Laser Ablation: Effects and Applications

LASERS AND ELECTRO-OPTICS RESEARCH AND TECHNOLOGY

LASER ABLATION: EFFECTS AND APPLICATIONS

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LASERS AND ELECTRO-OPTICS RESEARCH AND TECHNOLOGY

LASER ABLATION: EFFECTS AND APPLICATIONS

SHARON E. BLACK EDITOR

Nova Science Publishers, Inc. New York

Copyright © 2011 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com

NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Laser ablation : effects and applications / editor, Sharon E. Black. p. cm. Includes index. ISBN 978-1-61209-189-1 (eBook) 1. Laser ablation. I. Black, Sharon E. TA1715.L367 2010 621.36'6--dc22 2010041370

Published by Nova Science Publishers, Inc. © New York

CONTENTS Preface Chapter 1

Chapter 2

Chapter 3

Chapter 4

vii  Double-Pulse Laser Ablation of Solid Targets in Ambient Gas: Mechanisms and Effects G. Cristoforetti and V. Palleschi  Assessing Hunter-Gatherer Mobility in Cis-Baikal, Siberia Using LA-ICP-MS: Methodological Correction for Laser Interactions with Calcium Phosphate Matrices and the Potential for Integrated LA-ICP-MS Sampling of Archaeological Skeletal Materials Ian Scharlotta, Andrzej Weber, S. Andy DuFrane, Olga I. Goriunova and Robert Creaser  Modeling of Laser Ablation Induced by Nanosecond and Femtosecond Laser Pulses Tatiana E. Itina, Mikhail E. Povarnitsyn and Konstantin V. Khishchenko  Fabrication of Silicon Nanocrystal Based Structures with Nanosecond Laser Ablation Processings in Liquid Media V. Švrček 

1 

45 

99 

127 

Chapter 5

Ho:YAG Laser Lithotripsy Jinze Qiu, Thomas E. Milner and Joel M. H. Teichman 

Chapter 6

Computer Modelling of Femtosecond Laser Ablation of Semiconductors and Dielectrics D. P. Korfiatis and K.-A. Th. Thoma 

153 

Thermophysical Effects of Femtosecond Laser Ablation of Metal Target Ranran Fang and Hua Wei

163 

Chapter 7

143 

vi Chapter 8

Chapter 9 Index

Contents Formation of Nanoparticles under Laser Ablation of Solids in Liquids G. A. Shafeev Nanodiamonds from Laser Ablation in Liquid G. W. Yang 

191  227  267 

PREFACE Laser ablation is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma. Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough. This book presents current research in the study of laser ablation from across the globe. Topics discussed herein include double-pulse laser ablation of solid targets in ambient gas; using laser ablation ICP-MS and its potential in sampling archaeological skeletal materials; and numerical modeling of laser-matter interactions. Chapter 1 - Laser Ablation (LA) is used in a widespread range of applications, among them it is employed as sampling procedure for the elementary chemical analysis of materials. With this aim, many analytical techniques makes use of LA - such as LA Ion Mobility Spectrometry, Resonant LA, LA-Atomic Fluorescence Spectrometry and LA-Microwave Induced Plasma-Atomic Emission Spectrometry – among them the most popular ones are probably the Laser-Ablation Inductively Coupled Plasma (LA-ICP) techniques and the Laser Induced Breakdown Spectroscopy (LIBS). In all these cases, it is desirable to attain the largest possible mass removal from the target in order to lower the Limit Of Detection (LOD) of the technique. Such intent is particularly true in the LIBS case, where LODs for solid samples are usually in the range of ppm or tens of ppm, which are often inadequate for many applications. One of the possible ways to overcome this problem is the use of two laser pulses, temporally separated by a suitable delay, for the laser ablation. In this scheme, the laser beams can be arranged in a collinear geometry, where both the beams are aligned normally to the target and are focused on its surface, or in an orthogonal geometry (pre-ablation configuration), where the first pulse runs parallel to the target surface and is focused in the ambient gas in front of it, while the latter is aligned perpendicularly and ablates the target. In case of ns-laser ablation, in both these configurations, a substantial enhancement of mass removal and of its atomization with respect to the case where a single pulse with equal total energy has been observed, where these phenomena lead to an improvement of both the LODs and the reproducibility of measurements. Such experimental configurations lead also to a different thermodynamic and dynamic evolution of the plasma, which can be useful in case of LIBS analysis.

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Many works have been dedicated to Double Pulse (DP) Laser ablation, aimed to establish the optimal experimental conditions to be used in different applications and to understand the physical mechanisms, still not clear, leading to the observed mass removal enhancement [1]. The aim of the present chapter is presenting and resuming the ns-ns DP state of art, discussing the occurring physical processes. Experimental results of the authors, as well as of other scientists, will be shown and discussed to validate and run down hypotheses on the mechanisms involved. In particular, the interpretation of the lower plasma shielding, due to a rarefied ambient environment, experienced by the second laser pulse will be presented. The reduction of plasma shielding allows a larger part of the laser energy to reach the target surface, resulting in an enhancement of the mass removal. Besides, such an improved laser-matter coupling produces a stronger heating, which could lead the molten pool near the thermodynamic critical temperature and drive the onset of phase explosion mechanism. A summary of the effects produced in DP configuration by using short and ultrashort laser pulses is also mentioned, together with a brief discussion of the mechanisms involved. Chapter 2 - Micro-sampling and analysis of tooth enamel from faunal samples in the archaeological record has enabled research into the mobility and seasonality of animals in prehistory. However, studies on human tooth samples have failed to yield similar results. It is well understood that human tooth enamel does not fully mineralize in a strictly linear fashion, but rather entails five recognizable stages of mineralization. Until the enamel matrix fully crystallizes, the matrix remains an open chemical system, thus at each stage of mineralization, the geochemical composition of the enamel matrix can be altered. At present it is unclear if failure to mirror the results from faunal teeth with human teeth is a factor of mineralization rates or simply the result of the difference in enamel volume and formation time between human and herbivore teeth. Therefore, the applicability of chemical analyses to human teeth is a balance between micro-sampling analytical techniques and generating archaeologically relevant data. Yet limited case studies have been performed to examine the scale and extent of this problem in human teeth using laser-ablation ICP-MS. Five human molars from an Early Bronze Age cemetery on the shores of Lake Baikal, Siberia were serially sampled and analyzed by means of laser-ablation quadrupole and multicollector ICP-MS in order to examine the nature of geochemical changes within the enamel matrix. This sampling was performed in order to generate a statistically significant dataset to assess the effectiveness of two approaches along with published methodologies to counter known problems with attempts to assess Sr87. Recent research has demonstrated that among the methodological problems, there is isobaric interference at mass 87 caused by the formation of calcium phosphate (Ca40PO) in response to interaction between the laser and the enamel matrix. Correction procedures using Zr91 in tandem with Ba/Sr ratios are examined. Additionally, serial sampling of teeth from hypothesized mobile hunter-gatherers provides useful insight into the dynamic interplay between physical sampling limitations and the scale at which useful geochemical data can be recovered from organic minerals. Traditional utilization of geochemical data for mobility has relied on a local/non-local dichotomy in population level analyses; however, this approach is of limited utility with regard to mobile populations. The authors’ ability to effectively analyze skeletal materials at a micro scale provides their best hope at addressing the rift between recognition of an indirect relationship between biological intakes, mineral formation and being able to generate relevant analytical data.

Preface

ix

Chapter 3 - The chapter considers the problem of numerical modeling of laser-matter interactions. The main objective is to clarify the mechanisms of this extremely complex process. Comparison of femtosecond and nanosecond laser ablation is first presented. Thermal model is used for nanosecond ablation. The physical phenomena involved into the interaction of a laser-generated plasma plume with a background environment are furthermore studied. A three-dimensional combined model is developed to describe the plasma plume formation and its expansion in vacuum or into a background gas. The proposed approach takes advantages of both continuous and microscopic descriptions. The simulation technique is suitable for the simulation of high-rate laser ablation for a wide range of the background pressure. The model takes into account the mass diffusion and the energy exchange between the ablated and background species, as well as the collective motion of the ablated species and the background gas particles. The developed approach is used to investigate the ablation of aluminum in the presence of a background gas. The influence of the background gas on the expansion dynamics of the laser-generated plume is examined. Experimental density distributions are explained based on the simulation results. A detailed analysis of material decomposition in femtosecond regime is then performed by using a hydrodynamic model with a thermodynamically complete equation of state. As a result, several ablation mechanisms are observed. A major fraction of the ablated material is found to originate from the metastable liquid region, which is decomposed either thermally in the vicinity of the critical point into a liquid-gas-mixture or mechanically at high strain rate and negative pressure into liquid droplets and chunks. The calculation results agree with the results of previous molecular dynamics simulations and explain recent experimental findings. In addition, effects of the ultra-short laser excitations of wide band gap materials need a particular attention. In this case, material ionization through multi-photon excitation and electron-impact ionization should be considered. Laser interactions are simulated with a particular focus on the control over laser plume expansion process. The properties of the laser-generated plasma plume are shown to be strongly affected by the laser-mater interaction mechanism Chapter 4 - In this chapter a nanosecond (ns) laser ablation and fragmentation processing in water, pure and doped spin on glass (SOG) polymer-based solutions are discussed. The confinement of laser-generated plasma in liquids allows the silicon nanocrystals (Si-ncs) formation with a quantum confinement size effects. The author demonstrates that ns laser processes in liquid can be efficiently applied for the fabrication and tuning the optoelectronic properties of Si-ncs based nanostructures. The laser fragmentation in water induces the selfassembly and allows formation of closely-packed stable luminescent Si-ncs over ~ 200 μm. Contrary to the water, the laser ablation and fragmentation in pure and doped SOG solutions inhibit aglomeration and enhance the Si-ncs luminescence properties. Finally, the authior disccusses physics and dynamics of Si-ncs formation through the serial growth processes that occurred in liquid media confined ns laser generated plasma. Chapter 5 - The long-pulse Ho:YAG laser has been used for intracorporeal laser lithotripsy of urinary calculi since the mid-1990’s and is considered the “gold standard” modality for endoscopic laser lithotripsy. The authors present an overview of Ho:YAG laser lithotripsy. They begin with an introduction of the ablative mechanism of Ho:YAG laser lithotripsy, and compare to short-pulse (< 10 usec) laser lithotripsy. Ablative properties of

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Ho:YAG laser lithotripsy are reviewed and the authors summarize several practical problems and safety issues of existing optical fibers for Ho:YAG lithotripsy. Chapter 6 - Ultrafast laser ablation has proved to be a powerful tool for material processing. Combination of both experimental and theoretical efforts can lead to the determination of optimum values of laser parameters for the improvement of accuracy which is a most crucial aspect in micromachining. Besides experiment and theory, numerical simulation can also provide a significant research tool in the field for the calculation of parameters of great importance which influence the micromachining process. Such parameters are ablation threshold as a function of the laser wavelength, pulse duration and the properties of the particular material. Especially, the prediction through numerical simulation of the geometry and the dimensions of the craters formed and the damaged surrounding surface for known laser and material properties can lead to control and optimization of the micromachining process. Furthermore, numerical simulation can drop light to the microscopic processes through which femtosecond laser damage occurs. In this chapter, the numerical techniques currently used for the simulation of femtosecond laser ablation of semiconductors and dielectrics are presented and discussed. These techniques include molecular dynamics simulation, the Fokker-Planck approach and two temperature models. Chapter 7 - The electron-phonon relaxation time as a function of pulse width and fluence of femtosecond laser is studied based on the two-temperature model. The satisfactory agreement between our numerical results and experimental data indicates that the electronphonon relaxation time is reasonably accurate with the influences of pulse width and fluence of femtosecond laser. An improved two-temperature model to describe femtosecond laser ablation of metal target is also presented. The temperature-dependent heat capacity and thermal conductivity of the electron, as well as electron temperature-dependent absorption coefficient and absorptivity are all considered in this tailored two-temperature model. The satisfactory agreement between our numerical results and experimental data indicates that the temperature dependence of heat capacity, thermal conductivity, absorption coefficient and absorptivity in femtosecond laser ablation of metal target must not be neglected. This chapter finally presents a unified thermal model, which can describe the thermophysical effects with laser pulse width ranges from nanosecond to femtosecond. The satisfactory agreement between the authors’ numerical results and experimental results of vaporization threshold indicates that the unified thermal model is correct and reasonable. Chapter 8 - The process of nanoparticle formation under laser ablation of solids in liquids is described. Critical parameters are discussed that govern the properties of nanoparticles ejected into the surrounding liquid. These parameters are laser wavelength, pulse duration, interaction of individual nanoparticles with laser beam inside the liquid. A review of previous results is presented on the properties of nanoparticles of noble metals. Recent data on laserassisted generation of other metals is given, including the formation of alloyed nanoparticles. Micro- and nanostructuring of the target upon its laser ablation in liquid environment is discussed. Examples are given of the influence of the surrounding liquid on the chemical composition of generated nanoparticles. The laser control over the size distribution of nanoparticles in liquids is demonstrated either by spatial profiling of laser beam intensity or proper tuning of laser wavelength into plasmon resonance of nanoparticles. Recent results are

Preface

xi

given on excitation of high energy levels of media under laser exposure to laser pulses of picosecond range of duration. Chapter 9 - Laser ablation in liquid, i.e. pulsed-laser induced liquid-solid interface reaction (PLIIR) has been developed to synthesize diamond nanocrystals. Chemical and physical mechanisms of the nanodiamonds synthesis upon PLIIR are addressed based on the nucleation thermodynamics and growth kinetics. The author’s studies showed that PLIIR could be expected to be a general route to synthesize the nanocrystals with the metastable phases.

In: Laser Ablation: Effects and Applications Editor: Sharon E. Black

ISBN: 978-1-61122-466-5 © 2011 Nova Science Publishers, Inc.

Chapter 1

DOUBLE-PULSE LASER ABLATION OF SOLID TARGETS IN AMBIENT GAS: MECHANISMS AND EFFECTS G. Cristoforetti1 and V. Palleschi2 1

National Institute of Optics, Research Area of National Research Council, Via G.Moruzzi, 1 – 56124 Pisa (ITALY) 2 Applied Laser Spectroscopy Laboratory, Institute of Chemistry of Organometallic Compounds, Research Area of National Research Council, Via G.Moruzzi, 1 – 56124 Pisa (ITALY)

ABSTRACT Laser Ablation (LA) is used in a widespread range of applications, among them it is employed as sampling procedure for the elementary chemical analysis of materials. With this aim, many analytical techniques makes use of LA - such as LA Ion Mobility Spectrometry, Resonant LA, LA-Atomic Fluorescence Spectrometry and LA-Microwave Induced Plasma-Atomic Emission Spectrometry – among them the most popular ones are probably the Laser-Ablation Inductively Coupled Plasma (LA-ICP) techniques and the Laser Induced Breakdown Spectroscopy (LIBS). In all these cases, it is desirable to attain the largest possible mass removal from the target in order to lower the Limit Of Detection (LOD) of the technique. Such intent is particularly true in the LIBS case, where LODs for solid samples are usually in the range of ppm or tens of ppm, which are often inadequate for many applications. One of the possible ways to overcome this problem is the use of two laser pulses, temporally separated by a suitable delay, for the laser ablation. In this scheme, the laser beams can be arranged in a collinear geometry, where both the beams are aligned normally to the target and are focused on its surface, or in an orthogonal geometry (preablation configuration), where the first pulse runs parallel to the target surface and is focused in the ambient gas in front of it, while the latter is aligned perpendicularly and ablates the target. In case of ns-laser ablation, in both these configurations, a substantial enhancement of mass removal and of its atomization with respect to the case where a single pulse with equal total energy has been observed, where these phenomena lead to

2

G. Cristoforetti and V. Palleschi an improvement of both the LODs and the reproducibility of measurements. Such experimental configurations lead also to a different thermodynamic and dynamic evolution of the plasma, which can be useful in case of LIBS analysis. Many works have been dedicated to Double Pulse (DP) Laser ablation, aimed to establish the optimal experimental conditions to be used in different applications and to understand the physical mechanisms, still not clear, leading to the observed mass removal enhancement [1]. The aim of the present chapter is presenting and resuming the ns-ns DP state of art, discussing the occurring physical processes. Experimental results of the authors, as well as of other scientists, will be shown and discussed to validate and run down hypotheses on the mechanisms involved. In particular, the interpretation of the lower plasma shielding, due to a rarefied ambient environment, experienced by the second laser pulse will be presented. The reduction of plasma shielding allows a larger part of the laser energy to reach the target surface, resulting in an enhancement of the mass removal. Besides, such an improved laser-matter coupling produces a stronger heating, which could lead the molten pool near the thermodynamic critical temperature and drive the onset of phase explosion mechanism. A summary of the effects produced in DP configuration by using short and ultrashort laser pulses is also mentioned, together with a brief discussion of the mechanisms involved.

1. INTRODUCTION In the last decade, many papers, especially aimed at improving LIBS and LA-ICP figures of merit, focussed their attention on Double Pulse Laser Ablation [1]. However, the pioneering works on the subject go back to the seventies, where effects similar to those recently investigated had been already observed and discussed. The double pulse approach was firstly studied by Piepmeier and Malmstadt [2,3]. In both works a multiple spikes irradiation over an aluminium target in air from a Q-switched ruby laser (λ = 694.3 nm, τ ≈ 50 ns), with a separation between the pulses of 500 ns, was performed. The observed enhancement of Al II and Al III lines was associated to the absorption of the second pulse by the plasma ignited by the first pulse, resulting in a further excitation of the aluminium species. Maher and Hall [4] studied the effects produced by focussing two CO2 laser pulses, separated by a suitable temporal delay, on the same spot on different targets. When the delay between the pulses was in the 30-70 μs window, they observed a pulse-target coupling stronger with respect to that produced by a single pulse, resulting in a higher impulse delivered onto the target surface. In order to understand the reasons of such effect, the focal spots were slightly moved, so that they did not overlap; also in that case, the results obtained were very similar to the ones observed with overlapping spots, suggesting that a possible thermal explanation of the observed phenomena, based on the pre-heating of the target surface, could be ruled out. High-speed photographs, interferometric observations, target damage evaluations and impulse measurements led the authors to conclude that the stronger laser-target coupling is produced by a pronounced reduction in local medium density caused by shocks expanding from the first laser pulse interaction resulting in a more difficult ignition

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

3

of Laser Supported Detonation (LSD) regime and then in a weaker shielding of the second laser pulse. Pershin and coworkers [5-7] in the eighties studied the effects produced by focussing two Q-switched Nd:YAG pulses of 20 ns-duration, separated by 25 μs, on an aluminium target and on laser glasses in air, observing an intensity enhancement of target element lines and, at the same time, a reduction of line intensity from atmospheric species with respect to the signal obtained with a single laser pulse. The authors suggested that by an appropriate spatial selection of plasma emission acquisition should then be possible to enhance the spectral line signal-to-background and improve the limits of detections (LODs) of the analysis. The formation of a plasma mirror and/or of self-focussing of the radiation in the plasma, which both could be possible causes of the observed effects, were excluded by the experimental results. Scattering and transmission coefficients were measured for the plasmas produced by the first and the second pulse, finding a much lower intensity of scattered light in the latter case [6]. Such results, together with the lower emission from atmospheric species [7], led the author to the conclusion that double-pulse effects are produced by a reduction in the gas density following the interaction with the first laser pulse, allowing the radiation of the second pulse to reach the heated surface of the target and generate a more efficient breakdown. An interesting variant of the method, consisting of DP ablation of samples immersed in a liquid environment, was introduced by Nyga and Neu [8] and developed by Pichahchy et al. [9]. In this case, the first laser pulse produces a cavitation bubble on the target surface which expands, reaching the maximum extension after about 400 μs, and then collapses again; the plasma produced inside the bubble is rapidly quenched and the emitted lines are considerably broadened by pressure effects. If a second pulse is focussed onto the target in the correspondence of the cavitation bubble, a plasma plume is ignited and expands into the bubble vapour environment, in a condition similar to that of plasmas generated in ambient gas, resulting in emission lines much narrower and in a strong line intensity enhancement, which can reach up to two orders of magnitude. Such method was proposed for the elementary analysis of samples submerged in water (e.g. for geological samples located under water). From the nineties up to now, different experimental configurations for the double and multiple pulses approach have been tested and a large amount of papers have been published on the topic, modifying the geometry of the laser pulses; the duration, energy and wavelength of the pulses, including combinations between femtosecond, picosecond and nanosecond lasers, and the delay between them; the environment where the ablation is ignited and finally the target composition. Many of such works were devoted to a better understanding of the mechanisms underlying the DP effects, by means of a large variety of techniques for plasma diagnostics and for the examination of laser crater on the target, including spectroscopic analysis of plasma emission, direct or shadowgraphic imaging of the plume, interferometric analysis, microscopic target analysis, etc. Many other works focussed their attention on the possible applications of DP benefits, and most of them pointed to the improvement of reproducibility and sensitivity of LIBS and LA-ICP techniques. Such works were devoted mainly to the description of DP effects and to the determination of the most suitable experimental conditions and apparatus for their application. However, other applications were proposed, as for example the production of

4

G. Cristoforetti and V. Palleschi

nanoparticles by means of DP laser ablation of metal targets in liquid environment. Burakov et al. [10] showed that DP-LA provides a more effective ablation, and then a larger amount of nanoparticles with respect to SP-LA. Moreover, the mean size and the stability of nanoparticles could be controlled by a proper selection of the temporal delay between the laser pulses, which affects the decay time of the plasma plume and then the temporal window where the nucleation and growth of nanocrystals occur. Basically, three different laser beam geometries have been up to now investigated in the literature and will be considered here, despite other, more exotic configurations, have been also tested [11]. Most research efforts have been focused on the collinear configuration, which is certainly the easiest to arrange experimentally. In this configurations both beams are aligned normally to the target and are focused on its surface (Figure 1a). However, a large amount of works have been published dealing with an orthogonal arrangement of the beams; here, the first pulse (pre-ablation scheme) (Figure 1b) or the second one (reheating scheme) (Figure 1c) is directed parallel to the target surface and focused in the atmosphere in front of it, while the other is perpendicular and ablates the target. All such configurations, when utilized with a proper choice of experimental conditions, may result in an enhancement of plasma emission, which can be fruitfully used for the improvement of LIBS technique figures of merit. However, while the first two arrangements, with appropriate choice of experimental parameters, produce also a substantial increment of the mass ablated, the last one turns out only in a re-heating of the plasma produced by the absorption of the first laser pulse, so that, strictly speaking, it can not be classified as DP-LA method. Nevertheless, also the results obtained by such configuration will be rapidly summarized in the following for the completeness of the subject. Another important criterion of classification between the experimental configurations is the duration of the laser pulses, since the mechanisms occurring during the DP-LA are noticeably different depending on the pulse length. In most of the works, two nanoseconds pulses are used, whose utilization is certainly more accessible for portable instruments and stand-off applications; however, also combinations of nanosecond with ultrashort pulses have been tested [12,13]. A summary of the effects produced by DP-LA in the different experimental configurations and a critical review of the mechanisms proposed in the corresponding cases are reported in the following sections. Particular emphasis will be given to the ‘lower shielding’ mechanism, which is in our opinion the main reason of signal and mass removal enhancement in case of nanosecond-nanosecond pulses. The attention will be focussed mainly on the nanosecond-nanosecond laser ablation, which appears to be more accessible for applications, and whose mechanisms are noticeably different from those occurring in femtosecond and picosend DP-LA; however, a brief description of results, mechanisms and references related to combinations of laser pulses with different duration will also be given.

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

5

Figure 1. experimental geometries used in DP experiments

Finally, the present chapter is focussed to the case of DP-LA of solid targets, mainly metals, in ambient gas, although the technique has been successfully applied to solid targets immersed in a liquid environment [14,15], with important potential applications going from the target sampling and analysis to the synthesis of nanoparticles of controllable size distribution. DP-LA of liquids, gases and aerosols is not treated in the present chapter.

2. NANOSECOND-NANOSECOND PULSES COMBINATION: EFFECTS OF DP-LA a) Collinear Beams Configuration The collinear configuration, where both laser beams are aligned perpendicularly to the target surface, has been widely tested and studied and, from a practical point of view, is the most suitable for standoff applications [16,17]. Some works published in literature utilize also slightly different configurations, where the laser beams hit the target at non-normal angles of incidence [11]; however, since both the laser pulses produce the ablation of the sample and the physical processes involved are similar to those occurring in the collinear perpendicular configuration, such cases are included in the present section. The paragraph is subdivided in four parts, dealing with the effects of collinear DP-LA on the plasma emission, on the plasma dynamics, on the ablation mechanisms and on the relationship between ambient gas conditions and DP effects.

Effects on plasma emission All the works dealing with the spectroscopic analysis of plasmas induced by a ns-ns laser pulses combination evidence a large enhancement, depending on the matrix of the target, of the line intensities produced both by target atoms and ions, which can reach two orders of magnitude [18-23]. At the opposite, the intensity of spectral lines emitted by atoms and ions deriving from ambient gas (e.g. N and O species in ambient air) is markedly reduced. An example of plasma spectra in the range 250-400 nm obtained in SP and in collinear DP configuration by a Fe-Mn alloy is shown in Figure 2, taken from Ref.[25]; all the lines in the spectral range shown in the figure belong to Fe and Mn atomic and ionic species and exhibit an evident enhancement of 1-2 orders of magnitude.

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It is worth to emphasize, however, that the signal enhancement found in collinear DP-LA is dependent on the detector field of view [24], that can be easily explained by the different plasma volume and dynamics obtained in SP and DP configuration. It is also important to note that, usually, the enhancement is obtained by calculating the ratio of the line intensities observed in the same temporal acquisition window both in SP and in DP configurations; clearly, the value so-calculated depends strongly on the different dynamical and thermodynamic evolution of the SP and DP plumes which can be very different, so that such method is, to some extent, arbitrary and misleading. This should be taken into account for example when the comparison of plasma temperature and electron density values in the two cases is done. The observed enhancement results in a marked improvement of the LIBS LODs, which can reach a few ppm or lower depending on the element analysed and on the matrix where it is embedded. Sturm et al. [26] found detection limits below 10 μg g-1 for C, P, S, Al, Cr, Cu, Mn and Mo in steel samples. Piscitelli et al. [23] found an improvement of LIBS sensitivity for Pb embedded in several metal targets of about an order of magnitude. In a previous work [27], we calculated the limits of detection for several elements in aluminium and steel alloys using both the single and the collinear double pulse configurations of laser-induced breakdown spectroscopy. We used a dual-pulse Nd:YAG laser (λ=1064 nm, Δτ=12 ns), where the energy per pulse was set to 30 mJ (~3 GW cm-2) for the ablation, and an echelle spectrometer coupled to an intensified CCD camera (λ/Δλ=5000) for the spectral acquisition. Calibration plots were constructed for Mg, Al, Si, Ti, Cr, Mn, Fe, Ni, and Cu using a set of certified aluminium alloy samples and a set of certified steel samples. The investigation included the optimization of the experimental conditions, where the temporal separation between the pulses, the delay time and the gate of acquisition were varied, to furnish the best signal-to noise ratio in both geometries. The final LODs are reported in Table 1, evidencing that the improvement obtained in DP configuration depends on the matrix of the target, being much higher for aluminium alloys than for steel alloys.

Figure 2. LIBS spectra obtained from the SP ablation (black line) and the collinear DP ablation (grey line) of a Fe-Mn alloy target. The figure has been taken from Ref.[25]

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

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Table 1. Comparison of LIBS detection limits obtained in SP and DP collinear geometries for aluminium and steel alloys Element Aluminium alloys Mg Ti Cr Mn Fe Ni Cu Steel alloys Al Si Ti Cr Mn Ni Cu

Wavelength (Å) utilized

Single-pulse LOD

Double-pulse LOD

2852.13 3349.41 4254.33 2949.20 3719.93 3414.76 3247.54

30 ppm 100 ppm 100 ppm 0.1% 400 ppm 600 ppm 150 ppm

4 ppm 10 ppm 10 ppm 90 ppm 50 ppm 100 ppm 80 ppm

3961.52 2881.58 3088.02 4254.33 4823.52 3414.76 3247.54

30 ppm 100 ppm 50 ppm 70 ppm 300 ppm 100 ppm 25 ppm

20 ppm 40 ppm 25 ppm 50 ppm 120 ppm 40 ppm 5 ppm

Figure 3. ranges of enhancements RI1 and RI2 for a) ionic lines and b) neutral lines observed in the spectral range 200–900 nm at an inter-pulse delay of 1 μs. RI1 and RI2 indicate the enhancement values obtained in DP (pulse energies 60+60 mJ) with respect to the SP configuration with pulse energy of 60 mJ and of 120 mJ (corresponding to a zero interpulse delay), respectively. Taken from Ref. [28]

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In a subsequent work [28], where the same apparatus described above (but an energy per pulse of 60 mJ) was used, we studied more in detail the effect of the matrix composition on the emission enhancement observed in DP collinear LIBS for several pure metal targets (Al, Au, Co, Cu, Fe, Mn, Mo, Ni, Pb, Pt, Si and W). In Figure 3 the range of enhancement of line intensities for the different targets are reported; RI1 and RI2 indicate the enhancement values obtained in DP (pulse energies 60+60 mJ) with respect to the SP configuration with pulse energy of 60 mJ and of 120 mJ (corresponding to a zero interpulse delay), respectively. The measurement of the emission enhancement for neutral and ionic lines of all the samples showed a wide range of results, going from the lowest observed for Pb, Ni and Mn to the highest values obtained for Cu, Al and Au. It is then clear that the matrix of the target plays an important role in determining the effects produced by DP configuration, as will be discussed successively. A large amount of papers have been published to evidence in detail the dependence of line intensity enhancement on many parameters – e.g. the interpulse delay, the energy of the pulses, the ionization stage of the emitting species, the energy of the upper level of the transition, the properties of the target and of the environment gas, etc. It is clear that line intensity enhancement is affected by two factors, which are the increase of ablated mass in the plume – related to the mechanisms of mass removal from the target, and thus to the material and ambient gas properties – and the variation of thermodynamic parameters of the plasma, e.g. temperature and electron density, related to the ignition process of the plasma and to its dynamical evolution. In fact, assuming the Local Thermal Equilibrium (LTE) of the plasmas, the intensity enhancement R of a generic line in DP configuration with respect to single pulse can be written:

R=

N DP n DP Z (TSP ) 1 1 exp(− E k ( )) − N SP n SP Z (TDP ) k B TDP k B TSP

(1)

where N is the absolute total number of atoms of the chemical element considered, n is the fraction of these atoms corresponding to the emitting species (atomic or ionic), Z is the partition function, Ek is the upper energy level of the transition and kB is the Boltzmann constant. In turn, the ratio n DP n SP can be expressed in terms of the temperature and electron density values via the Saha equation. So, the variation of experimental parameters results in a variation of the emission enhancement via the increase of the ablated mass (NDP/NSP) or via the enhancement of plasma temperature (terms nDP/nSP and exp(-Ek(1/kBTDP1/kBTSP)) ). A collection of the main results, dealing with the variation of experimental parameters, which can be useful to understand the DP effects and causes, is reported in the following. The emission enhancement depends strongly on the separation between the laser pulses, as shown in Figure 4 for gold line intensities (λ=1064 nm, τ=12 ns), where the maximum value is obtained for an interpulse delay going from hundreds of nanoseconds up to a few microseconds, depending on the other experimental parameters and on the ionization state of the emitting species. At separation delays shorter than ~100 ns a depressive effect on the LIBS signal can be obtained, mainly due to the laser shielding of the second laser pulse by the plasma formed by the first pulse, as clearly shown by Mao et al. [29] (Si target, λ= 1064 nm,

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

9

Line intensity enhancement

τ = 4 ns). The enhancement of plasma emission persists up to interpulse delays of tens of microseconds, often being still significant at 80-100 μs. In general, the optimal interpulse delay is larger for ionic than for atomic lines, as visible in Figure 4 where the largest enhancement for the Au I and Au II lines is obtained for interpulse delays of 1-2 μs and 4 μs, respectively. Just to make some examples, in a previous work [30] (brass target, λ=1064 nm τ=10 ns), we found an optimum interpulse delay of about 0.7 μs for neutral copper lines and of approximately 2 μs for ionic copper lines. In a successive work [31], where an aluminium target was used, we found the largest enhancement for interpulse delays in the range of 1-4 μs, both from neutral and ionic lines. St-Onge et al. [18,20], operating on an aluminium target, found that the optimum delay is less than 1 μs for Al I lines and between 2 and 5 μs for Al II lines.

Au I 312.2 Au II 291.3

25

20

15

10

5

0 0

10

20

30

40

50

interpulse delay (μs) Figure 4. Dependence of line intensity enhancement on the separation between laser pulses for a gold target

Gautier et al. [32] (Al target, λ=532 nm) found that the optimum interpulse delay Δτ depends on the excitation energy of the transition, where for low-energy atomic lines the highest improvement is obtained for Δτ = 0.2 μs, while for ionic lines and atomic lines with excitation energy higher than 6 eV the largest emission enhancement is obtained for Δτ higher than 1 μs. Other works evidenced that, in most of the cases, the emission enhancement is higher for ionic than for atomic lines, as visible in Figure 3, and for high excitation energy than for low excitation energy lines. Such features are well evident in Figure 5, taken from Ref.[31], where several aluminium atomic and ionic lines were observed and their enhancement was plotted versus the energy of the transition upper level.

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By looking at Eq.(1), it is evident that the increasing trend of emission enhancement with the excitation energy of the upper level of the transition is due to an increase of plasma temperature. In Ref.[31], we calculated the plasma temperature in both SP and DP-LA plumes, finding a slight increase of the order of 10% in the latter case; it was then verified that such increment is compatible with the slope of the points plotted in Figure 5. It was also showed that the absolute values of emission enhancement can not be only attributable to such slight temperature increase but must be caused by a concomitant growth of the ablated mass by a factor of ~7, as will be discussed later. An increase of plasma temperature was also found in other works, i.e. by Sattmann et al. [22] (steel target, λ=1064 nm, τ = 20 ns) and De Giacomo et al. [33] (Ti target, λ1= 532 nm,λ2 = 1064 nm, τ = 8 ns). In Ref.[28] (acquisition delay = acquisition gate= 1 μs) we found an increase of plasma temperature in DP collinear geometry with respect to SP with half-energy for most of the targets analysed up to 1200K, where such enhancement was larger for targets showing a higher emission enhancement; at the opposite, no evident growth of plasma temperature with respect to SP with equivalent total energy was found. Other works evidenced no substantial change or even a slight decrease of plasma temperature [21,29]. It is then clear that the main contribution of the observed emission enhancement is not caused by the higher temperature of the plasma.

Figure 5. Emission enhancement obtained in DP collinear configuration for Al I at 265.3, 305.0, 308.2 nm and Al II at 281.6, 385.6, 466.3 nm lines, for an interpulse delay time of 4 μs. Taken from Ref.[31]

In Ref.[28] (acquisition delay = acquisition gate= 1 μs), we calculated the plasma electron density in DP and in SP configuration for a large quantity of metal targets, showing in all the cases a marked decrease in DP with respect to SP configuration at equivalent total

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

11

energy; on the other hand, the electron density found in SP at half energy and DP cases are comparable. Colao et al. [19] (Al target, λ=1064 nm, τ = 8 ns) and St. Onge et al. [20] (Al target, λ=1064 nm, τ = 6-12 ns) demonstrated that the electron density in plasmas induced by DPLA is consistently lower at early times of plasma evolution with respect to SP-LA at equivalent total energy, but exhibits a slower decay with time, so that the values in the two configurations approach, or electron density in DP plasma even becomes higher, at late times (e.g. 700 ns in Ref.[20] and after 1.2 μs in Ref. [19] after plasma ignition). The lower electron density found in DP-LA at short acquisition times can explain the largest emission enhancement observed for ionic lines with respect to neutral lines; in fact, if we assume LTE and consider the Saha-Eggert equation, a lower electron density results in a higher ionization degree of plasma species.

Plasma dynamics The expansion dynamics and the consequent morphology of plasmas induced in collinear DP configuration are noticeably different from those of plasmas produced by a single laser pulse. In Figure 6, time-resolved images (acquisition delay = 700 ns, gate = 400 ns) of plasmas from a brass target in air (λ=1064 nm, τ=12 ns), spectrally filtered around the Zn I @ 520 nm line, induced by a single pulse, two coincident pulses, and two delayed pulses (Δτ = 10 μs) and acquired by an Intensified CCD camera, are shown. Both from the experiments performed at atmospheric pressure and at 100 torr pressure, it is evident that the dimensions of plasmas induced by the DP configuration are much larger than those of plasmas produced by a single pulse and even by two coincident pulses. Moreover, the DP plasma tends to detach from the target much more than SP plasmas, denoting a very different expansion dynamic in the two cases. Two sequences of frames, describing the evolution of the plasma from a brass target in air produced by a single laser pulse and by two laser pulses delayed by 2 μs, are shown in Figure 7, taken from Ref.[34] (λ=1064 nm, τ=12 ns). The images are obtained by shadowgraphic technique, using a white flash light to back-illuminate the region of the plume. The exposure time of the single frame is 100 ns and the frames are separated by 500 ns. In SP-LA, the generation, evolution and decay of the plasma at the surface of the sample, together with the formation of a shock wave propagating in the surrounding atmosphere, are visible. According to Figure 6, the plasma plume is near the surface of the target during all its life. The shadowgrams of the plasma obtained in DP mode reveal that the second laser pulse, coming 2 μs after the first pulse, initiates a new plasma at the sample surface (frame 4), which very rapidly propagates, moving away from the target and almost fills the region encompassed by the shock wave front formed by the first pulse, resulting in a plume wider than in SP configuration. Rapid expansion is caused by the rarefied ambient gas, formed by the first laser pulse, where DP plasmas are generated and expand. Gas rarefaction is evidenced also by the failure of detecting the shock wave formed by the second laser pulse, because of the lower jump in density at the shock front [34].

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Figure 6. Time-resolved images of plasmas obtained from a brass target by a single pulse (SP), two coincident pulses (CP) and two delayed pulses (DP) obtained at atmospheric (a-c) and 100 torr (d-f) pressure. The emission is spectrally filtered around the Zn I @ 520 nm line. Delay time of acquisition is 700 ns and gate is 400 ns

Figure 7. Shadowgrams of the laser induced plasmas in single pulse (on the left) and double pulse configuration (on the right). In both sequences, the first photograph (left bottom) has been taken at a delay time of ~500 ns with respect to the first laser pulse, while the temporal delay between photographs is 500 ns. Taken from Ref.[34]

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13

A detailed study on the different dynamics of plasmas induced in SP and DP configuration was presented by De Giacomo et al. [33] (Ti target, λ1= 532 nm, λ2 = 1064 nm). In their experimental work (sided by a LIP expansion model), it was shown that the SP plasma expands up to ~ 1 μs and then stops, due to the counterpressure of the cold unperturbed ambient gas, at a distance of ~ 1.1 mm; differently, the DP plasma rapidly expands in the hot rarefied gas up to ~ 400 ns and then stops at a distance of about 1.7 mm from the target. De Giacomo et al. stress that the DP plasma expansion during the early hundreds of nanoseconds resembles a free expansion; however, the expansion inside a hot environment and the confinement produced by the shock wave formed after the first laser ablation, lead also to a smaller loss of plasma energy and then to a slower decay time of plasma temperature. Similar results and conclusions were also presented by Noll et al.[35], using a high-speed electro-optic camera to observe the spatial and temporal development of the plasma morphology and a Mach–Zehnder interferometer to detect the spatio-temporal changes of the refractive index of the plasma. They also showed that the second laser pulse interacts predominantly with the sample surface while the laser absorption by the residual plasma causing its re-heating is marginal. So, in conclusion, both the wider volume of plasma emission and the longer plasma lifetime obtained in DP configuration produce a situation more suitable for LIBS analysis than SP geometry.

Mass removal mechanisms, atomized ablated mass and effects of the matrix target Many works [18,29,36-39] report that DP collinear laser ablation produces deeper craters with respect to the SP ablation, where the ablation rate can increase from 2 to 20-fold. It was also observed that the crater rims are lower in DP case or, however, the ratio of volumes occupied by the rims (Volumeup) and that of the hole (Volumedown) is noticeably smaller than in SP. Caneve et al. [36] (copper-based alloy target, λ=1064 nm, τ = 8 ns) found ablation rates in the range 0.08-0.11 μm/shot and 1.78-3.2 μm/shot, for single and double pulse schemes respectively; moreover, the presence of rims is well visible in the former scheme and negligible in the latter. 3D reconstructions and 2D profiles of craters induced on an aluminium target in both configurations (λ=1064 nm, τ=12 ns, Δτ= 4 μs, pulses energy = 78 mJ), obtained by video-confocal microscopy and taken from Ref.[31], are reported in Figure 8. In such a paper, we reported an increase of crater volume (measured as the volume of the drilled hole) of 4-6 times in DP-LA, and a ratio Volumeup/Volumedown decreasing from the range 1.8-2.5 in SP-LA to 0.8-1.1 in DP-LA. Here, it is necessary to bear in mind that values larger than 1 of the ratio Volumeup/Volumedown are possible because the density of molten and re-solidified material can be lower than that of the target. Such observations demonstrate that DP-LA improves the laser-target coupling, reducing undesired thermal effects and phenomena produced by plasma-target interaction, such as melt displacement and splashing of the molten pool.

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Figure 8. 3D reconstructions and profiles of craters produced in SP (on the left) and in DP configuration (on the right). Taken from Figure [31]

The increase of mass removal is certainly one of the major contributions to the LIBS signal emission and one of the reasons for which DP-LA is investigated for LA-ICP application. However, the increase of the ablation rate and the reduction of thermal effects deserved attention also in the field of micro-machining of materials [39,40]. Usually, for such applications, thermal effects and re-deposition of debris on the surface are avoided by using ultrashort laser pulses, which are able to produce high-quality large aspect-ratio holes; however, fs laser pulses have several drawbacks, as the low ablation rate, the need for reduced pressure to avoid air breakdown due to the high irradiance and the high cost of instrumentation, which limited their commercial exploitation. Forsman et al.[40] (steel and Al targets, λ=532 nm) and Wang et al. [39] (steel target, λ=1047 nm, τ = 21 ns) observed significant enhancements of the drilling rate (3-10 times in Ref.[40] and more than one order of magnitude in Ref.[39] depending on the experimental parameters) by using markedly smaller energy (fractions of mJ), tighter focussing (tens of μm) and smaller interpulse delays (tens of nanoseconds) with respect to the values usually utilized in LIBS experiments. Wang et al. showed that the drilling enhancement strongly increases with the thickness of the sample, since SP drilling rate tends to saturate at depths larger than 400 μm while DP performance remains stable with depth.

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

15

For applications as LIBS or LA-ICP, where quantifying the enhancement of atomized ablated mass in the plume rather than the drilling rate is important, the analysis of craters depth and morphology is not directly informative about the effectiveness of the laser ablation process. In fact, crater analysis does not provide the measure of the mass atomized in the plasma because the volume of the laser induced crater is often comparable to that of the rim around the hole, so that the calculation of the removed mass as the difference between such volumes leads to large uncertainties. Furthermore, even a precise measure of the removed mass would not be able to discriminate between the mass atomized in the plasma and that ejected (and spectroscopically lost) in particles or large clusters. A more direct measurement of the ablated mass could be obtained by weighing the target before and after the ablation process, even if also this method does not discriminate molten droplets and clusters; however, such approach is hindered by the low effective ablation rate, which can be much lower than a μg per pulse, so that it implies the weighing of the sample after a large number of laser shots, as in Ref. [41]. Because of these disadvantages, in Ref.[28,31,42-44] we estimated the atomized mass in the plasma, scaled by an arbitrary factor, directly from the analysis of emission spectra, even if this approach is strongly affected by the uncertainties in the determination on the thermodynamic parameters and by the geometry of signal collection in the experimental setup. In Figure 9, where the same notation of Figure 3 is used, the enhancement of atomized ablated mass for several metal targets is shown. By a rapid comparison with Figure 3, it is evident that there is a strong relationship between the enhancements of LIBS signal and of ablated mass, suggesting that the main contribution of DP-LA benefits in LIBS is produced by the increase of mass in the plume, and, only in the second place, to the increase of plasma temperature. In Ref.[31], we calculated the contribution to lines intensity enhancement of both the variations of temperature and atomized ablated mass. We showed that the calculated small increment of temperature (~10%) resulted in a slightly higher population of higher energy levels and a slightly higher ionization, producing a negligible variation of the intensity of the neutral lines in the range 0.94-1.2 and an increase of the ionized lines in the range 2.3-3; the remaining enhancement, which was by a factor of ~7, was due the increase of mass into the plume. By comparing the measured crater enhancement with the enhancement of the ablated mass in the plume (measured via spectroscopic analysis or via ICP analysis), it is evident that the latter is usually much higher; even if such comparison is incorrect, since the measurements of crater dimensions can not be used to estimate the mass removed from the target (see above), such observation led some authors to infer that DP-LA produces also a higher atomization of the ablated mass and a finer aerosol. Such hypothesis seems also confirmed by the imaging of the plume (e.g. see Ref. [45]). Despite the evident dependence of emission and ablated mass enhancement on the matrix of the target (see Figures 3 and 9), very few works were devoted to correlate the effects of DP-LA with the composition and properties of the target. In a detailed work [21], Gautier et al. analysed the effectiveness of the DP LIBS for different materials (aluminium, synthetic glass, steel, rocks) separating the contribution of temperature and ablated mass increase. Their results suggested that the intensity enhancement tends to be higher for the matrices originating a cooler SP plasma.

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Figure 9. Calculated enhancement of ablated atomized mass RM1 and RM2 for all the targets. The uncertainty is ~25–30%. RM1 and RM2 indicate the enhancement values obtained in DP (pulse energies 60+60 mJ) with respect to the SP configuration with pulse energy of 60 mJ and of 120 mJ (corresponding to a zero interpulse delay), respectively. Taken from Ref. [28]

30

Au Al

ablated mass 1 enhancement RM

25 Pt

20 Si

15 10 5

Cu

W Mo Fe Co NiPb

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 2

-1

thermal diffusivity at melting point (cm s )

Figure 10. Ablated mass enhancement in DP LIBS (pulse energies = 60 mJ + 60 mJ) with respect to SP configuration (energy = 60 mJ) vs. thermal diffusivity at melting point. Taken from Ref.[28]

In Ref.[28], the effect of DP-LA on several metals and semiconductors characterized by different thermal, electronic and optical properties was studied. Other materials such as insulators and polymers were not considered because of their different ablation mechanism (volumetric heating) with respect to metals (surface heating). In particular, we made an attempt to correlate the increase of ablated mass, as reported in Figure 3, with the melting point and heat, the boiling point and heat, the reflectivity and the ionization energy of the metal. However, no evident correlation was found. At the opposite, a correlation was observed between the ablated atomized mass enhancement and the thermal diffusivity of the metal, as shown in Figure 10; in order to explain such correlation, we proposed a simple picture that will be described in the following sections.

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17

Dependence on ambient gas pressure As final paragraph, we report some results concerning the influence of the ambient gas on the DP-LA effects previously described, i.e. the plasma emission, the plume dynamics and the mass removal. Some DP-LA studies were conducted in ablation chambers and performed at ambient gas composition different from air, such as nitrogen, argon, helium or mixtures [45-47]. Some of such experiments were motivated by the application of DP-LA for ICP analysis [45]; some others, where gas was flushing the interaction region, were aimed to remove the particulate produced by precedent bursts [46]. The results obtained suggest that the gas density (at constant gas pressure) strongly affects the drilling rate of the ablation process, both in SP and in DP configurations, and that the magnitude of DP enhancement decreases with increasing the density of ambient gas. Here, however, we will focus our attention on the effects associated to the pressure of the ambient gas, which are stronger and more significant for the modelling of DP-LA process. In a previous paper [30], we studied the effect of DP configuration on plasma emission, by performing laser ablation at different air pressures, ranging from 0.1 Torr to atmospheric conditions. Two Nd:YAG laser pulses (λ=1064 nm, τ=12 ns) were separated by a time delay ranging from 0 (coincident pulses) up to 8 μs and focussed on the surface of a brass sample. Neutral and ionized lines originated both by species deriving from the target and from the air environment were analysed. The results, shown in Figure 11, evidenced a different behaviour of copper species emission versus air pressure value in single- and double-pulse- operation modes. The line intensity measured in SP case shows a maximum emission around 100 Torr pressure followed by a significant reduction at higher pressures. This effect can be explained, as discussed in literature, in terms of the laser shielding of the target operated by the plasma, which is strongly affected by the buffer gas. On the other hand, in DP scheme the emission signal from copper species do not decrease at higher pressures, so that an enhancement with respect to the SP case is present for pressures higher than 100 Torr. It is also noticeable the reduction of Cu emission obtained in DP configuration at pressures lower than 100 Torr. A similar behaviour, was also observed for other lines emitted by atoms originated from the sample (e.g., Zn I lines). The analysis of the O I 777.3 nm line, shown in Figure 12, reveals a completely different behaviour with respect to that of Cu I, Zn I, and Cu II lines. The signal obtained by using a single pulse or two coincident pulses (Δτ=0) does not reach a maximum, but, on the contrary, shows a monotone increasing trend with increasing the air pressure in the chamber up to 300 Torr followed by a slight saturation. Such results are easily explainable by considering that the oxygen atoms originate from the environment, so that their concentration in the plasma is not affected by the laser shielding effect. Therefore, the increasing trend of the emission of oxygen atoms versus pressure is due to the combined effect of the increase of their concentration and of the raise of plasma temperature produced by the laser-shielding effect. Similar results were obtained by Peter and Noll [47], which studied the ablation of material and the plasma emission induced by a single pulse and two pulses (λ = 1064 nm, SP 1x80 mJ, DP 2x40 mJ, Δτ = 6 μs, τ = 20-40 ns) on iron samples at different argon pressures. The measured ablation rate confirms the spectroscopic trends shown in Figure 11, i.e. the DP mass removal is much more efficient than SP one at pressures higher than 100 mbar (DP

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G. Cristoforetti and V. Palleschi

ablation rate is larger by a factor 4 at 1 bar) but the enhancement vanishes at pressures lower than such a value. Wang et al. [39], operating on a steel target at quite different experimental conditions (laser energies in the order of the mJ, λ = 1047 nm, Δτ = 52 ns, τ = 21 ns), also investigated the effectiveness of drilling in SP and DP modes at different air pressures, finding that the SP ablation rate at the pressure of 8 mbar is close to that produced in DP configuration in the open air.

Figure 11. On the left, Cu I 521.5 nm line intensity versus the air pressure obtained by using a single pulse (triangles), two coincident pulses (squares), and two laser pulses separated by 700 ns (circles). The acquisition windows are delayed by 2 μs with respect to the single laser pulse and to the second pulse. On the right, enhancement factor of Cu I 521.5 nm line expressed as the ratio of emission signal obtained in double pulse configuration (Δτ=700 ns) over that obtained with two simultaneous pulses. Taken from Ref.[30]

Figure 12. O I 777.3 nm line intensity versus the air pressure obtained by using a single pulse (triangles), two coincident pulses (squares), and two laser pulses separated by 700 ns (circles). The acquisition window is delayed by 700 ns with respect to the single pulse and to the second pulse. Taken from Ref. [30]

Such results suggest that the effects produced by DP-LA are strongly correlated with the environmental gas density in which the plume forms and expands. SP-LA at atmospheric pressure is affected by a strong laser shielding, i.e. a large part of the laser energy is absorbed before reaching the target, resulting in less ablated mass and a proportionally lower emission;

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

19

a stronger laser-target coupling in SP-LA is obtained at reduced pressure, where the optimal situation is achieved at around 100 Torr. On the contrary, in DP case, the second laser pulse ablation occurs in a rarefied medium, where the laser shielding is reduced and consequently the mass removed and the plasma emission are higher. Results slightly contradicting this interpretation were obtained by Krstulovic et al. [48,49] (Ti target, λ = 1064 nm, τ = 5 ns), which showed a 2/3-fold enhancement of crater volume and particle number density in the plasma in collinear DP-LA in vacuum (where evidently the rarefaction effect of the first laser pulse is not present). Such results are observed for interpulse delays shorter than those obtained in DP-LA at atmospheric conditions, where the optimal delay Δτmax for increasing crater volume and plasma number density were 370 ns and 1 μs, respectively. The authors suggest that such effects are caused by a lowering of the target ablation threshold produced by the heating produced first laser pulse. These results thus suggest that the pre-heating of the target, causing a modification of its optical properties (reflectivity, absorption) and a decrease of its ablation threshold, might have a role, though maybe not dominant, also in the DP-LA at atmospheric pressure. A more accurate picture of the processes occurring in the collinear configuration will be given in next sections, since we believe the discussion will be also useful for the understanding of the pre-ablation orthogonal DP-LA configuration.

b) Orthogonal Beams Pre-Ablation Configuration In the orthogonal pre-ablation (or pre-spark) configuration (Figure 1b) the first laser pulse is sent parallel to the target surface and focussed in front of it, producing a plasma, while the second pulse passes through it and ablates the target. The plasma produced by the first pulse, thus, is just composed by species deriving from the ambient gas and only the second pulse removes mass from the target. In this way, the role of the first pulse is only that of preparing the environmental conditions in which the successive ablation is more effective, because of a stronger laser-target coupling. From a practical point of view, such configuration is probably less viable than the collinear one because of the higher complexity of the apparatus. However, it is more suitable for understanding the processes occurring in the DP-LA, due to the clear separation between the roles of the two pulses. From the following discussion it will become clear that the effects produced by this configuration are similar to the ones shown in the previous paragraph; this suggests that the primary cause of DP-LA effects in the two cases is the same, as will be discussed in section 3. The pre-ablation DP scheme was proposed by the Angel’s group [50-53], who used two Q-switched Nd:YAG lasers (λ = 1064 nm, τ = 7 ns) both for producing the pre-spark (pulse energy = 210 mJ) and the ablation (pulse energy = 100 mJ). The first pulse was focussed 1-2 mm above the target and did not produce any appreciable ablation from the surface. Such configuration led to LIBS signal enhancement both in the case of metal target (11-fold and 33-fold for Cu and Pb, respectively [50], 16-fold for Zn [53]) and of non-conducting targets (from 11-fold to 20-fold for Ti, Al and Fe in glass [51]). The maximum enhancement was obtained for an interpulse delay of ~2.5 μs; however, the enhancement is strong for Δτ values up to ~100 μs and persists even at ~300 μs. An increase of plasma temperature up to ~5000 K was observed as well as an increase of crater volume up to ~30 times with respect to SP-LIBS

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G. Cristoforetti and V. Palleschi

(in this case the comparison refers to the case where only the second pulse operates). Both these effects show a clear dependence on the interpulse delay similar to that obtained for signal enhancement; however, the authors suggest that the primary cause of line emission enhancement is the raise of ablated mass rather than the temperature increase [52]. The authors also noted that the largest enhancement was observed for lines originating from transitions departing from high energy levels; evidently, such effect is produced by the increase of plasma temperature from SP to DP configuration. Similar results were obtained by Gautier et al. [54] who performed DP pre-ablation LA on an Al target, using two Q-switched Nd:YAG lasers (λ = 1064 nm, τ = 9 ns for producing the pre-spark and λ = 532 nm, τ = 9 ns for the ablation). The largest line enhancements were obtained in an interpulse range going from 10 to 35 μs and for lines originating from high energy levels. Lindner et al. [55] (Cu-Zn target, λ = 1064 nm, τ = 8 ns) measured the size distribution and the composition of the particles generated in pre-ablation DP experiments performed in atmospheric argon. While in SP scheme the proportion of large particles (> 0.1 μm) was predominant, ultrafine aerosols particles (< 50 nm) were generated in DP configuration representing practically the total mass impacted. Since ultrafine particles form through vapour-phase condensation while large clusters points to a fragmentary mass removal mechanism, the authors conclude that the DP scheme provides a better atomization, close to 100%, of the ablated matter. According to the authors, thus, the signal emission enhancement is produced by the increase of both the ablated mass and its atomization. In the pre-ablation configuration, the value of the distance d of the pre-spark from the target surface can be adjusted, which constitutes an additional experimental parameter to be optimized, with respect to the collinear scheme. The influence of the d-value was studied by our group in a previous paper [56], where the effect of its variation in a range between 0.1 and 4.2 mm together with that of the variation of the interpulse delay, were investigated by spectroscopic and shadowgraphic approaches. The importance of studying the influence of the d-value resides in the information on the DP-LA mechanisms which can be disclosed; in particular, in the hypothesis that DP effects are produced by atmospheric effects, the effects in the pre-ablation scheme should reduce to those obtained in the collinear case when the dvalue approaches zero. In the work cited above, the emission spectra of the air spark could not evidence a detectable features from target species even in the case of d = 0.1 mm, indicating that the target ablation was negligible for all the values of the parameter d. The laser sources were two Nd:YAG lasers, each one emitting a laser pulse in 8 ns FWHM at the wavelength of 1064 nm. The energies of the first and second laser pulses were adjusted to 140 and 240 mJ, respectively. A brass target with ~60% Cu, ~ 40% Zn was used. Spectroscopic analysis of plasma emission reveals that a significant signal enhancement for the neutral and ionic lines is observed for distances d lower than 1 mm. This behaviour is highlighted by plotting the maximum enhancement (obtained at different interpulse delay values) vs. distance d, as shown in Figure 13 for neutral and ionic Zn lines, where a sharp reduction occurs around the value d = 1.0 mm.

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

21

Figure 13. Maximum signal enhancement of Zn I 472.2 nm and of Zn II 255.8 nm vs. the distance d between the air spark and the target surface. Taken from Ref.[56].

Figure 14. Intensity enhancement of Zn II 255.8 nm line vs. the interpulse delay time obtained at different values of the distance d. Taken from Ref.[56]

Other interesting information could be drawn from the behaviour of the signal enhancement with the interpulse delay time. It is evident from Figure 14 that the signal enhancement begins to increase already at a delay time of 100 ns for d=0.1 and 0.4 mm while

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for increasing distances the signal enhancement occurs at progressively higher interpulse delay times. Moreover, for the higher values of the distance d, a reduction of the signal is visible (DP/SP signal ~0.35–0.6 for the Zn I 472.2 nm line and DP/SP signal ~0.10–0.4 for the Zn II 255.8 nm line) at values of the interpulse delay Δτ immediately before the onset of signal enhancement. A similar signal reduction was previously found by Stratis et al. [50] who hypothesized a shielding of the sample from the ablation laser pulse by the air plasma. The calculation of plasma thermodynamic parameters showed that signal enhancement is associated to a modest increment of the temperature (generally lower than 1000 K), when present, and to a marked decrease of the electron density. By hypothesizing Local Thermal Equilibrium and by considering Saha equation, such features lead to a higher plasma ionization, which is consistent with the larger enhancement observed for ionic lines with respect to atomic lines. In order to understand the origin of signal enhancement we estimated the increment in the total number of emitting atoms (as an indication of the ablated mass), from the intensity of the Zn 472.2 nm line and the calculated values of temperature and electron density. The socalculated ratio of mass ablated in DP over that in SP, is reported in Figure 15 for all the combinations of the interpulse delay Δτ and the distance d used. Again, a sharp reduction of the ablated mass enhancement for values of the distance d larger than 1 mm is evident. A comparison of Figure 15 with Figure 14 shows clearly that the general behaviour of the emission signal with the interpulse delay is the same of that of the ablated mass, suggesting that the origin of the signal enhancement is mainly the higher ablated mass in the DP configuration, as already observed in the collinear DP experiments. For studying the dynamic evolution of the plumes obtained by using different combinations of d-Δτ values, we acquired shadowgraphic images using a Hadland Photonics frame camera, back-illuminating the plasma during its evolution with a white light source. It was found that the plasma evolution is strongly dependent on the choice of the d-Δτ values, as can be observed from Figure 16 a–d.

