = 472 Simulation parametersfor the ’P spectrum (curves e,d) are the same as in Fig. 1. Simulation parameters for the ’Car spectrum (curves a - d) are: D = - 0.0285 cm-’, E = cm-’, g, = g, = 2.002, & = 2.000, AHx = 2.5 mT, AH, = 1.5 mT, AHz = 2.5 mT. Values of the best-fit angular parameters 6, y are given in Table 1.
Lm= 472
been additionally purified on a DEAE-Sephacel column (Figure le,f). The MPS spectra of the purified preparation are identical to that of the wild type RCs. We take this as a proof of conservation of the structure and properties of the primary donor in the RCs of the mutant R26, as compared to Rb. sphaeroides 2.4.1. Thus, the data of the single-crystal studies of Rb. sphaeroides R26 RCs [29] could be used in our calculations for relating the optical transition moment vectors and magnetic axes systems to the molecular frames. Figure 3 shows the results of the remaining three MPS experiments (Type 2 - Type together with the spectral simulations. A small fraction of the ’P signal could be observed overlapping the much larger ’Car signal at 100 K (Type 2 and Type 3 experiments). It is most probably due to incomplete transfer of the ’P to carotenoid and a small fraction of RCs lacking carotenoid [32]. Because of the sufficiently different D and E values of the ’P and 3Car EPR signals, it was not difficult to suppress the contamination by subtracting an adequate fraction of the primary donor triplet measured on the R26 reaction centers. Figure 3 shows the high-temperature signals after such subtraction. No indication of spin-lattice relaxation effects reported earlier [24] could be detected in the 3Car signals at 100 K. In spectral simulations, the EPR spectroscopic parameters of the ’P and ’Car signals (ZFS parameters, g tensors, linewidths) were kept constant, so that only the angular parameters were varied. Their values are collected in Table 1.The errors given represent maximum deviations, estimated by varying y until the difference between simulated and experimental spectra became apparent by eye.
3.2. Orientation of the spheroidene optical transition moment. the Type 4 experiment the orientation of the spheroidene optical transition moment D relative to the 3P magnetic axes is determined, which enables the calculation of the orientation of D relative to the molecular structure of the carotenoid. To this purpose, the average structural axes of P were first calculated (called earlier the “X-ray axes”[29]), and then the information on the ’P principal
664 axes orientation relative to the structural axes utilized (Table 4 in Ref. [29]). As result, the coordinates of D vector in the structural system are obtained, which can be used to overlay this vector on the molecular frame of spheroidene. Since spheroidene does not undergo conformational changes when promoted from the ground state into its triplet state [33,34] we can utilize the X-ray structure of the RC obtained for the ground state of the carotenoid for studying an excited state. Apart from the orientation of the optical transition moment of spheroidene D given in Table 1, there are three other orientations of D relative to the 3P principal axes system which correspond to the measured MPS spectra. To select a unique optical transition moment orientation, additional physical considerations have to be applied. In linear polyenes such as carotenoids in the all-trans state, the optical transition moment lies along the molecule. In the cis state present in RCs [35-371, an additional cis-band appears at shorter wavelength from the main + So transition [38]. The 472 nm band used for the excitation in the present work is the strongest vibronic peak of the main transition, it thus should correspond to a shift of the electron density on excitation along the molecular frame. For the approximate Ca symmetry of the 15,15'-cis RC carotenoid one then expects the 472 nm optical transition moment to be oriented perpendicular to its Cz axis. If we consider the carotenoid molecule bound to RC as a bow, the carotenoid optical transition moment D will be then directed along the bowstring. the photophysical properties of carotenoids are largely determined by the atoms participating in the double-bond conjugation [39,40], in the following we will consider only this region of the spheroidene molecule. The four calculated orientations of the optical transition moment D were compared with the average double-bond direction of spheroidene (that is, the line connecting the ends of the double-bond region, carbon atoms C2 - CS'). Two orientations were discarded as they deviated too far from the expected direction (by 2 40"). Later checks of these orientations for the triplet magnetic axes systems to which they correspond to (see below), showed that such operation was justified. The other two orientations are making angles within 10" and 20" with the average plane of the double-bond region. They were treated explicitly up to the stage of the 3P principal magnetic axes calculation. This enabled the determination of a single optical transition moment orientation (making an angle of 20") with the "bow string", which was in accord with the principal triplet system as well.
33. Calculation of the spheroidene triplet magnetic axes system relative to the molecular frame. The four types of the MPS experiments provide information of how the two vectors, and D, are oriented relative to the two triplet magnetic axes systems - that of 3P, whose orientation relative to the crystallographic coordinates has been determined [29], and to the 'Car system with unknown triplet axes coordinates. In principle, the values of the coordinates of two non-collinear vectors define an axes system, so it is possible to calculate the orientation of the 'Car magnetic axes system relative to the molecular frame as well. Here again one encounters the problem of multiple possible orientations of the optical transition moment vectors allowed by MPS. Thus, all possible orientations have to be considered and the corresponding principal triplet axes calculated, overlaid on the RC Xray structure, and then compared on the basis of general physical principles. The results of a detailed analysis [41] are displayed in Figure 4 and Table 2. In Figure 4 the calculated optical transition moments and principal triplet axes of P and Car are overlaid on the X-ray structure of the Rb. sphaeroides 2.4.1. reaction center (1PSS entry of the Protein Data Bank [30]). The two-fold symmetry axis Cz, constructed by connecting the center of the Mg-Mg line of the primary donor BChls and the Table 1. Best-fit angular parameters for wild type RCs. rel. axes 6 90" 10" y 90" 10"
rel. 'Car axes D rel. 'P axes 75" 3" 26" 2" 5" 58" 10"
D rel. 'Car axes 28" 2" 50" 7"
665
Figure 4. Stereo pair of the part of the black) molecular axes overlaid.
structure with the primary donor (shown in grey) and spheroidene (shown
non-heme Fez+ ion, is also shown. The Q direction is thought to be perpendicular to the [42]. For clarity of presentation only the photosynthetic membrane incorporating the RC BChls of the primary donor P and the spheroidene molecule are shown. In the case of P, the optical transition moment Qr coincides with the y triplet axis of primary donor and makes an angle of 89" with the Cz axis. For Car the optical transition moment D makes an angle of 73" with the Czaxis. It is tilted out of the average spheroidene plane by The orientations of the 3Car principal magnetic axes and of the optical transition moment D relative to the crystallographic axes of the lPSS structure, used for drawing Figure 4, are summarized in Table 2.
4. DISCUSSION 4.1. Principal triplet axes of spheroidene. Error estimation. The coordinates of the 3P magnetic axes system in the crystallographic axes have been determined earlier [33], and the uncertainty estimated as ca. 21". The orientation of the spheroidene triplet axes relative to the molecular structure has been determined here for the first time. It is thus especially important to estimate the precision of this determination. The procedure of finding the 3Car principal axes was numerical, precluding us to perform an analytical error calculation. Therefore we adopted an alternative approach. The triplet axes were calculated for each orientation of Qyand D Table 2. Projections of 3Car axes and of D vector onto the PDB coordinate system of the 1PSS structure used to construct Fig 4.
I 3 ~ ax-axis r 3 ~ ay-axis r 3 ~z-axis r D
PDBx-axis 0.3807 -0.8745 -0.2702 -0.4314
I
y- y-axis 0.7325 0.0989 0.6856 0.8662
I
PDB z-axis -0.5644 -0.4747 0.6760 0.2516
I
666
vectors relative to the 'P and 'Car magnetic axes that lie within the error limits of these vectors, with a step of 1" and 3", respectively. This operation produced ca. 3000 solutions for the 'Car axes orientation. The projections of these axes on the 'P axes system were found to be distributed around their average values in a bell-shape manner. In the next step the statistical formalism for Gaussian distributions was applied, and standard deviation values were obtained. This approach is not mathematically strict. However, it seems to us realistic enough for the following reason. The wings of the distribution correspond to the case when the errors estimated from MPS spectra are close to their maximum values. We assume that the situation when all or the majority of the angular parameters simultaneously reach their outermost values has low probability. This draws the calculated distribution of the axes system nearer to a statistical case. The uncertainty fields for the 'Car principal axes system relative to the molecular structure are then described as cones with an angle of 29". A possible source of systematic error is the non-selectivity of carotenoid excitation at 472 so that partial averaging of MPS could take place. This effect diminishes the error due to nonselective absorption. The data on MPS in Rb. sphaeroides 2.4.1 RCs (Figure 3), however, shows no admixture of non-photoselected spectra. We therefore conclude that the effect of non-selective absorption on the calculated angles is negligible.
4.2. Comparison with previous data. The orientation of the spheroidene optical transition moment and its triplet axes has been extensively studied before, both with optical and EPR spectroscopy. Photodichroism studies of Rb. sphaeroides 2.4.1. RCs enabled determination of an angle of ca. 70" between and D vectors [43], which is in very good agreement with the value of 68" i 10"calculated from our MPS measurements. The value of 67" was obtained also from optical linear dichroism measurements on single crystals of Rb. sphaeroides 2.4.1 RCs [44]. The finding that is parallel to the photosynthetic membrane [42,45] agrees nicely with the angle of 89" between vector and the axis derived from our MPS measurements. Investigations of the 'Car magnetic axes have been carried out for a number of years by H. Frank et al. [19,23,45,46]. Table 3 compares the orientations of the and D vectors relative to the 'Car magnetic axes obtained from MPS measurements b that group [23], and in the present work. In Table 4 such comparison is made for the angles of the YCar axes relative to the Cz axis of the RC. From Table 3 it is apparent that except for the orientation of D in the 3Car axes system, the data of the present work differ considerably from previous results. Also the angles for the orientation of 'Car axes relative to the Cz axis obtained in this work and those in Ref. [45] (Table 4) differ considerably. We believe that these differences are largely due to the inadequacy of the steady-state MPS technique with EPR used earlier for the studies of dynamic paramagnetic states. Let us consider this problem in more detail. The cw EPR spectra are essentially steady-state ones, thus they represent the timeaverage of photoinduced signals. As in the approach adopted in the present work and in Ref. [24], spectral simulation in previous studies [47] was started with the calculation of the 'isotropic' spectrum, i.e., the spectrum arising when all orientations of the absorbing molecules in a sample are excited with equal probability. Spin-lattice relaxation of 'Car was neglected because it was assumed to be slower than the triplet depopulation to the ground state, and the deviations of the canonical lines amplitudes from theoretical values was accounted for by introducing depopulation rate constants of the spin sublevels. The kinetic parameters obtained from simulation of the isotropic spectrum were
Table 3. Projections of the
and D optical transition moments onto the 'Car magnetic axes.
D Y Z Y Z X Ref. (231 0.72i0.08 0.60i0.07 0.3220.08 0.3720.03 0.4220.02 0.8220.03 This work* 0.25k0.09 0.93k0.04 0.2520.05 0.30i0.06 0.36k0.06 0.88i0.02 X
.
667
Table 4. Orientation of the 'Car triplet axes relative to the C, axis of the reaction center. Axes X
Y z aErrors 29"
This work" Ref. [45] 20 40 70 53 85 78
subsequently introduced into the MPS calculations, resulting in the angular parameters. However, the spectrum excited under standard EPR conditions cannot be considered as isotropic [24]. The assumption of its isotropic character brings errors into the estimated kinetic parameters, and finally into the values of the angular coordinates of the optical transition moments. In our calculations, the EIIBoand EIBospectra were measured separately, and the isotropic spectrum constructed as a sum of one EllBospectrum with two EIBospectra. In addition, demands for the simulation precision in the case of the DDEPR-detected spectra are higher than for the cw ones, which are in the derivative presentation. The 'integrated' DDEPR spectra have to be carefully calculated throughout the whole spectral range, in contrast to cw EPR spectra which show large amplitudes mostly around the canonical fields, where the derivative of the spectral line becomes large. Moreover, the DDEPR spectra are essentially free from spin-lattice relaxation effects. All this results in an improved reliability of the angular coordinates obtained from DDEPR MPS.
5. CONCLUSIONS This work shows that magnetophotoselection with direct-detection EPR enables one to obtain precise orientations of the optical transition moments relative to the triplet magnetic axes system even in the case of short-lived states with fast spin-lattice relaxation rate. The relative orientations of the optical transition moments and the triplet magnetic axes of the primary donor and carotenoid molecules in bacterial reaction centers of Rb. sphaeroides 2.4.1. were determined. For the first time, using the above techniques and earlier data on the 'P principal magnetic axes orientation in the molecular frame, we were able to determine the orientation of the optical transition moment and the triplet axes system of spheroidene in the molecular frame of spheroidene in the reaction center.
ACKNOWLEDGEMENT We are grateful to Mr. Bram Joosten for the RC preparations. This work was supported by the Netherlands Foundation for Chemical Research (SON), financed by the Netherlands Organisation for Scientific Research (NWO). I.B.K. thanks INTAS and NWO for travel support (grants No. 93-2849ext and 47-006-003). I.I.P. acknowledges financial support of FOM (Dutch Foundation for Fundamental Studies of Matter) and of the Russian Science Foundation RFBR (grant 99-04-48184).
REFERENCES 1. 2.
3. 4. 5. 6.
R. J. Cogdell, H. A. Frank, Biochim. Biophys. Acta, 895 (1987), 63. H. A. Frank, In The Photosynthetic Reaction Center; J. Deisenhofer, J. R., Norris, Eds., Academic Press, vol. 2,1993; 221. F. Boucher, M. van der Rest, Biochim. Biophys. Acta 461 (1977), 339. H. A. Frank, J. D. Bolt,; S. M. B. de Costa, K. Sauer, J. Am. Chem. SOC, 102 (1980), 4893. Frick, J.; von Schutz, J. U.; Wolf, H. C.; Kothe, G., Mol. Cryst. Liq. Cryst, 183 (1990), 269. H. Rademaker, A. J. Hoff,, R.van Grondelle, L.N.M. Duysens, Biochim. Biophys. Acta, 592,
668
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47.
(1980), 240. J-P. Chauvet, M. Bazin, R. Santus, Photochem. Photobiol. 41 (1985), 83. W. W. Parson, R. J. Cogdell, Biochim. Biophys. Acta 416 (1975), 105. M. C. Thurnauer, J. J. Katz, J. R. Norris, Proc. Natl. Acad. Sci. USA. 72 (1975), 3270-3274 A. J. Hoff, I. I. Proskuryakov, Chem. Phys. Lett. 115 (1985), 303. C.A.Wraight, J.S. Leigh, P.L.Dutton, R. H. Clayton, Biochim. Biophys. Acta 333(1974) 401 C. C. Schenck, R. E. Blankenship, W. W. Parson, Biochim. Biophys. Acta 680 (1982), 44. C. C. Schenck, P. Mathis, M. Lutz, Photochem. Photobiol. 39 (1984), 407. H. A. Frank, C. A. Violette, Biochim. Biophys. Acta 967 (1989), 222. A. De Winter, S. G. Boxer, J. Phys. Chem. 103B (1999), 8786. R. J. Cogdell, T. G. Monger, W. W. Parson, Biochim. Biophys. Acta 408 (1975), 189. T. G. Monger, R. J. Cogdell, W. W. Parson, Biochim. Biophys. Acta 449 (1976), 136. H. A. Frank, J. Machnicki, M. Felber, Photochem. Photobiol. 35 (1982), 713. H. A. Frank, J. Machnicki, R. Friesner, Photochem. Photobiol. 38 (1983), 451. D. E. Budil, M. C. Thurnauer, Biochim. Biophys. Acta 1057 (1991), 1. A. Angerhofer, In Chlorophylls; H.Scheer, Ed.; CRC Press: Boca Raton, FL, 1991; 945. A. J. Hoff, J. Deisenhofer, Phys. Rep. 287 (1997), 1. W. J. McGann, H. A. Frank, Biochim. Biophys. Acta 807 (1985), 101. I.V. Borovykh, 1.1. Proskuryakov, I.B. Klenina, P. Gast, A. J. Hoff, J. Phys. Chem. 104B (2000), 4222. D. Siefermann-Harms, Biochim. Biophys. Acta 811 (1985), 325. R. J. Cogdell, I. Durrant, J. Valentine, J. G. Lindsay, K. Schmidt, Biochim. Biophys. Acta 722 (1983), 427. M. K. Bosch, I. I. Proskuryakov, P. Gast, A. J. Hoff, J. Phys. Chem. 100 (1996), 2384. S. M. C. Otte, Doctoral Thesis, Leiden University, Leiden, The Netherlands, (1992). J. R. Norris, D. E. Budil, P. Gast, C. -H. Chang, 0. El-Kabbani, M. Schiffer, Proc. Natl. Acad. Sci. USA 86 (1989), 4335. T. 0. Yeates, H. Komiya, A. Chirino, D. C. Rees, J. P. Allen, G. Feher, Proc. Natl. Acad. Sci. USA 85 (1988), 7993. J.P. Allen, G. Feher, T.0 Yeates, H. Komiya; D.C. Rees Proc. Natl. Acad. Sci. USA 84 (1987), 5730. The photochemistry of carotenoids; Frank, H. A.; Young, A. J.; Britton, G.; Cogdell, R. J. Eds., Advances in Photosynthesis, vol. 8., Kluwer, Dordrecht, 1999. M. Lutz, L. Chinski, P. Y. Turpin, Photochem. Photobiol. 36 (1982), 503. B. Robert, W. Szponarski, M. Lutz, In Time-resolved vibrational spectroscopy; A. Laubereau, M. Stockburger, Eds., Springer, Berlin, 220 (1985). M. Lutz, I. Agalidis, G. Hervo, R. J. Cogdell, Biochim. Biophys. Acta 503 (1978), 287. Y. Koyama, M. Kito, T. Takii, K. Saiki, K. Tsukida, J. Yamashita, Biochim. Biophys. Acta 680 (1982), 109. B. Robert, Biochim. Biophys. Acta 1017 (1990), 99. L. Zechmeister, Cis-trans isomeric carotenoids. vitamins A and arylpolyenes; Academic Press, New York, (1962). A. Angerhofer, F. Bornhauser, A. Gall, R. Cogdell, Chem. Phys. 194 (1995), 259. B. E. Kohler, (1995) in: Carotenoids vol. 1B: spectroscopy, Birkhauser Varlag Basel, 3 I.V. Borovykh, I.B.Klenina, 1.1. Proskuryakov, P. Gast, A.J. Hoff, J. Phys. Chem. B, in press. J. Breton, Biochim. Biophys. Acta 810 (1985), 235. A. Vermeglio, J. Breton, G. Paillotin, R. Cogdell, Biochim. Biophys. Acta 501 (1978), 514. H. A. Frank, C. A. Violette, S. S. Taremi, D. E. Budil, Photosynth. Res. 31 (1989), 107. W. J. McGann, H. A. Frank, Chem. Phys. Lett. 121 (1985), 253. H. A. Frank, J. Machnicki, P. Toppo, Photochem. Photobiol. 39 (1984), 429. H. A. Frank, R. Friesner, J. A. Nairn, G. C. Dismukes, K. Sauer, Biochim. Biophys. Acta 547 (1979), 484.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
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Electron dipole-dipole interaction in ESEEM of biradicals S.A. Dzuba and L.V. Kulik
Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, 630090 Novosibirsk, Russia
Three new approaches to study electron-electron dipole-dipole interaction between two unpaired electrons in nitroxide biradicals are described. The first one involves Fourier transform of the standard primary ESE envelope modulation (ESEEM). It is shown that in organic glass the frequency spectra contain a small but detectable peak arising due to this interaction. The intensity of this peak is increased substantially in a new three-pulse Relaxation-Induced Dipolar Modulation Enhancement (RIDME) experiment. The third approach is based on performing a rapid stepping of the magnetic field in the resonator of ESE spectrometer (field-step ELDOR). The data are analyzed for a Fourier transform resulting in a Pake resonance pattern. The interspin distance is determined for nitroxide biradical in glassy toluene and in frozen nematic liquid crystal 4-cyano-4'-pentyl-biphenyl. The advantages and limitations of these techniques are discussed. 1. INTRODUCTION
Application of Electron Spin Echo (ESE) to study distances between unpaired electrons in biradicals and radical pairs in organic randomly oriented solids is now of great interest. ESE provides information in the very important nanometer range of distances. Several approaches in ESE has been used so far: double electron-electron resonance (DEER or PELDOR) [l-lo], 2 + 1 method [ll-131, multiple-quantum EPR [14, 151, solid-echo and Jeneer-Broekaert sequences [161. Here we describe three other approaches to study distances. The first one is a routine twopulse ESEEM (ESE envelope modulation) experiment. Recently, it was successfully applied to spin-correlated radical pairs appearing after a light flash in photosynthetic preparations [ 171. In these studies ([18] and refs. therein) a strong electron-electron modulation was obtained. In spite of the extreme simplicity of this approach, it was not used so far for organic biradicals in solids at thermal equilibrium. The obvious reason for that is the electron-nuclear hyperfine interactions, which strongly contribute to ESEEM of biradicals in solids [19]. Our results however will show that two-pulse sequence in favorable cases may be applied to study electron-electron dipole-dipole interactions in organic biradicals in thermal equilibrium. The second approach is based on the three-pulse stimulated echo sequence. The echo is measured as a function of the delay 5 between the first and the second pulses, at different
670
fixed delays between the second and the third pulses. The delays must be large enough to be comparable with the relaxation times 7'1. This experiment involves the non-resonant partner spin in the formation of ESEEM via a relaxation-induced change of the local dipolar field experienced by the resonant spin. An alternative to double-frequency ELDOR technique is the use of the rapid stepping of the magnetic field Bo in the sample v ~ l u m eto a new resonance position (field-step ELDOR). Although this approach was suggested long time ago in time-resolved EPR [20] and in ESE [21], it has not been applied so far to study electron-electron interactions. For this purpose, field-step ELDOR may be performed with a help of a three-pulse stimulated echo sequence The magnetic field jump, synchronized with an additional microwave pumping pulse, is positioned between the second and the third pulses of the stimulated echo sequence. The echo is measured as a function of the delay z between the first and the second pulses. The time interval between the second and third pulses must be larger than the rise time of the magnetic field jump.
1.1. Theoretical background For biradicals in thermal equilibrium, the theory developed in [23] predicts that the echo signal in the case of a complete microwave excitation by two 90' and 180" pulses separated by the time interval is: Am R
= --,cos(Az)
+
B2 ,cos(Rz)cos(Ar) R
+ -sin(Rr)sin(Az), R
where A=-
2 3
1 D(3c0s2 8 - 1) + 3 R = (Am' + =
~
,
and Am is the difference in resonance frequencies for the two spins in the absence of dipolar and exchange interactions, D represents the dipolar coupling, 8 is the angle between the line connecting the two species and the direction of the external magnetic field, is the value of the spin-exchange interaction (half the singlet-triplet splitting). In the point dipole approximation D is connected with the interspin distance value by the simple relation:
As cos(R and sin(R z) dephase quickly, they can be dropped beyond time of common spin-echo spectrometers. So, the echo amplitude is simply
Am2 E(2z) =-y-COS(AT).
100ns, the dead
(3)
67 1
m icrow ave n12
n
echo
m a g n e t i c field
Figure 1. The time scheme of field-step ELDOR experiment. The stimulated echo sequence is employed with zinterval varied. An additional pulse acts at a shifted resonance position. One can see that ESE is modulated with the frequency - D(3cos2 8 - 1) + . This has a simple physical meaning. The second pulse, except for refocusing the transverse magnetization, also changes the partner spin orientation and alters therefore the precessing frequency of the spin under observation. The difference between two frequencies is This theory is applicable when the whole spectrum is excited uniformly by the microwave pulses. If only a fraction is excited the electron-electron modulation will appear only for the pairs in which both spins are involved. This is another restriction for the two-pulse experiment. E.g. for nitroxides widely used as spin labels in different applications, the amplitude of the microwave field available in commercial ESE spectrometers is one order of magnitude smaller than the EPR linewidth. In the field-step ELDOR experiment the first pulse (see Fig. 1) creates transverse magnetization which precesses during the first z-period. The second pulse converts this transverse magnetization into longitudinal magnetization. The phase of precession is conserved before the third pulse of stimulated echo sequence is applied. The additional n: pulse, acting between the second and third pulses at a shifted resonance field position, changes the partner spin projection and, in consequence, the local dipolar field experienced by spins under observation. Therefore, after the third xJ2 pulse these spins precess with a frequency differing by A from its initial value. This results, while the time interval z between the first and second pulses is varied, in a modulation of the stimulated echo signal:
- cos(Az), which resembles Eq. (3). For polyoriented sample with the interspin distance fixed, Eqs. (3, 4) should be averaged over the angle 19.Its cosine Fourier transform results in a well-known Pake spectrum. One may obtain two characteristic frequencies, corresponding to 0 = and 0 [17]:
672
=
(2/3D+w)/2n
j j = (-4/3D+25)/2n. The value is determined by the positions of the singularities of the Pake spectrum while thefil value corresponds to the positions of the outmost edges. So, Eqs. (2,5) allow the values of D, r and to be determined.
2. EXPERIMENTAL
The experiments were performed on a Bruker ESP 380E pulse X-band EPR spectrometer using a homemade rectangular resonator with a quartz dewar containing liquid nitrogen. The resonator quality factor was adjusted to obtain the dead time of 72 ns. The lengths of the 7112 and pulses in the two-pulse sequence were 8 and 16 ns, respectively. For the three-pulse sequence all pulse lengths were 16 ns with n/2 turning angles. Microwave excitation was applied at the maximum of the nitroxide EPR spectrum. For creating the field jump, the current pulses were fed through Helmholtz coils stuck to resonator side walls made of nonmagnetic stainless steel. Conceptually, our equipment for creating a magnetic field jump is similar to that described in [24] and will be given in detail in a forthcoming paper [25]. The rise-time of the magnetic field jump in our experimental conditions is about -1 ps and is determined mostly by the eddy currents in the resonator walls. In our experiments we used biradical:
Biradical was synthesized and purified as described in [26]. Biradical was dissolved in toluene, or in deuterated toluene, or in nematic liquid crystal 5CB (4-cyano-4'-pentylbiphenyl):
All samples were measured in a glassy state obtained by fast freezing at 77K
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T=
0
f, MHz Figure 2. Modulus ESEEM frequency spectra of biradical in toluene glass. The time-domain signal within the dead time (72 ns) was restored (a) and dropped (b). The asterisks mark the the Pake spectrum singularity.
5
10
15
20
25
MHz Figure 3. Modulus ESEEM spectra of biradical in toluene glass obtained in three-pulse RIDME experiment at different (see text),.
3. RESULTS 3.1. Two-pulse ESEEM of biradical toluene solution Primary ESEEM time-domain traces for the biradical solution were extrapolated to zero z value, using a linear prediction method. It was fitted by an exponential function, which was then subtracted before Fourier transformation. The resulting modulus Fourier spectrum is given in Fig 2a. One can see peaks around the single and double proton Larmor frequencies (14 MHz and 28 MHz). In addition, a shallow but sharp negative peak with the minimum at 7.3 MHz is seen. This peak is absent for the spectrum of nitroxide monoradical TEMPON in toluene obtained in the same way (data not shown). One may suggests that this negative peak is induced by electron dipole-dipole interaction representing the singularity (8.7d2) in the Pake spectrum. Its unusual shape we ascribe interference of the weak electron-electron dipolar line with the huge wing of the electron-nuclear peak having a comparable intensity at this spectral position. This is readily supported by model calculation employing modulus Fourier transform of several damped harmonics differing in amplitudes [27]. The sign of the electron dipole-dipole peak in Fig. 2a may be corrected by dropping some initial data points of ESEEM time-domain traces prior to Fourier transform. Variation of
674
initial value changes the relative phase of electron-electron and electron-nuclear contributions in the frequency spectrum. Fig. 2b presents the modulus ESEEM spectrum with the data points omitted for the initial time interval z,=72 ns (which exactly corresponds to our experimental dead time). The Pake spectrum singularity appears now as a positive peak with the maximum at 7.1k0.2 MHz. Numerical calculation with two damped harmonics indicates that the positive peak in the modulus Fourier transform spectrum obtained in such way reflects the correct value of the frequency of the weak harmonic. We assumed that because substantial overlap of the unpaired electron orbitals is not expected for the biradical of such structure. The corresponding interspin distance calculated by Eq. (2) is 19.6k0.2 A.
3.2. Relaxation-Induced Dipolar Modulation Enhancement (RIDME) The small intensity of the electron dipole-dipole peak seen in Fig. 2 is a consequence of a relatively small excitation bandwidth. We estimated for our experiments a microwave amplitude of 7 G while the total width of the nitroxide EPR spectrum is 70 G. Only the fraction of pairs where both spins are excited by the microwave pulses (1 5-20%) contributes to dipolar modulation. The experiment proposed below shows that this fraction may be increased substantially. In this experiment a three-pulse stimulated echo sequence d 2 - r-z/2-T-d2- z-echo is used with fixed and variable This sequence is equivalent to the primary echo sequence with the only difference that the second pulse in the primary sequence is split into two halves. Let us consider a biradical in which one electron spin (spin A) is excited by the microwave pulse and takes part in echo formation while the other (spin B) is not. Suppose that during the time interval spin B flips under the action of longitudinal relaxation. For an odd number of flips, this results in alternation of the local dipolar field experienced by spin A. So, during the first and the second z interval the spin A precesses in different magnetic fields, with the difference determined by the electron-electron dipolar coupling. Thus, the echo intensity will be modulated with the same frequency as in Eqs. (3, 4). The fraction of spins B flipping an odd number of times is given by the relation
where is the spin-lattice relaxation time. Up to the half of the pairs may be involved in formation of ESEEM. This method may be called as Relaxation Induced Dipolar Modulation Enhancement, or RIDME [28]. Fig. 3 presents the modulus RIDME spectra of biradical in toluene glass. The spectra were normalized to the same total signal intensity using the following procedure. Experimental time-domain traces were fitted by an exponential decay function, which was subtracted prior to Fourier transform. Then the resulted modulus spectra were divided by the initial amplitude of this exponential function. The frequency position of the electron dipole-dipole peak in Fig. 3 is the same as that in Fig. 2, and does not change as is varied. Its intensity increases dramatically with increasing. As described above, this effect may be explained by sudden jumps of the local dipolar field, due to longitudinal relaxztion of electron spins. An odd number of electron spin
675
04 0
10
20
f;m Figure 4. Cosine Fourier transform of the time traces obtained in field-step ELDOR for biradical in toluene solution (see text).
Figure The same as in Fig. 4, for biradical in 5CB with the liquid crystal director d parallel to Bo.
flips will change the local dipolar field experienced by the partner spin. Some variation of the peak shape with increasing may be attributed to interference with the nuclear peak. Exactly the same three-pulse RIDME experiment was performed on TEMPON in toluene. Neither peaks at the frequencies other then proton Larmor frequency nor the change of the spectra with variation was found. This proves that RIDME is a characteristic feature of biradicals. 3.3. Field-step ELDOR Employing the rapid stepping of the magnetic field, we investigated biradical in deuterated toluene. In experiment the original traces were recorded twice, with the pumping pulse (see Fig. 1) on and off. The ratio of the two traces is believed to represent a pure electron-electron contribution to ESEEM. The result of its cosine Fourier transforms is shown on Fig. 4. The sharp peak reflects the singularity of the Pake spectrum at 871/2. The highfrequency edge (ti\) of the Pake spectrum is less pronounced and hardly could be distinguished from the noise and artifacts, because of incomplete excitation of the corresponding spin packets ql, estimated as is close to In addition, the edges of the Pake spectrum in disordered samples are of rather low intensity. We studied also biradical dissolved in 5CB for the sample with the liquid crystal director d parallel to Bo. In this case the statistical weight of parallel orientations increases. The Fourier transform is shown on Fig. 5. From the position of the maximum slope at the high frequency edge of the spectrum the valuefil = k (14.4 0.3) MHz was determined. With the determined above (suggesting that it does not depend on the solvent), Eqs. give = 10.7 0.2 MHz (r = 1.94 0.02 nm), and < 0.1 MHz.
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4. DISCUSSION Our results show that the electron dipole-dipole interaction in biradicals in randomly oriented organic media may be detected employing the routine two-pulse ESEEM. The method has both instrumental and theoretical simplicity. It is applicable when electronelectron and electron-nuclear anisotropic interaction are well separated in the frequency domain. However, it is not free from the dead-time problem. The effectiveness of the method may be substantially increased employing the proposed RIDME experiment. This experiment involves the non-resonant partner spin in the formation of ESEEM via a relaxation-induced change of the local dipolar field experienced by the resonant spin. This approach works well at normal experimental conditions (nitroxide biradical at liquid nitrogen temperature). It is interesting to note that the relative importance of the electron-electron and electronnuclear interactions in ESEEM is completely different for thermally equilibrated biradicals and spin-polarized radical pairs. The result of this work shows that in the former case the electron-nuclear contribution prevails to a great extent. For the latter case the situation is perfectly opposite [ 17, 181. We examined how X-band field-step ELDOR may be employed for investigation of the electron dipole-dipole interactions in biradicals. This is an alternative to conventional double frequency ELDOR which was widely used previously [ l - 101. While variation of the offset between the excitation and observation frequencies in double frequency ELDOR is technically restricted, the magnetic field jump amplitude may be easily varied in a wide range. As the magnetization in field-step EPLDOR is stored along the Z-axis during the T-period, the inhomogeneity of the additional magnetic field AB does not affect the phase of the observed signal. The rise-time in our experimental conditions is about -1 ps. That does not influence the time resolution of the method because decay of the stimulated echo for nitroxides at the X-band at 77K occurs at much longer times.
ACKNOWLEDGEMENTS Authors are thankful to Yu.D. Tsvetkov for the interest to this work, to Yu.A. Grishin for developing electronics for the magnetic field jump, to I.A. Grigoryev for synthesis of biradical. This work was supported by grants from Russian Foundation for Basic Research, numbers 00-15-97321 and 00-03-40124, from Ministry of Education of RF, number 2000.5.106, and from CRDF, number RC1-2056.
REFERENCES 1. 2. 3. 4.
A.D. Milov, A.B. Ponomarev, Yu.D. Tsvetkov, Chem. Phys. Lett., 110 (1 984) 67. A.D. Milov, A.B. Ponomarev, Yu.D Tsvetkov, Zh. Struct. Khim., 25 (1984) 51. R.J. Larsen, D.J. Singel, J. Chem. Phys., 98 (1993) 5 134. H. Hara, A. Kawamori, A.V. Astashkin, T. Ono, Biochim. Biophys. Acta, 1276 (1996) 140.
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5. V. Pfannebecker, H. Klos, M. Hubrich, T. Volmer, A. Heuer, U. Wiesner, H.W. Spiess, J. Phys. Chem., 100 (1996) 13428. 6. R.E. Martin, M. Pannier, F. Diederich, V. Gramlich, M. Hubrich, H.W. Spiess, Angew. Chem. Int. Ed.,37 (1998) 2834. 7. A.D. Milov, A.G. Maryasov, Yu.D. Tsvetkov, Appl. Magn. Reson., 15 (1998) 107. 8. A.D. Milov, A.G. Maryasov, Yu.D. Tsvetkov, J. Raap, Chem. Phys. Lett., 303 (1999) 135. 9. A.D. Milov, A.G. Maryasov, R.I. Samoilova, Yu.D. Tsvetkov, J. Raap, V. Monaco, F. Formaggio, M. Crisma, C. Toniolo, Dokl. Akad. Nauk, 370 (2000) 265. 10. M. Pannier, S. Veit, A. Godt, G. Jeschke, H.W. Spiess, J. Magn. Reson., 142 (2000) 331. 1 1. V.V. Kurshev, A.M. Raitsimring, Yu.D. Tsvetkov, J. Magn. Reson., 8 1 (1989) 44 1. 12. A.V. Astashkin, Y. Kodera, A. Kawamori, Biochim. Biophys. Acta, 1187 (1994) 89. 13. A.V. Astashkin, H. Hara, A. Kawamori, J. Chem. Phys., 108 (1998) 3805. 14. S. Saxena, J.H. Freed, Chem. Phys. Lett., 251 (1996) 102. 15. P.P. Borbat, J.H. Freed, Chem. Phys. Lett., 313 (1999) 145. 16. G. Jeschke, M. Pannier, A. Godt, H. W. Spiess, Chem. Phys. Lett., 331 (2000) 243 17. S.A. Dzuba, P. Gast, A.J. Hoff, Chem. Phys. Lett., 236 (1995) 595. 18. S.A. Dzuba, A.J. Hoff, in “Biological Magnetic Resonance” (L.J. Berliner, S.S. Eaton, G.R. Eaton, Eds.) Kluwer/Plenum, New York, 2000, p. 569. 19. S.A. Dikanov, Yu.D. Tsvetkov, “Electron Spin Echo Modulation (ESEEM) Spectroscopy”, CRC Press, Boca Raton, (1992). 20. S.K. Rengen, V R. Bhagat, V.S.S. Sastry, J. Magn. Reson., 33 (1979) 227. 21. S.A. Dzuba and Yu.D. Tsvetkov, Khim. Fizika, 1 (1982) 1197. 22. A.A. Dubinskii, Yu.A. Grishin, and K. Mobius, to be published 23. V.F. Yudanov, K.M. Salikhov, G.M. Zhidomirov, Yu.D. Tsvetkov, Teor. Eksper. Khim., 5 (1969) 663. 24. M. Willer, J. Granwehr, J. Forrer, and A. Schweiger, J. Magn. Reson., 133,46-52 (1998).
25. L.V. Kulik, Yu.A. Grishin, S.A. Dzuba, I.A. Grigoryev, S.V. Klyatskaya, S.F. Vasilevsky, Yu.D. Tsvetkov, to be published. 26. I.A. Grigoryev, S.A. Dikanov, G.I. Schukin, L.B. Volodarskiy, Yu.D. Tzvetkov, Zh. Struct. Khim., 23 (1982) 59. 27. S. Van Doorslaer, G.A. Sierra, A. Schweiger, J. Magn. Reson., 136 (1999) 152. 28. L.V. Kulik, S.A. Dzuba, I.A. Grigoryev, Yu.D. Tsvetkov, Chem. Phys. Lett., 343 (2001) 315.
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EPR in the 21" Century A Kawarnori, J Yarnauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
The influence of label spins on EPR spectra of charge separated states in photosynthetic reaction center Kev M. SALIKHOV', Stephan G. ZECH2 and Dietmar STEHLIK2
' Kazan Physical-Technical Institute, Kazan, Russian Federation Fachbereich Physik, Freie Universitaet Berlin, Germany
A strategy is discussed to study electron transfer in photosynthetic reaction centers by inserting a third electron spin into the system in terms of a cysteine spin label which we call an observer spin. Here, the influence of such an observer spin on cw- and pulsed EPR experiments on spin-correlated charge separated states is investigated. As revealed by numerical simulations of a thresspin system, the additional spin-spin interactions between radical pair partners and the observer spin give rise to specific changes in the time-resolved spectra of the radical pair state P'700A-1 in PS I. Furthermore, the modulation of the out-of-phase electron spin echo is expected to be even more sensitive to the presence of additional spin-spin coupling. Implications for the directionality of electron transfer in PS I are discussed.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
679
The structural analysys of photosystem I1 by PELDOR of three spin system H Hara', A Kawamori2 and N. Katsuta2 'Bruker BioSpin K.K. Tsukuba, 305-0051 Japan, 2Faculty of Science, Kwansei Gakuin University, Sanda, 669-1337 Japan We applied the PELDOR (Pulsed Electron Electron Double Resonance) pulse sequence to a three spin system, in which three radicals ( Y i , Y i and QA-) are generated, in PS 11. The distance between YZ and QA is determined to be 34 A using the distances of the other radical pairs, Y D ' - Y ~29 A and YD' - QA- 38 A.
INTRODUCTION In higher plants photosystem I1 (PS 11) is composed of several intrinsic and extrinsic membrane protein complexes. Among them, the heterodimer of D1 and D2 proteins is believed to bind almost all electron transfer components of PS 11. In Fig.1 a supposed model structure based on a bacterial reaction center is shown. On the donor side, the primary electron donor P680 (a chlorophyll dimer), the secondary donor YZ (tyrosine-161 in D1 subunit) and a cluster of four Mn atoms in water-oxdizing complex (WOC) are included [l]. On the acceptor side, one of two pheophytin molecules accepts an electron from P680 and denotes it to the primary electron acceptor plastquinone (QA). Besides, there are several additional redox-active components, cytochrome b559, Chlz (a monomer chlorophyll donor) and one more redox-active tyrosin residue YD (tyrosine-161 in D2 subunit) [l]. To elucidate the location of the electron carriers in PS 11, the several pulsed EPR methods are applied [2-71. In previous work, the dipole interactions between the paramagnetic species in PS I1 were probed by a PELDOR(Pu1sed Electron eLectron Double Resonance) method [8]. The pulse sequence employed was 3-pulse electron spin echo (ESE) method, but the microwave (m.w.) pulses with two different frequencies were used. This method was applied to estimate the distance, 39 A, between and QA [9]. Recently the crystal structure of PS 11 of Synechococcus elongatus was analyzed with a resolution of 3.8 A [lo] but still the detail structure is not derived. The distance between YD and YZwas estimated 29 A using '2t1' ESE method [ll]. The '2t1' ESE technique used in [ l l ] is a special case of general Figure 1. A model structure of PELDOR methods. It employs a sequence of three Photosystem
680
m.w. pulses with the same carrier frequency and is applicable when the EPR transitions of the studied paramagnetic centers can be efficiently excited by the pulses used. In this work a PELDOR technique was applied to different radical pairs, consisting three radical species QA, YDand YZ and the distance between QAand YZwas determined in addition to so far obtained distances of QA-YDand YD-Yz. 2. EXPERIMENTAL
Oxygen evolving PS I1 membranes were prepared from market spinach using the method of Kuwabara and Muratra [12]. To prepare the Y;-Yz'-QA- radical system, Tris treatment was performed by incubating PS I1 membranes on ice for 30 min in 0.8 M Tris buffer at pH 8.5 under room light. After centrifugation, the pellet was suspended in ZnCl;! containing buffer to substitute non-heme iron by Zn2+ [13]. The pellet was centrifuged again and was transferred into Suprasil quartz tubes of 4 mm inner diameter with final chlorophyll concentration was about 20 mg/ml. The radical pairs Y;-Yi-QA-, were trapped by illumination of PS I1 sample for 20 sec at 253 K and immediately was put into liquid nitrogen. The PELDOR experiments have been performed on a pulsed EPR spectrometer ESP380 (Bruker) with a pulse sequence shown in Fig.2. The spectrometer was equipped with a cylindrical dielectric cavity (ER4117DHQ-H, Bruker) and a nitrogen gas flow system (CF935, Oxford Instruments). The measurement temperature was about 80 K and m.w. pulses of 80, 152, 152 ns duration were used. The pulse amplitudes for this sequence are adjusted to give spin rotation angles go", 180" and 180", respectively.