Figure 15. Enhancement of the atomized ablated mass as a function of interpulse delay and at different values of the distance d. Taken from Ref. [56]

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Figure 16. Shadowgraphic images showing the evolution of the plume and of the first and second shock waves produced with an interpulse delay of 2 μs and a distance d of 4.2 mm (a), 2.8 mm (b) 1.9 mm (c), and with an interpulse delay of 4 μs and a distance d of 0.7 mm (d). The temporal delay between the frames is 500 ns. Taken from Ref.[56]

The four images nearly represent all the different situations which can occur in the orthogonal pre-ablation DP scheme. In all the cases, both the pre-spark and the ablation plasma produced near the surface by the second laser pulse are clearly visible, as well as the two shock waves SW1 and SW2 departing from them. In case a) the ablation laser is fired before the shock wave SW1 generated by the air spark arrives on the target surface. The expansion of the ablation plume and of the shock wave SW2 shows a slow expansion in the direction perpendicular to the surface and a faster expansion in the radial direction. SW2 slows down because it expands in a medium flowing in the opposite direction and, at early times, with a higher gas density at SW1 front (Sedov self-similar solution predicts that the SW radius r and the ambient gas density ρ are related to each other by r ∝

ρ −1 / 5 ). It is then clear that the expansion of both the second plume and

SW2 is faster in the radial directions, where there is no density increase with respect to atmospheric conditions. As a result the ablation plume and SW2 expand assuming a flat shape similar to a disc and do not coalesce with the air spark. In case b), the second pulse is fired just after the arrival on the target surface of the shock wave SW1. As in the previous case, the ablation plume has initially a flat shape, probably due to a slowdown caused by the interaction with the SW1 front; however, in this case, SW2 and the ablation plume find soon the rarefied slow moving region located beyond the SW1 front and in the middle of the air spark and accelerate their motion, assuming an elongated shape. This results in the coalescence of the two plumes; the mixing of the two plumes leads to a reheating of the air spark, evident by the stronger brightness of air plume in frames 5,6,7,8 of Figure 16b with respect to Figure 16a.

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In case c), the ablation laser pulse is fired well after the time taken by SW1 to reach the target. Here, the situation is similar to case b) but the elongation is much less evident since the rarefied region of the first plume is nearer to the target. The two plumes rapidly coalesce and the two shock waves become almost concentric. Finally, in case d) the ablation laser is fired well after both SW1 and also the air plasma have reached the target. In this case, the evolution is similar to case c) but the final plume is larger and much brighter even for larger values of the interpulse delay (frames 8–10 in Figure 16d). A rough estimation of the maximum radius of the air spark from the shadowgraphic images (the bright region coincides with the continuum emission zone) gives a value ~1 mm. By comparing this value with the range of distances d, derived from spectroscopic analysis, for which a large signal enhancement is obtained, it is possible to conclude that the large signal enhancement corresponds to the case d). In other words the d-Δτ values result in a considerable signal enhancement if the target ablation occurs inside the region of the air spark (i.e. the air plasma has reached the surface) and not only in the region encompassed by the shock wave SW1. Such result is significant for understanding the causes leading to the signal and the mass removal enhancement observed in this DP scheme and, for the similarity of the results, in the case of collinear pulses and will be discussed in section 3.

c) Orthogonal Beams Re-Heating Configuration The orthogonal re-heating configuration (Figure 1c), strictly speaking, is not a DP-LA technique since it does not produce a raise of the mass removed from the target. In this case, the interaction of the second laser pulse with the plume produced by the first pulse leads to a re-heating of the plasma and thus to an emission enhancement, that can be exploited in LIBS measurements. The scheme was originally introduced by Uebbing et al. [57] (Al and Mn in glass matrix; Mg and Mn in glass, copper and aluminium matrices, λ = 1064 nm, E1= 13 mJ, E2 = 115 mJ, τ1 = 8 ns , τ2 = 5 ns), with the purpose of improving the internal standardization effectiveness for different matrix targets. Uebbing et al., aiming at obtaining reliable quantitative analyses by SP LIBS, noted that the usage of the same calibration curve, obtained by internal standardization, for different matrices lead to large errors because of the different temperatures of plasmas induced on different matrices and because of the selective volatilization of particles and droplets of different elements in the plasma (e.g. Cu and Zn in brass, which have very different values of vapour pressure). The second problem could be overcome by choosing a long delay time of signal acquisition (tens of μs) when the atomization process is ended; however, the plasma emission in that temporal range is often very low and scarcely useful for analytical measurements because of the decay of the temperature. The authors, thus, proposed the re-heating of the atomized mass by a second laser pulse for reducing the problems related to fractionation of materials; at the same time, the temperature differences in plasmas induced on different matrices should also reduce because the temperature of the second plasma is only affected by the second pulse features (energy, duration and wavelength) and not by the properties of the target (reflectivity, thermal conductivity, absorption, etc.). By using the re-heating DP scheme in an Ar rarefied

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environment, with an interpulse time Δτ of 40 μs, Uebbing et al. demonstrated the validity of internal standard usage over three orders of magnitude of concentrations for metals embedded in different matrices (Cu and Zn in brass; Mg and Mn in glass, copper and aluminium matrix; Al and Mn in glass and steel). The effectiveness of DP re-heating configuration for enhancing LIBS signal was investigated by Gautier et al. [54,58] (Al target; ablation laser λ = 532 nm, τ = 9 ns; reheating laser λ = 1064 nm, τ = 9 ns), who studied the influence of the delay between the laser pulses and of the delay time of acquisition. The authors showed that the optimal value of the interpulse delay for enhancing ionic lines is ~200 ns, for which intensity enhancements up to 7 could be obtained. At that interpulse delay value, however, the intensity of neutral lines is reduced, except that of lines deriving from high energy levels which slightly increases. There is no experimental condition for which the intensity of neutral lines is significantly increased. Such results, together with the found increasing trend of line enhancement vs. the upper level energy, are evidently produced by the increase of plasma temperature resulting in the raise of the ionization degree and favouring the population of high-energy atomic levels. As a consequence, Gautier et al. show that the re-heating configuration leads to an improvement of LODs by 2-3 times, if ionic lines are used for the analysis. Despite the modest line enhancement, that is often lower than that obtained by other DP schemes, such configuration can be useful in LIBS technique in cases where it is necessary to reduce the damage on the sample or in case of re-heating of fs-plasmas, which have usually a low temperature and decay much faster than ns-plasmas; this second case, however, will be treated in section 4. A slightly different re-heating approach was proposed by Cheung and coworkers [59-62], where a second laser pulse intercepts and rekindles the ablation plume via resonantabsorption (Resonance-Enhanced LIBS or RELIPS). The technique was probed on different matrix targets (potassium iodate KIO3 pellets containing traces of Na as analyte; sodium bicarbonate NaHCO3 pellets doped with lithium as analyte; Al alloys with Cu, Mg, Pb and Si as analyte) where in all cases the resonant absorption was performed by tuning the wavelength of a dye laser on a particular transition of the major component of the matrix. The character of resonant-rekindling was evident by slightly detuning the wavelength of the dye laser, which resulted in a sharp reduction of the RELIPS signal. Both longitudinal and transversal interception of the second beam were probed, finding that the latter scheme provides signal 6 to 20-times larger signals and lower background noise. For LIBS to perform equally well, the sampling has to be ten times more destructive, which can be undesirable for some applications. The extent of the enhancement was found to depend on type and pressure of ambient gas, where an appropriate choice could result in the confinement and in the thermal insulation of the plasma, leading to a maximization of signal intensity. The sensitivity was found to depend critically also on the beam profile and on the spatial overlap of the laser beams. According to Cheung and coworkers, the primary mechanism of RELIPS enhancement reside in the large volume of re-heated plasma, where, at the opposite, in a classical re-heating DP mode (non-resonant) just the hot spots of the plasma, characterized by a large electron density (the mechanisms is in this case the inverse Bremsstrahlung process), tend to be reheated. The authors argue that this feature constitutes an advantage because it allows an

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uniform and stable final heating since local absorption is automatically capped whenever the excited population of the resonant transition is saturated. To quantify the performance, a mass LOD of about 100 amol for Mg, and a detection sensitivity of about 0.3% of a monolayer of aluminium oxide over a 1 mm2 probed area, were demonstrated.

3. NANOSECOND-NANOSECOND PULSES COMBINATION: DISCUSSION ON THE MECHANISMS While the mechanisms leading to signal enhancement obtained in DP-reheating configuration (either resonant or non-resonant) are clear, the mechanisms operating in collinear and orthogonal pre-ablation DP configurations are still uncertain and need to be discussed [33]. Although the obvious differences in the experimental setups of these last two schemes, strong similarities in the produced effects are obtained, as listed in the previous section, such as the similar line intensity enhancement, the large increase of crater depth and atomized mass in the plume, the scarce increase of plasma temperature, the larger ionization associated to a reduction of the plasma electron density, the similar expansion dynamics of the plume (when the parameter d is lower than 1 mm). Such similarities suggest the working hypothesis that the primary mechanism in collinear and orthogonal pre-ablation is substantially the same, where the latter scheme approaches essentially to the former when the distance of the pre-spark to the target surface tends to zero. As will be discussed in the following, the ‘lower shielding’ mechanism can fit well and is able to explain the gross of the results obtained in both the configurations; on the other hand, a minor role can be played by the pre-heating of the target in the collinear DP scheme. In both collinear and orthogonal pre-ablation schemes, it was found that LIBS enhancement is produced by an increase of atomized ablated mass and by a different expansion dynamics rather than by an increase of plasma temperature. The larger ablated atomized mass results in a larger amount of emitting atoms, which is even higher in case of ions because of the higher ionization degree. At the same time the expansion of the plasma in SP and DP schemes is completely different as remarked by De Giacomo et al.[33]: while a SP plasma expands in a cold environment, exchanging energy with it, a DP plasma expands in a rarefied hot environment composed by the same species, so that it can be considered a ‘closed’ system. In fact the limited energy loss due to dissociation of ambient gas molecules and to excitation of ambient gas species, together to a spatial confinement of the plume at late times by the shock wave SW1 produced by the first laser pulse, results in a slower decay of the plasma and in a condition more favourable to LTE condition. The reduction of plasma electron density, driving a higher ionization of the plume can be explained by two different, probably interplaying, mechanisms. Firstly, the DP plasma expands very rapidly during the early 300-400 ns, due to the rarefied environment encountered, and successively stabilizes to dimensions corresponding to the region encompassed by the shock wave SW1. The expansion results in a geometric reduction of electron (and atom) density which drives a further ionization of the plume. Secondly, the reduction of electron density is due to the lower contribution from the ionization of ambient gas species due to the rarefied environment where the second plasma forms. The contribution

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of electrons due to the ionization of ambient gas species is very uncertain, as well as the amount of the ambient gas species in the plume. However, recent papers suggest that their contribution is not negligible [63] and can be even dominant; in this case, the depletion of ambient gas species inside the bubble formed by the first laser pulse, which can be estimated of the order of 1/10 with respect to atmospheric conditions, results in a reduction of the electron density and thus, considering the Saha-Eggert equation, in a larger ionization of the plume. In both cases, the ionization is due to the fall of three-body recombination rate caused by the drop of electrons in the plasma. The effects produced by the different expansion dynamics of the plumes in the preablation orthogonal DP scheme, with respect to SP case, were analysed by Choi et al.[64]. In their experiment, pre-ablation and ablation laser pulses with significantly reduced energy were applied, so that the increase in ablated mass was negligible. In this condition, both the larger ionization of the plume and its prolonged lifetime, where both the effects are related to ambient gas rarefaction, were evidenced. At the same time, the results by Choi et al. suggest that the contribution to LIBS enhancement due to dynamic effects is modest except than at large acquisition time delays, leading to the conclusion that the primary cause of emission enhancement in DP-LA (operated at larger laser energies) is the increase of atomized ablated mass. At this point, it remains to understand why the DP schemes produce an increase of crater volume and a even higher enhancement of atomized ablated mass in the plume. Two main hypotheses have been suggested in literature, already quoted in the early work by Maher and Hall [4], i.e. the pre-heating of the target operated by the first laser pulse and the ‘lower shielding’ of the second laser pulse due to the environment rarefaction.

a) Target Heating Effects This hypothesis supposes that the second laser pulse hits the target surface when it is still hot and, for short interpulse delays, even melted, which results both in a sharp fall of target reflectivity and in a reduction of the energy to be supplied to reach the evaporation point. In the collinear scheme the heating of the target is evidently produced by the impact of the first laser pulse, which obviously can not apply to the orthogonal scheme. However, it is possible to imagine several other ways in which a LIP formation near a solid can heat the sample surface [65]. The most direct mechanism is the absorption of the broadband emission produced immediately after plasma formation, which can be very effective if there exists a significant overlapping between the absorption bands of the target and the radiative emission continuum. Other possible mechanisms are the target heating by the impact of the shock wave and by the plasma itself, which persists above the surface for several hundreds of microseconds. Bogaerts et al. [66], modelled the Double Pulse Laser Ablation in case of collinear configuration, taking into account the laser-solid interaction, the vapour plume expansion, the plasma formation and the laser-plasma interaction. Since the model was one-dimensional, the maximum interpulse delay considered was only 100 ns, after which the radial expansion of the plume could not more be neglected. By comparing the DP case with the case of SP with the same total energy (Cu target, λ=266 nm, τ = 5 ns, irradiance 0.5 GW cm-2 per pulse),

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Bogaerts et al. found that the surface temperature at the maximum is a bit lower in the DP configuration, because of the lower irradiance of one laser pulse, but it remains high during a longer time, because it rises again upon the second laser pulse. Also the calculated amount of vaporized material is somewhat lower in the DP configuration, which is attributed to the lower surface temperature. However, the only mass removal mechanism included in the model is target vaporization, while melt splashing and phase explosion are not taken in account. Noticing that evaporation depths are much lower than melt depths and even of measured crater drilled depths, the authors suggest that melt effects and phase explosion can bring a major contribution to the ablation process; such role could be enhanced in DP case where the target remains for a longer time in the molten state. The model of Bogaerts et al. shows also that at interpulse delay times larger than 70 ns the target material solidifies again in between the two pulses, which suggests that the preheating effects of the target become unimportant for delay times in the order of μs. This agrees with De Giacomo et al. [33], who noted that for interpulse delay times in the order of μs, the target surface has enough time to reach equilibrium. Dissonant results were obtained by Krstulovic and Milosevic [49] which showed a 3-fold drilling enhancement in case of ns-ns dual-pulse ablation of a titanium target in vacuum, where the pre-heating of the target is the only hypothesis to be applicable. The maximum enhancement was obtained for the interpulse delay of 370 ns, but a significant increase was obtained for delay times up to 20 μs. In order to reduce the effects of target pre-heating in the DP scheme, Maher and Hall [4], using two CO2 lasers (τ ~ 25 μs) tried to misalign one of the laser beams, so that the two spots in the surface did not overlap. When the spots on the target are tangent to one other, the temperature increase caused by the first laser pulse in the point where the second pulse is focussed is negligible. In fact, the thermal diffusion time is larger than the interpulse delay. Moreover, the target heating caused by the interaction with the first plasma is strongly reduced. At the same time, also in the case of non overlapping spots, the expansion of the preablation plume produces a depleted environment in front of the focusing spot of the second laser pulse, so that the presumed effect due to air rarefaction, such as a decrease of laser shielding, is still present. It means that in such geometry the pre-heating of the sample and the environmental rarefaction effects produced by the first laser pulse are separated. The experiment of Maher and Hall revealed that the damage on the target produced by nonoverlapping spots is similar to that produced by a collinear scheme, suggesting that the preheating effect in determining DP outcomes is scarce. In order to extend such results to the ns-ns case, we performed an experiment by using an apparatus similar to that used by Maher et al., by using two Nd:YAG lasers in the fundamental mode onto an Al target [67]. The energy of the first laser pulse was fixed to 90 mJ (fluence ~ 200 J cm-2), the interpulse delay to 1 μs and the energy of the second pulse was scanned in the range 3-125 mJ (fluence in the range 12-502 J cm-2). The enhancement of line intensity, the amount of ablated atomized mass in the plume and their trend vs. laser fluence obtained in the collinear and in the parallel non-collinear (non overlapping spots) schemes are the same, strongly detaching from the values obtained in SP configuration. However, an unexpected strong increase of crater drilling was found in the non-collinear configuration, which was explained by a hydrodynamic draining out from the crater of the aerosol and of the molten material, hindering its re-deposition (see Ref. [67]).

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The above results again confirm that the effects due to the target pre-heating are scarce unless, maybe, for short interpulse delays in the order of a few hundreds of nanoseconds.

b) Atmospheric Effects on Laser Shielding Laser supported detonation and laser ablation The works on the photoablation process begun in the seventies, driven by the need of optimizing material processing techniques. According to the notation used in the literature, laser-matter interaction is characterized by the figure of merit Cm, defined as the ablation pressure over the incident laser intensity or, which is the same, the momentum imparted to the target over the total laser energy, quantifying the mechanical coupling between laser and target [68]. During the photoablation process the momentum can be delivered to the target by the vaporization of material, by the shock wave produced over the surface and finally by spallation or boiling phenomena. By the experimental measurement of Cm parameter in vacuum, mainly by the ballistic pendulum method, it was observed initially by Gregg and Thomas [69], that the mechanical coupling Cm shows a maximum in correspondence of a determined laser irradiance Imax, typically between 108 and 109 W cm-2, and then noticeably decreases at higher laser irradiances. Such threshold is slightly higher than the laser irradiance Ip at which plasma ignition occurs and corresponds to the onset of a strong laser absorption in the plasma which establishes a plasma-mediated photoablation regime. The situation is more difficult to explain when laser ablation is performed in ambient gas environment where, for adequate laser irradiances, the Laser Supported Detonation (LSD) mechanism is ignited and a complex gasdynamic description must be included in the model [70]. In this case, a hot high-pressure plasma, which is initially composed by target species, is produced and drives a shock wave in the surrounding gas. When the laser irradiance is high enough to ignite the LSD mechanism, the passage of the shock wave heats and ionizes a new shell of ambient gas, allowing the laser absorption to occur in it until the plasma state is reached. In turn, laser absorption feeds a further expansion of the shock wave, resulting in a progressive propagation of the plasma plume in the surrounding gas [71]. A further problem arises for the description of LSD ignition in ns-regime laser pulses, since the initiation time of the laser-supported regime is comparable with the pulse duration, so that for a certain range of pulse durations and energies a stationary LSD regime cannot be reached, and a transient regime should be considered. Xu et al. [72] estimated that the LSD initiation time for a Nd:YAG 1064 nm, 10 ns laser pulse in the range of irradiance 108-109 W cm-2 is of the order of 3 ns, though such value is strongly dependent on laser irradiance [73]. Although different mechanisms of laser absorption waves (i.e. blast waves) can occur at irradiances lower than LSD threshold, it was shown by Hettche et al. [74] that the onset of LSD wave corresponds to the beginning of a strong plasma shielding. This occurs because most of the laser energy cannot pass through the high-absorbing region behind the SW front, so that the shielding of the surface can be nearly complete. Therefore, the onset of LSD corresponds to a maximum of impulse coupling Cm parameter which tends to decrease at large laser irradiances. The coincidence of the onset of LSD with strong plasma shielding was

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observed more recently by shadowgraphy and laser drilling experiments by Gravel et al.[75], showing that the strong laser absorption associated to LSD drastically reduces the ablation rate. Moreover, it is expected that the variation of laser-target coupling due to the onset of strong shielding also influences the mechanisms of mass removal and their thresholds. On one hand, melt splashing is favoured by the formation of a hot high-pressure plasma, on the other, phase explosion is hindered by the reduction of the effective irradiance on the target. In a previous work [42], we studied the transitions between different mass removal regimes varying the laser irradiance (λ = 1064 nm, τ = 50 ns, 2.4·108 < Irradiance < 1.2·1010 W cm-2), during the laser ablation in atmospheric air of an Al target. In a subsequent work [43], the investigation was extended to the SP ablation in air at reduced pressures and to the ablation in orthogonal DP pre-ablation configuration. The intensity of atomic and ionic lines and the crater volumes were measured in the different experimental conditions; moreover, the calculation of spatially-averaged temperature and electron density allowed also the estimation of the atomized ablated mass in the plume (see Figure 17).

Figure 17. Drilled volume of the crater and atomized ablated mass vs. laser irradiance used. Taken from Ref.[43]

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By looking at the data obtained in SP configuration at atmospheric pressure, the trends of line intensities (not reported here), atomized ablated mass and crater volume vs. laser irradiance evidence a significant change of regime at ~8-9·108 W cm-2. In a following work [44], where a shorter laser pulse (λ = 1064 nm, τ = 32 ns) was focused on an Al target, the discontinuity was found at a slightly lower irradiance value of ~7·108 W cm-2. At lower irradiances, line intensities, atomized mass and crater volume increases with increasing laser irradiance; on the contrary, above the threshold they markedly fall down, evidencing a less efficient ablation process. The observed discontinuity indicates that a transition occurs between a weak laser absorption in the plasma, beginning at the plasma ignition threshold, and a strong plasma shielding, where the larger part of the pulse energy is absorbed in the plume by inverse Bremsstrahlung processes or reflected at its surface. These latter effects result in a significant reduction of ablated mass. A similar discontinuity in crater drilling, although at larger irradiance values (~ 2.5 GW -2 cm ), was found by Gravel et al. [75], (brass target, λ = 1064 nm, τ = 22 ns) who also showed by shadowgraphic imaging that the threshold is associated to the onset of a Laser Supported wave mechanism. The thresholds obtained in our works [42-44] agree with the power density corresponding to the maximum impulse-coupling coefficient found by Xu et al. [72], calculated using the ballistic pendulum method in the ablation of an aluminium target by a 10 ns 1064 nm laser pulse. Such threshold was again successfully modelled by considering the occurrence of LSD wave with non-negligible initiation times. The morphology of the craters, reported in Ref.[42], suggests that the onset of strong shielding produces also a change in the mechanism of mass removal from the target. For irradiance values lower than LSD threshold, melt effects on the surface were negligible and phase explosion was the only mechanism able to justify the ablation rate (2–4 μm per pulse) found experimentally. This is possible because almost all the laser energy reaches the surface and is able to heat the target surface up to the critical temperature for phase explosion onset. Despite the extensive discussion that appeared in literature about the irradiance threshold of phase explosion, where often much larger threshold values were proposed, striking evidence of phase explosion occurrence in metals was found at values similar to the one obtained here. For example, Porneala and Willis [76] (Al target, λ = 1064 nm, τ = 5 ns), found evidence of phase explosion both by measuring a jump in the ablation rate and by observing a violent ejection of droplets with a shadowgraphic technique, at 1 GW cm-2. Differently, at laser irradiances above the LSD threshold, melt displacement and expulsion progressively become more relevant, as testified by the increase of crater rims formed by re-solidified material and by melt droplets splashed on the target surface around the crater. It is likely that the motion of molten material is the effect of a lateral pressure gradient induced by the high-pressure plasma above the target surface. Phase explosion could be inhibited or become less efficient since, in this range, the ablation rate strongly decreases, despite the increasing melt displacement and expulsion. It is then possible that above the LSD threshold the main mechanisms of mass removal are the melt splashing, which however produces mostly liquid droplets, and vaporization, which produces atomized mass in the plasma. Concluding the paragraph, we want to remark that the occurrence of Laser Supported Detonation mechanism during SP laser ablation in ambient gas strongly affects the process,

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reducing the mechanical coupling with the target and therefore the drilling and the LIBS signal on one hand, and modifying the mass removal mechanism inhibiting the phase explosion, on the other hand.

Laser supported detonation wave and ambient gas pressure Several parameters affect the occurrence of LSD during laser ablation, mainly the target properties and the ambient gas pressure. Maher et al. [77] noted that the laser ablation of samples that absorb much of the incident laser energy, such as SiO2 and Lucite, induces the formation of blast waves instead of LSD waves, resulting in a much lower plasma absorption. According to the authors, this occurs because of the large amount of vaporized matter that pushes and heats the ambient air, so that the absorption blast wave moves perpendicular to the target surface (note that, on the contrary, LSD moves toward the laser beam, an effect which is clearly observable for non normal incidence angles). Also the gas pressure affects both the LSD thresholds and their initiation times. Such behaviour is comprehensible, in view of the fact that the low air pressure corresponds to lowdensity gas, which makes less effective the absorption from air atoms and therefore the feeding of LSD mechanism. Maher et al. [77] calculated the LSD threshold varying the air pressures for different targets, showing that, for metal targets, the LSD ignition is more difficult at pressures lower than the atmospheric, resulting in a higher ignition threshold. This is in accordance with the trends of crater volume and atomized ablated matter obtained at reduced air pressures plotted in Figure 17, showing that the irradiance threshold of strong shielding, resulting in the discontinuity observed in the range 7·108-2·109 W cm-2, slightly increases when air pressure decreases. It is also evident from Figure 17 that the effectiveness of plasma shielding similarly decreases with air pressure, where the discontinuity in the plot is scarcely visible for pressures lower than 300 torr. As a result, the trends of crater volume, atomized matter in the plume, and plasma line intensities (not shown here) have a monotone increasing trend at 100 torr pressure. These results agree with the observation that, in single pulse LIBS, the emission and the ablation rate are maximum when the buffer gas density is lower than the atmospheric one, as shown in Figure 11 and found experimentally by Sdorra and Niemax [78] and by Iida [79]. The same result can be obtained in Figure 17 by choosing a laser irradiance above the LSD threshold and by progressively reducing the air pressure. The key mechanism that must be considered is again the laser shielding operated by the plasma, since a high buffer gas density favours the breakdown-cascade process, the formation of free electrons in the plasma and the absorption of laser radiation. It should also be noted that for irradiances lower than LSD threshold the values in Figure 17 are coincident in all the experimental configurations. This suggests a picture of laser ablation in 2-steps Step 1. From the beginning of the laser pulse to the time where the plasma electron density reaches a critical value, a weak laser absorption occurs in the plume mainly by IB processes. The ablation efficiency seems not to be severely affected by ambient gas density since the ablation rate and the plasma mass are the same in SP and in DP configurations. This agrees with the hypothesis that, at the beginning of laser-target interaction, the plume freely expands in ambient gas without being affected by background gas properties [44,80]; however, the progressive laser absorption in the plume increases the expansion speed of the

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plume and produces a pile-up of vapour atoms at the vapour-background gas interface, the socalled ‘snowplough effect’, which forms a shock wave. The following expansion of the plume and growing rate of electron density is progressively affected by the background density. Step 2. During the trailing part of laser pulse the electron density reaches a critical value producing avalanche ionization and electron cascade, driving a much stronger laser pulse absorption and reflection, and causing a drop of the ablation efficiency. The laser absorption feeds the shock wave expansion so that a Laser Supported wave mechanism begins. In this picture, if laser irradiance is lower than LSD threshold, only Step 1 occurs, and Step 2 is never reached, a condition which explains the similarities of results obtained at different gas pressures in Figure 17.

Laser absorption in double pulse configuration By comparing the plots in Figure 17, it is evident the strong similarity between the data obtained in SP at 100 torr air pressures and those obtained in DP configuration, suggesting that DP effects are mainly related to atmospheric effects. At this point, it is important to give a picture of the environmental situation found by the second laser pulse when it reaches the target. When the first laser pulse ends, the LSD mechanism clearly stops and a blast wave moves in the environment, progressively dissipating its energy for the expansion and for the heating-ionization of the ambient gas. As time elapses, the shock wave, initially in contact with the plasma region, detaches from it and continues to expand, while the plume stops at a radius of the order of 1 mm, depending on the experimental conditions. When all the available energy has been dissipated in the expansion and in the excitation/ionization processes, the pressure of the internal region equalizes with the external one and the shock wave becomes a sonic wave. At times larger than the laser pulse width, the situation can be well described by the point strong explosion theory, formulated by Sedov [81,82], which describes the effect of a large amount of energy delivered in a small volume of a homogeneous atmosphere during a short time interval. According to the theory, during the expansion of the shock wave produced by the sudden release of laser energy, most of the mass of the ambient gas is compressed in a thin layer near its front surface. Therefore, Sedov theory predicts that the shock wave front is characterised by a large mass density pileup, while beyond it the density steeply drops down to values much lower than that in the unperturbed medium. Unfortunately, the Sedov self-similar solution, while it can be effectively used to describe the shock wave radius and velocity, and the profiles of temperature and gas density in the region near the shock front, is unable to give quantitative and reliable values of such parameters in the core of the plasma. In fact, the basic theory of strong point explosion neglects internal heat transfer phenomena such as conduction, radiation as well as excitation and ionization of the gas, and predicts an infinite temperature and a null density in the core of the plume. To correct this problem, more sophisticated models [83,84] accounting for the internal heat transport have been developed. These models predict a finite temperature in the core of the plume. Moreover, considering the ideal gas state equation ρ ∝ P , a non-zero gas T density is also predicted.

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Figure 18. Qualitative sketch of the profiles of pressure, temperature and gas density along the preablation plume and shock wave. Taken from Ref. [56]

A qualitative sketch of the thermodynamic profiles in the air spark region is reported in Figure 18, where the steep increase of temperature at the plume border leads to an abrupt decrease of gas density in the core. The profiles plotted in the figure can also qualitatively represent the situation produced by the ablation of a solid target in air, where evidently the plasma expands from the target surface. The density profile (accounting for both the air and plume atoms) is consistent with the predictions and results of the numerical model by Bogaerts and Chen [85], although it is limited only to the early hundreds of nanoseconds after the laser ablation. The gas rarefaction in the core of the plume can be evaluated from the ideal gas relation:

n plume n atmospheri c

=

Pplume

Tatmospheri c

Patmospheri c

T plume

(2)

According to strong explosion theory, the pressure reaches a maximum PSW at the SW front and then rapidly decreases behind it toward a value Pplume≈0.365 PSW (this value is substantially independent on the blast wave energy down to a SW velocity around Mach 1.5). The corrections to the Sedov model accounting for the internal heat transfer processes do not bring significant variations in pressure profiles, showing at most a slight decrease of the value of Pplume; this can be also perceived from the good agreement of the measured shock wave radius with the predicted one, since the driving force of the shock is essentially the pressure behind it. Moreover, the SW front pressure PSW and the unperturbed pressure Patmospheric are linked by the relation

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

PSW =

Patmospheric (2γM 2 − γ + 1)

γ +1

35

(3)

where M is the SW Mach number and γ=1.4 is the adiabatic coefficient of air [81]. Calculating the Mach number by fitting the shock radius derived from Figure 16, we obtain M~2.2 and then PSW~5.5Patmospheric. Finally, substituting this value in Eq. (2) and considering Tatmospheric=300 K and Tplume~15,000 K (a reasonable value of the spark temperature) we obtain nplume/natomspheric = 0.04 [56]. By considering the images in Figure 7, it is possible to make the same calculations, as in Ref.[86], obtaining the value nplume/natomspheric = 0.07. Such values suggest that the gas density inside the first laser plasma is similar to the density of the gas at room temperature and at 50 torr pressure. At this point, it becomes immediate to observe that the occurrence of LSD mechanism becomes more difficult and less efficient for the second laser pulse, since laser ablation occurs in rarefied gas, resulting in a much lower plasma shielding of the target. Then, the DPLA process is to be compared to a SP-LA at lower air pressure, around 50-100 torr, which explains the similarities of the trends in Figure 17 between DP and SP low pressures configurations. In this way, the DP configuration avoids the onset or, at least, reduces the effects of LSD wave, in particular the strong plasma shielding, resulting in a much larger laser-target coupling and mass removal. In some sense, the first laser pulse in DP-LA has the effect of producing a sort of low-pressure chamber, which optimizes the laser-target coupling, but without the experimental complications inherent to the use of such apparatus. This mechanism is effective if the interpulse delay is larger than ~200 ns [29], when the capability of the first LIP to absorb the second laser pulse becomes unimportant and the rarefaction effects of ambient gas become dominant. This mechanism thus agrees with the range of interpulse delays suitable for generating DP-LA effects (see Figure 4), which are of the order of the plasma decay time. The mechanism proposed also allows the modelling of the influence of the distance of the pre-spark from the target surface. If such distance is large (d>1mm in Figure 13-15), so that only SW1 arrives on the target, this leads initially to a density growth at the surface–gas boundary, but successively the wave reflection leads to a modest gas rarefaction, resulting in a modest line intensity enhancement. However, if the hot plasma region produced by the pre-ablation pulse is able to reach the target surface (d<1 mm), the rarefaction produced at the surface boundary becomes much stronger. In that case, the pre-ablation configuration reduces to that of collinear scheme, the ablation process by the second pulse is very efficient and the rapid expansion of the new resulting plume in a rarefied medium leads to a large and dense plasma region. Finally, this mechanism agrees with the observed different expansion dynamics of DP plasmas generating a strong ionization at early times due to fast expansion and a more steady situation at longer times due to the confinement of SW1.

Mass removal mechanisms in DP schemes The morphology of craters, observed and discussed in Ref.[43], suggests that the DP scheme could also induce a change in the mass removal mechanism from the target. The works in this field almost exclusively refer to metal targets, from which some indication on

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the change of mass removal mechanisms occurring in DP scheme can be extracted. In case of metals, the shape and dimensions of the craters indicate that the strong laser shielding at atmospheric air pressure inhibits or makes less efficient the phase explosion mechanism in SP case [42]. The situation can be different in DP case where phase explosion could be triggered by the large amount of laser energy reaching the target surface. Such hypothesis agrees with the works by Colao et al. [19] and by Mao et al. [29] who hypothesize the occurrence of phase explosion in DP laser ablation. The change from vaporization to phase explosion by applying SP and DP schemes could also explain the relation between DP effects and thermal diffusivity, shown in Figure 10. In fact, supposing that in SP the pure vaporization mechanism is predominant, being it a surface process, the metals with lower thermal diffusivity are favoured in removing material from the target, because of the higher surface temperature reached. Moreover, in SP (where the effective irradiance is low because of the laser shielding) the threshold of the explosive-boiling regime can be more easily reached for metals with lower thermal diffusivity, which dissipate less heating in the interior of the target and for which a subsurface heating might be still possible. Both these effects concur to draw a picture where, other material properties being neglected, in SP configuration a larger ablation of matter is favoured for the lower thermal diffusivity metals. On the other side, in the DP configuration, where a much larger energy reaches the target, a different situation can occur, where all the targets probably reach the phase explosion threshold. Here, the metals with higher thermal diffusivity are favoured in expelling material because of the larger depth of the molten pool, in a situation opposite to that described for SP. The above considerations would corroborate, at least qualitatively, the trend obtained in Figure 10. The situation can be very different for non-metal targets, such as insulators or polymers, which are characterized by very different thermal, electronic and optical properties. In this case, the laser ablation is a volumetric heating process, where the temperature distribution in the target is mainly determined by the laser absorption length rather than by the thermal diffusion. This leads to significant differences in the ablation process, where other phenomena (i.e. the spallation of the target and the absence of LSD wave) can occur and should be taken into account for modelling purposes. Such differences results also in DP effects often dissimilar to those obtained for metals and often in lower emission and ablation rate enhancements (e.g. those obtained for the analysis of soils, rock and pressed pellets). It is therefore evident that a deeper understanding of DP-LA (and even SP-LA) of such targets still needs more and dedicated works. To conclude, it is evident that the atmospheric effects are able to explain most of the effects observed in ns-ns DP configurations, although the pre-heating of the target and the reheating of the plume can play a significant role at interpulse delays of the order of a few hundreds of nanoseconds. A theoretical model of DP laser ablation, reliable for interpulse delays of the order of microseconds and including the different mass removal mechanisms (vaporization, melt displacement, melt splashing, phase explosion), although very difficult to build, would be however very useful to confirm (or reject) the validity of the mechanisms suggested by the experimental results, and summarized above.

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

37

4. DOUBLE PULSE WITH SHORT AND ULTRASHORT LASER PULSES: EFFECTS AND MECHANISMS Some authors investigated the effects induced by the use of short and ultrashort laser pulses, separated by time delays going from hundreds of femtoseconds up to hundreds of microseconds. Because of the different nature of laser-target interaction and for the different mechanisms underlying the DP effects, such results will not be treated in detail in the present chapter. However, we will try to provide a concise summary of the phenomena occurring, as emerging from the literature, with the aim of delineating a general framework of results and mechanisms. For a more complete list of results and related works, the reader could refer to Ref.[1]. Two main reasons drove the studies of DP-LA with short and ultrashort pulses, i.e. the improvement of quality and control of micromachining of materials, avoiding the saturation of the ablation rate and the formation of recast material around the crater at the higher fluences [87], and the enhancement of the emission of a fs-plasma to be used for LIBS applications [12,88-92]. Besides, other possible applications emerged in literature as the enhancement of ion yield [93], the exclusion of air breakdown [94], the enhancement of the performance of Pulsed Laser Deposition techniques [95]. Both collinear and orthogonal beams configurations were tested, where most of the applications, except the enhancement of LIBS emission, involve the double-ablation of the target and then the former scheme. Several factors make at the moment very difficult to delineate a coherent and complete framework of the produced effects and of the underlying mechanisms associated to the collinear configuration. First of all, the mechanisms of mass removal which occur when a short/ultrashort laser pulse is focussed on a target surface are still unclear, where highly nonequilibrium processes need still to be understood. Moreover, it was shown that different ablation regimes occur depending on the laser fluence, evidently associated to different mass removal mechanisms, so that the effects produced by the DP-LA depends on the laser fluences used in the experiment. Finally, the still small number of publications on the subject, where different and often not comparable experimental conditions are used, makes it very difficult to draw an unified scheme of knowledge. Roberts et al.[96] presented a twotemperature ablation model, predicting the time dependence of electron and lattice temperatures in the target, and compared the outcome of the model with the ablation rates on a silver foil obtained using single and double 130 fs laser pulses; laser fluence spanned over a range of three orders of magnitude up to 900 J cm-2 and pulses separation in the range from 0.1 ps up to 3.4 ns. The results allowed the authors to sketch a rough scheme of the mechanisms occurring at different interpulse delays Δτ in collinear geometry, which agrees with the results of other published works. In their scheme several timescales should be taken into account for understanding DP-LA process, i.e. the electron-phonon relaxation time τei, the heat diffusion time τD and the time τT needed for the ejecta or the plasma produced by the first pulse to interfere with the second pulse. For Δτ < τei, when the second pulse is fired the energy of the first pulse has not been still transferred to the lattice of the target and no melting has been produced on the surface. Therefore, two pulses behave as a single pulse of fluence corresponding to the sum of the fluences. A rough estimate of crater depth d at different fluences F can be calculated using the

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G. Cristoforetti and V. Palleschi

relation d = K ln( F / Fth ) [97], where K is a constant and Fth is a threshold fluence; however, since the ablation rate has a non-linear dependence on the laser fluence, due to the onset of different mass removal mechanisms at different thresholds, the increase of crater depth can be much larger than that estimated in such a way [96]. The value of τei calculated by Roberts et al. is in the range 1-10 ps depending on the laser fluence. The results obtained by Semerok and Dotouquet [88] (Al and Cu target, τ = 50 fs, 160 fs, 675 fs) agree with the previous scheme, showing that for an interpulse delay smaller than 1 ps, the craters produced in the DP scheme (E = 2x20 μJ) were almost two times deeper than those produced by a SP (E = 20 μJ); the result does not depend on the duration of the laser pulse. For Δτ > τei, the ablation rate of DP falls with increasing the interpulse delay. The main reasons are the lower temperature of the target surface and the absorption/scattering of laser radiation by ejecta or by the plasma. The onset and the relevance of such phenomena depend on the heat diffusion time τD and on the time τT needed for the ejecta or the plasma induced by the first pulse to interfere with the second pulse. Roberts et al. [96] showed that such decrease begins at interpulse values depending on the laser fluence, in a range between 10 ps at low fluences and 0.1 ps at large fluences. Moreover, the authors evidenced that a completely different behaviour is obtained at fluences larger than 16 J cm-2, probably because of the onset of a different mass removal mechanism. A similar decreasing trend was also found by Semerok and Dutouquet [88] (Al and Cu target, τ = 50 fs, 160 fs, 675 fs), in the range 1 < Δτ < 10 ps, and by Chowdhury et al. [98] (fused silica target, τ=90 fs), in the range 0.1 < Δτ < 10 ps. In all the above works the ablation rate obtained at the lowest point was even lower than that obtained in SP scheme. Chowdhury et al. [98] measured also the transmission of the second pulse varying Δτ. They showed that the observed decrease of ablation rate is due to the absorption of the laser pulse by the plasma in front of the target. A similar conclusion was reached by Semerok and Dutouquet [88], who imaged the plasma plume produced in this temporal range with an ICCD camera, finding a much higher intensity and a higher reproducibility of the plasma emission in DP scheme, where the intensity rises with increasing the interpulse delay. The authors argued that the effect was produced by the re-heating of the plume by the absorption of the second laser pulse. Such plasma re-heating, which is optimal for Δτ ~100-200 ps according to Ref.[88], can be fruitfully used for fs-LIBS applications which are known to suffer from a low plasma emission. An interesting phenomenon observed in this interpulse delay range is also the enhancement of the low-energy ion yield, which can be fruitfully utilized for ion implantation in microelectronics and optoelectronics. Koudoumas et al. [93] (Si target, λ1 = 800 nm, τ1 = 180 fs; λ2 = 248 nm, τ2 = 0.5 ps) showed that the utilization of two ultrashort pulses with and an interpulse delay larger than ~1 ps results in the enhancement of the production of Si+ ions and in the increase of their thermal energy. The authors suggest that the effect is associated to the creation of a melted zone exhibiting modified optical coupling properties, where the second pulse induces a much higher temperature in a thinner surface layer. Many papers concerning DP-approach are devoted to the enhancement of fs-LIBS emission. The fs-LIBS has the potential advantage of reducing the ‘matrix effect’ of the technique and to assure a better stoichiometry of the ablation process. However, it often results in a low sensitivity caused by the lower temperature of the plasma and by its faster decay. In order to overcome these drawbacks some authors probed a double pulse approach

Double-Pulse Laser Ablation of Solid Targets in Ambient Gas

39

where a combination of ns and fs pulses are used. Scaffidi et al. [12] (Fe target, λ = 800 nm, τ = 100 fs; λ = 1064 nm, τ = 5 ns) reported a study where a combination of fs-ns pulses in collinear scheme was used; they found an enhancement of atomic emission at different focus positions, suggesting that different reasons, i.e. plasma-plasma coupling and reduction of air density during the second breakdown event, are the primary causes of such effects. Other experiments devoted to fs-LIBS emission enhancement were performed in the orthogonal configuration, where the processes of laser ablation and plasma re-heating are temporally well separated. Scaffidi et al. [91] (Al and Cu target, λ = 800 nm, τ = 100 fs; λ = 1064 nm, τ = 7 ns) probed different configurations where a fs or ns pulse is used for the ablation and the other is used for re-heating the plasma or for inducing a pre-spark in front of the target. The combination where the fs-plasma is re-heated by a ns pulse shows the largest emission enhancement (~30–fold for the Cu I 501 nm line and ~80 for the Al I 396 nm line) at an interpulse delay of 5 μs. Significant line enhancements were obtained by Santagata et al. [92] (Ti target, λ1 = 527 nm, τ1 = 250 fs; λ2 = 532 nm, τ2 = 7 ns) by using a similar configuration where a fs-plasma is re-heated by a ns pulse; in this case, however, the optimal interpulse delay, depending on the fs pulse energy, is noticeably larger than that found by Scaffidi et al. (Δτ = 500 μs for Epulse = 0.8 mJ and Δτ = 250 μs for Epulse = 3 mJ). On the other hand, the combination where the fs pulse induces a pre-spark in front of the target and the ns pulse ablates the surface shows a modest line enhancement which appears associated to the reduction of air density [90] and then substantially similar to the effects already discussed in the ns-ns orthogonal scheme.

PERSPECTIVES AND FUTURE DEVELOPMENT The large number of papers dealing with DP-LA recently appeared in literature attests the increasing interest on the subject, motivated by its fruitful usage in many applications, among which laser micro-sampling and laser micro-drilling. The experimental and theoretical results reported in the present chapter allow for a quite clear picture of the mechanisms underlying the process of ns-ns Double-Pulse laser ablation of solid targets. However, the quantitative prediction of the enhancement in ablation rates and optical emissions, as well as the theoretical determination of the thresholds observed in varying the laser pulse properties and the environmental characteristics still call for a reliable theoretical model of DP laser ablation, valid for interpulse delays of the order of microseconds and including all the different mass removal mechanisms (vaporization, melt displacement, melt splashing, phase explosion, spallation). Most of the results reported in this chapter concerns the LA of metals in ambient gas, to which prevailingly the above considerations and modelling refer to. The utilization of the DP scheme for the ablation of solid targets immersed in liquid is also very promising for many applications, going from analytical analyses to nanoparticles formation; however, for the different characteristics of the LA process, the issue is not treated here, so that we suggest the interested reader to refer to other specialized publications [14,15]. The understanding of DP-LA of non-metals targets still needs accurate and dedicated works, because of the substantial differences of laser ablation processes, which result also in dissimilarities in DP effects. More works are also needed in the case of DP-LA with short-

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ultrashort pulses, for the still scarce knowledge on the LA process obtained with fs and ps lasers.

ACKNOWLEDGMENTS The authors would like to acknowledge Elsevier B.V. for the use of excerpts and figures published in Spectrochimica Acta Part B. The authors acknowledge also IOP sciences for the use of excerpts taken from Journal of Physics D: Applied Physics.

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G. Cristoforetti and V. Palleschi Krstulovic, N; Cutic, N; Milosevic, S. Spectrochim. Acta Part B, 2009, 64, 271-277. Krstulovic, N; Milosevic, N. Appl. Surf. Sci., 2010, 256, 4142-4148. Stratis, DN; Eland, KL; Angel, SM. Appl. Spectrosc, 2000, 54, 1270-1274. Stratis, DN; Eland, KL; Angel, SM. Appl. Spectrosc, 2000, 54, 1719-1726. Stratis, DN; Eland, KL; Angel, SM. Appl. Spectrosc., 2001, 55, 1297-1303. Angel, SM; Stratis, DN; Eland, KL; Lai, T; Berg, MA; Gold, DM. Fresenius J. Anal. Chem., 2001, 369, 320-327. Gautier, C; Fichet, P; Menut, D; Lacour, JL; L’Hermite, D; Dubessy, J. Spectrochim. Acta Part B, 2005, 60, 265-276. Lindner, H; Koch, J; Niemax, K. Anal. Chem., 2005, 77, 7528-7533. Cristoforetti, G; Legnaioli, S; Pardini, L; Palleschi, V; Salvetti, A; Tognoni, E. Spectrochim. Acta Part B, 2006, 61, 340-350. Uebbing, J; Brust, J; Sdorra, W; Leis, F; Niemax, K. Appl. Spectrosc, 1991, 45, 14191423. Gautier, C; Fichet, P; Menut, D; Lacour, JL; L’Hermite, D; Dubessy, J. Spectrochim. Acta Part B, 2004, 59, 975-986. Chan, SY; Cheung, NH. Anal. Chem., 2000, 72, 2087-2092. Lui, SL; Cheung, NH. Appl. Phys. Lett, 2002, 81, 5114-5116. Lui, SL; Cheung, NH. Spectrochim. Acta Part B, 2003, 58, 1613-1623. Yip, WL; Cheung, NH. Spectrochim. Acta Part B, 2009, 64, 315-322. Cristoforetti, G; Lorenzetti, G; Legnaioli, S; Palleschi, V. Spectrochim. Acta Part B, 2010, 65, 787, 796. Choi, SC; Oh, MK; Lee, Y; Nam, S; Ko, DK; Lee, J. Spectrochim. Acta Part B, 2009, 64, 427-435. Scaffidi, J; Angel, SM; Cremers, DA. Anal. Chem., 2006, 78, 24-32. Bogaerts, A; Chen, Z; Autrique, D. Spectrochim. Acta Part B, 2008, 63, 746-754. Cristoforetti, G; Legnaioli, S; Palleschi, V; Tognoni, E; Benedetti, PA. Appl. Phys. A, 2010, 98, 219-225. Phipps, CR; Turner, TP; Harrison, RF; York, GW; Osborne, WZ; Anderson, GK; Corlis, XF; Haynes, LC; Steele, HS; Spicochi, KC. J. Appl. Phys., 1988, 64, 1083-1096. Gregg, DW; Thomas, SJ. J. Appl. Phys., 1966, 37, 2787-2789. Pirri, AN. Phys. Fluids, 1973, 16, 1435-1440. Root, RG. Modeling of post-breakdown phenomena,, in: J; Radziemski, A. Cremers, (Eds.), Laser-induced Plasmas and Applications, Marcel Dekker, New York, 1989, 69103. Xu, B; Wang, Q; Zhang, X; Zhao, S; Xia, Y; Mei, L; Wang, X; Wang, G. Appl. Phys. B, 1993, 57, 277-280. Walters, CT; Barnes, RH; Beverly III, RE. J. Appl. Phys., 1978, 49, 2937-2949. Hettche, LR; Tucker, TR; Schriempf, JT; Stegman, RL; Metz, SA. J. Appl. Phys., 1976, 47, 1415-1421. Gravel, JFY; Boudreau, D. Spectrochim. Acta Part B, 2009, 64, 56-66. Porneala, C; Willis, DA. Appl. Phys. Lett, 2006, 89, 211121. Maher, WE; Hall, RB; Johnson, RR. J. Appl. Phys., 1974, 45, 2138-2145. Sdorra, W; Niemax, K. Mikrochim. Acta, 1992, 107, 319-327. Iida, Y. Spectrochim. Acta Part B, 1990, 45, 1353-1367. Wen, SB; Mao, X; Greif, R; Russo, RE. J. Appl. Phys., 2007, 101, 023115.

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[81] Sedov, LI. Similarity and Dimensional Methods in Mechanics, CRC Press LCC, Moscow, 1993. [82] Zel’dovic, YB; Raizer, YP. Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena, Academic Press, New York, 1987. [83] Abdel-Raouf, AM; Gretler, W. Fluid Dyn. Res., 1991, 8, 273-285. [84] Ghoniem, AF; Kamel, MM; Berger, SA; Oppenheim, AK. J. Fluid Mech., 1982, 117, 473-491. [85] Bogaerts, A; Chen, Z. Spectrochim. Acta Part B, 2005, 60, 1280-1307. [86] Corsi, M; Cristoforetti, G; Giuffrida, M; Hidalgo, M; Legnaioli, S; Palleschi, V; Salvetti, A; Tognoni, E; Vallebona, C. Authors’ reply to Wen et al.’s comment, Spectrochim. Acta Part B, 2005, 60, 872-875. [87] Le Harzic, R; Breitling, D; Sommer, S; Fohl, C; Konig, K; Dausinger, F; Audouard, E. Appl. Phys. A, 2005, 81, 1121-1125. [88] Semerok, A; Dutouquet, C. Thin Solid Films, 2004, 453-454, 501-505. [89] Scaffidi, J; Pearman, W; Carter, CJ; Colston Jr., BW; Angel, SM. Appl. Optics, 2004, 43, 6492-6498. [90] Scaffidi, J; Pearman, W; Lawrence, M; Carter, CJ; Colston Jr., BW; Angel, SM. Appl. Optics, 2004, 43, 5243-5250. [91] Scaffidi, J; Pender, J; Pearman, W; Goode, SR; Colston Jr., BW; Carter, CJ; Angel, SM. Appl. Optics, 2003, 42, 6099-6106. [92] Santagata, A; Teghil, R; De Giacomo, A; Dell’Aglio, M; Parisi, GP; De Bonis, A; Galasso, A. Appl. Surf. Sci., 2007, 253, 7792-7797. [93] Koudoumas, E; Spyridaki, M; Stoian, R; Rosenfeld, A; Tzanetakis, P; Hertel, IV; Fotakis, C. Thin Solid Films, 2004, 453-454, 372-376. [94] Mannion, PT; Magee, J; Coyne, E; O’Connor, GM. Proc. SPIE, 2003, 4876, 470-478. [95] Pronko, PP; Zhang, Z; VanRompay, PA. Appl. Surf. Sci., 2003, 208-209, 492-501. [96] Roberts, DE; du Plessis, A; Botha, LR. Appl. Surf. Sci., 2010, 256, 1784-1792. [97] Chichkov, BN; Momma, C; Nolte, S; von Alvensleben, F; Tunnermann, A. Appl. Phys. A, 1996, 63, 109-115. [98] Chowdhury, IH; Xu, X; Weiner, AM. Appl. Phys. Lett, 2005, 86, 151110.

In: Laser Ablation: Effects and Applications Editor: Sharon E. Black

ISBN: 978-1-61122-466-5 © 2011 Nova Science Publishers, Inc.

Chapter 2

ASSESSING HUNTER-GATHERER MOBILITY IN CIS-BAIKAL, SIBERIA USING LA-ICP-MS: METHODOLOGICAL CORRECTION FOR LASER INTERACTIONS WITH CALCIUM PHOSPHATE MATRICES AND THE POTENTIAL FOR INTEGRATED LA-ICP-MS SAMPLING OF ARCHAEOLOGICAL SKELETAL MATERIALS Ian Scharlotta, Andrzej Weber, S. Andy DuFane, Olga I. Goriunova and Robert Creaser University of Alberta, Edmonton, Alberta, Canada

ABSTRACT Micro-sampling and analysis of tooth enamel from faunal samples in the archaeological record has enabled research into the mobility and seasonality of animals in prehistory. However, studies on human tooth samples have failed to yield similar results. It is well understood that human tooth enamel does not fully mineralize in a strictly linear fashion, but rather entails five recognizable stages of mineralization. Until the enamel matrix fully crystallizes, the matrix remains an open chemical system, thus at each stage of mineralization, the geochemical composition of the enamel matrix can be altered. At present it is unclear if failure to mirror the results from faunal teeth with human teeth is a factor of mineralization rates or simply the result of the difference in enamel volume and formation time between human and herbivore teeth. Therefore, the applicability of chemical analyses to human teeth is a balance between micro-sampling analytical techniques and generating archaeologically relevant data. Yet limited case studies have been performed to examine the scale and extent of this problem in human teeth using laser-ablation ICP-MS. Five human molars from an Early Bronze Age cemetery on the shores of Lake Baikal, Siberia were serially sampled and analyzed by means of laser-ablation quadrupole and multicollector ICP-MS in order to examine the

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Ian Scharlotta, Andrzej Weber, S. Andy Dufane et al. nature of geochemical changes within the enamel matrix. This sampling was performed in order to generate a statistically significant dataset to assess the effectiveness of two approaches along with published methodologies to counter known problems with attempts to assess Sr87. Recent research has demonstrated that among the methodological problems, there is isobaric interference at mass 87 caused by the formation of calcium phosphate (Ca40PO) in response to interaction between the laser and the enamel matrix. Correction procedures using Zr91 in tandem with Ba/Sr ratios are examined. Additionally, serial sampling of teeth from hypothesized mobile hunter-gatherers provides useful insight into the dynamic interplay between physical sampling limitations and the scale at which useful geochemical data can be recovered from organic minerals. Traditional utilization of geochemical data for mobility has relied on a local/non-local dichotomy in population level analyses; however, this approach is of limited utility with regard to mobile populations. Our ability to effectively analyze skeletal materials at a micro scale provides our best hope at addressing the rift between recognition of an indirect relationship between biological intakes, mineral formation and being able to generate relevant analytical data.

INTRODUCTION Strontium isotope analysis has traditionally relied on thermal ionization mass spectrometry (TIMS) due to its reliability and analytical precision. The advent of multicollector inductively-coupled-plasma mass-spectrometry (MC-ICP-MS) as an alternative to TIMS coupled with either a laser microdrill or a laser ablation (LA) unit for micro-sampling greatly expanded the possibilities for archaeometric research of Sr isotopes. Both laser microdrills and laser ablation are far less destructive and enable higher spatial resolution for analysis than traditional TIMS and MC-ICP-MS methodologies, however micro-sampling for solution preparation still requires significant lab handling for sample preparation whereas laser ablation requires virtually no special handling [1, 2]. In spite of several studies using LA on human bone and high-Sr apatites [3, 4], analysis of phosphate minerals by LA has not figured prominently in the scientific literature until recently [1, 5-10]. The goal of this study is to examine problems associated with the application of laser ablation as a sample introduction method for MC-ICP-MS on human skeletal materials. Previous research[1, 8, 10] has indicated a number of potential problems in gathering accurate strontium isotopic data from calcium phosphate matrices using LA-MC-ICP-MS. In addition to interference from rubidium (87Rb), doubly charged rare earth elements [11], and calcium dimers [10], there is the production of a polyatomic species of CaPO that interferes with the [87] Sr [1, 8, 9]. This polyatomic species is apparently unique to laser ablation as it is not apparent with solution mode (SM) MC-ICP-MS. The exact source of this polyatomic species is uncertain; however it is a relatively minor contributor of isobaric mass 87 to most geological samples. Unfortunately, many archaeological skeletal samples have very low concentrations of Sr and are thus susceptible to significant error terms as a result of this polyatomic species while using LA-MC-ICP-MS. Traditional Sr isotopic research has focused primarily on sedentary agrarian groups. With such groups, the focus of research is on identifying the local signature so that nonlocals (people, animals, etc.) can be recognized. Such an approach is perhaps of only limited utility for the broader study of hunter-gatherers, as many groups utilized large ranges of territory and

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would thus have more complex (averaged over larger areas) isotopic signatures reflecting their lifetime mobility. Such complexity drives interest in micro sampling of skeletal materials to access greater chronologically refined insight into mobile individuals. However, further research is needed to fully understand the dynamic interaction between direct chemical interaction with the biologically available strontium, the formation of skeletal tissues, and data recovery from these tissues. This study is focused on the data recovery side of this problem, examining the range of variability in strontium isotope ratios and trace element composition found within human teeth.