3. THEORY First, we consider a spin system composed of pairwise-distributed radicals, with every pair consisting of spins, A and B, SA = = 1/2. In the PELDOR method, three m.w. pulses were used to excite the spin system. The first and third pulses, separated by the time interval have a carrier frequency w1 resonant with the EPR transitions of spins A and form the primary ESE signal of these spins. The second pulse, separated from the first one by the time interval t (t 5: has a carrier frequency m. This pulse is resonant with spins B and changes and vice versa. If the magnetic dipole interaction between their projections from to C
Q)1
Figure 2. The pulse sequence of the PELDOR method.
Figure 3. Radical species excited by two m.w. carriers.
68 1
the pairwise-distributed spins is appreciable, the flip of the B spin changes the local magnetic field for its partner in the pair (A spin). a result, the magnetization of the A spins after the third pulse cannot be completely refocused at the time 2%and the amplitude the primary ESE signal exhibits an oscillating dependence on the second pulse position (i.e. on t). The spin Hamiltonian for this system can be written as;
in which WA and w are the resonance frequencies of the isolated A and B spins, and Dm represents the dipole coupling between A and B spins. The pulse sequence is shown in Fig.1. In the rotating frame with angular frequency w1, the spin Hamiltonian, including only these terms that affected the A spins, is;
W = (AWA + DmMsB)SzA
(2)
in which MsB is the quantum number associated with SZ" = * 112 and AWA= WA - w1 is the resonance offset. The echo amplitude normalized to unity at = 0, can thus be written as; V(2t) = C ~ S (Dmt )
(3)
where Dm is dipolar interaction between spins A and B [11. Second, we consider a special spin system composed of pairwise-distributed radicals, with every pair consisting of two of three different spins, A, B and C (Fig.3). We defined that 8 1 is the angle between the external magnetic field and the radius vector The angle between the external magnetic field and the radius vector rm , 8 3 are described by;
cost$ = cos8Acos81- sinOAsin81cosq
(4)
is the angle between the vector rm and r c A ,rm and ~ B Cand r c A and r B c (& +Bs +& And q is the rotation angle belonging to the vector rm. In this case, the spin Hamiltonian is written as;
= 180').
In a rotating frame with angular frequency terms that affected the A spins, is;
w
= (AWA + D
~
+ DM ~
w1,
the spin Hamiltonian, including only these
~M ~~ ~ ) s ~ ~
(6)
after calculating by density matrix formalism, the observed echo oscillation is shown as;
where DBAand DCAare z-component of dipolar interactions between each radical pairs. In non-oriented system, Eq.(7) is to be averaged over the angle 8 and
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From the dipole interaction constant D derived by Eq.(8), the distance between the spins A,B and C can be calculated using a point-dipole approximation.
4. RESULTS AND DISCUSSION The three radicals, and QA- were obtained by illumination of the Zn-substituted PS I1 at 253 K for 20 sec and then freezing at 77 K, as the radical already present in PS 11. The formation of the radicals was examined by continuous wave (CW) EPR as shown in Fig.4. However, it is difficult to trap these three radicals simultaneously and other radical pairs also QA-. present; and The PELDOR experiments was performed at a fixed value o f t = 1200 ns, with t varying from to 1200 ns. The magnetic field was fixed at the position = 9.685 GHz shown by an arrowhead (Fig. 4). YD and YZ signals were excited with first and third m.w. pulse and QA signals were excited with the second m.w. pulse. Open circles in Fig.5 show the dependence of the primary ESE amplitude on t, measured in the Zn-substituted PS I1 with trapped and QA- radicals. In our previous work, the distances between and YZ was determined to be 29 [ l l ] and the distance between YD and QA was determined to be 39 [9]. Using these values, we made simulation of the observed time profile to find the best fitted distance between YZ and QA. In a three spin system, the angles of the triangle encompassed by three radicals were fixed and should be taken into consideration as shown by eq.(4). The bold line shown in Fig.5 shows the values obtained by simulation. In the simulation, we estimated the ratio of each trapped radical systems were; QA- : 15 %, : 30 % and - QA : 55 %. The contamination of radical pair was generated due to incomplete Zn substitution. More than 30% of these particles still have QA- pair could be ascribed to incomplete calculated by eq.(3). The existence of illumination efficiency. As a result of these simulations, the dipole interaction constant D was found to be 1.32 MHz eq.(7). From the value D, the distance between YZ and QA is
3300
3320 3340 Magnetic Field (Oauaa)
3360
33 80
200
800
Tim
1000
1200
683
estimated to be 34 * 0.5 A. The error of 0.5 A has little important means, since it is much smaller than the molecular size of YZ and QA. Therefore, the round value r 34 A can be considered as a good fit for the distance between YZ and QA. Thus, in the three spin system, one can estimate the distance between and QA accurately
-
Figure 6. The distance relations between YD, and QA .
REFERENCES 1. A.-EMiller and G.W.Brudvig, Biochim. Biophys. Acta 1056 (1991) 1 2. R.G.Evelo, S.Styring, A.W.Rutherford, and A.J.Hoff, Biochim. Biophys. Acta 973 (1991) 428 3. J.B.Innes, and G.W.Brudvig, Biochemistry 28 (1989) 1116 4. Y.Isogai, S.Itoh and M.Nishimura, Biochim. Biophys. Acta 1017 (1990) 204 5. Y.Kodera, K.Takura and A.Kawamori, Biochim. Biophys. Acta 1101 (1992) 23 6. D.J.Hirsh, W.F.Beck, J.B.Innes and G.W.Brudvig, Biochemistry 31 (1992) 532 7. H.Hara and A.Kawamori, Appl. M a p . Reson.13 (1997) 241 8. H.Hara, A.Kawamori, A.V.Astashkin and T.Ono, Biochim. Biophys. Acta 1276 (1996) 140 9. KShigemori, H.Hara and A.Kawamori, J. Chem. Phys. 108 (1998) 3805 10. Zouni, H-T.Witt, J.Kern, P.Fromme, N.Kraub, W.Saenger and P.Orth Nature 409 (2001) 739 11. A.V.Astashkin, Y.Kodera and A.Kawamori, Biochim. Biophys. Acta 1187 (1994) 89 12. TKuwabara and N.Murata, Plant Cell Physiol. 23 (1982) 533 13. Jegerschoeld,C .MacMillan, ELubitz, W. and Rutherford,A.W., Biochemistry 38 (1999), 12439
684
EPR in the 2l‘Century A Kawarnori, J Yamauchi H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Application of pulsed ELDOR detected NMR measurements on the studies of photosystem H. MinoaTb,and T. Onoa a Laboratory for Photo-Biology (l),Photodynamics Research Center, The Institute of Physical and Chemical Research (RIKEN), 519-1399 Aoba, Aramaki, Aoba, Sendai 980-0845, Japan. b Graduate School of Science of Nagoya University, Furou, Chikusa, Nagoya, 464-8602, Japan.
Pulsed ELDOR measurements detected by spin-echo have been performed in order to detect NMR transitions. The pulsed ELDOR spectrum of tyrosine Do, radical in plant photosystem I1 (PS 11) showed well-resolved hyperfine structure, similar to that obtained by conventional pulsed ENDOR measurements. Mn2+ ions associated to the I1 membranes have been measured by pulsed ELDOR wide range of 10-1000 MHz.
1. INTRODUCTION Pulsed ENDOR is a well-established technique for obtaining information about nuclei that are magnetically coupled to an EPR-detectable species in frequency ranges of 0-1000 MHz. However, some special set up including the improvements of the ENDOR probes and preparation of many variation of RF amplifiers are required to measure the ENDOR spectra in the wide frequency range. ELDOR (Electron-electron doublet resonance) technique may be an alternative way to measure the nuclei transition. Schosseler et al. has proposed a pulsed ELDOR technique named “pulsed ELDOR detected NMR” as a new technique for detecting NMR transition [l], in which the ELDOR detected NMR transition is measured with FID irradiated with a Gaussianshaped pulse. In this work, we applied this technique to the biological system: plant photosystem 11. We present the obtained spectra and discuss the advantage of the pulsed ELDOR method, by comparison with the results obtained by pulsed ENDOR.
2. MATERIALS AND METHODS 2.1 Sample preparation The oxygen-evolving PS I1 membranes were prepared from spinach using the method of Berthold, Babcock and Yocum [2] with modifications described in [3]. The membranes were washed twice with a buffer (buffer A); 400 mM sucrose, 20 mM NaCl and mM Mes acid)/NaOH (pH 6.5), and
resuspended in the same buffer. For ELDOR measurement of Mn2+ bound to bio-membrane, the membranes were once washed with 30mM Tris/HCl (pH 9.7) and resuspended in the buffer A supplemented with MnC12.
2.2 Instruments Electron spin echo experiments were performed using a Bruker E580 pulsed EPR spectrometer equipped with a cylindrical dielectric cavity (ER4117DHQ-H, Bruker) and a gas flow temperature control system (CF935, Oxford Instruments). A microwave synthesizer (HP83751A, Hewlett-Packard) was used as a second m.w. source. The output of the synthesizer was fed into the resonator through the m.w. pulse-former unit of the pulsed EPR spectrometer and the TWT amplifier to produce the second pulse in the pulse sequence. In pulsed ENDOR measurements, the m.w. and radiofrequency (rf) pulse sequence introduced by Davies [4] was used. The rf pulse was amplified by a 500 W amplifier (EN1 500A).
3. RESULTS AND DISCUSSION Figure 1 shows the CW (a) and primary ESE (b) field swept ESR spectrum of the dark-stable Tyr Do, radical in Photosystem (PS) I1 membranes. The ESR spectrum has shown hyperfine splitting with an intensity ratio of approximately 1:3:3:1 which arises from coupling with one of the P-methylene protons and two equivalent C(3),(5)-ringprotons. Figure 2 shows the comparison of the pulsed ELDOR spectrum (b) and pulsed ENDOR (a) spectrum of the Tyr Do, radical at the magnetic field for the maximum of the field swept spectrum. Both spectra clearly show signals at 23-24 MHz and 27-28 MHz, which are ascribed to ring protons and one of the P-methylene protons, respectively.
time
Scheme 1: pulse sequence for pulsed ELDOR
686
Figure 3 shows the ESE field swept ESR spectrum of the membrane-associated MnZtions. Five transitions of electron spin states denoted as m s = -112 to +1/2 are overlapping in the spectrum. The center peaks in the spectrum arise from nuclear transitions of the six sublevel denoted as mI= -512 and mi= +5/2 of the electron spin transition m, = +112 ms= -112. Figure 4 shows the pulsed ELDOR spectra measured at the magnetic field of the respective peaks (a-f) in fig. 3. It is notable that very wide frequency ranges of 10-1000MHz can be covered by the pulsed ELDOR method. Based on the reported results of high frequency ENDOR of MnZt ions we assigned the peaks of about 120 MHz and 180 MHz to the transition of ms= -112 and +112, respectively. The peaks at higher magnetic field show peaks at higher NMR frequency, in a good agreement with the ENDOR results The peaks of about 350-380 MHz, 500-550 MHz and 700-750 MHz may correspond to the transition of ms= -312, +312 and -512, respectively. It is notable that the pulsed ELDOR method may give nuclear information equivalent to that for pulsed ENDOR, with much higher sensitivity and without any special modification. Therefore, the pulsed ELDOR method must be suitable for the measurements in various biological systems, of which spin density must be very low.
I
I
Magnetic field
Figure l:CW (a) and primary ESE field swept (b) ESR spectra of Tyr Do, radical in photosystem 11.
ELDOR Frequency [MHz]
Figure 2: Davies ENDOR spectrum (a) and pulsed ELDOR spectrum (b) of Tyr Do, radical in photosystem 11. Inside lines show simulation spectra.
687
Magnetic field
Figure 3: primar ESE field swept spectrum of Mn' ions associated with
I1 membranes.
I
,
I
.
I
,
I
.
0
ELDOR frequency [MHz]
Figure 4: pulsed ELDOR spectra of Mn2+ ions associated to PS membranes. The spectra were measured at the magnetic field for the respective peaks (a-f) in fig. 3.
REFERENCES 1. P. Schosseler, Th. Wacker, and A. Schweiger, Chem.Phys.Lett. 224 (1994) 319. 2. D. A. Berthold, G. T. Babcock and C. F. Yocum, FEBS Lett. 134 (1981) 231. 3. T. Ono and Y. Inoue, Biochim. Biophys. Acta 850 (1986) 380. 4. E. R. Davies, Phys. Lett. A47 (1974) 1. 5 . D. W. Randall, B. E. Sturgeon, J. A. Ball, G. A. Lorigan, M. K. Chan, M. Klein, W. H. Armstrong and R. D. Britt, J. Am. Chem. SOC 117 (1995) 11780.
688
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Pulsed-ENDOR cavities modified from the CW-ENDOR TM-mode and Pulsed-
ESR TE-mode cavities Jun Yamauchi" and Kanae Fujiib aGraduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan b
Analytical Instrument Service, JEOL Datum Co. Ltd., Akishima, Tokyo 196-8558, Japan
Facile modifications for pulsed-ENDOR measurements were investigated using conventional CW-ENDOR with a TMllo-mode cavity as well as the commonly-used pulsed ESR with a TEloZ-mode cavity. The high-loaded Q value of the CW-ENDOR cavity was reduced by over-coupling in the coupling iris and by inserting high dielectric material (water) inside the cavity. On the other hand, a Z-coil device was utilized for the introduction of an rf magnetic field to the TE-mode cavity. The ENDOR effect was observed in both cavities; however, both methods have advantages and drawbacks. On the basis of this investigation, a new type of TE-mode cavity was proposed.
1. INTRODUCTION The recent development of pulsed ESR spectroscopy has enabled us to investigate a variety of fields, one of which is the combination of a radio-frequency (rf) pulse among microwave frequency pulses. early as 1965, rf resonance transitions were induced during the time interval between the second and the third pulse of a stimulated echo sequence, causing a fall in the echo signal amplitude [11. Many pulsed-ENDOR devices, especially cavity systems, have been reported so far, many of which are special purposes [2-61. Recent techniques and progress were reviewed by Schweiger et al. [7, 81. Some textbooks have fully dealt with pulsed ENDOR explanations [9, 101. In this paper, facile modifications of conventional ENDOR or commonly used pulsed-ESR into pulsed-ENDOR methods were examined. We have made a preliminary report on pulsed ENDOR utilizing a typical continuous-wave (CW) ENDOR in which a TMllrmode cavity was modified in order to obtain pulsed-ENDOR signals [ll]. In this case, a pulsed rf wave was easily led into an rf coil within the cavity to generate enough NMR transitions to decrease electron spin echo intensities. However, some problems regarding high-loaded Q value and weak microwave power have to be circumvented for intense electron spin echo signals. for a conventional pulsed-ESR cavity, an rf coil becomes the most important attachment, regarding how to introduce rf power inside the cavity. Facile developments of these two
689
conventional methods into pulsed-ENDOR observation would be useful to obtain additional information by small variations. In this paper, those characteristics are summarized, and a new type of TE-mode cavity is proposed.
2. EXPERIMENTAL All experiments in this paper were carried out at room temperature. For the sake of proper echo intensity, we used a polycrystalline phenoxyl radical dispersed in the diamagnetic precursor phenol, which was found to be useful as a standard for ESR, ENDOR, and pulsed ESR investigation [121. The sample used exhibited spin-spin relaxation time (phase memory time), Tz=1.12 ,us, and spin-lattice relaxation time, T1=5.94 ,us. This long value at room temperature is helpful for stimulated echo observation and generally for two-pulse spin echo experiments. Pulsed-ENDOR measurements were performed using a JEOL PX1050 spectrometer furnished with a standard TEloz-mode cavity [12]. Many reports dealing with a TM-mode cavity for CW-ENDOR, especially TRIPLE resonance, have been published [12-171. A simple block diagram of our experiments is shown in Figure 1. The microwave pulses for the detection of spin echoes were supplied to the cavity through a TWT amplifier of 1 kW, and the rf pulse was generated in the mixer depending on the signal from a pulse generator, after which the rf wave was amplified using a broad-band power amplilier (EN1 M O O ) . In typical pulsed-ENDOR experiments, there exist two pulse sequences developed by Davies and Mims [18, 11. Because of the necessity of a long rf pulse duration, we mainly used Mims' pulse sequence, in which an rf pulse is added between the second and the third microwave pulse in the stimulated spin echo.
3. RESULTS AND DISCUSSION 3.1. Modification of the TM-mode cavity and the ENDOR effect Details of the structure of a TMllo-mode cavity were given earlier. A helical rf coil in the center of the cavity works for strong irradiation of an rf pulse [18, 191. Thus, a large ENDOR effect is anticipated. On the other hand, the microwave power around the samples inside the helix coil is reduced, which could cause the problem of lack of 90" rotation in full microwave power, resulting in a weak spin echo intensity. The most important point of the modification of conventional CWENDOR is reducing the loaded Q value of the cavity. This was circumvented by over-coupling (an enlarged iris) and the insertion of a high dielectric substance (water). The latter was also advantageous in reducing and deleting microwave ringing when arranging Figure 1. Block diagram of the pulsed ENDOR observation conditions. The latter is apparatus
690
shown in Figure 2, in which a position-adjustable devise as well as a measurable one is inserted from the optical irradiation window, and a helical rf coil for NMR transitions with a 10 mm diameter and 20 turns can be seen (variable depending on rf and microwave conditions). One of the severe drawbacks is the weak microwave power generated in the TM-mode cavity, resulting in insufficient echo intensity to observe a variety of samples, which necessitate long TI and Tz values. As a matter of fact, longer microwave pulse duration is needed even in the highest microwave power of the spectrometer. The following data is one example of the experimental conditions; Microwave frequency 9.4 GHz, Microwave pulse width 150 ns, rf 14.23 MHz, and rf pulse width 5 ,us. Each segment of the four pulses was 700, 200, 5100, and 650 ns, respectively, in Mims’ ENDOR pulse sequence. Due to the weak stimulated echo in this cavity, we could not extend the rf pulse duration by more than 10 ,us. By lowering the temperature, this situation would be improved. At the above-mentioned experimental conditions, the ENDOR effect (echo intensity ratio between the rf pulse on and off conditions) amounted to 60 %. This value is much better than that of the next TE-mood cavity. The reason may be attributable to a strong rf power generated from the helix coil in spite of the weak echo intensity.
3.2. Modification of TE-mode cavity and ENDOR effect The conventional pulsed ESR cavity adopts a TEloZ-mode rectangular cavity, in which high microwave power is advantageous. The modification deals with the introduction of an rf pulse around the samples. In order to satisfy mutual orthogonality for static, microwave, and rf fields, an rf coil bobbin was inserted in the cavity from the bottom. Figure 3 depicts the inserted coil and the device for the deletion of microwave ringing. The bobbin is 7 mm in diameter, and the two-turn coil has a 30 mm length. The coil of this system is a Z-coil, which, it was finally revealed, was unable to exert a high rf field on the samples. This means that there was a weaker ENDOR effect than with the TM-mood cavity. The following data is one example: Microwave frequency 9.05 GHz, Microwave pulse width 50 ns, rf 13.73 MHz, and rf pulse width 15 H S . Each segment of the four pulses was 350, 350, 15100, and 340 ns, respectively, in Mims’ ENDOR pulse sequence. Because of the strong echo height, the rf
Figure 2. TM-mode ENDOR cavity
Figure 3. TE-mode ENDOR cavity
69 1
pulse duration could be extended at room temperature as long as 15 ,LLS and more. Under these conditions, the ENDOR effect resulted in only 20 %, probably due to the weak rf power of the Z-coil system. However, one can accomplish a strong echo intensity in the TE-mode cavity so that one can obtain good S/N ratio in the ENDOR spectrum. When tthese data are considered collectively, along with the ease of variation, this type of modification is widely applicable to many samples. In the last section we will propose a new TE-mode cavity with an rf coil inside to solve the problem of the weak rf field.
3.3. ENDOR spectrum of a phenoxyl radical The rf frequency was swept under the observation of stimulated echoes, in which the echo intensity decreases when an NMR transition takes place, leading to an ENDOR spectrum. Pulsed ENDOR spectra were measured using a polycrystalline phenoxyl radical dispersed in the diamagnetic precursor phenol, for which CW-ESR, CW-ENDOR, and ESEEM spectra were reported [12]. The pulsed ENDOR spectra obtained under the conditions previously mentioned exhibited a broad ENDOR effect around the free proton frequency in both cavities. The spectra were not well resolved, compared with the CW-ENDOR spectrum. The reason for this may be attributable to a short rf pulse time. Theoretical ENDOR spin echo spectroscopy indicated that the duration of the rf pulse is very important in determining the ENDOR line width [20]. The line width of the ENDOR spin echo spectrum increases with decreasing the rf pulse time, and the effect is observed even if pulse strength is varied so as to maintain a constant turning angle. Because of the restriction of any pulse duration within 20 ,US in the spectrometer used at present, we could not show genuine pulsed ENDOR, but we obtained well-resolved spectra under a longer rf pulse, that is, continuous. Figure shows stimulated echo variations that depend on the radio frequency supplied and yield almost the
I
I
13
I
I
I
14
I
15
I
I
16
FREQUENCY/MHz Figure 4. Stimulated echo intensity variation depending radio frequency.
692 I
13
FREQUENCYMHz Figure 5. Differentiation of the central part of Figure 4 (dotted curve), which is comDared with the CW-ENDOR snectrum (ref. 12). same response as with the CW spectrum. Figure 5 gives a more detailed picture of the central part of the free proton region, which was differentiated to obtain a derivative spectrum comparable to the CW spectrum. A more careful manipulation of the experimental conditions, such as lower temperature can result in the achievement of sufficient ENDOR spectra using easily modified pulsed-ESR spectrometer. 3.4. Proposed ENDOR cavity
Further improvement of the TE-mode cavity for ENDOR may involve strengthening of the rf pulse field. The Z-coil system in this case has only two turns or a little more and quite limited rf power. In order to use a helical rf coil around the samples, one possibility is rotating the cavity mode so as to contain a vertical set-up of the coil. Using the same TEloz-mode, it is possible to arrange three mutually orthogonal magnetic fields, that is, a static field, microwave, and radio-frequency wave. Figure 6 indicates this set-up, containing an inserted helical coil in the vertical direction. The experimental results will be published elsewhere. The authors are grateful to Drs. Yukio Mizuta and Yoshio Iima of JEOL Co. Ltd. for giving them instructive suggestion and manufacturing various devices in this research.
693
Figure 6. TE-mode cavity with a helical rf coil in the vertical direction.
REFERENCES 1. 2. 3. 4. 5. 6.
W. B. Mims, Proc. Roy. SOC.London A, 283 (1965) 452. W. B. Mims, Rev. Sci. Instrum., 45 (1974) 1583. J. L. Davis and W. B. Mims, Rev. Sci. Instrum., 49 (1978) 1095. E. J. Reijerse and A. A. K. Klaassen, Rev. Sci. Instrum., 57 (1986) 2768. S. Pfenninger, J. Forrer, and A. Schweiger, Rev. Sci. Instrum., 59 (1988) 752. J. Forrer, S. Pfenninger, J. Eisenegger, and A. Schweiger, Rev. Sci. Instrum., 61 (1990) 3360. 7. A. Schweiger, Angew. Chem. Int. Ed. Engl., 30 (1991) 265. 8. C. Gemperle and A. Schweiger, Chem. Rev., 91 (1991) 1481. 9. A. J. Hoff, Advanced EPR-Applications in Biology and Biochemistry, Elsevier, 1989. 10. L. Kevan and M. K. Bowman, Modern Pulsed and Continuous-Wave Electron Spin Resonance, John Wiley & Sons, Inc., 1990. 11.J. Yamauchi, K. Kanemoto, and K. Fujii, Proceeding of the APES '99, (1999) 53. 12. J. Yamauchi, K. Katayama, M. Tamada, and S. Tanaka, Appl. Magn. Reson., 18 (2000) 249
13. W. Weltner, Jr. and R. J. Van Zee, Rev. Sci. Instrum., 57 (1986) 2763. 14. J. Yamauchi, K Okada, and Y. Deguchi, Bull. Chem. SOC.Jpn., 60 (1987) 483 15. J. Yamauchi and Y. Deguchi, Canad. J. Chem., 66 (1988) 1862. 16. J. Yamauchi, H. Fujita, and Y. Deguchi, Bull. Chem. SOC.Jpn., 64 (1991) 3620. 17. J. Yamauchi and H. Fujita, Bull. Chem. SOC.Jpn., 66 (1993) 2505. 18. E. R. Davies, Phys. Letters, 47A (1974) 1. 19. K. Mobius and R. Biehl, Multiple Electron Resonance Spectroscopy, ed. M. M. Dorio and J. H. Freed, Plenum Press, New York, 1979, Chapter 14. 20. A. E. Stillman and R. N. Schwartz, Mol. Phys., 35 (1978) 301.
694
EPR in the 21" Century A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Ferroelectric resonators for EPR spectrometers at 35,65 and 125 I. N. Geifman" and I. S. Golovinab "Oakton Community College, 7701 N. Lincoln Avenue, Skokie,
60077, USA
bInsititute of Physics of Semiconductors of NASU, Pr. Nauki 45, Kiev 03028, Ukraine
As it was shown recently [ 1, 21, application of a ferroelectric resonator made from singlecrystal potassium tantalate and placed on a sample holder in a standard EPR cavity TEoll, in X-band CW EPR experiments allows us to increase signal-to-noise ratio in 10-50 times, depending on the resonator type (geometry and shape). A ferroelectric resonator can significantly improve EPR sensitivity due to increasing the filling factor and the magnetic field at the sample, and may be especially useful for studying small, non-saturable samples. In the Pulsed EPR experiments the use of a ferroelectric resonator leads to the reduction of the incident power in 50 times, so, e.g. a ferroelectric W-band resonator could permit much shorter d 2 pulses than are now possible. Here we present the determination of the dimensions of the ferroelectric resonators for EPR spectrometers at high frequencies, 35, 65 and 125 GHZ. The results of the calculations and possible resonator materials are discussed. 1. INTRODUCTION
One of the most important characteristics of the EPR spectrometer is its sensitivity that can be expressed in terms of the signal-to-noise ratio (SIN). Advances in sensitivity can be provided by development of new types of dielectric resonators. It is known that the S/N is proportional to the square root of the incident power P, resonator quality Q and filling factor i.e. S I N P'"Qq (1) Here q=&2/fH2dV, where is the magnetic field on a sample, H is the microwave magnetic field in cavity, V is the cavity volume. Usually the magnitude of & equals the microwave magnetic field amplitude in the cavity HdH'dV, i.e. W=l. If one places in the cavity a material with a high dielectric constant E and low losses, Hs increases so that [3], and thus, q d 2 . increasing the filling factor leads to increasing the sensitivity, so does increasing the dielectric constant of dielectric insert (or resonator). Ferroelectric materials have an order-of-magnitude higher value of E than conventional dielectric ceramics used for current dielectric resonators have. When using a ferroelectric resonator (FR), the microwave magnetic field at the sample becomes higher considerably, according to (l), resulting in a dramatic signal enhancement. Low dielectric losses let the resonator quality Q be improved, also resulting in increasing S I N ratio. Earlier [2, 41 it has been shown that the ferroelectric resonators from KTa03 significantly improve EPR signal intensity at X-band frequency range. Since the last decades high-
695
frequency and multi-frequency EPR experiments became demanded for solving a lot of scientific problems, including in biology and medicine, it is important to expand the application of the ferroelectric resonators to high-frequency EPR. In this work we present the calculationsof dimensions of the resonators from KTaO3 for 65 and 125 frequencies. 2. TaEORY
The developed ferroelectric resonators from KTa03 tested at X-band [1, 2, 41 have a bore (or hole) for introduction the samples. The bore is drilled down the length, L, of the resonator. As a first approximation, in the calculations we consider an electromagnetic model of a solid cylindrical and rectangular resonators for symmetrical modes with continuous tangential components of the electromagnetic fields on the boundary surfaces. We assume that a ferroelectric resonator has the "magnetic" walls along any axis for cylindrical FR and axis z for rectangular FR as shown in Figure 1 [ 5 ] . It means that a wave does not reflect completely from the walls inside the ferroelectric resonator, i.e. a part of a wave penetrates the resonator boundary surfaces and exponentially decays out of it. In this case we will compute the resonant oscillations of modes, where 6 is a factor of wave attenuation outside resonator. We also assume that a standard cavity where a ferroelectric resonator is placed, does not affect the resonant frequency of a FR, so in electromagnetic model we consider a FR as an open resonant structure. The computation starts with the Helmholtz vector equation (quasi-stationary oscillations) [61: v2n'w) + (2) where k==lc and k=os'"lc is a wavenumber in air and in the ferroelectric resonator, respectively. The vector equation reduces to the scalar one for the solution in cylindrical coordinate system, i.e. instead of (2), we have: vzn("),+ Pn(m),=o (3)
v"),
+
n'"),
(4)
regarding to the longitudinal magnetic and electric components of Hertz vectors. Omitting unwieldy computations, the details of the model are explained elsewhere [6], we present here the final expressions, which were used in the calculations. For cylindrical FR: f = (p2+p:)1/2d(27cE1"), p,tg(LpJ2) = (p'-p0')'2, (5) where p=2B,,,,,lD, pz=7c6/L, p0=2xfjc;B, is the m-th root of a Bessel equation, is the order of a Bessel function; D is the resonator diameter, L is a height of the resonator; 6 is a factor of wave attenuation outside resonator, c is the speed of light; and E is the resonator constant. For rectangular FR: f = (pxz+pyz+p~)'~2c/(2~E1~z), pJg(LpJ2) = (pxz+pyz-pO')'", (6) where pX=mdA, Pz=z6L, p,=27cf/c; A, B, and L are the resonator width, length and height, respectively; m, and n are whole numbers corresponding to the Hmnsmode; 6 is a part of a half-wave in the resonator along the length (axis z). We assume that the hole for introduction the samples affects the resonant frequency and outside dimensions of the model resonators, so we add the cross-section square, S, of the hole to the calculated dimensions.
696
3. RESULTS AND DISCUSSION
Previously [1,2,4], eight ferroelectric resonators from single crystal of KTaO3 were made and investigated. This material was chosen due to the unique property - its dielectric losses decrease while the dielectric constant increases with decrease the temperature [7]. Changing the dielectric constant results in appearing multi-mode resonance at the temperature dependence of EPR signal intensity. We determined the type of Hms mode (i.e. m, n and 6) for each resonant peak observed in experiment. analysis of the obtained data indicated, in both rectangular and cylindrical resonators the lowest Hlos, H I Imodes ~ appear. Therefore, the calculations of the rectangular and cylindrical ferroelectric resonators, were made to obtain which could be employed in radiospectrometers at 35, 65 and 125 also the lowest modes, i.e. when m=l, and m=n=l. The results are presented in the Tables 1 and 2. The notations in the tables are explained in Figure 3 and Figure 4. In the present calculations, the temperature dependence of dielectric constant of KTa03 has been taken. As we can see from the tables, the dimensions of the resonators from KTa03 at high frequencies and especially at helium temperature, due to the grown dielectric constant, become extremely enough (0.2-0.5 micrometers). For example, a rectangular FR for 125 with dimensions of lxlxl mm3 at the temperature of 4.2 K has thickness of 0.18 or of 0.36 for Hlos and Hlls (6=0.988) modes, respectively. For a cylindrical resonator for 125 GHz with height = 2 mm and diameter D1 =1 mm, the thickness is 0.4 for Hlos and 0.55 for Hlls (6=0.994), respectively. Note that the higher mode leads to the increasing the thickness. Anyway, making such resonators from a single-crystal bulk seems difficult enough, so for high-frequency and low-temperature experiments it would be easier to develop thin-film FR or introduce additional chemical treatment of single-crystal FR to thin it down. The other way is making a resonator from the other material with a lower dielectric constant, e.g. rutile TiOz. To provide the possibility of applying various materials, with different E, for a certain working frequency we computed the values of L & vs. L/D for the cylindrical resonators for Hlos mode, presented in Figure 2
h
P
F xE Z
L Figure 1. Model of the field configuration along axis z of a FR.
Figure 2. Theoretical values of Lf& vs. L/D for cylindrical resonators (H~os mode).
Table 1 Rectangular ferroelectric resonators for EPR spectrometers at 35, 65 and 125 GHZ No ResoWidth A=B Height Length = S ofhole Depth v nator fll~ mm mm =L mm2 ofhole MI+
T
E
r n n
6
K
m 1.
#1
2.001
0.0632
2.001
3.0
4.0
3.0
37509
4.2
4000
1
0
0.987
2. 3. 4. 5. 6.
#1
2.002
0.0895
2.002
3 .O
4.0
3.0
37458
4.2
4000
1
0.987
#1
2.005
0.1415
2.005
3.0
4.0
3.0
36589
77
840
1 1
0
0.971
#1
2.010
0.2002
2.010
3.0
4.0
3.0
36573
77
840
1
1
0.971
#1
2.015
0.2454
2.015
3.0
4.0
3.0
37923
300
261
1
0
0.950
#1
2.030
0.3477
2.030
3.0
4.0
3.0
37852
300
261
1
1
0.950
~~~
7. 8.
#2 #2
1.0006 1.0012
0.0346 0.0490
1.0006 1,0012
1.0 1.o
1.0 1.0
1.0 1.0
68538 68443
4.2 4.2
4000 4000
1 1
0 1
0.978 0.978
9.
#2
1.003
0.0775
1.003
1.o
1.0
1.0
66916
77
840
1
0
0.953
10.
#2
1.006
0.1097
1.006
1.o
1.0
1.0
66856
77
840
1
1
0.953
11.
#2
1.010
0.1418
1.010
1.o
1.o
1.0
65985
300
261
1
0
0.918
12.
#2
1.020
0.2010
1.020
I .O
1.0
1.0
65835
300
261
1
1
0.918
13. 14.
#3 #3
1.0028 1.0056
0.0749 0.1060
1.0028 1.0056
1.o 1.0
1.0 1.0
124192 124105
300 300
261 261
I
0.954 0.954
#3
1.0009
0.0424
i.ooo9
1.0
1.0
122083
77
840
#3
1.0018
0.0600
1,0018
1.0
1.0
122007
77
S40
#3
1.00018
0.0190
1,00018
1.0
1.0
124762
4.2
4000
1 1 1 1
0 1
15. 16. 17. 18.
1.o 1.0 1.o 1.o 1.o
#3
1.00036
0.0268
1.00036
1.0
1.o
1.0
125088
4.2
4000
1
0
0.974
1
0.974
0
0.988
1
0.988
Table 2 ferroelectric resonators for EPR spectrometers at 35, 65 and 125 GHz No. ResoL Di d D h T nator mm mm m m m m mm MHZ 1. 2. 3. 4. 5.
#1 #1 #1 #1 #1
2.0 2.0. 2.0 2.0 2.0
2.0025 2.006 2.012 2.04 2.10
2.0 2.0 2.0 2.0 2.0
0.1000 0.15503 0.2194 0.4020 0.6403
2.0 2.0 2.0 2.0 2.0
36219 37000 36099 35438 35313
4.2 4.2 77 300 300
4000 4000 840 261 261
6. 7. 8. 9. 10.
#2 #2 #2 #2 #2
2.0 2.0 4.0 2.0 2.0
2.0007 2.002 1.008 2.012 2.03
2.0 2.0 1.0 2.0 2.0
0.0529 0.0890 0.1267 0.2194 0.3477
2.0 2.0 4.0 2.0 2.0
68438 64096 62360 64761 64706
4.2 4.2 77 300 300
4000 4000 840 261 261
11. 12. 13. 14. 15.
#3 #3 #3 #3 #3
2.0 2.0 2.0 2.0 2.0
1.0004 2.00055 1.0020 1.0066 2.008
1.0 2.0 1.0 1.0 2.0
0.0283 0.04691 0.0633 0.1151 0.1791
2.0 2.0 2.0 2.0 2.0
128021 122237 124907 123269 125432
4.2 4.2 77 300 300
4000 4000 840 261 261
t Wdth
Figure 3. Schematic picture of a rectangular
k Figure 4. Schema
699
4. CONCLUSIONS
The dimensions of the ferroelectric resonators (if made from KTa03) at high frequencies become too small for conventional cutting technique used before for making the resonators at X-band. For making the resonators for high-frequency and/or low-temperature EPR experiments it should be developed a thin-film technique, introduced additional chemical treatment,or applied the other material with lower dielectric constant than that of KTa03.
Acknowledgments The authors would like to thank T. V. Son’ko for growing the single crystals of KTa03 and express the special thanks to Prof Gareth R. Eaton for the attempts in his laboratory and fruitful discussions. REFERENCES 1. I.N. Geiifman, I.S.Golovina, V.I.Kofman, E.R.Zusmanov, Ferroelectrics, 234 (1999) 81. 2. I.N. Geifman and I.S. Golovina, Ferroelectrics(to be published). 3. P. Hedvig, ActaPhysicaHingaricae, 10 (1939) 115. 4. I.N. Geiifman, I.S. Golovina, E.R. Zusmanov and V.I. Kofman, Technical Physics, 45 (2000) 263. 5 . A. Okaya and L.F. Barash, Proc. 50 (1962) 2081. 6. M.E. Ilchenko (ed.), Dielectric resonators, Radio i Moskva, 1989. 7. I.M. Buzin, I.V. Ivanov, N.N. Moiseev and V.F. Chuprakov, Fiz. Tv. Tela, 22 (1980) 2057.
700
EPR in the 2 1 Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Fourier-transform ESR spectroscopy and observation of ultrafast spin-lattice relaxation by optical means T. Kohmoto. Y. Fukuda and M. Kunitomo Department of Physics, Faculty of Science, Kobe University, Kobe 657-8501, Japan Ultrafast spin dynamics in the ground state of the thulium ion doped in calcium fluoride are studied by using ultrashort laser pulses. Quantum-beat free-induction decay in the subnanosecond region and ultrafast spin-lattice relaxation in the picosecond region near room temperature were observed by the polarization spectroscopy with the optical pump-probe technique. Such ultrafast spin dynamics cannot be observed by the conventional electron spin resonance.
1. INTRODUCTION
We report on the Fourier-transform ESR spectroscopy and the observation of ultrafast spin-lattice relaxation in the ground state of Tm2+ in CaFz achieved by optical means without any use of a microwave or rf source. The magnetic circular dichroism of the optical transition to the absorption band is responsible for the creation and the detection of the magnetization. The magnetization is created in the ground state by a circularly polarized pump pulse, and, using a polarization sensitive technique, the dynamics of the magnetization in the subnanosecond and picosecond regions is detected through optical anisotropy by a probe pulse. Such ultrafast spin dynamics cannot be measured by the conventional ESR, whose time resolution is nanoseconds at best. The time resolution of our optical method is limited only by the temporal width of the light pulses, and investigation of ultrafast spin dynamics can be realized. The spin Hamiltonian of the ground state of Tm2+ ion in the magnetic field H is given by PI
with effective electron spin S=1/2 and Tm nuclear spin I = l / 2 , where g=3.451, A=-1101 MHz, and the nuclear Zeeman interaction is neglected. ESR free-induction-decay (FID) signal, which we refer to as quantum-beat FID, was observed as ground-state quantum beats in low magnetic fields (< 1 kOe). The magnetization perpendicular to the magnetic field is induced in the ground state by the pump pulse, and free precession of the magnetization is detected by the probe pulse. The Fourier transform of the observed FID signal gives the ESR spectrum. The spin-lattice relaxation time TI was obtained from the decay curve of the magne-
70 1
tization parallel to the magnetic field. The value of which is of the order of seconds at liquid helium temperatures, becomes of the order of picoseconds near room temperature. The spin-lattice relaxation in the higher-temperature region is caused by the Raman process of phonons, where the temperature dependence of the relaxation rate for the Kramers ion is expected as l/T1 T9. The values of l/T1 obtained in our experiment, however, are deviated from this T g dependence. To explain the experimental data, we consider a theory of the Raman process including the Debye temperature of the host crystal. Using this theory the observed temperature dependence of l/Tl is explained well.
2. EXPERIMENT The pump pulse is provided by the second harmonics of a TiSapphire regenerative amplifier and the probe pulse by a optical parametric amplifier. The circularly-polarized pump beam and the linearly-polarized probe beam are nearly collinear and focused on the sample (Tm2+:CaF2, 0.02% Tm2+) in a refrigerator. The waist size of the beams at the sample is about 100 pm. The wavelength, the pulse energy, and the pulse width at the sample are 400 nm, -1OpJ, and 0.2 ps for the pump pulse and 560 nm, -1pJ, and 0.2 ps for the probe pulse. Because of the selection rules of the optical transition, population differences (or magnetization) in the magnetic sublevels of the ground state, whose electron spin is l / 2 , are instantaneously created by the circularly-polarized pump pulse. Then the circular dichroism of the optical transition is induced in the sample. The created magnetization is detected by a quarterwave plate and a polarimeter as the change of the polarization of the linearly-polarized probe pulse due to the induced circular dichroism. The time evolution of the magnetization was observed by changing the optical delay between the pump and probe pulses. A chopper (180 Hz) for the pump beam and a lock-in amplifier were used to attain higher signal-to-noise ratio. In the experiment of the quantum-beat FID, magnetic field of 0 - 1 kOe is applied perpendicular to the laser beams. In the experiment of the ultrafast spin-lattice relaxation, on the other hand, a magnetic field of 200 Oe is applied parallel to the laser beams. The spin-lattice relaxation time was obtained from the decay curve of the magnetization. For long relaxation times (> 100 ns) at lower temperatures, the decay curve of the magnetization was monitored by a cw probe light of 610 nm from a dye laser, instead of the probe pulse, and the spin-lattice relaxation time was obtained from the waveform digitized on an oscilloscope.
3. QUANTUM-BEAT FID
Quantum-beat FID signals for = 0, 350, 640, and 1000 Oe observed at 60 K are shown in Figure l(a), where the Zeeman coherences in the ground state are created by the pump pulse, and the free oscillation of the magnetization is detected as the polarization change of the probe pulse versus the probe delay time. As the magnetic field is increased, the oscillation period is decreased to the order of hundred picoseconds. The decay time
702
Quantum beats
(b) Fourier spectra
.r(
?
350 Oe
.El
SO h
0
.r(
v1
640 Oe
0 I
m 1000 Oe
8 Delay Time (nsec)
0 0
00 8 Frequency (GHz)
Figure 1. (a) Quantum-beat FID signals of Tm2+:CaF2 in the magnetic field perpendicular to the pump beam observed at 60 K and (b) their Fourier transform.
of the quantum-beat FID signals is determined by the superhyperfine interaction between the Tm2+ ions and surrounding fluorine nuclei. The Fourier transform of Figure l ( a ) is shown in Figure l ( b ) , which gives the ESR spectra. As the magnetic field is increased, the resonance frequencies of the signals on the ESR spectrum are increased as the result of the increase of the Zeeman splitting. The expected energy splittings corresponding to the resonance frequencies in the strong-field limit are A/2 for = f 1 / 2 .
4. ULTRAFAST SPIN-LATTICE RELAXATION
4.1. Experimental result Decay curves of the magnetization at 140 K, 210 K, and 298 K are shown in Figure 2. The decay curve of the magnetization is fitted well to a single exponential. The obtained spin-lattice relaxation times are 60 ps, 10 ps, and 5 ps for 140 K, 210 K , and 298 K ,
703
n
1014
T!
rn
10'2
108
."
106
B
104
102 100 1
-20
0
40
80
Delay Time (ps) Figure 2. Decay curves of the magnetization in Tm2+:CaFz at (a) 140 K, (b) 210K, and (c) 298 K. The obtained spin-lattice relaxation times are 60 ps, 10 ps, and 5 ps, respectively.
10
100
1000
Temperature (K) Figure 3 . Temperature dependence of the spin-lattice relaxation rate in Tm2+:CaF2. 0 ; present work, x ; Sabisky and Anderson [a]. The solid lines are obtained (a) from Eq.(2) and (b) from Eq.
respectively. Temperature dependence of the spin-lattice relaxation rate l/Tl is shown in Figure 3. The data at the lower temperatures are those which have been obtained from the experiment of ESR [ 2 ] , where the spin-lattice relaxation is explained by the direct process and the Raman process of phonons. The value of TI ranges from seconds at liquid helium to picoseconds near room temperature. Thus, in this sample, the spin-lattice relaxation time over eleven orders of magnitude can be measured by using the present optical method. The spin-lattice relaxation at the higher temperatures is caused by the Raman process of phonons, and the temperature dependence of the relaxation rate for the Kramers ion is expected as l/T1 which is shown by the straight line (a) in Figure 3. The values of l/Tl obtained in our experiment at the higher temperatures, however, deviate from this dependence.
4.2. Analysis In the ESR study, the spin-lattice relaxation at the higher temperatures has been explained by the Raman process of phonons. The spin-lattice relaxation rate due to the Raman process for the Kramers ion has been fitted to the equation 1
-=
Ti
704
and the coefficient A have been determined for Tm2+:CaF2; A = 7.6~10-' sP1KP9 [2]. The straight line (a) in Figure 3 is obtained from Eq.(2). In our experiment at the higher temperatures, the values of the spin-lattice relaxation rate deviate from the Tg dependence. Here, to explain this experimental results, we consider the Debye model of the lattice vibration. Turning back to the beginning of the theory of the Raman process, the relaxation rate due to the Raman process for the Kramers ion is expressed as [3]
where is a constant, w is the phonon frequency, and w, is the maximum phonon Equation (3) means frequency corresponding to the Debye temperature OD (h.w,=lcOD). that the rate of the Raman process is due to the phonons with frequencies below w,. In the case of low temperatures (T << O D ) , the upper limit O D / T of the integral in Eq.(3) can be regarded as infinity, and the integral becomes a constant. Then the wellknown expression in Eq.(2) is derived. In almost all of the conventional ESR studies on the relaxation, << OD is a good approximation, and Eq.(2) is valid for the relaxation analysis. In the case of higher temperatures (T 2 O D ) , the integral in Eq.(3) can no longer be regarded as a constant but depends on the temperature. In such a case, as in the case of our experiment, we have to consider the relaxation rate in Eq.(3) taking account of the temperature-dependent integral. Equation (3) can be rewritten as
When O D / T + 00, the integral approaches 8!, and f ( T / O D ) + 1. Then Eq.(4) becomes same with Eq.(2). The Debye temperature for CaF2 is known as OD = 513 K [l].The curve (b) in Figure 3 is obtained from Eq.(4). Equation (4) explains our experimental result. This shows that the deviation from the T g dependence at higher temperatures is due to the lack of high frequency phonons; the distribution of the phonon frequency has the upper limit corresponding to the Debye temperature.
5 . SUMMARY
Ultrafast spin dynamics in the ground state of a rare-earth-ion doped crystal are studied by the polarization spectroscopy with the optical pump-probe technique. Quantumbeat FID signals in the subnanosecond region were observed. Their Fourier transform gives the ESR spectra. The spin-lattice relaxation time Ti is obtained from the decay curve of the optically-induced magnetization up to the room temperature, where the relaxation time is of the order of picoseconds. The observed temperature dependence of the
705
rate l/Tl near room temperature deviates from the well-known T9 dependence. Considering the effect of the Debye temperature of the host crystal on the Raman process, the observed temperature dependence of l/Tl can be explained well. Ultrafast spin dynamics such as observed in the present work cannot be observed by the conventional ESR, whose time resolution is nanoseconds at best.
ACKNOWLEDGMENT This work was supported by JSPS research grant for Future Program.
REFERENCES 1. C. H. Anderson and E. S. Sabisky, in Physical Acoustics, edited by W. P. Mason and R. N. Thurston, Academic, New York, 1971, Vol. VIII, Chap. 1. 2. E. S. Sabisky and C. H. Anderson, Phys. Rev. B (1970) 2028. 3. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon, Oxford, 1970, Chap. 10.
706
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Subnanosecond relaxation of optically-induced magnetization in aqueous solutions of transition-metal ions S. Fume", K. Nakayama", T. Kohmotob,Y. Fukudab and M. Kunitomob "Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan bDepartment of Physics, Faculty of Science, Kobe University, Kobe 657-8501, Japan Optically-induced magnetization in aqueous solutions of transition-metal ions is studied by the polarization spectroscopy. From the concentration and magnetic-field dependences of the magnetization signal in the picosecond and nanosecond regions, it is found the observed decay of the magnetization is caused by the spin cross relaxation. The quantum-beat free-induction-decay signals are observed under a transverse magnetic field. Fourier transform of the observed signals gives the ESR spectra.
1. INTRODUCTION To study fast spin dynamics in condensed matter near room temperature, optical pumping with a short laser pulse is very useful. Observations of the spin relaxation [l,2] and the free-induction-decay (FID) signal [3] in the nanosecond region have been reported, where the time derivative of the optically-induced magnetization was monitored by a pickup coil. However, the resolution time of the detection system in such experiments is of the order of 1 ns at best. In order to study ultrafast spin dynamics in the picosecond and femtosecond regions, we applied the polarization spectroscopy with the pump-probe technique for creation and detection of the magnetization. The time resolution of our optical method is limited only by the temporal width of the light pulses. In the present paper we report on optically-induced magnetization and its fast relaxation in aqueous solutions of transition-metal ions at room temperature. The magnetization is created in the ground state by a circularly-polarized pump pulse, and the spin relaxation time can be obtained from the decay curve of the magnetization. In a saturated solution without the external magnetic field, the magnetization precesses around the randomly-distributed internal field due to the surrounding spins, and the decay time of the magnetization becomes of the order of subnanoseconds. Longer decay times are expected when the longitudinal magnetic field larger than the internal field is applied, or when the concentration of the magnetic ion is lowered, where the distance between the ions becomes longer and the magnetic interaction becomes weaker. In a transverse magnetic field, the precession of the magnetization is observed as quantumbeat signals. The Fourier transform of the observed signals gives the ESR spectra. This method may be called optically-induced Fourier-transform (FT) ESR spectroscopy.
707
2. EXPERIMENT
The pump and probe pulses are provided by optical parametric amplifiers. The pump (circularly polarized) and probe (linearly polarized) beams are nearly collinear and focused on the sample, aqueous solutions of copper sulfate (CuSO4) in a cell with thickness of 1 mm. The wavelengths of the pump and probe pulses are 1.3 pm and 650 nm, respectively, which are selected by considering the absorption coefficient. The waist sizes of the beams at the focus are -100 pm. The pulse width is 0.2 ps. The magnetization in the ground state of the Cu2+ ion (electron spin S=1/2) is created by the pump pulse, and its time evolution is detected as the change of the polarization (circular dichroism) of the probe pulse, where the time separation between the pump and probe pulses is swept by an optical delay line. The change of the polarization is monitored by using crossed polarizers and a photomultiplier. The change of the electric-field amplitude corresponding to the observed signal is l o p 5 times as small as the field amplitude of the probe pulse. To attain higher signal-to-noise ratio, a photoelastic modulator for the pump pulse and a lock-in amplifier for the photomultiplier output are used. A magnetic field (0 5 kOe) is applied parallel or perpendicular to the pump beam.
-
3. SPIN CROSS RELAXATION
The observed decay curve of the magnetization created by the pump pulse in a saturated solution of CuSO4 a t zero magnetic field is shown in Figure 1. The observed decay curve in a diluted solution of CuSO4 is shown in Figure 2. For the lower concentration of the magnetic ion, the time constant becomes larger. In a magnetic field parallel to the pump beam, the decay time becomes longer. At 4 kOe the time constant is about 1 ns. These results suggest that the observed decay of the magnetization is caused by the spin cross relaxation due to the magnetic dipole interaction between the copper ions. If the decay is caused by the spin cross relaxation between the copper ions, it is expected that the decay curve is in the form of exp(-@) with the relaxation rate [4]. The solid
h
v A
B
-3
a
zo Delay Time t (
7
Figure 1. Decay curve of the opticallyinduced magnetization in a saturated aqueous solution of CuS04. (0.92 mol/l)
Delay Time t (
Figure 2. Decay curve of the opticallyinduced magnetization in a diluted aqueous solution of CuS04. (0.46 mol/l)
7
708
curves in Figures 1 and 2 are the best fits to the observed ones. The obtained relaxatior times l/re , which are defined by the time of l / e point, are 340 ps and 1.3 ns for tht saturated and diluted solutions, respectively.
4. QUANTUM-BEAT FID
Figures 3(a) - 3(d) show the magnetic-field dependence of the quantum-beat FIE signal for the magnetic field of 0, 1.6, 3.0 and 4.4 kOe perpendicular to the pump beam Their Fourier transform is shown in Figures 3(e) - 3(h), which gives the ESR spectra. Foi the higher magnetic field, the beat frequency of the observed quantum-beat FID signal which is of the order of ten gigahertz, becomes higher. The time constant of FID is severa hundred picoseconds.
Figure 3. Quantum-beat FID signals for 0, 1.6, 3.0 and 4.4 kOe [(a) - (d)] and their Fourier spectra [(e) - (h)].
709
The beat frequencies obtained from the ESR spectra are 5.5, 9.4 and 14 GHz for 1.6, 3.0 and 4.4 kOe, respectively. These correspond to the Zeeman splitting of the ground state (S=1/2), and lead to the g-factor is g = 2.15 0.05. Most of the hydrated salts of copper contain a [ C U ( H ~ O ) ~complex ] ~ + shows Jahn-Teller distortion, in which typical g-values are 911 2.4, g1 2.1 [5]. It is considered that the distortion axis of the copper complex is randomly distributed in the solution, and an averaged value of g is observed in the present experiment.
-
N
5. SUMMARY
Optically-induced magnetization in aqueous solutions of copper ions a t room temperature is studied by using the polarization spectroscopy with the optical pump-probe technique. The magnetization is created in the ground state by a circularly-polarized pump pulse. Fast spin relaxation of copper ions in the subnanosecond region was observed through the change of the polarization of the probe pulse in the longitudinal magnetic field. The dependences of the decay curve of the magnetization on the concentration and magnetic field show that the decay of magnetization is caused by the spin cross relaxation among the Cu2+ ions. In the transverse magnetic field, the precession of the magnetization was observed as quantum-beat FID signals. Their Fourier transform gives the ESR spectra in the gigahertz region. It is shown that this pure-optical method is a very useful method to study the spin dynamics in solutions of magnetic ions at room temperature.
ACKNOWLEDGMENT This work was supported by JSPS research grant for Future Program.
REFERENCES 1. J. P. van der Zeil and N. Bloembergen, Phys. Rev., 138 (1965) A1287. 2. Y. Takagi, Opt. Commun., 59 (1986) 122. 3. Y. Takagi, Y. Fukuda and T. Hashi, Opt. Commun., 55 (1985) 115. 4. T. Endo and T. Muramoto, Phys. Rev., B 29 (1984) 6043. 5. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon, Oxford, 1970.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors)
710
2002 Published by Elsevier Science B.V.
Detection of the internal electric field and relaxational magnetoelectric effect in chromium mesogen N.E. Domrachevaa, I. V. Ovchinnikova, A.Turanova and G.Lattermannb Kazan Physical-Technical Institute of the Russian Academy of Sciences, Sibirsky Trakt 1017,420029, Kazan, Russia (domracheva@mail. ru). Makromolekulare Chemie I, Universitat Bayreuth, Bayreuth, Germany
a
A chromium metallomesogen possessing the columnar mesomorphism has been studied using EPR, magnetic susceptibility measurements and dielectric spectroscopy. The anomalous nonlinear temperature dependencies of the EPR resonance field position and of the fine-structure parameter D in columnar mesophases were observed. It is shown that this anomaly is associated with the existence of the internal electric field and with the dynamical contribution from the soft mode of the crystal lattice. The dielectric spectroscopy data reveal the soft mode contribution and indicate the phase transition from the paraelectric state to the dipole-ordered one in COlxd mesophase. A significant increase of the chromium ion's spin polarization was observed for the glassy sample (sharply cooled from the dipole-ordered phase). Such phenomenon is due to the transfer of the dipole polarization to the spin polarization, i.e., due to a relaxational magnetoelectric effect.
INTRODUCTION Over the past few years much attention has been devoted to the synthesis and characterization of metallo-organic mesogens [ 11. Increasing interest to these objects is caused by their unique magnetic [2] and ferroelectric [3] properties widely used in practice. The paramagnetic chromium (111) metallomesogen LCrCl3 (L is the azacyclic ligand, is studied by EPR, magnetic susceptibility and dielectric spectroscopy methods. An unusual polymorphism of the columnar liquid-crystalline phases is observed ( phase transition temperatures are given in "C): K 42.0 colxd 68.5 Colhd 167 COlrd 223.5 I, where K is the crystalline phase; COlxd is an unknown mesophase, Colhd is a hexagonal mesophase, COlrd is a rectangular columnar mesophase, and I is the isotropic phase. 1. EPR The X- and Q-band EPR spectra were measured for the crystalline, isotropic and different mesomorphic phases, several types of the EPR spectra having been observed. All the EPR spectra of the chromium mesogen are described by the spin-Hamiltonian
H = 811 PHz Sz + g l p( Hx with S=3/2, where
and
+ HYSy ) + D[ Sz2 - S(S+1)/3] + E( Sx2 - S y 2 )
(1)
are the fine-structure parameters which characterize the axial and
This research has been supported by RFBR grant N 99-03-32716 and N 02-03-32179.
711
Q-band
X- band
B
P
3
Figure 1. Experimental and theoretical EPR spectra of the compound in the crystalline phase. the rhombic distortions, respectively, in the octahedron environment of the Cr 3+ ion. The EPR spectra for the crystalline phase at room temperature for the and bands are presented in Figure 1 (bottom lines). A computer simulation of these spectra (top lines in Figure 1) using the spin-Hamiltonian (1) gives the following values of the fine-structure parameters: g = 1.99, D = 0.17 cm , = 0.03 cm The columnar mesophases (Colrd , Colhd , Col,d ) were studied at slow cooling of the compound from the isotropic phase, because, according to the polarization microscopy data, the phase textures are most pronounced in this case. The EPR (X-band) spectrum in the isotropic phase is a single symmetric line (see Figure 2d). EPR line in the columnar mesophases become asymmetric (see Figures 2b, 2c), and the fine-structure corresponding to the 3/2+1/2 transition is observed in the spectrum. When the temperature is decreased, the left wing of the asymmetric line (3/2+1/2 transition) is displaced by -250 G (in the 180°C interval) to lower fields with respect to the central (*1/2) transition. When the phase transition (Col,d+K) point is approached, the EPR line is considerably broadened, becoming almost symmetric again. EPR spectra for different types of the columnar phases were simulated using spinHamiltonian (1). The microwave resonance fields Hre,, the individual line widths AH, and the fine-structure parameter D were calculated. The temperature dependencies of these parameters are presented in Figures 3a-3c. It is seen from Figure 3b that behaviors of the microwave resonance fields for parallel and perpendicular orientations in the region of existence of the columnar phases are different. Moreover, during the transition from colrd and Colhd phases to the columnar colxd phase, the resonance fields, the line widths, and the parameter D depend on the temperature in a nonlinear way. The behavior of the resonance fields and the line widths is analogous to the behavior of these parameters in anisotropic ferromagnets [4] and antiferromagnets [5] above the critical point, where such a behavior is due to the growth of the spin correlation effects [4]. To clarify the nature of these anomalies, we carried out the measurements of the magnetic susceptibility, the dielectric permittivity and the (dielectric) relaxation time for the compound at standard temperature regimes (cooling or heating the sample).
-'.
-'
712
2000
Co'xd
3000
4000
I
r
I
.
-l,o
-
I
I 2000
2000
3000 H (0)
4000
I
I 2000
3000
2000
Isotropic phase 3000
4000
(01
3000
,
4000 - 1,o
14,o
I
-l,o -
I
2000
3000
H (0)
4000
2000
3000
4000
(G)
Figure 2. Experimental and calculated (circles) EPR spectra of the metallomesogen in (c) COlrd ,and (d) isotropic phases. (a) ColXd ,(b) 2. MAGNETIC SUSCEPTIBILITY AND DIELECTRIC RELAXATION DATA Within the temperature interval of 4.2K to 400 K, the magnetic susceptibility is described by the Curie-Weiss law, C/(T -0) + Xdla ,with the constant C corresponding to S=3/2, Xdia = -1023 cm3/mole, and @= 0.65 K. This is an evidence of the weak ferromagnetic exchange interactions (J/k = 0.13 K) occuring between the chromium (111) ions. Such a value of the exchange integral is, however, too small for the anomalous changes in the resonance field HI, position and in the fine-structure parameter D detected in higher-temperature columnar phases to be explained by the magnetic ordering. The nature of the EPR-registered anomalies is, probably, associated the internal electric field acting upon the chromium atom. To check this assumption, dielectric measurements were carried out by Yu. Feldman and coworkers. At = 327.9 K, a phase transition (Figure 4) from the paraelectric state to the dipole-ordered one was observed. This transition was connected with the dynamical contribution from the soft mode. It was shown in [6-81 that the nonlinear temperature dependence of the EPR line position and of the fine-structure parameter D can be due to the contribution from the soft mode (the optical branch of the crystal lattice vibration spectrum, whose frequency decreases to zero on approaching to at a certain point of the Brillouin zone). The dependence of the soft mode frequency on the temperature, the wave vector, and the propagation direction is expressed by the formula [6,7]:
713
7
0;O
1.0450
5
04188
D1=O 00005 (cm"IK) 00008 (crn.'lK)
OD;
0-
'"1' HO=31513(GL H[-
o JB~(GIK). 102(O/K). T-327 8 K
fl415
0.0225
f"o~l 3ooo
q=2009
F 1 415 q=2OOQ
\
1
?,
(a) 350
400
450
5W
400
450
350
500
4M
450
5W
Figure 3. Temperature dependence of (a) the parameter D, (b) the resonance fields, and (c) the line widths in the columnar phases of mesogen. Solid lines are the approximation, according to equations(3) and (4), of the temperature dependence of the fine-structure parameter and of the position of HII . Approximation parameters are listed in the inserts.
where 0 is the angle between the direction of the spontaneous polarization and the phonon propagation direction. Carring out a calculation of the soft mode influence similar to that made in [6,7], we obtain the following expression for the temperature dependencies of the D tensor: D(T) = D
+
D ,T
+ D * T "1
IT
-
T,
rl
1''
. tan
' 1''
)I,
(3)
IT - T ,
where q = @, la), and is the maximum value of the wave vector q, Anomalous shifts of the resonance fields are closely related to the change in the D tensor. Taking the linear relation between the position of the EPR line and the value of D tensor into account [S, 91, one comes to an equation for H(T) in paraelectric phases:
The temperature dependencies of H 11 and D were approximated using equations (3) and (4) (see Figure 3a, 3b). The figure shows this approximation to adequately describe experimental data, thus confirming the assumption on the effect of the soft mode on the EPR spectra of chromium ions in the paraelectric phase. The values of the critical parameters y and q obtained by us are typical for the ferroelectric phase transitions detected by EPR [7, 81. So, the anomalous temperature dependencies of the parameter D and of the resonance field H 11 position for the compound in the paraelectric phase are due to soft-mode vibrations. The appearance of the soft mode, as a rule, testify to the existence of a multi-well potential in the system.
714
I
0,05L 200
,
,
,i
,
300
T OK
,
, 400
,
, 450
o,OOal 200 250 300
,;.
,
,
,
350 4Ml 450 T OK
Figure 4. Temperature dependencies of the change in the real part of the dielectric permittivity (a), and of the relaxation time (b) in the Colxd phase.
3. RELAXATIONAL MAGNETOELECTRIC EFFECT In such type of system, where simultaneously present the reorienting electric dipole moment, the magnetic moment and switching electric field, it is possible, in principal, to detect the mutual influence of magneto-electric properties. For this purpose the chromium mesogen was transferred to the electric dipole-ordered state and sharply cooled to the glassy state. Then the temperature dependence of the magnetic susceptibility was measured with a squid-magnetometer (from T=4.2K). The observed behavior of the magnetic susceptibility for such a glassy sample is presented in figure 5. We have also used EPR-spectroscopy to register the temperature dependence of the integrated EPR line intensity (which is proportional to the magnetic susceptibility) of Cr3' and observed the analogy of these two dependencies. Figure 5 shows that, within the temperature range of (4.2-ll)K, the values of the magnetic susceptibility (heating process) for this glassified (from the dipole-ordered phase) sample significantly exceed those for the cooling process (they follow the Curie-Weiss law). Similar behavior of magnetization have been detected for the non-equilibrium process at switching the electric field in paramagnetic systems possessing the reorienting electric dipole moments: (A13+-O-)centers in quartz [ l l ] and Co2+inSrO [lo]. Initial conditions for such effects (existence of non-equvalent potential wells and their non-equilibrium population) are similar for chromium and for earlier investigated systems, so we could use the results of previous works for our system. Two models [lo, 111 exist to explain the increase of the spin polarization. The first (the ((resonance))one) implies the potential wells moving when switching the electric field. The spin polarization increases due to tunneling channels coming into play when the spin levels belonging to different wells come into coincidence. Then the polarization is transferred from electric to magnetic dipoles producing a significant increase of magnetization. The second (((relaxation))) model explains the increase of the spin polarization by a relaxation process in the system. The conditions at which the increase of magnetization is possible are found from the rate equations for levels' populations.
715
5
10 T(K)
15
20
Figure 5. Temperature dependence of the magnetic susceptibility for glassy sample. For the non-equilibrium process we have investigated, the registered value of the spin polarization (see insert in figure almost two times exceeds the “equilibrium” one. The choice of a definite model will be reported in a separate work. However, the result obtained, irrespective of a concrete model taken for its explanation, is of interest in itself, since, for a molecular compound (possessing liquid-crystalline properties) the effect described here was observed for the first time. Besides (as opposed to previous works), one can observe it without any external electric field. ACKNOWLEDEGMENTS
The authors are grateful to Yu. Feldman (Jerusalem University) for carrying out the dielectric measurements, and to Marc Nilgels from the Illinois EPR Research Center (USA) for providing us with a program for calculating ((powder-like))EPR spectra. REFERENCES
1. S. A. Hudson and P. M. Maitlis, Chem. Rev., 93 (1993) 861. 2. A. Turanov, I. Ovchinnikov, Y. Galyametdinov and D. Bruce, Liq. Cryst., 28 (2001) 845. 3. J. Barbera, R. Iglesias, J. L. Serrano et.aL, J. Am. Chem. SOC.,120 (1998) 2908. 4. D. L. Huber and M. S. Seehra, Phys. Status Solidi B, 74 (1976) 145. 5. Y. Yokozawa, J. Phys. SOC. Japan, 31 (1971) 1590. 6. K. Tzuchida and R. Abe, J. Phys. SOC.Japan, 46 (1979) 1225. 7. D. Barb, N. M. Grecu, V. V. Grecu, and F. F. Popescu, Chem. Phys. Lett., 56 (1978) 355. 8. G. V. Mamin and V. N. Efimov, Ferroelectrics, 233 (1999) 111. 9. G. Burns, J. Appl. Phys., 32 (1961) 2048. 10. V. S. Vikhnin, L. S. Sochava, V. A. Krylov and et.aZ., JETP Lett., 40 (1984) 1248. 11. A. B. Brik in book: Radiospectroscopija Twedogo Tela, Naukova dumka, (1992) 202; A. B. Brik, Sov. Phys. Solid State, 27 (1985) 91.
Section 10 High Frequency and High Field
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EPR in the 21“ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
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Modern ESR methods in studies of the dynamic structure of proteins and membranes
Jack H. Freed Baker Laboratory of Chemistry and Chemical Biology, Cornell University and National Biomedical Center for Advanced ESR Technology (ACERT) A review of modern electron spin resonance (ESR) techniques for studying the dynamic structure of proteins and membranes using nitroxide spin labels is presented. In particular multiple quantum coherence FT-ESR, multifrequency ESR based on high frequency ESR and two dimensional ESR to study the structure and dynamics of bio-membranes and proteins are illustrated by several examples. Distances in biomolecules have been determined accurately. The details of complex dynamics in proteins and the dynamic structure of membranes were characterized.
1. INTRODUCTION
ESR spectroscopy, based on nitroxide spin labeling, is largely driven by the sensitivity of the nitroxide label to its surroundings. The interaction of the unpaired electron spin of nitroxide with the 14N magnetic nucleus = 1) leads to an electron nuclear dipolar (or hyperfine tensor, which for rapidly tumbling molecules averages to a nonzero value. This is also the case for the g tensor of the electron spin, which, when averaged, yields the isotropic g shift. If the investigated system (e.g., a membrane) is macroscopically aligned, then one can observe the different “single-crystal-like” spectra obtained for each orientation of the system with respect to the static magnetic field. The homogeneous line broadening of the spectra would then reflect the motional dynamics. ESR spectra are known to change dramatically as the tumbling motion of the probe slows, thus providing great sensitivity to fluidity in the neighborhood of the probe [ 11. An approach based on the stochastic Liouville equation, known as slow motion theory, has been developed [2], which shows that the dramatic line shape changes are particularly sensitive to the microscopic detail of the dynamics. These methods of analysis enable a quantitative assessment of ESR spectra in terms of motional rates of spinning of the nitroxide moiety, the end-over-end tumbling of the tagged molecule, and local environmental constraints on the motion. For a spin-labeled protein, for example, the motions could be the spinning of the tether attaching the nitroxide and the overall tumbling of the protein. In a bilayer, typical of a cell membrane, reorientation of the lipids is constrained by the surrounding lipids and other molecules. The motional rates lead to a rotational diffusion tensor, whereas the motional constraints lead to an orientational order parameter. The changes in magnitude of the splitting and g value can be used to monitor features of the local surroundings, such as its polarity. In addition, unpaired electron spins from different
720
nitroxides (on the same or different molecules) interact weakly through long-range magnetic dipolar interactions or strongly through short range Heisenberg spin exchange. In fluid media, these latter interactions can be used to monitor microscopic translational dynamics In frozen or very viscous media the dipolar interactions can be used to measure distances, either, by continuous wave (cw) ESR or pulsed ESR [4,5]. Often, one must deal with the complications of samples that are MOMD (i.e. microscopically ordered but macroscopically disordered)[2]. Membrane vesicles offer a property. Spin-labeled moieties in the different vesicle regions may physical example of be oriented at all angles with respect to the magnetic field, thus providing a “powder-like” spectrum with inhomogeneous line broadening superimposed on the hb. The degree of is determined by the extent of local (microscopic) ordering. This masks the resulting in reduced resolution to dynamic and ordering parameters. Despite this, modeling of the heterogeneity and of the dynamic effects to fit the ESR spectrum can yield important insights.
2. DOUBLE QUANTUM COHERENCE AND DISTANCE MEASUREMENTS The determination of intra- and inter-molecular distances has become an important application of contemporary ESR spectroscopy. Areas of interest include the structure of protein complexes and functional dynamics of proteins that are neither soluble nor crystallized
simulation m.06A
Real
A
-0.5
0.0
0.5
.o
(Left) The doubly labeled molecule is dissolved in a frozen amorphous matrix. Figure (Bottom) The maximum echo signal recorded as a function of tc= tz-tp(Seeref. [6a] for the 6 pulse DQC sequence applied). (Right) A Fourier transform of this echo signal yields the Pake-type dipolar spectrum in the frequency domain. The separation vL of the two sharp peaks is directly related to the distance r between the labels r v ~ - ’ ’ ~The . calculated value of the distance 28.8 A is close to the value 28.06 A obtained by a model simulation [6a].
72 1
[41. Strong Double Quantum Coherence (DQC) signals for a variety of bilabeled nitroxide
Lysozyme obtained from DQC experiment [6b]. (a) The DQC Figure 2. Long distances in signal obtained for the double labeled mutant 65/86. (b) This signal can be separated into (i) the intramolecular signal and (ii) a baseline from the intermolecular signal. (c) Fourier transformed spectrum from (i). Broken lines show the simulations with %, = 37.4 8, and AR = 2.7 A. (d) Molecular modeling showing three distances in three double labeled DQ experiments.
722
molecules in both disordered and oriented solids have been achieved recently [6a,6b] shown in Fig. 1. For a benchmark, (a distance of 28.8 0.05 between the two nitroxide groups) the rigid linear biradical, was obtained (versus 28.06 from molecular modeling). Another example, T4-Lysozyme double labeled mutant 65/86 shows a distance of 37 between the unpaired electrons on each of the two labels. The echo signal and its Fourier transformed frequency spectrum are shown in Fig. 2 (a), (b) and (c). In this case, a baseline signal due to the intermolecular DQ signal subtracted before Fourier transformation. In simulation the distribution of the distance has been assumed. The distances obtained are consistent with the known crystal structure of TCLysozyme and the conformations of the nitroxide tether. 3. MULTIFREQUENCY ESR BASED ON HIGH FREQUENCY ESR
3.1. Virtues of High Frequency ESR The snapshot feature of cw ESR encourages a multifrequency approach to the study of the complex modes of motion of proteins, DNA, and other polymers, enabling the decomposition of these modes according to their different time scales [7]. The degree and nature of the line broadening is a h c t i o n of the frequency applied. For example, in the case of proteins, the higher frequency ESR should "freeze out" the slow overall tumbling motion, leaving only faster internal modes of motion. Alternatively, ESR performed at lower frequencies would
Figure 3. (a) Nitroxide cw ESR spectra observed at 250 GHz change dramatically with temperature caused by the slowing down of the tumbling motion with decreasing temperature. The uppermost two spectra, -40 and -60 "C, correspond to the motional narrowing regime, the middle -81 and -100 "C to the slow motional regime, and the lowest - 119 and -129 "C to the rigid limit. (b) The derivative ESR simulations for characteristic ESR frequencies demonstrate the snapshot property of ESR when spin-labeled molecules are tumbling with correlation time = 1.7 x sec. The spectra observed at 15.6 to 125 in (b) correspond to the motional narrowing regime and the spectra observed at 500 and 1000 GHz to the slow motional regime [ 131.
723
be sensitive to the motions on a slower time scale. Fig. 3 (a) shows changes of spectra with decreasing temperature observed for PDTiToluene at 250 GHz. The spectra observed at -40 and -60 "C show three sharp hflines corresponding to the motional narrowing regime. The spectra at - 81 and -100 "C show a gradual change to the slow motional regime. Those at - 119 and -129 "C correspond to the rigid limit. Instead of changing temperature, a change in ESR frequency manifests almost the same phenomena shown in Fig. 3 (b). A motional process that looks fast at lower frequencies will look slow at higher frequencies. This means that for complex dynamics of proteins the fast motions will show up best at higher frequencies, whereas the slower overall and collective motions will show up best at lower frequencies.
3.2. Protein and DNA Dynamics Fig. 4 (a) illustrates the complex motions of spin labeled T4 Lysozyme in aqueous solution [7,8] which consist of spin label reorientation, side chain fluctuation and global tumbling. Fig. 4(b)[9] depicts the fluctuating side chain. In the MOMD model, the global tumbling rate tends to zero. The virtues of such an approach were demonstrated in a study at 9 and 250 GHz on spin-labeled mutants of T4 lysozyme in aqueous solution [S]. On the short time scale of the 250-GHz ESR experiment, the overall tumbling too slow to affect the spectrum; thus, a MOMD analysis provided a satisfactory modeling of the overall tumbling, and HF-enhanced spectral resolution reported on the internal dynamics (Fig. 5b). The analysis of the 9 GHz spectra was more complex, because the longer time scale did not freeze out the overall
Figure 4. (a) Protein Dynamics of Spin-labeled T4-Lysozyme: There are three kind of motions, spin-label reorientation, side chain fluctuations and global tumbling. (b) The SRLS model is illustrated including relevant motional parameters [7,9].
724
336033w
wdo
(0)
Figure 5. ESR spectra of spin labeled T4 Lysozyme. (a) 9 GHz spectra are affected by overall tumbling. (b) 250 GHz ESR spectra show nitroxide motion about the tether and of the peptide backbone [8]. The upper (lower) curves are for T4 lysozyme mutants labeled at site 44 (69).
tumbling motion. The slowly relaxing local structure (SRLS) model [7] simultaneously incorporated both the internal and overall motions. By fixing the internal motional parameters at the values obtained from 250 GHz data, fits to the 9-GHz line shapes generated by successfully yielded the rate for the global dynamics. This demonstrated how these different modes be distinguished using the multi-fiequency ESR approach. The analysis also indicated that R0l reflects the backbone protein motion. A related ESR study was performed on spin-labeled DNA oligomers [9]. The dynamic modes and spin label are illustrated in Fig. 6. The results show two conformers which are distinguished by their ordering. Rol decreases with increasing oligomer length, strongly suggesting that backbone motion is being sensed. Note that for DNA oligomers, the overall
Figure 6. (a) Dynamics of spin-labeled DNA, (b) Molecular formula of spin label DUTA [9].
725
Segmental, tno
Molecular difision
t
Figure 7. (a) The spin labeled PC experiences two kinds of motions: segmental diffusion and molecular diffusion. no shows the average direction of the membrane normal and n(t,) in the lower insert, shows the fluctuating normal due to slow director fluctuations. (b) shows the chemical [ll]. structure of spin label
tumbling may be analyzed by use of hydrodynamic theory. 3.3 Dynamic Structure of Membranes We have been studying model membrane vesicles consisting of di-palmitoylphosphatidyl-choline (DPPC) in excess water. The spin label is incorporated in the membrane and EPR spectra of the label are observed in various membrane systems over a range of temperatures. The dynamics of a labeled lipid in a membrane is illustrated in Fig.7. Cholesterol is an important regulator of the physical properties of biological membranes and their functions. When added to model membranes of pure lipid in the liquid crystalline (LC) phase, it leads to a new phase, the liquid ordered (LO) phase, which is more ordered yet fluid. Spin label 16PC (cf. Fig. 7b) was observed by ESR at 250GHz vs. temperature for pure DPPC vesicles (Fig. 8 left) and for vesicles of DPPC and cholesterol in ratio 1:l (Fig. 8 right), [111. These spectra and equivalent ones obtained at 9GHz were fit by the MOMD model to yield rotational diffusion tensors components R l and RII and order parameters, S (cf. Fig. 9). Based on these recent results [111, the following conclusions can be derived: (1) 250 vs. 9 GHz spectra reveal that simple MOMD analyses show typical discrepancies in their predictions of ordering and dynamics. (2) SRLS fits to 9 GHz do however lead to consistency with the 250 GHz MOMD results for the internal dynamics and ordering. In addition they show that overall tumbling rate constants are 1 - 3 x10 -'s-' and overall order parameters are approximately 0.5. (3) The LO phase shows faster end-chain motions over the LC phase and some increased local ordering, with some increase in overall ordering, but little change in overall motional parameters.
726
DPPC -Liquid Cryst.1
DPPCKbol = 1 -Liquid Ordered A
T 4 C
---L---
T41%
T=%%
T=3O*C
89OOO
89200
89100
89400
89300
Field (Gauss)
Field (Gauss)
Figure 8. 250 ESR Spectra of 16 PC in a liquid crystal and cholesterol membranes and their MOMD fits [ 1 11. The experimental results and fitting curves are almost coincident.
9 GHz MOMD Fit
250 GHz MOMD Fit
0
1
0
2
0
3
0
4
Temperature ("C)
0
5
0
o
i
o
z
o
~
a
m
Temperature CC)
Figure 9. Rotational Diffusion rates and Order parameters of 16PC in membranes from 250 GHz and 9 GHz ESR spectra obtained their respective MOMD fits [ 111.
121
Figure 10. Dispersion sample for illustration of MOMD model. Bo magnetic field, d director axis and Y director tilt angle. All angles Y exist in the macroscopically disordered sample.
3420 3440 5460
3340 3360
Field
Figure 11. Absorption cw-ESR MOMD spectrum which is composed of the component spectra shown a function of director tilt.
4. TWO DIMENSIONAL ELECTRON SPIN RESONANCE
4.1. Two-dimensional Fourier transform (FT) ESR and dynamic structure of membranes Two-dimensional time domain ESR methods allow one to study both dynamics and
I 0
I
0
I
TM=150ns
Figure 12. The 2D-ELDOR contours of (a) SW16PC (Liquid crystal) and (b) SWChoV16PC (Liquid ordered) [10,121.
728
ordering in membrane vesicles [10,12,13]. In general, 2D ELDOR spectra from membrane vesicles exhibit more dramatic variations due to changes in membrane properties than do spectra obtained by cw ESR. Fig. 10 illustrates the challenge of a macroscopically disordered (i.e., MOMD) sample, such from vesicles, micelles, micro-emulsions or LC polymers. We can define a local director axis d associated with the microscopic ordering. Then Fig. 11 shows the absorption cw-EPR spectra as a function of director tilt. The observed spectrum based on MOMD is shown at the top and results from the summation over all orientations. Fig. 12 shows the contours of the 2D ELDOR of 16PC observed in the LC and LO phases respectively in samples of (sphingomyelin) SW16PC in the LC phase and SWChol/l6PC in the LO phase [10,12], similar to those vesicle samples in the previous section. These sharply different spectra show that 2D-ESR dramatizes the differences in dynamic structure between liquid crystalline and liquid ordered phases. The results of
Figure 13. Two kinds of lipids; bulk and boundary lipids res~ctive*ysuffer different and y-orderings’
Figure 14. Comparison of cw and 2D FT ESR observed at 45C in pure and GA incorporated lipid vesicles [12].
729
this study may be summarized follows: (1) A MOMD analysis of 2D-ELDOR yields results closer to those 6-om 250 GHz cw than 9 GHz cw ESR. (2) 2D-ELDOR is more sensitive to the spin relaxation, which is dominated by the faster internal modes. (3) Time Domain ESR differentiates between homogeneous and inhomogeneous broadening.
4.2. Lipid Protein interactions studied by two-dimensional ELDOR The merits of 2D FT ESR spectroscopy are well demonstrated in studies of the effect of the peptide gramicidin A (GA) on the dynamic structure of model membranes. Besides bulk lipids, boundary lipids that coat the peptide are present but are difficult to resolve in cw ESR. Fig. 13 illustrates the two kinds of lipids in the dynamic structure of membranes containing GA. Figure 15. 2D FT ESR spectra (with Fig. 14 shows a comparison ofcw and 2D FT short dead time) of 16PC showing boundary ESR observed at 45°C in pure and GA and bulk lipids in the presence of GA [lo]. incorporated lipids [12]. Much more dramatic differences are observed in 2D FT ESR vesicles. 2D-FT ESR spectra obtained with very short dead times (25ns) show distinct signals from both boundary and bulk lipids [10, 121, cf. Fig. 15. These experiments show that the boundary lipids experience a moderate decrease in motional rates but substantial orientational ordering changes. ACKNOWLEDGEMENT The author is grateful to the ShinSedai Research Institute for providing the travel funds to present this lecture at APES’Ol and owes very much to Prof. Asako Kawamori, Chairperson of this symposium for the arrangement of this review. The work reported herein was supported by grants from NIWNCRR, NIWGM, and NSF, and was performed by many members of the Freed Research Group.