HUNTER GATHERER MOBILITY IN CIS-BAIKAL The Cis-Baikal region of Siberia denotes the geographic region including the western coast of Lake Baikal, the upper sections of the Angara and Lena river drainages, and the Tunka region adjacent to the southwestern tip of Lake Baikal (approximately between 52° and 58° N and 101° and 110° E). The topographic complexity of the rift valley that formed Lake Baikal led to the formation of a large number of microhabitats, with a variety of seasonally available resources [12-14]. The thermal capacity of Lake Baikal itself moderates the local climate, resulting in generally milder temperatures during the winter and cooler temperatures during the summer. As a result, the Angara River Valley remains relatively free of snow during the long winter which attracts various species of ungulates looking for forage and less restricted mobility [15]. There is a variety of large game found in the region including moose (Alces alces), red deer (Cervus elaphus), roe deer (Capreolus capreolus pygarus), reindeer (Rangifer tarandus), and mountain goat (Capra sibirica). Smaller species such as hare (Lepus sp.), suslik (Spermophilus citellus), wild boar (Sus scrofa sibiricus), marmot (Marmota sibirica), geese, and other waterfowl are also abundant in many areas around the lake. During the summer, large runs of black grayling (Thymallus arcticus) are found in the first section of the Angara River, and several fish species enter the tributaries of the Angara in large numbers to spawn. The shallow coves and bays in the Little Sea region of Lake Baikal, between Ol’khon Island and the west coast of the lake, also provide excellent opportunities for fishing and during the late winter when the lake is frozen, nerpa, the Lake Baikal seal (Phoca sibirica) can be hunted [16-19]. Ethnographic studies of boreal forest populations highlight the use of mushrooms, berries, and pine nuts as other non-medicinal resources [15, 20, 21]. There is very limited evidence for plant use during the Neolithic, however there is sufficient ethnographic evidence for the role that plants play in boreal forager subsistence systems around the world to speculate upon their usage. Within the Cis-Baikal region there are four main geological zones that roughly overlap with archaeological micro-regions (Figure 1). The main zones are 1) the Baikal basin, including the lake itself, the coastal areas as well as the Little Sea area enclosed by Ol’khon Island; 2) the drainage of the upper and middle Angara River bounded by the Eastern Sayan Mountains to the west and the Central Siberian Plateau to the east and extending north towards Bratsk; 3) the upper Lena river basin cutting through the Central Siberian Plateau as it heads northwards; and 4) the Tunka region covering a sizeable valley running south of the Eastern Sayan Mountains and broadly connecting the southwestern tip of Lake Baikal to Lake Khovsgol in Mongolia. The upper and middle sections of the Angara River flow through

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Mesozoic and Quaternary deposits, with expected 87Sr/86Sr values in the range of 0.7050.712. The upper Lena watershed and the surrounding Central Siberian Plateau are dominated by Cambrian and Precambrian limestones, with expected values fairly tightly clustered around 0.709 [22, 23]. Overall values for Lake Baikal water are reported as 0.7085 [24]. The Baikal basin includes the Primorskii and Baikalskii mountain ranges and is characterized by relatively high 87Sr/86Sr (~0.720-0.735) due to the presence of Archean and Proterozoic granites [12]. Bedrock of similar ages occur around the southwestern shores of Lake Baikal and drainages adjacent to the Eastern Sayan Mountains, however our preliminary data for environmental sampling of biologically available strontium isotopes in the Cis-Baikal region indicate that these two regions have quite different 87Sr/86Sr values [25]. Both zones overlap 87 Sr/86Sr ranges of neighboring regions (e.g., Angara Drainage), while only the Little Sea area exhibits values above 0.720. Further clarifications of the distinction between these two zones of similar age will be possible upon completion of regional sampling efforts.

Figure 1. Lake Baikal, Siberia showing the location of the KN XIV cemetery, cultural micro regions, and the age of the dominant bedrock formations

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KHUZHIR-NUGE XIV CEMETERY The KN XIV cemetery is located on the west coast of the Little Sea micro-region of the Lake Baikal basin, near the southern end of Ol’khon Island and c. 3 km southwest of the mouth of the Sarma River (53°04’58” N, 106°48’21” E). It occupies the southeast slope of a hill rising from a shallow bay. With 79 graves and a total of 89 individuals unearthed, KN XIV is the largest Early Bronze Age hunter-gatherer cemetery ever excavated in the entire Cis-Baikal region (Weber et al. 2007). All the graves were only c. 30-60 cm deep subrectangular pits filled with rocks and loamy sand, and covered by surface structures built of stone slabs still visible on the surface prior to archaeological excavation. Most graves contained single inhumations, seven were double, and two were triple interments. The north-south orientation of Grave 7 is consistent with the Late Neolithic Serovo culture of the Ol’khon region, while all the other graves show clear similarities with the mortuary tradition of the Early Bronze Age Glazkovo culture [22, 26, 27]. The most diagnostic Glazkovo characteristics include the generally west-east orientation of the burials and such grave goods as copper or bronze objects (rings, knives, needles, and bracelets), kaolinite beads, and rings and discs made of white nephrite or calcite [28]. A recent analysis of approximately 80 14C dates indicates that the KN XIV cemetery was used continuously by Glazkovo peoples for a maximum of 700 years between ~4650 and 3950 cal. BP but the majority of the burials (70%) date to between ~4450 and 4250 cal. BP [29]. Since the analysis did not reveal any obvious temporal trends in mortuary attributes, it seems to be justified to treat the cemetery with the exception of the much earlier Grave 7, as one analytical unit (McKenzie 2006; Weber et al. 2005). In previous studies [15, 22, 26] a sample of 25 individuals from KN XIV were analyzed for strontium isotope ratios and compared with 79 faunal samples collected throughout Lake Baikal and the Cis-Baikal region. Of these samples, there were 20 adult individuals for which all three molars and a femur sample were available, 5 subadult burials with only M1 and M2 crowns completed were included too. For the latter individuals (Burials 16, 35.2, 37.2, 39 and 45) the M3 was either not yet formed, or still forming. Partly developed crowns were not examined because such crowns are incompletely mineralized and this could have affected the isotope ratios. The five molar samples used for this research came from this pool of previously studied materials. Teeth from Burials 7, 12, 16, 35.1, and 35.2 were used in this study to provide continuity and comparability with previous studies on KN XIV. This previous work has helped to expand the possible applications of strontium isotope research and helped to identify an interesting general pattern with several mobility profiles within KN XIV individuals. Broadly speaking, it appears that there was a significant amount of movement of individuals during their lifetime, whereby people buried at KN XIV were frequently not born in the Little Sea region, but only migrated there as subadults or adults [15, 22]. There was significant variability within the cemetery itself as to the origin and age of migration to the Little Sea with indications of correlated mortuary patterning that is the subject of ongoing research.

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PRINCIPLE OF STRONTIUM CATCHMENT Strontium ratios in herbivore bone reflect the isotopic signatures in the plants that these animals eat and the water that they drink thus are a direct reflection of their bioavailable geochemical environment. An herbivore foraging range will therefore roughly equate to its Sr-catchment [30, 31]. The situation is slightly different for carnivores as their Sr-catchment will reflect their dietary intake rather than simply their geographical territory. The territory of a predator, human or otherwise, will intersect and contain portions of the territories of numerous prey animals, though will likely not encompass the full procurement ranges of these species. Therefore the strontium ratios of their prey animals may derive from geological regions outside of the predator’s geographic territory. This highlights the important concept of effective geochemistry as a step beyond bioavailable geochemical signatures. Herbivores provide direct translations of bioavailable geochemical values in plants, thus whatever portion of soil geochemistry can be mobilized into the food chain. Carnivores subsist largely or solely on other animals, thus their bioavailable geochemical values will not directly translate into either their actual movements on the landscape or their bounded procurement territory. The only evidence to directly relate carnivores with their physical territory is the water that they drink, any vegetal matter they may consume (e.g., berries), or small animals (e.g., rodents, lizards, etc.) whose entire range will be limited in scale and thus contained with the carnivore’s territory. Thus both human and animal predatory strontium signatures may reflect procurement ranges both larger and different from their actual territories. In geologically diverse regions, interpretation of bioavailable geochemistry can be complicated by the fact that animals with small home ranges (e.g., suslik) may have adjacent geographical territories but exist on different bedrock formations and thus have different strontium ratios. Larger species, such as moose and red deer frequently cross geologic boundaries during the course of an annual foraging cycle; averaging the 87Sr/86Sr ratios in their tissues. Thus interpretation of the bioavailable geochemistry can be unintentionally biased during sampling based on sample availability.

TOOTH MINERALIZATION Teeth are dynamic mineral structures whose complexities are still being unraveled. It has long been recognized that the incremental striae of Retzius represented some aspect of matrix deposition but that there is a disconnect between this matrix deposition and the final mineralization that will finalize the mineral matrix (e.g., [32]). At the time, microsampling of individual striae was not practical, thus the matter was largely ignored. However, there has recently been a resurgence of interest in the formation process of the incremental growth lines as reflections of the circadian rhythm of enamel matrix secretion with the advent of microsampling techniques such as laser ablation and microdrilling that could theoretically sample the enamel at such pertinent scales (e.g., [1, 4, 7, 33, 34]). Numerous hypotheses have been forwarded regarding the pattern and progression of enamel mineralization, however the common theme amongst all works is that the progression of mineralization of layers and/or maturation of matrices is patchy and effectively non-linear thus making the chronological relationship between incremental lines and geochemical signals rather tenuous [33-42]. Broad

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trends whereby mineralization begins at the tooth cusp and finishes at the cingulum are still present, though the intermediate pathways are debatable. Montgomery and Evans, [39] and Fincham et al., [37] provide excellent discussion of the biomineralization of tooth enamel with respect to Sr isotope analysis. The process of mineralization spans a series of five distinct phases wherein an organic gel or protein superstructure is transformed into a mineral matrix: 1) secretion; 2) assembly; 3) matrix formation; 4) resorption prior to maturation; and 5) maturation (see Fincham et al. [37] Figure 8; Bentley [35] Figure 18). Following assembly, nanospheres of apatite will remain largely intact until maturation, however although these individual crystal structures are reflexive of their formation environment, they will mingle with other crystals to form a heterogeneous lattice of crystals in the mature matrix. Effectively, at all stages prior to maturation, the enamel matrix remains an open chemical system vulnerable to alteration, overprinting, or simply averaging of the matrix at the scale of modern recovery techniques. The practical ramifications of this open chemical system is that while the formation of tooth crowns progresses at a well known rate and there are incremental growth lines to further support the logical conception of enamel matrix as a progressive linear formation, all work to date demonstrates that there is a disjunction between enamel formation and matrix maturation. So while we know that calcification of M1 begins at birth and ends with maturation and root formation between 3 – 4 years of age, we are left with nearly three years of that molar remaining an open chemical system and a potential averaging effect [33, 43]. In spite of this theoretical difficulty, there is still some promise to the concept of microsampling tooth enamel. That incremental growth lines do not mineralize in a similar incremental fashion has been well demonstrated, however we still have some broad guidelines that remain true: 1) the crown of a tooth will fully mineralize before the root; and 2) though accomplished in a patchy or wave-like fashion, there are still broadly linear trends in mineralization progressing from crown to cingulum. Ongoing research into this problem with herbivore teeth has demonstrated that there are long-term mixing effects in action during the formation and maturation of tooth enamel [32, 42, 44-51]. Such research has highlighted a secondary problem with efforts to access the microstructure of teeth and thus provenance their formational period, that while formation of enamel proceeds using available mineral components within the body and that available components come from the diet, there is a gap between intake of the raw ionic components of the mineral structure and their incorporation into mineral tissues. Specifically this is the problem of residence time in the body for different elements. Water has a short residence time in the body of only 14 days; however strontium, calcium, and lead can remain in the body for 800–1600 days, with 10% of traceable doses remaining active after 400 days [51-53]. Recent works (e.g., [46, 51, 54]) have demonstrated that this residence time in the body has an intriguing effect on isotopic signatures of a linearly-sampled herbivore tooth. Namely that an abrupt change in geochemical geography and/or diet will not manifest as a sharp transition in isotopic signals, but rather that there will be a gradual sloping change as contributions from different geochemical end-members vary within the body-water average being accessed for ionic component material in enamel formation. At face value, this combination of lag time in mineral formation and maturation with body-water averaging of over a year should render discussions of microsampling human teeth for interim provenance information between the crown and cingulum, however it should be noted that there is an important difference between herbivore and human teeth. While it is quite likely that the same mineralization primers are in

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effect for both human and herbivore teeth and that the non-linear progression of mineral maturation is effectively the same, the time spans involved are different. For example, each bovine molar will form and fully mineralize over a span of 12–18 months [51]. However, each human molar can theoretically span a time of 24–48 months between initial calcification and final mineral maturation, though it will likely occur in less than 36 months. Thus, we have a gap in comparative volumes and chronologies in discussing the differences between herbivore and human teeth. While many herbivore teeth are not good candidates for microsampling because their tooth formation rates will not outstrip uncertainties about residence time and mineralization rates, human teeth will likely exhibit some aspects of useful variability in isotopic signatures through formation time and thus through enamel mineral volume/geography. We still must keep in mind that at present it is impossible to overcome residence time for intake and maturation time for the mineral matrix, it may well be worth pursuing microsampling of human teeth in between cusp and cingulum.

LASER ABLATION OF TEETH The coupling of a laser ablation unit to either an ICP-MS or a MC-ICP-MS is no longer a novel concept to the fields of analytical chemistry and archaeology, and is rapidly becoming a mainstream tool for ongoing research and a key player in the advancement of microanalytical techniques (cf. [55-58]). Studies involving skeletal materials were fairly late additions to the field, in part due to latent concerns about the materials being analyzed and their potential for diagenetic alteration at the proposed scale of analysis. However, in the last decade or so, ICP-MS studies on teeth and bones have picked up significantly and are now part of a healthy academic field of research [1, 3-5, 7, 8, 51, 59-62]. Numerous studies have demonstrated the reliability of ICP-MS and MC-ICP-MS as compared to TIMS and INAA (cf. [57, 63-66]) using both laser ablation and solution mode sample introduction for both elemental composition and numerous isotopic series. ICP-MS and MC-ICP-MS are generally faster and less labor intensive than traditional analytical methods, however one of the tradeoffs is the tacit recognition of the need for corrections for a variety of interferences. The identification of and correction for the seemingly endless string of interferences across the mass spectrum is an extremely important part of researchers ability to use confidently ICPMS as an analytical tool. For single radiogenic isotopic series such as strontium, the list of potential problems includes known isobaric interferences (87Rb), doubly-charge rare earth ions [11], polyatomic species such as calcium dimers [10], calcium phosphate (CaPO) [1, 7-9, 67, 68], and other molecular species yet to be identified. While daunting and providing a divergence from the seemingly parsimonious relationship linking artifact and data via traditional analytical methodologies, identifying possible problems and monitoring for data quality are important parts of any analytical process and so should not be avoided. For Sr analysis, the largest if not most pernicious problems are isobaric interference from [87] Rb and a recently identified polyatomic interference from calcium phosphate, though an important distinction between the two should be made. Rubidium corrections are necessary for all ICP-MS and MC-ICP-MS analyses as the charged [87] Rb will carry the same masscharge ratio as its 87Sr counterpart, though this can be countered with accurate mass-bias calculations. This is not a problem associated with sample introduction methodology. On the

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other hand, polyatomics such as Ca dimers and calcium phosphate species are notably absent in solution-mode analysis as sample ions are held in acid and thus prevented from recombining as they are free to do in the carrier-gas environment of laser ablation chambers. From the perspective of an end-user, that such interference only manifest, or are only apparent at significant levels via laser ablation introduction both with and without aspirated acids introduced, is both interesting and discouraging for microsampling potentials. It is intriguing that complex molecules manifest in the highly charged plasma environment when introduced by a carrier gas but not as an aspirated solution, as both are theoretically entering into the plasma chamber as ionized particles. Woodhead et al. [10], Simonetti et al. [8], Horstwood et al. [1], and Vroon et al. [9] have all discussed the presence of significant interference on mass 87 from a previously unidentified source, thus impinged on researchers’ ability to accurately assess the 87Sr/86Sr ratios of phosphate matrices with laser ablation. As all mammalian skeletal tissues are varieties of phosphate mineral matrices, this is a major problem for efforts to access the life signals contained therein and thus in archaeologists’ and paleontologists’ ability to accurately interpret these signals and thus reconstruct the movement histories of these animals. It appears that the root of the problem is the excess of Ca and P present in the charged environment coupling with the oxide production rates within the MC-ICP-MS. In theory, Ca and P levels should be proportional in all parts of skeletal tissues, thus mineral replacements such as Sr, Ba, and the incorporation of other trace elements should be proportional as well, and interferences will be related to the oxide operational conditions of the instrument itself. This leaves us with several important points to consider: do we have reason to question any of these starting assumptions? Can we monitor the formation of CaPO during analysis and thus correct for it in ways other than those outlined in previous works? Can we recover useful geochemical information from skeletal tissues using laser ablation as a microsampling technique?

MATERIALS AND METHODS Tooth enamel samples consist of five human molars from the KN XIV cemetery that have previously been used for analytical work by the Baikal Archaeology Project. Samples included 4 second molars from graves 7 (Sample #1997.211), 16 (1997.217), 35-1 (1998.355), and 35-2 (1998.359), as well as 1 third molar from grave 12 (1997.225). All samples were previously analyzed by Haverkort et al. [22] and several were also analyzed via TIMS by Weber et al.[26]. All samples were analyzed for elemental composition using both SM-ICP-MS and LA-ICP-MS and likewise for 87Sr/86Sr ratios using SM-MC-ICP-MS and LA-MC-ICP-MS. All preparation and analyses were conducted at the Radiogenic Isotope Facility of the Department of Earth and Atmospheric Sciences at the University of Alberta. Solutions were prepared by extracting fragments of enamel (between ~0.020-0.060 g) as close to the cingulum as possible to provide comparability between teeth with various levels of wear. Fragments were mechanically removed using a diamond cutting disk (NTI Diamond disc, Interflex-double sided, 8 mm diameter, 0.15 mm thickness) fitted to a Dremel tool. If necessary, samples were abraded with the disk to remove any adhering dentine. Sample locations were not side-specific as these teeth have been previously sampled, thus samples

54

Ian Scharlotta, Andrzej Weber, S. Andy Dufane et al.

were taken where adequate materials remained, though largely stemming from areas immediately adjacent to previous sampling locations. Sample preparation occurred in a Class 100 clean room facility and followed procedures outlined in Simonetti et al. [8] and Haverkort et al.[22]. Samples were sonicated for 15 min in milliQ (MQ) de-ionized water and then in 5% acetic acid for 15 min. After an overnight leaching in 5% acetic acid, the acid was removed and samples were rinsed with MQ prior to transfer to clean Teflon vial. A known amount of 87Rb–84Sr spike was added, followed by 4 mL of 16 N HNO3 and 1 mL of 12 N HCl and capped to digest on an 80 C hotplate overnight. Digested samples were then dried overnight on the hotplate. Dried samples were dissolved in 3 mL of 0.75 N HCl and loaded into syringes with disposable filters. Filtered samples were loaded onto 10 cm cation exchange columns containing 1.42 mL of 200-400 mesh AG50W-X8 resin. Columns were rinsed with 3x1 mL of 0.75 N HCl, 3x1 mL of 2.5 N HCl and washed with 17 mL of 2.5 N HCl. Samples of 5 mL of 2.5 N HCl containing the purified Sr were collected into clean Teflon vials and left to dry overnight on the hotplate. Dried samples were dissolved with 1 mL of 2% HNO3 prior to necessary dilution for MC-ICP-MS analysis. Analysis was conducted on a Nu Plasma HR MC-ICP-MS with a DSN-100 nebulizer. Strontium isotope data were acquired in static, multicollection mode using five Faraday collectors for a total of 400 s, consisting of 40 scans of 10 s integrations. The ‘wash-out’ period following the analysis of a sample was approximately 5 min. Prior to the aspiration of a sample, a 30 s measurement of the gas (+acid) blank was conducted to correct for [86] Kr and [84] Kr isobaric interferences. The isobaric interference of [87] Rb was monitored and corrected online using the [85] Rb signal. Accuracy and reproducibility of the analytical protocol based on long-term repeated analysis of a 100 ppb solution of the NIST SRM 987 strontium isotope standard 0.710242 ± 0.000041. Elemental samples of similar size underwent similar handing, though without the [87] Rb–[84] Sr spike, and not loaded onto cation exchange columns. Digested samples were simply dissolved in 2% HNO3 prior to quadrupole-ICP-MS analysis. Sample solutions were analyzed for 57 elements (Li, Be, B, Na, Mg, Al, P, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, [90] Zr, [91] Zr, Nb, Mo, Ru, Pd, Ag, Cd, Sn, Sb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Pt, Au, Tl, Pb, Th, and U) using a Perken Elmer Elan6000 quadrupole ICP-MS, and instrument operating conditions as follows: RF power = 1200 W; dual detector mode; blank subtraction performed subsequent to internal standard correction; unit of measurement is cps (counts per second); auto lens on; use of 4point calibration curves (0, 0.25, 0.50, and 1.00 ppm for Ca, Mg, and Fe; 0.005, 0.010, and 0.020 ppm for the remaining elements); sample uptake rate (using a peristaltic pump) was ~1 mL; sample analysis consisted of 35 sweeps/reading, 1 reading/replicate and 3 replicates; dwell times were 10 ms for Al, Mn, and U, and 20 ms for the remaining elements; total intergration times (dwell time x number of sweeps) were 350 ms for Al, Mn, and U, and 700 ms for the remaining elements (Table 1). External reproducibility, based on repeated analysis of international whole rock standards is 5-10% (2σ level) for most elements. Laser ablation for elemental analysis of samples was conducted using the Perken Elmer Elan6000 quadrupole ICP-MS coupled to a UP213 nm laser ablation system (New Wave Research, USA). The instrument was optimized using the NIST SRM 612 international glass standard reference material (RF power 1200 W, peak hopping acquisition, 50 ms dwell time). Teeth were serially sampled (Figure 2) using LA-ICP-MS to examine the nature and extent of useful intra-tooth geochemical variability in tandem with attempts to monitor the formation of

Assessing Hunter-Gatherer Mobility in Cis-Baikal, Siberia Using LA-ICP-MS

55

the CaPO polyatomic species. This sampling included 8 sampling locations or groups on each tooth, offset but approximately equally spaced between the bottom of the crown and the cingulum. Groups consisted of 5 lines each with a combination of laser spot size and laser power settings were employed to assess the impact of potential laser-matrix effects. Half of the sampling groups were conducted using 50% laser power, while the other half was run at 100% laser power. Line groups consisted of reducing laser spot sizes of 100, 80, 55, 40 and 25 µm in sequence, with a repetition rate of 20Hz and an energy density of ~13 J cm-2. Experiments were conducted in a mixed He/Ar atmosphere (ratio of 0.5:0.1 L min-1) within the ablation cell, and mixed with Ar (1.03 L min-1) prior to entering the torch assembly. The laser ablation cell was flushed with a higher flow rate of He (up to 0.9 L min-1) for approximately 1 min in-between laser ablation runs to ensure adequate particle washout. The NIST SRM 612 glass standard was used as the external calibration standard. Quantitative results for 57 elements (Li, Be, B, Na, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, [90] Zr, [91] Zr, Nb, Mo, Ag, Cd, In, Sn, Sb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Au, Tl, Pb, Bi, Th, and U) were obtained and normalized to 24Mg, as measured by solution analysis, as the internal standard using the GLITTER® (XP version, Macquarie University) laser ablation software (Table 2). Mg was used instead of Ca in order to assess variability in calcium in these teeth. Laser ablation for isotopic analysis was conducted using a UP213 nm laser system coupled to the Nu Plasma HR MC-ICP-MS with the sample-out line from the desolvating nebulizing introduction system (DSN-100 from Nu Instruments) to allow for simultaneous aspiration of a 2% HNO3 solution. At the beginning of each analytical session, parameters for the introduction system and the ion optics were optimized by aspirating a 100 ppb solution of the NIST SRM 987 Sr isotope standard. Based on the results of the elemental laser ablation analysis, a full replication of the line groups was not done. Instead, overlying each full powered elemental sampling site, three parallel lines were analyzed using 100 µm laser spot size; 100% laser power; 20 Hz repetition rate; ~15 J cm-2 energy density (Table 3). Half powered elemental sampling sites were not sampled for isotopic data as the elemental data were deemed to be of poor quality. Strontium isotope data were acquired in static, multicollection mode using five Faraday collectors for a total of 400 s, consisting of 40 scans of 10 s integrations, for data reported. Testing for potential collector setups included attempts at using eight collectors in order to extend the monitored mass range to include masses 90 and 91. Similar efforts were also attempted using dual-acquisition, static analysis to similar effect following Horstwood et al. 1. Laser data were partially monitored by repeated analysis of a specimen of Durango Apatite with a reported value of 0.706327 ± 0.000724 by TIMS 1. In one analytical session, an average value of 0.706118 ± 0.000035 were observed, and in a second, 0.706244 ± 0.000028. This sample is currently being intensively sampled for its ongoing use as a mineral standard for strontium analysis at the Radiogenic Isotopic Facility, however at present has been analyzed fewer than fifty times and thus can only be viewed with moderate confidence. As such, no attempts were made to standardize data for enamel samples to apatite values. Quantification of the oxide levels during analyses using UO+/U+ demonstrated levels of approximately 0.7%.

56

Ian Scharlotta, Andrzej Weber, S. Andy Dufane et al. Table 1. Solution mode 87Sr/86Sr ratios and elemental data for KN XIV teeth. Local/Nonlocal determinations from Haverkort et al. 2008 Re

Os

Pt

Au

Tl

Pb

Th

U

0.001927

0.009514

0.001906

0.058862

0.001221

0.268555

2.340665

0.071605

Little Sea Origin Nonlocal

0.007496

0.043007

0.007412

0.298735

0.073691

0.079869

0.213768

0.015656

Local

0.005342

0.033591

0.007894

0.246172

0.04021

0.138083

0.198961

0.024269

Local

0.001878

0.028736

0.006105

0.186726

0.022941

0.054092

0.091068

0.005631

Nonlocal

0.002238

0.012796

0.002007

0.083647

0.007105

0.060767

0.030142

0.003889

Nonlocal

Figure 2. Sampling scheme used on sectioned teeth. Enlargement – laser ablation scars representing one sampling group

RESULTS AND DISCUSSION This research included a number of different aspects, beginning with efforts to demonstrate the value in trace element analysis of human teeth for provenance and/or mobility purposes. The treatment of organic minerals in a similar fashion to complex inorganic minerals for the purposes of provenance analysis is a fairly new and expanding area of research within the realm of provenance analysis (cf. [5, 69, 70]). The underlying concept is the same for any geochemical sourcing study, in that the range of variability must meet the strictures of the “Provenance Postulate” [71]. Though this has been demonstrated for populations on Rapa Nui [70], and in distinguishing African immigrants in a Mexican cemetery [5, 69]; the range of useful variability in trace element composition must be determined for each geographical region. Thus, for cemeteries in Cis-Baikal, a new database of locally useful elemental variability must be generated. This research consisted of only 5 samples, however still presents a beginning to the formation of such a database.

Table 2. Laser ablation elemental data for KN XIV teeth (Sample ID Laser Spot Size Sample Group) Sample ID_Spot Size_Group 97211_100_A 97211_80_A 97211_55_A 97211_40_A 97211_25_A 97211_100_B 97211_80_B 97211_55_B 97211_40_B 97211_25_B 97211_100_C 97211_80_C 97211_55_C 97211_40_C 97211_25_C 97211_100_D 97211_80_D 97211_55_D 97211_40_D 97211_25_D 97211_100_E 97211_80_E 97211_55_E 97211_40_E 97211_25_E 97211_100_F 97211_80_F

Li 0.364 0.49 0.65 0.3 1.08 0.452 0.41 0.44 0.28 1.44 0.5 0.36 0.4 0.67 0.19 0.342 0.52 0.32 0.6 1.56 1.1 1.37 3.13 5.44 25.95 0.35 1.2

Be 0.205 0.6 2.02 1.05 7.14 0.46 0.2 1.37 2.56 2.68 0.048 0.25 2.56 3.48 3.63 0.36 1.04 0.18 2.17 6.97 7.93 11.93 33.7 23.82 160.17 2.8 2.12

B 1.08 1.22 1.23 3.77 7.2 0.79 1.58 1.06 2.36 4.11 1.31 1.11 1.09 1.83 3.78 1.01 0.83 1.56 1.15 4.17 3.15 5.69 17.63 22.8 59.52 1.2 3.89

Na 3650.44 3547.81 3653.11 3716.37 3814.94 3657.98 3724.74 3741.95 3569.28 3175.41 3649.68 3641.77 3607.72 3416.2 3474.17 4132.01 4204.36 4010.44 4146.8 4206.67 3894.62 4215.7 3801.75 4827.18 5311.39 3803.64 3779.97

Al 241.22 43.92 6.41 47.85 18.7 21.52 25.71 39.4 75.03 265.42 14.92 21.53 35.86 43.02 94.97 19.07 37.76 51.79 62.98 148.6 10.1 392.19 1018.72 1198.29 2085.95 273.24 190.42

Si 207.24 117.65 104.64 146.62 403.09 104.25 55.41 120.28 83.01 476.86 101.74 69.43 135.81 129.46 287.02 101.29 95.69 129.55 156.79 272.57 154.08 403.74 850.41 1125.62 4583.92 75.26 323.51

P 64027.67 63294.82 61442.43 64486.52 65675.6 58902.48 61789.18 59194.78 59178.95 53285.48 55783.39 57419.32 55748.88 54201.63 55328.13 51666.73 58097.5 55033.9 56970.46 55208.66 48036.38 49273.86 49333.76 43452.75 48566.3 41278.58 41964.24

Ca 222294.11 237603.84 236211.53 234467.86 264530.88 218525.92 233481.7 230185.63 225280.58 195903.66 214245.89 228728.3 210288.25 203894.3 210873.78 201905.63 225548.22 214956.39 223171.36 202296.55 199831.05 190208.02 169428.05 166217.27 190663.23 155721.28 167564.7

Sc 0.09 0.163 0.38 0.44 1.99 0.083 0.158 0.33 0.45 1.37 0.076 0.154 0.257 0.45 1.48 0.099 0.193 0.29 0.43 1.41 0.88 1.95 4.42 5.23 20.43 0.44 1.28

Ti 4.27 1.69 4.81 5.07 21.41 0.85 1.47 4.78 5.9 15.23 0.66 1.82 2.65 4.25 17.57 0.82 1.53 2.74 4.14 17.12 8.08 15.33 33.18 57.2 242.12 4.44 18.32

V 2.88 2.99 3.11 2.47 3.5 2.6 2.39 2.81 1.69 1.9 2.25 2.37 2.11 2.1 1.53 2.2 1.94 1.94 2.28 2.01 3.12 1.96 4.45 5.8 23.43 3.33 2.6

Cr 1.01 0.72 1.62 2.11 9.87 0.88 1 1.52 2.12 7.14 1.05 1.44 1.18 2.28 7.03 0.93 0.75 1.7 2.25 6.81 4.24 8.88 22.43 26.83 116.43 1.91 7.42

Mn 35.32 9.61 4.16 12.8 9.85 5.81 5.95 9.29 14.62 31.62 5.91 7.58 11.68 16.71 23.61 10.03 9.76 15.9 43.31 214.22 1.51 26.38 444.45 205.11 829.46 12.23 48.77

Fe 112.83 57.92 42.41 52.47 44.14 55.75 58.15 50.59 65.95 128.41 54.41 53.47 50.49 59.75 78.31 51.05 49.51 65.86 83.66 161.45 41.3 122.1 247.13 193.95 611.92 99.04 126.33

Co 0.325 0.082 0.117 0.124 0.82 0.072 0.065 0.128 0.155 0.61 0.035 0.197 0.107 0.159 0.33 0.126 0.116 0.16 0.332 2.09 0.39 0.83 4.88 10.14 13.29 0.193 0.59

Cu 0.53 0.34 0.69 0.93 4.1 0.243 0.25 0.61 0.91 3.09 0.205 0.3 0.41 0.93 2.68 0.164 0.25 0.52 0.93 2.78 1.39 2.89 6.95 7.29 41.04 0.51 2.24

Zn 111.04 87.43 78.25 86.53 82.72 58.72 54.58 45.15 61.21 63.15 46.04 50.7 47.52 56.47 63.14 31.87 36.83 48.97 63.01 88.47 22.07 25.29 68.38 59.31 145.64 29.72 17.46

Table 2. (Continued) Sample ID_Spot Size_Group 97211_55_F 97211_40_F 97211_25_F 97211_100_G 97211_80_G 97211_55_G 97211_40_G 97211_25_G 97211_100_H 97211_80_H 97211_55_H 97211_40_H 97211_25_H 97217_100_A 97217_80_A 97217_55_A 97217_40_A 97217_25_A 97217_100_B 97217_80_B 97217_55_B 97217_40_B 97217_25_B 97217_100_C 97217_80_C 97217_55_C 97217_40_C 97217_25_C

Li 1.81 1.78 4.18 0.51 0.3 2.77 1.84 0.001 0.57 0.63 2.43 3.53 21.17 0.302 0.23 0.156 0.76 1.3 0.27 0.42 0.39 0.67 2.38 0.33 0.42 0.75 0.75 2.44

Be 0.001 5.31 17.92 1.31 0.45 12.88 0.001 19.03 0.28 5.88 12.35 67.22 92.14 0.049 0.25 2.23 4.03 9.3 0.57 1.03 3.77 5.33 3.59 0.51 1.81 2.2 0.55 5.37

B 9.57 6.65 12.3 1.58 1.18 6.89 6.83 6.33 1.17 3.18 10.22 42.16 72.65 1.25 1.3 2.63 3.13 4.74 1.55 1.61 1.87 3.21 7.52 1.2 2.14 1.95 1.75 9.74

Na 3236.03 2218.74 1287.24 3620.49 2540.82 2853.51 2510.86 2013.43 4636.57 5196.43 5194.39 4132.36 3925.68 5322.98 5153.04 5177.93 5336.97 5415.9 5164.93 5245.86 5329.83 5352.14 5516.12 5361.01 5488.83 5538.91 5787.83 5344.36

Al 452.1 460.72 357.72 176.83 275.93 332 281.65 1399.86 262.33 692.49 833.21 946.69 2336.83 4.03 2.83 0.84 1.83 4.69 4.97 5 13.07 8.4 6.8 7.6 21.42 27.02 42.73 70.34

Si 434.71 452.35 739.64 54.21 214.82 281.46 438.22 535.55 84.19 354.25 450.13 3006.45 3143.39 87.9 75.32 188.54 131.66 445.3 119.17 134.56 119.79 122.07 425.48 101.6 103.23 160.34 177.41 491.56

P 36926.37 25898.87 12963.16 37092.63 25102.88 31469.35 31199.55 16874.91 46825.67 53934.13 54286.59 46711.54 39578.45 73633.24 78360.02 76338.85 78017.85 79130.62 75013.55 79826.28 80309.66 81215.74 84018.63 72692.55 80016.89 79543.34 84828.95 83480.05

Ca 147313.73 100775.26 44549.54 147173.53 100285.7 129220.73 117070.11 56437.6 184185.75 205958.03 207796.53 206638.84 170792.42 288064.91 301801.22 301184.59 310309.25 312386.91 295278.63 316606.38 311098.38 313226.5 322509.84 287984.03 315531.25 305731.81 335817.41 322326.03

Sc 2.18 2.15 4.64 0.25 0.44 1.35 2.03 2.56 0.39 1.08 2.14 13.37 14.04 0.097 0.19 0.39 0.82 2.45 0.078 0.183 0.38 0.59 2.05 0.096 0.2 0.4 0.63 1.82

Ti 14.3 25.22 32.42 2.54 4.43 16.66 25.25 30.69 5.41 9.2 23.24 150.48 184.61 1.05 1.32 4.09 5.57 21.26 0.88 1.58 2.95 6.23 13.76 0.69 1.83 3.86 6.61 24.66

V 3.06 2.39 4.11 3.73 1.99 4.33 2.49 2.81 3.66 2.7 3.04 12.71 14.02 1.29 1.11 0.83 0.67 1.88 0.9 1 0.84 0.6 2.09 0.98 1.33 1.25 1.4 1.82

Cr 11.28 11.96 19.39 1.44 2.98 7.94 11.77 14.3 1.95 5.62 12.6 71.73 86 1.17 0.89 2.03 3.75 13.64 1.27 1.63 2.3 3.35 11.41 1.31 2.01 1.94 3.62 11.19

Mn 16.03 16.69 6.98 115.21 20.58 33.58 81.72 238.55 353.34 314.95 504.33 174.51 67.08 1.19 1.18 1.37 0.9 3.08 2.05 1.26 2.4 1.22 2.71 1.48 2.97 2.79 6.08 12.04

Fe 283.83 144.88 96.96 82.6 137.19 191.63 165.51 309.78 171.42 193.53 388.33 324.33 1350.36 86.75 81.11 67.95 77.8 97.92 74.66 73.82 58.88 61.34 108.25 67.52 72.63 54.57 97.04 64.64

Co 0.96 0.94 1.33 0.72 1.14 0.79 2.73 4.2 10.1 7.32 7.08 4.31 8.11 0.522 0.128 0.158 0.29 0.8 0.03 0.051 0.215 0.4 0.53 0.054 0.117 0.114 0.228 0.74

Cu 2.15 3.17 5.05 0.46 0.76 1.93 3.07 4.31 0.57 1.73 3.39 22.78 24.61 0.443 0.27 0.62 1.05 2.14 0.78 0.32 0.47 1.09 2.56 0.142 0.186 0.49 0.67 2.35

Zn 17.79 22.71 28.1 21.46 15.11 11.66 19.09 19.27 24.26 28.9 55.69 88.13 106.7 54.82 60.25 61.75 52.89 39.46 80.93 87.49 109.41 101.06 124.54 75.36 128.63 140 161.98 225.47

Table 2. (Continued) Sample ID_Spot Size_Group 97217_100_D 97217_80_D 97217_55_D 97217_40_D 97217_25_D 97217_100_E 97217_80_E 97217_55_E 97217_40_E 97217_25_E 97217_100_F 97217_80_F 97217_55_F 97217_40_F 97217_25_F 97217_100_G 97217_80_G 97217_55_G 97217_40_G 97217_25_G 97217_100_H 97217_80_H 97217_55_H 97217_40_H 97217_25_H 97225_100_A 97225_80_A 97225_55_A

Li 0.253 0.29 0.45 0.73 2.22 1.35 3.37 5.58 12.48 29.37 0.51 0.34 3.48 7.5 25.38 0.67 1.2 6.13 15.07 11.67 1.45 4.18 3.73 15.94 15.77 0.138 0.36 1.27

Be 0.15 1.12 2.44 3.93 7.97 0.73 12.55 29.31 46.07 340.96 5.72 11.48 25.14 38.1 90.59 4.74 20.96 9.41 60.66 185.76 5.01 28.56 23.31 43.94 49.86 0.21 0.81 0.88

B 1.16 0.83 3.84 1.3 8.81 3.36 8.4 13.91 38 89.22 3.36 9.56 13.56 15.21 53.91 1.63 10.35 12.93 25.88 112.51 2.94 15.91 11.66 27.01 73.04 2.15 2.79 2.69

Na 5927.98 6053.21 6359.43 6338.53 5876.09 4874.99 4940.43 4384.32 5117.52 6593.72 5447.38 4986.99 6080.22 5933.6 6416.94 5204.55 5310.13 5367.78 4478.11 5050.97 5566.45 4952.52 4308.03 6457.71 5397.13 7009.91 6991.29 7209.43

Al 17.49 17.32 49.05 43.1 25.42 1.95 4.08 8.34 18.48 46.4 1.32 8.92 96.24 15.44 30.51 1.22 5.06 7.75 19.24 65.11 1.24 8.03 6.13 16.53 54.8 11.65 5.8 11.45

Si 147.25 121.23 153.02 176.99 554.23 157.63 385.68 716.33 1674.86 5197.13 131.51 344.7 594.78 1192.35 2850.36 112.05 430.95 701.58 1402.86 6014.97 145.54 654.89 752.29 1401.29 4588.98 236.14 203.36 115.56

P 77479.99 82094.33 84394.7 83431.14 80796.29 74659.08 66046.16 62152.77 72106.77 73007.28 61558.16 56684.51 62864.18 59769.67 54528.75 61292.16 63336.43 63516.46 55760.45 71602.13 62848.21 64016.12 56822.76 63017.98 56168.67 118420.75 117615.88 115827.97

Ca 304195.91 330688.81 331941.84 331381.16 310824.81 288323.47 272802.16 253801.8 255346.58 252952.38 238079.16 229622.8 221898.14 229634.73 229070.47 230307.47 257116.75 232396.8 209294.88 211325.31 245832.11 269740.88 217050.23 239038.19 254952.69 547999.25 533000.56 534019.63

Sc 0.1 0.176 0.41 0.58 1.88 0.85 1.79 3.52 8.62 29.53 0.76 1.68 3.43 6.21 15.06 0.7 1.91 3.02 6.99 27.45 0.76 3.04 3.64 6.55 19.83 0.197 0.33 0.64

Ti 0.94 1.46 4.45 8.74 15.15 7.2 15.89 22.6 76.39 268.9 7.09 17.35 21.84 65.84 145.7 2.87 30.52 31.54 57.12 291.19 6.6 33.5 33.31 57.01 203.67 2.5 3.72 8.39

V 1.47 1.51 1.85 1.78 1.55 0.84 1.75 3.24 7.03 24.41 0.89 1.59 2.4 6.56 12.4 0.62 1.87 2.97 5.88 18.66 1.14 2.67 3.2 5.23 16.15 2.63 1.9 1.45

Cr 1.31 1.76 2.5 3.22 9.78 4.14 11.14 19.99 44.99 141.67 3.55 9.32 22.42 34.57 76.63 3.12 11.28 19.41 37.26 167 4.02 17.63 20.4 37.44 128.87 2.29 1.42 2.84

Mn 2.15 2.16 4.04 4.55 4.27 0.98 2.54 4.58 11.92 29.96 0.88 2.18 3.83 6.24 17.4 0.65 2.32 4.13 9.29 34.55 0.9 3.94 4.49 8.11 25.43 22.63 19.45 30.69

Fe 62.82 67.75 97.87 93.64 55.41 64.49 79.98 102.32 233.36 750.51 23.24 51.4 100.09 219.56 403.72 21.43 72.1 98.5 189.85 847.78 39.04 82.8 99 211.21 710.13 164.72 92.34 79.29

Co 0.054 0.088 0.134 0.215 0.31 0.39 0.83 1.12 3.91 7.09 0.215 0.61 1.33 3.17 5.81 0.197 0.77 2.23 1.56 9.54 0.256 1.03 1.67 2.33 7.25 0.267 0.203 0.29

Cu 0.166 0.32 0.48 2.99 1.91 1.05 2.72 5.22 11.55 45.08 0.9 1.92 8.17 9.38 22.22 0.87 3.09 5.71 10.36 48.97 1.2 5.71 7.16 10.75 42.1 0.54 0.41 0.74

Zn 89.66 133.34 161.51 171.33 213.6 18.04 15.44 31.78 44.87 157.48 29.97 39.49 30.27 63.28 77.88 27.64 16.15 22.12 41.25 146.95 25.61 20.12 24.28 39.32 127.89 233.38 197.71 180.02

Table 2. (Continued) Sample ID_Spot Size_Group 97225_40_A 97225_25_A 97225_100_B 97225_80_B 97225_55_B 97225_40_B 97225_25_B 97225_100_C 97225_80_C 97225_55_C 97225_40_C 97225_25_C 97225_100_D 97225_80_D 97225_55_D 97225_40_D 97225_25_D 97225_100_E 97225_80_E 97225_55_E 97225_40_E 97225_25_E 97225_100_F 97225_80_F 97225_55_F 97225_40_F 97225_25_F 97225_100_G

Li 1.3 2.1 0.26 0.39 0.53 0.35 6.26 0.21 0.14 0.82 0.94 5.34 0.34 0.12 1.63 1.23 1.83 2.38 6.32 0.001 54.37 65.04 2.36 5.63 13.96 20.93 41.89 2.66

Be 8.14 32.21 1.4 1.66 4.56 0.001 4.97 0.76 2.5 3.33 5.58 21.27 0.78 1.76 0.001 6.65 14.99 5.64 23.43 108.18 241.3 232.96 8.36 19.7 48.22 11.64 0.001 1.36

B 5.23 12.01 1.7 2.26 2.74 4.4 10 2.66 3.65 2.59 3.44 14.47 2.64 3.04 2.82 6.96 13.52 7.22 21.38 25.63 81.33 216.3 10.21 14.88 35.32 75.92 104.35 24.07

Na 7091.02 7298.59 7110.85 7101.9 7213.48 7262.51 6867.13 6903.09 6900.58 7091.58 6971.92 6917.97 7044.42 7369.27 7249.75 6012.49 6205.92 7142.03 6578.72 6918.53 7743.79 7593.37 6720.33 8880.7 7745.32 7110.64 13476.81 6693.77

Al 22.99 30.42 11.32 27.67 2.87 50.78 144.9 29.25 62.83 40.18 90.88 312.97 45.77 61.45 97.82 1168.06 1193.8 153.98 124.8 315.61 621.28 1749.5 112.88 485.34 780.71 855.26 1660.19 449.02

Si 406.28 969.63 188.03 173.43 221.78 334.47 1187.44 245.45 240.13 166.9 330.92 1007.41 252.37 193.14 324.94 2203.63 1523.9 717.12 729.19 1919.67 4511.08 7438.72 329.52 631.81 1511.97 2208.14 7062.54 196.77

P 116400.12 115523.09 105481.35 107869.02 103344.02 102750.77 99076.79 94364.73 96928.23 102025.57 98884.62 110471.86 87573.85 95663.72 95804.03 81548.66 81772.52 86554 80613.89 78710.49 102583.11 84279.17 78545.98 73909.46 85002.02 76997.26 73522.03 64489.92

Ca 528096.56 538233.38 486559.34 500983.44 493218.41 452287.69 433927.63 429419.22 447873.19 446393.53 439993.31 451093.31 407170.88 454880.19 437473.03 354900.31 382409.78 394904.47 377897.91 390870.03 454514.84 319535.22 347284.44 324531.34 358737.06 312071.31 303866.94 288153.59

Sc 1.06 3.77 0.231 0.46 0.61 0.89 2.96 0.23 0.27 0.58 0.85 3.48 0.278 0.31 0.56 0.65 2.3 1.44 3.66 7.92 20.08 35.12 1.24 2.91 7.42 11.41 38.06 0.72

Ti 15.75 49.6 3.32 4.6 6.87 22.02 32.24 1.82 4.24 5.72 9.97 40.78 2.47 2.81 5.18 199.39 41.36 67.73 32.64 129.8 287.86 436.9 11.31 31.28 134.15 308.31 324.19 30.89

V 2.51 3.75 1.52 1.73 1.37 1.63 2.44 1.91 2.29 2.15 1.55 3.03 1.75 1.48 1.56 3.84 3 2.46 3.34 6.66 17.18 26.82 2.58 2.7 5.78 7.37 28.36 4.19

Cr 5.28 19.59 2.09 1.84 3.04 4.32 15.95 2.16 1.63 2.63 4.42 17.02 2.08 1.38 2.79 3.44 12.07 6.6 17.86 48.22 112.72 185.53 6.04 14.9 36.92 55.22 181.28 3.44

Mn 51.46 70.57 72.05 116.74 41.39 139.16 191.49 90.3 114.56 117.72 133.45 165.93 66.22 71.25 96.46 198.46 317.94 25.13 55.97 417.99 47.27 252.2 215.38 324.34 5553.71 983.29 4583.88 995.24

Fe 105.56 115.7 89.8 101.37 72.48 129.37 255.37 108.12 195.85 124.74 246.59 284.82 154.96 104.73 162.87 1195.9 1467.85 183.52 122.69 525.76 554.84 1294.26 147.61 479.44 1667.19 465.53 3465.42 661.69

Co 0.34 0.91 0.361 0.57 0.24 0.36 1.72 0.94 0.62 0.34 0.61 1.11 1.71 0.43 0.82 1.45 4.88 0.69 2.54 10.67 7.48 14.38 2.67 3.26 111.06 18.13 94.51 10.7

Cu 1.62 6.09 0.4 0.67 0.72 1.29 14.63 0.59 1.16 0.7 1.16 3.99 0.56 0.53 0.63 2.29 4.52 24.17 12.67 28.57 49.34 166.2 5.66 22.62 34.72 23.08 63.43 7.49

Zn 148.62 160.13 110.46 133.36 136.73 137.3 121.88 63.11 83.93 100.05 127.31 159.73 50.85 82.18 109.97 147.13 209.36 44.22 43.21 46.12 191.91 175.58 32.93 38.32 45.03 58.93 166.3 49.91

Table 2. (Continued) Sample ID_Spot Size_Group 97225_80_G 97225_55_G 97225_40_G 97225_25_G 97225_100_H 97225_80_H 97225_55_H 97225_40_H 97225_25_H 98355_100_A 98355_80_A 98355_55_A 98355_40_A 98355_25_A 98355_100_B 98355_80_B 98355_55_B 98355_40_B 98355_25_B 98355_100_C 98355_80_C 98355_55_C 98355_40_C 98355_25_C 98355_100_D 98355_80_D 98355_55_D 98355_40_D

Li 6.55 22.54 22.79 97.52 3.32 7.57 13.67 13.86 13.25 0.54 1.01 2.44 2.97 14.16 1.19 0.98 1.47 3.55 1.27 0.6 0.9 1.93 2.61 2.72 0.95 1.58 1.29 3.25

Be 21.49 51.58 51.42 306.78 14.55 3.35 28.11 41.03 88.78 1.24 1.92 4.42 2.14 3.61 0.91 1.25 5.58 6.75 25.41 1.03 1.71 6.37 6.53 0.001 0.98 0.75 1.07 12.4

B 18.48 27.72 58.92 192.06 9.17 27.47 50.85 26.43 70.58 5.76 5.56 4.37 5.01 15.13 3.2 3.23 2.88 5.29 10.19 2.48 2.96 3.75 2.63 10.44 1.91 2.22 2.31 3.08

Na 6380.51 6193.42 5364.01 7696.5 6893.58 6836.5 8864.9 5652.05 5005.34 5715.82 5622.5 5663.76 5670.43 4686.16 5657.17 5615.04 5372.02 5249.11 6173.66 5582.27 5446.07 5575.09 5514.95 5778.55 6429.01 6421.12 6820.49 7765.45

Al 1021.31 859.05 1316.62 2135.23 1161.7 1539.04 1908.91 3108.64 5094.67 47.49 73.4 32.12 26.4 28.89 3.01 0.49 1.08 1.6 7.56 1.09 0.45 1.44 3.48 7.16 237.45 323.44 273.27 729.25

Si 1335.82 1466.06 1545.29 6449.39 905.22 1612.08 2008.06 3307.72 9918.99 160.44 147.51 171.84 237.2 588.75 148 126.5 191.85 142.09 527.63 105.88 57.24 191.56 132.94 483.09 1193.9 1267.38 1211.85 943.44

P 67508.48 70270.96 53421.58 58507.17 80838.57 87671.91 96971.75 65682.73 75493.9 76667.38 78834.41 77084.85 80196.4 75901.7 68496.59 68328.76 65910.07 68869.52 72705.4 62837.27 64233.39 63542.27 65848.53 71633.34 59013.73 60990.56 62857.28 70267

Ca 292432.97 350032.38 238860.69 299051.5 352329.72 401446.84 391857.13 304642.97 264445.81 416772.53 431454.03 409923.94 426649.38 387291.19 364693.19 369036.72 343270.53 350840.31 368005.75 323981.69 333070.44 319095 335529.44 355946.41 323913.41 341151.97 344753.59 347507.31

Sc 2.07 8.38 7.78 29.7 1.44 3.35 5.97 5.25 10.08 0.221 0.236 0.51 0.84 2.92 0.107 0.222 0.44 0.68 2.38 0.111 0.158 0.35 0.61 2.04 0.085 0.143 0.3 0.77

Ti 61.34 77 75.24 357.12 51.19 61.56 110.92 211.5 224.26 1.21 2.12 3.64 6.64 29.52 0.94 1.51 4.07 6.01 22.59 0.91 0.81 1.88 6.21 16 10.85 16.11 8.05 23.78

V 4.61 5.45 5.16 23.14 2.65 4.48 16.21 18.87 10.29 8.08 7.73 7.25 7.12 5.07 5.11 4.19 3.48 3.06 4.81 2.82 2.62 2.2 3.02 3.83 4.06 3.78 4.16 3.95

Cr 11.34 35.98 39.87 157.61 6.74 17.02 29.96 32.3 61.11 2.16 1.35 2.52 4.53 15.51 1.96 1.97 2.24 3.78 14 1.58 1.57 1.99 3.56 13.25 3.3 4.59 3.56 5.72

Mn 1122.96 146.26 587.92 76.04 714.47 2161.51 7760.16 11867.08 4544.06 30.84 27.08 20.22 15.82 13.3 10.49 5.33 3.39 4.13 4.17 2.88 3.19 2.66 3.05 4.09 28.16 54.91 30.31 23.01

Fe 1626.33 811.2 1544.88 1325.45 1753.68 2538.6 4277.3 5616.39 8637.84 96.47 65.61 74.58 48.68 85.65 48.69 34.46 45.33 43.18 64.66 58.47 30.25 39.47 36.14 67.77 245.76 369.97 306.27 456.15

Co 28.25 3.06 22.83 11.05 15.33 76.2 141.86 249.35 238.78 0.34 0.148 0.178 0.184 0.88 0.116 0.084 0.09 0.31 0.58 0.064 0.103 0.132 0.3 1.51 0.456 0.716 0.193 0.39

Cu 9.74 7.5 10.52 31.23 5.73 8.44 11.78 26.78 16.51 0.727 0.38 0.81 0.96 3.7 0.495 0.25 0.45 0.68 3.2 0.206 0.159 0.34 0.55 2.37 10.24 11.73 12.21 1.86

Zn 52.58 48.88 33.97 128.11 112.81 174.01 386.13 181.63 194.79 112.49 85.24 67.55 56.57 45.46 32.04 25.22 22.53 22.76 19.42 30.29 35.1 48.45 72.28 78.21 35.88 49.34 58.79 74.3

Table 2. (Continued) Sample ID_Spot Size_Group 98355_25_D 98355_100_E 98355_80_E 98355_55_E 98355_40_E 98355_25_E 98355_100_F 98355_80_F 98355_55_F 98355_40_F 98355_25_F 98355_100_G 98355_80_G 98355_55_G 98355_40_G 98355_25_G 98355_100_H 98355_80_H 98355_55_H 98355_40_H 98355_25_H 98359_100_A 98359_80_A 98359_55_A 98359_40_A 98359_25_A 98359_100_B 98359_80_B

Li 7.48 2.65 8.41 5.53 45.64 135.38 2.76 0.001 14.86 35.13 116.58 0.32 8.75 17.38 14.44 68.33 2.35 2.86 4.17 12.33 13.1 0.89 0.64 2.01 5.07 14.36 0.87 0.74

Be 28.57 7.15 12.24 7.48 123.51 518.4 2.74 34.52 57 67.4 206.26 1.72 16.82 33.42 55.57 0.001 6.39 27.29 23.26 21.93 0.001 0.28 3.05 6.43 5.39 7.48 0.28 2.87

B 10.06 7.09 9.7 27.77 43.72 106.08 4.8 10.28 26.19 43.86 159.77 4.2 9.83 21.87 23.03 44.58 3.47 8.03 18.14 8.09 90.89 1.97 2.62 2.34 5.68 18.51 1.78 1.58

Na 7226.71 7003.48 6643.06 6571.61 6222 7630.24 6357.18 6530.7 6482.76 5455.34 10722.04 6637.35 6358.15 6488.91 6861.44 6309.71 6866.28 6626.17 6971.95 5529.31 5899.21 5155.86 5219.42 5111.96 5250.39 4523.82 4936.14 5005.04

Al 816.36 451.6 164.06 110.82 51.48 80.32 17.83 31.94 29.75 49.25 100.66 9.56 14.6 12.41 16.2 74.92 233.88 472.36 577.83 2086.6 9426.62 83.85 41.29 1.61 1.49 5.66 23.98 2.6

Si 1445.24 304.93 450.82 1095.75 1935.38 7652.89 183.21 369.99 899.09 1417.57 6978.81 145.86 327.27 688.09 852.42 2313.8 124.11 370.88 1629.38 2253.96 16379.57 295.08 134.21 152.6 148.71 532.77 152.36 97.72

P 60617.42 73559.76 72134.31 77795.85 73093.86 67619.34 65250.62 69647.97 75102.91 57715.38 79516.3 62604.81 62025.96 59559.25 67326.55 61648.47 60993.12 61651.24 56879.88 48220.17 28831.54 71507.77 73027.66 69484.04 70842.09 69690.9 61188.7 64117.52

Ca 327554.84 387677.06 389539.25 403901.31 390653.22 390669.75 346684.97 371498.56 386510.13 302612.47 406200.09 323535.66 320332.09 312780.53 279488.56 272751.09 314303.63 316833.75 281299.06 201172.02 120600.91 375569.22 389421.38 366452.78 372658.91 341599.88 321109.91 331139.34

Sc 2.04 0.65 1.99 5.13 7.73 32.84 0.6 1.82 4.56 5.73 37.13 0.7 1.38 3.14 3.21 9.56 0.54 1.46 2.29 3.38 9.06 0.105 0.199 0.45 0.62 2.7 0.103 0.328

Ti 34.46 7.17 22.75 39.24 81.43 223.27 4.91 8.55 44.54 52.55 220.29 3.14 13.03 30.57 30.32 110.94 6.67 13.23 27.08 79.21 438.55 1.96 2.63 4.35 6.05 20.95 2.07 1.52

V 3.85 7.29 6.89 8.59 9.83 25.21 4.88 4.9 4.45 5.27 16.19 3.57 3.9 3.25 3.39 8.52 2.68 2.72 2.87 5.01 20.38 3.85 3.5 3.32 2.29 3.02 3.65 3.04

Cr 11.1 3.96 11.84 29.63 52.67 204.87 3.35 9.88 24.29 37.58 181.6 3.23 8.88 18.31 22.44 60.58 3.34 9.55 13.16 19.37 63.91 2.3 1.7 2.34 3.91 13.66 1.8 1.19

Mn 15.73 281.81 187.55 783.73 376.86 328.08 20.55 3.77 5.97 9.16 44.79 1.99 2.86 5.73 5.09 15.53 8.33 47.32 642.65 35.12 3839.48 7.37 8.23 2.86 1.4 3.32 1.81 1.03

Fe 523.33 159.94 70.34 138.07 244.42 907.71 39.85 56.89 111.98 217.75 844.32 32.76 46.4 81.55 129.05 283.86 188.03 383.12 736.87 1438.3 7596.34 87.05 53.1 27.35 26.45 69.27 73.71 28.83

Co 0.78 3.34 1.95 13.7 5.11 8.08 0.48 0.85 2.16 2.08 11.95 0.139 0.36 1.77 1.47 3.27 0.34 0.92 18.4 1.24 127.47 0.16 0.16 0.235 0.136 0.58 0.044 0.06

Cu 1.78 1.23 2.24 4.13 6.48 33.26 0.52 1.8 3.64 6.08 30.78 0.52 1.07 3.46 3.21 10.36 0.7 1.33 2.57 2.73 12.61 0.389 2.34 1.96 3.33 8.1 0.207 0.38

Zn 82.64 35.97 30.44 26.03 50.53 124.6 19.58 19.08 23.78 36.41 159.19 25.05 28.27 21.33 21.62 58.73 28.24 40.54 33.35 29.19 63.06 99.1 64.81 50.85 41.51 29.85 54.9 42.41

Table 2. (Continued) Sample ID_Spot Size_Group 98359_55_B 98359_40_B 98359_25_B 98359_100_C 98359_80_C 98359_55_C 98359_40_C 98359_25_C 98359_100_D 98359_80_D 98359_55_D 98359_40_D 98359_25_D 98359_100_E 98359_80_E 98359_55_E 98359_40_E 98359_25_E 98359_100_F 98359_80_F 98359_55_F 98359_40_F 98359_25_F 98359_100_G 98359_80_G 98359_55_G 98359_40_G 98359_25_G