REFERENCES 1. L.J.Berliner, Ed., Spin Labeling: Theory and Applications (Academic Press, New York, (1976). 2. D.E. Budil, S. Lee, S. Saxena, J.H. Freed, J. Magn. Reson. A120 (1996) 155. 3. J.H. Freed, Annu. Rev. Biophys. Biomol. Struct. 23 (1994) 1. 4. G.R. Eaton, S.S.Eaton, L.J. Berliner, Eds., Distance Measurements in Biological Systems
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by EPR, vol 19 of Biological Magnetic Resonance (Kluwer, NewYork 2000) 5. A.D. Milov, A.G. Maryasov, Yu. D. Tsvetkov, Appl. Magn. Reson. 15 (1998) 107 6a. P.P. Borbat, J.H. Freed, Chem. Phys. Lett. 3 13 (1999) 145; Ref. 4, Ch. 9 (2000). 6b. P.P. Borbat, H.S. Mchaourab, J.H. Freed, J. Am. Chem. SOC.(2002) submitted. 7. Z. Liang, J.H. Freed, J.Phys. Chem. B. 103 (1999) 6384 8. J.P. Barnes, Z. Liang, H.S. Mchaourab, J. H. Freed, and W.L. Hubbell Biophys. J. 76, (1999) 3298 9. Z. Liang, A.M. Bobst, R.S. Keyes. J.H. Freed, J.Phys. Chem. B. 104 (2000) 5372 10. P.P. Borbat, A.J. Costa-Filho, K.A. Earle, J.K. Moscicki, and J.H. Freed, Science, 291 (2001) 266 11. Y. Lou, M. Ge, J. H. Freed, J. Phys. Chem, 105 (2001) 11053 12. G. Patyal, R.H. Crepeau, J.H. Freed, Biophys. J. 73 (1997) 2201 13. J.H. Freed, Annu. Rev. Phys. Chem. 51 (2000) 655
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
73 1
High-frequency single-crystal EPR application to multifiequency approach: study of metalloproteins Sushi1 K. Misra Physics Department, Concordia University, 1455 de Maisonneuve Boulevard West, Montreal, Quebec H3G 1M8, Canada Single-crystal EPR spectra at very high frequencies (> 140 GHz) of transition-metal ions Mn" (S=5/2), Fe3' (S=5/2), and Ni2+ (S=l), with spin S > %, are discussed to demonstrate the virtues of very high frequency EPR. It is shown that a considerable simplification of the spectra, and thus their interpretation, is achieved in the high-field limit even for large zero-field splittings. On the other hand, broadening of lines at higher frequencies may hinder their observation. Simulated spectra for various spin systems at Xband (-9.5 GHz) and VHF (-249.9 GHz), will be shown. Two important special cases: (i) EPR of non-Kramers ions with integral spins at various frequencies, and (ii) the advantage of using multi-frequency approach in estimating SHP, are highlighted, in particular. Proposed VHF EPR study of single crystals and powder samples of four metalloproteins of biological interest is presented. The details of the iron centres involved in these metalloproteins, namely, non-heme mononuclear iron, Fe(II), 2Fe-2S cluster, diiron centre, and mononuclear Fe(I1) are provided.
1. INTRODUCTION Recently, there has been a resurgence of activity in very-high-frequency (VHF > 140 GHz) EPR due to advances in magnet and millimeter-wave technology [I]. However, there have been reported very few cases of single-crystal studies at VHF in contrast to those involving powders. In addition, most VHF research has involved organic systems in amorphous states, for which preparation of single crystals is not easy. In this paper are described advantages of using single-crystal EPR over powder EPR. The virtues, as well as disadvantages, of VHF EPR are illustrated by simulations and use of X-band EPR results for three ions, including both Kramers and non-Kramers ones.
2. SINGLE CRYSTAL VERSUS POWDER (POLYCRYSTALLINE) EPR Single-crystal EPR has distinct advantages over powder EPR. (i) It enables a more precise determination of spin-Hamiltonian (SH) parameters [ 1,2]. This is because there are available many more EPR line positions by rotation of a single crystal with respect to the external magnetic field (B) to fit to SH parameters [2], unlike that for a powder (polycrystalline) sample for which one only observes broad averages over all orientations at each field value with a concomitant loss of spectral resolution. (ii) Single-crystal EPR lines are much narrower than those of powder lines, which can be very broad, e.g. in the range of
732
m
1.0-
crystal
v
5
0.50.0-
5
I
-1.0
7J
.-
Magnetic field (kG)
Figure la. Simulated single-crystal spectrum for Mn" in ZnzVz07 at 249.9 GHz.
.-
LI
Magnetic field (kG)
Figure Ib. Simulated powder spectrum for MnZ+in ZnzVz07 at 249.9 GHz.
0.1 -lT in the presence of either large zero field splittings (zfs) and/or large g-tensor strain or anisotropy [3-51. This reduces seriously the signal-to-noise ratio of a powder spectrum over that of a single-crystal. (iii) The resonant susceptibility of a single-crystal sample is enhanced by the cooperative effect of many spins at a given orientation intensifying the transition amplitude of a single line as opposed to a powder sample, where the absorption is averaged over all orientations as noted above, and partitioning of spectral intensity among all transitions reduces the achievable signal-to-noise ratio for any one transition. (iv) Single crystals have a higher effective spin concentration of the crystalline lattice as opposed to that in the looser packing of polycrystalline powders. The effects of (ii) - (iv) can be illustrated by simulating the single-crystal fine-structure spectrum, as shown in Figure la, and then comparing it with the powder simulation, as shown in Figure 1 b for Mn2+(electron spin S=5/2) in ZnVz07 using the parameters listed in Section 5.2. It is seen from Figures la and lb that the powder spectrum is generally much broader, thereby reducing both signal sensitivity and spectral resolution. In particular, whereas all 5 fine-structure lines for MnZ+are prominent, and of comparable amplitude in the single-crystal spectrum, only the central line (-1/2 1/2) is not severely broadened in the powder. The non-central transitions in the latter are only a few percent of the amplitude of the central line which itself is less intense by about a factor of 3 than the single-crystal line. Given the typical expected signal-to-noise ratio of the experiments, the non-central transitions fine-structure transitions would not be observable unless there exist very high spin concentrations as those in neat samples, rendering it impossible to determine the zero-field splitting. This observation is consistent with previous studies on powder samples of MnZ+at 250 GHz [3,4].
3. VIRTUES OF VHF EPR 3.1. Determination of large zero-field splittings One requires to use an appropriately high frequency to observe the allowed fine-structure (AM = +1) transitions for the case of large zfs. For example, Mn3+was considered to be
733
1
3
4
5
Magnetic field (T)
Figure 2. A plot of the eigenvalues of the Kramers Fe3+ion in YCaA104, calculated using the indicated parameters, for values of B up to 5T showing all expected AM = 1 transitions at X (9.79 GHz) and Q (35.69 GHz) bands. The inset shows the eigenvalue plot for values of B up to 15T showing all expected AM = 1 transitions at 249.9 GHz.
“EPR silent” due to its large zfs, but its EPR signal was recently reported at VHF by Goldberg et al. [5]. High-frequency EPR is especially suitable to measure large zfs, which is more difficult at low frequencies. It is noted that at low frequencies, it may be possible to observe the forbidden AM = 0 transitions with the use of a dual-mode cavity [6]; however, being forbidden, they have much smaller intensities compared to those of AM = 1 transitions. 3.1.1. Case of Kramers ions (half-odd integral spins) For the Kramers case of the Fe3+ion in YCaA104, this can be seen in Figure 2, which shows the energy level scheme as well as those AM = 1 transitions that can be observed at 9.5 GHz. However, one notes from Figure 3, showing the angular variation of EPR line positions that it is possible to observe all possible AM = 1 transitions at 249.9 GHz with an external magnetic field that can be varied up to about 13T. At least one non-central AM = 1 transition, that is other than -1/2 %, is required to estimate the values of the zfs parameters accurately. In addition, when one tries to determine the absolute sign of D, and thus those of all the spin- Hamiltonian parameters since the least-squares fittingmatrix-diagonalization
734
h
v v)
Q) ii=
8
200-
Q
8 100-
-120 -90
-60 -30
0
60
90
120 150 180 210 240
Orientation of magnetic field (degrees) Figure 3. Angular variation of Fe3+ EPR line positions at 9.79 GHz in YCaA104 single crystal using the spin-Hamiltonian parameters listed in Section 5.3; the insets show angular variations at 35.69 and 249.9 GHz. procedure yields correct relative signs of all the parameters [2], this requires comparing the intensities of the two extreme lines belonging to the whole set of AM = 1 transitions 3.1.2. Case of Non-Kramers ions (integral spins) The result for the non-Kramers ions NiZ' (S=l) is shown in Figure 4, showing the energy levels versus the magnetic field. One can observe AM = k 1 transitions at X-band values up to about 40 GHz, from which D may be extracted from a rotational set of spectra. On the other hand, when D is significantly larger than 40 GHz, VHF EPR becomes essential, as was the case for another non-Kramers ion Mn3+with S = 2 ( D 66 GHz) [5].
-
3.2. Simplification of EPR spectrum at VHF At high-enough frequencies where the Zeeman interaction dominates, the EPR spectra are considerably simplified [3,4]. This will be seen in the spectra simulated in Section 5. 3.3. More accurate determination of g-tensor More precise determination of g-tensor is obtainable at VHF due to increased size of the Zeeman term in the spin Hamiltonian with respect to the ZFS terms. 3.4. Determination of absolute sign of Spin-Hamiltonian parameters Another virtue of single crystal studies is the determination of the absolute sign of spin-
735
100
1
.
.
.
.
.
. ' .
t
L
0 0 0.2 0.4 0.6 0 8 1.0 1.2 1.4 1.6
D=-30.97 GHz : dotted lines (site II)
- 249.9 GHz I
I
I
4
6
Magnetic field (T)
I
I
a
Figure 4. A plot of the eigenvalues of the Ni*' ion in NizCdCls . 12Hz0, calculated using the indicated parameters, versus the intensity of the exernal magnetic field (B) for values of B up to 11 T showing all expected AM = 1 transitions at 249.9 GHz. The inset shows the eigenvalues versus B plot for values of B up to 1.5 T showing all expected AM = 1 transitions at X-band (9.498 GHz).
*
Hamiltonian (SH) parameters [8] at a relatively higher temperature than that required at low frequencies, because the Boltzmann factor, which governs population differences, is enhanced considerably at VHF frequencies, decreasing exponentially with the microwave frequency. 3.5. More precise determination of spin-Hamiltonian parameters: multifrequency approach A more precise determination of spin-Hamiltonian parameters can be accomplished by combining low and high frequency EPR studies since, in general, the fine-structure zero-field splitting and hyperfine structure parameters A,B, are determined more precisely at lower frequencies [9], while the g-tensor is determined more precisely at higher frequencies.
4. DISADVANTAGES OF VHF EPR
As for disadvantages, the EPR linewidth increases at higher frequencies, which can mask small splittings between lines, such as those due to the hyperfine interaction. It is well known that there occurs enhancement of inhomogeneous broadening due to distribution of g- values in the crystal at higher frequencies. It can be overcome by using low-frequency EPR to study the hyperfine interaction (9). Furthermore, g-strain broadening can be another impediment at
736
Ni,CdCl, H20 249 9 GHz D=-3097 g,=2 244
00-
g
00
05:
; e
B.0;
05
N$CdCIe.1ZH20 919GHz
80
D= 97 GHz q=2.244 9,=2 207
15
1.0
8.5
7.0
,
.
0.m
.
~.
.
,
.
0.m
0
.
I
85
OOJ
0.m
,
.
,
,
.
.
,
.
M
0
Orientakn d magneticfield(degrees)
.
,
w
Owntabon d magnstr field (degrees)
Figure 5a. The simulated ular variation EPR line positions for the' N ! ion in NizCdC16.12HzO at 249.9 GHz calculated using the indicated parameters.
Figure 5b. The simulated an lar variation of EPR line positions for the Ni'ion in NizCdCl6-12HzOat X-band (9.6 GHz) calculated using the indicated parameters.
VHF. However, one may gain if the exchange-narrowing effect ("10/3 effect") at higher frequencies (6) becomes effective at the same time. 5. ILLUSTRATIVE EXAMPLES
-
5.1. Ni2+in NiZCdCla 12H20
NiZ'is a non-Kramers S = 1 ion. In the single-crystal host NizCdCls * 12Hz0, its EPR has been studied in detail at X-band (9.6 GHz) by Misra et al. (10). The spin-Hamiltonian, characterized by axial symmetry, is, in the usual notation:
H = p&llBzSz
+ a(BxSx+ BySy)]+ D [S?
-
S(S+1)/3] + E (Sx' - S:)
(1)
There were found two magnetically-inequivalent NiZ' ions, I and 11, characterized by the following values ofthe SH parameters at 295 K (10): Ion I: gll = 2.236, g~=2.291, D = -31.12 GHz; Ion 11: gll = 2.274, 2.323, D = -6.81 GHz. For ion I, with much larger zfs, it is still possible to get an observable transition at Xband, although the Zeeman interaction is small compared to the zero-field splitting of 31 [Above about 45 GHz it would not be possible to observe any transition at 9.6 GHz for magnetic field values below 1 TI. However, at VHF (249.9 GHz), the allowed transitions are well into the high-field regime where the Zeeman term dominates as seen from Figure 4. Angular variation of EPR line positions were calculated using the above parameters for rotations about an axis perpendicular to the crystallographic c-axis, as shown in Figures 5a and 5b at 9.6 and 249.9 GHz, respectively. Since the Zeeman interaction dominates the zerofield splitting, these spectra are very easy to interpret at 249.9 GHz. In fact, as seen from Figure the magnitudes of the zero-field splitting can be directly measured from the VHF spectra, where the splitting of the two lines is 2D for B along the magnetic z-axis, unlike the case for X-band spectra. The linewidths at VHF for this crystal are expected to experience the compensating
737
effects of two competing mechanisms: (i) g-distribution which increases linewidth with increasing frequency; and (ii) exchange narrowing, (i.e. the 10/3 effect) which decreases the linewidth with increasing frequency. [See [S], and references therein]. One can expect to obtain greater accuracy in determination of g-values at VHF, in this case, by an additional significant figure from VHF spectra at 249.9 GHz from that at X-band. Since, at 249.9 GHz, the g-tensor effects (Zeeman term) are greater by a factor of about 25 as compared to that at X-band. 5.2. Mn2+in a-ZnzV207 single crystal
There were observed two magnetically inequivalent, but physically equivalent, Mn" ions [ll]. Due to the 55Mnnuclear spin I = 5/2 (g,, = 1.382), each fine-structure line splits into six hyperfine lines. Both sets of EPR lines were described by the same set of SH parameters. The EPR line positions were fitted to a spin Hamiltonian appropriate to a monoclinic site symmetry in the usual notation [12]:
The values of the SH parameters at 295 K, as estimated using the least-squaredmatrixdiagonalization procedure [2], are: g = 2.008; b:= 5.75 GHz, b: = 1.09 GHz. Since this is a case of isotropic g-tensor, VHF EPR can only offer limited improvement in the accuracy of the average g-value. The simulated angular variations of the MnZCfine-structure line positions for one magnetically inequivalent MnZ+ion, with the above spin-Hamiltonian parameters are shown in Figures 6a and 6b, respectively, at 9.6 and 249.9 GHz. Again, it noted that there is greater simplicity of the simulated angular variations of the line positions at 249.9 GHz shown in Figure 6b versus those at 9.6 GHz in Figure 6a; see also References [3,4]. Fe3' in YCaA104 single crystal
Fe3+ EPR studies at X-band (9.79 GHz) were carried out on a YCaA104 single crystal doped with 0.2% Fe3', substituting for AI3+ion [14]. The ion is characterized by tetragoanl site symmetry in this crystal. The magnetic z-axis is parallel to the crystal c-axis. The spin Hamiltonian applicable to this crystal, characterized by tetragonal symmetry, has the same form and definitions as those given by Equation (2), with m = 0,4, n= 2,4 (m < n), and with no hyperfine terms. There are two main problems in the determination of the spin-Hamiltonian parameters from X-band EPR line positions in this crystal: (i) The EPR lines are almost completely broadened out for magnetic field values higher than 0.35 T at Xband; (ii) Full angular variation was observed only for the central +1/2++-1/2 transition; only the EPR line positions for this transition for the orientation of B near the X (Y) magnetic axis can be used to estimate b: , because for B near the X(Y)-axis the dependence
of the central transition on is in first order, due to the transformation of the spin operators [8,13]. Finally, the SH parameters determined at X-band are: gll = 1.991; = 2.021; = 34.7 GHz.
738
Zn,V,O,: Mn” 249.9 GHz
Orientation of magnetic field (degrees)
Figure 6a. Simulated angular variation of EPR line positions for the Mn” ion in ZnzVz07 single crystal at 9.6 GHz.
Orientation of magnetic field (degrees)
Figure 6b. Simulated angular variation of EPR line positions for the Mn2+ion in ZnzVz07 single crystal at 249.9 GHz.
The EPR line positions corresponding to the non-central allowed fine-structure transitions, required to estimate the zero-field splitting parameter b; (=D) precisely, can only be observed at sufficiently high frequency, e.g. 249.9 GHz. It is noted that at room temperature the non-central allowed transitions may not be observable at VHF due to their fast spin-lattice relaxation; one may have to lower temperatures to observe them. The simulated angular variation of EPR line positions at 9.79 and 249.9 GHz are shown in Figure 3. 6. METALLOPROTEINS
Metalloproteins, primarily in the disordered state, are biologically and medicinally interesting systems. Moreover, their EPR data are amenable to sophisticated computational analysis [ 17,181. A number of biologically interesting enzymes, including a variety of oxygenases, use bound Fe@I)in catalysis. The study of the mechanisms of these enzymes has been hampered by the lack of a suitable spectroscopic probe for Fe(I1). It is well known that the Fe(II) ion possesses large zero-field splitting (ZFS) in metalloproteins, and temperatures below 10 K are required for observation of its EPR signal The local order around the Fe (11) ion can be probed more sensitively by VHF EPR, thus providing information on the role that the iron ion plays in some metalloenzymes. This can be accomplished by exploiting the recently developed computational techniques [ 16,171 to simulate and fit EPR spectra of mononuclear Fez+and exchange coupled FeZC-Fe3+ centres in these amorphous materials. The electronic environment of the paramagnetic centre in the protein and the perturbations induced by ligand binding will be defined by the values of the spin-Hamiltonian (SH) parameters and the shape of the spectrum. We propose to study four specific Fe-containing proteins by VHF EPR at Cornell University’s Advanced Center for Electron Spin Resoanance Technology (ACERT) directed by Professor J. Freed: (i) biphenyl dioxygenase (BPDO) [IS], (ii) 2,3-dihydroxybiphenyI 1,2dioxygenase (DHBD) 11191, (iii) BphF, and (iv) phenol hydroxylase (PH) [20]. The samples will be prepared by Ms. N. Imbault and Professor J. Powlowski (Concordia University), and Professor L. Eltis (University of British Columbia). BPDO contains two metallocentres: a mononuclear Fe(I1) (S = 2) and a “Rieske-type” 2Fe-2S cluster with one Fe bound by two cysteines and the second by two histidines. The
739
reduced Rieske center has a net charge of I+ and the Fe(I1) and Fe(II1) ions are exchangecoupled so that the net spin is = %. Since the mononuclear iron center is believed to be the site of oxygen activation and insertion, EPR will be used to characterize its environment when different chlorinated biphenyl substrates and uncouplers are bound. DHBD enzyme is a type I Fe(I1)-dependent extradiol dioxygenase [21] that catalyzes the extradiol cleavage of 2,3-dihydroxybiphenyI (DHB), incorporating both atoms of dioxygen into the product, (HOPDA). DHBD is a limiting step in the aerobic degradation of PCBs by the bph pathway as it is competitively inhibited or suicide-inhibited by a number of chlorinated catechols that are produced during PCB-transformation, such as 3-chlorocatechol. The enzyme is thus the subject of directed evolution experiments to overcome these metabolic blocks and to engineer microorganisms for bioremediation. EPR data are expected to provide novel insights into substrate activation of the reaction by DHBD and related Fe(I1)-dependent enzymes. BphF is the Rieske-type ferredoxin associated with biphenyl dioxygenase. The 2Fe-2S cluster of BphF is structurally and electronically very similar to that of BPDO. The recently determined crystal structure of BphF, refined at 1.6 8, resolution, reveals that the protein has the same fold as other Rieske proteins, including the Rieske cluster domain of BPDO [22]. High field EPR studies of reduced and oxidized BphF will greatly facilitate studies of BPDO. The phenol hydroxylase enzyme (PH) [I91 is similar to the well-studied methane monooxygenase in terms of type of iron center, and the requirement for interaction with both a reductase protein and an activator protein. Progress on characterization of PH has been hampered by the lack of an x-ray structure, since the protein is not easily crystallized. It is precisely for this reason that EPR studies of phenol hydroxylase in different redox states and with various phenolic ligands should provide greater detail of the environment around the iron centres, and how it changes during binding of substrates, binding of interacting proteins, and during steps in the catalytic cycle. This type of information is not readily accessible using other techniques. In addition, a Mn-substituted form of phenol hydroxylase is available,which will also be examined using the EPR technique. CONCLUDING REMARKS
Single-crystal VHF EPR is a potentially powerful technique to study biological systems, e.g. metalloproteins, which contain transition metal ions characterized by large zero-field splitting parameters D, E, e.g. Fe2+, Mn3+. A multi-frequency approach has been found usehl to accurately estimate all of spin-Hamiltonian parameters by simultaneously fitting all resonant line positions observed at various frequencies. The g-values are determined more accurately at higher frequencies, whereas the fine- and hyperfine-structure parameters require lower frequencies for more precise determination. It is important to use complete and accurate diagonalization of the spin Hamiltonian matrix [2,4], rather than less accurate perturbation methods [3,6], and to fit all EPR line positions simultaneously. In a forthcoming publication by Misra et al. [ 1 I] experimental 249.9-GHz EPR studies on the samples discussed in Section 5 are reported. Acknowledgments The author is grateful to the Natural Sciences and Engineering Research Council of Canada for partial financial support, and to Professor J. Freed for many useful discussions. Computations were facilitated by the Cornell Theory Center.
740
REFERENCES 1. J.H. Freed, Annu. Rev. Chem., 51 (2000) 655. 2. S.K. Misra, J. Magn. Reson., 23 (1 976) 406. 3. W.B. Lynch, R.S. Boorse, J.H. Freed, J. Am. Chem. Soc., 115 (1993) 10909. 4. R.M. Wood, D.M. Stucker, L.M. Jones, W.B. Lynch, S.K. Misra, J.H. Freed, Inorg. Chem., 38 (1999) 5384. 5. D.P. Goldberg, J. Tesler, J. Krzystek, A.G. Montalban, L.-C. Brunel, A.G.H. Barrett, and B.M. Hoffman, J. Am. Chem. Soc., 119 (1997) 8722. Their powder of microcrystals aligned in the strong magnetic field, produced an effective single-crystal spectrum with the magnetic z-axis parallel to B in a VHF study of Mn3+ in Mn(TPP)Cl, Mn(ODMAPz)Cl, Mn(ODMAPz)DTC, and Mn(DD-IX-DME)CI at frequencies of 200 GHz and higher. 6. W.R. Hagen, in Advances in Inorganic Chemistry, 38, (1992) 165; as an illustration spectra for Mo-nitrogenese is recorded for both B1 B (no signal) and B1 11 B (a signal) configurations; W.R. Hagen, Coord. Chem. Revs., 190-192 (1999) 209. 7. S.K. Misra, New Methods of Simulation of MnZfEPR Spectra: Singli Crystals, Polycrystalline and Amorphous (biological) Materials, Vol. 18: Instrumental Methods in Electron Magnetic Resonance, Eds. C. Bender and L. Berliner (Kluwer AcademicPlenum Publications, 2002), in Press. 8. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, (Clarendon Press, Oxford, 1970). 9. S.K. Misra. PhysicaB, 240 (1997) 183. 10. S.K. Misra, L.E. Misiak, and P. Chand, Physica, B202 (1994) 31. 11. S.K. Misra, S. I. Andronenko, K.A. Earle, and J.H. Freed, Appl. Mag. Reson. (In press). 12. S.K. Misra and C. Z. Rudowicz, Physica, B147 (1988) 677. 13. S.K. Misra, in Handbook of Electron Spin Resonance, C.P. Poole, Jr. And H.A. Farach, Eds. (A.I.P. Press, Springer, New York, 1999), Chap. VII, p. 115. 14. S.K. Misra and S. I. Andronenko, Phys. Rev. B (In press) 15. S.K. Misra, Physica, 121B (1983) 193. 16. S.K. Misra, J. Magn. Reson., 137 (1999) 83-92. 17. S.K. Misra, J. Magn. Reson., 140 (1999) 179-188. 18. N.Y.R Imbeault., J.B. Powlowski, C.L.Colbert, J.T.Bolin, L.D Eltis, J. Biol. Chem.,. 275 (2000) 12430-12437. 19. J.T Bolin and L.D. Eltis “2,3-Dihydroxybiphenyl 1,2-Dioxygenase”. In Handbook of Metalloproteins (Eids. Messerschmidt et al.), 02000, Wiley. 20. J.B. Powlowski, J. Sealy, V. Shingler, and E. Cadieux, ”On the Role of DmpK, An Auxiliary Protein Associated with Multicomponent Phenol Hydroxylase from Pseudomonas sp. strain CF600”, J. Biol. Chem., 272 (1997) 945-951. 21. Eltis L.D. and Bolin J.T, J Bacteriol, 178 (1996) 5930-5937. 22. C.L. Colbert, M.M.-J. Couture, L.D. Eltis, and J.T. Bolin, Structure Fold. Des., 8 (2000) 1267-1278.
EPR in the 21" Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
74 1
EPR evidence of onset of the quantum critical point in CuGe03:Fe S.V.Demishevasb,R.V.Buntinga, H.Ohtac, S.Okubo', Y.Oshimad and N.E.Sluchankoa aL~w Temperatures Laboratory, General Physics Institute, Vavilov street, 38, 117942 Moscow, Russia bVentureBusiness Laboratory, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan 'Molecular Photoscience Research Center and Department of Physics, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan dGraduate School of Science and technology, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan Influence of doping by iron impurity on spin-Peierls state in CuGe03 is studied. EPR measurements for the frequencyltemperature domain 60-450 GHdl.8-300 K show that insertion of 1% of Fe completely destroy both spin-Peierls and antiferromagnetic orders. Damping of long-range magnetic order is accompanied by onset at K of +ewer asymptotic for magnetic susceptibility with the index a=0.35. This effect is characteristic to the limit of strong disorder for doped CuGe03 and may reflect onset of a quantum critical point in CuGeO3:Fe. Discovery of inorganic spin-Peierls compound CuGeO3 opened an opportunity to study influence of doping and disorder on the spin-Peierls state. Numerous experiments and theoretical studies have been carried out in this field up to now. However, from the theoretical point of view, most of the available data correspond to the limit of weak disorder when density of states have a pseudogap, i.e. a spin-Peierls gap filled by disorder-induced states [l]. In this case the expected temperature-concentration T-x phase diagram consists of uniform state, spin-Peierls state, antiferromagnetic state and the region, where antiferromagnetic and spin-Peierls orders coexist [l]. This structure of phase diagram was observed experimentally for Zn, Si, Ni, Co, Mg and Mn impurities [l-lo]. In the limit of a strong disorder the ground state of CuGeOs is gapless and the density of states diverges at E=O: P(E) cc I [l]. As a consequence the temperature dependence of magnetic susceptibility acquires the form [111
742
where a 4 . From the theoretical point of view the non-Curie asymptotic behavior of magnetic susceptibility reflects the onset of the Griffiths phase (GP) which thermodynamic properties are controlled by relatively rare spin clusters correlated more strongly than average [12-151. The GP appears in various spin systems below some critical temperature if the magnitude of random potential is strong enough to destroy transition to magnetically ordered phase [ 12151.
As long as any type of the long-range magnetic order in GP at finite temperature is smeared, the aforementioned physical situation is often discussed in terms of a quantum critical point. In this ansatz the disorder shifts the temperature of magnetic transition to absolute zero and in the vicinity of the “critical point” susceptibility diverges in accordance with Equation (1). A non-Curie type behavior of have been first reported for CuGeO3 doped with Zn [4]. However, experiments in Ref.4 were carried out for extremely low Zn concentrations corresponding to the weak disorder limit, and the observed deviations from the Curie law can not be related to the case of strong disorder. The aim of the present work consists in providing evidence of experimental realisation of a strong disordered limit in doped CuGe03. We argue that insertion of 1% of iron in CuGe03 matrix completely damp both spin-Peierls and antiferromagnetic transitions and gives rise to onset of a quantum critical point. Single crystals of Cuo99Feo.olGe03were obtained by self-flux technique [16]. The quality of crystals have been controlled by X-ray and Raman scattering data; the actual contents of Fe in crystals was determined by chemical analysis. The structure of the samples studied coincided with the structure of pure CuGe03 and the effect of doping on the Raman spectra confirmed that iron impurity substitute cooper [17]. Two types of experimental facilities were used to study magnetic properties of CuGe03:Fe. Magnetoabsorption lines for the frequencies up to 450 GHz were studied with the help of the magneto-optical facility at Kobe University. In this experiment we measured transmission through the sample as a function of magnetic field up to 16 T at fixed frequency for liquid helium temperatures 4.2 K and 1.8 K. Simultaneously the reference transmittance of a thin layer of DPPH powder have been recorded and both high-frequency EPR spectra of CuGe03:Fe and DPPH were analyzed quantitatively. Temperature dependence of the EPR spectrum in the range 1.8-140 K was measured using 60 GHz cavity spectrometer in the General Physics Institute [17]. For each temperature studied the accuracy of the temperature stabilization was better than 0.01 K. All magnetoabsorption experiments reported below were carried out in B I I a geometry. Typical magnetoabsorption spectra in transmission experiment are shown in Figure 1. Up to o / 2 x = 450 GHz neither additional “impurity” lines, nor AFMR modes have been detected and the spectrum for CuGe03:Fe consists of a single lorenzian EPR line. The corresponding g-factor value is close to that for Cu2+ions in the case B I I (see below). The resonant field for this line scales linearly with frequency, i.e. g-factor is frequency independent (see inset in Figure 1). Transmission data obtained at various frequencies were used to calculate EPR integrated intensities for CuGe03:Fe and DPPH. The main problem in the quantitative analysis of high frequency EPR line is a possible violation of the widely applied approximation [18]: . Indeed, in general case the integrated intensity for arbitrary electromagnetic wave frequency and resonant magnetic field B,es is given by [191
743
h
% 0.2 0
100
200
300
400
Frequency (GHz) Figure 1. Magnetoabsorption spectra for CuGeO3:Fe in transmission experiment.
Figure 2. Frequency and resonant field dependence of magnetic moment and ESR line width AB for CuGe03:Fe at K. Curve 1 correspond to best fit of using Equation (5).
and the deviations from Curie law deduced from may reflect a non-linearity of magnetic moment in strong magnetic field. It follows from Equation (2) that for each frequency
where indexes 0 and 1 denote characteristics of ESR lines for DPPH and CuGeO3:Fe respectively. Assuming that magnetic moment of DPPH is given by Brillouin function it is possible to calculate field or frequency dependence of magnetic moment = for CuGe03:Fe with the help of Equation (3). The result gives (Figure 2) that linear region Mi cc lasts up to = 6 T (w/271 e200 GHz). At higher resonant fielddfrequencies magnetic moment of CuGe03:Fe tends to saturate and above = 1 1 T (o/271 w 350 GHz) Mi starts to decrease with field (Figure 2). Along with the integrated intensity the width AB of the EPR line in CuGeO3:Fe was calculated. Contrary to the frequency independent g-factor this parameter demonstrates a considerable frequency dependence. It follows from Figure 2 that AB increases two times
744
when frequency is varied from o / 2 n = 60 GHz to w/271: = 450 GHz. Experimental data at T=1.8 K can be modeled by expression +C,
(4)
which is characteristic to Raman relaxation mechanism for S=1/2 ion [20]. The line 1 in Figure 2 correspond to best fit parameters in Equation (4) A =(3.2f0.4).10-7 T/GHz2 and =(0.063+_0.004)T. It is interesting, that attempts to model by expression for the direct process cc /tanh(Aw/2kBT) [20] have failed as long as the theoretical frequency dependence was too strong to fit the experimental data in Figure 2. Therefore it is possible to conclude that at low temperatures the dispersion of the relaxation time in CuGeO3:Fe is controlled mainly by the Raman process. Temperature measurements of the EPR in the 60 GHz cavity spectrometer were carried out on the same crystal as was investigated in the quasi-optical transmission experiment. For the precise determination of the g-factor a DPPH crystal was placed in the cavity together with the CuGe03:Fe sample. The results of the previous section indicate that at o/271: = 60 GHz the CuGeO3:Fe remains in the region of linear magnetic response and consequently a relation cc = is valid. At all temperatures studied a single absorption line of lorenzian shape was observed (Fig. 3), that is in agreement with the results of the quasi-optical experiment. Data in Figure 3 were used to calculate temperature dependences of the g-factor g(T), line width M ( T ) and integrated intensity (see Figure 4). For >20 K the value of the g-factor is g m2.15 and characteristic to Cu2' ions in CuGeO3 structure for geometry B I I a 1211. Below T=20 K g-factor starts to increase with lowering temperature and reach the value g=2.19 at T=1.8 K (Figure 4). It is worth to note, that in the case of Fe-doped crystal no giant changes of the gfactor like in Ni-doped CuGeO3 [S] are observed. The temperature dependence of the line width is non-monotonic: when temperature is lowered the first decreases, passes through a minimum at T=10-20 K, and finally starts to increase again. It is interesting that in pure CuGe03 the width of the EPR line decreases gradually with lowering temperature and the magnitude of at Figure 3. Evolution of the ESR absorption line with temperature measured in cavity spectrometer T=100 K is about 6 times smaller ( 0 / 2 n =60 GHz, mode TEoll, quality factor Q=104). than in Fe-doped crystal [21].
.-u)
L
. 0.1
i
Figure 4. Temperature dependences of the integrated intensity line width and g-factor obtained in cavity experiment.
The decrease of temperature makes a difference in more dramatic: at T=l.S K the line width for the Fe-doped CuGe03 is 200 times bigger than in pure crystal (see Figure 4 and data from [21]). The significant difference between pure and Fe-doped CuGeO3 is visible in the temperature dependence of the integrated intensity. It follows from Figure 4 that in the Fedoped crystal spin-Peierls transition is completely damped. For 70 K integrated intensity obeys Curie law (Figure 4, curve 1). In the temperature range 2570 K saturates, and at lower temperatures a power law asymptotic behavior with the index
a=0.35+0.03 is observed (Figure 4, curve 2). Summarizing experimental results of the present work, we wish to mark that observed low temperature behavior of the ESR line reflects intrinsic properties of Cu2' chains modified by Fe impurity rather than impurity paramagnetism caused by Fe ions. This conclusion can be deduced from the g-factor values characteristic to Cu2' (Figure 4) and the observation of the line width frequency dependence given by Equation (4). Indeed, for the Fe2+ion substituting Cu2+ion in S=1/2 chain a spin state with S=2 may be expected [22]. For the integer impurity spin the term proportional to in expression for the line width should vanish and will be frequency independent [20]. The expected "impurity behavior" for Fe2' contradicts to experimental data (Figure 2) and magnetic properties of CuGe03:Fe system are controlled by disordered Cu2' chains. The presence of the disorder in magnetic subsystem follows from the strong broadening of the ESR line with respect to the pure crystal (Figure 4 and Reference 21) and agrees with the results of the structural studies [17]. As long as the measured integrated intensity at 0 / 2 n = 60 GHz for CuGeO3:Fe is proportional to magnetic susceptibility the latter quantity diverges at T<20 K: , where ~ 0 . 3 5 In . the model of a quantum critical point the index a in Equation (1) is not universal and depends on the distribution h c t i o n of the spin clusters [12-151. However theoretical calculations [23] and experimental data [24] for this regime gives values a-0.30.33 which are close to that observed in the present work. The non-Curie asymptotic of magnetic susceptibility in CuGeO3:Fe not only corresponds to one expected for a quantum critical point, but also appears in the absence of any type of magnetic ordering known for the doped CuGeO3 [l]. As long possible Nee1 states are
746
damped by disorder together with the spin-Peierls state, it possible to conclude that inserting of of iron in CuGeO3 matrix really induces a strongly disordered limit of doping and leads to formation of a quantum critical point. Considering the formation of the Griffths phase in CuGe03:Fe one gets the value of as K (Figure Consequently some low temperature anomalies of EPR (Figure and Figure may reflect intrinsic properties of the GP. For instance possible interaction effects inside spin clusters of GP may be responsible for a weak temperature dependence of the gfactor observed at (Figure and unusual field dependence of magnetic moment (Figure 2). However this problem requires further theoretical investigation. Authors are grateful to L.I.Leonyuk’ for providing of CuGe03:Fe crystals. A.A.Pronin and N.A.Samarin are acknowledged for assistance. This work have been supported by programmes “Physics of Microwaves” and “Fundamental Spectroscopy” of Russian Ministry of Industry, Science and Technology.
Deceased. M.Mostovoy, D.Khomskii, and J.Knoester, Phys. Rev. B Y.Sasago, N.Koide, KUchinokura et al., Phys. Rev. B S.Koad, J.-G.Lussier, D.F.McMorrow, and D.McK Paul, J.Phys.: Condens. Matter K.Manabe, H.Ishimoto, N.Koide et al., Phys. Rev. B H.Nojiri, T.Hamamoto, Z.J.Wang et al., J.Phys.: Condens. Matter B.Grenier, J.-P.Renard, P.Veillet et al., Phys. Rev. B P.E.Anderson, J.Z.Liu, and R.N.Shelton, Phys.Rev. B V.N.Glazkov, A.I.Smirnov, 0.A.Petrenko et al., J.Phys.: Condens. Matter T.Masuda, A.Fujioka, Y.Uchiyama et al., Phys. Rev. Lett. P.E.Anderson, J.Z.Liu, andR.N.Shelton, Phys. Rev. B L.N.Bulaevskii, A.V.Zvarykina, Yu.S.Karimov et al., Sov. Phys. JETP R.B.Griffiths, Phys. Rev. Lett. D.S.Fisher, Phys. Rev. Lett. Phys. Rev. B Phys. Rev. B R.A. Hyman, K. Yang, R.N. Bhutt, S.M. Girvin, Phys. Rev. Lett., A.Rosch, in Abstracts of LT22, Helsinki, S.V.Demishev, L.Weckhuysen, J.Vanacken et al., Phys. Rev. B S.V.Demishev, R.V.Bunting, L.I.Leonyuk et al., JETP Letters S.A.Al’tshuler and B.M.Kozyrev, Electron Paramagnetic Resonance, Academic Press, New York and London, p. C.Kitte1, Introduction to Solid State Physics, Fourth edition, J.Wiley & Sons, New York, London, Sydney, Toronto, ch. A.Abragam and B.Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon Press, Oxford, S.V.Demishev, A.V.Semeno, N.E.Sluchanko et al., JETP W.Low, Paramagnetic Resonance in Solids, Academic Press, New York and London, H. Reiger, A.P. Young, B. Andraka, A.M. Tsvelik, Phys. Rev. Lett.,
EPR in the 2 1'Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
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Millimeter and submillimeter wave ESR measurement of spin ladder system Sr(Cu,-xZnx),O, S. Okubo", K. Hazukib, T. Sakuraib,H. Ohta".', H. Yoshidad, M. Azumad and M. Takanod "Molecular Photoscience Research Center, Kobe University, Rokkodai 1-1, Nada, Kobe 6578501, Japan T h e Graduate of School of Science and Technology, Kobe University, Rokkodai 1-1, Nada, Kobe 657-8501, Japan "Venture Business Laboratory, Kobe University, Rokkodai 1-1, Nada, Kobe 657-8501, Japan dInsitute for Chemical Research, Kyoto University, Uji, Kyoto 61 1-001 1, Japan
Non-magnetic impurity doped S=1/2 spin ladder systems Sr(Cu,~,Zn,),O, have been studied by millimeter and submillimeter wave ESR measurements. The 4% Zn doped system, which has the highest TN, showed the g-shift and the broadening of the linewidth at low temperature. AFMR observed at 1.8K turned out to be the easy axis type AFMR mode.
1. INTRODUCTION Since the discovery of Haldane system many spin gap systems have been found in the low dimensional quantum spin systems[l-71. The existence of the spin gap can be explained by the valence bond solid (VBS) picture. Recently, an antiferromagnetic order is observed experimentally in the non-magnetic impurity doped spin Peierls system ( C U , . ~ Z ~ . ) G ~ O It ,[~]. suggests that the non-magnetic impurity breaks the VBS, and the antiferromagnetic moments are induced. Many theoretical models, which explain the appearance of antiferromagnetic moments by non-magnetic impurities, are proposed[9]. Theorists also expect that antiferromagnetic moments can be induced by doping the non-magnetic impurity in S=l/2 2-leg ladder system[ lo]. Therefore, experimental studies for 2-leg ladder system are desired. SrCyO, is one of the model substance of S=1/2 2-leg ladder system. The crystal structure is described in reference [l 11. From the magnetic susceptibility measurements and the neutron measurements [12] of 2-leg ladder system SrCy03, the spin gap between the singlet ground state and the first excited state was estimated to be 550K. Azuma et have performed the magnetic susceptibility measurements of Sr(Cu,.,ZnJ,O,[ 131. Even the 1% doped system shows a cusplike anomaly suggesting the onset of an antiferromagnetic ordering at around 3K. The temperature, which shows an anomaly, increases as the impurity concentration is increased. The maximum temperature of anomaly is about 8K at 4% sample. From the specific heat measurements of 2% and 4%, samples the cusplike anomalies were also observed.
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These results suggest that the observed anomaly is the NCel Temperature. To clarify the antiferromagnetic state of Zn doped SrCyO,, we performed the submillimeter and the millimeter wave ESR measurements of 4% Zn doped Sr(Cu,.,ZnJ,O,. As Sr(Cu,.xZn,),O, are obtained by the high pressure synthesis, only the powder sample can be obtained. Therefore, we used the aligned powder sample to clarify the magnetic anisotropy.