Li 0.85 2.72 9.34 0.35 0.28 2.13 2.44 8.56 0.85 0.96 1.37 1.94 1.06 2.28 2.68 27.73 4.03 88.56 0.34 10.43 12.91 36.32 132.88 0.99 5.82 16.32 12.99 26.59

Be 6.15 1 36.26 0.97 0.76 4.14 9.48 40.81 0.87 2.6 3.78 6.63 5.84 6.7 25.98 44.3 91.09 0.001 6.06 20.45 4.96 71.31 261.06 1.84 22.91 8.32 36.17 20.52

B 1.84 2.62 17.2 1.7 1.93 2.5 4.4 7.12 1.82 2.04 2.29 3.11 13.21 4.99 7.94 19.64 34.21 199.35 11.18 10.92 16.16 38.18 213.89 7.84 7.43 21.05 25.21 23.14

Na 4997.4 5108.13 4932.36 4903.19 4943.03 4978.02 4939.95 4685.97 5348.59 5498.72 5397.37 5637.41 5009.01 5693.73 5703.57 6335.57 5890.06 6135.47 5376.1 5545.46 5018.12 5154.94 4990.08 6065.3 6486.02 6366.62 7365.09 6516.75

Al 0.9 1.48 4.83 1.42 0.4 0.78 1.26 4.62 0.371 0.41 0.83 2.74 8.41 1.73 3.71 8.38 12.74 52.47 2.45 4.28 7.04 18.9 54.35 1.48 3.49 8.73 11.35 14.08

Si 193.66 138.25 536.52 131.86 75.48 174.13 160.23 424.1 108.87 74.36 138.84 119.24 385.34 120.81 359.4 845.04 1292.55 4619.15 117.25 369.16 777.74 1514.04 5214.77 122.57 306.39 607.2 721.1 1466.4

P 62665.92 65434.02 65240.82 57371.66 59334.29 60393.65 60888.34 60645.27 56437.51 59835.91 57691.97 62415.65 60288.12 61014.26 59933.54 65396.54 58709.2 51783.14 54854.95 56081.06 56912.63 61955.3 88965.34 58369.95 61152.07 64143.27 68626 54682.33

Ca 320117.16 336027.06 323575.19 297024.63 307361.75 307603.44 308996.72 302050.69 289109.75 313633.91 298639.34 315019.06 296060.66 294051.94 293452.13 319323.88 274251.97 220297.39 270489.22 283891.38 285265.88 298124.88 299012.84 283876.56 304762.22 284995.59 258389.66 183066.75

Sc 0.4 0.7 1.89 0.084 0.205 0.35 0.49 1.9 0.069 0.141 0.32 0.48 1.41 0.6 1.48 2.87 5.54 19.94 0.56 1.75 2.89 7.33 20.08 0.62 1.24 2.74 2.63 6.14

Ti 3.85 6.11 19.11 9.34 1.4 1.94 5.86 15.54 0.4 1.35 2.48 3.42 10 5.77 8.44 24.87 46.59 209.49 3.38 11.38 25.69 39.52 128.23 5.77 11.51 22.78 32.38 46.8

V 2.83 2.6 3.12 2.68 2.65 2.83 2.47 2.84 2.44 2.76 2.76 2.13 3.11 0.87 1.07 2.47 3.59 12.45 1.33 1.43 2.92 4.4 24.21 2.29 2.48 3.24 3.48 3.69

Cr 2.3 3.71 12.41 1.44 0.96 2.03 3.95 12.05 1.68 2.29 2.41 3.43 11.95 3.19 8.7 22.56 35.73 125.24 4.39 9.97 19 41.9 140.9 3.19 8.59 16.17 19.71 39.59

Mn 1.14 1.08 3.04 5.56 0.66 0.92 0.9 2.59 0.541 0.66 0.74 1.6 2.57 0.77 2.03 4.73 8.45 28.88 0.71 2.15 4.46 9.36 34.68 0.88 1.73 3.46 4.13 9.15

Fe 20.95 41.69 57.78 25.07 28.16 27.2 27.81 48.3 26.33 25.96 16.67 18 51.89 22.66 37.9 85.3 184.32 612.7 22.55 49.12 74.04 169.8 693.69 35.08 59.96 69.29 99.63 167

Co 0.157 0.126 0.75 0.034 0.078 0.046 0.074 0.68 0.035 0.038 0.114 0.27 0.62 0.235 0.64 2.19 1.29 6.96 0.171 0.41 1.43 2.83 10.34 0.109 0.105 0.25 1 2.9

Cu 0.52 0.65 2.23 0.144 0.28 0.6 1.06 2.76 0.153 0.204 0.35 0.62 2.15 0.97 2.48 4.88 5.93 26.93 0.56 1.87 4.11 8.52 24.89 0.67 5.64 4.12 5.21 9.47

Zn 37.53 38.11 32.91 32.56 30.94 33.48 34.83 46.73 39.78 54.79 68.81 95.38 143.8 18.37 23.36 28.19 24.78 78.4 23.29 16.37 35.53 35.4 110.36 24.71 25.48 26.7 36.11 62.63

Table 2. (Continued) Sample ID_Spot Size_Group 98359_100_H 98359_80_H 98359_55_H 98359_40_H 98359_25_H Ga 0.289 0.28 0.205 0.23 1.4 0.164 0.201 0.227 0.356 0.99 0.173 0.159 0.227 0.25 0.67 0.179 0.146 0.33 0.26 0.86 0.44

Li 3.75 2.82 6.31 22.02 0.001

Ge 0.059 0.115 0.35 0.43 2.42 0.06 0.178 0.36 0.37 1.82 0.066 0.112 0.31 0.5 1.15 0.057 0.158 0.235 0.51 1.58 0.69

Be 2.99 0.001 38.17 43.49 74.28

As 0.26 0.59 1.08 1.33 8.53 0.25 0.51 1.21 1.29 4.97 0.3 0.54 1.02 1.46 5.52 0.227 0.69 1.07 1.42 4.82 2.2

B 4.01 9.87 16.94 20.41 23.4

Rb 0.113 0.101 0.264 0.29 1.53 0.048 0.119 0.226 0.32 1.1 0.048 0.093 0.163 0.28 0.95 0.043 0.089 0.194 0.31 0.85 0.5

Na 6224.87 6294.9 6217.4 12534.47 7188.94

Sr 97.83 99.13 99.69 99.84 113.04 98.87 99.7 99.58 98.14 90.62 94.49 93.42 89.69 88.4 91.61 94.98 98.26 94.41 100.4 98.18 115.82

Y 0.637 0.202 0.03 0.249 0.206 0.095 0.093 0.136 0.473 1.46 0.0663 0.115 0.272 0.564 1.29 0.146 0.098 0.302 0.365 1.08 0.106

Al 12.51 7.55 47.8 79.04 52.02

Si 207.87 238.55 711.08 857.8 1536.63

Zr90 0.118 0.027 0.064 0.095 0.45 0.0166 0.058 0.024 0.136 0.4 0.0216 0.0181 0.045 0.037 0.54 0.031 0.031 0.11 0.085 0.29 0.162

Zr91 0.195 0.027 0.77 0.53 3.81 0.092 0.206 0.22 0.36 2.61 0.08 0.114 0.35 0.44 1.63 0.099 0.34 0.5 0.41 1.85 1.47

P 58702.38 61310.42 59534.62 71561.28 42252.8

Nb 0.0091 0.0101 0.088 0.047 0.31 0.014 0.04 0.058 0.025 0.28 0.0155 0.03 0.031 0.059 0.26 0.0123 0.0092 0.036 0.062 0.143 0.112

Mo 0.132 0.187 0.31 0.47 2.68 0.129 0.156 0.27 0.42 2.98 0.115 0.136 0.35 0.3 2.09 0.111 0.107 0.191 0.59 1 1.26

Ca 285996.91 263535.66 281567.31 280557.31 179132.05 Ag 0.046 0.026 0.035 0.146 0.69 0.026 0.033 0.131 0.107 0.51 0.027 0.055 0.057 0.108 0.34 0.028 0.039 0.038 0.162 0.64 0.29

Sc 0.6 0.98 3.06 3.93 6.78

Cd 0.119 0.118 0.35 0.65 0.76 0.046 0.143 0.23 0.43 2.47 0.096 0.096 0.38 0.65 1.78 0.026 0.202 0.36 0.76 1.88 1.22

Ti 6.59 9 58.95 27.23 51.88

In 0.0095 0.0181 0.0248 0.058 0.32 0.0091 0.0175 0.024 0.03 0.06 0.0084 0.0216 0.03 0.066 0.256 0.0098 0.0119 0.06 0.078 0.25 0.089

V 2.59 1.89 2.67 5.46 4.79

Sn 0.174 0.134 0.164 0.32 1.14 0.099 0.226 0.223 0.29 0.95 0.113 0.123 0.158 0.25 0.92 0.162 0.184 0.193 0.32 0.98 0.66

Cr 3.5 6.58 18.7 86.26 41.22

Sb 0.025 0.028 0.1 0.106 0.98 0.032 0.04 0.131 0.27 0.76 0.0157 0.054 0.105 0.225 0.58 0.0282 0.046 0.083 0.135 0.19 0.34

Mn 1.68 1.44 4.28 5.75 10.26

Cs 0.01 0.0226 0.039 0.048 0.26 0.0078 0.0261 0.0259 0.055 0.16 0.0093 0.0124 0.0222 0.042 0.149 0.0099 0.0132 0.043 0.038 0.235 0.144

Ba 31.86 27.73 26.19 29.06 28.33 26.24 25.92 27.05 28.35 32.27 23.09 23.17 25.15 25.52 28.06 23.15 23.85 25.99 29.78 33.55 27.56

Fe 45.43 32.42 83.31 109.72 199.94 La 2.48 1.112 0.08 1.06 0.47 0.376 0.453 0.778 1.9 6.98 0.478 0.568 1.1 1.96 3.11 0.762 0.389 1.44 2.28 6.87 0.273

Co 0.204 0.5 1.28 1.69 3.52 Ce 2.59 0.471 0.064 0.454 0.187 0.295 0.322 0.4 1.36 3.72 0.301 0.297 0.514 0.952 1.41 0.379 0.186 0.722 1.02 6.91 0.139

Cu 3 1.61 5.5 17.16 10.06

Zn 60.05 104.34 133.56 239.34 117.26

Pr 0.386 0.126 0.024 0.189 0.145 0.0577 0.086 0.123 0.277 1.45 0.0706 0.078 0.108 0.353 0.491 0.1098 0.061 0.198 0.513 0.96 0.06

Table 2. (Continued) Ga 1.32 2.29 2.61 10.95 0.448 0.74 0.84 1.49 2.29 0.27 0.34 0.98 1.45 1.3 0.35 0.77 1.13 7.62 9.57 0.621 0.471 0.43 0.38 1.34 0.547 0.68 0.84 0.51 1.31 0.73

Ge 2.24 5.01 5.47 25.87 0.42 0.93 2.42 2.28 4 0.26 0.42 1.58 2.56 2.43 0.41 1.19 2.39 15.65 14.68 0.081 0.179 0.37 0.79 2.38 0.072 0.191 0.35 0.77 2.43 0.069

As 6.29 13.02 19.01 93.03 0.86 4.22 5.82 6.6 13.63 1.37 2.25 4.6 6.75 9.05 1.45 3.52 5.99 43.8 65.94 0.222 0.7 1.27 2.15 6.35 0.25 0.43 0.89 2.05 6.35 0.36

Rb 1.14 2.81 3.94 15.15 0.266 0.81 1.5 1.54 2.81 0.164 0.32 0.96 1.38 1.86 0.246 0.7 1.56 8.79 9.98 0.114 0.096 0.28 0.4 1.73 0.081 0.105 0.217 0.37 1.49 0.052

Sr 114.88 108.6 102.1 124.85 98.78 110.75 111.94 87.97 65.19 103.57 83.64 108.29 102.31 92.34 101.59 108.49 108.61 131.1 199.86 113.4 113.59 113.71 113.51 110.88 120.11 123.23 122.05 124.06 121.92 126.72

Y 1.93 3.74 4.43 7.55 1.072 2.2 3.57 2.89 1.75 1.18 1.93 3.67 1.6 3.72 1.25 2.75 3.88 5.01 23.33 0.0071 0.0161 0.1 0.027 0.232 0.0202 0.0208 0.062 0.062 0.117 0.0145

Zr90 0.34 1.76 2.36 10.16 0.177 0.29 0.44 0.77 0.52 0.104 0.107 0.42 0.31 1.11 0.112 0.22 1.23 2.52 4.61 0.0415 0.0188 0.082 0.147 0.71 0.026 0.053 0.111 0.135 0.47 0.0182

Zr91 1.56 5.07 7.56 20.66 0.72 2.98 1.99 2.75 4.32 0.23 0.83 1.3 3.33 3.31 0.51 1.75 2.76 11.45 14.81 0.112 0.224 0.64 1.16 3.23 0.116 0.319 0.5 0.86 3.04 0.165

Nb 0.47 0.77 0.94 5.44 0.022 0.5 0.43 0.3 0.84 0.086 0.073 0.34 0.41 0.3 0.077 0.34 0.32 1.74 2.25 0.0098 0.034 0.098 0.144 0.49 0.0144 0.045 0.133 0.186 0.33 0.0085

Mo 1.2 3.88 4.73 22.39 0.55 1.03 0.47 1.94 4.07 0.31 0.64 1.74 1.64 1.8 0.55 1.56 4.31 8.83 19.79 0.099 0.238 0.29 0.73 1.9 0.09 0.106 0.33 0.49 1.66 0.066

Ag 0.76 1.01 2.77 10.16 0.145 0.76 0.57 0.55 2.24 0.078 0.137 0.53 0.78 0.67 0.146 0.41 1.06 4.69 6.99 0.034 0.025 0.188 0.34 1.17 0.028 0.02 0.119 0.254 1.1 0.034

Cd 2.23 2.96 6.23 24.01 0.66 1.54 3.26 2.24 2.6 0.188 0.55 1.41 4.02 1.89 0.41 0.52 1.7 15.91 11.86 0.072 0.127 0.51 1.19 3.63 0.052 0.134 0.071 0.96 1.37 0.067

In 0.267 0.43 0.83 2.5 0.0166 0.255 0.42 0.235 0.39 0.032 0.108 0.194 0.33 0.35 0.061 0.15 0.105 1.39 2.54 0.0096 0.0236 0.037 0.115 0.196 0.0115 0.0117 0.043 0.074 0.185 0.0071

Sn 0.97 2.11 3.45 13.34 0.28 1.24 1.48 1.98 2.14 0.202 0.38 1.26 1.39 2.22 0.33 0.88 2.5 7.67 11.67 0.231 0.142 0.33 0.57 1.75 0.216 0.144 0.255 0.42 1.49 0.189

Sb 1.02 2.34 1.09 11.62 0.078 0.43 0.91 1.4 1.45 0.074 0.051 0.42 1.06 1.05 0.107 0.55 0.94 3.61 4.65 0.04 0.07 0.285 0.42 0.38 0.029 0.074 0.155 0.266 1.14 0.025

Cs 0.239 0.51 0.82 3.15 0.051 0.258 0.34 0.31 0.42 0.026 0.062 0.121 0.23 0.33 0.051 0.124 0.25 1.49 2.36 0.0059 0.0222 0.052 0.087 0.36 0.01 0.02 0.062 0.091 0.28 0.0137

Ba 35.36 49.17 46.24 81.83 33.08 38.03 39.03 36.26 38.63 32.91 32.04 40.59 34.87 52.26 34.27 42.02 53.94 54.43 178.6 85.83 78.45 71.86 73.39 64.21 87.27 94.47 101.74 104.38 99.09 102.59

La 5.16 9.96 9.84 18.26 3.9 6.8 12.31 13.53 16.01 5.22 9.71 13.1 10.14 17.38 3.87 5.68 10.85 17.58 64.67 0.0104 0.0129 0.03 0.054 0.186 0.0284 0.034 0.029 0.049 0.117 0.0178

Ce 2.97 6.19 8.43 23.55 1.91 3.12 4.9 4.43 4.56 1.48 2.42 4.24 3.46 6.8 2.89 2.8 8.85 13.22 31.76 0.0439 0.0161 0.027 0.048 0.088 0.0251 0.0321 0.044 0.062 0.103 0.0299

Pr 1.26 1.51 2.6 2.48 0.805 1.22 2.28 1.98 2.37 0.693 1.52 2.35 1.81 2.94 0.549 1.36 1.86 5.65 14.75 0.0063 0.0126 0.036 0.053 0.181 0.0063 0.0164 0.028 0.034 0.207 0.0065

Table 2. (Continued Ga 0.78 0.97 1.38 1.27 1.24 1.12 1.21 1.51 1.11 0.54 1.13 1.99 4.95 15.88 0.77 1.08 2.71 4.11 8.42 1.11 1.36 2.16 3.5 11.52 1.35 1.82 1.87 3.91 12.77 1.81

Ge 0.148 0.33 0.61 2.18 0.097 0.198 0.46 0.5 1.98 1.03 2.54 3.68 11.7 29.5 0.8 1.92 3.94 6.78 18.06 0.82 2.12 3.75 7.68 37.92 0.67 3.45 3.82 8.13 22.53 0.216

As 0.38 1.26 1.89 5.65 0.33 0.43 1.43 1.35 5.93 1.87 5.05 14.72 22.35 84.77 2.13 2.56 8.82 16.77 44.23 1.45 4.51 7.95 20.3 103.24 2.45 9.34 7.56 20.01 74.21 0.45

Rb 0.108 0.212 0.43 1.14 0.103 0.117 0.222 0.35 1 0.46 1.02 2.1 4.47 15.29 0.32 1.1 1.64 2.79 8.29 0.28 0.98 1.96 3.52 17.55 0.35 1.65 2.13 3.55 12.26 0.232

Sr 129 128.36 138.29 124.89 142.7 147.03 150.12 148.55 141.15 107.97 99.03 94.71 84.06 133.39 121.7 106.33 109.06 121.88 131.03 108.09 127.42 117.05 103 131.44 118.94 114.98 95.27 91.21 100.22 206.73

Y 0.0205 0.048 0.114 0.143 0.0087 0.0209 0.0155 0.025 0.261 0.111 0.123 0.147 1.22 3.27 0.086 0.42 0.53 0.001 1.34 0.131 0.38 0.63 0.4 3.79 0.144 0.133 0.47 0.3 0.7 0.318

Zr90 0.052 0.109 0.019 0.25 0.0254 0.067 0.144 0.072 0.108 0.17 0.42 1.17 1.53 5.03 0.22 0.53 5.2 1.59 1.59 0.019 0.47 1.36 2.7 5.82 0.31 0.89 1.61 1.78 4.39 0.055

Zr91 0.288 0.33 0.65 3.62 0.086 0.175 0.102 0.86 3.15 0.77 1.45 3.37 6.92 32.21 0.6 4.16 6.15 9.65 13.18 0.97 2.13 3.08 10.58 45.6 1.41 4 4.6 6.09 39.75 0.177

Nb 0.0273 0.053 0.066 0.71 0.0035 0.0267 0.016 0.025 0.28 0.034 0.5 0.48 0.77 6.03 0.159 0.52 0.69 1.7 0.83 0.148 0.46 0.47 1.32 4.03 0.029 0.067 0.99 1.87 2.63 0.0157

Mo 0.122 0.27 0.5 1.62 0.131 0.192 0.41 0.47 2 0.6 2.96 3.44 10.78 17.75 0.46 2.96 4.56 12.3 17.83 0.76 1.67 4.17 6.76 15.84 0.55 4.43 4.41 6.75 15.57 0.292

Ag 0.049 0.146 0.33 0.88 0.036 0.074 0.195 0.25 0.54 0.33 0.57 0.94 3.6 16.76 0.181 0.31 0.76 3.77 3.27 0.208 0.65 0.82 3.22 11.34 0.215 1.22 1.72 1.22 8.62 0.063

Cd 0.185 0.27 0.61 2.31 0.067 0.236 0.34 1.05 2.82 1.04 2.08 6.83 9.24 24.8 0.65 1.84 2.83 7.39 10.07 0.64 2.82 4.06 9.28 19.97 0.76 4.28 3.47 6.5 12.95 0.431

In 0.0039 0.032 0.096 0.36 0.0104 0.037 0.056 0.052 0.31 0.066 0.43 0.6 1.46 1.96 0.104 0.253 0.39 0.48 1.61 0.072 0.184 0.59 0.91 1.4 0.149 0.35 0.4 1.05 2.98 0.0153

Sn 0.211 0.225 0.61 1.64 0.225 0.215 0.27 0.39 1.11 0.47 1.22 2.45 7.33 19.28 0.48 1.25 3.5 3.56 10.88 0.36 1.62 2.34 5.16 18.79 0.39 2.02 2.39 4.89 12.67 0.352

Sb 0.051 0.151 0.279 0.43 0.052 0.106 0.162 0.258 0.55 0.232 0.36 1.01 4.14 9.62 0.36 0.71 0.97 2.34 1.6 0.143 0.63 1.81 1.79 10.92 0.29 1.43 2.12 3.07 10 0.059

Cs 0.034 0.045 0.048 0.31 0.0126 0.0158 0.059 0.063 0.3 0.133 0.242 0.59 1.08 3.12 0.088 0.152 0.36 0.71 1.8 0.05 0.38 0.45 0.77 4.49 0.072 0.38 0.41 0.83 3.05 0.0156

Ba 123.08 137.93 177.35 160.01 154.58 160.86 179.69 184.3 191.44 93.47 82.7 86.54 90.61 126.97 140.69 143.84 376.5 193.63 195.51 119.4 152.91 141.88 124.83 165.22 157.4 115.34 104.97 113.29 88.85 163.9

La 0.07 0.075 0.159 0.168 0.0097 0.025 0.053 0.081 0.145 0.027 0.215 0.25 1.23 2.56 0.048 0.135 0.21 0.44 1.04 0.066 0.169 0.49 0.48 3.59 0.079 0.223 0.36 0.31 2.43 0.447

Ce 0.064 0.057 0.065 0.26 0.0212 0.0183 0.038 0.043 0.128 0.077 0.134 0.22 0.97 2.26 0.021 0.037 0.254 0.55 2.99 0.034 0.258 0.3 0.43 1.83 0.049 0.197 0.035 0.6 1.38 0.363

Pr 0.0131 0.028 0.051 0.164 0.0038 0.0039 0.036 0.036 0.068 0.06 0.04 0.244 0.38 1.77 0.033 0.09 0.35 0.31 1.02 0.032 0.165 0.098 0.58 1.44 0.067 0.31 0.218 0.33 2.16 0.0649

Table 2. (Continued) Ga 1.53 1.55 1.63 3.32 1.4 1.59 1.09 1.66 2.33 1.44 1.45 1.76 1.93 2.5 1.09 1.06 1.47 2.1 3.04 1.41 2.54 6.74 12.2 25.27 2.36 2.66 12.23 7.62 20.67 2.93

Ge 0.27 0.62 1 4.37 0.132 0.29 0.6 1.02 2.53 0.119 0.207 0.58 0.99 3.47 0.13 0.275 0.48 0.61 2.31 1.14 3.87 9.46 22.17 43.11 1.09 3.31 7.5 10.44 35.41 0.58

As 0.76 2.12 2.64 8.97 0.3 0.83 1.36 1.95 5.66 0.35 0.62 1.47 1.92 9.58 0.34 0.47 1.13 1.98 6.26 2.89 9.25 19.98 49.42 97.02 2.77 6.48 15.72 28.82 68.97 1.56

Rb 0.34 0.7 1.15 4.4 0.161 0.29 0.55 0.83 2.98 0.139 0.226 0.43 0.76 2.67 0.85 0.222 0.44 0.83 1.72 0.94 2.57 6.25 15.86 24.64 0.85 1.89 4.86 6.98 24.42 0.57

Sr 196.19 205.11 198.42 194.21 193.9 194.45 189.39 178.93 161.1 186.09 184.64 180.38 181.43 180.83 177.93 186.82 185.44 159.81 160.61 162.01 153.97 167.69 193.57 199.85 151.26 157.07 173.82 155.48 152.54 155.8

Y 0.139 0.201 0.605 1.29 0.176 0.324 0.059 0.68 1.67 0.527 1.09 0.571 0.83 1.82 0.635 0.73 0.602 1.9 6.99 1.08 2.06 6.31 9.23 15.14 2.82 9.13 12.57 12.15 15.98 10.67

Zr90 0.139 0.136 0.37 1.13 0.04 0.094 0.181 0.34 1.15 0.047 0.147 0.091 0.215 1 0.033 0.105 0.193 0.214 0.88 1.8 1.4 3.23 5.85 23.78 0.37 1.22 2.6 2.52 6.04 0.82

Zr91 0.34 1.05 0.2 5.12 0.234 0.3 0.58 0.97 4.01 0.095 0.219 0.41 1.19 5.19 0.095 0.212 0.76 0.79 1.78 2.1 1.59 7.28 27.95 46.47 0.96 3.17 10.91 9.16 54.48 0.7

Nb 0.03 0.053 0.163 0.64 0.0227 0.0157 0.14 0.149 0.5 0.0145 0.048 0.025 0.149 0.4 0.0145 0.033 0.054 0.172 0.27 0.163 0.31 1.58 1.72 3.81 0.208 0.49 1.45 2.47 8.38 0.169

Mo 0.52 0.39 0.84 3.29 0.388 0.31 0.53 0.22 1.83 0.241 0.319 0.61 0.77 2.9 0.219 0.168 0.32 1.17 1.73 0.83 3.07 8.15 18.08 21.26 0.76 1.78 6.12 6.38 30.58 0.42

Ag 0.76 0.229 0.23 0.87 0.056 0.094 0.181 0.37 1.78 0.042 0.069 0.017 0.204 0.82 0.055 0.134 0.138 0.252 0.98 1.27 0.87 3.28 4.46 14.85 0.33 1.02 3.03 2.99 15.18 0.32

Cd 0.26 0.53 1.31 7.81 0.248 0.158 0.74 0.41 1.15 0.65 0.161 0.43 0.87 1.9 0.069 0.27 0.45 0.86 1.83 1.79 10.95 9.13 23.34 33.56 0.98 0.84 6.8 8.67 33.86 0.76

In 0.0069 0.064 0.159 0.44 0.0091 0.025 0.071 0.119 0.45 0.0183 0.023 0.071 0.103 0.32 0.0164 0.037 0.1 0.084 0.22 0.129 0.24 1.41 2.43 5.21 0.144 0.44 1.06 1.4 2.75 0.096

Sn 0.333 0.32 0.59 2.24 0.32 0.296 0.46 0.44 1.7 0.262 0.308 0.27 0.55 2.07 0.379 0.437 0.32 0.47 1.26 1.91 2.01 5.16 14.76 23.53 0.71 1.42 4.46 6.23 18.62 0.48

Sb 0.139 0.122 0.3 2.39 0.042 0.121 0.145 0.276 0.65 0.038 0.087 0.231 0.38 1.45 0.026 0.102 0.211 0.221 0.7 0.111 1.52 2.01 7.72 10.46 0.263 0.87 2.99 3.81 7.45 0.134

Cs 0.036 0.084 0.148 0.45 0.121 0.042 0.051 0.06 0.234 0.0143 0.0221 0.055 0.134 0.37 0.0126 0.0238 0.062 0.069 0.27 0.158 0.51 0.89 2.28 1.89 0.151 0.223 0.81 1.44 3.03 0.073

Ba 148.62 171.93 164.96 182.34 142.54 158.96 136.55 156.83 160.34 163.42 174.71 166.84 180.55 179.16 123.46 125.96 131.01 159.81 264.05 190.43 192.38 217.35 268.64 375.55 177.84 227.77 737.75 276.48 742.59 307.13

La 0.267 0.459 0.705 1 0.281 0.544 0.085 1.15 2.37 0.83 1.94 0.98 1.63 3.14 0.782 1.03 0.722 4.38 12.94 1.4 2.4 6.41 9.73 18.25 2.25 6.31 9.57 7.38 13.73 10.27

Ce 0.177 0.353 0.501 1.25 0.342 0.419 0.172 1.05 2.81 0.93 1.4 1.03 1.49 2.86 1.74 0.702 0.97 3.84 13.27 1.23 2.2 5.17 9.64 16.68 2.75 9.1 56.14 14.34 104.21 10.94

Pr 0.0459 0.094 0.131 0.252 0.0523 0.101 0.024 0.074 0.41 0.165 0.395 0.232 0.301 0.71 0.18 0.174 0.201 0.781 2.72 0.195 0.62 1.57 2 4.18 0.713 2.13 2.05 2.86 6.49 2.51

Table 2. (Continued) Ga 2.72 4.08 4.71 14.98 3.09 6.39 12.6 18.47 11.85 2.32 2.47 2.78 1.71 1.46 2.03 1.99 2.11 1.6 2.01 1.69 1.8 1.6 1.83 2.53 1.843 1.75 1.55 1.43 1.84 2.73

Ge 2.42 6.68 7.5 32.33 1.56 3.66 5.16 4.94 9.31 0.105 0.209 0.51 0.89 3.1 0.089 0.206 0.47 0.75 3.59 0.067 0.217 0.25 0.67 2.52 0.073 0.161 0.36 0.89 3 0.72

As 4.8 20.86 14.09 80.38 2.57 8.1 14.87 10.8 19.6 0.25 0.59 0.98 1.88 5.17 0.238 0.41 1.24 1.76 4.46 0.198 0.4 0.76 1.53 3.9 0.203 0.32 0.77 1.18 4.17 1.93

Rb 1.45 4.56 4.63 19.07 0.85 1.88 3.39 3.5 6.67 0.09 0.182 0.38 0.59 2.26 0.071 0.163 0.26 0.52 2.1 0.052 0.108 0.258 0.7 1.44 0.615 1.5 0.69 0.42 1.12 0.43

Sr 156.39 181.93 132.64 148.55 186.2 201.28 208.04 208.2 222.18 164.29 168.34 168.1 170.07 155.8 167.05 160.26 152.23 151.87 160.76 146.77 145.79 137.35 137.33 139.54 210.62 221.73 217.34 165.43 145.39 192.06

Y 10.98 15.09 14.19 21.68 12.91 21.79 18.21 44.51 55.27 0.0442 0.041 0.072 0.075 0.135 0.0088 0.0162 0.052 0.088 0.234 0.0095 0.0169 0.048 0.084 0.29 0.452 0.43 0.435 0.769 0.75 1.05

2.44

1.62

6.13

1.18

194.28 0.202

Zr90 1.16 1.78 2.63 9.03 0.95 0.95 1.92 5.02 5.62 0.036 0.095 0.127 0.228 0.54 0.053 0.083 0.079 0.134 1.13 0.0144 0.059 0.102 0.128 0.44 0.29 0.395 0.479 0.366 0.39 0.238

Zr91 2.91 6.1 9.72 23.52 1.11 3.5 6.19 7.52 10.81 0.143 0.255 0.41 1.29 3.54 0.19 0.34 0.36 0.62 2.33 0.149 0.35 0.88 0.4 2.85 0.198 0.281 0.48 0.78 2.06 1.27

Nb 0.27 1.23 1.49 6.27 0.24 0.38 1.73 0.76 1.66 0.0216 0.047 0.089 0.225 0.53 0.0128 0.0175 0.078 0.038 0.61 0.0143 0.0237 0.0068 0.09 0.31 0.0184 0.081 0.047 0.145 0.39 0.039

Mo 2.31 4.5 4.46 18.71 1.53 1.97 3.48 3.46 7.44 0.152 0.186 0.128 0.31 0.78 0.097 0.191 0.56 0.48 1.8 0.089 0.121 0.26 0.92 0.26 0.084 0.156 0.26 0.75 1.4 0.4

Ag 1.08 3.66 2.56 6.59 0.95 1.39 2.01 2 3.04 0.04 0.088 0.166 0.211 1 0.013 0.085 0.18 0.143 0.67 0.027 0.089 0.135 0.169 0.82 0.0362 0.047 0.156 0.225 0.9 0.259

Cd 2.94 4.05 8.01 16.78 1.12 3.05 5.39 3.08 6.62 0.252 0.203 0.174 0.8 2.21 0.075 0.152 0.32 0.39 1.46 0.059 0.138 0.3 0.64 2.19 0.1 0.103 0.34 0.78 1.6 0.8

In 0.146 1.1 1.2 4.1 0.216 0.37 0.38 0.93 1.41 0.0059 0.0214 0.066 0.119 0.201 0.0136 0.04 0.034 0.071 0.202 0.0026 0.0085 0.027 0.117 0.28 0.0142 0.0209 0.056 0.077 0.251 0.089

Sn 1.35 4.39 4.69 17.17 0.7 2.26 3.19 3.07 7.8 0.252 0.158 0.29 0.4 1.64 0.157 0.131 0.3 0.32 1.54 0.238 0.145 0.235 0.43 1.37 0.167 0.323 0.2 0.37 1.44 0.45

Sb 0.125 1.54 1.53 7.74 0.3 1.5 1.67 1.17 5.63 0.035 0.044 0.175 0.123 0.87 0.036 0.06 0.127 0.153 0.81 0.033 0.077 0.033 0.147 0.87 0.031 0.076 0.135 0.111 0.63 0.159

Cs 0.22 0.6 0.96 4.04 0.134 0.25 0.43 0.71 1.13 0.0097 0.0229 0.066 0.095 0.32 0.0071 0.0185 0.0206 0.081 0.36 0.0093 0.02 0.054 0.081 0.32 0.0287 0.0423 0.034 0.08 0.28 0.069

Ba 354.16 250.35 256.57 274.65 249.58 437.42 1199.26 1671.06 1419.87 250.44 262.66 252.21 245.61 223.98 238.67 230.61 216.93 214.98 221.25 202.42 199.97 183.53 192.41 199.11 197.17 199.39 182.75 211.52 198.71 369.39

La 13.44 12.07 15.86 14.36 18.37 26.89 26.84 58 72.04 0.425 0.524 0.105 0.136 0.257 0.0503 0.0176 0.026 0.064 0.169 0.0368 0.029 0.045 0.064 0.207 0.663 1.024 0.771 1.55 1.76 6.69

Ce 25.84 5.77 10.76 16.65 18.96 52.44 163.57 186.59 301.29 0.212 0.105 0.027 0.048 0.162 0.427 0.0111 0.024 0.04 0.262 0.196 0.0273 0.031 0.038 0.198 0.599 1.148 0.789 1.48 1.98 1.81

Pr 3.47 2.63 3.54 5.86 3.55 6.68 6.77 12.35 17.47 0.0386 0.0209 0.03 0.038 0.179 0.0202 0.0219 0.0261 0.063 0.118 0.0089 0.0079 0.0169 0.03 0.102 0.14 0.191 0.141 0.413 0.389 0.453

0.45

0.27

0.164

1.57

0.82

0.34

0.28

1.42

0.41

0.218

348.85

0.206

0.184

0.041

Table 2. (Continued) Ga 3.09 5.39 16.58 2.3 3.29 2.46 4.01 21.01 2.01 2.13 2.48 2.37 5.64 2.15 2.78 3.18 2.05 12.52 1.397 1.48 1.06 0.9 1.56 1.178 1.095 0.94 1.22 1.45 1.044

Ge 6.52 11 43.92 0.57 2.01 4.15 6.7 38.44 0.74 1.26 3.84 3.58 10.9 0.58 2.08 2.9 4.79 7.97 0.081 0.193 0.44 1.01 1.89 0.124 0.243 0.4 0.8 1.95 0.107

As 9.67 29.6 90.07 1.48 4.83 10.83 13.82 73.54 1.98 2.66 10 8.03 26.22 1.56 2.58 5.67 8.97 19.99 0.253 0.49 0.92 1.5 3.69 0.21 0.41 1.15 0.99 5.88 0.223

Rb 3.32 5.97 25.02 0.38 1.16 2.7 3.98 23.54 0.42 0.85 1.83 2.23 6.47 0.35 1.13 1.4 2.43 7.77 0.087 0.114 0.273 0.4 1.57 0.099 0.101 0.228 0.34 1.49 0.156

Sr 214.18 195.33 186.24 183.41 201.3 206.12 161.84 257.53 192.01 199.72 172.15 173.45 170.76 147.27 153.53 138.43 110.57 116.7 110.03 113.44 110.51 111.12 103.01 100.03 99.25 99.12 104.09 95.76 99.42

Y 0.49 0.77 3.23 0.08 0.151 0.5 0.15 3.88 0.055 0.251 0.166 0.48 1.3 0.887 1.85 0.97 2.69 4.21 0.344 0.141 0.054 0.067 0.062 0.0223 0.029 0.051 0.061 0.209 0.0112

Zr90 2.37 4.07 12.05 0.108 0.16 1.52 3.1 10.26 0.119 0.31 0.62 1.45 3.96 0.117 0.61 0.74 1.3 4.87 0.038 0.039 0.164 0.144 0.067 0.038 0.036 0.031 0.089 0.45 0.0296

Zr91 6.91 10.83 32.04 0.46 1.5 6.04 8.22 47.15 0.29 1.44 4.03 8.17 12.85 0.76 1.62 1.95 4.58 9.68 0.086 0.178 0.53 1.14 1.84 0.122 0.233 0.095 0.85 3.58 0.078

Nb 1.05 1.65 0.71 0.036 0.228 0.75 1.25 5.87 0.118 0.028 0.38 0.078 1.96 0.044 0.35 0.194 0.35 0.46 0.0089 0.039 0.057 0.101 1.55 0.0264 0.0205 0.077 0.131 0.214 0.012

Mo 3.82 6 10.91 0.89 1.18 2.74 4.15 21.45 0.43 1.14 3.18 0.41 10.18 0.43 1.82 2.19 0.73 15.4 0.119 0.172 0.42 0.74 1.82 0.13 0.132 0.4 0.84 1.66 0.089

Ag 2.83 0.72 22.76 0.189 0.97 1.44 2.4 7.94 0.16 0.94 1.18 0.98 4.61 0.224 0.95 0.37 1.43 2.85 0.036 0.053 0.248 0.195 0.95 0.026 0.049 0.127 0.048 1.38 0.0233

Cd 4.35 10.79 20.2 0.41 1.34 4.41 2.86 24.33 0.49 0.91 2.55 3.66 9.98 0.59 2.06 2.47 1.78 8.69 0.079 0.16 0.193 0.84 2.9 0.055 0.105 0.23 0.54 1.32 0.086

In 0.69 0.88 2.6 0.065 0.243 0.49 0.82 3.6 0.063 0.165 0.33 0.54 1.28 0.062 0.264 0.225 0.46 0.79 0.0122 0.0291 0.053 0.076 0.186 0.0122 0.0135 0.0195 0.06 0.34 0.0128

Sn 3.13 6.57 21.29 0.37 1.12 3.23 4.36 20.77 0.34 0.93 2.19 2.48 7.01 0.34 1.28 1.59 2.12 7.67 0.193 0.222 0.31 0.5 1.53 0.229 0.137 0.25 0.53 1.44 0.181

Sb 1.74 2.2 8.07 0.232 0.38 1.25 1.47 11.96 0.278 0.37 1.03 1.2 4.64 0.195 0.112 0.87 1.25 4.31 0.031 0.046 0.097 0.241 0.59 0.012 0.043 0.159 0.272 0.54 0.029

Cs 0.82 1.24 3.12 0.055 0.273 0.48 0.7 3.52 0.097 0.172 0.37 0.46 1.41 0.079 0.211 0.29 0.44 1.17 0.0177 0.0277 0.049 0.079 0.225 0.0095 0.0141 0.039 0.099 0.29 0.0109

Ba 399.64 358.22 328.93 275.99 296.07 325.8 250.62 302.23 279.18 291.95 273.52 267.88 299 217.4 244.32 323 289.22 941.25 149.34 157.5 157.73 160.42 140.3 136.64 134.46 139.1 142.2 137.8 131.46

La 0.62 0.56 2.33 0.047 0.154 0.156 0.42 2.8 0.056 0.105 0.207 0.224 2.16 1.268 2.3 2.29 3.85 10.73 1.344 0.733 0.039 0.039 0.37 0.0099 0.0065 0.045 0.044 0.151 0.0169

Ce 0.45 1 2.08 0.03 0.016 0.39 0.69 0.58 0.05 0.162 0.186 0.234 1.06 0.949 1.75 6.69 3.44 57.26 0.95 0.287 0.04 0.061 0.149 0.0369 0.0108 0.0124 0.039 0.018 0.037

Pr 0.133 0.77 3.24 0.023 0.131 0.25 0.29 1.38 0.028 0.073 0.204 0.239 0.22 0.372 0.549 0.53 0.95 2.63 0.236 0.063 0.001 0.033 0.199 0.0044 0.0083 0.0252 0.031 0.104 0.0043

Table 2. (Continued) Ga 1.072 1.22 1.03 1.95 0.909 0.83 0.722 0.99 1.16 1.26 1.97 2.12 2.95 11.22 1.33 0.97 2.32 3.56 13 1.43 0.83 1.49 1.52 3.65 0.76 1.15 1.87 1.99 3.5

Ge 0.165 0.33 0.63 2.21 0.061 0.152 0.43 0.7 1.86 0.55 1.43 4.5 6.9 24.79 0.59 1.65 4.86 7.75 26.08 0.6 1.83 2.68 3.79 9.72 0.59 1.33 3.52 4 9.31

As 0.35 0.89 1.02 3.11 0.194 0.42 0.95 1.59 4.03 1.97 3.11 7.5 17.56 41.45 1.12 4.63 5.4 14.22 39.31 1.15 3.65 4.83 10.52 20.04 1.57 1.92 7.56 9.2 11.1

Rb 0.084 0.171 0.33 1.23 0.04 0.097 0.161 0.34 0.91 0.31 0.97 2.29 2.91 11.19 0.32 0.88 2.1 3.57 13.17 0.35 0.86 1.51 1.75 3.66 0.25 0.64 1.85 1.97 3.98

Sr 98.43 100.79 98.44 95.29 97.5 97.72 91.98 95.94 87.44 114.69 114.12 123.07 109.47 102.25 113.21 120.06 126.26 130.86 102 111.29 112.48 103.07 101.63 63.2 91.15 84.59 88.8 83.46 65.71

Y 0.0041 0.032 0.054 0.189 0.0028 0.0147 0.03 0.075 0.208 0.021 0.25 0.3 0.26 3.83 0.082 0.085 0.55 0.53 2.01 0.0071 0.02 0.35 0.48 1.12 0.079 0.096 0.57 0.135 0.78

Zr90 0.033 0.145 0.165 0.66 0.015 0.032 0.065 0.114 0.37 0.077 0.31 1.06 2.18 5.85 0.078 0.185 1.2 1.68 4.34 0.122 0.46 0.34 0.84 2.1 0.28 0.83 4.86 8.82 70.15

Zr91 0.214 0.33 0.53 1.87 0.068 0.145 0.42 1.47 0.91 0.74 2.47 4.84 9.93 32.64 0.46 2.7 2.73 12.09 19.73 0.73 1.22 1.87 4.67 5.51 0.86 1.02 4.87 6.39 10.87

Nb 0.0031 0.051 0.055 0.29 0.0043 0.0223 0.079 0.032 0.26 0.114 0.237 0.75 0.77 5.77 0.072 0.241 0.42 0.84 3.05 0.043 0.33 0.37 0.72 1.48 0.147 0.15 0.067 0.99 0.52

Mo 0.122 0.38 0.35 1.22 0.096 0.117 0.42 0.17 1.94 0.6 1.16 2.79 4.04 21.71 0.93 1.27 4.45 7.66 16.15 0.6 1 2.79 0.92 9.06 0.36 0.24 3.28 4.56 4.49

Ag 0.064 0.171 0.225 0.79 0.041 0.043 0.154 0.221 0.137 0.222 0.43 2.06 1.5 9.85 0.198 0.67 1.43 3.66 8.47 0.222 0.37 1.03 1.84 2.38 0.167 0.29 1.49 2.19 2.88

Cd 0.078 0.085 0.28 1.2 0.088 0.093 0.19 0.47 1.02 0.47 0.91 4.4 3.19 17.14 0.092 1.42 3.04 4.93 9.78 0.33 1.36 2.2 1.21 7.12 0.5 0.61 2.57 0.57 6.1

In 0.0215 0.027 0.062 0.46 0.0113 0.0238 0.024 0.043 0.139 0.061 0.166 0.63 0.71 2.2 0.077 0.258 0.5 0.45 3.26 0.074 0.3 0.206 0.39 1.02 0.045 0.085 0.23 0.46 0.64

Sn 0.235 0.251 0.48 1.22 0.222 0.123 0.249 0.27 1.32 0.31 0.97 2.16 3.48 16.52 0.35 1.02 1.97 3.8 18.68 0.38 0.91 1.85 1.65 4.23 0.41 0.62 2.11 5.73 5.17

Sb 0.056 0.087 0.075 0.98 0.036 0.054 0.078 0.137 0.77 0.037 0.53 0.91 2.28 7.08 0.214 0.59 1.26 1.11 6.5 0.093 0.46 0.65 1.03 2.57 0.153 0.44 0.76 0.86 2.55

Cs 0.0198 0.052 0.032 0.29 0.0113 0.0125 0.039 0.068 0.223 0.058 0.131 0.54 0.36 2.11 0.04 0.207 0.27 0.94 3.83 0.052 0.106 0.34 0.38 0.78 0.083 0.126 0.37 0.53 0.78

Ba 131.73 140.09 132.39 127.06 108.44 105.28 101.3 106.17 95.92 168.93 186.32 189.52 174.35 135.46 177.44 177.52 209.42 189.63 190.86 147.23 155.12 143.27 144.46 109.49 94.55 93.22 81.3 86.45 50.98

La 0.0191 0.059 0.055 0.136 0.0087 0.0085 0.025 0.054 0.123 0.026 0.147 0.5 0.72 1.38 0.021 0.228 0.282 0.56 1.33 0.03 0.044 0.249 0.34 0.4 0.097 0.266 0.158 0.097 0.56

Ce 0.0171 0.001 0.024 0.122 0.0045 0.0134 0.001 0.077 0.111 0.034 0.025 0.033 0.056 2.14 0.061 0.069 0.099 0.71 1.3 0.034 0.113 0.224 0.178 0.63 0.206 0.086 0.185 0.3 0.62

Pr 0.0108 0.0167 0.038 0.094 0.0049 0.0127 0.015 0.0212 0.148 0.046 0.144 0.3 0.217 1.9 0.033 0.079 0.195 0.61 1.41 0.037 0.087 0.172 0.193 0.39 0.029 0.068 0.142 0.228 0.48

Table 2. (Continued) Nd 1.35 0.541 0.099 0.38 0.67 0.129 0.23 0.39 1.1 4.66 0.154 0.362 0.451 0.63 1.62 0.404 0.21 0.77 1.42 3.91 0.36 4.82 5.06 10.96 24.06 2.7 4.08 7.68 8.7 9.83 2.82

Sm 0.183 0.092 0.282 0.33 1.39 0.065 0.153 0.117 0.31 1.25 0.053 0.119 0.174 0.51 1.29 0.106 0.119 0.262 0.39 1.25 0.48 1.88 4.03 4.54 16.18 0.57 1.54 1.69 2.32 2.7 0.31

Eu 0.0481 0.0174 0.044 0.046 0.37 0.0098 0.028 0.029 0.07 0.25 0.017 0.0187 0.03 0.041 0.224 0.0127 0.0255 0.042 0.043 0.194 0.109 0.166 0.75 0.79 1.52 0.13 0.197 0.42 0.37 0.33 0.08

Gd 0.127 0.058 0.084 0.129 0.41 0.022 0.048 0.073 0.266 0.55 0.0228 0.056 0.082 0.156 0.78 0.0277 0.04 0.083 0.124 0.58 0.46 0.76 1.42 1.73 5.8 0.254 0.59 1.03 0.44 1.26 0.434

Tb 0.0211 0.0075 0.0249 0.03 0.213 0.0046 0.0071 0.0284 0.0215 0.111 0.0034 0.0118 0.0225 0.04 0.164 0.0049 0.0118 0.015 0.046 0.135 0.062 0.093 0.37 0.37 1.23 0.061 0.112 0.2 0.163 0.27 0.031

Dy 0.108 0.045 0.15 0.189 0.23 0.027 0.068 0.099 0.169 0.58 0.033 0.058 0.088 0.199 0.31 0.03 0.051 0.091 0.148 0.192 0.33 0.8 2.24 1.93 6.48 0.199 0.48 1.71 1.1 1.62 0.282

Ho 0.0202 0.0122 0.053 0.047 0.13 0.0084 0.0132 0.025 0.039 0.083 0.0059 0.0177 0.0224 0.055 0.191 0.0089 0.0154 0.0159 0.058 0.144 0.124 0.129 0.68 0.64 2.27 0.046 0.084 0.25 0.3 0.57 0.05

Er 0.0345 0.024 0.111 0.083 0.27 0.0102 0.023 0.073 0.118 0.42 0.0159 0.047 0.036 0.104 0.232 0.0161 0.0287 0.047 0.134 0.177 0.197 0.42 0.68 0.82 1.37 0.089 0.5 0.52 0.48 0.6 0.097

Tm 0.0046 0.005 0.0258 0.038 0.156 0.0058 0.0105 0.017 0.0274 0.033 0.0058 0.0071 0.0253 0.034 0.132 0.005 0.0087 0.022 0.0254 0.023 0.102 0.168 0.39 0.54 1.81 0.039 0.024 0.214 0.208 0.28 0.029

Yb 0.0449 0.052 0.019 0.091 0.86 0.0159 0.029 0.081 0.106 0.39 0.0168 0.067 0.104 0.199 0.51 0.0206 0.041 0.04 0.121 0.22 0.31 0.2 0.49 2.58 6.1 0.143 0.39 1.17 0.28 1.04 0.118

Lu 0.0056 0.0082 0.0168 0.058 0.234 0.0061 0.0136 0.0255 0.024 0.149 0.0053 0.0106 0.0186 0.044 0.114 0.0053 0.013 0.0286 0.027 0.122 0.097 0.178 0.53 0.41 2.35 0.058 0.124 0.23 0.125 0.36 0.036

Hf 0.0164 0.0321 0.034 0.149 0.7 0.0201 0.036 0.102 0.134 0.28 0.0174 0.035 0.041 0.119 0.26 0.0276 0.03 0.108 0.088 0.49 0.275 0.34 1.54 1.44 11.74 0.109 0.29 1.05 0.42 1.18 0.121

Ta 0.0076 0.0192 0.045 0.062 0.18 0.0127 0.0106 0.055 0.05 0.165 0.0051 0.0144 0.031 0.069 0.021 0.0072 0.0215 0.039 0.073 0.033 0.093 0.31 0.39 0.67 4.48 0.034 0.236 0.25 0.3 0.4 0.046

W 0.029 0.083 0.098 0.179 0.84 0.04 0.039 0.049 0.17 0.62 0.0269 0.093 0.068 0.13 0.41 0.027 0.02 0.118 0.305 0.44 0.35 0.74 1.19 0.22 13.74 0.064 0.25 0.66 0.91 1.06 0.077

Re 0.0162 0.045 0.039 0.117 0.39 0.017 0.047 0.109 0.105 0.44 0.0118 0.033 0.069 0.081 0.42 0.0069 0.044 0.052 0.069 0.31 0.216 0.53 0.85 1.47 4.26 0.086 0.27 0.53 0.52 0.93 0.048

Au 0.029 0.116 0.3 0.45 1.32 0.034 0.108 0.302 0.56 1.66 0.036 0.088 0.127 0.34 0.41 0.056 0.086 0.154 0.25 0.8 0.63 0.94 1.52 1.85 15.1 0.34 0.97 0.83 0.81 3.97 0.166

Tl 0.0197 0.061 0.103 0.164 0.77 0.0242 0.051 0.106 0.138 0.5 0.0226 0.046 0.1 0.181 0.51 0.0233 0.049 0.104 0.178 0.47 0.29 0.58 1.59 2.35 9.89 0.11 0.44 0.75 0.82 1.4 0.099

Pb 0.335 0.122 0.086 0.105 0.67 0.072 0.049 0.079 0.12 0.35 0.062 0.068 0.086 0.208 0.48 0.102 0.037 0.066 0.126 0.4 0.192 0.57 1.25 1.13 5.04 0.167 0.6 0.73 0.77 0.97 0.053

Bi 0.009 0.0231 0.041 0.068 0.38 0.0053 0.0235 0.057 0.06 0.222 0.0125 0.0193 0.035 0.047 0.255 0.0112 0.0236 0.049 0.084 0.18 0.057 0.28 0.85 1.03 2.82 0.069 0.272 0.208 0.37 0.57 0.052

Th 0.0106 0.0142 0.047 0.025 0.115 0.0104 0.0077 0.0058 0.028 0.103 0.0077 0.0126 0.0158 0.043 0.134 0.0062 0.0152 0.027 0.044 0.115 0.04 0.167 0.38 0.33 3.1 0.027 0.1 0.148 0.144 0.24 0.024

U 0.0345 0.0112 0.0052 0.056 0.206 0.0095 0.0129 0.0335 0.027 0.327 0.006 0.0147 0.0214 0.035 0.064 0.0102 0.0047 0.0225 0.037 0.095 0.027 0.124 0.32 0.24 0.74 0.038 0.067 0.169 0.168 0.159 0.043

Table 2. (Continued) Nd 6.63 9.97 4.67 11.23 2.39 5.26 5.2 12.41 55.41 0.043 0.092 0.124 0.32 1.08 0.022 0.08 0.118 0.35 0.096 0.044 0.055 0.116 0.214 0.98 0.04 0.039 0.176 0.2 0.6 0.25 0.63

Sm 0.67 1.23 2.69 2.78 0.34 1.2 1.26 11.78 13.9 0.054 0.188 0.27 0.93 2.53 0.079 0.162 0.39 0.67 1.79 0.091 0.17 0.25 0.47 1.23 0.079 0.205 0.47 0.35 1.5 1.01 2.08

Eu 0.199 0.49 0.28 0.37 0.085 0.3 0.31 1.18 2.16 0.0076 0.0164 0.094 0.069 0.33 0.0119 0.03 0.052 0.034 0.44 0.0085 0.0069 0.029 0.094 0.144 0.0123 0.012 0.054 0.107 0.185 0.111 0.098

Gd 0.64 1.01 0.93 1.3 0.224 0.52 1.01 1.96 8.21 0.0253 0.024 0.014 0.32 0.89 0.0037 0.047 0.074 0.168 1.32 0.045 0.043 0.096 0.251 0.81 0.033 0.068 0.103 0.067 0.27 0.03 0.73

Tb 0.075 0.203 0.114 0.223 0.047 0.147 0.145 0.96 1.51 0.0066 0.001 0.049 0.039 0.269 0.0079 0.0141 0.0053 0.036 0.102 0.0048 0.0079 0.0205 0.0102 0.123 0.0034 0.0229 0.044 0.035 0.057 0.078 0.111

Dy 0.24 0.47 0.85 0.84 0.203 0.51 1.07 4.1 4.61 0.033 0.069 0.266 0.169 1 0.024 0.113 0.179 0.222 1.33 0.029 0.094 0.153 0.33 0.55 0.048 0.044 0.164 0.3 0.64 0.43 0.83

Ho 0.06 0.184 0.157 0.3 0.054 0.143 0.133 0.89 1.62 0.0041 0.01 0.052 0.073 0.287 0.0054 0.0213 0.039 0.066 0.3 0.0073 0.0181 0.054 0.041 0.262 0.0076 0.0109 0.053 0.066 0.3 0.118 0.119

Er 0.179 0.52 0.36 0.47 0.135 0.241 0.4 3.04 5.55 0.0092 0.03 0.121 0.125 0.6 0.025 0.045 0.034 0.046 0.57 0.0267 0.044 0.113 0.114 0.39 0.032 0.033 0.07 0.113 0.091 0.249 0.179

Tm 0.045 0.14 0.066 0.144 0.0222 0.125 0.187 1.22 0.91 0.008 0.0013 0.023 0.041 0.282 0.0058 0.0208 0.0218 0.0051 0.053 0.0051 0.0145 0.0215 0.04 0.128 0.0091 0.0107 0.023 0.052 0.03 0.047 0.117

Yb 0.31 0.25 0.98 0.44 0.128 0.3 0.99 2.07 2.29 0.019 0.047 0.109 0.34 1.77 0.0196 0.07 0.18 0.253 0.25 0.0242 0.069 0.145 0.189 1.37 0.0202 0.041 0.156 0.177 1.18 0.225 0.56

Lu 0.041 0.148 0.07 0.39 0.041 0.132 0.276 1.5 2.37 0.006 0.0103 0.0243 0.044 0.211 0.0062 0.0191 0.04 0.069 0.242 0.0076 0.0108 0.046 0.073 0.272 0.0056 0.0161 0.0245 0.088 0.016 0.087 0.174

Hf 0.144 0.28 0.58 0.71 0.077 0.31 0.78 4.23 4.46 0.0195 0.034 0.21 0.247 0.84 0.0246 0.062 0.13 0.07 0.64 0.0303 0.035 0.09 0.194 0.44 0.026 0.064 0.159 0.181 0.21 0.33 0.4

Ta 0.014 0.23 0.29 0.088 0.077 0.199 0.26 1.42 1.84 0.008 0.0197 0.057 0.083 0.53 0.0102 0.006 0.069 0.014 0.33 0.0072 0.0145 0.043 0.046 0.37 0.0129 0.0264 0.074 0.074 0.158 0.067 0.29

W 0.224 0.28 0.64 1.73 0.119 0.58 1.21 2.33 8.46 0.0135 0.052 0.017 0.38 0.41 0.031 0.056 0.117 0.108 1 0.0271 0.055 0.281 0.212 0.68 0.0188 0.081 0.123 0.28 0.84 0.36 0.88

Re 0.098 0.51 0.88 0.83 0.095 0.208 0.43 4.06 4.29 0.0076 0.045 0.076 0.158 0.6 0.0209 0.0188 0.093 0.144 0.67 0.0238 0.048 0.058 0.132 0.65 0.032 0.041 0.077 0.142 0.37 0.181 0.5

Au 0.39 0.93 1.11 2.75 0.206 0.92 1.21 7.97 5.93 0.0258 0.109 0.33 0.59 1.56 0.045 0.066 0.196 0.53 1.18 0.0129 0.091 0.134 0.5 1.79 0.033 0.094 0.247 0.276 1.93 0.41 0.71

Tl 0.163 0.45 0.91 0.83 0.108 0.41 0.74 4.53 4.25 0.0203 0.056 0.122 0.264 0.68 0.027 0.043 0.128 0.205 0.8 0.027 0.054 0.095 0.192 0.56 0.028 0.052 0.121 0.215 0.67 0.28 0.53

Pb 0.154 0.39 0.62 0.95 0.158 0.26 0.65 4.3 3.75 0.036 0.044 0.094 0.184 0.57 0.073 0.027 0.119 0.136 0.59 0.062 0.063 0.067 0.215 0.57 0.029 0.047 0.093 0.115 0.45 0.24 0.55

Bi 0.065 0.153 0.43 0.31 0.064 0.166 0.32 1.53 3.57 0.0097 0.0237 0.055 0.077 0.37 0.0096 0.028 0.033 0.099 0.167 0.0122 0.019 0.033 0.113 0.36 0.0138 0.03 0.065 0.027 0.31 0.071 0.3

Th 0.05 0.096 0.052 0.24 0.027 0.073 0.216 0.83 0.75 0.0065 0.0113 0.0143 0.048 0.162 0.0082 0.0169 0.0127 0.074 0.061 0.0047 0.0031 0.0243 0.063 0.205 0.004 0.012 0.0174 0.059 0.124 0.091 0.129

U 0.075 0.151 0.088 0.115 0.058 0.192 0.328 0.79 1.76 0.002 0.0076 0.0179 0.056 0.074 0.0064 0.0161 0.0238 0.029 0.027 0.0055 0.0078 0.0164 0.0163 0.138 0.004 0.014 0.0247 0.039 0.204 0.079 0.087

Table 2. (Continued) Nd 1.78 2.27 12.95 0.097 0.56 1.21 3.16 1.77 0.27 0.28 1.42 2.82 12.12 0.32 0.71 1.06 0.32 12.89 0.393 0.181 0.44 0.54 0.95 0.191 0.475 0.228 0.74 1.67 0.648 1.36

Sm 2.58 8.57 29.72 0.61 1.56 2.64 5.13 10.84 0.6 1.24 2.53 3.55 30.53 0.71 3.48 3.52 2.49 16.09 0.107 0.214 0.43 0.62 3.18 0.127 0.229 0.44 0.67 1.62 0.142 0.287

Eu 0.64 0.99 2.31 0.086 0.026 0.37 0.22 1.33 0.098 0.3 0.31 0.3 0.82 0.071 0.49 0.65 1.06 2 0.0148 0.0199 0.074 0.078 0.199 0.0181 0.021 0.091 0.119 0.33 0.0188 0.038

Gd 1.7 1.88 6.2 0.163 0.071 0.29 2.62 5.05 0.131 1 1.44 1.65 10.05 0.269 1.32 0.88 2.32 5.35 0.047 0.043 0.114 0.21 0.85 0.04 0.082 0.17 0.32 0.87 0.09 0.255

Tb 0.26 0.4 2.3 0.078 0.171 0.126 0.32 0.31 0.04 0.067 0.253 0.35 2.64 0.041 0.164 0.27 0.35 1.19 0.0069 0.0198 0.025 0.045 0.17 0.0104 0.0123 0.024 0.056 0.133 0.011 0.0178

Dy 2.35 4.59 8.07 0.37 0.74 1.13 3.12 3.49 0.32 0.79 1.33 3.05 9.29 0.3 1.41 1.4 3.04 8.57 0.044 0.07 0.237 0.46 1.04 0.052 0.106 0.3 0.3 0.93 0.057 0.202

Ho 0.52 0.61 2.84 0.053 0.259 0.23 0.6 0.98 0.03 0.33 0.192 0.66 3.66 0.075 0.47 0.45 0.54 2.14 0.0155 0.027 0.026 0.104 0.32 0.007 0.0264 0.057 0.095 0.25 0.0164 0.0298

Er 0.82 2.57 4.23 0.223 0.45 0.134 1.46 3.45 0.22 0.4 1.14 2.26 10.89 0.185 0.74 0.85 1.13 3.67 0.038 0.037 0.176 0.196 1.09 0.0181 0.088 0.13 0.219 0.43 0.06 0.084

Tm 0.192 0.6 1.39 0.064 0.104 0.226 0.34 1.61 0.03 0.184 0.188 0.53 2.78 0.023 0.299 0.2 0.53 2.71 0.0062 0.0121 0.037 0.065 0.254 0.0026 0.013 0.025 0.059 0.243 0.0054 0.0175

Yb 0.91 1.09 9.38 0.247 0.076 1.32 0.84 2.62 0.244 0.25 1.79 1.78 7.65 0.35 0.38 0.94 1.77 9.98 0.042 0.081 0.133 0.058 1.71 0.03 0.062 0.205 0.187 0.67 0.058 0.109

Lu 0.203 0.45 1.47 0.067 0.19 0.45 0.88 1.7 0.031 0.195 0.34 0.39 2.4 0.064 0.36 0.51 0.55 1.81 0.0066 0.018 0.047 0.068 0.077 0.0134 0.0193 0.0045 0.062 0.21 0.0079 0.0183

Hf 0.66 2.06 9.59 0.126 0.62 1.1 2.03 4.8 0.144 0.63 1.29 3.39 7.8 0.256 0.59 0.96 2.21 8.31 0.037 0.083 0.199 0.271 0.87 0.038 0.077 0.121 0.202 0.48 0.028 0.064

Ta 0.209 0.85 1.98 0.104 0.147 0.28 0.44 2.8 0.045 0.41 0.38 0.53 3.22 0.0082 0.35 0.4 0.74 2.97 0.0088 0.03 0.082 0.112 0.44 0.0127 0.037 0.078 0.102 0.44 0.0081 0.037

W 1.02 2.04 7.42 0.32 0.78 0.85 1.81 1.76 0.223 0.65 2.24 2.8 8.54 0.32 1.83 1.05 2.57 9.09 0.033 0.095 0.138 0.242 0.38 0.034 0.097 0.228 0.221 1.05 0.031 0.07

Re 0.63 1.62 4.61 0.107 0.34 1.01 1.45 3.44 0.126 0.43 0.67 1.01 7.51 0.217 0.57 0.53 1 3.6 0.031 0.061 0.121 0.275 0.45 0.03 0.065 0.134 0.159 0.53 0.03 0.044

Au 2.33 5.75 11.94 0.38 1.4 1.66 1.32 4.82 0.35 0.78 1.93 2.21 13.42 0.36 1.76 1.64 4.35 7.07 0.088 0.155 0.148 0.45 2.29 0.081 0.127 0.243 0.33 0.79 0.045 0.024

Tl 1.14 2.6 9.24 0.185 0.56 0.92 2.03 4.19 0.186 0.73 0.98 2.34 9.52 0.262 1.02 1.06 2.13 6.01 0.033 0.062 0.138 0.243 1.14 0.027 0.069 0.15 0.157 0.75 0.028 0.05

Pb 1.08 2.51 7.05 0.197 0.28 0.81 1.56 3.51 0.105 0.46 1 1.14 9.85 0.089 0.61 0.86 1.6 3.69 0.094 0.06 0.147 0.225 0.77 0.054 0.065 0.106 0.218 1.17 0.125 0.14

Bi 0.68 1.01 4.44 0.039 0.33 0.38 0.88 2.69 0.094 0.28 0.49 1.11 4.48 0.079 0.28 0.36 0.87 3.11 0.0226 0.031 0.086 0.135 0.46 0.0148 0.036 0.063 0.061 0.58 0.0133 0.0197

Th 0.3 0.81 1.54 0.04 0.11 0.248 0.37 0.88 0.032 0.142 0.35 0.7 3.48 0.081 0.263 0.21 0.13 0.22 0.0114 0.0069 0.039 0.071 0.189 0.0067 0.0192 0.037 0.062 0.253 0.0056 0.0134

U 0.247 0.45 1.79 0.047 0.171 0.118 0.35 0.59 0.0217 0.095 0.194 0.38 1.17 0.0084 0.125 0.143 0.46 1.74 0.0161 0.0121 0.0222 0.046 0.127 0.0094 0.0128 0.025 0.041 0.169 0.0305 0.065

Table 2. (Continued) Nd 0.6 1.12 2.79 0.642 0.71 0.43 3.3 10.01 1.3 1.74 5.9 11.72 19.63 2.39 7.6 5.05 6.3 14.91 10.59 9.99 12.64 17.06 20.52 13.94 21.2 25.07 42.94 57.18 0.13 0.133

Sm 0.47 0.55 3.14 0.129 0.255 0.46 0.64 1.76 0.84 2.47 5.85 19.43 26.36 0.87 2.2 7.57 8.51 24.65 1.21 3.6 4.52 5.48 22.98 3.01 2.97 6.02 9.13 9.83 0.101 0.12

Eu 0.071 0.118 0.257 0.0316 0.042 0.056 0.096 0.32 0.179 0.47 0.72 1.59 1.22 0.152 0.57 0.54 0.64 3.79 0.52 0.39 0.79 1.35 4.61 0.58 0.88 1.41 1.33 3.7 0.0136 0.0243

Gd 0.132 0.26 0.69 0.099 0.113 0.115 0.52 1.01 0.28 1.03 3.33 3.68 7.07 0.53 1.7 2.48 4.72 10.12 1.61 1.81 2.1 2.08 6.16 1.84 3.83 2.23 9.46 9.75 0.0215 0.066

Tb 0.033 0.096 0.21 0.0129 0.021 0.034 0.045 0.265 0.104 0.068 0.72 1.84 2.16 0.077 0.258 0.31 1.21 1.34 0.228 0.254 0.64 0.42 1.33 0.355 0.31 0.52 0.89 0.91 0.008 0.0143

Dy 0.176 0.45 0.91 0.077 0.082 0.241 0.34 1.08 0.37 1.35 1.79 5.81 11.43 0.48 1.46 3.28 4.84 13.38 1.1 1.07 2.35 3.37 9.99 1.62 2.48 2.4 6.25 5.92 0.027 0.107

Ho 0.072 0.073 0.23 0.0148 0.039 0.027 0.069 0.303 0.091 0.38 1 1.72 4.66 0.118 0.34 0.67 0.85 2.89 0.306 0.4 0.75 0.77 2.49 0.257 0.72 0.61 0.68 2.11 0.0071 0.0246

Er 0.13 0.31 0.32 0.041 0.061 0.113 0.145 0.75 0.271 0.87 2.66 4.17 4.52 0.32 0.58 2.3 2.93 8.6 0.84 0.73 1.43 1 3.71 1.07 2.74 1.38 2.99 4.15 0.0152 0.033

Tm 0.05 0.042 0.222 0.0081 0.0128 0.026 0.034 0.152 0.063 0.072 0.76 1.38 2.29 0.116 0.135 0.47 1.08 1.64 0.105 0.149 0.32 0.76 1.42 0.114 0.3 0.36 0.39 0.65 0.0099 0.0108

Yb 0.118 0.027 0.75 0.044 0.061 0.126 0.228 0.87 0.3 0.79 2.09 10.37 7.71 0.3 1.33 1.45 4.61 1.13 0.54 0.77 2.3 3.67 11.67 0.76 1.64 1.37 2.88 3.78 0.0183 0.074

Lu 0.045 0.044 0.235 0.0049 0.0135 0.056 0.051 0.196 0.095 0.175 0.8 1.45 3.42 0.086 0.32 0.6 0.33 2.45 0.101 0.151 0.62 0.5 1.49 0.122 0.27 0.28 0.48 0.86 0.0075 0.0231

Hf 0.12 0.246 0.76 0.0053 0.088 0.128 0.116 0.64 0.31 1.13 0.001 4.71 13.56 0.197 0.58 1.12 1.65 9.72 0.175 0.35 2.02 1.15 11.84 0.32 0.51 0.9 1.78 1.94 0.052 0.054

Ta 0.078 0.101 0.54 0.0081 0.036 0.064 0.107 0.214 0.126 0.33 0.88 1.94 2.46 0.115 0.269 0.93 1.36 3.27 0.059 0.142 0.48 1.06 3.45 0.163 0.36 0.35 0.52 1.12 0.0158 0.0145

W 0.263 0.118 0.83 0.043 0.096 0.14 0.179 1.14 0.127 0.88 3.29 7.28 8.56 0.123 1.24 1.74 2.56 12.26 0.169 0.75 2.55 3.09 12.94 0.28 0.79 2.41 2.75 0.86 0.027 0.055

Re 0.067 0.193 0.42 0.0218 0.0164 0.071 0.204 0.61 0.295 0.45 1.87 4.14 8.14 0.22 0.51 1.25 2.6 6.97 0.178 0.38 1.45 1.43 5.37 0.25 0.89 1.32 0.86 1.51 0.0249 0.036

Au 0.241 0.61 0.82 0.039 0.124 0.29 0.26 0.59 0.26 1.29 3.13 13.03 17.65 0.212 2.2 4.72 3.7 19.82 0.226 0.54 1.41 1.81 18.55 0.5 1.78 3.13 3.67 6.65 0.026 0.056

Tl 0.112 0.212 0.63 0.03 0.057 0.124 0.219 0.48 0.31 0.88 2.15 6.29 10.45 0.28 0.66 2.02 2.64 9.51 0.212 0.31 1.79 1.77 5.95 0.43 0.72 1.38 1.5 2.48 0.0217 0.04

Pb 0.105 1.13 0.77 0.12 0.063 0.102 0.202 0.54 1.79 0.7 1.49 5.01 10.01 0.312 1.24 1.38 2.19 7.93 0.335 0.84 1.7 1.72 8.68 0.79 1.1 3.57 1.43 3.15 0.063 0.041

Bi 0.063 0.096 0.51 0.0132 0.026 0.047 0.092 0.29 0.092 0.34 0.83 2.24 6.14 0.117 0.37 0.67 1.65 2.38 0.053 0.231 0.7 0.69 4.09 0.118 0.5 0.38 0.53 1.4 0.0059 0.0184

Th 0.031 0.061 0.229 0.0059 0.0132 0.061 0.119 0.101 0.129 0.169 0.32 0.99 1.64 0.024 0.237 0.47 0.49 1.65 0.041 0.135 0.34 0.48 1.41 0.134 0.258 0.37 0.59 0.95 0.0025 0.0106

U 0.03 0.051 0.156 0.0282 0.035 0.026 0.08 0.373 0.061 0.178 0.63 0.92 2.2 0.129 0.52 1.09 0.96 1.56 0.57 0.72 0.67 0.99 0.98 0.76 1.08 0.89 2.24 3.39 0.0032 0.0158

Table 2. (Continued) Nd 0.177 0.32 1.51 0.041 0.052 0.11 0.188 1 0.02 0.067 0.143 0.25 1.05 0.528 0.653 0.73 1.13 2.1 1.54 0.082 0.2 2.32 4.21 0.241 0.91 1.06 0.46 11.65 0.234 0.44

Sm 0.36 0.76 2.38 0.093 0.115 0.35 0.59 2.02 0.063 0.16 0.259 0.4 1.56 0.112 0.11 0.36 0.44 1.39 0.55 2.05 4.23 5.13 32.8 0.56 1.3 3.02 3.9 21.06 0.56 0.96

Eu 0.056 0.123 0.41 0.014 0.0163 0.023 0.084 0.31 0.0089 0.0149 0.045 0.056 0.283 0.0344 0.0235 0.042 0.053 0.3 0.085 0.43 0.46 0.73 5.28 0.076 0.202 0.33 1.11 3.66 0.021 0.137

Gd 0.087 0.078 1.13 0.0148 0.063 0.19 0.32 0.61 0.0028 0.041 0.123 0.154 0.086 0.056 0.077 0.115 0.194 0.47 0.311 0.52 0.17 1.3 2.42 0.208 0.68 0.91 2.62 7.09 0.143 0.53

Tb 0.0232 0.042 0.094 0.0082 0.0052 0.041 0.049 0.226 0.0037 0.0175 0.0187 0.033 0.195 0.0078 0.0112 0.0247 0.031 0.142 0.0191 0.112 0.037 0.43 2.53 0.026 0.069 0.48 0.29 2.63 0.053 0.08

Dy 0.142 0.256 0.86 0.041 0.083 0.089 0.261 0.8 0.028 0.066 0.141 0.38 1.47 0.063 0.06 0.107 0.253 0.62 0.38 0.84 2.91 3.72 15.58 0.224 0.73 0.85 2.45 17.52 0.27 0.177

Ho 0.036 0.12 0.43 0.0073 0.0148 0.029 0.066 0.201 0.0041 0.0213 0.0204 0.072 0.173 0.0113 0.0152 0.032 0.082 0.109 0.062 0.173 0.52 0.54 4.78 0.079 0.258 0.43 0.87 3.32 0.033 0.152

Er 0.108 0.237 0.65 0.0156 0.032 0.067 0.161 0.29 0.008 0.05 0.137 0.109 0.83 0.0258 0.043 0.081 0.123 0.47 0.172 0.64 0.48 3.44 3.4 0.189 0.55 0.64 1.07 7.05 0.1 0.263

Tm 0.0249 0.045 0.213 0.0072 0.0103 0.0219 0.053 0.038 0.0057 0.0188 0.035 0.05 0.121 0.0053 0.012 0.0087 0.047 0.108 0.026 0.12 0.29 1.13 1.92 0.0277 0.09 0.161 0.35 2.31 0.047 0.086

Yb 0.016 0.088 0.73 0.0033 0.05 0.106 0.181 0.46 0.0273 0.064 0.097 0.069 0.58 0.043 0.067 0.056 0.194 0.3 0.184 0.16 0.19 0.72 4.03 0.189 0.43 0.62 1.68 6.42 0.043 0.32

Lu 0.037 0.048 0.226 0.0077 0.0155 0.023 0.04 0.02 0.0074 0.0141 0.0213 0.065 0.182 0.0056 0.0128 0.034 0.035 0.162 0.057 0.256 0.29 1.2 2.04 0.029 0.096 0.222 0.52 2.26 0.06 0.13

Hf 0.088 0.223 0.75 0.031 0.073 0.109 0.227 0.49 0.0199 0.033 0.1 0.125 0.6 0.0132 0.0247 0.061 0.116 0.38 0.231 0.42 2.06 2.1 11.68 0.137 0.32 1.27 1.11 0.93 0.163 0.23

Ta 0.061 0.127 0.37 0.0145 0.0254 0.031 0.075 0.28 0.0057 0.0231 0.049 0.071 0.297 0.0075 0.0099 0.027 0.094 0.265 0.054 0.34 0.93 0.65 2.73 0.039 0.221 0.3 0.86 5.18 0.019 0.031

W 0.134 0.032 0.001 0.026 0.079 0.118 0.109 0.76 0.031 0.051 0.109 0.33 0.66 0.029 0.065 0.1 0.255 0.59 0.207 0.66 0.65 2.51 10.66 0.151 0.73 1.15 3.32 8.97 0.255 0.18

Re 0.116 0.269 0.4 0.0194 0.028 0.045 0.142 0.42 0.0108 0.04 0.077 0.096 0.39 0.0144 0.0231 0.088 0.143 0.35 0.18 0.46 0.8 1.78 5.39 0.142 0.58 0.81 1.35 4.13 0.064 0.29

Au 0.13 0.252 1.57 0.046 0.094 0.163 0.278 1.28 0.037 0.122 0.107 0.152 0.11 0.035 0.037 0.031 0.178 0.82 0.35 1.12 0.64 2.88 20.84 0.212 0.98 2.28 4.26 17.86 0.31 0.82

Tl 0.097 0.195 0.78 0.033 0.04 0.132 0.163 0.72 0.0206 0.041 0.096 0.11 0.5 0.022 0.038 0.093 0.178 0.58 0.139 0.58 1.57 2.46 9.6 0.143 0.47 0.77 1.87 10.37 0.151 0.32

Pb 0.104 0.202 0.77 0.114 0.05 0.092 0.143 0.72 0.0366 0.036 0.091 0.122 0.65 0.298 0.235 0.213 0.167 0.37 0.17 0.62 1.33 1.77 8.75 0.134 0.35 1.08 1.34 6.29 0.16 0.3

Bi 0.055 0.164 0.33 0.0132 0.0202 0.037 0.073 0.18 0.0079 0.0131 0.044 0.049 0.4 0.0083 0.0164 0.037 0.05 0.185 0.084 0.27 0.77 0.91 4.27 0.073 0.179 0.29 0.49 3.25 0.073 0.149

Th 0.042 0.0118 0.147 0.0053 0.0054 0.0115 0.073 0.136 0.00128 0.0129 0.0105 0.069 0.203 0.027 0.032 0.0168 0.056 0.104 0.052 0.117 0.115 0.45 0.54 0.027 0.087 0.203 0.89 1.58 0.045 0.032

U 0.0162 0.077 0.098 0.0033 0.0045 0.0077 0.035 0.092 0.0037 0.0123 0.0132 0.04 0.052 0.0143 0.0197 0.0247 0.029 0.1 0.0236 0.079 0.27 0.049 1.8 0.0258 0.059 0.138 0.231 2.42 0.0218 0.014

Table 2. (Continued) Nd 0.86 0.93 3.89 1.61 1.3 1.95 5.68 10.82 1.018 0.31 0.161 0.28 0.46 0.0175 0.05 0.072 0.183 0.89 0.033 0.018 0.001 0.087 0.98 0.036 0.044 0.09 0.064 0.51 0.223 0.43

Sm 2.91 2.88 8.61 0.63 1.4 2.38 3.26 9.16 0.155 0.119 0.205 0.72 1.52 0.047 0.18 0.36 0.23 1.38 0.052 0.165 0.223 0.68 1.25 0.046 0.177 0.227 0.35 1.46 0.35 0.78

Eu 0.47 0.55 1.22 0.041 0.218 0.41 0.31 1.42 0.0218 0.0169 0.0144 0.108 0.31 0.0077 0.0222 0.034 0.015 0.106 0.0149 0.0116 0.0085 0.0137 0.36 0.0065 0.0195 0.04 0.0067 0.108 0.033 0.36

Gd 1.48 1.23 3.35 0.24 0.44 0.72 1.04 5.64 0.096 0.046 0.196 0.172 0.59 0.0225 0.043 0.092 0.021 0.29 0.0204 0.0106 0.086 0.019 0.49 0.0179 0.025 0.11 0.192 0.88 0.193 0.53

Tb 0.159 0.32 1.13 0.052 0.085 0.188 0.22 0.54 0.0097 0.0141 0.036 0.037 0.127 0.0068 0.0092 0.034 0.034 0.163 0.0044 0.0045 0.0099 0.03 0.147 0.0038 0.0022 0.0234 0.05 0.133 0.0117 0.112

Dy 1.39 1.99 2.56 0.29 0.79 0.95 1.37 3.26 0.063 0.087 0.129 0.278 1.11 0.036 0.09 0.149 0.255 0.5 0.0026 0.0149 0.114 0.184 0.46 0.0236 0.05 0.072 0.31 0.92 0.18 0.6

Ho 0.49 0.2 0.96 0.066 0.14 0.196 0.3 1.18 0.0091 0.0243 0.032 0.08 0.196 0.0105 0.0225 0.0215 0.064 0.102 0.0082 0.013 0.029 0.046 0.114 0.0059 0.0153 0.036 0.084 0.206 0.032 0.15

Er 0.74 0.56 2.88 0.14 0.42 0.62 0.51 1.77 0.045 0.046 0.097 0.17 0.42 0.0222 0.06 0.064 0.045 0.53 0.0143 0.0186 0.085 0.097 0.48 0.0034 0.053 0.022 0.095 0.31 0.095 0.37

Tm 0.099 0.4 1.09 0.046 0.097 0.287 0.24 1.01 0.0096 0.0151 0.001 0.056 0.137 0.0052 0.014 0.0029 0.0097 0.175 0.0047 0.0091 0.034 0.0172 0.112 0.0058 0.0012 0.0178 0.0042 0.102 0.031 0.148

Yb 1.43 1.93 2.14 0.168 0.47 0.37 1.15 1.29 0.0269 0.073 0.029 0.051 0.66 0.0248 0.019 0.102 0.246 0.08 0.0091 0.044 0.064 0.154 0.001 0.0197 0.042 0.035 0.212 0.2 0.15 0.71

Lu 0.026 0.3 1.16 0.0139 0.103 0.124 0.179 0.62 0.0072 0.0092 0.041 0.042 0.145 0.0123 0.0257 0.0225 0.054 0.186 0.0067 0.0104 0.017 0.027 0.266 0.0062 0.0092 0.0189 0.057 0.152 0.074 0.091

Hf 1.04 1.57 1.87 0.16 0.68 0.82 0.59 2.03 0.0257 0.065 0.136 0.276 0.39 0.0255 0.035 0.181 0.179 0.43 0.0164 0.071 0.139 0.112 0.68 0.0143 0.052 0.088 0.109 0.61 0.109 0.42

Ta 0.34 0.28 1.09 0.065 0.239 0.166 0.48 1.17 0.0079 0.0143 0.032 0.079 0.052 0.0127 0.0199 0.001 0.073 0.176 0.0094 0.0257 0.028 0.045 0.159 0.0058 0.0123 0.0102 0.063 0.249 0.052 0.21

W 0.94 1.69 5.2 0.25 0.14 1.12 0.93 4.54 0.041 0.016 0.249 0.029 0.76 0.027 0.078 0.118 0.201 1.19 0.052 0.027 0.156 0.36 0.51 0.0154 0.049 0.199 0.07 0.57 0.303 0.83

Re 0.66 0.87 2.38 0.109 0.38 0.45 0.74 1.61 0.0096 0.042 0.117 0.155 0.66 0.0144 0.034 0.102 0.124 0.49 0.0185 0.031 0.067 0.141 0.31 0.0081 0.03 0.05 0.062 0.35 0.124 0.42

Au 1.33 1.1 2.94 0.179 0.76 0.86 1.87 2.98 0.0117 0.032 0.36 0.31 1.33 0.029 0.079 0.207 0.24 0.7 0.059 0.051 0.159 0.182 1.11 0.033 0.07 0.177 0.253 1.01 0.31 0.7

Tl 0.89 1.39 2.39 0.09 0.41 0.49 0.74 2.91 0.031 0.051 0.099 0.198 0.83 0.0238 0.048 0.083 0.133 0.55 0.0195 0.045 0.073 0.154 0.54 0.019 0.044 0.07 0.123 0.45 0.131 0.45

Pb 0.59 0.98 1.63 0.112 0.34 0.53 0.71 3.01 0.142 0.049 0.103 0.153 0.58 0.049 0.042 0.052 0.089 0.43 0.0215 0.047 0.109 0.056 0.55 0.0431 0.03 0.062 0.14 0.25 0.133 0.211

Bi 0.45 0.56 1.63 0.085 0.217 0.39 0.34 1.42 0.0241 0.0185 0.039 0.131 0.194 0.0073 0.0244 0.052 0.063 0.33 0.0082 0.0224 0.045 0.072 0.277 0.0083 0.0124 0.031 0.063 0.145 0.078 0.193

Th 0.165 0.193 0.91 0.025 0.188 0.129 0.23 0.77 0.0034 0.0103 0.0087 0.038 0.018 0.0027 0.0095 0.035 0.049 0.016 0.0064 0.0059 0.0269 0.031 0.108 0.0039 0.0118 0.0241 0.042 0.097 0.004 0.116

U 0.049 0.061 0.51 0.066 0.091 0.191 0.13 0.59 0.0056 0.0142 0.015 0.026 0.025 0.0049 0.0036 0.0057 0.024 0.219 0.0034 0.0105 0.0133 0.03 0.051 0.0028 0.0039 0.012 0.0141 0.037 0.021 0.081

Table 2. (Continued) Nd 0.15 1.5 9.89 0.141 0.47 1.17 2.84 3.68 0.156 0.64 0.73 0.82 1.67 0.134 0.29 1.21 0.13 0.34

Sm 4.18 6.35 10.28 0.4 1.2 2.97 5.11 18.64 0.28 1.24 2.27 1.8 2.13 0.52 0.89 3.07 2.46 4.19

Eu 0.24 0.67 1.79 0.044 0.257 0.37 0.3 1.88 0.069 0.232 0.46 0.57 0.52 0.104 0.09 0.041 0.3 0.52

Gd 0.13 1.83 4.92 0.242 0.71 0.71 2 5.16 0.135 0.001 1.09 1 1.44 0.143 0.076 1.04 1.18 2.46

Tb 0.191 0.39 1.05 0.037 0.151 0.34 0.6 1.1 0.029 0.068 0.035 0.261 0.43 0.0165 0.118 0.41 0.178 0.61

Dy 0.85 0.63 7.96 0.196 0.154 0.67 3.23 4.82 0.218 0.51 0.59 1.47 1.9 0.189 0.231 0.97 1.56 1.88

Ho 0.209 0.74 1.15 0.049 0.051 0.33 0.74 1.3 0.03 0.061 0.29 0.37 0.48 0.033 0.1 0.172 0.39 0.37

Er 0.27 1.28 3.44 0.12 0.49 0.5 2.2 9.51 0.094 0.31 0.88 0.98 1.74 0.04 0.172 0.72 0.16 1.71

Tm 0.06 0.155 0.62 0.039 0.162 0.164 0.46 2.37 0.0042 0.012 0.144 0.162 0.66 0.0177 0.0087 0.169 0.159 0.46

Yb 2.2 2.86 2.22 0.133 0.45 1.57 2.69 8.02 0.21 0.115 0.98 1.1 1.58 0.157 0.38 0.25 1.83 2.7

Lu 0.22 0.45 2.4 0.042 0.14 0.3 0.69 1.77 0.057 0.077 0.191 0.172 0.5 0.0188 0.12 0.36 0.203 0.85

Hf 0.1 1.8 6.83 0.119 0.33 0.81 3.2 10.1 0.152 0.25 0.87 0.98 1.99 0.162 0.28 1.02 0.94 1.6

Ta 0.41 0.42 3.21 0.056 0.107 0.33 1.3 1.03 0.024 0.103 0.35 0.4 0.47 0.025 0.114 0.26 0.77 0.8

W 0.17 1.67 6.35 0.221 0.14 0.92 3.16 6.67 0.3 0.41 0.32 1.29 1.87 0.186 0.148 0.59 1.08 1.85

Re 0.41 0.84 6.76 0.136 0.37 0.57 1.3 4.1 0.088 0.29 0.71 0.56 1.48 0.132 0.161 0.59 0.86 1.97

Au 1.68 2.44 16.04 0.32 0.77 1.65 2.69 11.98 0.188 0.74 0.85 1.9 3.37 0.194 0.75 1 1.61 3.87

Tl 0.81 1.55 3.86 0.11 0.37 0.91 1.57 5.72 0.122 0.27 0.83 0.7 2.06 0.129 0.31 0.53 0.75 1.88

Pb 0.51 1.16 3.41 0.119 0.33 0.81 1.7 5.06 0.108 0.31 0.71 0.85 1.21 0.404 0.3 1.77 2.3 2.64

Bi 0.29 0.43 3.24 0.08 0.213 0.33 0.49 3.8 0.104 0.149 0.36 0.23 0.83 0.075 0.1 0.34 0.44 0.74

Th 0.278 0.28 1.87 0.0164 0.127 0.156 0.3 1.96 0.004 0.139 0.138 0.309 0.32 0.044 0.05 0.099 0.182 0.54

U 0.196 0.35 1.32 0.042 0.063 0.156 0.38 1.38 0.0209 0.07 0.138 0.11 0.39 0.024 0.054 0.198 0.13 0.22

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Ian Scharlotta, Andrzej Weber, S. Andy DuFrane et al. Table 3. Laser ablation 87Sr/86Sr ratio data for KN XIV teeth (Sample ID_Sample Group) Sample ID 97.211_A1 97.211_A2 97.211_A3 97.211_B1 97.211_B2 97.211_B3 97.211_C1 97.211_C2 97.211_C3 97.211_D1 97.211_D2 97.211_D3 97.217_A1 97.217_A2 97.217_A3 97.217_B1 97.217_B2 97.217_B3 97.217_C1 97.217_C2 97.217_C3 97.217_D1 97.217_D2 97.217_D3 97.225_A1 97.225_A2 97.225_A3 97.225_B1 97.225_B2 97.225_B3 97.225_C1 97.225_C2 97.225_C3 97.225_D1 97.225_D2 97.225_D3 98.355_A1 98.355_A2 98.355_B1 98.355_B2 98.355_B3 98.355_C1 98.355_C2

total Sr (V) 0.5155146 0.5160899 0.4953065 0.5237191 0.5002587 0.4872413 0.5060082 0.4307859 0.5063735 0.4994893 0.46193 0.540535 0.4115777 0.4041834 0.4004222 0.3322841 0.3403646 0.3680073 0.2988586 0.4335352 0.4269052 0.3627065 0.3776331 0.379205 0.2559062 0.2326417 0.2678432 0.2201758 0.2348739 0.2483402 0.2104877 0.2590772 0.2533799 0.1873117 0.2179966 0.1321271 0.2093049 0.1914608 0.17089 0.195499 0.2208219 0.170033 0.1643442

88Sr (V) 0.45866608 0.453345569 0.428867147 0.436220764 0.4155926 0.4048075 0.420384 0.3578929 0.4208142 0.4150779 0.3837917 0.4492097 0.3417825 0.3355515 0.3324746 0.2759177 0.2825703 0.3055199 0.2481763 0.359995 0.3544793 0.3011957 0.3135448 0.3147506 0.212446 0.1931096 0.2222689 0.182762 0.194932 0.206077 0.1746781 0.2149743 0.2102729 0.1554626 0.1808601 0.1097235 0.174305 0.1597384 0.1419118 0.1623537 0.1833904 0.1412572 0.1365448

85Rb (V) 2.89E-04 2.41E-04 2.70E-04 2.80E-04 1.92E-04 2.08E-04 1.77E-04 1.41E-04 1.77E-04 2.26E-04 1.12E-04 1.67E-04 3.00E-04 3.99E-04 4.29E-04 2.38E-04 2.91E-04 3.38E-04 2.15E-04 2.94E-04 2.86E-04 1.91E-04 2.07E-04 2.35E-04 3.16E-04 2.65E-04 3.13E-04 3.03E-04 3.21E-04 3.51E-04 3.30E-04 3.62E-04 3.90E-04 2.53E-04 3.44E-04 1.64E-04 5.31E-05 5.37E-05 5.08E-05 5.24E-05 6.25E-05 4.84E-05 6.00E-05

87Sr/86Sr 0.7123743 0.7125077 0.7125393 0.7122774 0.7120285 0.71261 0.7116353 0.7115308 0.7117711 0.712428 0.7119169 0.7114825 0.7158983 0.7170207 0.7172126 0.7163448 0.7168815 0.7174497 0.7153606 0.7158572 0.7151173 0.7151284 0.715732 0.7160216 0.7144163 0.7161518 0.7163261 0.7155843 0.716525 0.7161068 0.715738 0.7153563 0.7159872 0.7150012 0.7166502 0.7152321 0.7132161 0.7152994 0.7126116 0.7120714 0.7119601 0.7116431 0.7121804

2σ error 0.0001806 0.0001684 0.000204 0.000198 0.0001622 0.0001414 0.0001572 0.000202 0.000164 0.0001718 0.0001888 0.000214 0.0001974 0.000274 0.0002 0.00032 0.0001302 0.000204 0.000214 0.00024 0.000204 0.000234 0.000195 0.000236 0.000294 0.000312 0.000208 0.000296 0.000252 0.0003 0.0003 0.000252 0.00028 0.00032 0.000348 0.000672 0.000366 0.00042 0.00045 0.000302 0.000278 0.000414 0.000448

84Sr/86Sr 0.05591817 0.05651735 0.05614837 0.05598387 0.05664957 0.05541669 0.05709169 0.05717407 0.05517618 0.05474164 0.05678069 0.0562032 0.05696245 0.05821956 0.05708164 0.05776875 0.05858407 0.05726491 0.0579626 0.05737652 0.05757585 0.05827334 0.05776602 0.05894893 0.05954526 0.05905727 0.05987656 0.05914619 0.05998222 0.05992156 0.06010092 0.06026456 0.05949909 0.06019127 0.06077109 0.05440657 0.03778981 0.02281661 0.06041714 0.05998997 0.05933711 0.05909435 0.05917739

2σ error 0.000181 0.000202 0.000192 0.000182 0.000115 0.000272 0.000112 0.000116 0.000191 0.000404 0.000109 0.000262 0.000175 0.000114 0.000112 0.000196 0.000147 0.000129 0.000155 0.000145 0.000102 0.000167 0.000131 0.00015 0.000222 0.00019 0.000232 0.000166 0.000208 0.000216 0.000246 0.000159 0.000169 0.00026 0.00023 0.000842 0.001754 0.001328 0.00036 0.000192 0.00024 0.00024 0.000226

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Table 3. (Continued) Sample ID 98.355_C3 98.355_D1 98.355_D2 98.355_D3 98.359_A1 98.359_A2 98.359_A3 98.359_B1 98.359_B2 98.359_B3 98.359_C1 98.359_C2 98.359_C3 98.359_D1 98.359_D2 98.359_D3

total Sr (V) 0.1749326 0.1417553 0.1710117 0.154442 0.08855514 0.09535257 0.09603494 0.07893794 0.0850022 0.09881024 0.07774534 0.09139404 0.0969025 0.0784394 0.08522065 0.08934028

88Sr (V) 0.1453045 0.1177061 0.1420012 0.1282176 0.07353076 0.07918973 0.07977681 0.06555476 0.07058874 0.08204339 0.06454512 0.07587025 0.08046143 0.0651374 0.07078526 0.0742008

85Rb (V) 6.72E-05 5.70E-05 1.05E-04 6.71E-05 3.41E-05 3.17E-05 3.54E-05 2.82E-05 2.74E-05 2.92E-05 2.07E-05 1.97E-05 2.68E-05 1.65E-05 2.26E-05 3.15E-05

87Sr/86Sr 0.7126502 0.713514 0.7136801 0.7146961 0.7117397 0.7127162 0.7118754 0.7117544 0.7129407 0.7129047 0.7141845 0.7134008 0.7137356 0.7133296 0.7123873 0.7126201

2σ error 0.000346 0.000632 0.00039 0.000464 0.00069 0.00066 0.000672 0.00071 0.000916 0.000712 0.001052 0.00068 0.00061 0.00086 0.00088 0.0008

84Sr/86Sr 0.05839649 0.05987801 0.05868803 0.05890335 0.06180479 0.05925366 0.05880733 0.06094116 0.06024675 0.06003406 0.05964469 0.05944841 0.05919033 0.06141835 0.06142619 0.06015517

2σ error 0.000288 0.000342 0.000278 0.00024 0.00049 0.00047 0.000402 0.000444 0.000504 0.00044 0.000504 0.000456 0.000352 0.000484 0.000472 0.000414

We must first establish the veracity of hypothesized expectations for Cis-Baikal inhabitants. Within each sampling group, five lines of different sizes: 100, 80, 55, 40, and 25 µm, were drawn to demonstrate the impact that sample size has within a microsampling environmental. Figures 3 and 4 show the results of different laser spot sizes being used on strontium and rubidium at full laser power; rubidium being representative of elements of low concentration (below 20 ppm) and strontium of elements of higher concentration. The results are not surprising as the direct relationship between the physical amounts of sample introduced into the ICP-MS is integral to the functioning of the equipment; however it does provide a reminder that mass ranges with low concentrations are subject to significantly larger error terms as beam size is reduced. As such, the use of low concentration mass ranges (i.e., rubidium or zirconiuim) for correction factors must be taken carefully as the sampling methodology can have a major impact on the effectiveness of such a correction factor. The difference in variability is strictly the results of resultant signal strength, which can be equally altered by changes in the spot size and the laser power. This variability viewed as confidence ellipses on bivariate plots will yield relatively larger or smaller ellipses. Figure 5 demonstrates this as a compounded effect of both laser power and spot size as each ellipse represents one group of five laser lines of different size. The extent of variability related to laser effects was somewhat surprising, not in that the concentrations were more variable, but rather that these data formed significantly different shapes in statistical space. This could be interpreted as reflecting significant internal variability within a single tooth, greater than may actually be present, simply as an effect of the laser power. As these data came from prehistoric hunter-gatherer teeth, it is possible that such variability is reflective of provenance shifts during the life of the individual, however with the neat divide between groups measured with 50% and 100% laser power, it is reasonable to assume that the ranges seen are in fact the result of the laser settings and not fully reflective of internal variability within the enamel matrix.

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Figure 3. Rubidium values by spot size

Figure 4. Strontium values by spot size

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Figure 5. Impact of laser effects shown by groups’ variability clustering by laser power rather than by internal variability

Figure 6. Strontium and zinc bivariate plot, suggests a transitional period during the early period of molar formation followed by a period of relative stability in geography and diet and the beginning of another transition towards the end of molar formation

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Figure 7. Barium and manganese bivariate plot shows an intriguing separation between paired groups A-B and C-D demonstrating a clear shift in geography or diet between the first and second half of the molar growth period

The next question is whether there is adequate variability reflected in the geochemical data from serial sampling of a tooth to potentially address the underlying concern of the disjunction between enamel matrix formation and the supposed dietary source of these signals. Figures 6 and 7 demonstrate that there is significant variability within the span of a single tooth. As noted by Britton et al. [46] and Montgomery et al.[51], the incorporation of a sudden change in geochemical input signal will lead to a moderately sloped interchange reflecting both the old and new end-members of the geochemical signal, so any significant change in the elemental data is likely outstripping the visibility of this effect, or showing snapshots along the transition slope still reflecting different values and statistical morphology. This is highly suggestive of the presence of useful variability in trace element composition in the hunter-gatherer population of Cis-Baikal comparable to effects noted in agrarian groups by Cucina et al. [5, 69] and Dudgeon [70]. However, one interest aspect of the range of variability within a single tooth raises some concerns about the extent of the validity of the assumption that the hydroxyapatite matrix contains relatively stable quantities of Ca and P. Figures 8 and 9, show calcium and phosphorous projected against strontium, two elements that are supposed to be present in a fairly constant ratio throughout the tooth, should show similar patterning. The predictable nature of calcium phosphate matrices is the primary feature that enables geochemical research to be conducted on skeletal tissues. The range of variability visible in this one sample is still within the ranges to be expected for normal Ca:P ratios of teeth not significantly altered by diagenetic processes, however the distribution and differences in statistical morphology is intriguing.

Assessing Hunter-Gatherer Mobility in Cis-Baikal, Siberia Using LA-ICP-MS

Figure 8. Strontium and calcium are strongly correlated as expected as interchangeable mineral components

Figure 9. Surprising variability in the phosphorus values both in extent and direction with correlation not following suit of Sr/Ca ratios as is generally hypothesized

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After establishing that trace element analysis is a useful tool for analyzing huntergatherers from Cis-Baikal, we must identify elements that may mirror or enhance provenance information acquired from 87Sr/86Sr ratios. Strontium isotope ratio is a well developed analytical approach for provenancing skeletal tissues, however it has one major caveat in its ability to elucidate either origins or mobility of an individual; that it can only operate on the scale of the dominant bedrock formation and/or geologic zone. In some areas of the world, this is more than adequate to answer all of the current research questions relating to available cemetery populations. This is particularly true for agrarian groups, where questions are dominated by a local/nonlocal dichotomy where the primary goal is to establish the local signal and thus identify immigrants in a population, with the provenancing of the immigrants falling to secondary level of investigation. Geologically complex areas such as Cis-Baikal are broadly speaking, quite amenable to such a research approach as there are geologic formations spanning three major epochs in fairly well defined and non-overlapping geography, however we encounter several problems in this situation. Previous studies in CisBaikal have demonstrated that the 87Sr/86Sr technique works in the region, but also that there are two situations that cannot be clarified without further research: that there are some individuals who, though likely mobile, stayed within a single geological zone; and that there are two major zones of similar age and thus theoretically indistinguishable, leaving a rather difficult scenario where hypotheses regarding regional population exchange will inevitably be hampered by an inability to separate out individuals from these two regions. Two potential solutions to this problem are intensive environmental sampling in order to improve the comparison map available for samples, and the addition of another elemental and/or isotopic series to provide statistical depth to the data and enable multivariate analyses. Within CisBaikal, four elements appeared to meet the criteria for their ability to enhance the 87Sr/86Sr data: rubidium (Figure 10), cesium (Figure 11), barium (Figure 12), and rhenium (Figure 13). That Rb concentrations can mimic 87Sr/86Sr ratios is not too surprising as most radiogenic formations also contain higher levels of Rb and Sr, however this does not inhibit its value as an elemental signal in helping to elucidate further provenancing of samples within a radiogenic zone as there is still considerable variability in the raw concentrations of the element encountered in the environment. Rhenium functions in a similar fashion. Barium and cesium do not replicate 87Sr/86Sr data as effectively, demonstrating instead their usefulness in discriminating between groups within a single zone (Cs) or between individuals who all come from an area with similar 87Sr/86Sr values, but markedly different Ba concentrations, and thus likely from different areas within another zone. Examined more closely, rhenium and zinc values, for example, show a fair amount of variability throughout the geography of a single tooth (Figure 14), illustrating this individual’s continual presence within a single geologic zone, in this case the Little Sea, however also showing that they had notable variability in their interaction with rhenium through their environment via either dietary or mobility changes. Discussions over methodological approaches to obtaining accurate 87Sr/86Sr data using laser ablation frequently include debates over diagenetic alterations and the presence of various interferences. Doubly-charged rare earth elements frequently fall within the mass ranges monitored for 87Sr/86Sr analysis, thus their presence is of great concern. Theoretically, the presence of elements unable to make direct replacements in the mineral matrix strongly supports proponents of the view that all rare earth elements are diagenetic in origin and thus represent both contaminated samples and a dead end avenue for geochemical research.

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However, there are very often anomalies within any mineral matrix, especially so in organic matrices such as calcium hydroxyapatite, and these areas of imperfect mineral matrix are effectively traps for other mineral constituents, including trace elements in general. Furthermore, studies demonstrating the utility of trace elemental analysis on human teeth tend to overshadow concerns of diagenetic overprinting or alteration of samples preventing the recovery of useful compositional data from teeth. Following in light of this debate, we attempted to see if there were significant correlations between the strontium and barium values. Sr2+ replaces calcium within enamel at a rate not exceeding 1 in 10 ions, and so has a limited capability for accumulation within skeletal tissues even if abundant in the body water supply at the time of enamel formation. Similarly, Ba2+ sometimes substitutes strontium in the same position, again at a fraction of the potentially available positions in the matrix. So, significant shifts in the Sr:Ba ratios should hypothetically be a signal that there is diagenetic alterations that could render normal interference calculations for rare earth elements or other interferences inaccurate. However, in the teeth analyzed for this research, no significant correlations could be found linking Sr:Ba ratios with other forms of interference in 87Sr/86Sr analyses.

Figure 10. Rubidium replicates Sr isotope groupings

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Figure 11. Cesium values largely replicate Sr isotope groupings, however suggest greater internal variability for “local” groupings than is suggested by isotopic analysis

The last major goal of this research was to generate adequate data to test the potential for an online or in situ measurement of interference on mass 87 from the polyatomic molecule [40] Ca [31] P[16]O. Previous research has demonstrated that the formation of this polyatomic species is the source of significant interference for laser ablation analyses of strontium isotopes at such a scale that interpretations can be biased through methodological fault. As the [40] Ca [31] P [16] O is the result of interactions between the enamel surface, the laser and the charged oxidation environment of the plasma, however it remains unclear which element in the system is primarily responsible for this interference, or if it is truly an unavoidable consequence of having excess amounts of Ca, P and O in a charged environment. There are several uncertain variables in this equations, thus the easiest way to measure [40] Ca [31] P [16] O production during analysis would be to measure the related species [44] Ca [31] P [16] O that will skew values of mass 91 in relative proportion to the level of interference on mass 87. In order to measure the interference at mass 91, we need to compare the mass peaks of zirconium 90 and 91. A comparison of [90] Zr and [91] Zr between solution mode and laser ablation quadrupole-ICP-MS clearly shows the presence of the hypothesized offset between the two masses of Zr (Figure 15). The LA values include the data from small laser spot sizes as well as larger ones, so there is significantly more variability in the laser data than the solution data, though there is still a readily apparent offset and linear trend in the offset that can be used for a correction. Such a correction follows the logic that the visible offset in Zr values will correlate with the 87Sr/86Sr differences between laser and solution data.

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Figure 12. Barium values suggest greater variability in “nonlocal” interpretation than is indicated by Sr isotope values.

As a test of this concept, we first utilized the published dataset from Simonetti et al.8 to see if using reported Sr concentrations we could “correct” the LA data and get results within the original error terms of the analysis. The published data contained Sr concentrations, but no information on Zr, so an added step was needed in this experiment. A linear relationship between the concentrations of Sr and Zr were drawn from the KN XIV analyses and applied to the reported Sr values in order to generate the expected offset of [91] Zr. This offset was then compared to the differences in 87Sr/86Sr data to gain a second linear relationship for the expected error from [40] Ca [31] P [16] O based on the levels of Sr and Zr (Figure 16). The dataset lacked information on specific corrections used, however the laboratory protocols from the time did not incorporate REE or Ca-dimer corrections, so additional blanket correction values were included in the process, however the correction procedure largely followed that outlined by Horstwood et al. 1. Due to the number of variables missing, there is a fair amount of uncertainty in the accuracy of the final “corrected” data, yet the new LA data largely fall on or near the SM data reported. Several of the values did not end within the original error terms, showing the difficulties in applying corrections to data with significant amounts of uncertainty attached and compounding concerns over the comparability of microsampling locations with the masses of enamel homogenized for solutions. The fact that the majority of samples fell surprisingly close to the SM values strongly supports the potential for this avenue of online correction. The difficulty with this situation is that these data were drawn from quadrupole-ICP-MS analysis that is equipped with different electronmultipliers and will have different operating oxide conditions than both another quadrupoleICP-MS and a MC-ICP-MS. This problem could theoretically be overcome if the quadrupole-

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ICP-MS and the MC-ICP-MS were connected to the same ablation chamber and online correlations could be drawn between 87Sr/86Sr values and Zr concentrations, however the RIF laboratory is not set up in such a fashion.

Figure 13. Grouping via rheniuim replicates Sr isotope groupings

Figure 14. Rhenium values for a single “local” individual illustrate their continued residence within a geologic region, though with some smaller scale mobility

Assessing Hunter-Gatherer Mobility in Cis-Baikal, Siberia Using LA-ICP-MS

Figure 15. Comparison between 90Zr/91Zr between solution mode and laser ablation ICP-MS for KN XIV teeth

Figure 16. Zirconium difference extrapolated from strontium concentrations and compared with observed laser ablation– and solution mode–MC-ICP-MS differences for Simonetti et al.2008 data

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Ian Scharlotta, Andrzej Weber, S. Andy DuFrane et al. Table 4. Laser ablation 87Sr/86Sr ratio data for KN XIV teeth with correction results Sample ID 87Sr/86Sr 97.211_A1 0.7123743 97.211_A2 0.7125077 97.211_A3 0.7125393 97.211_B1 0.7122774 97.211_B2 0.7120285 97.211_B3 0.71261 97.211_C1 0.7116353 97.211_C2 0.7115308 97.211_C3 0.7117711 97.211_D1 0.712428 97.211_D2 0.7119169 97.211_D3 0.7114825 Standard Deviation 97.217_A1 0.7158983 97.217_A2 0.7170207 97.217_A3 0.7172126 97.217_B1 0.7163448 97.217_B2 0.7168815 97.217_B3 0.7174497 97.217_C1 0.7153606 97.217_C2 0.7158572 97.217_C3 0.7151173 97.217_D1 0.7151284 97.217_D2 0.715732 97.217_D3 0.7160216 Standard Deviation 97.225_A1 0.7144163 97.225_A2 0.7161518 97.225_A3 0.7163261 97.225_B1 0.7155843 97.225_B2 0.716525 97.225_B3 0.7161068 97.225_C1 0.715738 97.225_C2 0.7153563 97.225_C3 0.7159872 97.225_D1 0.7150012 97.225_D2 0.7166502 97.225_D3 0.7152321 Standard Deviation 98.355_A1 0.7132161 98.355_A2 0.7152994 98.355_B1 0.7126116 98.355_B2 0.7120714

Zr Corrected #1 0.708214083 0.708232427 0.708248213 0.708167282 0.708107142 0.708259837 0.707992527 0.707963213 0.708035033 0.708211639 0.708071365 0.707968825 0.000113406 0.709163308 0.709466918 0.709530354 0.709282721 0.709433961 0.709599039 0.709011616 0.709152765 0.70895875 0.708948668 0.709119498 0.709208743 0.000223459 0.708755271 0.709239481 0.709286307 0.709074139 0.709337331 0.709231976 0.709117164 0.709023225 0.709190372 0.708913961 0.709373185 0.708993929 0.000184104 0.708425491 0.709003086 0.708261768 0.708117652

Zr Corrected #2 0.711088326 0.7112217 0.711406978 0.711007949 0.710774713 0.711366711 0.710362712 0.710267076 0.710558893 0.711157536 0.71069914 0.710286141 0.000415611 0.714625791 0.715773901 0.716033087 0.71507506 0.715642341 0.716232863 0.714099739 0.714610899 0.713868661 0.713851615 0.71446194 0.714729529 0.000830401 0.713143106 0.714888032 0.715136798 0.714320011 0.715261989 0.714866517 0.714457597 0.714078762 0.714739576 0.713722761 0.715376629 0.71401245 0.000673628 0.711944233 0.714034025 0.711343396 0.710820199

Sr Corrected 0.710569148 0.710718353 0.710756634 0.710484925 0.710245952 0.710826027 0.709787549 0.709668758 0.709856572 0.710586686 0.710117022 0.709633692 0.00044487 0.714258617 0.715382757 0.715575752 0.7147632 0.715324752 0.715883702 0.713830202 0.714343247 0.71359879 0.713702191 0.714330123 0.714636228 0.000763181 0.713245991 0.714950852 0.715151286 0.714376255 0.715318674 0.714884275 0.714504436 0.71411776 0.71473358 0.71373858 0.715419111 0.713996315 0.000667163 0.711898467 0.713999032 0.711305823 0.71073572

2σ error 0.0001806 0.0001684 0.000204 0.000198 0.0001622 0.0001414 0.0001572 0.000202 0.000164 0.0001718 0.0001888 0.000214 0.0001974 0.000274 0.0002 0.00032 0.0001302 0.000204 0.000214 0.00024 0.000204 0.000234 0.000195 0.000236 0.000294 0.000312 0.000208 0.000296 0.000252 0.0003 0.0003 0.000252 0.00028 0.00032 0.000348 0.000672 0.000366 0.00042 0.00045 0.000302

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Table 4. (Continued) Sample ID 87Sr/86Sr 98.355_B3 0.7119601 98.355_C1 0.7116431 98.355_C2 0.7121804 98.355_C3 0.7126502 98.355_D1 0.713514 98.355_D2 0.7136801 98.355_D3 0.7146961 Standard Deviation 98.359_A1 0.7117397 98.359_A2 0.7127162 98.359_A3 0.7118754 98.359_B1 0.7117544 98.359_B2 0.7129407 98.359_B3 0.7129047 98.359_C1 0.7141845 98.359_C2 0.7134008 98.359_C3 0.7137356 98.359_D1 0.7133296 98.359_D2 0.7123873 98.359_D3 0.7126201 Standard Deviation

Zr Corrected #1 0.708087172 0.707992121 0.708145311 0.708280574 0.708529766 0.708583201 0.70886989 0.000328141 0.70802341 0.708290944 0.70808271 0.708028108 0.708352823 0.708341993 0.708692376 0.708478361 0.708591637 0.708455316 0.708200505 0.708271653 0.000214107

Zr Corrected #2 0.710714156 0.710377001 0.710937714 0.711487117 0.712218521 0.712382943 0.7134102 0.001154633 0.710463167 0.711456789 0.710661269 0.710486623 0.711697772 0.711632711 0.712909064 0.712154703 0.71248943 0.712055396 0.711126393 0.711417852 0.00077966

Sr Corrected 0.710585614 0.710239802 0.710771703 0.711191809 0.712354224 0.712548372 0.713553632 0.001243646 0.710068174 0.711076884 0.710208528 0.709975754 0.711152791 0.711115233 0.712398622 0.711602994 0.711965841 0.711520369 0.710580792 0.7107383 0.000759206

2σ error 0.000278 0.000414 0.000448 0.000346 0.000632 0.00039 0.000464 0.00069 0.00066 0.000672 0.00071 0.000916 0.000712 0.001052 0.00068 0.00061 0.00086 0.00088 0.0008

Figure 17. 87Sr/86Sr ratios by sample for SM, LA, and LA corrected for REE, calcium dimer and CaPO

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In order to enable in situ correction of mass 87 using mass 91 for 87Sr/86Sr analysis, the same machine must monitor both masses simultaneously. Unfortunately, the MC-ICP-MS used for this research was not sensitive enough to measure peaks at masses 90 or 91 without special tuning. It remains theoretically possible that this machine could be effectively used for LA 87Sr/86Sr analysis using a Zr offset correction; however the special tuning would render the procedure impractical and likely interfere with regular operations of the instrument. Thus at present, efforts to correct 87Sr/86Sr data for [40] Ca [31] P [16] O interference remain most ably demonstrated with the procedures used by Horstwood et al. 1. Without the same depth of suitable reference materials and sample runs, we can still examine new data, but must approach it with some measure of caution. Taking the LA data for the Cis-Baikal samples and applying several different approaches to correction, we find a significant amount of variability. Applying the correction equation used for the Simonetti et al. [8] directly, but with measured Zr concentrations, we clearly have an overcorrection (Table 4). There is likely to be a significant divergence of the laser data from the single solution datum as we realize the disconnect between internal variability of a tooth’s formation and the homogenizing effects of solution preparation, however are overcorrected by a 87Sr/86Sr ratio of approximately 0.002. A second attempt at using zirconium differences as a correction yield better results, which are well within the realm of uncertainty regarding internal variability relative to the solution data available. We find similar results from a correction drawn directly from strontium concentrations and bypassing zirconium offsets altogether. These later two corrections are difficult to assess without a coupled set of microsampled set of solutions. Given the added uncertainty of drawing these data from a different instrument, we were inclined to have greater faith in the strontium corrected laser data in this case. The strontium data are likely to be more similar between the two instruments and thus less prone to measurement errors within the scope of this experiment. These data also highlight a fair amount of internal variability regardless of which correction is used. Of the five teeth sampled, there is an average deviation of 0.000773 for 87Sr/86Sr ratios, with some individuals exhibiting significantly more variability than others. This is strongly suggestive of 87Sr/86Sr ratios indicative of mobility across geologic zones and/or changes in dietary geochemical interactions of these individuals.

CONCLUSION Microsampling of complex minerals is a delicate balance between microsampling methodology, analytical precision and the formation events responsible for the initial complexity. This will be true whether the mineral matrix is organic or inorganic. For skeletal materials, there is greater uncertainty attached to the formation process and the contributing ionic pool of resources used to form the final matrix. Dietary intakes will be averaged within the body, for over a year for elements such as strontium, and the forming mineral structure remains an open chemical system for weeks or months after the formation of the structures visible as Retzius lines. This makes progress difficult to impossible for efforts to generate useful provenance and/or dietary data for individuals at a temporal scale smaller than a tooth as a whole. Evidence from herbivore teeth strongly suggest that lag times can outstrip effective mineralization rates, making intensive microsampling an unnecessary effort as the

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same trend lines can be generated with a fraction of the total analyses involved in intensive microsampling. The same may or may not be true for human teeth as they take longer to form and in a smaller volume than herbivore teeth, thus potentially incorporating geochemical information at a temporal scale either within the body residence time of heavy elements or sufficiently long that changes in body averages can become visible using mixing models to interpret such data effectively. The data generated for this research strongly support two conclusions: 1) that trace element analysis can provide a useful contribution to understanding provenance/mobility data for skeletal tissues; and 2) that microsampling of human teeth is a worthwhile effort, with laser ablation being a reasonable option with appropriate corrections applied.