2. EXPERIMENTAL The powder samples of Sr(Cu,~,Zn,),O, are synthesized under high pressure. The sample synthesis is described in Reference 11. Aligned powder samples, whose binding resin is Stycast 1266, are used in ESR measurements. Procedure of making aligned powder sample is described in Reference 14. The ESR measurements are performed by the transmission method. Detailed experimental set up is shown in Reference 15. The millimeter and the submillimeter wave ESR measurements are performed using pulsed magnetic field up to 16 T. The observed temperature region is from 1.8K to 80K. Measured frequencies are from 30GHz to 3 15GHz. 3% Zn doped system is used for powder sample measurements. 4% Zn doped system is used for aligned powder sample measurements. 3. RESULTS AND DISCUSSION
Figure 1 shows the ESR spectra of the powder sample and the aligned samples observed at 1.8K for 160GHz. The ESR spectrum of powder sample shows typical Cu” ion powder pattern. It is difficult to observe the resonance shift in the temperature dependence measurements for the powder sample. Therefore, we have performed the temperature dependence measurements of the 4% Zn doped aligned sample with TN=8K.The ESR spectra of aligned sample show that the sample is aligned very well. Signal at around 5.36T seems to be the paramagnetic impurity signal, because it’s intensity increases as decreasing the temperature. For the m a l i g n sample, the resonance at around 5T, which is coming from the component of H//align seems to remain due to the aligned imperfection. The intrinsic resonances are the lower field resonance for H//align and the higher field resonance for malign. The resonance shift is observed clearly for 30GHz below 7K for H//align. It suggests the development of the internal field. From the frequency dependence measurements at 1.8K, the distinct frequency dependence is observed for Hilalign. We plotted the difference between the resonance field and the EPR as a function of the resonance field as shown in Figure 2(a). The g-value of EPR are estimated to be g,,=2.279 and g,=2.033 from the frequency field relations observed at 70K. The AH increases as the frequency is decreased. It suggests that the frequency field relation of H//align follows the behavior of the typical AFMR mode. The solid line is a fitting line of AFMR mode (o/y)=H2+c. From the fitting an antiferromagnetic gap is estimated to be 0.64T. On the other hand, Figure 2(b) shows the difference between the resonance field and the EPR for m a l i g n . It shows that the frequency field relation is similar with the EPR line. We performed an AFMR mode analysis of these resonances by the easy axis type AFMR mode. If we assume that the AFMR mode of Hlialign as the hard axis, the AFMR mode of m a l i g n is the mixture of the easy axis and the 2nd easy axis. Then the AFMR mode of
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m a l i g n will be the same with EPR. However, figure 2(b) shows that AH is not completely zero. If we choose 2nd easy axis as the Hiialign, the AH can be interpreted as Figure2(b). An antiferromangetic(AF) gap of 2nd easy axis mode at zero magnetic field is estimated to be o/y0.64(T) by the fitting as shown in Figure 3. The easy axis mode can be estimated by the spin flop field which is equal to the AF gap of 2nd easy axis mode. Therefore, the observed AFMR mode for m a l i g n can be considered as the mixture of the easy axis and the hard axis. The inset of Figure 3 shows the fitting results of U a l i g n mode as the average of the easy axis and the hard axis. The AF gap of the hard axis mode is estimated to be 0.81(T).
4
4.5
5
5.5
6
6.5
7
H(T) Figure 1. Typical ESR spectra of powder sample and aligned samples at 1.8K for 160GHz.
4. CONCLUSION
Millimeter and submillimeter wave ESR measurements of non-magnetic impurity doped S=1/2 2-leg ladder spin system Sr(Cu,.xZn.J203have been performed. Distinct AFMR modes are observed in the 4% Zn doped aligned powder sample. The observed AFMR mode turned out to be the easy axis type. AF gaps are estimated to be 0.81T and 0.64T for the hard axis and 2nd easy axis, respectively. 0.20 0.15 g0.10
5
0.05 0.00
0
2
4
6
8
10 12 14 16
Resonance field (T) Figure 2. The difference between the resonance field at 1.8K and the EPR for H//align(a) and €€Lalign(b).
H, (TI Figure 3. The frequency field relation observed at 1.8K. Lines show the AFMR analyses. See text for the detail.
ACKNOWLEDGEMENT This work was supported by Grant-in-Aid for Scientific Research on Priority Areas (A) (No. 12046250 Novel Quantum Phenomena in Transition Metal Oxides) and (B) (No. 13130204) from the Ministry of Education,Culture, Sports, Science and Technology of Japan.
REFERENCES 1. F. D. M. Haldane, Phys. Rev. Lett., 50 (1993) 1153. 2. S. Yamamoto, Phys. Rev., B 52 (1995) 10170. 3. J. P. Renard, M. Verdaguer, L. P. Regnault, W. A. C. Erkelens, J. Rossat-Mignod and W. G. Stirling, Europhys. Lett. 3 (1987) 945. 4. T. Sakaguchi, K. Kakurai, T. Yokoo, J. Akimitsu, J. Phys. SOC.Jpn., 65 (1996) 3025., S. Kimura, H. Ohta, M. Motokawa, T. Yokoo, J. Akimitsu, J. Magn. Magn. Mat., 177-181 (1998) 624. 5. M. Hase, I. Terasaki, K. Uchinokura, Phys. Rev. Lett., 70 (1993) 3651. 6. S. Kimura, S. Hayashi, H. Ohta, H. Kikuchi, H. Nagasawa, N. Nojiri, M. Motokawa, Physica B, 246-247 (1998) 565. 7. M. Azuma, T. Saito, Y. Fujishiro, Z. Hiroi, M. Takano, F. Izumi, T. Kamiyama, T. Ikeda, Y. Narumi, K. Kindo, Phys. Rev., B 60 (1999) 10 145. 8. M. Hase, K. Uchinokura, R. J. Birgeneau, K. Hirota, G. Shirane, J. Phys. SOC.Jpn., 65 (1996) 1392. 9. H. Fukuyama, T. Tanimoto, M. Saito, J. Phys. SOC.Jpn., 65 (1996) 1182. 10. Y. Motome, N. Katoh, N. Furukawa, M. Imada, J. Phys. SOC.Jpn., 65 (1996) 1949., H. Fukuyama, N. Nagaosa, M. Saito, T. Tanimoto, J. Phys. SOC.Jpn., 65 (1996) 2377. 11. Z. Hiroi, M. Azuma, M. Takano, Y. Bando, J. Solid. State Chem., 95 230-238 (1992). 12. M. Azuma, Z. Hiroi, M. Takano, K. Ishida, Y. Kitaoka, Phys. Rev. Lett., 73 3463-3466 (1994). 13. M. Azuma, Y. Fujishiro, M. Takano, M. Nohara, H. Takagi, Phys Rev., B 55 8658-8661 (1997). 14. S. Okubo, H. Ohta, M. Hayashi, T. Yamada, H. Nojiri, T. Sakon, M. Motokawa, I. Mogi, K. Watanabe, H. Kikuchi, H. Nagasawa, N. Kitamura, phys. stat. sol., (b) 215 (1999) 1099. 15. M. Motokawa, H. Ohta, N. Makita, Int. J. Infrared & MMW, 12(2) (1991) 149-155., S. Kimura, H. Ohta, M. Motokawa, S. Mitsudo, W-J Jang, M. Hasegawa, H. Takei, Int. J. Infrared & MMW, 17(5) (1996) 833-841., N. Nakagawa, T. Yamada, K. Akioka, S. Okubo, S. Kimura, H. Ohta, Int. J. Infrared & MMW, 19(2) (1998) 167-176.
EPR in the 2 1" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
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High frequency ESR on quantum spin systems by using single shot and repeating pulsed fields H. Nojiri Department of Physics, Okayama University, Okayama 700-8530, Japan
High frequency ESR system has been developed by using both single shot and repeating pulsed fields. Single shot pulsed fields have been used to generate high fields up to 40 T. Temperature of specimen can be changed between 400 K and 400 mK. To enhance the sensitivity of the system, a repeating pulsed field generator has been also used, which is capable of generating 30 T every 2 seconds. Signal to noise ratio of ESR spectra has been improved more than one magnitude. The new method provides a way to improve the sensitivity of a high frequency ESR system. By using the high frequency ESR system, the spin gap in Sr,,Cu,, Oqlhave been studied.
1. INTRODUCTION Magnetic excitations in quantum spin systems have attracted much attention in these decades. ESR has been considered as a unique tool to study magnetic excitations among various means such as inelastic neutron scattering, Raman scattering and far-infrared spectroscopy. One of the reasons is that high energy-resolution of peV-order can be achieved by using monochromatic radiation sources such as a Gunn oscillator or a far-infrared laser. A capability of high field measurement enables us to examine new phases appear in high magnetic fields. Different from a conventional microwave ESR equipment, a high-energy magnetic excitation up to 30 meV can be observed by use of THz-ESR. On the other hand, sophisticated high sensitive devices such as a resonator or a cavity are scarcely used in a wide-band THz-ESR. It is because such spectrometer covers the extremely wide frequency between Q-band and several-Tela-Hz. It has been desired to develop an alternative mean to enhance the sensitivity of a THz-ESR system. A repeating pulsed field is a useful method to improve a signal to noise(S/N) ratio of a measuring device working under pulsed magnetic fields. It is capable of generating pulsed fields up to 30 T every two seconds. This system had been developed to perform high field pSR as well as high field neutron diffraction, in which pulsed beam sources with intense instantaneous intensities are obtained. Recently, the system is applied for the THz-ESR to improve the S N r a t i o by averaging several hundreds of pulsed field shots. In the following, new features of high frequency ESR spectrometers at Okayama-university by using both single shot and repeating pulsed fields are presented. Several examples of recent activities on quantum spin systems are also shown.
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2. INSTRUMENTATION 2.1. Radiation source Figure 1 shows the schematic set up of the ESR equipment. For the insulating compounds, a simple transmission method is employed. As the radiation source, three kinds of devices are combined. Below 100 GHz, conventional G u m oscillators are used. For frequency between 250 GHz and 7 THz, an optical pumped fir-infrared laser is employed. Several backward travelling wave tubes are also used in 35 and 400 GHz. The available frequencies of the laser are shown in Fig. 2. To cover the wide frequency range, a proper choice of molecular gases in the laser is most important. Such extremely wide frequency range is covered by four different kinds of detectors as follows:InSb, GaAs, a field-tuned InSb and GeGa. 2.2.ESR spectrometer with single shot pulsed field For generation of magnetic fields, a compact capacitor bank of 90 kJ has been developed and pulsed fields up to 40 T are generated. The capacitor bank is designed by Okayama-group aiming for the spread of a commercial high frequency ESR system with a pulsed field generator. The pulse duration time is 5 msec. It can be extended to 15 msec by using a crowbar circuit. The inner diameter of the cryostat can be changed between 11 mm and 25 mm depending on the maximum field. A magnetic field up to 20 T can be generated by the split type pulsed magnet and it is used for the experiments for the Voight configuration. The lowest temperature down to 0.4 K can be achieved by using a 3He cryostat.
Figure 1. Schematic view of high frequency ESR eqipment.
Figure 2. Frequency coverage of high frequency ESR equpment. Dashed line shows the g=20 resoance line.
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2.3. ESR spectrometer by using a repeating pulsed field As mentioned in the introduction, a resonator and a cavity are difficult to be combined with a wide band and full tunable high frequency ESR equipment. An alternative method should be employed to enhance the sensitivity of the system, which does not disturb the tunability of the system. A repeating pulsed field is employed for this purpose. A repeating pulsed field was developed to perform high field pSR and neutron diffraction experiments[l]. It is capable of generating pulsed fields up to 30 T at every 2 seconds. Figure 3 shows the schematic view of generated magnetic fields by the system. In the present case T=2 sec and z=1 msec. The S/N ratio can be improved by the averaging of numbers of pulsed field sweeps. Figure 4 shows example spectra showing an effect of the averaging. After a quarter an hour, the quality of ESR signal is much improved. On the other hand, in a single shot measurement, the detail of the line shape is hardly resolved. The advantage of this method is that the repeating pulsed field system can be used for any frequency. By using present technique, it becomes possible to built a high sensitive and full tunable high frequency ESR system.
3. RESULTS AND DISCUSSION 3.1. Spin gap in Sr,,Cu,, O,, A magnetic ladder has been interested in this decade because of the interesting intermediate magnetic network between one and two-dimensions. Theoretical prediction of high superconductor is another reason of the interests. Actually, a superconducting phase was found in Sr,,Cu,, 04,by hole doping. This compound is made up of two kinds of magnetic networks. One of networks is a magnetic ladder and the other is a magnetic chain. The ground state of chain is singlet. The origin of the gap is the alternation of the intra-chain exchange couplings by the charge ordering. The spin gap of the chain has been studied by different methods such as inelastic neutron scattering, magnetic susceptibility and X-band spectroscopy [ 2 ] . In the latter two experiments, the value of the gap is indirectly evaluated by the fitting of the temperature dependence of the data. In the inelastic neutron scattering measurement, a splitting of the spin gap is observed and it has been desired to determine the value of spin gap more accurately. Magnetic field
T
k
405
1.1
4.15
42
4
4
Ii
Figure 3. Schematic view of repeating pulsed field. T is the period of repetition and is the pulse duration time.
Figure 4. Improvement of S/N ratio by the averaging. The frequency is 95 GHz and the sample is thin Co-multilayer.
3200
0
6
4
8
.. . . , .. . . ,. .
. .
~. . . . , . . . .
10
B(T)
B (TI
Figare 5. ESR spectra of Sr,,Cu,,O,, at 2.52 THz for H//c direction. El and E2 show signals for two gaps.
Figure 6. Frequency-field diagrum of Sr,,Cu,,O,,. Rectangulars and circles the larger and smaller gaps, respectively.
As is well known, the ESR transition between the ground singlet state and the excited triplet state due to the mechanism of magnetic dipole transition is forbidden in usual cases[3]. When a sizable non-secular term such as staggered field or Dzyaloshinskii-Moriya(DM) interaction exist, the selection rule is broken. Figure 5 shows the ESR spectra of Sr,,Cu,, O,, at low temperatures. As the temperature is lowered, the signal intensity increases. This temperature dependence indicates that the absorption is caused by the transition from the ground state. The frequency-field diagram shown in Fig. 6 clearly shows the existence of two zero field gap. The values of two gaps are 2330 GHz(9.635 meV) and 2625 GHz(10.856 meV), which are consistent with the previous values by the neutron experiment. The observation of a weak forbidden transition in the present result indicates the high performance of high-frequency ESR in THz region. ACKNOLEDEMENT A part of this work has been carried out in Institute for Material Research, Tohoku University as a collaboration research with M. Motokawa. We also express our acknowledgements for J. Akimitsu for supplying us a sample. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan. REFERENCES
1. H. Nojiri, M. Motokawa, K. Takahashi and M. Arai, IEEE Trans. Appl. Supecond. 10(2000)534-537. 2. M. Matsuda, H. Hagiwara and K. Katsumata, Phys. Rev. B 53(1998)2183-2189. 3. H. Nojiri, H. Ohta, S. Okubo, 0. Fujita, J. Akimitsu and M. Motokawa, J. Phys. SOC.Jpn. 68( 1999)3417-3423.
EPR in the 21%Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
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ESR study on magnetic ordering of spin-frustrated antiferromagnet ZnCr,O, single crystal
H. Kikuchi', H. Ohtab,S. Okubo I. Kagomiya', M. Toki', K. Kohn' and K. Shiratori 'Department of Applied Physics, Fukui University, Fukui,
Japan
bMolecularPhotoscience Research Center, Kobe University, Kobe, 'Department of Physics, Waseda University, Tokyo,
Japan
Japan
High field ESR spectra on the single crystal of spin frustrated spinel antiferromagnet ZnCr,O, were measured. The single crystal was revealed to be consisted of several crystalline domains. The easy plane type AFMRresonances were observed. The values of the anisotropy parameters were estimated from the frequency-field diagram. 1. INTRODUCTION The magnetic ions onthe octahedral B sites of the cubic spinel structure form a corner-sharing tetrahedral lattice, so that strong spin frustration is expected if the dominant nearest neighbor interactions are antiferromagnetic. ZnCr,O, is one of the normal spinels with magnetic B sites occupied by Cr3+ions. In spite of the large Weiss temperature 400 K, the K. We have measured the low antiferromagnetic order does not occur down to about temperature X-ray diffraction pattern of powder sample of ZnCr,O, and found the structural phase transition occurred at [I]. We have discussed this magneto-structural correlation in term of the partial release of the spin frustration by the structural distortion. The macroscopic number of spin degeneracy inherent in the spin frustration will be lifted if the lattice structure is distorted. In order to elucidate the nature of the magnetic phase transition of ZnCr,O,, whichis accompanied bythe structural transition, we have measured themagnetic susceptibility, muon spin rotation [2] and ESR on the polycrystalline ZnCr,O, The easy-plane type antiferromagnetic resonance (AFMR) was detected below in the previous ESR research. ESR method was found to be very effective to investigate the magnetism of ZnCr,O, and more detailed ESR experiments using single crystal was desired. Since we have succeeded to grow the single crystal of ZnCr,O,, high field ESR measurements on the single crystal were carried out and the experimental results are reported in this paper.
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2. EXPERIMENTAL DETAILS
Single crystals of ZnCr,O, were grown from high temperature solution in molten Bi,O,. The highest temperature was 1250°C. The raw material was polycrystalline ZnCr,O, prepared by an ordinary ceramic method. Millimeter wave ESR measurements of ZnCr,O, single crystal have been performed in the frequency region from 40 to 315 GHz using the pulsed magnetic field up to 16 T. GUM oscillators are used as light sources and the observed temperature region is from 1.8 to 80 K. The shape of the sample we have measured was almost regular octahedron. The magnetic field was applied along two axes of the crystal ; one is the axis passing through two opposite vertices (axis l), and another is the axis perpendicular to the axis 1 (axis 2).
3. RESULTS The ESRspectra for // axis1 at various temperatures from 1.8 to 80 K at 160 GHz were shown in Figurel. The pronounced electron paramagnetic resonance (EPR) spectra were 2.004 observed above TN. The EPR spectrum at 80 K had double peaked structure with and 1.961. The linewidth of EPR increased slightly as decreasing temperature without divergence near AFMR spectra were obtained below The linewidth of AFMR signals was narrower than that observed for the powder samples. In Figure 2, the temperature I
I
I
I
H
axis2
160 GHz
79.7 K 60.OK 49.8 K
h
.-C Y
30.0 K 19.9 K
15.1 K 12.3 K 10.5 K v
8.5 K v
4.2 K v
1.8 K
2
3
4 5 6 Magnetic Field (T)
7
8
Figure 1. Temperature dependence of the spectra at 160 GhZ in magnetic field parallel to the axis1 .
I
I
,
I
2
3
4
5
'
I
6 7 Magnetic Field (T)
I
8
Figure 2. Temperature dependence of the ESR spectra at 160 Ghz in magnetic field parallel to the axis2.
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dependence of ESR for // axis2 is shown. The features of EPR and AFMR spectra for // axis2 were essentially same as those observed for axis 1. The small resonance signals near DPPH line appeared below except AFMR signals. Similar small signals were reported in the X-band ESR measurement on the single crystal Figure 3 shows the AFMR spectra at // axis1 at 1.8 K. Almost same frequency various frequency from 50 to 315 GHz in dependence was observed for axis2. From these results, we have drawn the frequency-field diagram for /I axis 1 using g=2.0, shown in Figure 4. DISCUSSION
The frequency-field diagram (Figure shows some features characteristics of the AFMR with easy plane type anisotropy. However, the AFMR mode denoted by solid square and respectively, where z-axis circle in Figure is expected to be observed for // and is hard-axis of magnetization. In our measurements,two AFMRmodes are observed regardless of the field orientation against the single crystal, suggesting that the crystal contains several crystalline grains whose crystal axes differ among them. This inference is consistent with the observation that the EPR signals were double peaked rather than single one. [5] has derived Hereafter, brief analysis of the AFMR modes will be presented. Ohta et the expressions for the resonance conditions for the easy plane type antiferromagnet in the
Wlaxisl 1.8K
12 10 8 h
Ez
. r 6 4
2 0 0
2
4
6
8
1
0
1
2
B (TI
Figure 3. Frequency dependence of AFMR spectra from 50 to 3 15 GHz at 1.8 K in magnetic field parallet to the axisl.
Figure 4. Frequency-fielddiagram for HNaxis 1 . The intense AFMR resonances are plotted in solid circles and squares. The small signals are plotted in open squares. The solid lines are fitting results using expressions (1) and (2).
75 8
frame of the molecular field approximation as
for
lzand
( w ~ / Y= )H~2 b + - 3- A } '+r'
2
+ 6(A' -r')(A +A')Mi
A
for // where A means inter-sublattice molecular field coefficient, A' and are principal values of the tensors for the anisotropic terms of the inter- and intra-sublattice molecular field coefficients, respectively. is a sublattice magnetization and y is the gyromagnetic ratio. The in-plane anisotorpy was neglected and both of A' and are assumed to be much smaller than A . As shown by the solid line in Figure 4, the calculated curves using these expressions where agree quite well with the experimental results. Provided that M0=3pg/Crand A is the magnetic susceptibility per one Cr ion, then A' and estimated to be A'-r--lO-'A. Here, emdmol obtained fiom our measurement at 5 K. The signs of the both of the anisotorpy term A' and are negative, being consistent with the our assumption that the AFMR has easy-plane type character. The structural transition from cubic to tetragonal or orthorhombic crystalline system at TN may be responsible for the origin of the easy-plane anisotropy. We need to know the precise spin structure for more detailed analysis. In summary, we have carried out the high field ESR on the single crystal of frustrated antiferromagnet ZnCr,O,. The single crystal was revealed to be consisted of several crystalline domains. The easy plane type AFMRresonances were observed. The values of the anisotropy parameters were est hated.
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ACKNOWLEDGMENTS This work is supported by Grant-in-Aid for Scientific Research on Priority Areas (T3) (No. 13130204) from the Ministry of Educations, Culture, Sports, Science and Technology, Japan. REFERENCES 1. H. Kikuchi and M. Mamiya, Proceedings of ICF 8 September, 2000, Kyoto, Japan. 2. H. Kikuchi, H. Fukushima, W. Higemoto and K. Nishiyama, submitted to Hyperfine Interactions. 3. H. Ohta, S. Okubo, H. Kikuchi and S. Ono, Can. J. Phys., 79 (2001) in press. 4. H. Martino et al, Phys. Rev. B, 64 (2001) 024408. 5. H. Ohta, M. Sumikawa, M. Motokawa, S. Noro and T. Yamadaya, J. Phys. SOC.Jpn., 64 (1995) 1759.
EPR in the 2 1 Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
759
ESR study of frustrated spin chain T. Kunimoto”, T. Kamikawab, S. Okuboc, H. Ohtaa,cand H. Kikuchid &VentureBusiness Laboratory, Kobe University, Rokkodai 1-1, Kobe 657-8501, Japan. hGraduateSchool of Science and Technology, Kobe University, Rokkodai 1-1, Kobe 657-8501, Japan. ‘Molecular Photoscience Research Center, Kobe University, Rokkodai 1-1, Kobe 657-8501, Japan. “Department of Applied Physics, Fukui University, Bunkyo 3-9-1, Fukui 910-8507. X-band, millimeter and submillimeter ESR measurements of frustrated spin chain [Cu(bpy)H,O] [Cu(bpy)(mal)H,O] (CIO,), were performed at various temperatures. An anomalous narrow absorption line corresponding to the EPR of Cu2+was observed. The slightly anisotropic g-values were obtained as g,,,=2.164 and gmn=2.101. The one-dimensional character of this system is discussed from the temperature dependence of g-values and line width of EPR spectra.
1. INTRODUCTION Recently, experimental studies of low dimensional quantum spin systems have attracted much attention due to the discovery of model substances. In this paper, we study new one-dimensional spin system called “delta-chain”. The delta-chain is composed of S=1/2 spin trimers and these trimers are stacked like a saw-tooth which is shown in Figure l(a). The delta chain having antiferromagnetic coupling is considered to be a fully frustrated quantum spin system [ 11. Theoretical studies for this spin system have been made to investigate the ground state and the lowlying excitation. The ground state is essentially singlet dimer state and has a high degeneracy due to the full frustration. The low-lying excitation is considered to be a topological excitation [2], which has a finite energy gap between the ground state, resulting in a Schottkey-type low temperature peak in specific heat [ 11 and the zero susceptibility at zero temperature The model substance of delta-chain was synthesized by Ruiz-PCrez et al [4]. The crystal is monoclinic and the structure is made up of uncoordinated perchlorate anions and malonate-bridged zigzag Cu chains running parallel to the
Figure 1 . (a) Schematic drawing of delta chain. Open circle indicates S=1/2 spin. (b) Schematic spin structure of delta chain with two kinds of exchange interactions (AF) and J,(F)
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axis. There are two kinds of nearest-neighbor exchange interactions, between the basal spins is antiferromagnetic (AF), while between the apical and the basal spins are ferromagnetic (F). Therefore the spin system of this substance has a frustration because of the competition of AF and F interaction in the spin trimer as shown in Figure l(b). XT product of this compound shows a maximum around 20 K, which suggests that the dominant exchange interaction changes from F to AF below 20 K [4]. To investigate the magnetic properties and spin dynamics of the delta-chain compound [Cu(bpy)H,O] [Cu(bpy)(mal)H,0](C1O4),,X-band, millimeter and submillimeter ESR measurements were performed at various temperatures.
2. EXPERIMENTAL X-band ESR measurements have been preformed using Bruker spectrometer with Oxford He-flow cryostat. Submillimeter wave ESR measurements have been performed using pulsed magnetic fields up to 30 T and frequency between 60-3 15 GHz.[5-71We have employed a simple transmission method with Faraday configuration. Single crystals [Cu(bpy)H,O][C~(bpy)(mal)H,O](CI0~)~ were synthesized by one of the author (H. K.). The synthesized crystals were analyzed by X-ray diffraction.
3. RESULTS AND DISCUSSION In order to determine the g-values, angular dependence measurement of g-values using Xband was preformed. A narrow absorption line for Cu2+is observed. As shown in Figure 2, this absorption signal has small g-value anisotropy with the value ranging from 2.101 to 2.164. The signal splits into two signals when the magnetic field was applied in the a-c plane as shown in
2.145
2.170 2.160
2.140
2
2.150 2.135
2.140
3
?
2.130
2.130 2.120
2. I25
2.110 2.120
2.100 0
40
80
120
160
0
40
80
120
160
Angle (deg.) Angle (deg.) Figure 2. Angular dependences of g-values. (a) Single crystal was rotated around the x direction in the c plane, which is denoted in the inset (solid square), and rotated around the c-axis (open triangle). (b) Single crystal was rotated around the b-axis
76 1
Figure 2(b). The g-values of the crystallographic axes are obtained as g,=2.140 and gc=2.124. The g-value of the a-axis could not be determined because of two absorption signals observed in the // a spectra. The detailed crystal structure is very complicated and there is a possibility that the two signals are related to bicrystal. Therefore it is difficult to determine the g-values of principal axes at present. To investigate the one-dimensional( 1D) characteristic of the delta chain, the line profile of observed signal was analyzed. The angular dependence of the linewidth does not show the magic angle, which is the typical character of 1D magnet.[8] In comparison with typical 1D AF magnet TMMC[8], the line profiles are close to Lorentzian as shown in Figure 3. The variation of line profile of our delta-chain is similar to that of 1D AF magnet RbMnC1,*2H2Owith non-negligible interchain exchange couplings.[9] From the view point of crystal structure, each chain is well isolated in our compound.Thus it suggests that the intra-trimer exchange interaction is effective on the spin dynamics in the delta-chain compound. The narrow linewidth is can be interpreted by the exchange narrowing effect. The temperature dependencies of g-value and linewidth are shown in Figure 4. The obtained g-values for the b and x-directions show the postitive and negative shifts, respectively, and the linewidth increases in low temperature region. These facts suggest that the typical ID characteristics[10, 111 appear on the ESR spectra. The gradual g-shift and increase of linewidth in higher temperatures are probably due to the developing of 1D spin correlation. However, the temperature dependence of integrated intensity of absorption signal and x [ 121 do not show any broad maximum at low temperature. Thus the dynamical properties of EPR spectra shows criti-
h
30
-
25
-
20
-
Gaussian
z 15 9
Y
10
Lorentzian
I
0 0
I 4
I
l 8
l
I 12
l
-
l 16
(H-HoYAH,,2 Figure 3. The inverse of the line profile as a function of magnetic field. Line profile and magnetic field are given in units of maximum intensity and half-width at half maximum, respectively.
0
20
40
60
80
100 120 140
Temperature (K) Figure 4. Temeprature dependence of (a) g-value and (b) linewidth. The magnetic field was applied to the b-axis (open circle) and the x-direction (open triangle). Dotted lines are the guide to eye.
762
cal behavior even though there seems to be no sign of short range order in spin susceptibility. In addition, from the frequency-field diagram in the range from 60 to 3 15 GHz, the absorption line is determined to be EPR of Cu(l1) ion and the magnetic order is not found even at 1.8 K. Thus, there is a possibility that frustration based on the competition of AF and F interaction affects the statical and dynamical properties of Cu spins, however more detail experimental and theoretical studies are necessary to discuss this point.
4. SUMMARY X-band, millimeter and submillimeter ESR measurements of frustrated spin chain [Cu(bpy)H,O] [Cu(bpy)(mal)H,O](ClO,), were performed at various temperatures. A very narrow absorption line corresponding to the EPR of Cu2+was observed. The slightly anisotropic gvalues were obtained as g,,,=2.164 and g,,“=2.101. By analyzing the line profile of the spectra, the exchange interaction is dominant in this compound. The critical behaviors in linear chain appear in the temperature dependence of g-values and line width of EPR spectra.
ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Scientific Research on Priority Areas (B) (No. 13130204 “Field-Induced New Quantum Phenomena in Magnetic Systems”) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors wish to thank Venture Business Laboratory of Kobe University for the financial assistance.
REFERENCES I . K. Kubo, Phys. Rev., B48 (1993) 10552. 2. T. Nakamura and K. Kubo, Phys. Rev., B53 (1996) 6393. 3. D. Sen, B. S. Shastry, R. E. Walstredt and R. Cava, Phys. Rev., B53 (1996) 6401. 4. C. Ruiz-Pkrez, M. Hernandez-Molina, P. Lorenzo-Luis, F. Lloret, J. Can0 and M. Julve, Inorg. Chem., 39 (2000) 3845. 5. M. Motokawa, H. Ohta and N. Makita, Int. J. Infrared & MMW, 12(2) (1991) 149-155. 6. S. Kimura, H. Ohta, M. Motokawa, S . Mitsudo, W-J Jang, M. Hasegawa and H. Takei, Int. J. Infrared & MMW, 17(5) (1996) 833-841. 7. N. Nakagawa, T. Yamada, K. Akioka, S. Okubo, S. Kimura and H. Ohta, Int. J. Infrared & MMW, 19(2) (1998) 167-176. 8. R. E. Dietz, F. R. Merritt, R. Dingle, D. Hone, B. G. Silbernagel, P. M. Richards, Phys. Rev. Lett., 26 (1971) 1186 9. K. Nagata and T. Hirosawa, J. Phys. SOC.Jpn. 40 (1976) 1584. 10. K. Nagata and Y. Tazuke, J. Phys. SOC.Jpn., 32 (1972) 337 ; K. Nagata, J. Phys. SOC.Jpn., 40 (1976) 1209. 11. T. T. P. Cheung, Z. G. Soos, R. E. Dietz and F. R. Merritt, Phys. Rev., B 17 (1978) 1266. 12. H. Kukuchi, unpublished.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
763
Millimeter wave ESR measurement of diamond chain substance azurite Tomohisa, KAMIKAWA", Takashi, KUNIMOTOb, Susumu, OKUBO", Hitoshi, OHTAbsC , Hikomitsu, KIKUCHId "The Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai, Kobe 657-8501, Japan. bVenture Business Laboratory, Kobe University, 1-1 Rokkodai, Kobe 657-8501, Japan. 'Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai, Kobe 657-8501, Japan. dDepartment of Applied Physics, Fukui University, Bunkyo 3-9-1, Fukui 910-8507, Japan.
Azurite Cu, (OH), (CO,), is a candidate of the model substance for diamond chain with antiferromagnetic interaction. The millimeter and submillimeter wave ESR measurements of Cu, (OH),(CO,), single crystal have been performed in the frequency region from 50GHz to 315GHz and in the temperature region from 4.2K to 80K. Very broad absorption lines are observed in the measured temperature region. The g-shift was observed below 20K, which is corresponds to a peak of the magnetic susceptibility. The decrease of linewidth was observed below 30K. These temperature dependences will be discussed in connection with the short range order.
1. INTRODUCTION
Last decade, low dimensional quantum spin systems have attracted much attention since the discovery of the model substances. In this paper, we introduce a new model substance of one-dimensional (1D) diamond chain. Takano [ 11 investigated analytically the ground state for S=1/2 diamond chain (Figure 1). There are three ground state phases for h=J'/J, such as the ferrimagnetic (FM) phase, the tetramer-dimer (TD) phase, and the dimer-monomer (DM) phase. Recently, azurite (Cu,(OH),(CO,),) is expected as a candidate substance for the 1D diamond chain. Azurite takes the monoclinic structure, and the lattice constants are a=4.97& b=5.84& c=10.29& and p=87" [2,3]. From proton NMR measurements below 30K, T, was estimated to be 1.86K [4], and there was no critical field up to 900mT [5]. The magnetic susceptibility measurements of a single crystal were performed in the low temperature region from 1.55K to 4.2K [6], the easy axis was estimated to be c' axis which is a direction of 25" from the c-axis and 63" from the a-axis. Recently, Kikuchi performs the magnetization
764
measurement up to 36T and the magnetic susceptibility measurements from 5K to 300K of single crystal [7]. The magnetization curve for H // c at 4.2K shows that the magnetization increases linearly with increasing the magnetic field. The differential magnetization shows a peak at 30T. The temperature dependence of magnetic susceptibility shows a broad peak at 20K. Although the magnetic susceptibility increases below 10K, it is unknown whether the origin of increase in the magnetic susceptibility is from impurities or the critical behavior towards Nee1 temperature. The Weiss temperature is estimated to be 20K by fitting of the Curie Weiss low x=C/(T-O) from 70K to 300K. The ground state of azurite seems to be gapless as the values of the magnetic susceptibilities take finite one when the temperature approaches zero. The behavior of the magnetization does not suggest the ferrimagnetic phase. Therefore, it is expected that the azurite is a model substance for 1D diamond chain and its ground state is in the DM phase. Most of the magnetic measurements of azurite were performed in '50-'60 year [4-61, and it is not considered as the model substance of diamond chain at that time. Therefore, in order to reveal the magnetic properties of the 1D diamond chain, we have performed the millimeter and submillimeter wave ESR measurements of azurite single crystal. In this paper, we report the results of the temperature dependence of azurite, and discuss the temperature dependence of linewidth and g-value.
2. EXPERIMENTAL The malachite Cu,(OH),(CO,) is easy to aggregate with the natural mineral azurite Cu,(OH),(CO,),. To select single phase azurite, we checked the diffraction pattern of the X-ray diffraction measurements. The millimeter wave ESR measurements are performed using the pulsed magnetic field up to 16T. The frequency region was from 5OGHz to 315GHz using G u m oscillators as light sources. The measured temperatures are from 4.2K to 80K. Detailed experimental set up of our ESR system is illustrated in refs. [8-10 1. 3. RESULTS AND DISCUSSION
-
Figure 2 shows the temperature dependence of the ESR spectra for 160GHz. Applied field direction is b axis which is the chain direction. Very broad absorption lines are observed in
b axis
J Figure . The spin structure of the diamond cain. The circles correspond to the spin sites.
765
comparison with usual Cu2’ absorption lines such as Bi,CuO, [ l 11. The high field ESR measurement is necessary to observe such broad absorption lines. The resonance fields shift to higher field below 20K, which corresponds to the peak of the magnetic susceptibility. Another absorption lines at around 8T (B in Figure 2) develop below 8K. This corresponds to the increase of the magnetic susceptibility below 1OK. We assume that increase of the magnetic susceptibility below 10K is due to ESR signal B. Figure 3 shows the temperature dependence of the linewidth. The linewidth below 8K is taken as 2 times of half of half width at half maximum of ESR signal B, because the absorption lines below 8K become asymmetric. The linewidth is almost constant about 3-4T from 80K to 40K and become sharper rapidly below 30K. The g-values for 160GHz are shown in Fig. 4. The g-values become smaller below 20K. It is well known as a g-shift in the low dimensional system such as CsMnC!, [12,13] which shows the g-shift for H // chain to lower field as the temperature is decreased. The temperature dependence of g-value is similar to the low dimensional antiferromagnetic system. However, 5.0
-
4.5 4.0
3.5
79.7K 70.4K 60.1K
3.0 .d
2.5
50.OK 40.OK 35.OK 30.OK 25.OK 19.9K 15.1K 1 O.OK
2.0
1. 0
-1 20
40 60 T(K)
80
100
Figure 3. The temperature dependence of linewidth for 160GHz.
8.OK 6.OK 4.2K
0
2
4
6
8 10 12 14 16 (TI
Figure 2. The temperature dependence of the ESR spectra for 160GHz. A very sharp absorption line is DPPH signal, which is the magnetic field marker.
T (K)
Figure 4. The temperature dependence of g-values for 160GHz.
166
we have to make measurements for other directions. Although the typical low dimensional antiferromagnet shows divergence of the linewidth at low temperature, the linewidth of azurite is almost constant below 20K. Below 10K two ESR signals are observed, and the g-value of them are almost constant below 10K. From our preliminary experimental results, the frequencyfield relation at 4.2K is linear crossing the origin. Therefore ESRsignals seems to be paramagnetic even at 4.2K.
4. CONCLUSION The millimeter wave ESR measurements of azurite Cu, (OH), (CO,), have been performed in the temperature region from 4.2K to 80K. The g-value starts to shift at 20K which corresponds to the peak ofthe magnetic susceptibility. This behavior seems to correspond to the development of short range order. However, the temperature dependence of linewidth does not show the typical behavior of the low dimensional antiferromagnet. Acknowledgment
The authors thank Dr. M. Azuma, Mr. T. Saito and Prof. M. Takano for let us using X-ray equipment with imaging plate. This work was supported by Grant-in-Aid for Scientific Research on PriorityAreas (B) (No. 13130204 "Field-Induced New Quantum Phenomena in Magnetic Systems") from the Ministry of Education, Culture, Sports, Science and Technology of Japan. REFERENCES
1. K. Takano, K. Kubo and H. Sakamoto, J. Phys. Condens. Matter, 8 (1996) 6405. 2. H. Brasseur, Z. Krist., 82 (1932) 195. 3. Von G. Gattow and J. Zemann, Acta cryst., 11 (1958) 866. 4. Spence, R. D. and Ewing, R. D., Phys. Rev., 112 (1958) 1544. 5. Van der Lugt, W. and Poulis, N. J., Physica, 25 (1959) 1313. 6. Garber, M. and Wagner, R., Physica 26 (1960) 777. 7. H. Kikuchi, unpublished. 8. S. Kimura, H. Ohta, M. Motokawa, S. Mitsudo, W.-J. Jang, M. Hasegawa and H. Takei, Int. J. Infrared Millimeter Wave, 17 (1996) 833. 9. Submillimeter EPR of Co:Rb,MgF, and Anomalous g-Values M. Motokawa, H. Ohta and N. Makita, Int. J. Infrared & MMW, 12(2) (1991) 149. 10. Millimeter and Submillimeter Wave ESR System Using 30 T Pulsed Magnetic Field N. Nakagawa, T. Yamada, K. Akioka, S. Okubo, S. Kimura and H. Ohta, Int. J. Infrared & MMW, 19(2) (1998) 167. 11. H. Ohta, K.Yoshida, T. Matsuya, T. Nanba, M. Motokawa, K. Yamada, Y. Endo and S. Hosoya, J. Phys. SOC.Jpn., 61 (1992) 2921. 12. K. Nagata and Y. Tazuke, J. Phys. SOC.Jpn., 32 (1972) 337. 13. K. Nagata, J. Phys. SOC.Jpn., 40 (1976) 1209.
EPR in the 21'' Century A Kawamori, J Yamauchi H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
767
High field ESR of (Ca,~ySry),~,CuO, with edge-sharing CuO, chain K. Kawakami", A. Ueda", H. Ohta',', S . Okubo',', Z. Hiroid, M. Takanoe "The Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan. 'Molecular Photoscience Research Center and Depertment of Physics Kobe University, Kobe 657-8501, Japan. "Venture Business Laboratory, Kobe University, Kobe 657-8501, Japan dISSP, Tokyo University, Kashiwa277-8581, Japan. "Institute for Chemical Research, Kyoto University, Uji 61 1-001 1, Japan. We performed submillimeter wave ESR measurements on the powder sample of quasi-onedimensional cupric oxides (Ca,.,Sry),.xCuO,(y=O, 0.10,0.12,0.15). We studied Sr concentration dependense of g-values from the powder pattern EPR analysis at high temperature. The results were discussed in connection with the crystal field. Moreover, antiferromagnetic resonances (AFMR) below were observed and the results show that AFMR gap decreases as the Sr density increases.