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[57] Speakman, RJ; Neff, H. Laser ablation-ICP-MS in archaeology research. University of New Mexico Press: Santa Fe, 2005. [58] Tykot, RH; Young, SMM. Archaeological Applications of Inductively Coupled Plasma - Mass Spectrometry. In Archaeological Chemistry, AM; Pollard, C. Heron, Eds. Royal Society of Chemistry: Cambridge, 1996, 116-130. [59] Budd, P; Montgomery, J; Cox, A; Krause, P; Barreiro, B; Thomas, RG. The distribution of lead within ancient and modern human teeth: implications for long-term and historical exposure monitoring. The Science of the Total Environment, 1998, 220, 12136. [60] Copeland, SR; Sponheimer, M; le Roux, PJ; Grimes, V; Lee-Thorp, JA; de Ruiter, DJ; Richards, MP. Strontium isotope ratios (87Sr/86Sr) of tooth enamel: a comparison of solution and laser ablation multicollector inductively coupled plasma mass spectrometry methods. Rapid Communications in Mass Spectrometry, 2008, 22(20), 3187-3194. [61] Copeland, SR; Sponheimer, M; Lee-Thorp, JA; Le Roux, PJ; De Ruiter, DJ; Richards, MP. Strontium isotope ratios in fossil teeth from South Africa: assessing laser ablation MC-ICP-MS analysis and the extent of diagenesis. Journal of Archaeological Science, 2010, 37(7), 1437-1446. [62] Trotter, JA; Eggins, SM. Chemical systematics of conodont apatite determined by laser ablation ICP-MS. Chemical Geology 2006, 233, 196-216. [63] Christensen, JN; Halliday, AN; Lee, DC; Hall, CM. In situ Sr isotopic analysis by laser ablation. Earth and Planetary Science Letters, 1995, 136, 79-85. [64] Kin, FD; Prudêncio, MI; Gouveia, MÂ; Magnusson, E. Determination of Rare Earth Elements in Geological Reference Materials: A Comparative Study by INAA and ICPMS. Geostandards and Geoanalytical Research, 1999, 23(1), 47-58. [65] James, WD; Dahlin, ES; Carlson, DL. Chemical compositional studies of archaeological artifacts: Comparison of LA-ICP-MS to INAA measurements. Journal of Radioanalytical and Nuclear Chemistry, 2005, 263(3), 697-702. [66] Revel, G; Ayrault, S. Comparative use of INAA and ICP-MS for Environmental Studies. Journal of Radioanalytical and Nuclear Chemistry, 2000, 244(1), 73-80. [67] Horstwood, MSA; Evans, JA. Complications of LA-MC-ICP-MS Sr isotope analysis of phosphate matrices. In Eighth International Conference on Plasma Source Mass Spectrometry, University of Durham, UK, 2002. [68] Horstwood, MSA; Nowell, GM. Multi-collector devices. In ICP-MS Handbook, Nelms, S., Ed. Blackwell Scientific Publications: Oxford, 2005, 54-68. [69] Cucina, A; Neff, H; Tiesler, V. Provenance of African-born individuals from the colonial cemetery of Campeche (Mexico) by means of trace element analysis. Dental Anthropology, 2005, 17, 65-69. [70] Dudgeon, J. The genetic architecture of the late prehistoric and protohistoric Rapa Nui. Doctoral Dissertation, University of Hawai'i, Manoa, 2008. [71] Weigand, PC; Harbottle, G; Sayre, EV. Turquoise Sources and Source Analysis: Mesoamerica and the Southwestern U.S.A. In Exchange Systems in Prehistory, TK; Earle, JE. Ericson, Eds. Academic Press: New York, 1977, 15-34.

In: Laser Ablation: Effects and Applications Editor: Sharon E. Black

ISBN: 978-1-61122-466-5 © 2011 Nova Science Publishers, Inc.

Chapter 3

MODELING OF LASER ABLATION INDUCED BY NANOSECOND AND FEMTOSECOND LASER PULSES Tatiana E. Itina1*, Mikhail E. Povarnitsyn2 and Konstantin V. Khishchenko2 1

Hubert Curien’s Laboratory, CNRS 55216, 18 rue de Professeur Benoît Lauras, Bat. F, 42000, Saint-Etienne, France 2 Joint Institute for High Temperatures RAS, 13 Bd. 2, Izhorskaya street, Moscow, Russia

ABSTRACT The chapter considers the problem of numerical modeling of laser-matter interactions. The main objective is to clarify the mechanisms of this extremely complex process. Comparison of femtosecond and nanosecond laser ablation is first presented. Thermal model is used for nanosecond ablation. The physical phenomena involved into the interaction of a laser-generated plasma plume with a background environment are furthermore studied. A three-dimensional combined model is developed to describe the plasma plume formation and its expansion in vacuum or into a background gas. The proposed approach takes advantages of both continuous and microscopic descriptions. The simulation technique is suitable for the simulation of high-rate laser ablation for a wide range of the background pressure. The model takes into account the mass diffusion and the energy exchange between the ablated and background species, as well as the collective motion of the ablated species and the background gas particles. The developed approach is used to investigate the ablation of aluminum in the presence of a background gas. The influence of the background gas on the expansion dynamics of the lasergenerated plume is examined. Experimental density distributions are explained based on the simulation results. A detailed analysis of material decomposition in femtosecond regime is then performed by using a hydrodynamic model with a thermodynamically complete equation *

Corresponding author: Email: [email protected].

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Tatiana E. Itina, Mikhail E. Povarnitsyn and Konstantin V. Khishchenko of state. As a result, several ablation mechanisms are observed. A major fraction of the ablated material is found to originate from the metastable liquid region, which is decomposed either thermally in the vicinity of the critical point into a liquid-gas-mixture or mechanically at high strain rate and negative pressure into liquid droplets and chunks. The calculation results agree with the results of previous molecular dynamics simulations and explain recent experimental findings. In addition, effects of the ultra-short laser excitations of wide band gap materials need a particular attention. In this case, material ionization through multi-photon excitation and electron-impact ionization should be considered. Laser interactions are simulated with a particular focus on the control over laser plume expansion process. The properties of the laser-generated plasma plume are shown to be strongly affected by the laser-mater interaction mechanism

1. INTRODUCTION Shortly after the demonstration of the first laser, the most intensely studied theoretical topics dealt with laser-matter interactions. Many experiments were undertaken to clarify the mechanisms of this extremely complex process. At the same time, numerous models, both analytical and numerical, were proposed to describe these interactions. In these models, different experimental conditions were considered, and several terms were proposed to denote the processes occurring during laser action on different materials. Thus, "laser ablation", "evaporation", "desorption" or "sputtering" – all these terms are relevant to the interaction of a laser beam with a solid (or a liquid) surface that results into transition of the surface particles into a gas phase. For simplicity, here we will use only the term "laser ablation". Laser ablation has found a number of industrial applications, such as laser cleaning, micromachining, molecular mass spectrometry, plasma technology, laser surgery, etc. One of the main advantages of this technique is the simplicity of the experimental set-up. The other advantage is the possibility of adjusting the experimental conditions in order to obtain the desirable treatment quality. Among the important experimental parameters one should mention the followings • • • • •

laser pulse parameters (fluence, beam dimensions, duration, time-shape) target material target-substrate distance ambient gas (pressure, atomic mass, temperature) substrate characteristics

Recently, a particular attention has been attracted to various surface-treatment applications, such as laser micromachining, marking, modification of the surface properties, etc. One of the major problems that arise in the development of these applications is connected with prevention of collateral thermal damage of the treated material. One way of the minimization of the undesired thermal effects is the decrease of laser pulse duration. That is why starting from the beginning of 80th, the interest of researches turned to lasers with nanosecond pulse duration. After recent commercialization of laser systems with short (picosecond) and ultra-short (femtosecond) laser pulse duration, the attention of researches has been naturally focused on the advantages of these systems [1-7]. The rapid evolution in laser system development has opened new horizons for laser applications. Laser wavelength

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now varies from infrared to ultra-violet and starts to penetrate into X-ray. Laser power has grown considerably, so that smaller focusing is required. Among other important adjustable parameters, one can note laser pulse shape, polarization and beam quality. Therefore, lasers are currently used not only for surface treatment, cleaning, micro and nano-machining, structuring, but also for nanoparticle formation, surface analysis, optics and photonics, microelectronics, nanoplasmonics, nano-bio-technology, atomic physics, chemistry, medical applications, etc. In particular, femtosecond laser pulses provide unique opportunities for such applications as laser spectroscopy (LIBS), nanoparticle synthesis both in vacuum, in gas, and in liquid, laser surgery, nano-fabrication, etc. [8,9,10]. One one hand, cluster formation by laser ablation provides an attractive avenue for fabrication of nanostructured materials and for medicine [15,11,12,13,14]. The applications based on cluster deposition include fabrication of nano-crystalline or cluster-assembled films and coatings, deposition of metal particles for catalysis, or composite compound semiconductors for electro-optical applications. On the other hand, the presence of clusters or particulates in the ablation plume can be harmful for the quality of thin films grown in pulsed laser deposition (PLD) [15]. In addition, the formation of debris and re-deposition of ejected particulates can cause problems in manufacturing of surface microstructures. Clusters in the expanding plume can scatter the incident laser light and lead to a shielding of the target surface in the case of long laser pulses or in the multi-pulse irradiation regime. For all these reasons, it is crucial to be able to predict and control the parameters of clusters formed in laser ablation, such as particle size distributions, velocities, and temperature at the time of deposition. Despite rapid development in laser physics, one of the fundamental questions still concerns the definition of proper ablation mechanisms. Apparently, the progress in laser systems implies several important changes in these mechanisms, which depend on both laser parameters and material properties. Among the more studied ablation mechanisms there are thermal, photochemical and photomechanical ablation processes [11]. Frequently, however, the mechanisms are mixed, so that the existing analytical equations are hardly applicable. In this case, numerical simulation is needed to better understand and to optimize the ablation process [16,17,18]. So far, thermal models are commonly used to describe nanosecond (and longer) laser ablation [19, 20]. In these models, the laser-irradiated material experiences heating, melting, boiling and evaporation. Thermal effect plays therefore a major role, particularly in the case of metals with high thermal conductivity. In this case, the ablation flux can be described by a Hertz-Knudsen equation. The interaction of femtosecond pulses even with metals implies, however, a change in the ablation mechanism due to not only the absence of equilibrium between electrons and lattice-ions during the pulse, but also because the heating is too fast. The Hertz-Knudsen equation is inapplicable in this case and a numerical study is for the careful optimization of the laser processing of metals. Recently, many experimental and theoretical investigations of the mechanism of femtosecond (< 100 fs) ablation have been performed [21,22,23,24]. Evidently, the interest in femtosecond lasers was caused by numerous exciting laser applications listed above. Among the main advantages of these short pulses is the possibility of laser treatment of any materials, even these that were considered to be transparent, in a possibility of control over electronic excitations, and in the minimization of non-desirable thermal effects. However, the mechanisms of these interactions are often rather different from that of longer pulses, and many difficulties first arouse in the corresponding numerical modeling. The main point that

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makes ultra-short interactions difficult to model, is that the pulse duration is shorter than the electron-phonon/ion relaxation time tei~1-10 ps. As a result of strong difference between the electron and ion mass, the mean electron energy rises much faster and higher that that of ion subsystem. The terms “electron and ion temperatures”, however, require equilibrium in each sub-system that also may take longer time to establish. If these terms can be applied, electron temperature is much larger that ion temperature during the interaction. The knowledge about the required equation of state (EOS) is also very limited. In addition, metastable matter states, such as superheated liquid one, seem to play a role. All these points limit the information about model parameters and make computer simulation rather difficult. In studies of femtosecond interactions with metals, new ablation mechanisms have been proposed both for metals and semiconductor materials [21,22,23,25-34]. In particular, for semiconductor and/or dielectric materials, such processes as multi-photon/tunneling ionization, electron-impact, or avalanche ionization, charge separation, optical breakdown, material damage and ablation have been investigated [35]. Previous theoretical studies used either the detailed Boltzmann’s equation [35,36] or simplified rate equations to describe laser excitation kinetics [37-41]. For metals, two-temperature model [42,43] has yielded information about electron and lattice temperature evolution. However, the validity of the classical TTM model is limited due to the conditions of equilibrium to be established in each of two sub-systems and because of the heat diffusion equations that do not describe the heat front. As a result, the general conclusion was that the conventional TTM can be used for a bulk metal target at relatively high intensities. At smaller laser intensities, there is no equilibrium in the electron sub-system. In addition, for metal films, the ballistic electron transport should be considered [44]. In general, even in the case of a bulk target, the heat diffusion equation disregards the finite time of the heat propagation. In addition, the calculation of model parameters represents a challenge because of the lack of knowledge of electron distribution and temperature-dependency of such parameters as heat capacity, thermal conductivity, electron-phonon coupling, etc in the absence of both thermal and electron-ion equilibrium. Only recently, ab-initio calculations were performed [45] to account for the excitation of d-band electrons [46] in some metals, such as gold, silver, nickel. However, these first results need more analysis and verification To calculate material motion (and not only its temperatures), three numerical approaches were used, such as •

•

Atomistic approach, based on such methods as molecular dynamics (MD) [25,47,48,49] and Direct Monte Carlo Simulation (DSMC) [50,51,52].Typical calculation results provide detailed information about atomic positions, velocities, kinetic and potential energy; Macroscopic approach based hydrodynamic models [18,53,54] This model allows the investigations of the role of the laser-induced pressure gradient, which is particularly important for ultra-short laser pulses. The models are based on a one fluid two-temperature approximation and a set of additional models (equation of state) that determines thermal properties of the target. Recently electron-phonon coupling parameter was calculated as a function of electron temperature by using abinitio quantum mechanical methods [45];

Modeling of Laser Ablation Induced by Nanosecond and Femtosecond… •

103

Multi-scale approach based on the combination of two approaches cited above was developed by several groups and was shown to be particularly suitable for laser applications.

The ejection of liquid and/or solid particulates has been studied for metals [55,56,57,58], semiconductors [59,60,61,62], dielectrics [63,64], and organic materials [11,65]. The parameters of the ejected particles are found to have a strong dependence on the laser irradiation conditions and the background gas pressure. A number of scenarios of cluster formation in laser ablation have been discussed in the literature. In many cases, observation of small clusters is attributed to the collision-induced condensation in the dense regions of the ejected plume [62,64,66,67]. Some evidences of the direct ejection of nanoparticles by ultrashort laser pulses were also obtained. Before considering the modeling details, we would like to emphasize that the laser-target interaction is an extremely complex process involving more than one physical phenomenon. These phenomena include the absorption of laser radiation, the creation of a high pressure and temperature region in the solid, the propagation of the compression and thermal waves, phase transitions, material decomposition and ionization, the ejection of electrons, ions and/or neutrals, laser plume expansion, the interaction of the ablated species with background gas, chemical reactions, cluster formation, etc… That is why it is difficult to formulate a complete and self-consistent model to interpret all observations. In general, laser ablation models can be subdivided into two categories • •

modeling of the interaction of the laser beam with a surface, where attention is focused on the primary mechanisms of the material ejection; modeling of the formation of a laser plume (secondary mechanisms) and its expansion in vacuum or in a background gas.

These categories are closely connected, since the first one provides initial conditions for the second one. Models that describe all the ablation process self-consistently and combine both calculations are exceptional. Some models are based on approaches with considerable mathematical complexity, some rely on simple physical descriptions, and others are phenomenological. Most of the theoretical studies do not provide insights directly applicable to improve the experimental conditions. In addition, the calculations contain many parameters, some of which are unknown and most of them are temperature dependent. Nevertheless, as far as several particular characteristics of the process are concerned, the quantitative comparison with experimental data turns out to be possible. The latter is valid especially for the experiments with either very small or very large laser fluence. This former case is of a particular interest because of a number of technical and scientific applications. Herein, we examine the different approaches used in the modeling of laser ablation and the most interesting results that were obtained in this field.

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2. MODELING OF NANOSECOND LASER ABLATION In this section we consider the modeling of laser ablation induced by nanosecond laser pulses. The description consists of two parts that correspond to the two categories of the ablation models.

2.1. Primary Mechanisms of the Material Ejection under Nanosecond Laser Action As we have noted in the Introduction, the most common and simplest model that describes the laser-solid interaction under low power density is based on a “thermal effect”. The problem of the evaporation of a metal surface heated up to a certain temperature T0 was considered by a number of authors, for example, by Anisimov et al [68], and by Ready. A simplified one-dimensional model which neglected the presence of a liquid phase was used in the first papers. Later, more realistic models of target heating and evaporation were developed, where moving boundary conditions were used. One can note, for example, the model of Luikov et al [69]. More recently, temperature, pressure and density discontinuity across Knudsen layer were considered by Anisimov [70]. To describe the heat transport one can use the heat flow equation that can be written in one-dimensional form as follows [71]:

c(T ) ρ (T )

∂T ∂ ⎛ ∂T ⎞ = ⎜ K (z , T ) ⎟ + μ I (z , t ) , ∂t ∂z ⎝ ∂z ⎠

(1)

where I is the absorbed laser radiation given by (1.1). For simplicity, the quantities c, ρ and K are frequently assumed to be temperature and space independent. If one neglects, furthermore, surface melting and the movement of the evaporation front, the boundary conditions for a surface laser source and a semi-infinite solid target can be written as follows:

z = 0 I = (1 − R) I 0 ,

(2)

z = 0, t = 0 T = T0 , and the solution of Eq. (1.6) obtained analytically using Green function technique is

T ( z, t ) =

1

ρ c πχ

t

∫ 0

I 0 (t − ψ )

ψ

⎛ z2 ⎞ ⎟⎟dψ + T0 exp⎜⎜ − ⎝ 4 χψ ⎠

Then, the time-evolution of the surface temperature is given by the equation:

(3)

Modeling of Laser Ablation Induced by Nanosecond and Femtosecond…

T (z, t ) =

A

ρc

t

πχ ∫

I (t − ψ )

ψ

0

dψ + T0

105

(4)

where A=1-R is the absorption coefficient. This equation allows the calculation of the surface temperature for a given time-evolution of the laser intensity. Using the solution obtained one can estimate the minimum (threshold) laser intensity Ith for evaporation by using the equation Tmax= TVap, where Tmax is the maximum surface temperature obtained from Eq. (1.9), and TVap is the material vaporization temperature. For example, for a top-hat laser pulse it gives:

I th =

ρ c πχ 2A τ

(TVap − T0 ) .

(5)

If thermal desorption mechanism is assumed, the initial velocity distribution is halfMaxwelian with the surface temperature T(t). Since the desorbed current is cosine distributed, the ablated flow velocity U0(t)=VT(t)/2, where VT (t ) = 8kT (t ) / πm , k is a Boltzmann’s constant and m is the particle mass. The desorption flux Φ(t ) = n0 (t )U 0 (t ) , where n0(t) is the density of the desorbed material immediately in front of the surface. If surface melting takes place, phase transition must be considered. To consider solidliquid or liquid-solid transitions, two boundary conditions are required at the interface where the phase transition occurs. First of these is an energy balance:

ρΔH m (Ttr )υ int = K sol

∂T ∂z

z + int

− K liq

∂T ∂z

(6) z − int

where Ttr is the temperature at which the transition takes place, Ksol and Kliq are thermal conductivities of solid and liquid phase respectively, ΔHm is the heat of melting at T= Ttr , and

υ int is the solid-liquid interface velocity. Moreover, the thermodynamics of crystallization

(Pererlongo et al., Ref.71) requires that υ int should be a function of the supercooling Ttr- Tm,

υ int = f (Ttr − Tm ) .

(7)

During the vaporization the surface recedes with velocity υ r . It is, however, still possible to label its position with z=0 in the reference frame moving with a receding surface. Then, neglecting mass accumulation and temperature dependence, Eq. (1) becomes

cρ

∂ 2T ∂T ∂T + μ I (z, t ) = K 2 + cρυ r ∂z ∂t ∂z

(8)

106

Tatiana E. Itina, Mikhail E. Povarnitsyn and Konstantin V. Khishchenko A simple way to compute υ r is to consider that liquid is in thermal equilibrium with its

saturated pressure. In this case the number NV of particles vaporizing per unit time and area is

NV =

p

(2πkTm)1 / 2

CS ,

(9)

where p is the gas pressure, and CS is the sticking coefficient. If one assumes that this equation also holds in a nonequilibrium situation as when particles are emitted into vacuum, then the velocity

υr =

p

ρ (2πkT / m )1 / 2

CS

(10)

The relation between the equilibrium vapor pressure and the temperature may be obtained from the Clausius-Clapeyron equation in the limit Vliq<
⎧ ΔH (T )m ⎛ 1 1 ⎞⎫ p = p b exp⎨ υ b ⎜⎜ − ⎟⎟⎬ , k ⎝ Tb T ⎠⎭ ⎩

(11)

where pb=1atm, ΔH υ (T ) is the heat of vaporization that can be assumed approximately equal to the value at the boiling temperature Tb at 1atm. During last decade, new mechanisms of the material ejection stage were investigated. For instance, Ostland and Orlander [72] considered nonequilibrium surface processes. A so-called “phase explosion” mechanism was proposed by Martynyuk [73]. Then, Kelly et al. [74] proposed the mechanism of thermal-shock-induced exfoliation. Luk’yanchuk et al. [75] studied photophysical ablation of organic polymer materials.

2.2. Secondary Mechanisms: Plasma Plume Expansion after Nanosecond Laser Pulse If the ablation flux is large enough, the emitted particles tend to move according to the laws of gas-dynamics. The application of the gas-dynamical equations requires the presence of a thermodynamical equillibrium in the considered region of the flow. Since, the outer part of the plume always goes to free-molecular flight, these methods can not describe all the flow. Although, if the density of the evaporated material is sufficiently large, the hypothesis of the thermodynamical equilibrium is justified for a major part of the flow until a certain time. Several analytical models of plume expansion were proposed. First analytical models based on a system of gas-dynamical equations were developed by Dawson [76] and by Singh and Narayan [77]. Later, a more accurate self-similar adiabatic expansion model was

Modeling of Laser Ablation Induced by Nanosecond and Femtosecond…

107

proposed by Anisimov et al. [78] to adequately describe plume evolution in vacuum. A number of numerical solutions of the system of gas-dynamical equations were furthermore proposed. One can note the modeling of the plume dynamics performed by Peterlongo et al. [79], Bulgakov and Bulgakova [80], and Le et al. [81]. These studies demonstrated numerous gas-dynamic effects taking place during the plume expansion both in vacuum and in the presence of a background gas. An alternative approach, particularly suitable to study of the effects of gas-phase collisions, based on the Direct Simulation Monte Carlo method (DSMC) [82] was used in a few papers. In this method, the time evolution of the gas cloud is obtained by following simultaneously a number of representative flow particles (103 to 106) which undergo collisions (and chemical reactions). The method turns out to be very effective for describing non-equilibrium flows with high density-gradients and strong time-variations of the parameters. In addition, chemical reactions, internal mode excitation and exchange can be directly included in the simulation procedure. That is why, recently it was successfully used to study pulsed laser desorption for low and moderate evaporation fluxes. For example, NoorBatcha et al. [83], with the aid of the one-dimensional DSMC algorithm, considered the characteristics of the desorption flow containing atoms and molecules with internal degrees of freedom. Furthermore, Urbassek and Sibold [84] investigated the one-dimensional pulsed laser desorption problem with recondensation. As a result, the numerical solution was obtained for a first time to this problem for a wide range of Knudsen number. Later, this solution was compared with analytical results obtained by Kelly [85] and with analytical solution proposed by Sibold and Urbassek [86]. A three-dimensional Monte Carlo simulation of the desorption from a finite-sized laser spot was carried out by Sibold and Urbassek [87] for a top-hat thermal desorption flux. The effects of the jet formation and of the segregation of heavy and light components were shown to result from the collisions in the laser plume. Later, by the author with the co-workers the DSMC method was used to show the effects of recombination-dissociation in the plume on the angular distribution of the ablated particles [52]. In addition, combined DSMC-Random Trajectories method was developed to calculate the stoichiometry distribution of thin films deposited by laser ablation of binary targets [50]. The Monte Carlo methods also were used to analyze the results of time-of-flight (TOF) experiments [88]. The microscopic methods have thus yielded a wealth of information about the ablated plume. The main limitation of these methods is, however, connected with the density and size of the plume, since the calculation resources needed for the DSMC method are proportional to Kn-n, where Kn=l/L is the Knudsen number; l is the mean free path of the species; L is the characteristic size of the plume; and n is the dimensionality of the problem. To effectively study plume expansion into a background gas, a novel hybrid method has been recently developed [51]. This method combines both continuous and microscopic descriptions of laser-induced plasma expansion and is applicable for different background pressures, starting from the plume expansion into vacuum and up to the regime of strong shock waves. The method is particularly useful to study the transitional background pressure regime. It takes advantages both of the gas dynamical description of the plume at the early stage, and of the DSMC simulation at the late stage of the plume expansion, avoiding the above-described challenging issues of these techniques. As a result, the expansion of a highdensity plume in the presence of a background gas can be tackled during a time as long as 510 microsecond, at ambient gas pressures from zero to hundreds Pa. The developed model is three-dimensional and uses axial symmetry. We assume that a laser beam with radius R0 and

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Tatiana E. Itina, Mikhail E. Povarnitsyn and Konstantin V. Khishchenko

pulse duration τ interacts with a plane target (z ≤ 0). The half-space z > 0 is filled by a background gas (pressure Pb, density ρb). Under laser irradiation, the target absorbs a part of the laser energy; the target material is heated and ablated forming a laser plume near the target surface.

2.2.1. Combined LP -DSMC method At the first stage, we consider the plasma plume as a non-viscose and non-heatconductive vapor containing atoms, ions and electrons. In the plume, the temperatures of electrons, Te, deviates from the one of ions and neutrals, Ta, because of the slow rate of energy transfer between electrons and heavy particles. Based on these considerations, we adopt an approach of one-fluid two-temperature gas-dynamics to describe the movement of the laser plume. In this case, the system of gas-dynamical equations can be written in the divergent form as follows (the plume expands along the Z axis):

(

)

(12)

(

)

(13)

r ∂P ∂ρ V + div ρ V W + = 0, ∂t ∂r

(

)

(14)

r r ∂ρ E a + div ρ E a W + div PaW = Qei + Qea , ∂t

(15)

r ∂ρ + div ρ W = 0 , ∂t r ∂P ∂ρ U + div ρ U W + = 0, ∂t ∂z

(

)

(

(

)

)

( )

r r ∂ρ E + div ρ E W + div PW = − F , ∂t

(16)

r 2 r 2 E = ε a + ε e + W / 2 , Ea = ε a + W / 2 ,

(17)

P = Pa + Pe , Pa = (γ − 1)ρ ε a , Pe = (γ − 1)ρe ε e

(18)

r where ρ is the density, U and V are Z and R components of the velocity vector W ; γ is the specific heat ratio;

ε a is the thermal energy of ions and neutrals; ε e is the thermal energy of

electron gas; Pa and Pe are the partial pressures of the heavy (ions and neutrals) and light (electrons) gases. In Eq.(15), Qei and Qea describe the energy exchange between the electron gas and the gas composed of ions and neutrals, and F is the energy lost by electrons for ionization. In Eq. (12-18), a local thermodynamic equilibrium is assumed, and ideal-gas equations of state are used.

Modeling of Laser Ablation Induced by Nanosecond and Femtosecond…

109

To solve numerically the system (17)-(23) we use the method of large particles (LP) adapted to the case of laser-solid interaction [89]. The advantages of this method are the uncoupling of physical processes and the combination of the Euler and Lagrange approaches, which gives the possibility of following initial plume expansion into a background gas in an affordable computer time. The heat obtained by ions and atoms in elastic collisions with electrons is

Qei + Qea =

2me 3 k (Te − Ta )ne (vei + vea ) , m 2

(19)

where vei and vea are the mean frequencies of electron-ion (e-i) and electron-atom (e-a) collisions respectively. The collision frequencies are

4(2π ) ni e 4 Λ C , vei = 3m1e / 2 (kTe ) 3 / 2 1/ 2

c Σn a n e

v ea =

2

,

(20)

(21)

where Λ C is the Coulomb logarithm, and c is the average relative velocity, and Σ is the electron-atom collision cross-section. The energy lost by electrons for ionization

F=

2ne na ε i k ei , 3

(22)

where

k ei = 2

(2πme kTe )3 / 2 k 3

h ne

rec

⎛ ε ⎞ exp⎜⎜ − i ⎟⎟ ⎝ Te ⎠

(23)

is the e-impact ionization rate constant, and is the three-body recombination constant.

4π (2π ) 9

1/ 2

k rec =

e10 n 9/ 2 e me1/ 2 (kTe )

(24)

Initial boundary conditions at the surface z = 0 are calculated as a heat source resulting from the absorption of the laser radiation corresponding to a laser pulse with gaussian temporal shape and the pulse duration τ. The thermal evaporation model is used, so that the vapor pressure at the surface is obtained from the Klasius - Clapeyron equation. The

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temperature distribution along the laser-irradiated area is assumed to be uniform. Electron gas temperature at the beginning is set to be higher than the one of heavy atoms, because electrons absorb laser radiation. The initial flow parameters at z = 0 and r < R0 are calculated using the jump conditions at the Knudsen layer boundary [90,91]. The flow velocity at the Knudsen layer boundary UK is assumed to be equal to the velocity of sound aK, so that the Mach number M = 1 at z = 0 [91]. For one-atomic vapor (γ = 5/3), the Knudsen layer parameters are TK = 0.65 TS, nK = 0.62 nS, UK = 1.31 TS, where index S refers to the parameters at the surface, and K to the border of the Knudsen layer. The large particle calculations are performed until a certain time t0, and then, the motion of the ablated and background species is followed by using a microscopic Monte Carlo method. On one hand, the switch time t0 should be long enough for the plume density to diminish by several orders of magnitude with respect to the initial values. As a result of the plume expansion, the characteristic Knudsen number drops down to the values required for the effective application of the DSMC method. Moreover, the t0 should be long enough to account for the ionization-recombination processes in the plasma plume. On the other hand, the switch time should be short enough, such that the mass diffusion and the heat exchange between the plume and the background gas are insignificant during the t0. For t > t0, the ablated plume expansion into a background gas is calculated using the Direct Simulation Monte Carlo (DSMC) method. In this method, gas flows are modeled by a representative ensemble of atoms or molecules (typically from 103 to 106). The DSMC procedure uses the uncoupling of the molecular motion and the intermolecular collisions. The method has the same limitations as kinetic theory of gases, which includes Boltzmann equation. The principal limitations are the assumption of molecular chaos and requirement of a dilute gas. The details of the method can be find in Ref.82.

2.2.2. Results of the combined LP -DSMC calculations The developed model has been used for a series of calculations of laser plume expansion in the presence of a background gas. Hereafter, we present the simulation results obtained for laser ablation of solid targets (Al) in the presence of several background gases. At small background pressure, initial plume expansion is slightly affected by the gas, and then the ablated material transport becomes diffusion-driven. Starting from a certain pressure, which can be determined from our simulation, a snowplow effect can be observed in the twodimensional contour maps of the plume and gas densities (Figure 1). This effect implies the compression of the background gas in front of the expanding plume. In addition, a compression of the plume front also takes place. If laser fluence is such that plume expansion is under-sonic, the compression does not imply shock-wave formation, and diffusion is strong. Monte Carlo simulation, which easily treats diffusion, is well suitable for these cases. If, furthermore, laser fluence several times exceeds the ablation threshold, laser plume expansion is super-sonic, and two shock waves are formed. One of them (external) propagates in the background gas, whereas the other (internal) moves backward inside the plume [92,93,94,]. Gas-dynamic description including a turbulent stage of Rayleigh Taylor instability [95] become relevant when the sizes of all shock waves are much larger than corresponding local mean free paths.

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Figure 1. Calculated contours of number density. (above) plume species; (below) gas species. The numbers are in m-3. Results are obtained for Al target in the presence of oxygen with laser spot radius of 1 mm, laser energy of 15 mJ, at background gas pressure of 70 Pa, and at t = 2.6 μs after the beginning of the laser pulse

Simulation results show, furthermore, that the dimensions of the laser spot affect the expansion process (Figure 2). For instance, at the same laser fluence, the larger is the laser spot, the higher is the density in the compressed plume front. At larger spot, the plume expansion in radial direction is weaker; the thermal energy of the plume is transferred into the kinetic energy of the outward motion, so that the flow velocity is larger.

Figure 2. Maximum plume front density as a function of background pressure. The calculations are performed for Al in O2 at two different radius of the laser spot and same laser fluence of 20 J/cm2. The calculation results are obtained at t = 2.6 μs after the beginning of the laser pulse

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(a)

 

(b) Figure 3. (a) Calculated spatial distribution of the product of the number densities of Al and O2, np (r, Z ) = n Al nO . (b) Experimentally measured contour maps proportional to the AlO ground-state 2

population density. The results are obtained at t = 3 μs after the beginning of the laser pulse, at laser spot radius of 0.15 mm, laser fluence F~20 Jcm-2, and background pressure of 13 Pa

If, furthermore, ambient pressure is small, the plume-gas mixing is stronger than in the snowplow mode. The plume expansion mode with an enhanced plume-gas mixing is preferable for the reactive laser ablation. Typical background pressures used to produce molecules by laser ablation depend on the molecular weight of the background gas. For aluminum ablation in oxygen, these pressures range from 10 to 30 Pa. This regime, which was particularly difficult for previous gas-dynamic simulations, is considered below. 0

To determine the flow regions with potentially maximum n AlO production rate, we consider the calculated distribution of the product of the number densities of Al and O2, n p (r, Z ) = n Al nO , (Figure 3). The np distribution reveals a surprising similarity with the 2

experiments, in spite of the fact that np represents only one of the factors determining the production of AlO. Most remarkably a spatial distribution with similar two maxima at the plume periphery can be observed at t = 6.3 μs. These maxima can be explained if we compare the calculated np with the velocity flow-field. The comparison indicates that one of the maxima, which is closer to the target, results from the backward particle fluxes. In fact, the backward fluxes, which are composed of both ablated atoms and gas molecules, have been observed in our calculations. The maximum of these backward flows is at the position of the first np maximum. The second maximum results from the dynamics of the plume expansion and mixing with the gas. The ablated particles flying near the plume axis are stronger decelerated by the gas than the ones flying at oblique directions because of the preferential

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plume expansion in the direction of the outward target normal. At a certain time, the plume starts preferentially expanding in radial (oblique) directions. As a result, the plume-gas mixing is more intense at the plume periphery than near the plume axis.

3. MODELING OF SHORT AND ULTRA-SHORT (FEMTOSECOND) LASER ABLATION In this part we consider the aspects of modeling of laser ablation under action of femtosecond laser pulses. For simplicity, we consider only metals. The description contains the same parts corresponding to the two main categories of the ablation models.

3.1. Primary Mechanisms of the Material Ejection under Femtosecond Laser Action 3.1.1. Metallic targets In the case of femtosecond laser, the heating is so fast that the material temperature can approach and even overcome the critical temperature of the material. In this case, no welldefined interfaces between different phases (melting and evaporation fronts) exist. These boundaries are well defined only at relatively small energy absorbed, when material temperature is much smaller than the critical temperature. Because thermal processes, such as thermal evaporation, are rather slow and take at least several nanoseconds, a different mechanism should be considered in the case of femtosecond laser interaction with metals. To bring more light on the mechanisms of material decomposition in the femtosecond laser ablation of metals, we performed a detailed hydrodynamic modelling [53,54]. Remind, that compared to atomistic techniques, hydrodynamic models allow calculations for much larger systems and take much shorter computer time. Our numerical model is based on the solution of a system of Eulerian hydrodynamic equations by a high-order multi-material Godunov’s method [96]. The equations were extended to the case of two-temperature hydrodynamics with laser energy absorption source, electron heat conductivity and electronphonon energy exchange terms [54]. A Gaussian temporal profile is used to simulate the laser energy deposition. The electron-phonon/ions energy exchange term, the reflectivity coefficient and the optical penetration depth are derived from the wide-range frequency of electron-phonon collisions [18]. For the completeness of our model, we use a semi-empirical thermodynamically complete EOS for aluminum with separated components of electrons and lattice (heavy particles). The EOS meets the following requirements: (i) to describe experimental results on compression and expansion for a wide range of densities and temperatures including data on critical and triple points; (ii) to contain separate information about electron and ion/lattice sub-systems; (iii) to represent changes of thermodynamic parameters during phase transitions. In addition, we switched between two different modifications of the EOS: (i) with metastable states; and (ii) without metastable states. To account for kinetic processes, an estimation of the realistic lifetime of metastable liquid state was introduced as we describe in what follows.

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In our model, when the binodal line is crossed, we include a particular treatment for each of the following two competitive effects: (i) for the spinodal decomposition, a criterion of the metastable liquid lifetime, based on the theory of homogeneous nucleation [97] is used; (ii) for the fragmentation, a mechanical failure algorithm of Grady [20] is applied. In the first case, we estimate the metastable liquid lifetime as τ = (CnV ) −1 exp(W k BT ) , where C = 1010 s1

is the kinetic coefficient, n is the concentration, V is the volume, W = 16πσ 3 3ΔP 2 is the

work needed to cause the phase transition, Δ P is the difference between saturated vapor pressure at the same temperature and the pressure of substance, kB is the Boltzmann constant and T is the temperature of the sub-system of heavy particles. The temperature dependence of the surface tension is described as σ = σ 0 (1 − T Tc )1.25 , where Tc is the temperature in critical point (CP), σ0 – the surface tension at normal conditions. In this case, as soon as the lifetime τ in the volume V is expired, the phase state in this point is no more metastable. The EOS with metastable phase states is therefore no more relevant in this volume, so that we continue to calculate the thermodynamic properties by using the stable EOS. To account for the second effect, a fragmentation criterion is used for the liquid phase with the spall strength 1/ 3 1/ 3 1 Ps = (6 ρ 2 c 3σε& ) and the time to fracture t s = (6σ ρε& 2 ) , where ρ is target density; ε& is c the strain rate and c is the sound speed. When this criterion is satisfied we introduce vacuum into the cell and relax the pressure to zero. Both of these criteria are used simultaneously and each of them can prevail in a given computational cell depending on the substance location on the phase diagram.

Figure 4. Phase diagram of aluminum with stable and metastable (in parenthesis) phase states and trajectories of different target layers. The phase trajectories 1 – 7 correspond to the depth of 5, 15, 20, 30, 50, 80, 130 nm from the free surface of the target, respectively. Here, the laser pulse parameters are τL = 100 fs, λ = 800 nm, and F = 5.0 J/cm2

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For the analysis of the target material evolution after the laser irradiation, it is convenient to use a phase diagram in the temperature-density plane (T, ρ) given by the EOS (Figure 1). The binodal line corresponds to the gas-liquid equilibrium (saturated vapor curve). The spinodal line (dashed) gives a limit of thermodynamic stability of the matter. The regions between the binodal and the spinodal lines show the metastable states of matter: the superheated liquid (SHL) and the supercooled vapor (SCV) state. Initially, the ablated target of aluminum is in a solid state and is subjected to a laser pulse with F = 5 J/cm2, τL = 100 fs pulse width, λ = 800 nm wavelength. The matter absorbs laser energy in a very thin region (skin-depth, on the order of 10 – 30 nm, trajectories 1 – 4 in Figure 4). Then, a heat wave propagates into the bulk, which is followed by a shock wave (SW). This moment corresponds to the right-hand deviation of the trajectories 4 – 7 in Figure 1. The SW goes into the target and it is followed by a rarefaction wave (RW). The RW leads to the material expansion (decreasing of the density along trajectories, Figure 1). The initial temperature in the vicinity of the target surface (depth < 15 nm) is high enough (up to T ~ 30 kK), so that the phase trajectories from this layer go above the CP and the target material is directly transformed into the gas phase (trajectories 1 – 2 in Figure 1). Then, these trajectories penetrate into the SCV region, where condensation begins leading to the appearance of a liquid-gas mixture. Note, however, that this layer represents a very small fraction of the ablated material, so that the condensation degree is insignificant. Next layer in the target (depth from 20 to 30 nm) is first transformed into a metastable liquid state. The upper part of this layer is heated to a high temperature (~ 25 kK), so that the corresponding phase trajectories cross the binodal line in the vicinity of the CP and enter the metastable region (SHL). Under these conditions, the target material is thermodynamically unstable. The lifetime of this state is estimated as described above. Thermodynamic instabilities are known to occur near the CP (particularly, at 0.9 Tc < T < Tc, where Tc = 6595 K for Al) leading to a rapid decomposion (several picoseconds) of the matter into a liquid-gas mixture. This process is similar to the phase explosion [25,26,47], though occurs during simultaneous material expansion and cooling. It better corresponds to the critical point decomposition or spinodal decomposition outlined in [22]. This mechanism, however, concerns only quite a thin slice of the ablated target (≈ 10 nm). In fact, the melted layer is much thicker than the one, which trajectories enter the SHL region near the critical point. The rest of the trajectories (trajectories 5 – 7 in Figure 4), which originate from the deeper lying melted layers, also enter the metastable SHL region, but their temperatures are much smaller than the critical one. In this case, the matter can stay in the metastable state much longer. As a result, mechanical decomposition occurring under the action of a tensile stress (negative pressure) starts to dominate over thermal effects (time to fracture is shorter than the metastable liquid lifetime). As soon as the time to fracture is exceeded, the material is decomposed into clusters and chunks (metastable liquid decay). The mechanism is similar to the dynamic fragmentation due to the mechanical origin of the decomposition. This mechanism concerns the major part of the ablated material at the considered laser fluence. The dynamics of the target decomposition is presented in Figure 5. Initially the free surface of the target is located at x = 0 nm. Upon electron energy relaxation, a SW propagates into the target (pressure peak in Figure 2), leading the material expansion in the opposite direction through a RW, where pressure decays and can drop below zero. At a delay t = 10 ps after the beginning of the laser pulse, the density profile is still continuous [Figure 5(a)]. At

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t = 20 ps, the trajectories originated from the skin-layer (thickness ≤30 nm) reach the binodal line and penetrate into the metastable liquid region, where thermal material decomposition sets in [Figure 2(b)]. This process is completed by t~ 40 ps. Then, only mechanical mechanisms are involved into the material decomposition process. By t ~ 80 ps [Figure 5(d)], the mechanical decay of the liquid phase is completed. This result corresponds to the final vibrations of the trajectories 5 – 7 in Figure 1 that take place in the neighborhood of the binodal curve, where pressure is zero. The fractions of the target material ablated due to the described mechanisms depend both on the material properties and on laser parameters. We present here the analysis of these fractions as a function of laser fluence (F). At very small fluences (F < 0.25 J/cm2 for aluminum) only melting occurs. When laser fluence is slightly above this value, material spallation takes place, in agreement with the previous MD simulations.. When laser fluence is further increased, both metastable liquid decay and spallation mechanisms play a role. At larger F, all three mechanisms (i) direct atomization; (ii) thermal decomposition (critical point or spinodal decomposition); and (iii) mechanical decomposition (metastable liquid decay and spallation) occur in different regions of the target. The third mechanism, however, is found to be dominant (up to 80% of ablated mass) at all considered laser fluences (from 0.1 to 5 J/cm2).

Figure 5. Contour plots of density (solid) and pressure (dashed) for different time moments after irradiation: (a) – 10 ps, (b) – 20 ps, (c) – 30 ps and (d) – 80 ps. Here, 1 – is thermal (spinodal) decomposition zone, 2 – is mechanical decomposition zone. The laser pulse parameters are the same as in Figure ‘

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Figure 6. Melted (open triangles, solid line) and ablated (black triangles) depths as a function of laser fluence for aluminum. Experimental data are taken from [98] (black stars) and [100] (open stars). Results of the simulation [17] are also shown (squares)

Finally, we propose several predictions from our simulations and compare our results with the available experimental findings [98,99,100]. It turns out, that the consideration of only critical point or spinodal decomposition results in a significant underestimation of the ablation depth (several times) with respect to the experimental values. This underestimation can be explained by the fact that only 10-20% of the target material is ablated due to these critical effects, whereas most of the ablated material is ejected due to the mechanical decomposition of the liquid phase. These results confirm that the decomposed melted zone contributes strongly into the estimation of the ablated zone (Figure 6).

3.1.2. Dielectric targets The density of the conduction band electrons is typically too small in wide band gap materials for the efficient laser energy absorption. Nevertheless, focused ultra-short laser pulses can induce electronic excitations and ionize these materials. Therefore, to model the ultra-short laser interactions with dielectric materials, we consider the laser excitation processes that provide conduction band electrons. In addition, laser energy absorption and propagation are calculated. The present study is focused only on the case of moderate laser intensities typically used in industrial applications. Therefore, a system of simplified rate equations can be used instead of the detailed Boltzmann equation. The corresponding system of one-dimensional differential equations accounts for the multi-photon ionization process (MPI), for the electron tunneling effect, the electron-impact (or, avalanche) ionization, as well as for the formation of self-trapped excitons (STEs), and plasma relaxation processes[30,101,102,103]

n ∂ne (nV − ne ) Φ + ai ne I + σ m n S I m − e , = nv te ∂t

(25)

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n ∂n S = −σ m n S I m + e , ∂t tS

(26)

(n − n e ) ∂I = −khω V Φ −αaI , ∂z nv

(27)

where z is the depth below the laser-irrdiated surface; t is time; ne(t,z) is the number density of conduction band electrons; ω is the laser frequency; I(t,z) is the laser intensity; nv is the number density of valence band electrons in the non-excited dielectric; Φ(t,z) accounts for the laser field ionization (MPI/tunneling) processes; ai is the electron avalanche parameter obtained as in Ref.36; nS is the number density of STEs; m is the number of photons needed for STE excitation (m=5); σm is the multi-photon cross sections for STEs; te is the electron plasma life-time due to recombination and trapping (between 100 fs and several ps, here 1ps); ts is the characteristic trapping time; and h is Plank’s constant. For STE excitation, σ5 is set to be 10-93m10s5/J5. The field ionization term Φ(t,z) is calculated as a function of laser field intensity I(t,z) by using a well-known Keldysh equation.[102] According to the Keldysh’s formalism,[102] the transition from the multi-photon to the tunneling ionization process occurs when the parameter γ =

ω m* E g

drops well below unity, where e is the electron

eE L

charge; m* is the reduced electron mass 1 / m * = 1 / me * + 1 / m h* , index e corresponds to electrons and h to holes (we assume m*=0.5 me,); ω is laser frequency; EL(t)= 2nr I / cε 0 is

electric laser field; nr is the refractive index; ε0 is vacuum permittivity; c is the speed of light; and Eg is the bandgap. The system is solved by using the fourth-order Rounge-Kutta method. In the present work, the calculations are carried out for fused silica since this material was widely used in previous studies. For simplicity, the plasma relaxation time, te, is assumed to be equal to ts. As a result of a parametric study, this parameter was set to be 1 ps because this value gives a reasonable agreement with the experimental results that we used. More precise value can be obtained only based on additional experimental results. The laser wavelength is 800 nm in all the calculations. For wide band gap materials, the complex dielectric function contains contributions from the unexcited solid and the response of the laser-induced free-electron gas

⎛

n ⎞

, ε * (ne ) ≅ 1 + (ε g − 1)⎜⎜1 − e ⎟⎟ − e −1 ⎝ nV ⎠ nc 1 + i (ωτ c ) where

n

1

(28)

ε g = nr2 is the dielectric function of non-excited material; nr is the refractive index; τc

is the characteristic collision time; nc = ω me ε 0 / e is the critical plasma density; and ε0 is 2

the vacuum permittivity. The collision time τ c

2

−1

= ν ee + ν eph + ν imp , where ν ee is the

electron-electron collision frequency; ν eph =3×1014 s-1 is the frequency of the electron-phonon

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ν imp is the frequency of electron collisions with the impurities. In the present

calculations, we neglect

ν ee and ν imp . Both reflectivity R and the absorption coefficient αa

are calculated by using the Fresnel’s equations as follows (normal incidence)

R=

[k1 (t, z ) − 1]² + k 22 (t, z ) , [k1 (t, z ) + 1]² + k 22 (t , z )

(29)

2ωk 2 (t , z ) , c

(30)

αa =

where k1 and k2 are real and imaginary parts of the complex refraction index that are calculated using Eq. (4), so that k1 = (ε 1 + ε 12 + ε 22 ) / 2 , and k 2 = ( −ε 1 + ε 12 + ε 22 ) / 2 , where ε 1 and

ε 2 are the real and imaginary parts of the complex dielectric function,

respectively. We present the calculation results obtained for single-shot excitations. These calculation results can be used as a reference for the analysis of more complex laser excitations. First, we consider fused silica and determine the optical breakdown threshold (Ith, or OBT) based on the maximum number density of conduction band electrons, Ne, reached in the calculations [104]. The optical breakdown is supposed to occur when Ne reaches the critical density, or 21 -3 n c = ω 2 m e ε 0 / e 2 . For fused silica at 800 nm, nc=1.74×10 cm . Figure 7 shows the calculated Ne as a function of the incident laser intensity I0 (peak value). A very sharp growth can be observed in the dependency at I0>Ith. At this intensity (here, ~5 × 1013 W/cm2), Ne overcomes the plasma critical density nc. Interestingly, the experimentally measured plasma radiation from the breakdown region was shown to increase similarly with laser intensity. These results confirm the validity of our model.

Figure 7. Maximum density of the conduction band electrons as a function of the peak laser intensity. Here, calculations are performed for 50 fs laser pulse at 800 nm

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The calculated optical breakdown threshold as a function of the laser pulse width is shown in Figure 8. One can see that the threshold intensity rises for laser pulses shorter than 100 fs. This result is in good qualitative agreement with recent experimental observations, where similar pulse width dependency and intensities values were observed [105]. Larger threshold values that obtained in the experiments for ultra-short pulses (<100 fs) can be attributed to the fact that that in this regime ionization takes place during shorter time, so that larger intensities are needed to reach the critical electron density. Furthermore, because free electrons oscillate in the electric laser field, the effective ionization potential e 2 EL2 increases with laser intensity [35], where E is the band-gap, ω and E the g L E g* = E g + 4m*ω 2 frequency and amplitude of electric laser field, respectively, and m* is the reduced effective mass. The calculated values are slightly smaller because we consider only surface damage (electron density is maximum just below the surface), whereas the pulses were focused 150 µm inside the fused silica sample in Ref. 29. In the latter case, the laser beam propagation can be strongly affected by material ionization, beam dispersion and other effects resulting in a higher threshold. When laser fluence is used to determine the OBT, the threshold fluence decreases with pulse shortening below 100 fs. As we have indicated in the description of our model, the Keldysh parameter varies as a function of laser intensity during the pulse. Considering the peak laser intensity, one can calculate the corresponding γ and estimate which process, multi-photon ionization or tunneling effect, is expected to prevail if one neglects the avalanche ionization process (Figure 8). The results of this estimation show that an intermediate regime is realized at the considered peak intensities. Typically, multi-photon ionization is considered to be a dominant process in this case, resulting in a small overestimation of the ionization probability. In a multi-pulse regime, the STE excitations also provide seed electrons. The contribution of the electron impact (or avalanche) ionization process rises with the temporal pulse width, and, is dominant for high peak intensities, in agreement with Ref. [106].

Figure 8. Threshold laser intensity (peak value for Gaussian pulse) as a function of temporal pulse width (circles). The Keldysh parameter values corresponding to the peak threshold intensity are shown by triangles

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3.2. Secondary Mechanisms: Plasma Plume Expansion after Femtosecond Laser Pulse The initial axial density distribution in the gas phase is thus given by the dynamics of the shock wave propagation and the rarefaction wave formation driving the femtosecond laser ablation The calculations of the two-dimensional plasma expansion in vacuum [107] with the initial conditions obtained from the 1D model of femtosecond interaction show that the plasma is much more forward directed (Figure 9). This result agrees with the experimental observations. In addition, the plume density distribution is more extended (prolonged) here compared with that created by nanosecond pulses. These results can be attributed to the different initial conditions caused by the femtosecond interaction mechanism. At the same laser fluence, both temperature and pressure gradients are much larger in the femtosecond ablation. Because of the propagation of pressure wave and fast cooling due to the electron heat conduction, the species ejected earlier travel longer distances than the ones that are ejected later. As a result, the density distribution is prolonged. Furthermore, ablation induced by a femtosecond pulse continues for several τr, which is still much shorter than the ablation time under nanosecond pulse. The ablation depth is, however, comparable in both cases. As a result, the initial plume density and the axial density gradient are much larger than these after the femtosecond pulse.

Figure 9. Two-dimensional density profile obtained in the calculations of the laser plume expansion at t=1 µs after a femtosecond laser pulse.

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CONCLUSION To summarize, several physical phenomena that take place during pulsed laser ablation are modeled by different numerical methods. Each of these methods has its advantages and limitations. Therefore, only combinations of different methods may describe the ablation process for a wide range of the experimental conditions. To study femtosecond laser ablation of metal targets hydrodynamic simulations is shown to be promising. The obtained results show that when laser fluence is sufficiently high, three following mechanism can play a role: (i) direct atomization; (ii) thermal decomposition (near critical point); (iii) mechanical decomposition. Relative importance of these processes varies as a function of depth. In addition, we have recently used this model with calculation of absorption in the expanding plasma to explain an interesting effect of the decay in the ablation depth observed in double-pulse experiments. For transparent materials, we have started developing a model to describe non-linear electronic excitations, ionization, nd absorption. For this “excitation” part we have used the Boltzmann-equation, the system of rate equations. The properties of the laser-generated plasma plume are shown to be strongly affected by the laser-mater interaction mechanism. Experimental diagnostics of the expanding plasma may help to better understand these mechanisms. The calculations demonstrate furthermore the crucial role of strong pressure gradients induced in the metals during femtosecond laser ablation. This study can be useful for different applications dealing with both laser treatments of surfaces and laser plasma technologies. Despite considerable efforts focused on the understanding of femtosecond interactions and mechanisms involved in laser nano-cluster formation and nanostructuring, many points need further much more detailed investigations. The definition of electron-phonon interactions and electron conductivity are still puzzling. New models with wider limits of applicability should be developed and used in future multi-scale simulation.

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Nolte, S; Momma, C; Jacobs, H; Tünnermann, A; Chichkov, BN; Wellegehausen, B; Welling, H. JOSA B, 1997, vol. 14, Issue 10, 2716-2722. Küper, S; Stuke, M. Appl. Phys. B, 1987, vol. 44, 199. Pronko, P; Dutta, S; Squier, J; Optics Commun, 1995, vol. 114, 106. Banks, PS; Feit, MD; Rubenchik, AM; Stuart, BC; Perry, MD. Appl. Phys. A, 1999, vol. 69, S377. Chichkov, BN; Momma, C; Nolte, S; von Alvensleben, F; Tünnermann, A. Appl. Phys., A, 1996, vol. 63, 109. Kautek, W; Krüer, J. Proc. SPIE, 1994, vol. 2207, 600. Teghil, R; D'Alessio, L; Santagata, A; Zaccagnino, M; Ferro, D; Sordelet, DJ. Appl. Surf. Sci., 2003, vol. 3-4, 307. Short Pulse Laser Interactions with Matter: An Introduction, P. Gibbon, Imperial College Press, London, 2005. Femtosecond Laser Spectroscopy, Peter Hannaford, Springer, 2004.

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In: Laser Ablation: Effects and Applications Editor: Sharon E. Black

ISBN: 978-1-61122-466-5 © 2011 Nova Science Publishers, Inc.

Chapter 4

FABRICATION OF SILICON NANOCRYSTAL BASED STRUCTURES WITH NANOSECOND LASER ABLATION PROCESSINGS IN LIQUID MEDIA V. Švrček* Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan

ABSTRACT In this chapter a nanosecond (ns) laser ablation and fragmentation processing in water, pure and doped spin on glass (SOG) polymer-based solutions are discussed. The confinement of laser-generated plasma in liquids allows the silicon nanocrystals (Si-ncs) formation with a quantum confinement size effects. We demonstrate that ns laser processes in liquid can be efficiently applied for the fabrication and tuning the optoelectronic properties of Si-ncs based nanostructures. The laser fragmentation in water induces the self-assembly and allows formation of closely-packed stable luminescent Sincs over ~ 200 μm. Contrary to the water, the laser ablation and fragmentation in pure and doped SOG solutions inhibit aglomeration and enhance the Si-ncs luminescence properties. Finnally, we disccuss physics and dynamics of Si-ncs formation through the serial growth processes that occurred in liquid media confined ns laser generated plasma.

Keywords: silicon nanocrystals, ns laser ablation and fragmentation in liquid, photoluminescence, quantum confinement.

1. INTRODUCTION Since pulsed laser ablation of solid materials is easily carried out as a result ablation almost any kinds of solid materials has been widely investigated and developed [1, 2]. *

Corresponding author: Tel: +81-29-861-5429 Fax: +81-29-861-3367. Email: [email protected],

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However, most researchers have focused on pulsed laser ablation of solid targets in vacuum and in a diluted gaseous ambient [3-6]. In such a way pulsed laser ablation has been successfully employed to fabricate novel microstructures, wide band gap semiconductors, nanocrystals, clean and elaborate surface of many types of materials [7-10]. Between the nanostructures nanoparticles with quantum confinement effects increased interest many scientific communities [7]. It has to be noted that nanoparticles synthesized in gas phase are very often agglomerated due to unstable surface properties [5, 6]. Last decade laser ablation conducted directly in liquid media has shown to be a peculiar technique to generate the nanoparticles [11,12]. This approach offer strong advantages comparison to the ablation in vacuum or gas atmosphere. Generated nanoparticles are obviously charged possessing a high zeta potential (>30 mV), which avoids agglomeration and results in high nanoparticles stability. Even more nanoparticles directly produced and dispersed in liquid media are not inhalable and thus leads to facilitating of the process safety requirements. Moreover, the chemical precursors are not necessary therefore compared to laser ablation in gas atmosphere enhanced purity of colloidal particles is widely reported [13, 14]. It has to be stress that nowadays besides element metal, alloying, and oxide nanocrystals also organic fullerene-like nanostructures have been produced by laser ablation in liquids [15, 16]. Silicon nanocrystals (Si-ncs), since the discovery of visible room-temperature photoluminescence (PL) from anodized porous silicon, have attracted much attention due to their potential application in many fields (i.e. optoelectronics, photovoltaics, biology). Laser ablation in liquid can be employed in producing of highly luminescent Si-ncs with strong quantum confinement effects [17-19]. As Si-based technology will keep playing an crucial role for device fabrication, also silicon dioxide (SiO2) will remain a fundamental interface material. Direct processing in water or oxide based liquid media might allow engineer Sidioxide interface at Si-ncs at superior quality by cost effective way. Thus might consent to control and even enhance overall Si-ncs properties. Environmental and human body compatible procedure based on laser processing in water can be another factor for producing non-toxic, room temperature luminescent Si-ncs, which might at the same time provide a further foundation for applications in biology or medicine. In this chapter cheap and scalable nanosecond-laser ablation and fragmentation processing in liquids is applied for the fabrication of Si-ncs based nanostructures. The confinement of laser-generated plasma in liquids allows the Si-ncs formation with quantum confinement size effects. Particularly, we discuss aspects of the Si-ncs preparations in water and liquid transparent polymers i.e. pure and doped spin on glass (SOG). We demonstrate that compared to the water the SiO2-based SOG inhibits aggregation and enhance the photoluminescence properties of Si-ncs. In order to enhance the Si-ncs rate formation, an effective way to prepare luminescent Si-ncs by pulsed laser-induced fragmentation of Si micrograins prepared by electrochemical etching in water and SOG is outlined. We demonstrate that nanosecond-pulsed laser fragmentation in water can be efficiently used to induce the self-assembly of closely-packed and stable luminescent nanocrystals. Finnaly, the mathematical description of dynamical Si-ncs formation within laser plasma confined by liquids is applied to describe obtained results.