1. INTRODUCTION Extensive researches have been performed so far on high superconductors which led to our knowledge that two dimensional carrier doped CuO, plane or Cu,O, ladder plane is essential for the superconductivity realized in high copper oxides. In connection with these aspects, it is also interesting to study the one dimensional CuO, chain with hole carrier. It can be considered to provide a key to understand the mechanism of high superconductivity from a different point of view. The title compound (Ca,,Sry),.,CuO, is a good candidate for hole-doped CuO, chain because of its 1-D nature with edge-sharing CuO, chain and the capability of variable hole number which allows us to study the effect of doping systematically. The magnetic properties of y=O compound have already been examined by means of susceptibility[ 1-41 and specific heat measurements[ 11. To explain the susceptibility data, Hiroi proposed a new model that both I-D antiferromagnetic chains and spin dimers exist in the y=O system[l]. On the other hand, the specific heat measurement resulted in the observation of long range magnetic ordering at 12 K. Moreover, in our previous measurement of submillimeter wave ESR performed on the y=O powder sample, antiferromagnetic resonance (AFMR) modes were confirmed below 12 K[5]. In addition with this AFMR signal, we also observed another ESR absorption, and the temperature dependence integrated intensity was
768
well fitted by the dimer model with small amount of impurity. These results suggest the coexistence of AF chain and dimer state in the y=O system. In this paper, we report the results of submillimeter wave ESR measurements performed on the powder samples (Ca,.ySry),. ,Cu02 with various concentrations of y=O.OO, 0.10, 0.12 and 0.15. The substitution of Ca" with Sr2+causes the reduction of 1-x value and, in principle, the increase of hole number. This is due to the the fact that the radius of S?' ion (1.13 A) is larger than that of Ca2+ion (0.99 A)
m
h .3
c
3
$ v
0 v)
2
F
4.0
4.5
5.0 5.5 H (TI
6.0
6.5 2. EXPERIMENTAL
Figure 1. Typical electron paramagnetic resonance (EPR) absorption line of y= O' 60GHz and 6o
ESR measurement have been performed on the powder samples in the temperature range from 1.8 to 70 K. The spectra were traced using pulsed magnetic field up to 15 T. The Gunn oscillators, backward traveling wave oscillator and InSb were employed as the light sources and signal detector, respectively. [6, 71 3. RESULT & DISCUSSION Figure 1 shows the typical electron paramagnetic resonance (EPR) absorption line of (Ca,,Sr,),.,CuO, (y=0.12) at 160 GHz and 60 K. The experimental data were well reproduced by the curve with a powder pattern[S], as indicated by the solid line in this Figure. The shoulder corresponds to g, while the peak corresponds to g,. Table 1 shows the g-values deduced from the powder pattern analysis of EPR absorption line observed for each y sample at 60 K. It is apparent that both g,/ and g , tend to decrease as y is increase. Then we consider the origin of y dependence of g-value in terms of crystal field theory within the point charge model. Each copper ion in this compound is surrounded by the oxygen coordination with distorted tetragonal symmetry elongated along z-axis. Thus the g-values were expressed by the following equations
where ge=2.0023and ;1is spin-orbit coupling constant. Using equations (1) and experimentally obtained g-values, the energy differences A,, and A, for each y compound are evaluated to be as listed in Table 1. It is noticed that both A, and A,, namely the energy gap lODq between ground and excited state (Figure 2), increase with Sr concentration y. If we consider the cubic
769
Figure 2. Energy diagram of 'D state in a tetragonally distorted cubic field field system for simplicity, the crystal field potential is proportional to lla' where is the bond distance. Therefore we can say that the doped Sr alters the local symmetry in such a way that C u - 0 bond distance shrinks, because an increase of lODq can be regarded as an increase of Figure 3 shows the typical temperature dependence of ESR absorption line observed at 3 15 GHz for y=O. 10 compound. Below about 10 K, the spectrum becomes broader and shows the shift of resonance field toward lower temperature. Similar behaviors were also seen for y=O. 12 compound. These results suggest that an antiferromagnetic ordering is also realized in the y=O. 10 and 0.12 compounds with slightly lower transition temperature than y=O one. At low temperature 1.8 K, many traces with various frequencies ranging from 50 to 420 GHz result in the frequency-field diagram of resonance points as shown in Figure 4. It is suggested OK OK OK h
v1
a?
C
9K OK 2K IK
m
2K
E
OK OK
D
2
v
s
400
I
.
y=0.15 y=0.12 y=0.10 + y=0.00
OK F.
2K
W
0
5
10
1
H (TI Figure 3. Temperature dependence of ESR absorption lines observed at 3 15GHz for y=O. 10 compound.
0
5
10
-
15 H fT) Figure 4. Frequency-Field diagram of observed resonances for y=O, 0.10, 0.12 and 0.15 at 1.8K
770
that the AFMR modes with uniaxial anisotropy are observed on y=O.lO compound and zero-field resonance frequency (w,) is estimated to be about 140 GHz which value is smaller than that of y=O compound (240GHz)[5]. We cannot clearly see AFMR in y=0.12 and 0.15 compounds due to weakness of ESR signal. Allowing an experimental ambiguity, however, as is estimated to be about 70 GHz for y=O.15 compound. Reduction of both w, and increasing of concentration y manifest that the doping of S? ion suppress the system to order magnetically. In summary, we performed submillimeter wave ESR measurements on the powder sample of quasi one dimensional antiferromagnetic chain systems (Ca,,Sr,),.,CuO, with y=O, 0.10, 0.12, 0.1 5. At high temperature, the increasing of g-values were observed with increasing of y. From the analysis based on the conventional crystal field theory, we suggested the shrinking of Cu-0 bond length due to substitution of Sr for Ca. At low temperature 1.8 K, we observed AFMR which seems to show the systematic change in against the Sr concentration. However the detailed mechanism to explain this behavior still remains to be unclear. Further information is needed to clarify the physical properties of this interesting compounds.
ACKOWLEDGMENT This work was supported by Grant-in-Aid for Scientific Research on Priority Areas (A) (No. 12046250 Novel Quantum Phenomena in Transition Metal Oxides) and Grant-in-Aid for Scientific Research on Priority Areas (B) (No. 13 130204 "Field-Induced New Quantum Phenomena in Magnetic Systems") from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
REFERENCES 1. Z. Hiroi, M. Okumura, Y. Nabeshima, T. Yamada, M. Takano: J. Phys. SOC.,Jpn., 69 (2000) 1824. 2. J. Dolinse, D. Arcon, P. Cevc, M. Miljak, I. Aviani: Phys. Rev., B 57 (1998) 7798. 3. G. I. Meijer, C. Rossel, E. M. Kopni, M. Willemin, J. Kaepinski, H. Schwer, K. Conder, P. Wachter: Phys. Rev., B 58 (1998) 2678. 4. A. Hayashi, B. Batlogg, R. J. Cava: Phys. Rev., B 58 (1998) 2678. 5. A. Ueda, H. Ohta, S. Okubo, Z. Hiroi, M. Okumura, M. Takano: Appl. Magn. Reson., 19 (2000) 399. 6. S. Kimura, H. Ohta, M. Motokawa, S. Mitsudo, W. J. Jang, M. Hasegawa, and H. Takei: Int. J. Infrared MMW, 17 (1996) 833 7. N. Nakagawa, T. Yamada, K. Akioka, S. Okubo, S. Kimura, and H. Ohta: Int. J. Infrared MMW, 19 (1998) 167 8. H. Ohta, N. Yamauchi, T. Nanba, M. Motokawa, S. Kawamata, and K. Okuda: J. Phys. SOC.Jpn., 62-2 (1993) 785
EPR in the 21“ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
771
High field ESR measurements (VOXP,O, Yuta NAGASAKX, Takashi KUNIMOTOb, Susumu OKUBO‘, Hitoshi 0HTAb,‘, Touru YAMAUCHI’, and Yutaka UEDA’ “The Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan venture Business Laboratory, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan “Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai, Nada, Kobe 6578501, Japan ’Institute for Solid State Physics, University of Tokyo, 5-1-5 Kasiwanoha, Kasiwa, Chiba 277-8581, Japan The X-band, millimeter and submillimeter wave ESR measurements of (VO),P,O, (ambient pressure phase) single crystal have been performed. From the angular dependence measurements of g-values at room temperature, the principle g-values are determined to be g,=1.929(6), gb=l.975( l), gc=l.974(6). The comparison of these g-values with those of high pressure phase is discussed. The g-shift was observed below 15K for all axes.
1. INTRODUCTION S=1/2 1D Heisenberg chain compound (VO),P,O, (VOPO) have two different crystal structures, ambient pressure phase (AP phase) and high pressure phase (HP phase). Both AP-VOPO and HP-VOPO consist of pairs of VO, pyramids and PO, tetrahedra, and the vanadium ions are in an oxidation state of 4+ with spin 112. The VO, pyramids and PO, tetrahedra are aligned along the c axis in a manner of up-up-down-down-up-up direction for the AP phase. In contrast, the direction combination is up-down-up-down for the HP phase [ 1,2]. The HP phase has a simple structure with only one V4+site in comparison with the AP phase which is reported to have multiple sites, four in the orthorhombic (Pca2,) structure for a powdered sample [3] and eight in the monoclinic structure (P2,) for a single crystal [4]. Both phases have singlet ground state at low temperature. At the beginning, AP-VOPO was considered to be one of the typical materials for two-leg ladder model. But recent inelastic neutron scattering measurements on single crystals revealed that it is best described as an alternating spin chain system with the chains running along the rung direction of the ladder model. The neutron scattering results also revealed that the interaction along the leg was found to be ferromagnetic and there exist two spin gaps of 36K and 67K Although the
772
magnetization measurement shows one critical field H,,,=18T for HP phase, it shows two critical fields Hc,,=2ST and H,,,=46T for AP phase. The existence of two critical fields in AP phase was explained by the coexistence of different spin gaps caused by two different bond alternating chains [6]. Recently, the angular dependence of g-value of the single crystal HP phase was performed and obtained principle g-values are g,=1.928(1), gb=l.974(1), g,=1.971(1) [ 11. We have performed millimeter and submillimeter wave ESR measurements of AP phase. And we will report on the angular, temperature and frequency dependence in this paper. The difference of g-values between AP and HP phase will be discussed also.
2. EXPERIMENTAL Angular dependence was performed by Bruker X-band EPR Spectrometer EMX081 with a TE,,, rectangular cavity. Temperature and frequency dependence were performed by using high field millimeter and submillimeter ESR measurements system. The experimental set up could be found in Ref.[7-91. The measurements have been performed in the temperature region from 4.2K to 80K and the frequency region from SO to 31SGHz by using Gunn oscillators. Single crystals of AP phase were used for this study .
3. RESULTS AND DISCUSSION Figures 1(a) and (b) shows the angular dependences of g-values at room temperature from a- to b-axis and from a- to c- axis, respectively. The solid curve shows least-squares fitting curve for observed g-values. The principle g-values are determined to be g=l.929(6), gb=l.975( I), g,=1.974(6) by line fitting. As we mentioned above, the AP g-values are slightly larger than HP one. It is considered that the difference of the g-values is caused by the crystallographic difference. The site of V4' ions has a tetragonal symmetry, therefore 2D multiplet splits into five energy levels whose energies are denoted as E, *, E,, E,, E,. The ground state is E,, and E,>E,>E,,,. However, the V - 0 distance along the V-01 direction of AP phase is slightly shorter than that of the HP phase [2,3] which means the pyramids are strained along the a axis in AP phase. Therefore, the energy difference of E4 - E, and E,,, - E, will be increased. I98 I91 1 96 Iu
2
?
195
I 94 I 93 I 92
0
50 100 150 200 Angle (degree)
250
Figure 1. Angular dependence of g-value at room temperature, the single crystal was rotated around (a) the c axis and (b) the b axis.
773
Using spin Hamiltonian approximation, g-values are obtained as the following expressions,
where is g-factor of free electron, h is spin-orbit coupling, and El is the i-th energy level as denoted above. Therefore, the g-values for AP phase are larger than those of HP phase. On the other hand, the VO, pyramids of AP phase are inclined each other from the a-axis which means the observed g-value should be smaller than the principle g-value estimated by the averaging of exchange interaction. However, we consider qualitatively that the increase of g-value for AP phase caused by the distortion of VO, pyramid is still larger than the decrease of g-value caused by the inclination of VO, pyramid. The line shape and angular dependence of linewidth did not show any one dimensional characteristic, which may suggest that the exchange interaction is dominant in this system. However, the temperature dependence of integrated intensity is consistent with the magnetic susceptibility results which reveal broad maximum around 80K Both results show typical temperature dependence of antiferromagnetic chain. We also performed the temperature dependence high field ESR measurements for AP phase. Figures 2(a) and 2(b) show absorption spectra of c-axis at various temperatures and frequency-field diagram, respectively. A simple EPR absorption line was observed as shown in Figure 2(a) and the g-values for each three axes were obtained as ga=l.934(8), gb=l.972(4), g,=1.976(1) as shown in Figure 2(b). These values are consistent with the result of X-band ESR considering the error bars. Below 15K, the g-values for three axes increase with decreasing the temperature as shown in Figure 3. The positive g-shifts for H parallel to a and b axes, which are perpendicular to the chain direction, are inconsistent with the typical behavior of one-dimensional antiferromagnet [lo]. However, the width increases below 15K which is the typical behavior of one-dimensional antiferromagnet. 4. CONCLUSION
X-band and high field ESR measurements of AP-VOPO single crystal have been performed.
-m
g
la)
Y
70 4K
60 IK
-
300
b-axis ---c-axis -e
50 1 K
Figure 2. (a) The temperature variation of absorption spectra of AP phase sample observed at 160GHz. Applied magnetic field is parallel to the c-axis. (b) Frequency-field diagram at 4.2K for three axes.
774 2.00 199 198 197
2 1.96 I95
I .94
1.93 I .92
10 20 10 40 50 60 70 80 Temperature ( K )
Figure 3. The temperature dependence of g-values at 16OGHz. The g-values are determined to be ga=l.929(6), gb=l.975( I), g,=1.974(6). It turned out that the AP g-values are slightly larger than HP one.The temperature dependence of high field ESR result is discussed in connection of the one dimensionality. ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Scientific Research on Priority Areas (A) (Novel Quantum Phenomena in Transition Metal Oxides), and Scientific Research on Priority Areas (B) (No. 13130204 "Field-Induced New Quantum Phenomena in Magnetic Systems") from the Ministry of Education, Culture, Sports, Science and Technology of Japan. REFERENCES 1 T. Saito, T. Terasima, M. Azuma, M. Takano, T. Goto, W. Utsumi, P. Bordet and D. C. Johnston, Journal of Solid State Chemistry., 153 (2000) 124. 2 M. Azuma, T. Saito, Y. Fujishiro, Z. Hiroi, M. Takano, F. Izumi, T. Kamiyama, T, Ikeda, Y. Narumi, K. Kindo, Phys. Rev., B60 (1999) 10145. 3 Z. Hiroi, M. Azuma, Y. Fujishiro, T. Saito, M. Takano, F. Izumi, T. Kamiyama and T. Ikeda, Journal of Solid State Chemistry., 146 (1999) 369. 4 P. T. Nguyen, R. D. Hoffman, A. W. Sleight, Mater. Res. Bull., 30 (1995) 1055. 5 A. W. Garrett, S. E. Nagler, D. A. Tennant, B. C. Sales and T. Barnes, Phys. Rev. Lett., 79 (1997) 745. 6 T. Yamauchi, Y. Narumi, J. Kikuchi, Y. Ueda, K. Tatani, T.C. Kobayachi, K. Kindo and K. Motoya, Phys. Rev. Lett., 83 (1999) 3729. 7 M. Motokawa, H. Ohta and N . Makita, Int. J. Infrared & MMW., 12(2) (1991) 149-155. 8 S. Kimura, H. Ohta, M. Motokawa, S. Mitsudo, W. J. Jang, M. Hasegawa and H. Takei, Int. J. Infrared & MMW., 17(5) (1996) 833-841. 9 N. Nakagawa, T. Yamada, K. Akioka, S. Okubo, S. Kimura and H. Ohta, Int. J. Infrared & MMW., 19(2) (1998) 167-176. 10 K. Nagata and Y. Tazuke, J. Phys. Soc. Japan., 32 (1972) 337.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
775
ESR measurements on triangular antiferromagnets CsCu,,Co, C1, Toshio Onoa, Hidekazu Tanaka", Hiroyuki Nojirib and Mitsuhiro Motokawa" "Department of Physics, Tokyo Institute of Technology, Tokyo 152-855 1, Japan bDepartment of Physics, Okayama University, Okayama 700-8530, Japan 'Institute for Materials Research, Tohoku University, Miyagi 980-8577, Japan CsCuCI, is a ferromagnetically stacked S = 1/2 triangular antiferromagnet with the weak easy plane anisotropy, and undergoes a field-induced magnetic phase transition due to the competition between the easy plane anisotropy and the quantum fluctuation. From our previous magnetic measurements, it was found that a new ordered phase appears in the ion substituted system CsCu,,Co,C1, with small Co2+ion concentration x. In order to investigate the spin dynamics, high field and high frequency ESR measurements were performed on CsCu,,Co,CI, for the samples with x = 0.015 0.032. Experimental results are discussed in comparison with the results for pure CsCuC1,.
-
1. INTRODUCTION
In frustrated antiferromagnetic systems, manifold degeneracy of the ground state often remains, when they are treated within the classical mean-field theory. In such cases, the quantum fluctuation plays an important role in the determination of the ground state. Theoretical studies [ 1,2] predict that a magnetization plateau appears at one-third of the saturation magnetization M, in the S =1/2 2D or ferromagnetically stacked triangular antiferromagnets in the isotropic limit. CsCuC1, is a ferromagnetically stacked S = 1/2 triangular antiferromagnets with weak planar anisotropy [3,4]. For the magnetic field parallel to the c-axis, this compounds undergoes a phase transition from the 12O0-structurein c-plane to a new coplanar structure, which is stabilized by the quantum fluctuation [2,5]. Since the energy gain due to the quantum fluctuation increases with increasing magnetic field, and then it overcomes the classical anisotropy energy, so that the field-induced phase transition occurs 121. Therefore, it is very interesting to study how the phase transition changes, when the macroscopic anisotropy of CsCuCI, is varied continuously. With this motivation, we have investigated the phase transitions in the mixed system CsCu,,Co,CI, with a few percent of Co2+ions, which is expected to produce the easy-axis anisotropy. In order to investigate the phase transitions in CSCU,~CO,CI,,we measured the temperature and the field dependence of magnetization for the samples with 0.015 Ix 5 0.032 [6]. We found two ordered phases in the present systems. Figure 1 shows the phase diagram of CsCu, ,Co,CI, (x = 0.015, 0.023 and 0.032) for H II The small amount of Coz+ion dopant produces a new ordered phase in the low-temperature and low-field region. The area of phase is enlarged with increasing Co2+ion concentration. Magnetic structures in the ordered phases I and I1 were determined by the neutron scattering experiments [7]. In phase I, the magnetic structure is identical to that in the ordered state of CsCuCl,, i.e., spins form 120"-
776 0tt
I
I
+I
I
I "1 V l ,
cscu .xcOxC13
CL
Hllc
c
t r
A
x=0.015
E *L
10
Figure 1. Magnetic phase diagram of cscu,,co,c1,.
Figure 2. Oblique triangular antiferromagnetic structure realized in phase 11. Angle ydenotes the half angle between S, and S,. structure within the c-plane. The phase I1 is an oblique triangular antiferromagnetic phase, in which the spin plane spanned by the spins on a triangular lattice is tilted from the c-plane as shown in Figure 2. For the sample with 0.03, tilting angle $ was estimated as $ = 44" at T = 1.6 K. The microscopic mechanism leading to the successive phase transitions has not been clear. In order to investigate the spin dynamics of the present system, we carried out the high frequency ESR measurements for the frequency 95 5 s 525.8 GHz:In this paper we report the results.
-
2. EXPERIMENTS Single crystals of CsCu,,Co,Cl, were prepared by the vertical Bridgman method from a melt of CsCuC1, and CsCoCI,. The details of the sample preparation have been described in Reference [ 61. The ESR measurement was performed at Institute for Materials Research, Tohoku University, using a multilayer pulse magnet, which produces magnetic fields up to 30 T. Optical-pumped far infrared gas lasers (525.8 - 326.1 GHz), backward traveling wave tubes (- 200 - 240 GHz and 330 - 380 GHz), and Gunn oscillators (- 95 - 190 GHz) were used as light sources. The Faraday configuration was taken in the measurement. The transmitted light power was detected by an In-Sb detector. ESR signals were taken at T = 1.6K in the magnetic field parallel to the c-axis.
-
3. RESULTS AND DISCUSSION Figure 3 shows the ESR absorption spectra observed at T = 1.5 K for the samples with = 0.015, 0.023 and 0.032. Arrows indicate the resonance fields. The vertical dashed lines denote the magnetic phase transition fields H, detected by the magnetization measurements. For all samples, each ESR signal is composed of a broad strong absorption line and a weak critical resonance at For the samples with x = 0.023 and 0.032, resonance fields increase monotonously with increasing frequency. One of the noticeable features of the absorption
777
5
j
zE s
c
0
5
10
15
0
5
10
15
0
5
10
15
I
Figure 3. ESR spectrum of CsCu,,Co,CI, for the samples with x = 0.015 (a), 0.023 (b) and 0.032 (c). Dashed vertical lines indicated by H, denote the phase transition field determined by the magnetization measurements. spectrum is that the line width broadens significantly, when the resonance field is located at around H,. Figure 4 shows the temperature variation of the resonance field for the sample with x = 0.023. Dot dashed line indicated by TNdenotes the phase transition temperature detected by magnetization measurements. With increasing temperature, the resonance field shifts abruptly toward higher field side, when the temperature passes through TN.Below TN,the resonance field reaches a value corresponding to g = 2.06. Similar results were obtained for other systems. The resonance data obtained for x = 0.032 (see Figure 3(c)) are summarized in the frequency versus field diagram as shown in Figure 5. In this diagram, the magnetic field is
h
!
I
I
10
I
I
50
T
Figure 4. Temperature dependence of the resonance field for H II c and = 135 GHz for the sample with x = 0.023. Dot dashed line indicates the phase transition temperature determined by the magnetic measurements.
5
H
m
15
Figure 5. Frequency versus field diagram for the sample with x = 0.032 measured at T = 1.6 K for H II c-axis.
778
Table 1. Magnetic planar anisotropy fields obtained by fitting of the data for CsCu,. xCoxCI,and CsCuC1, [4]. Other magnetic parameters were fixed as HE,,= 21 [TI, HE1= 11 [TI and HDM= 1.9 [TI.
HA [TI
x = 0.015
x = 0.023
x = 0.032
cscuc1,
0.203
0.174
0.174
0.18
normalized by the g-factor. Above H > 5 T, the resonance point approaches gradually the EPR line with increasing frequency. Solid curves labeled by and LO. denote the ESR modes calculated for the 120"-structure within the c-plane as observed in CsCuCI, [4]. When the effective fields due to the Dzyaloshinsky-Moriya interaction HDMand the planar anisotropy HA are much smaller than the exchange fields due the interactions along the chain (H,) and between the chains (&), the resonance conditions for the and modes are expressed as H, We fix the values of H,, HE1 and HDM to H , = 21 [TI, HEI= 11 [TI and HDM= 1.9 [TI, which are the same as those in CsCuCI, 141, since the concentration of Coz' ion is sufficiently small, and the period of helical spin arrangement is close to that in CsCuCI, [7]. Therefore, only the anisotropy fields HA are treated as the adjustable parameter. We see that the experimental results for H 5 T are well described by equation (1). Table 1 shows the values of HA obtained by fitting the data for H L 5 T to equation (I). On the contrary to our expectation, there is no significant change in HA. The resonance point below H < H, deviates from the theoretical curves for the and modes. The ESR signals obtained at lower field region H < 4 T are supposed to be the antiferromagnetic resonance modes, which are characteristic of the oblique triangular antiferromagnetic structure. Further investigations such as mode calculation for the new magnetic structure in phase I1 or the experiments for the lower frequency region v 5 95 GHz are needed.
REFERENCES 1. A.V. Chubukov and D. I. Golosov, J. Phys.: Condens. Matter, 3 (1991) 69. 2. T. Nikuni and H. Shiba, J. Phys. SOC.Jpn. 62 (1993) 3268. 3. K. Adachi, N. Achiwa and M. Mekata, J. Phys. SOC.Jpn., 49 (1980) 545. 4. H. Tanaka, U. Schotte and K. D. Schotte, J. Phys. SOC.Jpn., 61 (1992) 1344. 5. H. Nojiri, Y. Tokunaga and M. Motokawa, J. Phys. (Paris) 49 Suppl. C8, (1988)1459. 6. T. Ono, H. Horai and H. Tanaka, J. Phys.: Condens. Matter, 12 (2000) 975. 7. T. Ono, T. Kato, H. Tanaka,A. Hoser, N. StuBer and U. Schotte, Phys. Rev., B 63 (2001) 224425.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
779
Gyrotron ESR in CsFeCls up to 40 T M. Chiba", K. Kitai*, S. Mitsudob, T. Ideharab, S. Ueda" and M. Todad 'Department of Applied Physics, Fukui University, Fukui 910-8507, Japan bResearch Center for Development of Far Infrared Region, Fukui University, Fukui 9108507, Japan "Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan dResearch Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan
More than ten years ago, an anomalous magnetization was observed in CsFeC13 under the magnetic field exceeding 33 T. Usually, the spin state of Fez+ in CsFeCl3 is treated the fictitious spin S = 1 with the singlet ground state due to the crystallographic single ion anisotropy. The magnetization saturates at the magnetic field of about 10 T applied parallel to the crystal c-axis. The anomalous magnetization can not be expected within the framework of the fictitious spin S = 1. In order to study the relation between the anomalous magnetization and the excited spin multiplet = 2, an ESR experiment was performed under magnetic fields up to 40 T by use of a pulse magnet. The submillimeter wave was supplied by Gyrotron FU IV at Research Center of Far Infrared Region, Fukui University. The ESR absorption lines were observed corresponding to the anomalous magnetization jump. 1. INTRODUCTION
An ABX3 type hexagonal compound CsFeC13 has been attracted an attention relating to the magnetic frustration caused by the triangular-lattice antiferromagnetism. Further, the Fe2+spin in this material is treated the fictitious spin S = 1 and has the characteristics of the singlet ground state [l].At zero magnetic field the spin states are composed of the singlet ground- and doublet excited states separated by due to the crystallographic single-ion anisotropy. The material does not have any long range order at zero magnetic field reflecting the nature of the singlet ground state. On the other hand under a magnetic field B applied parallel to the crystal c-axis, ( B // c-axis), one of the states of the doublet excited state comes down to cross the ground state at 7.5 T. The long range magnetic order is reported to occur around the level-cross field below 2.5 K [2,3]. More than ten years ago a high field magnetization was measured at 1.3 K with Bllcaxis [4]. An anomalous magnetization has been observed under high magnetic field around 33 T [4]. The anomalous magnetization suggests the appearance of a new magnetic
780
a m
CsFeCl
4-
B I/ c-axis T=1.3K
10
20
30
(T)
Figure 1. Crystal structure of CsFeCl3.
Figure 2. High field magnetization in CsFeC13. After ref.[4].
structure, which can not be explained within the framework of the fictitious spin S = 1. In order to clarify the nature of the above-mentioned anomalous magnetization, an ESR experiment was performed under magnetic fields up to 40 T. The ESR experiment has an advantage that a direct observation is possible on the behavior of the electronic spin states. A preliminary results for the operating frequency of 301 GHz have been already reported [ 5 ] . 2. HIGH FIELD MAGNETIZATION
The present ESR experiment in CsFeC13 under high magnetic fields is motivated by an anomalous magnetization jump around 33 T observed previously by one of the authors (41. Here we briefly survey the high field magnetization observed at 1.3 K with B//c-axis. The crystal structure of CsFeCla is shown in Figure 1. At that time, in order to determine the g-factor from the saturation magnetization, a high field magnetization has been measured up to 40 T. The magnetization curve is shown in Figure 2. Under the magnetic field from zero to T the magnetization is weak with a slight linear increasing. The weakness of the magnetization is consistent with the fact that the ground state is singlet. The slight increasing in the magnetization with increasing magnetic field is considered to be due to the Van Vleck paramagnetism. The magnetization increases rapidly with increasing magnetic field from 4 to 11 T. The linear increasing of the magnetization in this region suggests the appearance of the magnetic order around the field of the ground state crossover. These features are consistent with the theory by Tsuneto and Murao [6]based on the exchange coupled S = 1 singlet-ground state system. The magnetization seems to saturate at 11 T. Afterwards, up to about 32 T the magnetization increases gradually affected by the Van Vleck paramagnetism. Thus below 32 T the framework of the fictitious spin = 1 has been found to work well for the explanation of the magnetization. However, under the magnetic field around 33 T an anomalous magnetization has been observed. This behavior in the magnetization can not be explained in the framework of the fictitious spin S = 1.
781
sz = sz = * l
sz = * z
cubic
I
Spin Energy Level of F e in CsFeCll 40
100
yr)
spin mbll
Figure 4. Calculated electron spin energy level diagram of Fez+ ion in CsFeC13. B//c-axis. After ref.[4].
Figure 3. Spin state of Fe2+ in CsFeC13.
SPIN STATE OF Fe2+IN CsFeC13 The free ion state of 3d6 is 5D. The energy levels are split by the effects of the crystalline field and the spin-orbit interaction as is indicated in Figure 3. At low temperatures under low magnetic fields, only the lowest levels, namely, singlet ground- and doublet excited states, are taken into account and are treated as the fictitious spin S = 1. Within the framework of fictitious spin = 1, the Hamiltonian describing a single ion spin state under the configuration of B//c-axis is given by ?-I
=
+ gpuBBS,,
(1)
where is the single ion anisotropy energy, g the g-factor and p~gthe Bohr magneton. Here, 0) brings a singlet ground- and doublet excited states. The octahedrons of C1- ions surrounding a Fez+ ion make a chain along the c-axis. The exchange coupling between Fe2+ ions along the chain is ferromagnetic, while the interchain coupling is antiferromagnetic, where 0.05 171. Since the effect of overcomes in the case of CsFeCl,, the material does not exhibit any long range order at zero magnetic field. The 3D-long range order is realized only under the applied field where the ground state crossover occurs. In order to consider the possibility that one of the levels in the excited spin multiplet = 2 crosses the ground state under the applied field, the single ion energy level scheme of Fe2+ is calculated with the following Hamiltonian 181, N
?-I = -kX1
.
-
- 2/3)
+
+
(2)
Here X is the spin-orbit coupling energy, k the orbital reduction factor, b the magnitude of the trigonal distortion. The result is shown in Figure where the following parameters = -2 are used: k = 0.9, I X I= 103 cm-‘ and I 6 / k X I= -2 191. The excited state comes down with increasing magnetic field but never crosses the ground state = -1 up to 100 T. Thus as far as the calculated energy level scheme in Figure 4 is concerned, no anomalous effect is expected under the available magnetic field up to 40 T. One of the possible
782
LY Sr= 1
sz =
CsFeC13 B // c-axis T=
K
sz= -
01 0
'
I
,
I
10 Bcl
20
B (T)
Figure 5 . High field ESR in CsFeCla. magnetization appears.
,
I
t
,
i
I
40
Bc2
Bc2 is the field where anomalous
mechanisms of the anomalous magnetization is an intrachain (along c-axis) ferromagnetic coupling. The ion Fez+ sees a large molecular field added to the applied magnetic field, = -2. which will enhance the lowering of the energy level of 4.
GYROTRON ESR EXPERIMENT
An ESR experiment was performed under pulsed magnetic fields up to 40 T with operating frequencies of submillimeter region. The experiment was performed at Research Center for Development of Far Infrared Region, Fukui University. The submillimeter wave was supplied by Gyrotron FU IV operating in the superconducting solenoid up to 12 T. A typical cw operation mode of the gyrotron at the frequency of 301 GHz was TEo3 mode with the output power of 20 W. The submillimeter wave was guided to the specimen by a light pipe. The absorption signal was detected by an InSb hot electron detector operating at the liquid helium temperature. The magnetic field up to 40 T was generated by a pulse magnet driven by a capacitor bank of 30 kJ. The magnet is immersed in the liquid nitrogen. The bore where the magnetic field is generated was 12 mm in diameter. An insert Dewar was settled inside the magnet for cooling down the specimen by use of the liquid helium. The inner diameter of the insert Dewar at the center of the magnet was 7.6 mm. The experiment was carried out at 4.2 K. The magnetic field was applied parallel to the c-axis of a single crystal CsFeCls. 5 . EXPERIMENTAL RESULTS AND DISCUSSION
In the experiment of ESR several branches of absorption spectrum were observed as is denoted by A, B, C, D and E in Figure 5 . The branches A and B are consistent with
783
the experiment carried out by another group [10,11]. Other branches C, D and E are the spectrum observed for the first time. Now we consider the branches A, B and C in the framework of fictitious spin = 1. The branches A and B correspond to the transitions from the state = 0 to = -1, and from the state = 0 to = 1, respectively. = 7.5 T appears the Under the magnetic field exceeding the first level cross field = 1 to = 0. The branch C which corresponds to the transition from the state slope of the straight lines in Fig. 5 corresponds to g = 2.6, which is consistent with that determined from the saturation magnetization [4]. The origin of the groups of the resonance lines, which belongs to neither A nor B, are not interpreted so far. One of the possible mechanisms is the contribution from some collective spin mode. According to the results of high field magnetization [5] the value of the anomalous magnetization is a little larger than 4 pB/Fe2'. By assuming that g is about 2, the = -2 at the field above 33 T. How the energy level of = possible ground state is -2 comes down? The branches D and E composed of strong absorption spectral lines were observed around 33 T coinciding the field where the anomalous magnetization jump appears, namely, the second level cross field The straight lines in branches D and E are drawn to fit the experimental data. If we formally determine the g-factor from their slopes, we obtain g = 3.7 for both branches of D and E. The value is too large considered from the calculated energy level diagram in Fig. 4. This fact means the rapid decreasing of the energy level = -2 in = 2 spin multiplet enhanced by the strong ferromagnetic intrachain coupling as has been proposed by Hori et al. [12]. The detailed qualitative discussion will be presented in a separate paper. References 1. H. Yoshizawa, W. Kozukue and K. Hirakawa, J. Phys. Soc. Jpn., 49 (1980) 144. 2. T. Haseda, N. Wada, M. Hata and K. Amaya, Physica, 108B (1981) 841. 3. M. Chiba, S. Ueda, T Yanagimoto, M. Toda, and T. Goto, Physica B, 204-208 (2000) 284. 4. M. Chiba, T. Tsuboi, H. Hori, I. Shiozaki and M. Date, Solid State Commun., 63 (1987) 427. 5. M. Chiba, A. Aripin, K. Kitai, T. Idehara, S. Ueda and M. Toda, Physica B , 294-295 (2001) 64. 6. T. Tsuneto and T. Murao, Physica, 51 (1971) 186. 7. M. Steiner, K. Kakurai, W. Knop, B. Dorner, R. Pynn, U. Happek, P. Day and G. McLeen, Solid State Commun., 38 (1981) 1179. 8. N. Suzuki, J . Phys. Soc. Jpn., 50 (1981) 2931. 9. W. B. Euler, C. Long, W. G. Moulton and B. B. Garrett, J . Magn. Resonance, 32 (1978) 23. 10. H. Ohta, N. Makita, K. Yoshida, T. Nanba and M. Motokawa, Int. J. Infrared Millim. Waves, 13 (1992) 457. 11. H. Ohta and M. Motokawa, Recent Advances in Magnetism of Transition MetalCompounds, (eds. A. Kotani and N. Suzuki, World Scientific, Singapore, 1993), p.316.. 12. H. Hori, I. Shiozaki, M. Chiba, T. Tsuboi and M. Date, Physica B, 155 (1989) 299.
784
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Magnetic properties of Fe12 ring : ESR and magnetization measurements Y. InagakP*,T. Asano", Y. Ajiroa, Y. Narumib,', K. KindoCsb,H. Nojirid,", M. Motokawad, A. Cornia" and D. Gatteschif "Department of Physics, Kyushu University, Fukuoka, 8 12-8581, Japan bCREST JST Corporation, Kawaguchi, Saitama 332-0012 Japan 'KYOKUGEN, Osaka University, Toyonaka, Osaka 560-853 1 Japan kstitute for Materials Research, Tohoku University, Sendai, 980-8577, Japan "Department of Chemistry, University of Modena, Modena, Italy Qepartment of Chemistry, University of Florence, Firenze, Italy The results of ESR and magnetization measurements on the powder sample of Fe12 molecular magnetic ring are reported. A discrete structure of energy levels of the system in lower energy part is revealed directly through magnetization measurements up to 55 T at 0.1 K. The ESR spectrum shows anomalous temperature- and frequency-dependent behaviors which may be attributable to the formation process of magnetic ring.
1. INTRODUCTION Recent progress on the synthesis technique of assembled metal cluster has provided a rich variety of substances, enable to investigate the fundamental properties of molecular magnets. Recently, dodecanuclear Iron(II1) ring cluster [Fe(OCH,),(dbm],, (abbreviated as Fe12), where dbm = dibenzoylmethane, was synthesized successfully [11. The crystal structure is monoclinic (space group P2,ic) and each molecule is composed of twelve number of Fe3' (S=5/2) ions forming a ring with chemically equivalent bridging rigands. The data of magnetic susceptibility shows broad maximum around 150 K and was well fitted by the S=5/2 quantum antiferromagnetic chain model, except for the temperature regime below about 10 K, in which decreases suddenly, reflecting the discrete quantum energy levels with singlet ground state characteristic to this finite size system. In this paper, we report the results of high-field magnetization and electron spin resonance (ESR) measurements performed on powder samples of this interesting molecular magnet, Fe12.
Present address; *Graduate School of Science and Technology, Kobe University, Kobe 6578501, Japan. **Department of Physics Okayama University, Okayama 700-8530, Japan.
785
2. MAGNETIZATION High-field magnetization was measured by induction method, using pulsed high magnetic field up to 55 T, at the temperature down to 0.1 K. FIGURE 1 shows the magnetization curve for increasing field H up to 55 T at 0.1 K, together with the field derivative of magnetization Five distinct magnetization steps are clearly seen; the magnetization is zero at low field below about 10 T and it then increased by about 2 p, at each step with the field separation of about 10 T. The behavior is characteristic of the system with discrete energy levels with different total spin states I&+, 1, 2, ...30>, which is one important aspect of the quantum nature. In increasing each of the Zeeman level crossings raises the value of quantized magnetic moment in the ground state by one unit. The fact that the crossovers occur at regular interval implies that the lowest energy levels for every states are given by a Lande interval rule. We can determine the energy gap AE between the singlet ground state /ST=O>and the lowest excited state IST=l> to be 13.6 K, directly from the first transition field at about 10 T, using A = g p B q , and g = 2.02 from ESR. Using the relation of AE = with N = 12, the exchange interaction within a ring is also determined to be 40.7 K. These values of A E a n d J should be compared to the values of 10.7 K and 3 1.9 K estimated from the susceptibility data [ 11. In principle, the present determination is more direct and therefore more accurate in view of that the finite size effect should have to be considered in the analysis of susceptibility data. It is of interest to notice that the dM1dH curve is somewhat unusual in a sense that second peak is higher and narrower than others as seen in FIGURE 1. Besides, we found that the magnetization curve at 1.3 K exhibits some unusual features with an anomalous hysteresis for increasing and decreasing field; the magnetization change associated with each step occurs at two stages leading to a characteristic winged hysteresis loop with a plateau region These anomalous behaviors are discussed in detail elsewhere [2-41. h
12 . 0 , . ,
. , .
I . ,
.
I
.
2.08
1000
,
CJ
. 2
'-2.06
v
+! 0 0 0 0 0
x x x x x
f
g-value 0
10 20 30 40 50 60 Field (T)
FIGURE 1. curve and its field derivative curve observed at 0.1K.
-
- 0
0
2.04 .
: ::2.02 s -
2.00
1.98 50 100 150 200 250 300 Temperature (K)
FIGURE 2. Temperature dependence of signal intensity line width DH and g-value observed at X-band frequency.
3. ESR Submillimeter wave ESR measurements were performed up to about 800 GHz and in pulsed magnetic field up to about 30T at the temperature down to 1.7 K. Additional ESR measurements were performed at X-band, using a standard equipment. FIGURE 2 shows the temperature dependence of the signal intensity Z, g-value and line width at X-band (9.0 GHz) frequency. The signal intensity decreases rapidly accompanying with the increasing line width below 150 K which corresponds to the maximum of , and the resonance position shows a marked g-shift below about 20 K which corresponds to the sudden drop of These temperature-dependent behaviors are attributable to the formation process of magnetic cluster. At high temperature, the ESR signal is nothing but the transition of the exchange coupled paramagnetic Fe3' ions in a sense that the total spin states are obscured in the thermal window. As the system becomes to correlate below = 200 K, the character of signal gradually changes. In the intermediate temperature regime, the observed signal is the collection of various transitions within the excited low-lying multiplets which are sufficiently populated by thermal excitations. In this situation, however, there still exist thermally induced transitions between the different multiplets which is sufficient to cause the average of spectrum and therefore only the single averaged spectrum is observed. This averaging effect diminishes with decreasing temperature, causing the broadening of the line width. At the same time, the signal intensity also decreases because of a depopulation of higher energy states with large total spins. At sufficiently low temperatures where magnetic cluster is formed to give a singlet ground state and a characteristic energy gap becomes to be well defined in the thermal window, we expect to observe eventually a resolved structure. However, at these temperatures, only the first excited state is populated, giving rise to a single line without averaging effect. This explains the reason why the g-shift occurs below about 20 K. Around this temperature, the transitions within = 2 multiplet lose intensity and the triplet signal within = 1 state moved from the averaged position to its own position without the effect of other transitions. It is noteworthy that the observed temperature dependence of ESR intensity is not the same as that of susceptibility; the intensity decreases more rapidly than the susceptibility, suggesting that the transitions within some multiplets could not be detected practically due to the broadening. FIGURE 3 shows the temperature dependence of ESR transmission spectrum at 190 GHz. The main absorption line at g = 2 disappears around 10 K with decreasing temperature, accompanying with the g-shift. At the same time, a weak signal appears at low field as indicated by the solid triangle. It is now clear that the main line comes from the excited state resonance because of the loss of intensity with decreasing temperature. FIGURE 4 shows the frequency dependence of ESR spectrum at 1.7 K. A very broad absorption line with a typical powder pattern due to the anisotropy of about 0.8 K are clearly seen at high field where the ground state is ST=2multiplet. The broad line is attributable to the transition within the quintuplet state. Interesting enough, the low-field mode mentioned above coincides with the g = 4 line at high field, however, it seems to have a gap at zero field as indicated by the dotted line. This gap mode appears simultaneously with disappearing of main absorption. This behavior suggests that the origin of this mode is related to the cluster-
787
60K 55K 50K 45K 40K 35K 3OK 25K 20K 15K 1OK 4.2K 3.5K 3.OK 2.5K 2.OK
A
m
.3
? P
3
v
s
.-
m
c="
6 9 Field (T) Field (T) FIGURE 3 . Temperature dependence of ESR FIGURE 4. Frequency dependence of transmission spectrum at 190 GHz. Weak sig- ESR spectrum at 1.7 K nal indicated by the solid triangle is observed at low temperatures. 0
3
ing within a ring. In summary, we performed the magnetization and ESR studies on the powder sample of Fe12 over wide field and temperature range. From the magnetization at very low temperature, the discrete energy level of the system was revealed and the gap AE was estimated to be 13.6 K directly. From the ESR measurements, we discussed the formation process of magnetic cluster with decreasing of temperature. ACKNOWREDGEMENT
The authors would like to thank H. Nakano and S. Miyashita for useful discussions. This work was performed at KYOKUGEN, Osaka University and IMR, Tohoku University, and supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Y.I. was financially supported by Japan Society for the Promotion of Science (JPSJ). REFERENCES
Caneschi, Cornia, A. C. Fabretti and D. Gatteschi: Angew. Chem. Int. Ed. Engl. 38 (1999) 1295 2. H. Nakano and S. Miyashita: J. Phys. SOC.Jpn. 70 (2001) 2151. 3. H. Nakano and S. Miyashita: To be published in J. Phys. Chem. Solids. 4. Y. Inagaki et al. to be published. 1.
EPR in the 2 1'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
788
High magnetic field ESR measurements of ACu2(PO& (A=Ba, Sr) Masayuki Hataa, Seitarou Mitsudob, Toshitaka Ideharab and Mamoru Mekataa "Faculty of Engineering, Fukui University, Fukui 910-8507, Japan bResearch Center for Development of Far Infrared Region, Fukui University Bunkyo 3-9-1, Fukui 910-8507, Japan High magnetic field ESR measurements of BaCu2(PO& have been performed using a Gunn oscillator (120 GHz) at temperatures from 4.2 K to 140 K. Three resonance lines have been observed below 20 K, while only one resonance line has been observed above 20 K. Above 20 K, ESR integrated intensity was in good agreement with 4 spin cluster model. The best fit of ESR integrated intensity data was obtained with
= 25 K and
= 55 K. High magnetic field ESR measurements of SrCu2(PO& have been performed
using a Gunn oscillator (120 GHz) and Gyrotron (301 GHz) at temperatures from 4.2 K to
77 K. Both integrated intensity and magnetic susceptibility have a broad maximum at 40 K and in the best fitting of both integrated intensity and magnetic susceptibility is obtained with = 30 K and = 40 K for 4 spin cluster model.
1. INTRODUCTION It is known that A C U ~ ( P O (A=Ba, ~)~ Sr) crystallizes into a triclinic structure and belongs to the P-1 space group. Cu2+ions occupy four crystallographically different sites forming zigzag chains in the a-b plain [l].The A2+ ions separate Cu2+ chains and the magnetic coupling along the c-axis is diminished due to PO4 tetrahedra. The spacing of the Cu2+ions in a chain has sequence of -3.80A-3.60A-3.47A-3.58A-3.80A-
so
that the four spins form a relatively isolated group. Based on the results of x-ray diffraction, it is expected that Sr compound has the same crystallographic structure as Ba compound. Results of magnetic susceptibility measurements of both Ba and Sr compounds suggest that both of them have a singlet ground state. A broad maximum in magnetic susceptibility was observed at 60 K for Ba and 40 K for Sr compounds. Experimental results for Ba
789
compound were best fitting using the ladder model with energy gap of 33 K. On the other hand, experimental results of Sr compound were best fitting using 4 spin cluster model = 30 K and = 40 K The differences in the magnetic properties of these with two compounds have been investigated by ESR technique.
2. EXPERIMENTS ESR measurements were performed using powder samples of Ba and Sr compounds. The equipment used in experiments allowed to perform ESR measurements with pulsed magnetic field up to 35 T. Measurements could be performed in 4.2 K to 140 K temperature range. Experiments were performed at a fixed frequency of 120 GHz provided by a Gunn oscillator, and at 301 GHz provided by a gyrotron. A gyrotron is a electro magnetic wave radiation source with an output power in millimeter and submillimeter wave range. Output frequency of gyrotron can be step varied allowing for certain tunability, and its output power can be from several 10 W to several kW
I
'
I
~
I
'
I
'
I
'
w
77 K .1
66 K
h . I
E
?
60 K
-_
x - _ - _ _
-56K
* '
m
v
48 K
a . I
. I
E
-
,
-
34K
,
~~
a
-
_-
-
25 K
m
20 K
b
11 K I
4.2
DPPHl I
,
1
,
I
,
1
1
,
I
,
Figure 1. Temperature dependence of the ESR spectrum of BaCu2(PO&
790
3. RESULTS AND DISCUSSIONS
3.1. BaCuz(PO4)z High magnetic field ESR measurements of BaCuz(PO& have been performed using a Gunn oscillator (120 GHz) at temperatures from 4.2 K to 77 K, as is shown in Figure 1. Three resonance lines have been observed below 20 K, while only one broad resonance line has been observed above 20 K. It has also been found that the temperature increase from 20 K to 60 K leads to a slight shift of the resonance position toward the lower fields side and increasing temperature further from 60 K to 77 K results in the resonance position shift back towards higher field side. The results of g-value estimation from experimental data for temperatures above 20 K are shown in Figure 2. It turns out that g-value has a sharp peak at 60 K which coincides with the temperature of a broad maximum of magnetic susceptibility. Figure 3 shows the temperature dependence of integrated intensity for BaCu*(PO&. The circle point in Figure 3 shows the integrated intensity of ESR resonance lines, above 20 K. The temperature dependence of integrated intensity shows a broad maximum around 60 K. Below 60 K, as temperature decrease, it toward to zero at 0 K. These features are in agreement with the results obtained from the magnetic susceptibility measurements [ 2 ] . The line of (a), (b), and are the fitting lines using ladder model, alternating chain model, and 4 spin cluster model, respectively [4-61. The best fit of experimental data was obtained with Jl/kB = 25 K and J2/ks = 55 K in 4 spin cluster model. The triangle point in Figure 3
*
2.101
il 2.05
i
1.95 0
20
40 60 80 100 120 14 Temperature (K)
Figure 2. Temperature dependance of g-value for BaCuz(PO&.
0
(a) : ladder (b) : alternating chain (c) : 4 spin cluster (d) : Curie term
20 40 60 80 100 120 140 160 180 20(
Temperature (K)
Figure 3. The temperature dependence of integrated intensity for BaCu2(P04)2.
79 1
shows the integrated intensity of ESR resonance lines, below 20 K. These points are in agreement with calculated value using Curie term (d).
3.2. SrCuz(PO& High magnetic field ESR measurements of SrCu2(PO& have been performed using a Gunn oscillator (120 GHz) and Gyrotron (301 GHz) at temperatures from 4.2 K to 77 K. Since similar results were obtained at both frequencies, only results at 120 GHz are shown in Figure 4. Only one resonance line ( P I ) was observed at 77 K. Also, g-value of P I dose not depend on temperature and is equal 2.20. Below 40 K, two new resonance lines ( P 2 , P3) appeared. Figure 5 shows the temperature dependence of integrated intensity for SrCu2(PO4)2. The circle point in Figure 5 shows the integrated intensity of ESR resonance lines of PI. The temperature dependence of integrated intensity shows a broad maximum around 40 K. Below 40 K, as temperature decrease, it toward to zero at 0 K. This behavior is in agreement with the result obtained from the magnetic susceptibility measurements. The solid line is the fitting lines using 4 spin cluster model. The best fit of ESR data was obtained with JIIkB = 30 K and J 2 l k ~= 40 K. The triangle point in Figure 5 shows the integrated intensity of ESR resonance lines of
P2
and
P3,
below 40 K. These points are in
agreement with calculated value using Curie term (dotted line).
0 3
20
30 40 50 Temperature (K)
70
80
4 5 Magnetic field (T)
Figure 4. Temperature dependence of the ESR spectrum of SrCu2(PO4)2 at 120 GHz.
10
Figure 5. The temperature dependence of integrated intensity for SrCu2(P04)2.
792
4. CONCLUSION Integrated intensity of ESR absorption for both Ba and Sr compounds showes good agreement with 4 spin cluster model. It is consistent with crystallographic symmetry. However, the values of exchange interaction constants
obtained form the 4 spin
cluster model are different. Sr compound has smaller alternating exchange interaction in 4 spin cluster model than Ba compound one. In order to clarify the reason for experimentally observed differences between Ba and Sr compounds it is necessary to further investigate the crystal structure of the Sr compound. And also at 4.2 K, the difference of ESR spectrum between Ba and Sr compounds may be come from these differences.
REFERENCES 1.A. Moqine, A. Boukhari, and J. Darriet, J. Solid State Chem. , 107 (1993) 362. 2. M. Mekata, T. Hanabata, K. Nakaya, and Y. Ajiro, to be published in J. Magn. Magn. Mater. , 2001. 3. T. Idehara, I. Ogawa, S. Mitsudo, M. Pereyaslavets, N. Nishida, Y. Yoshida, IEEE Trans. Plasma Sci. , 27 (1999) 340. 4. M. Troyer, H. Tsunetsugu, and D. Warts, Phys. Rev. B, 50 (1994) 13515. 5. W. E. Hatfied, J.Appl.Phys., 52 (3) March 1981. 6. M. Hase, K. M. S. Etheredge, S. Hwu, K. Hirota, and G. Shirane, Phys. Rev. Lett. , 70 (1993) 3651.
EPR in the 21” Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
793
ESR transmission experiments on P’-(ET)2SF5CF2S03and (ET)2SF5NN02, investigations of spin-Peierls systems. I.B. Rutela, J.Brooks”, B.H. Ward’, D. VanDerveer“, M.E. Sitzmannd, J. Schluetef, R.W Winter‘, G. Gardf aDepartment of Physics, FSU/NHMFL, 1800 E. Paul Dirac Dr., Tallahassee, FL 32310, USA ’School of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 323 10, USA ‘School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA dNaval Surface Warfare Center, Indian Head, MD 20640, USA ‘Chemistry and Materials Science, Argonne National Laboratory, Argonne, IL 60439, USA fDepartment of Chemistry, Portland State University, Portland, OR 97207, USA An anisotropic temperature dependent ESR investigation of the organic systems (ET)2SF5CF2S03and (ET)2SF+J”N2 has led to the indication of a spin-Peierls transition at TSP= 33, and 5 K, respectively. We present the details of our investigation and place our data in the context of the general spin-Peierls theory. We also show the dc susceptibility SQUID measurements revealing novel behavior in the second material. Note that to access the high field regime, large fields and high frequencies are needed.
1. INTRODUCTION This paper discusses two organic materials, and the investigation of whether the spinPeierls transition describes the behavior of each using magnetic resonance techniques. Following the introduction, data collected on the two compounds P’-(ET)zSF&SSO3 (CF2), and (ET)2SFsNNOz (”Oz), (where ET is will be presented in the general context of spin-Peierls theory. Crystals of both salts were grown by electrocrystallization methods previously described. The spin-Peierls (SP) transition is a second order magnetic transition that is defined by a structural distortion and subsequent dimerization of the lattice. The transition is best described as a one-dimensional (1D) chain of Heisenberg antiferromagnetic spins that at a given temperature, TSP,progressively dimerize into spin singlet pairs. Consequently, both the anisotropic behavior in the susceptibility is seen, as well as the susceptibility vanishing with
*
794
decreasing temperature. This is easily distinguished from a three-dimensional (3D) antiferromagnetic ordering by the decreasing of the susceptibility to zero along all three crystallographic axes in the SP material as opposed to just the easy axis in an antiferromagnetic material.2 Higher temperature behavior, antiferromagnetic fluctuations, is best described by Bonner-Fisher theory (BF), from which the coupling constant, can be determined.3
2. EQUIPMENT AND TECHNIQUES Three techniques were used in characterizing the materials, a cavity resonance transmission technique and a direct transmission technique. The cavity resonance technique was used in conjunction with a Millimeter-wave Vector Network Analyzer (MVNA), with a continually tunable (8-18 GHz) YIG source. Schottky diodes frequency multiply and mix the microwave source to frequencies ranging from 29-220 GHz. These signals then propagate through waveguides to a resonant cylindrical cavity that is 9.55 x 9.31 mm (height x diameter) exciting the fundamental mode (TEoll) of 41 GHz4,5 . ,Ouxlo* The experimental set-up is nearly 95
5
identical for the direct transmission technique, but allows 85 for measurements, which are impractical 75 with resonant cavities, since the cavity inn isn 2511 3uu "> l/T [KI] volume decreases [Kl inversely with Figure 1. a) SQUID data for the CF;, (open diamonds) material with frequency. The probe the BF fit. b) SQUID data for both samples plotted on an Arrhenius has the mixer diode plot, showing the possible onset of two separate transitions, in the replaced with an NZ "02 sample (filled diamonds) and CF2. cooled detector, a Gunn oscillator multiplies the diode output, and the sample is placed in the waveguide allowing only radiation that passes through the sample to be detected. The resonant cavity is replaced with two 45" mirrors that direct the radiation to the detector. =1
YO
I
02
03
"4
"3
795
A dc SQUID measurement was also carried out on the "02 material, using a Quantum Interference Design SQUID, where the field was held at a constant 1 T, and the temperature varied from 350 to 1.7 K.
I
I
.M
3.
5
5 4000A The ~ ' - ( E T ) Z S F ~ C F ~isSaOshiny ~ 0055 s . + black rectangular crystal with a p' packing 0 3000 crystal structure. Previous investigation of 0 ; 2 l CF2 has been performed to determine the A 0 nature of the observed magnetic transition - -0.05 2000 at 33K4. Previous dc susceptibility measurements, SQUID, indicate the -0.10 1000 susceptibility vanishes with exponential T [K] . +-Hllc-axis behavior (Figure la). Further Hlla-axis investigation was performed in order to I determine the characteristics of the 100 l o T[K] transition. ESR experiments were performed on Figure 2. The temperature dependence of the all three crystallographic directions EPR integrated absorption for the resonance providing the data in Figure 2 . The runs line at 67.5 GHz for all three orientations were performed between 10 and 50 K. The (Hlla,Hllb, and Hllc ) for CF2. Areas show spectra contain two distinct absorption anisotropic intensities due to coupling lines corresponding to the ET and DPPH differences of the crystal with the cavity mode signals, where the DPPH was used as a as the sample was rotated. The top inset marker to calibrate the magnetic field. shows a perspective view of the ET cation The spectra were analyzed by fitting layer in the crystal looking down the long axis the ESR absorption signal to a Lorentzian of the molecule. The bottom insert shows the line shape and then integrated. The g-shift normalized by the g value at 9 K. The integrated area, which is proportional to largest change is seen along the b axis the magnetic susceptibility, is then plotted showing direct support for the ET molecules against temperature in Figure 2. The being dimerized and forming a 1D chain susceptibility clearly displays a vanishing along the b axis. Symbols correspond to axis exponential behavior along all three orientation in for both graphs. Open triangles directions showing the characteristic spinindirnte dntn t n k m nt 3 3 8 5 GH7 Peierls behavior. Measurements of both increasing and decreasing temperature show no hysteretic behavior (i.e. 1St order behavior). The activated gap values and relating transition temperatures were found using an exponential function, fit over the transition, ranging from 3 to 33 K6. This calculation yields a gap (A,,) and TSPof 114 (*21) K and 33 (*7) K respectively, where the uncertainty arises from fitting over slightly different ranges of temperatures. The transition onset seems to be lower than 33K, but our value fits well within experimental error. A BF analysis was also performed in 4-
Y
--
* -
796
the high temperature regime of the SQUID plot yielding a magnetic exchange constant JBFof 257 K (Figure 3b). In the inset of Figure 2, we investigate the temperature dependent g-shift of the CF2 material. The largest change in the g-value occurs along the b-axis (-0.12%), where the baxis is along the plane (bc) of dimerized ET molecules. The J3' crystal packing symmetry results in the initially dimerized ET planes. These ET dimers then show long-range antiferromagnetic correlation and singlet ordering, consistent with spin-Peierls theory. This leaves the ET dimers paired with neighboring ET dimers, predominantly in the b direction (some distortion in the a direction is reported by Pigos et al.7, evident in the inset by a small change in the a-axis g-shift as well). Overall, the lattice distortions have dimerized quartets of ET molecules, which a detailed Xray analysis can confirm.8 For completeness we now turn to the Figure 3. The temperature dependence of higher frequency (i.e. field) investigation. The the EPR integrated absorption for the high field runs were carried out using a direct resonance line at 79.9 GHz for all three transmission probe with frequencies >220 orientations (Hlla, Hllb, and Hllc ) for GHz. By performing experiments at these " 0 2 . The dashed line is an exponential frequencies we are able to probe the field vs. fit to the data along orientation 1. The temperature phase diagram, the high field top inset shows the orientation of the regime, and the universality of the spin-Peierls sample with respect to the magnetic field transition. (i.e. Hlllong axis (a) for orientation 3). Our experiments were carried out at 228.5 The second inset is the crystal structure GHz and -1 1 T. Our data show (open triangles of the "02 material, stacking is along in Figure 2) the transition temperature moving the a axis shown in perspective, into the with the increase in field. We have also seen a page. The final inset is the temperature broadening of the line width with decreasing dependent g-shift, where the onset of 3D temperature, which may suggest the onset of a lattice distortion is evident as a first order transition at 22 K as well, providing percentage change from the g-value at 30 evidence of a possible intermediate high field K, for all 3 directions. Note the feature phase. (dip) in Orientation 2, see text for details. A similar analysis has been camed out on the (ET)2SFsNN02 materialg, and can be seen in Figure 3. The sample has been placed in a cavity varying the position to line up each crystallographic axis with the magnetic field. This material is a dark brown whisker like crystal with a primitive space group, and preliminary dimensions of 6.52, 15.91, and 15.97 8, for a, b, and c respectively, with angles of 97.2",
797
90.3", and 90.2" for and A view of the crystal structure is provided in the inset of Figure 3. We are uncertain of the packing symmetry due to the preliminary nature of the crystallography. The sample was placed in the same cavity from the CF2 investigation, and the ESR data was collected at a frequency of 79.87 GHz corresponding to a field of 2.54 T, from 1.5 to 50 K. The material shows a precipitous drop in the susceptibility at 8 K. This drop is seen along all three of the principal axes and is confirmed to be second order in nature (from similar hysteresis arguments as above), but has not been observed to reach zero (as opposed to the CF2 sample). The signal is weakest when the field is oriented along the stacking direction, aaxis, and when normalized has the largest error, seen in the plot scatter. The transition temperature and gap value of TSP= 4.6 K, and A. = 8.07 K respectively, has been calculated using a linear fit on a plot of SQUID data of In vs. inverse temperature (i.e. an Arrhenius plot) as shown in Figure lb. A coupling constant of JBF= 341 K was also determined using BF theory, but does not fit well in the high temperature regime. Analysis of the g-shift shows marked distortion along all principle axes below 8 K. This is consistent with the precursor lattice distortion seen just prior to the onset of the spin-Peierls transition. It appears that the quasi ID chain axis is not confined to any one of the principle crystal axes. Also of note, is the g-value shift in the second orientation, which distorts in a non-monotonic manner at just above the precursor onset of the transition; suggesting a possible frustration in the lattice from a higher temperature ordered state. Evidence of this state is seen in the SQUID data (Figure 2b) which clearly shows a second exponential transition at 53 K. This high temperature order is unexpected, and not seen in the CF2 material, or other spin-Peierls systems and deserves further investigation. 4. DISCUSSION
A universal phase diagram has been suggested for spin-Peierls materials1°-13. The two well understood phases are the uniform (U),and Dimerized (D) phases shown in Figure 4. The (U) phase has the spins equidistant, and each spin has equal magnetic coupling (i.e. a single Jvalue). In the Figure 4. Plot of Phase Diagram for Dimerized phase, the spins are distorted and now both CF2 and " 0 2 samples. alternate coupling constants between intra-dimer and Universality arguments are inter-dimer interactions.2. Guided by previous data introduced with broken lines for presented for compounds like CuGeOa, we present both compounds. Figure 4, and the proposed universality arguments for both compounds described here. The final phase is the (I) phase, and has an intermediate, or incommensurate state. Given a BCS-like transition a general rule for the relationship between critical temperature for = 0, and the critical field for = 0 is approximately
798 = kB/b (i.e., the ratio of the Boltzmann constant to the Bohr magneton). We might expect that would be of the order 35 and 4 T, for CF2 and "02 respectively. High field and frequency measurements will be carried out to better understand the universality of both compounds and the nature of the (I) phase itself.
5. CONCLUSION Two different organic class materials show strong evidence of spin-Peierls behavior. The ESR data show that these materials behave in a Bonner-Fisher like manner in the high temperature regime, which is then followed by an exponential isotropic decrease in the susceptibility with temperature around the transition. We note the details of the crystal structure are not well determined, and there is also a feature in the higher temperature susceptibility. Due to the high temperature of the spin-Peierls transition and the ESR up to 11 T, it seems evident that further high magnetic field ESR will be necessary to compare these materials with others, already studied. Acknowledgement. Work at NHMFL-FSU was supported by NSF Grant No. DMR-99-71474 and NHMFL/MRP 500/503 1. The NHMFL is supported through a contractual agreement between the NSF through Grant No. NSF-DMR-95-27035 and the State of Florida. Work at ANL is supported by US-DOE, Office of Basic Energy Sciences, Division of Materials Sciences under contract W-31-109-ENG-38. Research at PSU is supported by NSF grant No. CHE-9904316 and the Petroleum Research Fund ACS-PRF 34624-AC7. Thanks also to Sergei Zvyagin and Fivos Drymiotis.
REFERENCES B. H. Ward, J. A. Schlueter, U. Geiser, Chem. Mat. 12, 343 (2000). J. W. Bray, L. V. Interrante, I. S. Jacobs, in edited by J. S. Miller (Plenum Press, New York, 1982), Vol. 3, p. 353. J. C. Bonner and M. E. Fisher, Phys. Rev. 135,640 (1964). B. H. Ward, I. B. Rutel, J. Brooks, J. Phys. Chem. B 105, 1750 (2001). S. Hill, J. S. Brooks, J. S. Qualls, Physica B 246-247, 110 (1998). 6 A. C. Rose-Innes and E. H. Rhoderick, (Pergamon Press, New York, 1988). J. M. Pigos, B. R. Jones, J. L. Musfeldt, Chemistry of Materials 13, 1326 (2001). J. A. Schlueter, et al. to be published . B. H. Ward, et al. crystal data to be published. l o J. Northby, H. Groenendijk, and L. Jongh, PRB 25,3215 (1982). V. Kiryukhin, B. Keimer, and D. Moncton, PRL 74, 1669 (1995). l 2 U. Ammerahl, T. Lorenz, B. Buchner, Zeitschrift Fiir Physik 102, 71 (1997). l3 T. Hijmans, H. Brom, and L. Jongh, PRL 54, 1714 (1985).
EPR in the 2 1" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
799
High field ESR measurements on molecular oxygen Shojiro Kimura and Koichi Kindo KYOKUGEN, Osaka University, Machikaneyama 1-3, Toyonaka, Osaka 560-853 1 Japan
The high field ESR measurements have been performed on solid molecular oxygen in the frequency region from 94 to 1400GHz using the pulsed magnetic field up to 55T at 4.2. It turned out that the observed antiferromagneticresonance modes can be explained by the molecular field theory for the two-sublattice model.
INTRODUCTION Among the simple diatomic molecules, molecular oxygen 0 2 is of particular interest because it has the magnetic moment. The ground state of the molecular oxygen is state with While the molecular oxygen shows simple paramagnetic nature in the gas and the liquid phases, the solid molecular oxygen below the melting temperature Tmp=54.4K shows two successive structural phase transitions at T p ~ 4 3 . 8 Kand Tap=23 . 9 K accompanied with drastic changes of the magnetic properties. It is known that the antiferromagnetic ordering of the magnetic moments realizes in the low temperature phase below Tap, whereas there is no long range order in the p and the y-phase above Tap. The crystal structure of a-oxygen is monoclinic, space group C2/m [ 11. There are two molecules in the unit cell and the figure axis of the molecule is parallel to the c*-axis. The magnetic properties of a-oxygen have been investigated by the magnetic susceptibility, neutron scattering, high field magnetization and the far infrared (FIR) spectroscopy measurements [2-61. The neutron scattering measurement shows that, the magnetic structure of a-oxygen is the two-sublattice type, and the easy axis turned out to be the b-axis [21. The spin-flop transition at &=6.9T was observed by the high field magnetization measurement [ 3 ] , and it was revealed from the
spectroscopy
800
Figure 1 . Block diagram of our experimental setup. measurements, that two antiferromagnetic magnon modes exist in far infrared region at 190GHz and 810GHz [4-61. However, the field dependencies of these magnon modes are not so clear at this time. In this study, to clarify the antiferromagnetic resonance (AFMR) modes of a-oxygen , we have performed the high field ESR measurements on solid molecular oxygen in the frequency region from 94 to 1400GHz using the pulsed magnetic field up to 55T.
2. EXPERIMENTAL The high field ESR measurements on solid molecular oxygen were performed using the far-infrared laser (FIR laser) and the 94GHz Gunn oscillator, and the pulsed high magnetic field up to 55T at 4.2K. The magnetically enhanced InSb detector made by
QMC instruments was used. Figure 1 shows the block diagram of our experimental setup. The light from the laser and the Gunn oscillator is guided by the light pipe to the sample, which is located at the center of the magnet, and the transmitted light through
80 1
.............................
1182.OGHz
H(T) 10
20
30 40 H(T)
50
C
Figure 2. ESR absorption lines of a oxygen observed at 4.2K.
Figure 3. Frequency-field diagram of oxygen observed at 4.2K. Solid circle and open square show this study and the results by the FIR spectroscopy measurement [6], respectively. Solid lines correspond to the theoretical lines obtained from the molecular field theory.
the sample is reflected by the mirror just below the sample. The reflected light passes through the pin hole on the 45" mirror above the cryostat, and is detected by the magnetically enhanced InSb detector. In the measurement on the molecular oxygen, Si02 glass plated with silver is used for the light pipe of the cryostat. The oxygen gas is condensed on the bottom of the light pipe at liquid nitrogen temperature, and then slowly cooled down to 4.2K. Obtained solid molecular oxygen is a polycrystal.
3. RESULTS AND DISCUSSION Figure 2 shows the frequency dependence of the ESR absorption lines of a-oxygen observed at 4.2K. The ESR absorption lines accompanied with several peak structures, indicated by the arrows, are observed. Even using the polycrystalline sample, one can distinguish the resonance absorption lines for the field applied along the magnetic principal axis as such structures, because the density of state of the magnon for the direction of the each principal axis is large [7]. Two absorption lines B and C are observed near the resonance field of g=2. With increasing the frequency, the resonance
802
fields of those increase, and new absorption line A appears above 847.OGHz. The resonance field of the absorption line A approaches to that of the B, as the frequency is increased. The broad structure, observed at 1182.OGHz and 1392.8GHz in the high field region, may come from the interference effect and is considered to be not intrinsic. The origin of a broad one between the absorption lines A and B is, however, not clear at the moment. Figure 3 shows the frequency-field diagram of the observed ESR absorption lines. Open squares and dotted line in Figure 3 show the frequencies of the antiferromagnetic magnon at zero magnetic field, observed from the FIR spectroscopy measurements [4-61, and the spin-flop field Hc [3], respectively. The obtained AFMR modes are consistent with the results of the far infrared spectroscopy measurements, and are typical of the two-sublattice antiferromagnet with the orthorombic anisotropy. As shown in Figure 3, the AFMR modes can be explained by the conventional molecular field theory for the two-sublattice model using the parameters which were determined in ref. [3] from the high field magnetization and the FIR spectroscopy measurements. More detailed discussion will be published elsewhere.
ACKNOWLEDGMENTS This work was supported by Reseach Fellowship of the Japan Society of the Promotion
of Science for Young Scientists.
REFERENCES 1. C. S. Barrett, L. Meyer and J. Wasserman, J. Chem. Phys., 47 (1967) 592. 2. R. J. Meiyer and R. B. Helmholdt, Phys Rev., B 29 (1984) 1387. 3 . C.Uyeda, K. Sugiyama and M. Date, J. Phys. SOC.Jpn., 54 (1985) 1107. 4. T. G. Blocker, M. A. Kinch and F. G. West, Phys. Rev. Lett., 22 (1969) 853. 5. E. J. Wachel and R. G. Wheeler, Phys. Rev. Lett., 24 (1970) 233. 6. R. J. Meier, J. H. P. Colpa and H. Sigg, J. Phys. C: Solid State Phys., 17 (1984) 4501. 7. H. Ohta, N. Yamauchi, T. Nanba, M. Motokawa, K. Kawamata and K. Okuda, J. Phys. SOC.Jpn 61, (1992) 149.
EPR in the 21“ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
803
High field ESR measurements of Cu,(C,H,,N,),Cl, under high pressure M. Saruhashi”, T. Sakurai”,’, K. Kiritaa,b,T. Kunimoto’, S. Okubo‘,‘, H. oh tab^', H. Kikuchid, Y. Uwatoko” ’The Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan bVenture Business Laboratory, Kobe University, Kobe 657-8501, Japan “Molecular Photoscience Research Center and Department of Physics, Kobe University, Kobe 657-8501, Japan dDepartment of Applied Physics, Fukui University, Fukui 91 0-8507, Japan eInstitute for Solid State Physics, University of Tokyo, 5-1 -5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan We have performed high field ESR measurements of Cq(C,H,2N,)2C14single crystal under ambient pressure and 2 k bar pressure for H//g,. The measurement is performed at 80 GHz, and temperature region is from 1.8 to 6.0 K. The integrated intensity of the absorption lines decreases rapidly as the temperature is decreased for ambient pressure, while the integrated intensity shows the upturn below 4 K for 2 k bar. On the other hand, the temperature dependence of the linewidth under ambient pressure increased as the temperature is decreased, while that under 2 k bar was almost independent of the temperature.
1. INTRODUCTION Low-dimensional antiferromagnets are interesting system where quantum fluctuations play a important role. An important class of such systems are the S=1/2 spin-gap systems and
Cu,(C,H,,N,),CI, (abbreviation as CHpC) is one of such system. Crystal structure of CHpC takes the monoclinic system and space group P2,/c [l]. The Cu dimer formula units in CHpC stack to form double chains along the [loll direction. Therefore, CHpC is proposed to be a S=1/2 spin ladder system [2]. Correspondingly, the magnetic interactions between Cu spins are thought to yield a spin ladder with a spin Hamiltonian written as
,=,
where
J,
,=I ,=I
and
J,,
are the exchange constants along the rung and the leg, respectively, and s, ,
804
is an S=112 spin on the i-th leg and the j-th rung. was evaluated to be about five by the analysis of the magnetic susceptibility and the magnetization data [3]. The temperature dependence of the magnetic susceptibility shows a maximum around 8 K and it tends to zero below 8K which shows that CHpC has the singlet ground state. The energy gap of one dimensional Heisenberg antiferromagnet (1D HAF) between the singlet ground state and the first excited state can be suppressed under a certain applied magnetic field Hcl. From the magnetization measurements at 0.42 K, the magnetization appears above the critical field Hcl=7.5 T and it saturates at H,,=13 T. Our previous high field ESR measurements at 1.8 K revealed that there exists a magnetic phase transition not only at Hcl but also at Hm=lO.l T.141. We also found the direct transition between the singlet ground state and the excited state below 1 K 151. The mechanism of these high field ESR results are still under discussion. Recently the magnetic susceptibility measurements of CHpC under pressures have been performed [6]. Under the pressure, CHpC has organic structure. The magnetic properties changes are expected in CHpC. An upturn was observed at several k bar as the temperature is decreased below 8 K. The upturn of the magnetic susceptibility disappears when the pressure is released to ambient pressure. Therefore, it cannot be considered as a simple paramagnetic impurity centers formed by the pressure, and the origin of these behaviors under pressure is not clear at the moment. Recently we have developed high field ESR system under high pressure using the pulsed magnet [7j. The main aim of this study is to clarify the origin of the above anomaly of the magnetic susceptibility under high pressure. We performed high field ESR measurements of CHpC single crystal under ambient pressure and 2 k bar pressure.
2. EXPERIMENTAL Single crystals of CHpC were synthesized by means of.the slow evaporation method from a methanol solution of CHpC. Detail of synthesis was described in Reference [ 11 and [4]. The millimeter wave ESR measurements of CHpC single crystal under ambient pressure and 2 k bar pressure are performed using high pressure ESR system of Kobe university. High pressure is generated by the piston cylinder type cell. Detailed describe of the high pressure ESR system can be found in Reference [7]. The applied magnetic field is perpendicular to the crystal plane which corresponds to Hllg, 141. The millimeter wave ESR measurements are performed using the pulsed magnetic field up to 16 T and Gunn oscillators of 80 GHz as light sources . The measured temperature range is from 1.8 K to 6.0 K [8,9,10]. 3. RESULTS AND DISCUSSION Figure 1 shows the temperature dependence of the ESR spectra under the ambient pressure for 80 GHz. The intensity of absorption lines decrease with decreasing the temperature. This correspond to the temperature dependence of the magnetic susceptibility which has a peak at 8 K [2j. Figure 2 shows the temperature dependence of the ESR spectra under 2 k bar pressure for 80 GHz. The intensity of absorption lines increase distinctly with decreasing the temperature. It is clearly seen as compared with the paramagnetic behavior of DPPH signal.
805
1.8K
v ,
2.5
.
.
.
.
B
#
.
.
.
.
I
.
.
.
.
3.5
2
2.5
3.5
3 B (T)
Figure 2. The temperature dependence of the Figure 1. The temperature dependence of the absorption lines under lbar for 80GHz. absorption lines under 2kbar for 80GHz.
.
3.0: 2.5: 2.0-
25 :
0
20 :
.
15 :
1.5: 10:
1.0:
.
0
.
0.5 : n ' .
I
2 3 4 5 6 7 8 T (K) Figure 3. The temperature dependence of normalized integrated intensity.
"0
1
2 3 4 5 6 7 8 T (K) Figure 4. The temperature dependence of linewidth.
"0
1
For 2.0 K ESR spectra of Figure 1 and Figure 2 deformed back ground are observed. It comes from the unstable bubbling of liquid He because the sample is immersed in liq. He in the pumping cryostat. The temperature dependence of the integrated intensity of the absorption lines under ambient pressure and 2 k bar are shown in Figure 3. The integrated intensity under ambient pressure decreased rapidly as the temperature is decreased below 5 K, while that under 2 k bar showed the upturn below 4 K. This behavior seems to be consistent with the results of Reference [6]. The temperature dependence of the g-value for ambient pressure takes almost constant value of about 2.047 above 6 K, and increases below 6 K as the temperature is decreased [I I]. On the other hand, the temperature dependence of the g-value for 2 k bar pressure seems to stay constant around 2.047. We would like to point out that we did not observe new impurity resonance, and the pressure affected only the temperature dependence not the g-value. Moreover, the temperature dependence of the linewidth under
806
ambient pressure increased as the temperature is decreased, while that under 2 k bar was almost independent of the temperature (Figure 4). The results of the temperature dependences of g-value and linewidth seem to suggest that the short range ordering in the low dimensional antiferromagnet is suppressed by the pressure in CHpC. The origin of this mechanism remains as a future problem. 4. CONCLUSIONS
The high field ESR measurements of CHpC have been performed in the temperature region from 1.8 K to 6.0 K under ambient pressure and 2 k bar pressure. The temperature dependence of the integrated intensity under pressure is consistent with the magnetic susceptibility and we observed no new impurity resonance. However the temperature dependence of the g-value and linewidth under pressure suggested the suppression of the short range ordering in CHpC. ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research (B) (No. 10440109), and Grant-in-Aid for Scientific Research on Priority Areas (A) (No.11136231, 12023232 Metalassembled Complexes, No. 12046250 Novel Quantum Phenomena in Transition Metal Oxides) and (B) (No. 13130204 “Field-Induced New Quantum Phenomena in Magnetic Systems”) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. REFERENCES 1. B. Chiari, 0. Piovesana, T. Tarentelli and P. F. Zanazzi, Inorg. Chem., 29 (1990) 1172. 2. G. Chaboussant, P. A. Crowell, L. P. Levy, 0. Piovesana, A. Madouri and D. Mailly,
Phys. Rev. B, 55 (1997) 3046. 3 . M. Hagiwara, Y. Narumi, K. Kindo, T. Nishida, M. Kaburagi and T. Tonegawa, Physica B, 246-247 (1998) 234. 4. H. Ohta, T. Tanaka, S. Okubo, S. Kimura, H. Kikuchi and H. Nagasawa, J. Phys. SOC.Jpn., 68 (1 999) 732. 5. H. Ohta, Y. Oshima, T. Sakurai, S. Okubo, T. Tanaka, K. Koyama, M. Motokawa, H. Kikuchi, H. Nagasawa and J. P. Boucher, J. Magn. Magn. Mater., 226-230 (2001)439-440. 6. M. Mito, Private communication and Meeting Abstracts of the Phys. SOC.of Japan (2001). 7. H. Ohta, S. Okubo, T. Sakurai, T. Goto, K. Kirita, K. Ueda, Y. Uwatoko, T. Saito, M. Azuma, M. Takano and J. Akimitsu, Physica B, 294-295 (2001) 624. 8. M. Motokawa, H. Ohta and N. Makita, Int. J. Infrared & MMW, 12(2) (1991) 149-155. 9. S. Kimura, H. Ohta, M. Motokawa, S. Mitsudo, W-J. Jang, M. Hasagawa and H. Takei, Int. J. Infrared & NMW, 17(5) (1996) 833-841. 10. N. Nakagawa, T. Yamada, K. Akioka, S. Okubo, S. Kimura, and H. Ohta, Int. J. Infrared &NMW, 19(2) (1998) 167-176. 11 H. Ohta, S. Okubo, S. Kimura, T. Sakurai, S. Takeda, T. Tanaka, H. Kikuchi and H. Nagasawa, Applied Magn. reson., 18 (2000) 469.
EPR in the 2 l XCentury A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
807
ESR at ultra-low temperatures and observation of new mode in Cu-Benzoate T. Sakona, H. Nojirib, K. Koyama", T. Asano', Y. Ajiro' and M. Motokawa" a
b
Institute for Materials Research, Tohoku University, Sendai, 980-8577 Japan Department of Physics, Okayama University, Okayama, 700-8530 Japan Department of Physics, Kyushu University, Fukuoka, 8 12-8581 Japan
The development of the ESR measurement system at ultra low temperature using a 3He4He dilution refrigerator and a vector network analyzer has been achieved. For the first experiment, the ESR was measured on a quantum spin chain Cu-Benzoate down to 160 mK. 1. INTRODUCTION
Low dimensional quantum spin systems have attracted much attention by interesting quantum phenomena. A magnetic resonance measurement is powerful tool to observe the dynamical physical properties in spin system such as quantum fluctuations. NMR or pSR measurements have been carried out at ultra low temperatures and many notable results were achieved [l]. On the contrary, as for ESR measurement, there are hardly any experiments using dilution refrigerator. This is because of the technical difficulties such as heat leakage through the wave-guide, or heating by microwave power. In this study, the development of the ESR measurement system at ultra low temperature by using a 3He-4He dilution refrigerator and a vector network analyzer, which has ultra-high sensitivity, has been achieved. As the first application of our equipment, we have studied low temperature behavior of Cu benzoate, that is one of the ideal one-dimensional = 112 quantum spin systems. Many ESR and other studies have been done since the beginning of 1960s by Date and his collaborators [2 - 41. As for low temperature ESR studies, Oshima et a1 performed pioneer studies using 3He cryostat more than two decades ago [2]. In addition to the paramagnetic resonance, they observed a new resonance lines at low temperature and they interpreted as an appearance of the long-range antiferromagnetic order (LRO). In the specific heat measurement, however, no obvious transition to antiferromagnetic state was observed. Recently a broad peak was found when magnetic fields were applied [ S ] , but this was not a clear indication of the long range magnetic order. On the other hand, Oshikawa and Affleck [6, 71, and Essler and Tsverik [S, 91, proposed a theory that a gap is induced by an applied magnetic field caused by breather excitation and a crossover occurs from the paramagnetic state to the breather gap state in magnetic field.
808
Sample
Cavity
Figure 1. The schematic picture of the
system using a 3He-4Hedilution refrigerator
This theory was confirmed by specific heat and neutron scattering investigations According to this theory, a precise study was performed and a clear anomaly that considered to be due to the field induced breather gap was found at 0.5 K [lo]. This experiment was done applying external magnetic field parallel to the c-axis, that is the direction of the largest field-induced gap. In order to confirm this phenomenon, similar experiment must be done in another direction. In the case of H//b-axis, however, the magnitude of the gap is very small even in a magnetic field conventionally available and 0.5 K is not a sufficiently low temperature to observe the breather mode. This is the motivation of this experiment. 2. APPARATUS
Figure 1 shows a schematic picture of the ESR system. ESR measurements have been performed using a vector network analyzer (Al3 millimetre Co., Ltd.) in the frequency region between 54 GHz and 111 GHZ using a 20 T superconducting magnet. Using the vector network analyzer, full tunability of frequency enabled us to purge a mechanical tuner to adjust the length of a cavity, which is very difficult to do at ultralow temperatures. High-sensitive measurements are possible using a cylindrical resonant cavity. The loaded @value is 10000 at lowest temperature. In order to achieve a good thermal contact between the sample and the cavity, the sample was fixed directly on the endplate of the cavity. [ 1I ]
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The cavity is installed in a 3He-4Hedilution refrigerator (Kelvinox System, Oxford Co., Ltd.). The cooling power is 25 pW at 100 mK. The cavity holder was fixed directly on the Cu plate, which is connected to the mixing chamber by a copper pipe covered with a stainlesssteel pipe. Between the cavity and the analyzer, rectangular wave-guides WR 22 made from Cu-Ni alloy of low thermal conductivity were used for the microwave transmission. In order to suppress the heat leakage from the room-temperature area, several heat anchors were attached at the still heat exchanger and the mixing chamber. Moreover, a sapphire plate and black paper were inserted as a cold filter, as shown in Figure 1. In order to cool the sample more efficiently, meshed Cu wires were connected between the cavity and the mixing chamber. The temperatures were measured by a ruthenium oxide thermometer (Scientific Instruments Co. Ltd.) attached to the cavity. The lowest temperature of the cavity was 155 mK. The power of the microwave in the cavity is estimated to be one pW, which is much smaller than the cooling power of the dilution refrigerator. Varying the input power by more than 40 dB, we checked the heating of the sample. The heating due to the microwave was only 2 or 3 mK at the lowest temperature. When the ESR measurement was carried out, the field sweep speed was very slow, about 0.05 TI min, in order to avoid the Eddy current heating of the cavity. All measurements were performed in the Faraday configuration, where a propagation vector of the radiation is parallel to the external magnetic fields. Single crystals grown by the diffusion method and rectangular-shaped samples with typical dimensions of 0.5 x 0.5 x 0.1 mm3 were used for the measurements. The sample quality was confirmed by X-ray diffraction and magnetic susceptibility measurements.