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2. EXPERIMENTAL DETAILS 2.1. Laser Ablation Our experimental set-up for the synthesis of Si-ncs with strong quantum confinement effect is based on a recently developed nanosecond laser ablation technique described elsewhere [17,19]. In this approach the laser-produced plasma with high pressure (~GPa) is confined in liquid media [17,18]. Figure 1 represents schematic sketch of our experimental set-up. The Si-ncs are fabricated by using a single-crystal silicon wafer fixed in glass ware filled with liquid medium. Both types of (n-type, p-type, <100>, resistivity 0.1 cm, thickness 0.525 mm) as a target could be used. To induce laser ablation and the Si-ncs formation a third-harmonic of Nd:YAG laser (Spectra Physics LAB-150-30, 355 nm, 30 Hz, τ = 8 ns) or KrF excimer laser ((KrF, 245 nm, 20 Hz, 10 ns).) are used to irradiate onto the target immersed in liquid media (water, SOG solution) at room temperature. It has to be noted that the SOG polymers solutions are commercially available (Si-59000 Tokyo Ohka Kogyo Co., Ltd.). The laser beam is focused on the target by a lens (f = 250 nm) [17, 18]. Relatively low laser fluences are used to obtain a spherical shape of the plume, to limit excessive bubble formation, and to avoid generation of strong shock waves in liquid [19, 20]. During the ablation process the container with liquid and immersed Si target was rotated. To observe visible PL the aging processes of Si-ncs in water was assured by keping them in aqeuos solution at ambient temperature [17, 18]. The colloidal suspension prepared by laser ablation in SOG was sonicated for 10 min and solidified in air atmosphere at 323 K for 24 h, resulting in the formation of self-supporting samples [21, 22].

Figure 1. Schematic sketch of the experimental set-up used for fabrication of silicon nanocrystals (Sincs) by laser ablation of crystalline silicon target in liquid media. In this work two types of ns lasers have been applied independently (Nd:Yag and excimer KrF laser)

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Figure 2. Sketch of the experimental set-up used for preparation of Si-ncs by ns laser fragmentation of silicon micrograins prepared by electrochemical etching

2.2. Laser Fragmentation Figure 2 shows sketch of experimental set-up used for preparation of Si-ncs by ns laser fragmentation method of Si-ncs micrograins prepared by electrochemical etching. The technique involves the etching of a silicon wafer (p-type boron doped, h1 0 0i, 0.1 X cm, thickness 0.525 mm) for 1 h at 1.6 mA/cm2 constant current in HF:ethanol electrolyte (1:4). After the etching process, the resulting porous silicon wafer has been subsequently mechanically pulverized [21]. After mechanical pulverization of several porous silicon wafers the powder with Si-ncs micrograins is collected. The fragmentation of Si-ncs micrograins have been proceed in water, pure and phosphorus doped SOG (Liquid pure and doped SOG are commercially available from Tokyo Ohka Kogyo Co., Ltd.)) [23-25]. Colloidal solutions were placed into a glass ware, and irradiated by ns pulsed lasers (Nd:YAG, or KrF) independantly at room temperature. The laser beam in both cases was focused on the liquid surface. During irradiation, the glass container with colloidal solutions was closed and rotated. Fragmentation by ns pulsed laser requires a homogenous dispersion of the Si-ncs micrograins [25], therefore in the case that the Si-ncs micrograins were observed to disperse poorly in water, a small amount of ethanol (20 drops) was used to wet the micrograins surface prior to the introduction of water. The color of homogenously dispersed aqueous and SOG Si micrograins solution is yellow. When the nanosecond pulsed laser irradiation is applied, fragmentation of the micrograins occurs and at prolonged laser irradiation, the solutions become almost transparent [25]. After fragmentation, the transparent colloidal dispersions were used to prepare two types of samples. Upon preparation in water the Si-ncs have shown to self-assemble in specific structures when let it dry on glass substrate. In the case of SOG after irradiation, the colloidal suspensions was dried for solidification [23] in air at 323 K for 24 h to obtain self-supporting SOG films.

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Figure 3. Photos of blue luminescent Si-ncs prepared by excimer KrF laser in water and aged in water for 7 months. Image (a) represents Si-ncs prepared from p-type and photo (b) from n-type doped crystalline silicon wafer

3. SI-NCS AND SI-NCS BASED COMPOSITES PRODUCED BY LASER ABLATION IN LIQUID MEDIA Si-based technology keeps and most likely will keep playing a key role for electronics and photovoltaic industry. Due to the compatibility, non toxicity and most importantly purity of the nanoparticles surface highly luminescent Si-ncs with quantum confinement effects prepared by laser ablation in water with Si-dioxide surface termination might play an important role for development novel types of devices [17, 19, 20]. Surface passivation achieved by laser ablation in water leads to stable PL properties [19], that can facilitate the integration within existing Si based technologies. The employment of doped Si-ncs prepared by laser ablation can expand the possibilities of utilization of Si-ncs. Figure 3 represents photographs of room temperature luminescent Si-ncs prepared by excimer KrF laser in water and aged in water for 7 months. Image (a) represents Si-ncs prepared from p-type and photo (b) from n-type doped crystalline silicon wafer. Visible blue-room temperature PL is observed from both colloids under He:Cd laser excitation at 325 nm. At ambient conditions Si-ncs in colloidal solution show a stable blue-bands with maxima centered at ~420 nm. Compared to the Si-ncs made from p-type doped wafer, the PL band for n-type doped is stronger more than 2 times. Since for n-type doped silicon a smaller surface recombination velocity has been reported compared to p-type Si wafers [26], similar effects could be expected for Si-ncs and different surface termination due to the dopant most likely influences the PL intensity. In order to overcome aging process and allow to appear the PL ns laser ablation in SiO2 based polymer solution semms to be an effective way [18]. Our results showed that we can successfully prepare Si-ncs directly in SiO2 based polymers, which decreased the aging time from months to several hours [18]. The Figure 4 (a) shows the chemical formula of the polymer solution. The polymer consists of the mixture of thylpolysillicate (C2H5O(SiO)C2H5O)2n(C2H5), ethanol, and ethylacetate. The contents of these chemicals in SOG are 9%, 71%, and 20%, respectively. Such transparent solution has refractive index of 1.44, which allows ablation process directly in solution without considerable deterioration of polymer quality. The fabrication of Si-ncs directly in polymer solutions opens also flexibility

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for variety of sample structures. From Si-ncs/polymer colloidal solutions we can prepare thin films at low-cost (Figure 4 (b), either by spin coating or printing technique, in principle, on any type of the substrate. On the other hand, by simple solidification of the Si-ncs/polymer colloidal solution we can form self-supporting samples (Figure 4.c) with high concentrations of Si-ncs and different architectures. It has to be noted that this SOG polymers can be doped and doping level might help easy to control the PL properties of embedded Si-ncs [27]. It is observed that at low ablation intensities [28] irregular Si-nc fragments obtained by laser ablation in water are stabilized into regular-spherical particles. Figure 5 shows typical SEM images of Si-ncs prepared by laser ablation in water (a) by Nd:Yag and (b) by KrF excimer laser, respectively. In both cases SEM images shows that Si-ncs aggregates just after the preparation. As depicted, spherical aggregates reaching size distributions ranging from 2 to 100 nm, which is rather similar for both lasers. It was clearly observed that the smaller irregular aggregates of Si-ncs were obtained at higher laser fluences [28]. At higher fluences the formation of spheres is inhibited mostly because of enhanced fragmentation processes. Such fragmentation is induced by the increased density and intensity of shock waves that propagate through the liquid at higher laser fluence. Detailed structural analysis revealed that agglomerates contain Si-ncs with irregular shape smaller than the strong quantum confinement limit for silicon (<5 nm) [17]. Namely that both electrons and holes are spatially confined in such a small Si-ncs.

Figure 4. (a) Chemical formula of the spin on glass (SOG) polymer. (b) Schematic representation of using the colloidal solution for the thin film formation and (c) self-supporting samples based on Si-ncs directly produced in SOG by laser ablation

Figure 5. Typical SEM images of Si-ncs prepared by ns laser ablation in water (a) by Nd:Yag and (b) by KrF laser, respectively

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In the case of the polymer the Si-ncs do not agglomerate and remain separated and homogenously dispersed within the polymer matrix. Compare to the water produced Si-ncs, visualization of single Si-ncs by HRTEM is more difficult due to the presence of SOG polymers. Surrounding amorphous SOG did not permit a clear visualization of single Si-ncs, and we could not directly identify the crystalline lattice interplanar distance. However, dark separated areas of < 5 nm were clearly observed from HRTEM images [18]. The excess of silicon content by elemental analysis indicated increased Si content within SOG matrix. Independently to that, the electron diffraction pattern and XRD confirmed the presence of Sincs with single diamond like crystalline fraction. It has to be noted that in such composites we have also observed four additional strong peaks be possibly assigned to a silicon oxide that surrounds the silicon nanocrystals [18]. Those peaks were assigned to be silicon mono-oxide. It must also be noted that no diffraction spots were recorded from solidified SOG without Si-ncs after laser irradiation and subsequent drying; and that only the halo pattern of the amorphous phase was exhibited. It is supposed that as the SOG matrix allows proper Si-ncs dispersion during the laser ablation process. As a results dispersed Si-ncs are well exposed to SOG polymer and might enhance the formation of the silicon oxide in crystalline phase. However, at this stage of the research the mechanism of the formation the silicon oxide in crystalline phase is not clear yet, and further investigations are underway. The most important feature of Si-ncs prepared either in water or polymer based solution is their visible blue PL. Figure 6 compares typical PL spectra from Si-ncs in solidified SOG polymer and in water after aging. Blue triangles represent the PL spectrum of a selfsupporting sample prepared by laser ablation of the silicon crystalline target in SOG solution (Nd:Yag @ 0.76 J.cm−2) and subsequent solidification. Red open circles denote PL that from a Si-ncs solution prepared by laser ablation of the Si target in water under the same laser irradiation conditions and subsequent aging in water (6 months). The PL peaks are normalized to the PL maxima intensity. In both solution the PL from Si-ncs is centered at around 2.9 eV at room temperature. The PL spectrum in SOG was red-shifted by 0.2 eV and broadened by 250 meV compared to that in water. It has to be noted that contribution from SOG matrix to overall PL properties is negligible. Black symbols indicate PL signal of solidified SOG treated by ablation process. No PL in this spectral region was recorded. It is believed that the Si-ncs prepared in SOG contain most likely more defects and larger size distribution of formed Si-ncs that broader the PL spectrum. Indeed, the luminescence dynamics can give important inside and help in identifying the nature of an emission from the Si-ncs [20]. Our results showed that the PL decay clearly indicates the contribution of a very fast decay component induced by light scattering [20]. This effect disappeared for emission at lower energies (<2.5 eV). By using conventional analysis software (DAS 6), we were been able to remove the contribution of the scattering effect and estimate the PL decay time of Si-ncs [20]. The correct value of the PL decay time lies between around ~6 ns, that strongly support the quantum confinement phenomenon indicated from structural analysis and temperature depended PL spectroscopy [19].

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Figure 6. Blue symbols represent the PL spectrum of a self-supporting sample prepared by laser ablation of the silicon crystalline target in SOG solution. Red circles denote PL spectrum from a Si-ncs water solution prepared by laser ablation of the Si target in water and subsequently aged in water for six months. Black symbols indicate PL signal from a self-supporting SOG only treated by ablation process and is shown for comparison

4. INDUCED SELF-ASSEMBY & TUNING OF SI-NCS OPTICAL PROPERTIES BY NS LASER FRAGMENTATION IN LIQUID MEDIA As we discussed above a termination of Si-ncs surface by silicon oxide is favorable for stabilizing PL properties of Si-ncs. One of the choice that can enhances the Si-ncs rate production is exploration of fragmentation of Si-ncs micrograins prepared by independent technique with higher production rates e.g. electrochemical etching. In order to achieve Sincs passivation by silicon oxide (mono-, di-oxide) formation ns laser fragmentation is conducted in SiO2 dioxide polymer or water is applied [23]. Laser fragmentation processing in SOG polymer improves the PL properties and allows tuning of the PL spectra position as a function of ns laser fluence. The polymer doping with silicon compatible dopands (i.e phosphorus, boron) can considerably influence the optical properties of the Si-ncs [27]. Figure 7 summaries the PL intensity as a function of the wavelength for Si-ncs embedded in pure (blue symbols) and phosphorous-doped SOG polymer (black symbols) after laser fragmentation and solidification. The PL spectrum of Si-ncs micrograins in solid SOG before irradiation is shown for comparison (red symbols). With laser fluence increases, an enhancement in blue shift of PL peak accompanied with PL intensity increase (more than 10 times) is recorded. It is assumed that the blue shift (from 1.7 eV to 2.8 eV) and the PL intensity enhancement are due to the formation of higher concentration of Si-ncs with quantum confinement effect. Small sizes Si-ncs contributed to increase both the surface states and the efficiency of radiative recombination. It also been observed that in the case of doped SOG the PL intensity is weaker more then 6 times compared to the pure SOG polymer. The origin of this decrease could be related to phosphorous that might acts as a nonradiative recombination center or prevent proper Si-ncs oxide surface passivation [24]. Similar to the Si-ncs prepared by laser ablation (Figure 6) observed bright PL of Si-ncs obtained by fragmentation is mainly due to the simultaneous effects of surface states and quantum confinement. This is also supported by performed extensive structural analysis [23] i.e. XRD, HRTEM. Elemental and structural analysis revealed that fabricated Si-ncs embedded in both polymers posses a silicon crystal structure in cubic phase. One can argue that increased concentrations of smaller Si-ncs

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increases specific surface area of Si/SiOx, that can also be efficient source of blue PL [29]. Of course thus contribute to the PL intensity enhancement however the contribution compared to the quantum confinement effects from Si-ncs is minor [18]. We also verified that laser irradiation and solidification of only polymers their self in liquid phase are not responsible for observed blue PL. As mention above, laser processing in water is environmental and human body compatible therefore, the Si-ncs laser fragmentation directly in water can be beneficial for many biological and medical applications. Figure 8(a) is a photo of aqueous colloidal solution with Si-ncs before fragmentation process. When the colloidal solutions is excited by He:Cd laser a strong orange PL can be observed from Si-ncs attached on the micrograins or freely dispersed in solution. Basically the colloidal solution is yellowish and not transparent. The beam of the excitation laser cannot penetrate into solution and remain to excite only at the beginning of the ware. The color of the colloidal solution varies during the fragmentation process. Color of solution, initially yellowish, changes into more transparent during ns laser irradiation. As the micrograins got fragmented and colloidal solution become more transparent,CdHe laser beam penetrate mor into the solution and overal PL emission of the solution is improved (Figure 8b). It has to be noted that prolonged fragmentation time and increased laser fluence can lead to the quenching of the PL emission. Similar to laser ablation in water followed by aging in water is observed. After several weeks the blue PL emission from colloidal solution produced by fragmentation of Si-ncs micrograins can be seen (Figure 8(c)). However in this case the PL intensity is rather weak.

Figure 7. The photoluminescence (PL) intensity as a function of wavelength for Si-ncs embedded in pure solid SOG (blue symbols) and phosphorous-doped SOG matrices (black symbols) after laser fragmentation in polymer solution (Nd:Yag @ 5.9 mJ/pulse for 2 hours). The PL spectrum of Si-ncs micrograins in SOG before fragmentation is shown for comparison (red symbols)

Figure 8. Photos represent aqeuous colloidal solutions with silicon nanocrystals (Si-ncs) micrograins (a) before and (b) after ns laser fragmentation processing in in water. (c) Photo of Si-ncs/water colloidal solution after increased laser fragmentation fluence and aging in water for several months is shown. In all cases the colloidal solutions are excited by He:Cd laser at wavelength 325 nm

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Figure 9. (a) Typical SEM image of Si-ncs based self-assemblies induced after ns laser fragmentation in water. (b) Corresponding Micro-Raman spectra of the self-organized structures (blue symbol line). Black line represents the reference from crystalline silicon

Other important feature that we could observe is that the drop of fragmented-aqueous luminescent colloidal solution on not pattern substrate results self-assembly formation. Figure 9 (a) shows SEM image of Si-ncs prepared by ns laser fragmentation of Si-ncs micrograins in water. Since in ethanol or SOG well done separation of Si-ncs is achieved [30], the selfassembled structures are only observed when the ns processing is conducted in water. It has to be noted that extended interconnected structures exceeded several hundred micrometers in length. Furthermore, micro-Raman was employed to verify the presence of Si-ncs in the networks. Micro-Raman investigations revealed that the self-assembled patterns contain Sincs with quantum confinement features. Compared to crystalline silicon (black line in Figure 9b), the spectrum of the self-assembled structure exhibits a shift in the maximum of the peak and broadening of the spectrum (Figure 9(b), symbol line). The peak width is related to the Si–nc mean size and was evaluated to be around ~ 4 nm in diameter [31]. However, detailed micro-Raman investigation revealed as well as that some parts of the self-assembled branches consisted of larger silicon particles (>10 nm) and amorphous silicon tissue produced during laser fragmentation process [30]. On the other hand, those self-assembled structures showed to possess a good mechanical stability when exposed to a low-temperature and low-pressure plasma [30]. The selfassembled networks do not possess any organic surfactant layer that could hinder subsequent surface functionalization. A clean surface can then enhance the interaction between the laser elaborated nanoparticles. Self-assembly in our case is most likely induced by multiple mechanisms that act at the same time. The most significant process that promotes selfassembly is a dewetting process [32]. It has been shown that the interactions between hydrophobic particles and water can be affected during water evaporation [30]. On the other hand, the spontaneous self-assembly of isometric nanoparticles 1D-arrays can be produced with intrinsic magnetic [33] or electric dipoles [34]. However, contrary to metallic nanostructures, the semiconductor nanoparticles exhibit intrinsic electric dipole. The link between dipole interactions and self-assembly mechanisms remains uncertain and is still under debate [35]. Anyway it is believed that electric dipoles produced in the surface charge during laser fragmentation of the Si-ncs micrograins in water play a crucial role for selfassembly. In ethanol SOG polymer, the formation of electric dipoles is reduced due to the immediate stabilization of the Si-ncs micrograin surfaces by the organic layer that leads to

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increase in zeta potential. Contrary to that our studies shown an where zeta potential decrease after processing in water [28]. Low zeta potential helps to aggregation of the laser-elaborated particles as the oxygen based vacancies are formed [28]. The fragmentation process in water induces the ineffective screening of the Si-ncs micrograin dipoles and increases inter-particle electric dipole–dipole interactions. Due to the non-uniform dipole distribution [30] random self-assembly can be achieved.

5. PHYSICS OF THE NANOSECOND-LASER PROCESSINGS IN LIQUID MEDIA Though, the formation mechanism of Si-ncs is not completely understand yet, the laser ablation process in water and/or liquid SOG solutions can be schematically describe by following way. Figure 10 represents sketch of a ns laser ablation mechanism and formation of Si-ncs in a liquid medium (i.e. water). The absorption of ns pulses in a Si wafer immersed in liquid led to rapid heat generation. As a result a dense cloud of Si atoms spread over the plume is formed on the Si surface. Embryotic particles are produced in such a cloud as well. The initial rapid attachment of silicon atoms to ejected embryotic particle continued until all silicon atoms in the confined plum area are completely consumed. When particles escape from the plume into the water or SOG polymer solution, the nanocrystal growth is suppressed. The reasons for suppression are multiple but most important suppression in our opinion is due to the SiOx. Those molecules are widely presented in both solutions and immediately stack on the Si-nc surface. As a result Si-ncs with size of few nanometers with strong quantum confinement effects are formed. The dissipated kinetic energy of the material ejected by pulsed laser ablation is proportional to the Gibbs energy of as-formed nanocrystallites [36]. Then ablation rate R for particle formation can be written through the Gibbs energy (G) and can be written as follow

⎛G⎞ R = const. exp⎜⎜ ⎟⎟ ⎝ Ea ⎠

(1)

where Ea is the activation energy [37]. The kinetic energy differences between the embryotic particle initially ejected and that ejected at the end of the flight when the Si-nc enters into liquid media is proportional to the G. Then the Gibbs energy of as-formed spherically condensed Si-ncs with diameter d is linearly proportional to the initially ejected velocity (v0) and to the pressure (P) induced in the plume. As reported elsewhere [18], after performing some simple algebraic operations, the G can be expressed by

G ≈ α .d .v0

(2)

The slowing coefficient α is proportional to the particle geometrical cross section and Si atom density within the plume. The mathematical description of dynamical formation

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processes indicating that the initial size of the embryotic particle ejected by nanosecond pulsed laser irradiation is the determinant for the final Si-nc size [18].

Figure 10. Schematic representation of the laser ablation process and dynamical Si-ncs formation in liquid media (i.e.water)

In the case of ns laser induced fragmentation of Si-ncs micrograins in liquid media similar processes likely happen when the laser beam directly interact with micrograns. In that case of an instantaneous point interaction results laser ablation and plum generation in liquid. Additionally, we consider that thermal heating alone could not be responsible for Si fragmentation. Rapid heating and expansion of the plasma generate high-pressure shock waves. Shock waves that propagate from the laser spot [23, 38] through the liquid medium cause the mechanical deformations at regions relatively far from the irradiation spot [23]. The shock waves can develop after propagation over a characteristic distance (L)

L≈

ρ.c.τ 2π .ε .P

(3)

where ρ is the medium density, τ the duration of a transient stress wave inversely proportional to laser pulse frequency, c the speed of sound (c = 1.5 km/s), ε the permeability of liquid media, and P the pressure. We believe that such shock waves are at least responsible for the dissociation of fine Si-ncs that are weakly attached on Si micrograins. When the shock wave moves a sufficiently large distance away from the point of explosion, the fragmented particles continue to expand in liquid media. Furthermore, L linearly depends on the density of the liquid medium that is higher for SOG polymeric solution than for water. As a result the shock waves in SOG can propagate farther from the point of explosion and leads to more efficient fragmentation [23].

CONCLUSION In symmary, the fabrication and stabilization of blue photoluminescent silicon nanocrystals (Si-ncs) by nanosecond (ns) Nd:YaG and excimer KrF laser processing has been

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disccussed. In both cases the ns laser induced ablation in liquid media offers fabrication conditions with unique surface chemistry that allowed formation of luminiscent Si-ncs with quantum confinrment effects. We have shown that the self-limiting oxidation in water and doped spin on glass (SOG) polymer solutions provide oxide shell for stable surface resulting in room temperature blue PL. Particularly electron-hole recombination in the Si-ncs with high quality surface passivation in water promotes carriers trapping in oxygen-related localized states. Produced Si-ncs in water and SOG based solutions exhibited room-temperature PL peaked at around ~400 nm with a lifetime of about 6 ns, which could be attributed to the quantum confinement effect of Si-ncs embedded in oxide-based surroundings. The processing in SOG polymers can successfully prepare thin films and self-supporting samples containing blue luminescent Si-ncs at high concentrations. Importantly, through the processing in SOG based polymers the aging time to obtain blue photoluminescence from Si-ncs is significantly shortened. Furthermore, we showed that luminescent Si-ncs can be efficiently produced by ns pulsed laser fragmentation of electrochemically etched Si micrograins. We succeeded in preparing and tuning the luminescent Si-ncs properties by both lasers (Nd:YaG and excimer KrF laser) in water, pure and doped SOG polymers. Interestingly, we could produce self-assembled networks of the Si-ncs on no-patterned substrate after fragmentation in water. We demonstrated that even fragmentation process in SOG (ethanol) is more efficient, however, only the fragmentation in water leads to such self-assembled nanoarchitectures. A Si-nc formation scheme, which describes serial processes of rapid formation and growth of embryotic Si particles, consecutive oxidation in water or SOG and growth termination by quenching was applied to interpret obtained results. Laser plasma confined by liquid media significantly increases the Gibbs free energy of as-formed silicon nancrystallites and the Si-nc producing rate. We discussed a dynamical formation processes pointed out that the initial size of the embryotic particle ejected by ns pulsed laser irradiation is the determinant for the final Si-nc size. In the case of the laser fragmentation laser irradiation energy over the band gap of silicon provokes an optical absorption and resulting plasma superheat, together with strong shock waves, induce Si micrograins fragmentation. One can expect that this relatively simple and affordable ns laser fabrication of Si-ncs with strong quantum confinement could be useful procedure in a wide variety of applications. For instance direct preparation of fine blue luminescent Si-ncs in silicon technology compatible doped SOG polymers might help the formation of films with high Si-ncs concentrations and enhance the stimulated emission processes (i.e. optical gain). On the other hand, using of the Si-ncs self-assembled architectures prepared by laser fragmentation in water can be very useful for biological and photovoltaic applications.

ACKNOWLEDGMENTS We are deeply indebted to all colleagues who contributed to this work over the past years. Especially we wish to mention the members of groups in Japan (Nanoarchitectonics Research Center, Photovoltaics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba) and Prof. D. Mariotti at University of Ulster, U.K. This work was also partially supported by a JSPS fellowship and NEDO project

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REFERENCES [1] [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] [27] [28] [29] [30] [31]

Root, RG. Laser-induced plasma and applications, Marcel Dekker, New York, 1989. Bauerle, D. Laser processing and chemistry (3rd ed.), Springer-Verlag, Berlin, 2000. Geohegan, DB; Puretzky, AA; Duscher, G; Pennycook, SJ. Appl Phys Lett, 1998, 73, 438. Lowndes, DH; Rouleau, CM; Thundat, TG; Duscher, G; Kenik, EA; Pennycook, SJ. J Mater Res, 1999, 14, 359. Wu, KT; Yao, YD; Wang, CRC; Chen, PF; Yeh, ET. J Appl Phys., 1999, 85, 5959. Link, S; El-Sayed, MA. Int Rev Phys Chem., 2000, 19, 409. Fogarassy, E; Lazare, S. Laser ablation of electronic materials—basic mechanisms and applications, Elsevier, Amsterdam, 1992. Chrisey, DB; Hubler, GK. Pulsed laser deposition of thin solid films, WileyInterscience, New York, 1994. Lu, F; Song, WD; Ang, BW; Chan, DSH; Low, TS. Appl Phys A, 1997, 65, 9. Tornan, V; Zafiropulos, V; Vainos, NA; Fotakis, C. Optics Laser Eng., 2000, 34, 309. Fojtik, A; Henglein, A. Ber. Bunsen-Ges. Phys. Chem., 1993, 97, 252. Mafuné, F; Kohno, J; Takeda, Y; Kondow, T; Sawabe, H. J. Phys. Chem. B, 2000, 104, 8333. Mafuné, F; Kohno, J; Takeda, Y; Kondow, T; Sawabe, H. J. Phys. Chem. B, 2000, 104, 9111. Kabashin, AV; Meunier, M. J Appl Phys., 2003, 94, 7941. Izgaliev, AT; Simakin, AV; Shafeev, GA. Quantum Electron, 2004, 34, 47. Barcikowski, S; Hahn, A; Kabashin, AV; Chicjkov, BN. Appl. Phys. A, 2007, 87, 47. Švrček, V; Sasaki, T; Shimizu, T; Koshizaki, N. Appl. Phys. Lett, 89 (2006) 213113. Švrcek, V; Sasaki, T; Shimizu, Y; Koshizaki, N. J. Appl. Phys., 2008, 103, 023101. Švrček, V; Mariotti, D; Kondo, M. Optics Express, 2009, 17 , 520. Švrcek, V; Sasaki, T; Katoh, R; Shimizu, T; Koshizaki, N. Appl. Phys., B, 2009, 94, 133. Švrcek, V; Slaoui, and A; Muller, JC. J. Appl. Phys., 2004, 95, 3158. Švrcek, V; Slaoui, A; Muller, JC; Rehspringer, JL; Hönerlage, B; Tomasiunas, R; Pelant, I. Physica E, 2003, 16, 420. Švrcek, V; Sasaki, T; Shimizu, Y; Koshizaki, N. Chem. Phys. Lett, 2006, 429, 483. Švrcek, V; T. Sasaki, Y. Shimizu and N. Koshizaki, Physica E, 40. (2007) 293. Švrček, V. Pure and Applied Chemistry, 2008, 80, 2513. Yablonovitch, E; Allara, DL; C> Chang, C; Gmit-ter, T; Bright, TB. Phys. Rev. Lett, 1986, 57, 249. Švrcek, V; Slaoui, A; Rehspringer, JL; Muller, JC. J. of Lumin, 2003, 101, 269. Švrcek, V; Sasaki, T; Shimizu, Y; Koshizaki, N. Journal of Laser Micro/Nanoengineering, 2007, 2, 15. Fauchet, PM. J. of Lumin., 1996, 70, 294. Švrček, V; Mariotti, D; Kalia, K; Kondo, M. Chem. Phys. Lett, 2009, 478, 224. Kanemitsu, Y; Uto, H; Masumoto, Y; Matsumoto, T; Futagi, T; Mimura, H. Phys. Rev. B, 1993, 48, 2827.

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[32] Deegan, RD; Bakajin, O; Dupont, TF; Huber, G; Nagel, SR; Witten, TA. Nature, 1997, 389, 827. [33] Tlusty, T; Safran, SA. Science, 2000, 290, 1328. [34] Tang, ZY; Kotov, NA; Giersig, M. Science, 2002, 297, 237. [35] Jackson, AM; Myerson, JW; Stellacci, F. Nat. Mater, 2004, 3, 330. [36] Oraevsky, AA; Letoshkov, VS; Esenafiev, RO. Pulsed LaserAblation of Biotissue: Review of Ablation Mechanisms, Springer, Berlin, 1991. [37] Geohegan, DB. Appl. Phys. Lett, 1992, 60, 2732 . [38] Zeldovich, YB; Raizer, YP. Physics of Shock Waves and High- Temperature Hydrodynamic Phenomena, Dover Publications, Inc, New York, 2001.

In: Laser Ablation: Effects and Applications Editor: Sharon E. Black

ISBN: 978-1-61122-466-5 © 2011 Nova Science Publishers, Inc.

Chapter 5

HO:YAG LASER LITHOTRIPSY Jinze Qiu1, Thomas E. Milner1 and Joel M. H. Teichman2 1

Dept. of Biomedical Engineering, The Univ. of Texas at Austin, 107 W Dean Keeton, Austin TX, USA 78712 2 Dept. of Urologic Sciences, University of British Columbia, and St. Paul's Hospital, 1081 Burrard St., Burrard Bldg. C307, Vancouver BC, Canada V6Z 1Y6

ABSTRACT The long-pulse Ho:YAG laser has been used for intracorporeal laser lithotripsy of urinary calculi since the mid-1990’s and is considered the “gold standard” modality for endoscopic laser lithotripsy. We present an overview of Ho:YAG laser lithotripsy. We begin with an introduction of the ablative mechanism of Ho:YAG laser lithotripsy, and compare to short-pulse (< 10 usec) laser lithotripsy. Ablative properties of Ho:YAG laser lithotripsy are reviewed and we summarize several practical problems and safety issues of existing optical fibers for Ho:YAG lithotripsy.

INTRODUCTION Clinical application of intracorporeal laser lithotripsy to human urinary calculi began in the mid 1980s[1-3]. The surgical technique of visualizing a calculus in the urinary tract by inserting a ureteroscope and fragmenting the stone by fiber delivered laser energy has undergone significant advances over the past twenty years[4]. The principle advantage of intracorporeal laser lithotripsy over previous surgical techniques is the use of small diameter flexible optical fibers (<400 um) that can be passed through the working channel of a small caliber ureteroscope (typically less than 3 mm diameter) for pulsed laser energy delivery to achieve stone fragmentation. The laser lithotripsy procedure is considered to be minimally invasive and provides access to both ureteral and renal calculi.[2] A variety of lasers were

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introduced and applied clinically before the advent of the Ho:YAG laser: these included 1) the Q-switched Nd:YAG laser (wavelength: 1064 nm; pulse duration: tens of nanoseconds); 2) the Q-switched alexandrite laser (wavelength: 755 nm; pulse duration: hundreds of nanoseconds); and 3) the flashlamp pumped pulsed dye laser (FPDL, wavelength: around 540nm; pulse duration: several microseconds).[1, 3] These three short-pulse lasers, with their pulse durations ranging from several nanoseconds to several micro-seconds, fragment stones using a photomechanical mechanism.[5] Application of these lasers for lithotripsy produce large sized fragments (fragments larger than 4mm in diameter were found) and fragmentation of hard urinary calculi, such as calcium oxalate monohydrate and cystine stones, is poor[6, 7]. The Ho:YAG laser has been used for laser lithotripsy since the mid 1990s. For laser lithotripsy, pulse duration of the Ho:YAG laser is set at several hundred micro-seconds and fragmentation of stones uses a photothermal mechanism. Unlike the short-pulsed lasers mentioned above, the Ho:YAG laser fragments all stone compositions and produces smaller sized fragments. In recognition of the superior ablation characteristics, the Ho:YAG laser is widely recognized by urologists as the “gold standard” for intracorporeal laser lithotripsy of urinary calculi.[7-18]

MECHANISM OF HO:YAG LASER LITHOTRIPSY Short-pulse lasers fragment urinary calculi using a photomechanical mechanism.[1, 5, 19, 20] In short-pulse laser lithotripsy, plasmas are generated on the targeted area immediately following onset of the laser pulse. The plasmas expand to a hemispherical bubble and collapse or implode within a few hundred microseconds. In nanosecond Q-switched Nd:YAG or Q-switched alexandrite laser lithotripsy, both plasma expansion and bubble collapse generate shock waves. The resulting shock waves, with pressures that exceed 100 bars in magnitude, fragment stones as they traverse stone surfaces. In microsecond pulsed dye laser lithotripsy, fragmentation only occurs upon the shock wave produced by cavitation bubble collapse. Pressures produced from the shock wave resulting from plasma expansion are too weak to generate fragmentation. Existing Ho:YAG lasers for laser lithotripsy operate with pulse durations of 250 to 700 us with most in the range of 250-350 us. Optical intensity profiles of two typical Ho:YAG pulses are displayed in Figure 1. The fragmentation process of long-pulse Ho:YAG lithotripsy is governed by a photothermal ablative mechanism instead of photomechanical or photoacoustic mechanisms.[21] Laser energy is absorbed first by water between fiber tip and stone surface to form a vapor bubble, a phenomenon called the “Moses effect”. The remaining laser energy is transmitted through the vapor channel and absorbed by the stone. The short duration of the laser pulse ensures heat confinement within the irradiation area and causes melting and ejection of debris. Within a few hundred microseconds, the vapor bubble expands and collapses asymmetrically to multiple loci, producing weak pressure transients, typically 8 bars or less. Magnitude of these pressure waves are substantially weaker than those produced by short-pulse lasers.[22, 23]

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Figure 1. Optical intensity profiles of Ho:YAG laser pulses measured by a photodetector(EG&G J1218C-R250U).[24]

Various studies have confirmed a photothermal mechanism and the paucity of photomechanical effects for long-pulse Ho:YAG laser lithotripsy. In experiments reported by Chan et al. [21], direct irradiation of stone surface was shown as the ablative mechanism rather than shockwave induced ablation. Ablation was more efficient for dehydrated stones in air than for hydrated stones in air or in water. High-speed imaging showed that ablation occurred only after the laser beam was transmitted through the vapor bubble to the stone surface, an evidence of the “Moses effect”. The most efficient lithotripsy was achieved by minimizing the separation distance between the fiber tip and the stone surface, while a small separation between the fiber tip and the stone surface (~1 mm) was required to maximize ablation efficiency for the short-pulse lithotripsy.[21, 25] Moreover, for the long-pulse Ho:YAG laser, no lithotripsy effect was attributed to pressure waves. Pressure transient measured by a hydrophone was less than a few bars and about 1% of that from short-pulse induced shock wave lithotripsy, corroborating reports from other groups.[23, 26] No fragmentation occurred when the fiber was oriented parallel to the stone surface, but in this configuration, maximal bubble expansion occurred and the pressure transient corresponding to the bubble collapse was about 20 bars, larger than that produced by orientating fiber perpendicular to the stone surface. Thus, a larger vapor bubble did not increase Ho:YAG stone fragmentation. In contrast, short-pulse lasers can fragment stones with the delivery fiber oriented parallel to the stone surface since shock waves propagate spherically outward in all directions.[13, 22] Post-lithotripsy products were found to have components resulting from thermal decomposition, a result that was also confirmed by Dushinski et al.[14] and Schafer et al.[27]. Cystine stones were discovered to produce free sulfur and cysteine after lithotripsy and Uric acid stones were discovered to produce cyanide. Teichman et al. reported that application of the Ho:YAG laser yields smaller fragments and symmetric ablation craters whereas short-pulse lasers produce asymmetric ablation craters, indicating the ablation mechanism of the two types of lithotripsy lasers is different.[7, 15]

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PROPERTIES OF HO:YAG LASER LITHOTRIPSY Compared to short-pulse shock wave induced lithotripsy, Ho:YAG laser lithotripsy has at least four characteristic properties, making it more suitable for intracorporeal lithotripsy. First, the Ho:YAG laser can fragment all stone compositions.[7] Second, debris produced with the Ho:YAG laser are smaller than those produced by photomechanical lithotripsy devices. Third, Ho:YAG lithotripsy is clinically safe because of high absorption of Ho:YAG light by water, reducing damage by stray radiation. Fourth, Ho:YAG lithotripters cause minimal stone retropulsion compared to short-pulsed laser devices. [28, 29] The main limitation of Ho:YAG lithotripsy is a reduced speed compared to short-pulse shock wave induced lithotripsy. A less significant limitation is stone retropulsion. Debris ejected during the course of Ho:YAG laser lithotripsy travel along the normal to the stone surface. The stone being targeted thus moves in the opposite direction due to conservation of momentum and results in stone retropulsion.[30, 31] Increased operating time is needed for urologists to move the ureteroscope and fiber to the more proximal stone position. Lee et al. showed that retropulsion was greater for larger diameter fibers at a given energy or for larger energies for a given fiber diameter.[31, 32] Based on vector analysis, the geometry of the crater explains why larger diameter fibers cause increased retropulsion. A large diameter fiber produces a wide and shallow crater. Since fragments eject along the normal, a wide and shallow crater ejects fragments in a more uniform direction off the stone surface, producing greater momentum to the stone in the opposite direction. Similar results were reported by White et al.[30] Finley et al. reported dependence of stone retropulsion on Ho:YAG pulse duration in vitro[33]. Similar results were reported by Kang et al by a detailed measurement of laser pulse duration.[24] Longer pulses (FWHM: 280us) produce less stone retropulsion than shorter pulses (FWHM: 150us) regardless of pulse energy when a single laser pulse is applied. One possible reason is the occurrence of recoil pressure during debris ejection.[34] Debris ejection is associated with high temperature and pressure gradients of the ablated area caused by intensive laser irradiation. Based on conservation of momentum, debris ejection produces a so-called recoil pressure on the stone sample. The recoil pressure is proportional to radiant power[35] and therefore inversely proportional to pulse duration for a fixed pulse energy.

FIBERS FOR HO:YAG LASER LITHOTRIPSY Optical fibers, because of their flexibility and high efficiency in transmitting laser radiation, are widely used in biomedical applications. Ho:YAG laser radiation, highly absorbed by hydroxyl groups, is delivered by low OH silica fiber to the stone surface in intracorporeal laser lithotripsy.[36] Potential problems when using these fibers were investigated by several groups in order to mitigate potential damage to fibers and injury to patients.[37-40] This section reviews several important factors of optical fibers for the safe delivery of laser radiation, including laser-to-fiber coupling, fiber damage at extreme bending conditions and fiber tip degradation during lithotripsy. Diameter of the fiber core used for lithotripsy is variable from 150 um to 940 um, normally much smaller than diameter of the collimated laser beam. NA of Ho:YAG fiber

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cores is approximately 0.2 to 0.22[37]. Fluorine doped silica, good at Ho:YAG light transmission, is the main choice of cladding materials.[36] The beam is focused through a convex lens and launched into the fiber core. Two parameters, diameter and numerical aperture (NA), are critical to obtain high efficiency laserto-fiber coupling.[36, 41] Diameter of the beam at the fiber surface is fairly large but smaller than the core-diameter to prevent direct fiber damage by high fluence or an overfilled fiber core.[37] NA of a fiber determines maximal divergence angle a beam can have before entering the fiber[36]. Optical fiber transmits radiation using total internal reflection at the core-cladding interface. Excessive divergence of the incident beam causes some light to penetrate the fiber cladding and possibly damage the fiber or be lost as a radiation mode. The proximal end of the fiber is directly attaches to the clinical Ho:YAG laser. A standard connector, termed subminiature version A (SMA), is used to protect the proximal end so that fibers with different diameters can easily connect to the same laser. An SMA connector is designed with an air filled cavity surrounding the proximal end of the fiber to protect from leakage-related damage, a simple and low cost method called an “air-well” approach. A more detailed description of the air-well approach are documented by Nazif et al.[36] Small diameter fibers (core diameter less than 300 um) are normally used for upper urinary-tract lithotripsy where more flexibility is required to allow the fiber to access the ureter and kidney through a small caliber ureteroscope (less than 3 mm diameter). The beam diameter of most clinical Ho:YAG lasers are 100-300 microns at the exit aperture. For example, the beam diameter of the Lumenis Ho:YAG laser is approximately 160um, as reported by the manufacturer. To prevent fiber damage from energy leakage or overfill, the proximal end is designed with a core-diameter larger than fiber diameter to form a large-tosmaller taper.[37] Unfortunately, fiber damage due to low laser-to-fiber coupling is not uncommon in the case of small core diameter fibers (less than 300 um). Two reasons commonly contribute to damage of the proximal fiber end. First is the misalignment between the laser beam and fiber. A small misalignment may be acceptable for larger core-diameter fibers as long as the focused beam is within the fiber core, but can result in overlap between the laser beam and cladding of small core-diameter fibers and damage. A second source of fiber damage at the proximal end is beam leakage through the taper cladding. Significant leakage of laser radiation into the cladding can cause fiber damage if the fiber is misaligned[37]. When Ho:YAG laser radiation enters the proximal end of a fiber without damage, the beam propagates through the fiber and is transmitted onto the stone surface from the distal end of the fiber. When treating upper urinary-tract calculi, small core diameter fibers (less than 300um) are needed to pass through the working channel of small caliber ureteroscopes. Typically the ureteroscope is less than 3 mm diameter, and the working channel is often on the order of 1 mm diameter. Further, these small caliber fibers may be bent extremely if the ureteroscope itself is bent extremely, as required for retrograde ureteroscopic management of lower pole stones. At extreme bending condition, risk of radiation leakage into the cladding increases and can result in fiber damage, so-called thermal breakdown.[36, 38] In a series of in vitro experiments, fibers from several different manufacturers were compared with respect to thermal breakdown.[38] Threshold pulse energy for thermal breakdown was documented at a bending diameter of 1 to 3 cm. Large core diameter fibers were reported to have lowest damage thresholds. Small diameter fibers from different manufacturers performed differently

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even at the same bending diameter. Fiber failure always occurs at the point of maximal bending condition and only during laser transmission and never from mechanical fatigue alone. The results of studies investigating fiber damage at different bend diameters are critical for urologists to select correct fibers for different surgical procedures. When the Ho:YAG beam transmits through the fiber without fiber damage, the beam can be targeted to the stone surface for lithotripsy. No optical elements are positioned between the distal fiber tip and the stone. During lithotripsy, fiber tip degradation can occur at high pulse energy output.[15, 40] Fiber tip degradation may cause decrease of beam transmission through the fiber, decrease ablation efficiency and require intraoperative replacement of the fiber. Fiber replacement lengthens operating time, results in a resource loss, and compromises process efficacy. A recent study compared fiber degradation among different manufacturers.[40] Fiber tip degradation was an energy density related phenomenon and was significant for small core diameter fibers (200-273um) and at pulse energies larger than 1.0J(pulse energy at fiber input). Fibers with high output energies tended to have high fiber degradation. Fiber tip degradation might result in a less collimated output beam and an irregular tip surface.

SAFETY ASPECT OF HO:YAG LASER LITHOTRIPSY Ho:YAG laser lithotripsy has a good clinical safety profile. Sofer et al. reported a high stone-free rate of 97% in Ho:YAG lithotripsy for 598 patients with upper urinary-tract calculi.[42] Only 4% of patients had complications. Intraoperatively, 13 patients had complications, including laser related ureteral perforation in 1 patient and laser fiber breakage within the ureteroscope in 3 patients. Thus, 4 of 598 (0.7%) patients experienced laser related complications. Watterson et al. showed a similar stone-free rate in their procedures and reported that 29 out of 30 procedures were completed with no significant complication.[43] Most large series studies of Ho:YAG laser lithotripsy for ureteral calculi show similar stonefree outcomes (typically greater than 90% are stone-free) with minimal complications. Ureteral perforation caused by direct irradiation is a serious potential complication during Ho:YAG lithotripsy. An ex vivo comparison of four commonly used lithotripters showed that a Ho:YAG laser beam could easily perforate the ureter wall, but the experiment was completed under the extreme conditions of direction absorption, and the fiber tip was normally oriented to the ureteral mucosal surface. Perforation did not occur when the fiber tip was maintained at least 2mm away from the ureteral mucosal surface and separated by water because the Ho:YAG wavelength (2.1 um) is well absorbed by water.[44] In clinical circumstances, the optical fiber is usually oriented tangential to the ureteral mucosal surface (low fluence to the wall), and working under visual guidance allows urologists to maintain the fiber tip in contact with the stone surface and not the ureteral mucosa. In the authors’ experience (Teichman), laser related perforation of the ureteral wall is an uncommon complication. In the kidney, direct absorption of laser energy to the kidney epithelium is usually of little clinical concern as the depth of penetration is minimal (<0.4 mm). Another potential concern is the potential risk of chemical byproducts of photothermal ablation. Photothermal ablation of uric acid calculi can produce cyanide, a deadly poison if

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produced in sufficient concentrations but has never been observed clinically.[45-47] No evidence of cyanide toxicity was found in reports by multiple groups.

CONCLUSIONS The free-running Ho:YAG laser is currently considered the ”gold standard” by urologists for intracorporeal laser lithotripsy. In this article we have summarized several important aspects of clincial Ho:YAG laser lithotripsy. Ablation mechanism and ablation properties of Ho:YAG laser lithotripsy were reviewed. Documentation of fiber-related problems and safety issues provide important practical knowledge for administering successful clinical procedures.

REFERENCES Rink, K; Delacretaz, G; Salathe, RP. "Fragmentation process of current laser lithotriptors," Lasers in Surgery and Medicine, 1995, 16(2), 134-146. [2] Chan, KF; Choi, B; Vargas, G; Hammer DX; Sorg, B; Pfefer, TJ; Teichman, JMH; Welch, AJ; Jansen, ED. "Free electron laser ablation of urinary calculi: an experimental study," IEEE Journal on Selected Topics in Quantum Electronics, 2001, 7(6), 10221033. [3] Marks, AJ; Teichman, JMH. "Lasers in clinical urology: state of the art and new horizons," World Journal of Urology, 2007, 25(227-233) [4] Teichmann, H; Herrmann, TR; Bach, T. "Technical aspects of lasers in urology," World Journal of Urology, 2007, 25(221-225. [5] Rink, K; Delacretaz, G; Salathe, RP. "Fragmentation process induced by nanosecond laser pulses," Applied Physics Letters, 1992, 61(22), 2644-2646. [6] Qiu, JZ; Teichman, JMH; Kuranov, RV; Mcelroy, AB; Wang, T; Paranjape, AS; Milner, TE."Near infrared femtosecond laser ablation of urinary calculi in water," in Photonics West 2009, SPIE, San Jose, CA. [7] Teichman, JM; Vassar, GJ; Bishoff, JT; Bellman, GC. "Holmium:YAG lithotripsy yields smaller fragments than lithoclast, pulsed dye laser or electrohydraulic lithotripsy," Journal of Urology, 1998, 159(1), 17-23. [8] Liu, MK; Liu, PL; Yiu, TF; Chan, AYT. "Clinical experience with holium:YAG laser lithotripsy of ureteral calculi," Lasers in Surgery and Medicine, 1996, 19(103-106. [9] Razvi, HA; Denstedt, JD; Chun, SS; Sales, JL. "Introcorporeal lithotripsy with the holium:YAG laser," Journal of Urology, 1996, 156(3), 912-914. [10] Das, A; Erhard, MJ; Bagley, DH. "Intrarenal Use of the Holmium Laser," The Journal of Urology, 1998, 160(2), 630-631. [11] Adams, DH. "Holmium:YAG laser and pulsed dye laser: A cost comparison," Lasers in Surgery and Medicine, 1997, 21(1), 29-31. [12] Teichman, JMH; Rao, RD; Rogenes, VJ; Harris, MJ. "Ureteroscopic Management of Ureteral Calculi: Electrohydraulic versus Holmium:YAG Lithotripsy," The Journal of Urology, 1997, 158(4), 1357-1361. [1]

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[13] Teichman, JMH; Rao, RD; Glickman, RD; Harris, JM. "Holmium: YAG Percutaneous Nephrolithotomy: The Laser Incident Angle Matters," The Journal of Urology, 1998, 159(3), 690-694. [14] Dushinski, JW; Lingeman, JE. "High-Speed Photographic Evaluation of Holmium Laser," Journal of Endourology, 1998, 12(2), 177-181. [15] Vassar, GJ; Teichman, JMH; Glickman, RD. "Holmium:YAG lithotripsy efficiency varies with energy density," Journal of Urology, 1998, 160(2), 471-476. [16] Teichman, JM; Vassar, GJ; Glickman, RD. "Holmium:yttrium-aluminum-garnet lithotripsy efficiency varies with stone composition," Urology, 1998, 52(3), 392-397. [17] Teichman, JMH; Rogenes, VD; McIver, BJ; Harris, JM. "Holmium:YAG laser cystolithotripsy of large bladder calculi," Journal of Urology, 1997, 50(1), 44-48. [18] Teichman, JMH; Vassar, GJ; Glickman, RD; Beserra, CM; Cina, SJ; Thompson, IM. "Holmium:YAG lithotripsy: photothermal mechanism converts uric acid calculi to cyanide," The Journal of Urology, 1998, 160(2), 320-324. [19] Rink, K; Delacretaz, G; Salathe, RP. "Influence of the pulse duration on laser induced mechanical effects," in SPIE, 1994, 181-194, San Jonse. [20] Rink, K; Delacretaz, G; Salathe, RP. "Fragmentation process induced by microsecond laesr pulses during lithotripsy," Applied Physics Letters, 1992, 61(3), 258-260. [21] Chan, KF; Vassar, GJ; Pfefer, TJ; Teichman, JMH; Glickman, RD; Weintraub, ST; Welch, AJ. "Holmium:YAG laser lithotripsy: A dominant photothermal ablative mechanism with chemical decomposition of urinary calculi," Lasers in surgery and medicine, 1999, 25(1), 22-37. [22] Jansen, ED; Asshauer, T; Frenz, M; Motamedi, M; Delacretaz, G; Welch, AJ. "Effect of pulse duration on bubble formation and laser-induced pressure waves during holmium laser ablation," Lasers in Surgery and Medicine, 1996, 18(3), 278-293. [23] Schafer, SA; Durville, FM; Jassemnejad, B; Bartels, KE; Powell, RC. "Mechanisms of biliary stone fragmentation using the Ho:YAG laser," Biomedical Engineering, IEEE Transactions on, 1994, 41(3), 276-283. [24] Kang, HW; Lee, H; Teichman, JMH; Oh, JO; Kim, J; Welch, AJ. "Dependence of calculus retropulsion on pulse duration during Ho:YAG laser Lithotripsy," Lasers in Surgery and Medicine, 2006, 38(8), 762-772. [25] Zhong, P; Tong, HL; Malenbaum, J; Cocks, FH; Preminger, GM. "Transient cavitation and acoustic emission produced by different laser lithotripters," Journal of Endourology, 1998, 12(4), 371-378. [26] Beghuin, D; Delacretaz, G; Schmidlin, F; Rink, K. "Fragmentation process during Ho:YAG laser lithotripsy revealed by time-resolved imaging," in SPIE, 220-224, San Jose, 1998. [27] Schafer, SA; Durville, FM; Jassemnejad, B; Bartels, KE; RC. Powell, "Mechanisms of biliary stone fragmentation using the Ho:YAG laser," IEEE Trans On Biomedical Engineering 41(3), 276-283 (1994) [28] Charles G, M; Jeff C, S; Patrick, SW; James O, L; Songlin, Z; Pei, Z; David, A; M; Glenn M, P. "In vitro comparison of stone retropulsion and fragmentation of the frequency doubled, double pulse Nd:YAG laser and the holmium:YAG laser," The Journal of Urology, 2005, 173(5), 1797-1800.

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[29] Spindel, ML; Moslem, A; Bhatia, KS; Jassemnejad, B; Bartels, KE; Powell, RC; O'Hare, CM; Tytle, T. "Comparison of holmium and flashlamp pumped dye lasers for use in lithotripsy of biliary calculi," lasers in Surgery and Medicine, 1992, 12(482-489. [30] White, MD; Moran, ME; Calvano, CJ; Borhan-Manesh, A; Mehlhaff, BA. "Evaluation of retropulsion caused by holmium:YAG laser with various power settings and fibers," Journal of Endourology, 1998, 12(2), 183-186. [31] Lee, HO; Ryan, RT; Teichman, JMH; Kim, J; Choi, B; Arakeri, NV; Welch, AJ. "Stone Retropulsion During Holmium:Yag Lithotripsy," The Journal of Urology, 2003, 169(3), 881-885. [32] Lee, H; Ryan, RT; Kim, J; Choi, B; Arakeri, NV; Teichman, JMH; Welch, AJ. "Dependence of Calculus Retropulsion Dynamics on Fiber Size and Radiant Exposure During Ho:YAG Lithotripsy," Journal of Biomechanical Engineering, 2004, 126(4), 506-515. [33] Finley, DS; Petersen, J; Abdelshehid, C; Ahlering, M; Chou, D; Borin, J; Eichel, L; McDougall, E; Clayman, R. "Effect of holmium:YAG laser pulse width on lithotripsy retropulsion in vitro," Journal of Endourology, 2005, 19(8), 1041-1044 [34] Bauerle, D. Laser processing and chemistry, Springer-Verlag Berlin, 2000. [35] Dieter Bauerle, Laser Processing and Chemistry, Springer, Berlin (2000). [36] Nazif, OA; Teichman, JMH; Glickman, RD; Welch, AJ. "Review of laser fibers: a practical guide for urologists," Journal of Endourology, 2004, 18(9), 818-829. [37] Marks, AJ; Mues, AC; Knudsen, BE; Teichman, JMH. "Holmium:Yttrium-AluminumGarnet Lithotripsy Proximal Fiber Failures From Laser and Fiber Mismatch," Urology, 2008, 71(6), 1049-1051. [38] Knudsen, BE; Glickman, RD; Stallman, KJ; Maswadi, S; Chew, BH; Beiko, DT; Denstedt, JD; Teichman, JMH. "Performance and safety of holmium: YAG laser optical fibers," Journal of Endourology, 2005, 19(9), 1092-1097. [39] Mues, AC; Teichman, JMH; Knudsen, BE. "Evaluation of 24 Holmium:YAG Laser Optical Fibers for Flexible Ureteroscopy," The Journal of Urology, 2009, 182(1), 348354. [40] Mues, AC; Teichman, JMH; Knudsen, BE. "Quantification of Holmium:Yttrium Aluminum Garnet optical tip degradation," Journal of Endourology, 2009, 23(9), 14251428. [41] Harrington, JA. Infrared Fibers and their Applications, SPIE Press, Bellingham, 2004. [42] Sofer, M; Watterson, JD; Wollin, TA; Nott, L; Razvi, H; Denstedt, JD. "Holmium: YAG Laser Lithotripsy for Upper Urinary Tract Calculi in 598 Patients," The Journal of Urology, 2002, 167(1), 31-34. [43] Watterson, JD; Girvan, AR; Cook, AJ; Beiko, DT; Nott, L; Auge, BK; Preminger, GM; Denstedt, JD. "Safety and Efficacy of Holmium: Yag Laser Lithotripsy in Patients With Bleeding Diatheses," The Journal of Urology, 2002, 168(2), 442-445. [44] Santa-Cruz, RW; Leveillee, RJ; Krongrad, A. Ex vivo comparison of four lithotripters commonly used in the ureter: what does it take to perforate? 1998. [45] Corbin, NS; Teichman, JMH; Nguyen, T; Glickman, RD; Rihbany, L; Pearle, MS; Bishoff, JT. "Laser Lithotripsy and Cyanide," Journal of Endourology, 2000, 14(2), 169-173.

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[46] Teichman, JMH; Champion, PC; Wollin, TA; Denstedt, JD. "Holmium: YAG Lithotripsy of Uric Acid Calculi," The Journal of Urology, 1998, 160(6, Part 1), 21302132. [47] Corbin, NS; Teichman, JM; Nguyen, T; Glickman, RD; Rihbany, L; Pearle, MS; Bishoff, JT. "Laser lithotripsy and cyanide," Journal of Endourology, 2000, 14(2), 169173.

In: Laser Ablation: Effects and Applications Editor: Sharon E. Black

ISBN: 978-1-61122-466-5 © 2011 Nova Science Publishers, Inc.

Chapter 6

COMPUTER MODELLING OF FEMTOSECOND LASER ABLATION OF SEMICONDUCTORS AND DIELECTRICS D. P. Korfiatis and K.-A. Th. Thoma Physics Department, University of Patras 26500 Rio, Patras, Hellas, Greece

ABSTRACT Ultrafast laser ablation has proved to be a powerful tool for material processing. Combination of both experimental and theoretical efforts can lead to the determination of optimum values of laser parameters for the improvement of accuracy which is a most crucial aspect in micromachining. Besides experiment and theory, numerical simulation can also provide a significant research tool in the field for the calculation of parameters of great importance which influence the micromachining process. Such parameters are ablation threshold as a function of the laser wavelength, pulse duration and the properties of the particular material. Especially, the prediction through numerical simulation of the geometry and the dimensions of the craters formed and the damaged surrounding surface for known laser and material properties can lead to control and optimization of the micromachining process. Furthermore, numerical simulation can drop light to the microscopic processes through which femtosecond laser damage occurs. In this chapter, the numerical techniques currently used for the simulation of femtosecond laser ablation of semiconductors and dielectrics are presented and discussed. These techniques include molecular dynamics simulation, the Fokker-Planck approach and two temperature models.