3. RESULTS AND DISCUSSTION
The temperature dependence of the ESR spectrum for b is shown in Figure 2. The microwave frequency was 57.423 GHz and the lowest temperature of the sample was 160 mK. Although the microwave power was very weak, clear ESR absorption line was detected. According to the theoretical prediction, it changed by varying temperature. At high temperature around 4.2 K, a sharp absorption line due to a conventional paramagnetic resonance was observed. By lowering temperature, the absorption line became broader and weaker. At low temperature around 0.6 K, a new absorption line appeared. However, the line is very broad. The absorption became stronger and sharper as the temperature decreased. The crossover temperature from paramagnetic region to low temperature region for was same as that obtained by the specific heat measurement as well as the case of [5]. Then the behavior for was identical with that of previously reported and the new mode was considered to be the gap mode (breather excitation). Further we measured the frequency dependence of the ESR spectrum at = 220 mK. The peak field increased with increasing frequency, and the frequency dependence of the resonance field was nonlinear. Theoretically, the field dependence of the resonance field is written as [ 121,
810
800 mK
mK
W
-
c
-500 mK 400 mK
250 mK
B B
mK 190 mK
160 mK
1.9
2.0
Magnetic Field (T)
Figure 2. The ESR spectra at the frequency of 57.423 GHz for
b
where, is a energy gap of the breather excitation, v is a ESR frequency, g is a g-value in a paramagnetic state = 2.06). Figure 3 shows the field dependence of the energy gap which was obtained from ESR resonance. The is well explained by the function of denoted by solid line, which is consistent with theoretical prediction, The gap obtained from the specific heat measurement (reference is as same value as that obtained from this ESR investigation.
81 1
30 25
20
15 10 5
0
Figure 3. The field dependence of the breather energy gap for at Solid circles obtained from this ESR investigation and solid line is a fitting line.
=
220 mK.
4. SUMMARY
In summary, an ESR measurement system at ultra-low temperatures has been developed by using a 3He-4He dilution refrigerator and a Vector Network Analyzer. We used a cylindrical resonant cavity with high-Q value for ESR measurement. In order to cool the wave-guide and avoid the heat leak efficiently, some thermal anchors and filters were used. Moreover, to decrease the power of the microwave, attenuators were installed. The lowest temperature was 160 mK. We have studied the ESR experiment on Cu benzoate. Due to cooling down the sample to ultra-low temperatures, a well-defined breather excitation has been observed for as well as for in previous study [lo]. The field dependence of the energy gap of the breather excitation agrees well with the results of the specific heat measurements. ACKNOWLEDGEMENT The authors would like to thank to Mr. M.Yoshida for helping our experiments. This investigation has been performed at the High Field Laboratory for Superconducting Materials, Tohoku University. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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REFERENCES 1. NMR: H. Okura, K. Ishida, Y. Kawasaki, Y. Kitaoka, Y Yamamoto, Y. Miyako, T. Fukuhara and K. Maezawa, Physica B, 281-282 (2000) 61. pSR: A. Koda, W. Higemoto, R. Kadono, K. Ishida, Y. Kitaoka, C. Geibel and F. Steglich, Physica B, 281-282 (2000) 16. 2. K. Oshima, K. Okuda and M. Date, J.Phys. Soc. Jpn., 44 (1978) 757. 3. M. Date, M. MotokawaandH. Yamazaki, J. Phys. Soc. Jpn , l 8 (1963) 911. 4. M. Date, H. Yamazaki, M. Motokawa and S. Tazawa, Suppl. Prog. Theor. Phys., 46 (1970) 194. 5. D. C. Dender, P. R. Hammar, D. H. Reich, C. Broholm and G. Aeppli, Phys. Rev. Lett., 79 (1997) 1750. 6. M. Oshikawa and I. Affleck, Phys. Rev. Lett., 79 (1997) 2883. 7. I. Affleck and M. Oshikawa, Phys. Rev. B, 60 (1999) 1038. 8. F. H. L. Essler and A.M. Tsverik, Phys. Rev. B, 57 (1998) 10592. 9. F. H. L. Essler, Phys. Rev. B, 59 (1999) 14376. 10. T. Asano, H. Nojiri, Y. Inagaki, J. P. Boucher, T.Sakon, Y. Ajiro and M. Motokawa, Phys. Rev. Lett., 84 (2000) 5880. 11.G.Griiner (eds.), Topics in Advanced Physics 74, Millimeter and Submillimeter Wave Spectroscopy of Solids, p 127 (Springer-Verlag, Berlin, Heidelberg, 1998). 12. M. Oshikawa, I. Affleck, Phys. Rev. Lett., 82 (1999) 5136.
EPR in the 21‘ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
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ESR spectrometer using frequency tunable gyrotrons as a radiation source Seitaro Mitsudo”, Kazuaki Kanazawa”, Masayuki Hata”, Isamu Ogawab, Tomohiro Kanemaki” and Toshitaka Idehara” ”Research Center for Development of Far Infrared Region, Fukui University, Bunkyo 3-9-1, Fukui 910-8507, Japan bCryogenic Laboratory, Faculty of Engineering, Fukui University, Bunkyo 3-9-1, Fukui 910-8507, Japan
The Gyrotron FU series in Fukui University is being developed to provide high frequency source for far-infrared spectroscopy and plasma diagnostics. The series has achieved the frequency step tunability in wide range from 38 GHz to 889 GHz by many single mode operations at fundamentals and the second and third harmonics of electron cyclotron frequency. Gyrotron FU IVA in the series has achieved maximum frequency 889 GHz and has most wide frequency tunable range. Also, Gyrotron FU IV has achieved complete cw operation. Submillimeter wave ESR spectrometer using Gyrotron FU series as an electromagnetic wave radiation source and pulse magnet for high field up fo 35 T has been developed. The ESR spectrometer has been successfully applied to several ESR measurements.
1. INTRODUCTION Electron spin resonance (ESR) spectroscopy in the millimeter to submillimeter wave region using high magnetic fields is one of the most powerful tools for study of the magnetic properties of materials. However, there are many technical problems that we have to overcome, namely: 1) as the frequency is increased, the power of radiation sources and sensitivity of available detectors are decreased rapidly. 2) High magnetic field with intensity up to several tens Tesla is necessary. Klystrons and solid state oscillators have been used up to now as millimeter wave sources, whereas frequency multiplied solid state source, backward traveling wave oscillator (BWO) and FIR laser were employed as submillimeter wave sources. Gyrotrons are new sources in the millimeter to submillimeter wave region. Internationally, gyrotron development is being directed mainly towards the efficient generation of high-power millimeter waves for the electron cyclotron heating of thermonuclear plasmas. The gyrotron programs at the Fukui University have different aims. The goal is to develop moderately high power sources tunable over broad frequency ranges in millimeter to submillimeter wave region [l]. Such sources have many advantages for applications to various fields including the far-infrared spectroscopy, the measurement of material properties, polarization-enhanced NMR spectroscopy [2] and plasma diagnostics [3,4].
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In order to construct a new ESR device, we have developed a pulse magnet with a maximum field intensity of 35 T. The development of high power, high frequency ESR spectrometer using a frequency tunable gyrotron for the radiation source and pulsed high magnetic fields, is described in this paper. This high frequency and high power technique enables us to measure an ESR spectrum during one pulse of magnetic field, instead of the integration technique using a lock-in amplifier.
2. SUBMILLIMETER WAVE GYROTRONS Gyrotrons included in Gyrotron FU series are frequency step-tunable sources covering a wide frequency range from millimeter to submillimeter wave region. The output powers are several hundreds watt to several tens kilowatt in fundamental operation and from several tens watt to several kilowatt in second harmonic operation. This is not so high, compared with the output power of other high power millimeter wave gyrotrons. However, these powers are much higher than other radiation sources employed for ESR experiments in this frequency region. Gyrotrons usually operate near the fundamental of electron cyclotron frequency, w - - ,eB0 (1) where and e are the electron rest mass and charge, and is the relativistic factor. Because the maximum frequency that can be obtained is limited by the maximum available magnetic field. Operation at second harmonic offers the possibilities of doubling the frequency while using the same superconducting magnet. For a 17 T superconducting magnet, the frequency is
Vacuum layer
Sample
Figure 1. The schematic diagram of ESR spectrometer using Gyrotron FU IVA as a radiation source and pulse magnet with maximum field intensity of 35 T.
815
limited below about 440 GHz at fundamental operations of the electron cyclotron resonance, but harmonic operations can extend the range dramatically. Gyrotron FU IVA in the Gyrotron FU series has achieved maximum frequency 889 GHz at the second harmonic operation and has most wide frequency tunable rage. It consists of a magnetron injection type electron gun, a drift tube, a resonant cavity, an output waveguide and an output window. It is installed in the 70 mm room temperature bore of the 17 T superconducting magnet. We are improving the operation performances of Gyrotron FU series. Gyrotron FU IV has achieved many advantages as a high quality radiation source, for example, complete cw mode operations, modulations in frequency and amplitude, high stabilities of frequency and amplitude, and so on. When the gyrotron is applied to any measurements, these advantages are very convenient. Specially, the complete cw operation with high stabilities of frequency and amplitude is useful for the application to ESR measurement. The operation condition is limited by the heat capacity of the tube. As the consequence, the output power is several tens of watts.
3. ESR SPECTROMETER Figure 1 shows the schematic diagram of our ESR spectrometer using Gyrotron FU IVA as the electromagnetic wave radiation source. The electromagnetic wave from the Gyrotron FU IV A is transmitted by an oversize circular waveguide system with three quasi-optical bends, and fed on a sample located at the center of the pulse magnet. The power transmitted through the sample is measured by an InSb hot electron detector. The pulsed magnetic field is produced by discharging a capacitor bank of 30 kJ into the magnet coil. When the capacitor
Gyrotron 301 GHz cw
B 11 c-axis
theory B 11 c-axis theory8 l e - a x i s
Gunn 114.7 GHz
0
5 10 15 Field intensity B (T)
Figure 2. AFMR spectrum of MnF2 single crystal.
Figure 3. Frequency versus field diagram of MnF2.
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(a) 610 GHz, TE,,, Mode, 2nd harmonics
(b) 323 GHz, TEo3Mode, fundamental
? 20
10
B (Tesla)
Figure 4. Typical ESR spectrum of standard sample of DPPH. Gyrotron FU IVA was operated in (a) TE93 mode (2nd harmonics) and (b) TEo3 mode (fundamental)
Table 1. Operating parameters of Gyrotron FU IVA. Operating mode Harmonics Frequency (GHz) Beam current (mA) Cathode voltage (kV) Anode voltage (kV) Magnetic field (T)
TEo3
TE93
1st
2nd
323
610
500
400
-9.9
-9.8
11.705
11.177
bank is charged up to 5 kV (30 kJ), the field intensity is increased up to the maximum of 35 T and decreased. The field intensity pattern is similar to a half sinusoidal one with the width of 2.5 ms. The signals from the pickup coil for measurement of magnetic field intensity and from the InSb detector are recorded in digital oscilloscope as functions of time. Thereafter, the computer arranges automatically both signals for constructing an ESR spectrum as a function of magnetic field intensity. The temperature of the sample can be varied from about 100 K down to the liquid helium temperature by the heater surrounding the sample The temperature is measured by the FeAu 0.007 at.% - Ag thermocouple. The Gyrotron FU IVA is operated in pulse mode with pulse width of 4 ms. A single pulse triggers the operation of a high voltage supply for gyrotron, while the condenser bank for magnet is triggered by a delayed pulse. Then, the pulsed magnetic field can be synchronized with the gyrotron operation. The ESR spectrometer using the Gyrotron FU IV and Gyrotron FU IVA as radiation source has been applied to ESR measurements[5-71. Typical results of the experiments are introduced. In the first case, Gyrotron FU IV was used as radiation source. The gyrotron is operated in cw mode. The operation conditions are as follows. The cathode potential and the beam current are varied from -14 to -16 kV and from 50 to 100 mA, respectively. Figure 2 shows typical ESR spectrum of well-studied antiferromagnetic compounds MnF2 single crystal. Pulsed magnetic fields were applied along c-axis, which is antiferromagnetic easy axis. The measurements temperature was 4.2 K. At the frequencies 114.7 GHz and 120 GHz, the Gunn oscillators were used as radiation sources with output powers 25 mW. When Gyrotron was used as radiation sauce, the output power was attenuated to avoid increasing temperature of sample. As seen from Figure 2, Gyrotron output has enough stability for ESR measurements. Figure 3 shows the frequency versus magnetic field intensity diagram. AFMR resonance line
817
at each frequency is in good agreement with theoretical value. In the second case Figure 4 shows typical ESR spectrum of standard sample DPPH (its g-value is 2.0036) measured by this system. The ESR absorption line of DPPH is indicated by an arrow. the frequency of 610 GHz in Figure 4(a), a Gyrotron FU was operated in TEg3mode at second harmonics. The sharp and strong absorption is caused by DPPH. Other week and broad dips of transmission power come from fluctuation of the output power from gyrotron, because the output power of second harmonics operation is sensitive to operation parameter, mainly high-voltage power supply. However, we can distinguish between signal and noise by comparing a pattern in the phase of increased magnetic field with one of decreased field. The operating parameters of Gyrotron FU are shown in table 1.
REFERENCES 1. T. Idehara, I. Ogawa, S. Mitsudo, M. Pereyaslavets, N. Nishida and Y. Yoshida, IEEE Trans. Plasma Sci., 27 (1999) 340. 2. D. Hall et al., Science, 276(1997) 930. 3. I. Ogawa, M. Iwata, T. Idehara, K. Kawahata, H. Iguch and A. Ejiri, Fusion Eng. and Des., 34-35 (1997) 455. 4. I. Ogawa, K. Yoshisue, H. Ibe, T. Idehara and K. Kawahata, Rev. Sci. Instrum., 65 (1994) 3145. 5. S. Mitsudo, Aripin, T. Matsuda and T. Idehara, Int. J. Infrared and Millimeter Waves, 21 (2000) 661. 6. Aripin, S. Mitsudo, T. Shirai, K. Matsuda, T. Kanemaki, T. Idehara, T. Tatsukawa, Int. J. Infrared and Millimeter Waves, 20 (1999) 1875. 7. M. Chiba, Aripin, K. Kitai, S. Mitsudo, T. Idehara, S. Ueda and M. Toda, Physica B, 294-295 (2001) 64.
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EPR in the 21%Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
High-frequency (W-band) EPR studies of biological samples KBichi Fukui? Tomohiro Itob and Hiroaki Ohya"" "Regional Joint Research Project of Yamagata Prefecture, Yamagata Public Corporation for the Development of Industry, Matsuei 2-2-1, Yamagata 990-2473, Japan bGraduate School of Science and Engineering, Yamagata University, Yonezawa, Japan. 'Institute for Life Support Technology, Yamagata Public Corporation for the Development of Industry, Matsuei 2-2-1, Yamagata, Japan This paper reports a new W-band (-94 GHz) EPR spectrometer which is constructed for measurements of raw or fresh biological samples. Measured biological samples include plant seeds, leaves, and bacterial pellets, and their size is typically lmm x lmm x Imm. The setup and performance of the spectrometer are described, and some remaining problems are discussed. Results for a raw seed, a freshly cut leaf, and a TEMPO-absorbed leaf are presented. The former two exhibited well-resolved six-line signals due to trace Mn", which were not observed or only weakly observed in X-band spectra The latter one exhibited a spectrum where two signals from TEMPO in aqueous media and lipid media are clearly separated, which demonstrates good g resolution of W-band EPR. 1. INTRODUCTION
Electron paramagnetic resonance (EPR) is a powerful technique for investigation of radical and metal-iodcomplex species in biological systems. In fact, EPR with low microwave frequency such L band or even lower frequency band has been shown to be usehl for in-vivo measurements. In contrast, applying high-frequency (HF) EPR to biological samples, in particular raw or fresh biological samples, is still a difficult task because of severe dielectric loss of such samples. HF EPR, however, has several unique advantages such as (i) high sensitivity because of larger Zeeman splitting, and (2) better g resolution [ 11. Furthermore, particular usefulness in investigating mononuclear high-spin Mn(I1) iodcomplexes has been pointed out [2]. Hence, it is valuable to develop a HF EPR system that is easily applicable to raw (hopefully live) biological samples. We have been developing a W-band (-94 GHz) EPR spectrometer for measurements of raw or fresh biological samples. Specifically, biological samples which we are interested in are, for example, raw plant seeds, freshly cut leaves, and bacterial pellets. Although there have been published a considerable number of reports on HF EPR so far [1-4], it seems that no HF-EPR experiments have been performed for such samples. In this paper, we report details of our newly built W-band EPR spectrometer, and some of the results obtained using the spectrometer.
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2. EXPEIUMENTAL 2.1 W-band EPR spectroscopy
A block diagram of our W-band spectrometer is shown in Figure 1. The milliwave supplied with a cavity-stabilized Gunn oscillator (Keycom Co., CSO-01) is divided into two, where one is given to a mixer (MRI Co., model BMRlW) as Local signal and the other is lead to a Fabry-Ptrot cavity (fabricated in Keycom Co.). The Fabry-Perot cavity is made up of one concave mirror (upper mirror) and one flat mirror (lower mirror). The support of the lower mirror is connected to three stepping motors (Suruga Seiki, Co.), which allows threedimensional position adjustment. Around the cavity a coil is wound to provide field modulation (100 kHz). Dielectric waveguides (Keycom Co., DW110A Series) are used to connect the cavity and the milliwave main circuit, which allows flexible arrangement of the circuit. The output signal from the cavity and the local signal are mixed at the mixer to provide a DC output (modulated by 100 kHz owing to the field modulation). The DC output is first amplified by a low-noise preamplifier (NF electronic Instruments, SA430F5), and subsequently by a lock-in amplifier (NF electronic Instruments, 561OB). A sample is placed at the center of the lower (flat) mirror of the Fabry-Perot cavity. The position of the lower mirror is adjusted to achieve the optimal resonance condition. The actual frequency of the milliwave and the power of the output from the cavity are monitored using a spectrum analyzer (Hewlett Packard 8562E equipped with a 11970 harmonic mixer). Magnetic field is applied by a wide-bore superconducting magnet (Suzuki Shokan Co., A960077; bore diameter, 100 mm). The superconducting magnet was directly swept for EPR measurements, and the superconducting current is monitored with a digital multimeter (Iwatsu Co., VOAC7513). For calibration of the magnetic flux density, Mn2+ in MgO is used standard. All measurements were performed at ambient temperature.
._ Mixer
Oscillator
-5 I
Connector
--- ^-plifier
Superconducting Magnet 1-5T Fabry-PBrot Cavity
100 kHz Modulation
\
I Ill
I
Moduhtiom Coil Amp.
Sample Stage
Figure 1. Schematic diagram of our W-band EPR spectrometer
Computer
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2.1 Samples Brown rice, white radish sprouts, and gypsophila seeds were purchased from local stores. Cotoneaster leaves were taken from cotoneasters planted in a laboratory yard. Gypsophila seeds were of the size of 1 mm x 1 mm x 1 mm, so that one whole seed was used as a sample. After EPR measurements, gypsophila seeds were watered for six days. Cotyledon leaves thus germinated were cut off, and measured.
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3. RESULTS AND DISCUSSION 3.1 Instrument We have employed a Fabry-Perot cavity resonator in this construction of our W-band spectrometer, although cylindrical cavity resonators seem to be usually used in recent W-band spectrometers. A defect of the cylindrical cavity for W band is that it has very limited sample space and thus is not suited for measurements of biological samples such as leaves and seeds. The Fabry-Perot cavity, on the other hand, provides larger sample space and is expected to accept much wider range of samples. In our spectrometer, one only needs to put a sample on the flat mirror and place it on a support for measurements. The sample sizes eligible for our spectrometer are typically lmm x lmm for leave samples, lmm x lmm x lmm for seed samples, and 1 yl for aqueous solution samples. Of course, much larger samples can be measured when the samples are dried or dissolved in organic non-polar solvent. Our Fabry-PCrot cavity has two holes on the upper mirror, where one is used for milliwave input and the other is for output. Since the input and output lines are separated, we need not use a circulator in the milliwave circuit. This also helps the milliwave circuit to keep simple and inexpensive. In typical settings, the input power is 7 dBm and the output power under unloaded condition is - -8 dBm. Hence the power loss due to the (unloaded) cavity is 15 dB. As long as the sample size did not exceed the above-descried ones, the loss did not increase so drastically, still staying I 2 0 dB. However, when the size of a watercontaining sample became larger than these, the loss increased drastically and EPR measurements became difficult. Another factor which influences the output milliwave is the shape of the sample. When the sample is flat and thin, the phase of the EPR signal is expected. For example, solution samples, which were spread on the flat mirror and covered with a thin glass for measurements, generally provided correct-phased signals. However, when the sample was not flat and had an irregular shape, as may be usual for biological samples, the signal was a mixture of the dispersion and absorption signals. The phase of the signal could not be corrected with a phase shifter. This admixture most likely occurs because the irregular-shaped sample severely interfered the milliwave electromagnetic field. At present, we digitally correct the phase of the signal when it is severely distorted. The sensitivity of our spectrometer (minimal detectable spin number) was estimated Nmin 2x10" spins/gauss 1 Hz from room-temperature spectra of TEMPOL in H20. This value may be compared with that for X-band spectrometer (-10" spindgauss). Values of N,in = lo7-lo9 spins/gauss were reported in literature for sensitivities of W-band EPR spectrometers [1,2]. However, these values were obtained from data of nonaqueous samples. Very few data seem to be available in literature for W-band EPR spectra of aqueous samples. One example was nevertheless reported by Wang et al. [2], who estimated Nmin = 2.4~10" spindgauss for their W-band spectrometer from spectra of TEMPOL in HzO. Compared with their value, the sensitivity of our spectrometer is one-order worse. However, this
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difference most likely comes from simply the difference in the cavity employed. Their spectrometer employs a cylindrical cavity, which is advantageous with respect to sensitivity. We have adopted a cavity-stabilized oscillator as a milliwave source of the W-band EPR spectrometer. This oscillator provides milliwave of (i) negligible frequency drift and (ii) very sharp frequency profile. The half-height-full-width peak width of the milliwave was -0.2 MHz as measured with a spectrum analyzer. This peak width corresponds to a linewidth of 0.007 mT, which is negligibly small in usual experiments. (In the spectrum of TEMPOL in H20, the sharpest line (lowest-field line) had a peak-to-peak width of = 0.17 mT, which is practically the same as the linewidth in X-band spectra. This shows that inhomogeneity in magnetic field is also negligible in usual experiments) One drawback of our system is that we can not vary the frequency of the oscillator. Accordingly, to achieve the resonance condition, we adjust the spacing between the two mirrors of the Fabry-PCrot cavity, where the lower (flat) mirror is constructed to be movable using stepping motors. A problem around this tuning method is that it is difficult to include automatic-tuning control (ATC) system in it (in fact, at present, our spectrometer is equipped with ATC system). One solvent may be to use piezoelectric actuators to control the mirror position, or to include an additional oscillator for frequency adjustment and modulation. Improvement of our spectrometer in this point is under way. Nevertheless, it was reported that automaticfrequency control (AFC) may disturb EPR signals when narrow signals (such as nitroxyl signals in fast-motional regime) are concerned, and that such signals may have to be recorded without AFC after careful tuning [5]. Hence, the absence of AFC (or ATC) in our spectrometer would not affect the quality of our data itself, which were of course measured after careful tuning.
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3.2 Mn(I1) in plants One of the useful applications of W-band EPR is a study of mononuclear high-spin Mn" ionskomplexes. In W-band EPR, intensities of the forbidden transitions and the effects of zero-field splitting are greatly reduced, which makes the Mn" six-line signal much more clearer than in X-band EPR [2]. Mononuclear high-spin Mn" species are in fact abundantly found in plants, and the coordination environments and the roles of the Mn" species have been a subject for investigation. It is therefore expected that W-band EPR provides invaluable information about these questions. Figure 2 shows W-band EPR spectra of a raw seed and a fresh cotyledon leaf of gypsophila. X-band EPR spectra are also shown for ~
~
W
'0
Seed
3335
3350
ImT
3365
3380
3-..~ 3 '
315
330
345
360
375
ImT
Figure 2. W-band and X-band EPR spectra of one whole gypsophila seed and freshly cut cotyledon leaf measured at ambient temperature. W-band: v = 93.994 (seed), 93.996 (cotyledon) GHz. X-band: v = 9.442 GHz.
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Table 1 g and HFC parameters of Mn(I1) species Samples g Gypsophila Cotyledon Leaf 2.0007 MnS04 in H20 2.0008 Cotoneaster Leaf, Fresh 2.0008 Brown Rice Embryo 2.0009 Gypsophila Seed 2.001 1 ConcanavalinA 2.0009 Mn2+in MgO 2.0010
A(55Mn)/mT 9.57 9.56 9.56 9.39 9.32 9.25 8.71
Ref. This work This work This work This work This work 6 1
comparison. From the W-band spectra, the g values and the "Mn hyperfine coupling parameters A("Mn) were determined (Table 1). EPR parameters determined from W-band data for some other Mn" species are also listed in the table. It should be noted again that the determination of the EPR parameters from X-band data for these samples is very difficult or impossible because only a broad or no signal was observed from X-band EPR. The EPR parameters of the leaves are very close to those of MnSOs in H2O. Hence the Mn signal is most likely due to a free Mn" ion (Mn(H20):+). For the seed and rice embryo, on the other hand, the HFC parameters are remarkably different from that of MnS04 in H2O. This may indicate that the Md' ions in these samples are bound to some peptides or proteins. 3.3. Spin probes in biological samples Another advantage of W-band EPR is that it has better g resolution than X-band EPR. Figure 3 well demonstrates this advantage. The figure shows a W-band EPR spectrum of a TEMPO-absorbed white radish cotyledon leaf. The spectrum clearly displays two sets of three-line signals; one is a relatively sharp signal with g = 2.0058 and A('%) = 1.73 mT, and the other is a relatively broad signal with g = 2.0063 and A(I4N) = 1.55 mT. As is wellknown, the g and A('%) values of nitroxyl radicals depend on the hydrophobicity / hydrophilicity of medium, where the ,q value increases and the A(I4N) value decreases with the increase- of hydrophobicity of-the medium [7]. Hence, the sharp signal is attributed to TEMPO in aqueous phase and the broad peak is attributed to TEMPO in lipid phase. To our knowledge, this is the first clear observation that spin probes such as TEMPO are actually distributed in leaves to more than one type of matrix. We also performed X-band EPR measurements for the same sample. However, the two 6lmT signals were almost overlap in the X-band Figure 3. W-band EPR spectrum for a white spectrum with the highest-field line only radish cotyledon leaf. The leaf was floated split slightly (not shown). Also notable on a 5 mM TEMPO solution for a few hours = is the difference in linewidth between the before measurement. Conditions: two signals. Analyses of the linewidth 93.996 GHz, room temperature.
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may provide further information concerning the difference between the environments around the two types of TEMPO. 4. SUMMARY
We have described our new W-band EPR spectrometer that is constructed for biological applications. The biological samples we have measured are raw plant seeds, freshly cut leaves, bacterial pellets, etc. One of the powerful biological applications would be investigation of Mn(I1) species in plants. It is known that there are many Mn-related biological activities such Mn excess and Mn deficiency symptoms of plants [8], lignin synthesis and decomposition [9], sugar-binding activities of lectins [6]. Furthermore, Wband EPR may be used with spin probes to investigate the lipid content and viscosity (or also cytosol viscosity) of living cells and tissues.
REFERENCES 1. 2. 3. 4. 5.
6. 7. 8. 9.
0. Burghaus, M. Rohrer, T. Gotzinger, M. Plato, and K. Mobius, Meas. Sci. Technol., 3 (1992) 765-774. W. Wang, R. L. Belford, R. B. Clarkson, P. H. Davis, J. Forrer, M. J. Nilges, M. D. Timken, T. Walczak, M. C. Thurnaucer, J. R. Norris, A. L. Morris, and Y. Zhang, Appl. Magn. Reson., 6 (1994) 195-2 15. R. T. Weber, J. A. M. Disselhorst, L. J. Prevo, J. Schmidt, and W. Th. Wenckebach, J. Magn. Reson., 81 (1989) 129-144. M. J. Nilges, A. I. Smirnov, R. B. Clarkson, and R. L. Belford, Appl. Magn. Reson., 16 (1999) 167-183. T. I. Smirnova, A. I. Smirnov, R. B. Clarkson, and R. L. Belford, J. Phys. Chem., 99 (1995) 9008-9016. E. Meirovitch, Z. Luv, and A. J. Kalb, J. Am. Chem. SOC., 96 (1974) 7538-7546. C. F. Polnaszek, S. Schreier, K. W. Butler, and I. C. P. Smith, J. Am. Chem. SOC.,100 (1978) 8223-8232. Z. Rengel, A. Sigel and H. Sigel (eds.), Metal Ions in Biological Systems. Vol. 37, pp. 57-88, Marcel Dekker, New York, 2000. (a) G. Engelsma, Plant Physiol., 50 (1972) 5 9 9 4 0 2 . (b) B. Halliwell, Planta, 140 (1978) 81-88.
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A ABREGO, F.C., 614 AJIRO, Y., 784, 807 ALLAYAROV, S.R., 373,378 AMMERLAAN, C.A.J., 33 ANANDALAKSHMI, H., 225 ANZAI, K., 562 213,219 ARAKAWA, M., ARAO,S., 466 ASANO, T., 784,807 ASANO-SOMEDA, M., 355 AZUMA, M., 747 B BABA,M., 282 BABA,M., 361 614,624 BAFFA, O., BAKHANOVA, E.V., 628 BELTRAN-LOPEZ, V., 63 BERLINER, L.J., 503 BHAT, S.V., 145 BOROVYKH, I.V., 659 BRECHT, M., 437 BROOKS, J., 793 BUHRKE, T., 437 BUNTING, R.V., 741 C CAI, J., 93 CHANDRASEKHR, A.V., 575 CHEN, S., 389 CHEN,Z., 421 CHERNYSHEVA, T.E., 378 CHIBA, M., 93,779 CHOCK, P.B., 446 CHOH, S.H., 288 CHOI,D., 288 CHUMAK, V.V., 628 CORNIA, A,, 784
D DEMISHEV, S.V., 741 DHANASEKARAN, R., 139 DIKANOV, S.A., 488 DINH, N.L., 412 DINH, P.T., 412 DOMRACHEVA, N.E., 710
DREW, S.C., DZUBA, S.A., E EBISU, H., EHARA,M., ENDO, T., ENDOH, N., EST, A.V.D.,
39 669 213,219 567 133, 139, 145, 157 483 355
F FOSTER, M.A., 515 437 FOERSTER, S., FRANCAF, H.K., 585 FREED, J.H., 719 FRIEDRICH, B., 437 FUJII, K., 688 552 FUJISAWA, M., FUJITA, T., 247 302 FUJIWARA, Y., FUKUDA, A,, 97 FUKUDA, Y., 700,706 FUKUI, K., 384,817 706 FURUE, S., FURUKAWA, S., 79 FURUNO, N., 264 FURUTA, H., 306 FURUTA, M., 306 FUSE,T., 349 G GAST,P., 659 GARD,G., 793 GATTESCHI, D., 784 GEIFMAN, I.N., 694 GLASER, R., 471 GOLOVINA, I.S., 694 GORDON, D.A., 373 GRAEFF, C.F.O., 624 GRIGOREV, I., 5 15 GRUN,R., 613 GUDENKO, S.V., 127 H HA,V.M., 412 HAGIWARA, M., HAN, J.Y., 533
73
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HARA,H., 679 HARADA, Y., 93 HARIMA, H., 306 HASHIZUME, A., 133 HATA, M., 788,813 HAYASHI, Y., 556 HAZUKI, K., 747 HIDA, A.I., 552 HIGASHI, N., 151 HIGUCHI, Y., 437 HIRAO,T., 306 HIRATA, T., 93 333 HIROTA, N., HIROI, Z., 767 264 HIROTSU, K., HOA, T.T.K., 259 HOFF, A.J., 659 HONDA, M., 201 HONDA, S., 168 HORI,H., 259 69 HOSOKOSHI, Y., I IDEHARA, T., 168,779,788,813 784 INAGAKI, Y., INOUE,K., 69 IONNIDIS, N., 466 ISHIHARA, K., 89 ITO, T., 818 ITOH, K., 133, 145 133, 139, 145, 157 IWASAKI, S., IWASAKI, T., 488 J JIN,G., 389 JIN,S., 389 JINMON, A., 355
K KAGOMIYA, I., 755 KAINARA, V.V., 293 KAIzU,Y., 355 KAJI, H., 186 KALAUOVA, A.S., 595 KALE, R.D., 145 KALINOVSKAYA, I.V., 276 KAMADA, H., 556
KAMEKAWA, M., 552 KAMIKAWA, T., 759,763 KANAZAWA, K., 168,813 KANEMAKI, T., 813 446 KANG, S.O., KANG, Y.S., 477 KANG,K., 288 KARASEV, V.E., 276 KASAHARA, M., 322 KASSAB, L.R.P., 585 KATO,N., 403 KATO, R., 197,312 KATO,Y., 93 KATOH,K., 69 KATSUTA, N., 679 KATZ, E.A., 174 KAWABATA, T., 552 KAWAI,A., 349 KAWAI, J., 384 KAWAKAMI, K., 767 KAWAMATA, N., 201 KAWAMORI, A,, 466,679 KAWAMOTO, T., 302 KAWASHIMA, T.,., 306 KEMPINSKI, W., 174 KEVAN,L., 105 KHAN,%, 125 KHRAMTSOV, V.V., 515 KIKUCHI, H., 755,759,763,803 KILIBAYEV, M.B., 640 KIM, S.S., 288 KIMURA, S., 799 KWDO, K., 784,799 KINOSHITA, A., 614,624 KIRITA, K., 69, 803 KITAI,K., 779 KLENINA, I.B., 659 KOHMOTO, H., 133, 139,145,157 KOHMOTO, T., 700,706 KOHN,K., 755 KOIDE,T., 302 KOJIMA, K., 247 KOJIMA, Y., 264 378 KONOVALIKHIN, S.V., KOTAKE, Y., 322 KOUNOSU, A., 488 KOYAMA, K., 807
827 KOYAMA, R., 298 KUANG, T., 429 KUDAIBAYEV, K., 595,640 KULIK, L.V.,669 KUNIMOTO, T., 298,759,763,771 803 KUNITOMO, M., 700,706 KURODA, S., 79, 113,367 KURYAVYI, V.G., 276 KUSPANOVA, B.K., 595,640 KUWABARA, M., 59 L LATTERMANN, G., 710 LEE, C., 446 LEE, D.K., 477 LEE, M.C., 562 LI,T., 133 LI,Y., 326 471 LIKHTENSHTEIN, G.T., LIU,K., 429 LILT,Y., 429 LILT, Z.L., 421 LOS, S., 174 LOZINSKY, E., 471 437 LUBITZ, W., LURIE, D.J., 515
M MAKINO, K., 483 MANABE, T., 79 MARUMOTO, K., 79,367 MATSUBARA, A., 97 MATSUDA, K., 168 MATSUOKA, H., 264 MEKATA, M., 788 MEYERSTEIN, D., 471 MIKUNI, T., 567 MILOV, A.D., 647 MINO,H., 684 MINO, M., 85,89 MIROCHNITCHENKO, O., 542 MISAKI, Y., 192 731 MISRA, SK., 168,779,788,813 MITSUDO, S., MITSUYAMA, Y., 556 MIYAMOTO, R., 3 16
27,48 MIYASHITA, S., MIYAZAKI, H., 562 MIZUSAKI, T., 97 MORI,M., 213 MORII, T., 567 MORITA, Y., 384 MOTOKAWA, M., 775,784,807 MURAKAMI, K., 542 MURALI, A., 242 N NAGASAKA, Y., 771 NAIDENOVA, I., 456 NAJIMA, H., 567 494 NAKAGAWA, K., NAKAJIMA, A,, 483,556 NAKAMURA, M., 151 NAKAMURA, T., 63,270 NAKAMURA, T., 192, 197 NAKASUJI, K., 384 NAKAUE, Y., 145 NAKAYAMA, H., 298 NAKAYAMA, K., 706 NAKAZAWA, S., 384 562 NAKAZONO, K., 207,253 NARASIMHULU, K.V., NARUMI, Y., 784 NASIROV, R.N., 595,640 NEERAJA, P., 225 NGUYEN, V.T., 412 NISHIDA, S., 384 NISHIDATE, I., 282 NOGAMI, Y., 192 NOJIRI, H., 751,775,784,807 NOVOSELSKY, A., 471
0 OGASAWARA, A,, 27,48 OGAWA,A., 93 OGAWA, I., 813 OGURA,F., 247 OHMI,T., 97 OHNISHI, T., 54 OHTA, H., 59,69, 197,298,302 312, 741, 747, 755,759 763,767,771,803 OHTA,K., 282
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OHTA,M., 270 OHYA,H., 818 OHYA-NISHIGUCHI, H., 556 OKADA,S., 552 OKAZAKI,N., 54 OKUBO, S., 197,298,302,741 747,755,759,763 767,771,803 ONO,T., 684 ONO,T., 775 OSHIKAWA, M., 15 OSHIMA, Y., 312,741 OVCHNNIKOV, I.V., 7 10 OZAWA, T., 562 P PARK, I.W., 288 PARK, S.M., 477 PETKOVA, G., 456 PETROULEAS, V., 466 PHAM, T.V., 412 PHU, N.H., 259 PIEKARA-SADY, L., 174 PILBROW, J.R., 39 PING,Z., 389 634 PIVOVAROV, S., PRAKASH, P.G., 242 PRASUNA, C.P.L., 253 PROSKURYAKOV, I.I., 659 PUNNOOSE, A,, 162 R RAGOGNA, P., 355 RAJENDIRAN, T.M., 225 RAJU, B.D.P., 207 RANGUELOVA, K., 395 RAO, J.L., 207,242,253 RAO, P.S., 225 RAO, T.V.R.K., 253 RAVIKUMAR,R.V.S.S.N., 575 REDDY, B.J., 157,575,589 REDDY, G.S., 589 589 REDDY, R.R.S., REDDY, S.L., 589 REDDY, Y.P., 575 ROMANOWSKI, W.R., 282 ROMANYUKHA, A.A., 603
ROWLANDS, C.C., 63 RUDOWICZ, C., 3 RUKHIN,A., 634 RUTEL, I.B., 793
S SAIFOUTDINOV, R., 525 SAKAI,T., 54 SAKON, T., 807 SAKURAI, T., 69, 197,747,803 SAKURAI, Y., 157 SALIKHOV, K.M., 678 201 SAMEJIMA, K., SARATANI, H., 139 SARJONO 282 SARUHASHI, M., 803 SATO,H., 316 SATO, K., 264,384 SCHAURE, D.A., 603 SCHLUETER, J., 793 SEEHRA, M.S., 162 SEKI,K., 344 SEO, K.W., 477 SEREDAVINA, T., 634 SHAHABUDDW, M., 133 SHAMES, A.I., 127, 174,471 SHARMA, P.K., 125 SHEN, J.R., 466 SHEN,Y., 389 SHIBUYA, K., 349 SHIMIZU, H., 151 SHIMOYAMA, Y., 180, 186 SHIOMI, D., 264,384 SHIRATORI, K., 755 SHOJI, H., 562 SHOLOM, S.V., 628 174 SHTUTWA, S., 614 SILVA, N.A., SINGH, R.J., 125 SINLAPADECH, S., 105 SITZMANN, M.E., 793 SKRYLNIK, P.G., 236 SLUCHANKO, N.E., 741 SOKOLSKA, I., 282 STEHLIK, D., 678 STEIN,M., 437 316 SUDOH, S.,
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SUEISHI, Y., 322 SUN, J., 429 SZAMAROV, S.S., 595
V VANDERVEER, D., VENKATESAN, R.,
T TADA, M., 145, 157 TAGUMA, K., 361 TAI, N.T., 259 TAJIMA, K., 483 TAKAHASHI, N., 63 TAKAMI, M., 270 TAKANO, M., 747,767 TAKEDA, Y., 302 TAKEMURA, S., 54 TAKENAKA, H., 97 TAKESHITA, K., 533 TAKEUCHI, H., 213,219 TAKEUCHI, N., 367 TAKEYAMA, T., 63 TAIU,M., 259 TAKUI, T., 264,384 79,213 TANAKA, H., TANAKA, H., 775 TANAKA, K., 192 TANIGUCHI, M., 192 TATSUTA, M., 567 TATUMI, S.H., 585 TOBISAKO, H., 322 TODA,M., 779 TOKI,M., 755 TSUCHIHASHI, Y., 306 TSUKAMOTO, M., 85,89 TSUKUDA, H., 85 TSVETKOV, Y.D., 647 TURANOV, A., 710
W WADA,N., 247 WARD, B.H., 793 WAKI,Y., 97 WAKUTA, M., 139 WANG, J.S., 326 WANG,Q., 326 WELLS, J.-P.R., 201 WINTER, R.W., 793 WU, G.S., 326 WU,L.M., 421
U UEDA,A., 767 UEDA,S., 779 UEDA, Y., 483,556 UEDA,Y., 770 231 UEKI, S., 139 UTHAYAKUMAR, S., UTSUMI, H., 533,542,548 UWATOKO, Y., 803
793 225
Y YAGI,M., 344 YAKUBOVSKY, A.Y,, 127 YAMADA, J., 157 YAMADA, S., 466 YAMAGA, M., 201 YAMAGUCHI, K., 133 YAMAMOTO, H.M., 312 YAMAMOTO, K., 247 322 YAMAMOTO, S., YAMAOTO, I., 344 YAMASAKI, M., 133 YAMASHITA, M., 79 YAMAUCHI, J., 231,361,575,688 YAMAUCHI, T., 771 YAMAZAKI, H., 85,89 YAMAZAKI, M., 247 YANG,L., 421 YASUKAWA, K., 548 YIM, H.S., 446 YIM,M.B., 446 YOKOYAMA, H., 556 YONEMITSU, K., 59 YORDANOV, N.D., 395,456 YOSHIDA, H., 747 151 YOSHIDA, K., YOSHIDA, A,, 306 298,302 YOSHIKAWA, J., YOSHIMURA, T., 403 YOSHINO, F., 562 YOUNGDEE, W., 515
830
Z ZADOROZHNAYA, A.N., 276 ZECH, S.G., 678 ZNANG, Q., 429 ZHDANOV, A., 634 ZHOU,B., 421 ZHU, H., 421 ZIATDINOV, A.M., 236,293