1. INTRODUCTION Ultrashort-pulse lasers have proved to be a powerful tool for material processing offering many advantages in comparison to long pulses [Amoruso et al 2004, Farsari et al 2005]. Thus, precise micromachining with minimal heat affected zone can be achieved as a result of

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small thermal diffusion lengths associated with ultrashort pulses. Furthermore, ultrashort pulses offer low ablation thresholds because of energy localization and the absence of plasma plume absorption. Energy is localized in a small depth mainly due to the reduction of light penetration depth because of high laser intensities achieved with femtosecond pulses resulting to a major contribution of multiphoton absorption processes. Also, a major drawback of nanosecond laser ablation which is screening of the laser energy, reported to reach up to a one-half [Bulgakova et al 2004], is totally absent for the case of femtosecond laser pulses, since in this case plasma is formed after the end of each pulse. . The understanding of the ultrafast ablation process, the determination of optimum values of laser parameters for the improvement of the accuracy of micromachining, as well as the calculation of the associated laser fluence thresholds are of great importance and efforts towards this goal combining experiment, simulation and theory are highly required. In studying physical and physicochemical phenomena one of the big advantages of simulation (often referred in the literature as numerical experiment) in general is that the various processes involved can be studied separately. Through simulation one can also calculate parameters which may not be possible to measure through experiment. Furthermore, a large number of numerical experiments can be performed easier than the conventional experiments to reveal the relations between the parameters involved in the specific problem. The numerical techniques that are currently used for the simulation of femtosecond laser absorption and will be discussed explicitly in the following are molecular dynamics simulation, the Fokker-Planck approach and two temperature models. Special emphasis is given to the later as it is most frequently found in the literature. Finally, the shape and dimensions of craters formed by ablation are calculated, the calculations based on recent publications. In molecular dynamics simulation the solid is treated microscopically as an assembly of atoms that absorb energy from the incident radiation. As a result the bond length is altered causing a phase transition or the emission of some atoms from the solid lattice. In Fokker-Planck modelling, the electron energy distribution in the conduction band of semiconductors or dielectrics is described by the Fokker-Planck approximation of the Boltzmann equation. The free electron production due to the intense laser field is described through the variation of the electron energy distribution function. In two-temperature models a system of partial differential equations is constructed which describes the interaction between laser radiation and the solid target. The construction of the model is based on the conservation of the total energy of the system radiation-solid. The solid is divided in two subsystems, the lattice and the electron-hole plasma which is created due the absorption of the incident photons. For ultrashort pulses where the pulse width is comparable or shorter than the lattice-plasma energy relaxation time, it is generally accepted that the two subsystems attain two different temperatures, hence the name of the models.

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2. MOLECULAR DYNAMICS SIMULATION Molecular Dynamics (MD) simulation is a computational method to investigate the behaviour of materials by computing the molecular or atomic motion under the influence of a given potential. The general approach of MD is to obtain at the instant t+dt atomic positions, velocities, etc, based on positions, velocities, and other dynamic information at the instant t [Xu et al 2004]. A commonly used potential for the description of atom-atom interaction in semiconductors and dielectrics is the Lennard-Jones potential given by the relation [Perez and Lewis 2003]:

⎡⎛ σ ⎞12 ⎛ σ ⎞ 6 ⎛ σ ⎞12 ⎛ σ ⎞ 6 ⎤ V (r ) = 4ε ⎢⎜ ⎟ − ⎜ ⎟ − ⎜⎜ ⎟⎟ + ⎜⎜ ⎟⎟ ⎥ for r ≤ rc ⎝ r ⎠ ⎝ rc ⎠ ⎢⎣⎝ r ⎠ ⎝ rc ⎠ ⎥⎦ and V (r ) = 0 for r > rc

(1)

where rc is the cutoff distance at which the potential has been adjusted to vanish and ε ,

σ are the energy and length scales respectively.

Alternatively, a diamond-like structure has been modelled by using the Stillinger-Weber potential [Herrmann et al 1998, Holenstein et al 2004]. In the Stillinger-Weber potential twobody and three-body interactions are considered. The two-body term is of the form:

(

V2 (r ) = A1r

−4

)

− A2 e

⎛ σ ⎞ ⎜ ⎟ ⎝ r −a ⎠

(2)

where a is the cutoff distance, A1 , A2 are energy constants and σ is a length constant. The three-body term consists of exponential functions and an angular term favouring the tetrahedral arrangement of the four neighbours of a diamond-like structure.

V3 (rij , rik , θ jik ) = λ ⋅ e

⎛ γσ ⎞ ⎜ ⎟ ⎜ rij − a ⎟ ⎝ ⎠

⋅e

⎛ γσ ⎞ ⎜ ⎟ ⎜ r −a ⎟ ⎝ ik ⎠

1⎞ ⎛ ⋅ ⎜ cos θ jik + ⎟ 3⎠ ⎝

2

where rij , rik are the distances between the particles, θ jik is the bonding angle and

(3)

λ an

energy constant. Coulomb potential for ion-electron interaction is considered in addition to the potential describing ion-ion interaction. The lattice of the solid is simulated through an ensemble of a certain number of atoms bound via one of the above mentioned potential functions. Each incident photon has a finite probability of being absorbed by any of the atoms. This probability depends on the density of the atoms and the linear and multiphoton absorption coefficient of the material. After the absorption of a photon, the atom by which the photon is absorbed is marked as an excited atom. Changes in the potential and therefore in the crystal structure, with increasing degrees of excitation are implemented as a number of randomly broken bonds matching the degree of excitation. A broken bond is simulated by removing the attractive part of the two-body potential discarding also the three-body part of the potential [Herrmann et al 1998, Holenstein

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et al 2004]. For a solid with valence equal to 4, which is the case of silicon, an atom with a degree of excitation exceeding 3 is considered not to be bound at all [Herrmann et al 1998]. If after repeated absorption the total energy absorbed by an atom exceeds the workfunction then the atom is considered as ionized and an electron-hole pair is added to the system [Holenstein et al 2004]. Ablation threshold is defined as the minimum laser fluence value required for a certain number of ions to be ejected from the lattice. A third approach is the study of the increase of the mean bond-length as atoms absorb energy from the incident laser radiation. In this case the fluence threshold for melting and ablation can be defined by using the Lindemann criterion [Jeschke et al 2002]. A typical value of bond-length increase above which the lattice becomes unstable, leading to melting or ablation, is about 15%.

3. THE FOKKER-PLANK APPROACH The Fokker-Plank approximation of the Boltzmann transport equation for the electron distribution function is given by [Apostolova and Hahn 2000, Azzouz 2004, Oh et al 2006]:

∂f (ε , t ) ∂J (ε , t ) + = S (ε , t ) ∂t ∂ε

(4)

where f (ε , t )dε is the number density of electrons with kinetic energy between ε and

ε + dε at time t and S ( ε , t ) is related to electron generation and recombination through: S (ε , t ) = Rimp (ε , t ) + R rcc (ε , t ) + R pi (ε , t )

(5)

where Rimp is the impact ionization term, Rrcc is the Auger recombination term and R pi

is the multiphoton absorption term. The current in energy space, J (ε , t ) , is the number of

electrons per unit volume whose energy increases from a value less than ε to a value greater than ε per unit time. A complete analysis of these terms can be found in the literature [Apostolova and Hahn 2000]. In Fokker-Plank approach, damage threshold is usually defined in terms of a critical plasma density as given in section 3 through (14) [Azzouz 2004] or semi-empirically as a percentage of valence electron concentration.

4. TWO-TEMPERATURE MODELS In two-temperature models, it is assumed that energy from ultrashort laser radiation is absorbed by free carriers at a first step, leading to an increase of their temperature and density with time. At a second step a transfer of energy by free carriers to the lattice is considered, followed by an increase of the lattice temperature. The above procedure is governed by an

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energy relaxation time [Yoffa 1980]. This type of model was proposed for the first time in 1975 in order for the radiation-solid interaction to be described when the pulse width is shorter than the energy relaxation time [Anisimov et al 1975]. Usually, the two-temperature models appearing in the literature are 1D since lateral diffusion of carriers and heat can be neglected in the ultrafast time scale. 2D models though have also been used in certain cases [Sim et al 2007]. In two-temperature models, electrons and holes are assumed to be at a common temperature. The thermalization time of the carriers is of the order of 10 fs . This time sets the lower limit for the laser pulse width in order for the two-temperature model to be valid. The creation of electron-hole pairs is due to both linear and multi-photon (usually twophoton) absorption and also to avalanche ionization. For intense laser radiation multi-photon absorption is usually dominating over linear absorption, the former being the predominant mechanism for plasma creation in semiconductors. Multi-photon absorption in dielectrics results to the creation of an initial plasma density, crucial for avalanche ionization to take place. Electron and hole concentrations are considered as equal in all cases, as Dember field created by charge separation prohibits electron and hole concentrations from becoming significantly different [Van Driel 1987, Othonos 1998, Chen et al 2005]. The main recombination mechanism for high plasma densities is Auger recombination (the inverse phenomenon of avalanche ionization) with the recombination time given by a relation of the form:

τR = where

1 γ 3n 2

(6)

γ 3 is the bipolar Auger coefficient and n is the electron-hole pair concentration. 21

−3

For high enough plasma densities (higher than 10 cm ), due to screening of the Coulomb interaction, Auger recombination time tends to take a constant value independent of the plasma density. A typical two-temperature model for the description of femtosecond laser absorption by semiconductors and dielectrics consists of the following equations:

cL

T ( z, t ) − TL (z, t ) ∂T ( z, t ) ∂ ⎡ ∂T ( z, t ) ⎤ +L c = ⎢κ L ⎥ ∂t ∂z ⎣ ∂z ⎦ τE

∂n(z , t ) ∂ ⎡ ∂n(z , t )⎤ I (z, t ) I (z, t ) n( z , t ) = ⎢ Dα +α +β − + δ ⋅ n( z , t ) ⎥ ∂t ∂z ⎣ ∂z ⎦ τR hf 2hf

(7)

2

(8)

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2hf − Eg ⎞ ∂Uc ( z, t ) ∂ ⎡U c ( z, t ) ∂n( z, t ) ⎤ ⎛ hf − Eg Dα α+ βI (z, t ) + α FC ⎟⎟I ( z, t ) + ⎜⎜ = ⎢ ⎥ 2hf ∂t ∂z ⎣ n( z, t ) ∂z ⎦ ⎝ hf ⎠ + Eg

n( z , t )

τR

−L

Tc ( z, t ) − T ( z, t )

τE

∂I ( z, t ) = −(α + β I ( z, t ) + α FC )I ( z, t ) ∂z

(9)

(10)

In the right hand side of (7) the first term describes the heat conduction through the lattice and the second one the exchange of energy between lattice and plasma. In the right hand side of (8), diffusion, generation through linear and two photon absorption, recombination of the carriers and generation due to avalanche ionization, have been taken into account. The terms in the right hand side of (9) represent the diffusion of energy through carrier diffusion, the increase of carriers' energy because of the excess of photon energy over the energy gap, free carriers' absorption and Auger recombination. The last term denotes the exchange of energy between lattice and plasma. Finally, equation (10) represents the attenuation of light intensity due to linear, non-linear and free carriers’ absorption. The value of the absorbed light intensity I , which is really incident on the sample, is obtained after subtraction of the reflection. The relation between carriers’ energy and temperature and thus the specific heat of carriers can be obtained from classical statistics as [Lietoila and Gibbons 1982]:

U c (z , t ) = 3k B n(z, t )Tc ( z , t ) and cc = 3k B n( z, t )

(11) (12)

More precisely, the Fermi-Dirac statistics can be used for the determination of the carriers’ specific heat [Chen et al 2005]. As already mentioned, exchange of energy between carriers and lattice is governed by an energy relaxation time. The energy relaxation time is increased with increasing plasma density because the rate of energy exchange is reduced due to screening effects. The factor L in the energy exchange term in equations (7) and (9) is usually considered as equal to the specific heat of the carriers [Lietoila and Gibbons 1982, Van Driel 1987]. A more symmetric relation which is also used [Klossika et al 1996, Korfiatis et al 2007a] is:

L=

cc c L cc + c L

(13)

The meaning of all symbols used in equations (7)-(10) can be found in Table 1. The subscript L is for lattice and the subscript c is for carriers.

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159

Table 1 Symbol

T c

Meaning Temperature Specific heat

Symbol

κ Da

n

Plasma density

Eg

a

Linear absorption coefficient

β

Free carrier absorption coefficient Impact ionization coefficient

U

a FC

δ

hf

Meaning Thermal conductivity Ambipolar diffusion coefficient Energy gap Two-photon absorption coefficient Energy Photon energy

The boundary conditions required for solving the above system of partial differential equations can be that of the semi-infinite medium, namely, at the sample rare boundary lattice and plasma temperature are fixed at room temperature and plasma concentration at its intrinsic value. Alternatively, the boundary conditions can be set by defining zero fluxes of energy and carriers between the irradiated film and the surrounding, namely vanishing derivatives of lattice and plasma temperatures, as well as plasma density. A combination of the two types of boundary conditions can also be used. The initialization of ablation (ablation fluence threshold) has been defined in several ways in the relevant literature. Some researchers [Pronko et al 1998, Allenspacher et al 2003] define the ablation fluuence threshold as the point at which plasma density resumes its so called critical value for which the plasma frequency becomes equal to the laser angular frequency: (14) ωL = ω p This definition of critical plasma density has the disadvantage that the critical density obtained depends mainly on laser frequency and not on the material parameters. A more rational definition of critical plasma density results through the alternative approach given in the following. The main ablation mechanisms are thermal vaporization (strong ablation) and Coulomb explosion (gentle ablation) [Jiang and Tsai 2003]. For the case of strong ablation the melting of the solid target under the influence of the intense laser radiation can be described by the system of equations (7)-(10). The solid to liquid transition can be treated numerically as a moving boundary. Then, the vaporization of the liquid can be treated in a similar manner. Most semiconductors in liquid phase behave as metals, so a two-temperature model suitable for the description of metal-radiation interaction can be used. This is of the form [Chichkov et al 1996, Korfiatis et al 2009]:

∂T ( z , t ) ccl ⋅ c Il Tc (z , t ) − T ( z , t ) c = l ∂t cc + c Il τ El l I

(15)

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D.P. Korfiatis, K.-A. Th. Thoma

ccl

∂Tc ( z, t ) ∂ ⎛ ∂T ( z, t ) ⎞ ccl ⋅ c Il Tc ( z , t ) − T ( z , t ) = − ⎜ κ cl c ⎟− ∂t ∂z ⎝ ∂z ⎠ ccl + c Il τ El

(16)

where the superscript l is for liquid, the subscript I is for ions and the other symbols have the usual meaning. In this framework, the definition of ablation threshold comes directly as the minimum laser fluence required for the onset of vaporization of the liquid [Glover 2003]. Two-temperature models have also been used for the study of generalized thermoelastodynamics of materials subject to ultrafast laser heating [Qi and Suh 2009, 2010a, 2010b]. Finally, it must be pointed out that a combination of both molecular dynamics simulation and two-temperature models has been used in some cases [Yamashita et al 2006].

5. COMPUTATION OF CRATER GEOMETRY Ablation threshold is of great importance in laser micromachining since it can lead to the prediction of the shape of the crater created by ablation on the target surface. In the following the geometry of a crater created by ablation is calculated, the analysis based on two recent publications [Korfiatis et al 2007b, 2009]. Neglecting free carriers’ absorption and taking into account only linear and two-photon absorption, the attenuation of light intensity in the irradiated sample is given through the relation:

∂I = −(α + β I )I ∂z

(17)

For a rectangular pulse in time I is relating to the incident fluence through:

F0 =

Iτ p

1− R

(18)

where τ p is the pulse duration and R the reflectivity. Integration of (17) can lead to the calculation of damage depth by setting F0 = Fth at the damage depth [Korfiatis et al 2007b], where Fth is the fluence threshold. The damage depth is given through:

d=

⎡ F0 (ατ p + β (1 − R )Fth )⎤ ln ⎢ ⎥ α ⎣⎢ Fth (ατ p + β (1 − R )F0 )⎦⎥ 1

(19)

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161

For F0 → ∞ damage depth tends to a saturation value given by the relation:

d max =

⎛ ατ p + β (1 − R )Fth ln⎜⎜ α ⎝ β (1 − R )Fth 1

⎞ ⎟⎟ ⎠

(20)

For a Gaussian pulse in space:

F (r ) = F0 e

−

2r 2 w02

(21)

where w0 is the laser beam waist. So, eq. (19) can be written as:

⎡ ⎤ ⎢ ⎥ r2 −2 ⎢ F e w0 2 (ατ + β (1 − R )F ) ⎥ 1 p th ⎥ 0 d (r ) = ln ⎢ 2 r α ⎢ ⎛ ⎞⎥ −2 ⎢ F ⎜ ατ + β (1 − R )F e w0 2 ⎟ ⎥ 0 ⎟⎟ ⎥ ⎢ th ⎜⎜ p ⎠⎦ ⎣ ⎝

(22)

The radius of the damaged region can be calculated directly from (22) as the radius at

which F (r ) = Fth :

1 ⎛ F0 ln⎜ 2 ⎜⎝ Fth

rth = w0

⎞ ⎟⎟ ⎠

(23)

The last equation is used extensively by experimentalists for determining the ablation threshold given the radius of the ablated region. Integrating d (r ) over the entire crater surface, the volume of the crater results to be:

⎧

πw0 2 ⎪ 2rc 2 Vc = ⎨ 2α ⎪ w0 2 ⎩

⎡ ⎛ F ⎞ ⎛r ⎢ln⎜⎜ 0 + y ⎟⎟ − ⎜⎜ c ⎢⎣ ⎝ Fa 2 ⎠ ⎝ w0 where y ≡

⎞ ⎟⎟ ⎠

2

2r ⎛ − c2 ⎤ ⎜ ⎥ + L2 (− y ) − L2 ⎜ − ye w0 ⎜ ⎥⎦ ⎝

2

β (1 − R )F0 aτ p

⎞⎫ ⎟⎪ ⎟⎟⎬ ⎠⎪⎭

(24)

(25)

and L2 ( z ) is the dilogarithm function [Abramowitz and Stegun 1965]: 0

L2 ( z ) = ∫ z

ln (1 − t ) dt t

(26)

Obviously, by setting the ablation threshold as Fth in the above equations the shape of the crater created by ablation can be calculated. If instead of the ablation threshold the melting threshold is used the shape of the entire heat affected zone can be calculated.

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As precision is a major advantage of using ultra short pulses in material micromachining, it is of great importance that simulation techniques offer the possibility of calculating the shape and dimensions of the craters formed by ablation in the irradiated materials.

REFERENCES A. Lietoila and J. F. Gibbons, J. Appl. Phys. (1982) 53 3207. A. Othonos, J. Appl. Phys. (1998) 83 1789. B. N. Chichkov et al, Appl. Phys. A (1996) 63, 109. D. P. Korfiatis, K.-A. Th. Thoma, J. C. Vardaxoglou, Appl. Surf. Sci. (2009) 255, 7605. D. P. Korfiatis, K.-A. Th. Thoma, J. C. Vardaxoglou, J. Phys. D: Appl. Phys. (2007a) 40, 6803. D. P. Korfiatis, K.-A. Th. Thoma, J. C. Vardaxoglou, Metamaterials, October 22-26 (2007b) Rome, Italy. D. Perez and L. J. Lewis, Phys. Rev. B (2003) 67, 184102. E. J. Yoffa, Phys. Rev. B (1980) 21, (6), 2415. H. M. Van Driel, Phys. Rev. B (1987) 35 8166. H. O. Jeschke et al, Appl. Surf. Sci. (2002) 197-198 839. H. S. Sim, S. H. Lee and J. S. Lee, J. Mech. Sci. Technol. (2007) 21, 1847. I. M. Azzouz, J. Phys. B: At. Mol. Opt. Phys. (2004) 37 3259. J. J. Klossika et al, Phys. Rev. B (1996) 54, (15), 10277. J. K. Chen, D. Y. Tzou and J. E. Beraun, (2005) Int. J. Heat Mass Tran. 48 501. L. Jiang and H. L. Tsai, NSF Workshop on “Unsolved Problems and Research Needs in Thermal Aspects of Material Removal Processes”, June 10-12 (2003) Stillwater, OK. M. Abramowitz, I. A. Stegun, Handbook of Mathematical Functions, Dover, 1965. M. Farsari et al, J. Micromech. Microeng. (2005) 15, 1786. N. M. Bulgakova, A. V. Bulgakov, L. P. Babich, Appl. Phys. A (2004) 79, 1323. P. Allenspacher, B. Huettner and W. Riede, Proc. SPIE (2003) 4932, 358. P. P. Pronko et al, Phys. Rev. B (1998) 58, 2387. R. F. W. Herrmann, J. Gerlach and E. E. B. Campbell, Appl. Phys. A (1998) 66, 35. R. Holenstein et al, Proc SPIE (2004) 5579, 688. S. Amoruso et al, Appl. Phys. A (2004) 79, 1377. S. I. Anisimov, B. L. Kapeliovich and T. L. Perel’man, Sov. Phys. JETP (1975) 39, 375. T. Apostolova, Y. Hahn, J. Appl. Phys. (2000) 88, 1024. T. E. Glover, J. Opt. Soc. Am. B (2003) 20, 125. X. Qi and C. S. Suh, Int. J. Heat Mass Tran. (2010a) 53, 41. X. Qi and C. S. Suh, Int. J. Heat Mass Tran. (2010b) 53, 744. X. Qi and C. S. Suh, J. Therm.Stresses (2009) 32, 477. X. Xu, C. Cheng and I. Chowdhury, J. Heat Transfer (2004) 126, 727. Y. M. Oh et al, Int. J. Heat Mass Tran. (2006) 49, 1493. Y. Yamashita et al, Fusion Eng. Des. (2006) 81, 1695.

In: Laser Ablation: Effects and Applications Editor: Sharon E. Black

ISBN: 978-1-61122-466-5 c 2011 Nova Science Publishers, Inc.

Chapter 7

T HERMOPHYSICAL E FFECTS OF F EMTOSECOND L ASER A BLATION OF M ETAL TARGET Ranran Fang1 ∗and Hua Wei2 College of Mathematics and Physics, Chongqing University of Posts and Telecommunications, Chongqing 400065, China 2 Department of Physics, Chongqing University, Chongqing 400044, China 1

Abstract The electron-phonon relaxation time as a function of pulse width and fluence of femtosecond laser is studied based on the two-temperature model. The satisfactory agreement between our numerical results and experimental data indicates that the electron-phonon relaxation time is reasonably accurate with the influences of pulse width and fluence of femtosecond laser. An improved two-temperature model to describe femtosecond laser ablation of metal target is also presented. The temperaturedependent heat capacity and thermal conductivity of the electron, as well as electron temperature-dependent absorption coefficient and absorptivity are all considered in this tailored two-temperature model. The satisfactory agreement between our numerical results and experimental data indicates that the temperature dependence of heat capacity, thermal conductivity, absorption coefficient and absorptivity in femtosecond laser ablation of metal target must not be neglected. This chapter finally presents a unified thermal model, which can describe the thermophysical effects with laser pulse width ranges from nanosecond to femtosecond. The satisfactory agreement between our numerical results and experimental results of vaporization threshold indicates that the unified thermal model is correct and reasonable.

PACS 52.38.Mf; 52.50.Jm; 79.20.Ds ∗

Email address: [email protected]

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Introduction

It is well known that using femtosecond pulses in laser ablation has two major advantages over nanosecond pulses: much lower fluences are needed to accomplish ablation [1, 2, 3, 4, 5, 6, 7, 8] and considerably sharper contours can be achieved [9, 10]. Femtosecond laser ablation is useful in various applications concerning laser modification of surfaces (e.g. drilling, cutting, patterning), film growth by pulsed laser deposition (PLD), generation of nanoparticles and so on [11, 12, 13, 14]. In the past two decades, the femtosecond laser ablation of metals and its nonequilibrium energy transport have been very active research topics [15, 16, 18, 19, 17, 20, 21, 22, 23, 24]. In most theoretical treatments only electron-phonon collisions are taken into account for the electron diffusion process during and after femtosecond laser pulse ablation [25, 26, 27, 28, 29]. However, the recent pump-probe measurements have revealed that the role of electron-electron collisions becomes essential for the electron relaxation at electron temperature around or above 1 eV [26]. The electron-phonon collisions dominanting electron diffusion are valid only in the low electron temperature regime. Thus, extending the conventional two-temperature model from the low temperature regime dominated by electron-phonon collisions to the high temperature regime through modifying the electron thermal conductivity associated with the electron-electron collisions is crucial. Experiments have confirmed that the features of the femtosecond laser ablation process are greatly influenced by the quantity of laser energy absorbed by the target [30]. The absorption coefficient introduced by the Lambert-Beer’s law accounts for the attenuation of the absorbed energy in the target [31], and the absorptivity refers to the magnitude of the ratio of absorbed energy to incident energy [32]. They both describe the ability of absorbing laser energy for target. For the thermal mechanism dominates almost all the physical effects in the process of femtosecond laser ablation, the absorption coefficient and the absorptivity are important optical parameters in the femtosecond laser ablation. A lot of literatures consider the absorption coefficient and the absorptivity as constant in femtosecond laser ablation. However, the effect of electron temperature on the absorption coefficient and the absorptivity is distinctive. For the important application in technology and the rich subject in which to study, the researches of pulsed laser ablation and laser processing of materials develop very fast employing from the nanosecond to the femtosecond time regime [1, 5, 33, 34]. Both the experimental and theoretical studies have clearly demonstrated the presence of two different ablation mechanisms: nonequilibrium ablation [35, 36, 37] and equilibrium ablation [38, 39, 40, 41, 42, 43, 44, 45, 46], whose criteria is related with ratio of the electronphonon coupling time (τR ) and the laser pulse width (τL ), that is, as ττRL  1, it is the non-equilibrium ablation, while ττRL  1, it belongs to the equilibrium ablation, furthermore ττRL ∼ 1, it can be called a mixed ablation. In section II, we report the numerical solution of the electron-phonon relaxation time as a function of the heat source term parameters with two-temperature model(TTM). For a certain laser fluence the shorter the pulse width, the shorter the electron-phonon relaxation time is. However, the electron-phonon relaxation time becomes long for low laser fluence when the pulse width is fixed. Besides, the variation of heat-affected zone per pulse with laser fluence is performed. Our numerical results are in good agreement with the experi-

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mental data [16]. In order to describe the thermophysical effects of femtosecond laser ablation of metal target, a tailored theoretical model including temperature-dependent heat capacity, thermal conductivity of the electron, the absorption coefficient and absorptivity is presented in section III. The modified model is used to simulate femtosecond laser ablation of copper target. The satisfactory agreement between our numerical results and experimental data of ablation rate indicates that temperature dependence of heat capacity, thermal conductivity, absorption coefficient and absorptivity in femtosecond laser ablation of metal target should not be neglected. In section IV, we present a unified thermal model which can depict the process. The electron-phonon coupling time τR and laser pulse width τL are introduced in this model as the criteria to distinguished the two kinds of mechanisms. The well agreement between our numerical results and experimental results of vaporization threshold indicates that the unified thermal model is reasonable and satisfactory.

2.

Effect of Pulse Width and Fluence of Femtosecond Laser on the Electron-phonon Relaxation Time

In this section, the electron-phonon relaxation time as a function of pulse width and fluence of femtosecond laser is studied on the basis of two-temperature model. The twotemperature model is solved using a finite difference method for copper target. The temperature distribution of the electron and lattice along with space and time for a certain laser fluence is presented. And the time-dependence of lattice and electron temperature of the surface for different pulse width and different laser fluence are also respectively performed. Moreover, the variation of heat-affected zone per pulse with laser fluence is obtained. The satisfactory agreement between our numerical results and experimental data indicates that the electron-phonon relaxation time is reasonably accurate with the influences of pulse width and fluence of femtosecond laser.

2.1.

Two-temperature Model

The interactions of femtosecond laser pulses with metals are very complicated. The energy transport process in femtosecond laser heating of metals consists of two stages. The first stage is the absorption of the laser energy through photon-electron interactions within the femtosecond pulse duration. It takes a few femtoseconds for electrons to reestablish the Fermi distribution. Within the duration of a single femtosecond pulse, the change of lattice temperature is generally negligible. The second stage is the energy distribution to the lattice through electron-phonon interactions, typically on the order of tens of picoseconds. Although the electron-phonon collision time may be comparable to the electron-electron collision time, it takes much longer to transfer energy from free electrons to phonons, because the phonon mass is much larger than the electron mass. The characteristic time for the free electron and the lattice to reach thermal equilibrium is called the electron-phonon relaxation time [20]. In order to describe this phenomenon, TTM which takes account of the source term is used. This model considers the energy relaxation between electrons and phonons and the thermal diffusion, i.e. Fourier law, as follows:

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Ce (Te )

∂ ∂2 Te = ke (Te ) 2 Te − g(Te − Tl) + (1 − R)αbI0 (t) exp(−αb x) ∂t ∂x

Cl

∂ Tl = g(Te − Tl) ∂t

(1)

(2)

where Te and Tl are the electron and lattice temperature , respectively. Cl is the specific heat of lattice. Ce (Te) and ke (Te ) are the specific heat and the thermal conductivity of electron, respectively. Ce (Te) = Ae Te , and ke (Te ) = ke,0 Te /Tl [10, 47]. g is the electronphonon coupling coefficient. R and αb are the reflectance and the absorption coefficient of the target. In order to describe the incident laser intensity more accurately, we adopt the laser incident intensity expressed by a Gauss function [35], i.e., I0 (t) = I0 exp[−

(4 ln 2)t2 ] (τp)2

(3)

where I0 is the maximal laser power density, τp is the laser pulse width. For the equation (1) and (2), the initial condition is Te (x, 0) = Tl (x, 0) = T0.

(4)

and the boundary conditions can be expressed as: −ke

∂Te |x=0 = (1 − R)I0(t) ∂x

(5)

∂Te (6) |x=d = 0 ∂x where d is the target thickness, and T0 = 300K is the initial temperature which is uniform across the target. The analytical solution of electron-phonon relaxation time τep used in many papers [16, 47, 48, 49, 50] can be deduced from TTM, but this analytical solution without considering the fluence of femtosecond laser, and τep is expressed as −ke

τep ≈

τe τp(τp + τl ) τe τl + τe (τp + τl ) + τpτl τe + τl

(7)

where τi = Cl /g and τe = Ce /g are the electron cooling and lattice heating times, respectively. From equation (7), we can find that the analytical solution of the electron-phonon relaxation time is not fluenced by the fluence of the source term. For low laser intensities the analytical solution should be effective. However, for high laser intensities equation (7) will be invalid. It is the high time for us to obtain the numerical solution of the electron-phonon relaxation time. The effect of pulse width and fluence of femtosecond laser on electron-phonon relaxation time is studied depending on TTM in the following section.

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Table 1. Thermal and optical properties of copper R 0.92 Cl (J/m3K) 3.46

αb (m−1 ) 7.1 × 107 g(W/m3K) 2.6 × 1017

Ae (J/m3K 2 ) 96.6 Tm (K) 1356

ke,0(W/mK) 401 Tb (K) 2811

Figure 1. (a) Electron temperature distribution and (b) lattice temperature distribution at different times and position predicted by the two-temperature model for the copper target irradiated by a 100f s, 800nm pulse at 0.4J/cm2.

2.2.

Electron Thermal Diffusion Length

For high laser fluence, the thermal diffusion of electron is driven by the temperature gradient. The hot-electron bath is cooled down by electron-phonon interaction, the strength of which limits the diffusion range [10]. The electron thermal diffusion length can be expressed as [16, 47, 48, 49, 50], l=

p

Dτep

(8)

where D is the thermal diffusion coefficient, D = ke (Te )/Ce (Te ). The numerical solution of τep is calculated by virtue of TTM in this section.

2.3.

Results and Discussion

The thermal and optical properties of copper in Table I are adopted from Ref. [47]. 2.3.1. The space- and time-dependence of electron and lattice temperature of target The space- and time-dependence of electron and lattice temperature in target is represented in Fig.1. We can see from Fig.1(a) , at a fixed location, the electron temperature firstly

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Figure 2. The time-dependence of electron and lattice temperature of the surface for the copper target irradiated by a 100f s, 800nm pulse at 0.4J/cm2. rapidly increases along with the ablation time, while suddenly decreases when it reaches the maximum. As shown in Fig.1(b), the temperature of the lattice rises gradually. Then, the lattice temperature doesn’t change when it reaches the maximum. For a fixed time, the temperature of electron and lattice decrease along with depth. We can analyze the phenomenons as follows: When the femtosecond laser irradiates the target, it takes a few femtoseconds for electrons to absorb of the laser energy through photon-electron interactions. Within the duration of a single femtosecond pulse, the change of lattice temperature is generally negligible. Then the energy will be distributed from the electron to lattice through electron-phonon interactions. The lattice reaches to the same temperature as electron temperature eventually. The evolvement of electron and lattice temperature in this section is consistent with many experiments [26, 51, 52]. 2.3.2. The numerical solution of electron-phonon relaxation time As shown in Fig.2, it presents the evolvement of electron and lattice temperature of the surface along with time for the copper target irradiated by a 100f s, 800nm pulse at 0.4J/cm2. Along with increasing time, the electron temperature rises firstly, then it suddenly decreases when it reaches the maximum. However, the temperature of lattice firstly increases gradually, then it reaches to the same temperature as electron temperature. The electron-phonon relaxation time τep is the time for the electrons and the lattice to reach thermal equilibrium. The electron-phonon relaxation time τep ≈ 2ps can be obtained from Fig.2.

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Figure 3. The time-dependence of electron and lattice temperature of the surface for different pulse width with pulse fluence of 0.75J/cm2 and laser wavelength of 800nm. 2.3.3. Effect of pulse width of femtosecond laser on the electron-phonon relaxation time The evolvement of electron and lattice temperature of the surface along with time for different pulse width is shown in Fig.3. The time used by the electron temperature reaching the maximum is the corresponding pulse width. For the same pulse fluence, the shorter the pulse width, the bigger the electron temperature maximum is, and the shorter the electronphonon relaxation time is. Because the total incident laser energy is fixed, the laser energy per unit time irradiating the target becomes large for short pulse width. Absorbing much more laser energy per unit time, the electron temperature will be higher, and the corresponding electron kinetic energy will be bigger. Then the effective electron-ion collision frequency νei becomes higher. The time for energy transfer from electron to ions is expressed as tenergy = (νei me /mi )−1 [17]. We can obtain from this equation that the higher the effective electron-ion collision frequency, the shorter the time for energy transfer from electron to ions is. As a result, the electron-phonon relaxation time becomes short for short pulse width. 2.3.4. Effect of fluence of femtosecond laser on the electron-phonon relaxation time As shown in Fig.4, it presents the evolvement of electron and lattice temperature of the surface along with time for different laser fluences. For higher laser fluence, equilibrium temperature of electron and lattice becomes higher accordingly. It is obvious that the ultimate temperature of the target will be higher when it absorbs much more energy. However,

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Figure 4. The time-dependence of electron and lattice temperature of the surface for the copper target irradiated by a 100f s, 800nm pulse at different laser fluences. the lower fluence laser, the longer the electron-phonon relaxation time is. The reason is that when the high intensity laser irradiates the target, the kinetic energy of the electron will be bigger. The electron-phonon relaxation time becomes longer for low fluence laser. 2.3.5. Heat-affected zone per pulse as a function of laser fluence When considering the transport of absorbed energy into the depth of the material, we have to distinguish two processes occurring in successive time intervals. The first process is electron thermal diffusion by electron-electron collisions and electron-phonon interactions. The second interval is reached when electrons and the lattice stayed in thermal equilibrium, where the common thermal diffusion drives the heat dissipation into the material. Fig.5 shows how the heat-affected zone depends on laser fluence. The experimental data come from Ref. [16]. Considering the effect of pulse width and fluence of femtosecond laser on the electron-phonon relaxation time, the numerical solution of electron thermal diffusion length is obtained. The numerical solution of the heat-affected zone is plotted in Fig.5. It is clear that the simulated results are proper to describe the ablation process. In section II, the effect of pulse width and fluence of femtosecond laser on the electronphonon relaxation time is studied depending on TTM. As an example of copper target, the numerical solutions are obtained by solving TTM with a finite difference method. For a certain laser fluence the shorter the pulse width, the shorter the electron-phonon relaxation time is. However, the electron-phonon relaxation time becomes long for low laser fluence when the pulse width is fixed. Furthermore, a quite good agreement of numerical results with the experimental data has been obtained. This suggests that it is significant to in-

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171

Figure 5. Dependence of the thickness of heat-affected zone following the laser fluence. vestigate the effect of pulse width and flunce of femtosecond laser on the electron-phonon relaxation time.

3.

Improved Two-temperature Model and its Application in Femtosecond Laser Ablation of Metal Target

This section presents a unified thermal model, which can describe the thermophysical effects with laser pulse width ranges from nanosecond to femtosecond. Take gold target as an example, the numerical solutions are obtained from the unified model using a finite difference method. The temperature distribution of the electron and the lattice along with space and time at a certain laser fluence is presented. The time-dependence of lattice and electron temperature on the surface for different laser fluence is also performed. The satisfactory agreement between our numerical results and experimental results of vaporization threshold indicates that the unified thermal model is correct and reasonable.

3.1.

The Effect of Temperature on Heat Capacity and Thermal Conductivity of the Electrons

Normally, the thermophysical properties of electron such as the heat capacity and thermal conductivity of the electrons are all temperature dependent. The electron heat capacity is usually a linear function of the electron temperature, given by Ce (Te) = Ce0 Te ,

(9)

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where Ce0 being a constant. The electron thermal conductivity can be described by κ(Te, Tl) = Ce υF2 τ /3,

(10)

where υF is the Fermi velocity, and τ is the electron momentum relaxation time. For good conductors, such as metals, the electron-electron collision frequency can be determined by υe−e = ATe2, whereas the electron-phonon collision frequency is proportional to Tl, namely, υe−ph = BTl . Here A and B are constants, and both contribute to the electron collision frequency υ. A relationship between the electron momentum relaxation time τ and the electron-electron and electron-phonon collision frequency for electron temperature below the Fermi temperature is given by [53] 1 = υ = υe−e + υe−ph = ATe2 + BTl . τ

(11)

Substituting Eq. (11) into the expression of the electron thermal conductivity, we can obtain the following equation:

κ(Te, Tl) =

BTe 1 υF2 Ce0 Te = κ0 , 3 ATe2 + BTl ATe2 + BTl

(12)

where κ0 = υF Ce0 /(3B) is the electron heat conductivity at room temperature ( Te = Tl = 300K).

3.2.

Electron Temperature Dependences of the Absorption Coefficient and the Absorptivity

It is assumed that a pulsed laser beam irradiates on a target surface vertically. The electric field intensity in the target can be described as follows:

E = E0 exp[iω(

x − t)], υ

(13)

E0 is the electric field intensity at x = 0, ω is the incident laser angle frequency, υ is the laser propagation velocity in target satisfying the following relation: υ = c/nc ,

(14)

where c is the laser propagation velocity in vacuum. nc is the complex refractive index, which can describe the optical characters of material, nc = n + iκ,

(15)

where n is the refractive index, κ is extinction coefficient. Substituting Eqs. (14) and (15) into Eq. (13) yields E = E0 exp(−iωt) exp(−iω

xn xκ ) exp(−ω ). c c

(16)

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By the operation of module to Eq. (16), we have |E|2 = EE ∗ = |E0|2 exp(−2ω

xκ ). c

(17)

According to Lambert-Beer-Bouguer Law, when laser irradiates the target surface, the laser intensity attenuates in exponent in the target, which can be expressed by [54] I(x, t) = βI(0, t) exp(−bx),

(18)

I(0, t) is the intensity of the incident laser, β is the absorptivity of the target, b is the absorption coefficient of the target. In the target, the intensity equals to the square of the module of the electric filed intensity, namely |E|2 = I(x, t). Based on this relation, comparison of Eq. (17) with Eq. (18) gives the following relation: b=

2ωκ 4πυκ 4πκ = = , c c λ0

(19)

where λ0 is the laser wavelength in vacuum. It is well known that Maxwell equation in the isotropical medium can be described by[55] ¯ ¯ ×E ¯ = −µµ0 ∂ H , 5 ∂t ¯ ∂ ¯ ×H ¯ = σE ¯ + εε0 E , 5 ∂t ¯ ·H ¯ = 0, 5 ¯ ·E ¯ = 0, 5

(20)

where µ and µ0 are the magnetic conductivities of the target and vacuum. σ is the target conductance, ε is the target relative permittivity, ε0 = 8.85 × 10−12 C 2 N −1 m−2 is the permittivity of vacuum. Substituting Eq. (16) into Eq. (20), we obtain the refractive index n and the extinction coefficient κ as the functions of the conductance σ and the permittivity ε as follows: σ 1 n2 = ε{[1 + ( )]0.5 + 1}, 2 ωεε20 1 σ )]0.5 − 1}, κ2 = ε{[1 + ( 2 ωεε20

(21)

where 0 = 8.85 × 10−12C 2 N −1 m−2 , is the permittivity of vacuum. The conductance of general metal is generally in the range ∼ 108 − 1012Ω−1 m−1 , the relative permittivity is in the range 0-100, and in the the conditions of this section, the laser angle frequency is 2 × 1015, we can regard that (

σ 2 )  1. εε0 ω

(22)

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Ranran Fang and Hua Wei So Eq. (13) can be simplified into n'κ'

r

σ , 2ωε0

(23)

σ can be presented as a function of electron temperature Te [56] σ(Te) =

σ0 , 1 + α(Te − T0)

(24)

where σ0 is the target conductance at initial temperature T0, α is the target temperature coefficient of resistance. For general metal, α is in the range ∼ 400×10−5−700×10−5K −1 [57]. Substituting Eqs. (23) and (24) into Eq. (19) yields s r 4πσ 4πσ0 4πk (25) = . b(Te ) = = λ0 ε0 λ0c ε0 λ0c[1 + α(Te − T0 )] This is an important relationship that we derived in this section. From this formula, it can be found that the absorption coefficient monotonously decreases following target temperature when the resistance temperature coefficient and irradiated laser wavelength are both constants. With the same point of view, we discuss the absorptivity, namely the ratio of the absorbed intensity to incident intensity. When a laser irradiates a target surface vertically, the target absorptivity is generally described as follows[56]: β = 1−R = 1−

(n − 1)2 − κ2 2κ2 = , (n − 1)2 + κ2 (n − 1)2 + κ2

(26)

where R is the target reflectivity. Substituting Eqs. (22) and (24) into Eq. (25) gives r

β(Te ) = 2

4πυε0 = σ

s

4πcε0(1 + α(Te − T0 )) . λ0σ0

(27)

This is another important relationship that we derived indicating the relation between the resistance temperature coefficient, the laser wavelength, the target electron temperature and the absorptivity. S(x, t) is the heat source, which can be expressed as follows: S(x, t) = bβI(0, t) exp(−bx).

(28)

In experiments, the laser beam indeed has a transverse intensity profile. Transverse mode describes the energy distribution of laser spot. Transverse mode is divided into base transverse mode and high-order transverse mode. Beam quality of the base transverse mode

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is the best. Base transverse mode is Gaussian beam. Gaussian pulse can be used to accurately describe the incident laser intensity [18, 19, 17]. In our section, we adopt the Gaussian pulse to describe the incident laser: I(0, t) = I0 exp[−

(4 ln 2)t2 ], (τp)2

(29)

here I0 is the maximal laser power density. Substituting of Eqs. (25), (27) and (29) into Eq. (28), we get the heat source as follows: S(x, t) = bβI(0, t) exp(−bx) =

4πI0 (4 ln 2)t2 4πσ0 exp[− ] × exp[−[ ]0.5 x], λ0 (τp )2 ε0 λ0 c[1 + α(Te (x, t) − T0 )] = C exp[−

(4 ln 2)t2 D ]0.5 x], ] × exp[−[ (τp )2 1 + α(Te (x, t) − T0 )

(30)

where C = 4πI0/λ0, D = 4πσ0/ε0 λ0c. The effects of the copper d-band on femtosecond laser absorption are discussed in literature [18]. Copper has a fully-occupied d-band, a few electron-volts below the Fermi level. The Fermi level is located in the half-occupied, nearly free-electron (NFE) s-band, 7-8 eV above the bottom of the conductivity band. In copper, the d-band is 3-4 eV wide, and its upper edge lays 2.0-2.2 eV below the Fermi level. Thus, the presence of the d-band does not affect the NFE character of copper for the 800nm central wavelength of the laser at electron temperature below 1 eV. It is acceptable that it need not to consider the d-band absorption for the 800nm central wavelength of the laser at electron temperature below 1 eV [18].

3.3.

Verification of Absorption Coefficient Analytic Approximation

In order to verify the validity of the absorption coefficient analytic approximation, we present a comparison between the experimental results [19] and theoretical predictions for the absorption. The theoretical predictions include theoretical data of the Maxwell equations and theoretical data of our analytic approximations. The following are Maxwell equations for radiation propagation in the medium [18], ¯ = 0, ¯ ×H ¯ + i ω ε(x, t)E ∇ c (31) ¯ = 0, ¯ ×E ¯ − i ω µH ∇ c Q(x, t) =

1 Re{σ(x, t)} · |E(X, T )|2, 2

(32)

4πσ(x, t) (33) , ω where ε(ω, x) is the dielectric permittivity of the target at the incident radiation frequency ω, and σ(x, t) is the conductivity of the target. The absorption coefficient A is evaluated as [19] R 2τ R ∞ dt Q(x, t)dx A = 0 R 2τ0 (34) I(t)dt 0 ε(ω, x) = 1 + i

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Figure 6. Comparison of the experimental results with the theoretical predictions for 800 nm wavelength. Fig.6 presents a comparison between the experimental results and the theoretical predictions for the absorption of λ = 800nm wavelength, 50 fs FWHM duration laser pulses in high-quality commercial Cu targets[19]. Solid curve represents the theoretical data of the Maxwell equations. Dash dot curve indicates the theoretical data of our analytic approximations. Fig.6 shows that two different theoretical curves are both consistent with the experimental data. The two theoretical curves and the experimental curve show the increase movement in absorption. However, there is a big difference between the theoretical data of our analytic approximations and the experimental data after laser intensity exceeds 3 × 1014W/cm2. From the above analysis, we can clearly see that our analytic approximations are accurate when laser fluence is not very high (0.2 − 0.4J/cm2). Our analytic approximations are not very well for high fluence, it should be upgraded to obtain better approximations in the following research.

3.4. The Improved Two Temperature Model The original two-temperature model was formulated by S.I. Anisimov et al[58]. The following is the improved two temperature model, which considers the effect of electron temperature on the electron heat capacity, electron thermal conductivity, absorption coefficient and absorptivity.

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∂ (4 ln 2)t2 BTe ∂2 Te − g(Te − Tl ) + C exp[− ] Te = κ0 2 2 ∂t ATe + BTl ∂x (τp)2 D × exp[−[ ]0.5x], 1 + α(Te (x, t) − T0)

(35)

Ce0 Te (Te )

Cl

∂ Tl = g(Te − Tl ). ∂t

(36)

For the Eqs. (27) and (28), the initial condition is Te (x, 0) = Tl (x, 0) = T0,

(37)

and the boundary conditions can be expressed as: −ke

∂Te |x=0= (1 − R)I0 (t), ∂x

(38)

∂Te (39) |d = 0, ∂x where T0 = 300K is the initial temperature which is uniform across the target, d is the depth of the target. −ke

3.5. Results and Discussion Under the initial condition (37) and the boundary conditions (38) (39), Eqs. (35) and (36) are numerically solved by a finite difference scheme. The thermal and optical properties of gold in Table 3 are adopted from [59]. Table 2. Thermal and optical properties of copper coefficient of the electron heat capacity (Ce0 ) lattice heat capacity (Cl ) e-ph coupling coefficient (g) electron heat conductivity at room temperature (κ0 ) coefficient of e-e collision frequency (A) coefficient of e-ph collision frequency (B) target temperature coefficient of resistance (α)

96.6J/m3 K2 3.5 × 106 J/m3 K 1 × 1017W/m3 K 400W/mK 1.75 × 107K−2 · S−1 1.98 × 1011K−1 · S−1 4.3 × 10−3K−1

3.5.1. The Time-dependence of Electron and Lattice Temperature of Target The temporal evolution of the electron and lattice temperatures at the metal surface for different laser fluence is shown in Fig.7. The maximum temperature of the electron temperature at the surface is in the range 5500 − 8000K for the laser fluence varying from

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Figure 7. Temporal evolution of electron temperature (a) and lattice temperature (b) at the surface for the copper target irradiated by a 100f s, 800nm pulse at 0.2J/cm2 , 0.3J/cm2 , 0.4J/cm2 , respectively. 0.2 − 0.4J/cm2. The surface electron and lattice temperature increase with increasing laser fluence. For a fixed laser fluence, the electron temperature firstly rapidly increases along with the ablation time, while suddenly decreases when it reaches the maximum in Fig.7(a). And the temperature of the lattice raises gradually in Fig.7(b). 3.5.2. Ablation rate per pulse as a function of laser fluence In order to argue the correctness of the improved two-temperature model in this section, we compare our calculated ablation rate with the experimental data and the result of the model without considering electron temperature-dependent heat capacity, thermal conductivity of the electron, absorption coefficient and absorptivity, shown in Fig.8. Curve 1 represents how the ablation rate depends on laser fluence based on our model Eqs. (30) and (31). Curve 2 stands for the model without considering electron temperature-dependent heat capacity, thermal conductivity of the electron, absorption coefficient and absorptivity. In our simulation, it is assumed the thermal conductivity of electron to be 318W/mK, specific heat of electron to be 67.7J/m3K, the absorptivity to be 0.06, and the absorption coefficient to be 7.88 × 105/m [47]. In the experiment, the intensity autocorrelator and the powermeter at the exit of the amplifier periodically monitored the drifts in the pulse width and output energy in order to ensure a stable pulse energy on the Cu target. An 80 mm focal length fused silica convex lens is used to focus the attenuated laser pulses onto the Cu substrate. The pulse width and the center wavelength are 100fs and 800nm, respectively. The ablation rate is estimated as an averaged value over 300 sbsequent pulses. The ablation depths after laser ablation are measured using a mechanical stylus (Dektak-3030)[16]. In the simulation, the pulsewidth τp and the center wavelength λ0 are 100fs and 800nm in the the heat source Eq.(22). The range of the maximal laser power density I0 is from 0.1J/cm2 to 2J/cm2 . The details on how the ablation dynamics was modeled are given as follow: When the femtosecond laser irradiates the target, it takes a few femtosecond for electrons to absorb the

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Figure 8. Dependence the ablation rate on the laser fluence, curve ◦ (numerical data 1) represents the improved model, see text in detail. laser energy through electron-photon interactions. Within the duration of a single femtosecond pulse, the change of lattice temperature is generally negligible. Then the energy will be distributed from the electrons to lattice through electron-phonon interactions. The lattice reaches to the same temperature as electron’s temperature, eventually. When the electron temperature and lattice temperature are equal, the common thermal diffusion drives the heat dissipation into the material. The electron temperature and the lattice temperature are both functions of target depth x and time t, that is to say, T = T (x, t). x refers to the spatial coordinate in the direction perpendicular to the target surface. The ablation rate is the depth x when the lattice temperature reaches the boiling point, 2811 K for copper in the simulation [16]. Fig.8 displays the relationship between the ablation rate and laser fluence. The ablation rate is estimated from the depth which becomes heated above the boiling point, 2811 K for copper in the simulation [16]. Both curves 1 and 2 ascend with the increasing time. As curve 1 takes into account the effect of the variation of heat capacity, thermal conductivity of the electron, the absorption coefficient and the absorptivity, it is obviously higher than curve 2 for the increment of absorbed energy. The experimental data come from Ref.[16]. Obviously, this figure shows a better agreement between our model (curve1) and the experiment than curve 2. Hirayama and Obara said that the MD simulation results are not well consistent with the experimental results of the ablation rate in their paper[16]. They also explained that the difference in the ablation rate between the experiment and the simulation may be due to optical parameters of the material, especially the absorption coefficient which has been

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considered as constant. They indicated that the absorption coefficient may be changed due to the heating up of the laser-irradiated surface[16]. In our section, based on the electron-phonon and electron-electron collision frequency, we can obtain the temperature-dependent heat capacity and thermal conductivity of the electron. The electron temperature-dependent absorption coeffcient and absorptivity can be deduced through a combination of optical properties of solids and Maxwell’s law. The temperature-dependent heat capacity and thermal conductivity of the electron, as well as electron temperature-dependent absorption coeffcient and absorptivity are all considered in this improved two-temperature model. The tailored two-temperature model is solved using a finite difference method. It is very easy to precisely define the initial and boundary conditions in the finite difference method. The simulation results of the ablation rate are consistent with the experimental results. In section III, in order to describe femtosecond laser ablation of metal target, an improved thermal model considering electron temperature-dependent heat capacity, thermal conductivity of the electron, absorption coefficient and absorptivity has been developed in this section. The present model is used to simulate femtosecond laser ablation of copper target. The satisfactorily good agreement between our numerical results and experimental results of ablation depth confirms that the tailored model is a much more satisfactory theoretical framework in femtosecond laser ablation of metal target. The analysis to the ablation characteristics is important to understand the basic mechanisms involved in the femtosecond laser-target interaction. We hope the present model will be helpful for the experimental investigation of the application of femtosecond laser.

4.

A Unified Thermal Model of Thermophysical Effects with Pulse Width from Nanosecond to Femtosecond

This section presents a unified thermal model, which can describe the thermophysical effects with laser pulse width ranges from nanosecond to femtosecond. Take gold target as an example, the numerical solutions are obtained from the unified model using a finite difference method. The temperature distribution of the electron and the lattice along with space and time at a certain laser fluence is presented. The time-dependence of lattice and electron temperature on the surface for different laser fluence is also performed. The satisfactory agreement between our numerical results and experimental results of vaporization threshold indicates that the unified thermal model is correct and reasonable.

4.1. The Unified Model 4.1.1. Physical background When laser irradiates the target, the first stage is the absorption of the laser energy through photon-electron interactions. It takes a few femtoseconds for electrons to reestablish the Fermi distribution. The second stage is the energy distribution to the lattice through electronphonon interactions, the characteristic scales of which is called the electron-phonon coupling time, typically on the order of tens of picoseconds [20].

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For nanosecond laser, the pulse width of which is much more than the electron-phonon coupling time. This kind of ablation is called thermal equilibrium ablation. However, the ablation undergoes nonequilibrium process for the femtosecond laser due to that the pulse width is less than the electron-phonon coupling time. As for the picosecond laser ablated process, the corresponding mechanism is very complex due to both of the two ablation mechanisms are coexisting. We can learn that the ratio of the electron-phonon coupling time and laser pulse width is the important criteria. As τR  τL, i.e., exp[−a ττRL ] → 0, the ablation is nonequilibrium ablation. Our unified model changes into the simplified two temperature model which can represent the phenomenon of nonequilibrium ablation. When τR  τL , i.e., exp[−a ττRL ] → 1, it is equilibrium ablation. The unified model can be transferred into the classical equation of one-dimensional heat-conduction, and the equilibrium ablation can also be described by this model. However, if τR ∼ τL , i.e., 0 < exp[−a ττRL ] < 1, it is the mix ablation including equilibrium and nonequilibrium ablation, the complicated ablation mechanism can be presented by our unified model as well. Therefore, an exponential term is inserted in the TTM model to make it valid from fs to ns pulse duration. 4.1.2. The contents of our model The following equations are the unified thermal model, which can describe the thermophysical phenomenon of laser ablation process with laser pulse width ranges from nanosecond to femtosecond:

Ce

∂ τR ∂ 2 Te = ke exp[−a ] 2 Te − g(Te − Tl) + (1 − R)αb I0 (t) exp(−αb x), ∂t τL ∂x

(40)

∂ (41) Tl = g(Te − Tl ), ∂t where Te and Tl are the electron and lattice temperature, respectively. Ce and Cl are the specific heat of electron and lattice. ke is the thermal conductivity of electron. g is the electron-phonon coupling coefficient. R and αb are the reflectance and the absorption coefficient of the target, respectively. a is an undetermined coefficient, which is variant for different material. The value of a is determined by fitting theoretical results to experimental data. The effect of the thermal excitation of electrons on the thermophysical properties is sensitive to the details of the electron structure of the target material. However, when Te is below 3000K, the influence of the thermal excitation of electrons on the Ce and ke is very little [60]. On the other hand, when we deal with melting, and in order to simplify the problem to stress the unified model, it is necessary to assume Ce and ke are independent of the electron temperature. Since in metal the thermal conduction is dominated by electron, the diffusion term can be neglected for lattice in Eq. (41) [10]. In order to describe the incident laser intensity more accurately, we adopt a Gaussian pulse of duration τp (FWHM) with an intensity [11]: Cl

I0 (t) = I0 exp[−

(4 ln 2)t2 ], (τp )2

(42)

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here I0 is the maximal laser power density. When τR  τL , i.e., exp[−a ττRL ] → 0, Eqs. (40) and (41) can be transferred into Ce

∂ Te = −g(Te − Tl ) + (1 − R)αb I0 (t) exp(−αb x), ∂t

(43)

∂ (44) Tl = g(Te − Tl ), ∂t they are the equations of the simplified two temperature model (TTM). For the long pulses, the ablation is thermal equilibrium process, i.e., Te = Tl = T , so Eq. (41) can be neglected. Under the condition that τR  τL , i.e., exp[−a ττRL ] → 1, Eq. (40) can be changed into Cl

Cv

∂ ∂2 T = k 2 T + (1 − R)αb I0 (t) exp(−αb x), ∂t ∂x

(45)

where Cv = Ce +Cl , k and T are the specific heat, the thermal conductivity and the temperature of the target, respectively. Eq. (45) becomes the classical equation of one-dimensional heat-conduction. The first one of the right part in Eq. (45) is the heat-conduction term of the target. When 0 < exp[−a ττRL ] < 1, it is the mix process including equilibrium and nonequilibrium ablations. Therefore, the unified thermal model can well describe the whole thermal conduction phenomenon with laser pulse width ranged from nanosecond to femtosecond timescale. For the Eqs. (40) and (41), the initial condition is Te (x, 0) = Tl (x, 0) = T0,

(46)

and the boundary conditions can be expressed as follows, −ke

∂Te |x=0= (1 − R)I0 (t), ∂x

(47)

∂Te (48) |δ = 0, ∂x where T0 = 300K is the initial temperature which is uniform across the target. δ is the thermal diffuse depth. In terms of the low-fluence ablation case, it has been considered that the number density of the hot electrons is low enough that the energy transfer occurs only within the area characterized marked by the skin depth δ = 1/αb . However, if the laser q −ke

fluence is very high, the electron thermal diffusion length should be δ =

ke Ce τR

[49] .

4.2. Results and Discussions Under the initial condition (46) and the boundary conditions (47) (48), Eqs. (40) and (41) are numerically solved by a finite difference scheme. The thermal and optical properties of gold in Table 3 are adopted from Refs. [53, 49, 20, 5].

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Table 3. Thermal and optical properties of gold wavelength(λ) thermal conductivity of electron (ke ) specific heat of electron (Ce ) specific heat of lattice (Cl ) absorption coefficient (αb ) reflectance (R) e-ph coupling coefficient (g) the electron-phonon coupling time ( τR )

1053nm 318W/mK 67.7J/m3K 2.30 × 106J/m3K 7.88 × 105m−1 0.94 16 2.1 × 10 W m−3 K −1 6ps

Figure 9. Damage threshold fluences versus pulse width for the gold target irradiated by 1053nm pulse(experimental and computed modeling results). 4.2.1. Determination of the value of a Damage threshold fluences versus the pulse width for gold is presented in Fig.9. The pulse width dependence of threshold fluences measured by Stuart et al. on a 200-nm-thick gold with laser pulses is generated by a 1053nm Ti:sapphire CPA system. The front end of the system produced 1-ns stretched pulses as much as 60 mJ at 10 Hz in a T EM00 Gaussian mode. The pulses were then compressed in a single-grating compressor of variable length. Pulses of continuously adjustable duration from 300 fs to 1ns can be obtained by varying the dispersive path length of the compressor. The energy delivered to the damage sample was adjusted with a half-wave plat before compression. The energy of each pulse was recorded

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from calibrated leakage through a mirror. After irradiation, Nomarski microscopy was used to inspect the sample for damage. The definition of damage includes any visible permanent modification to the surface observable with the Nomarski microscopy [1]. The simple one-dimension heat-conduction model is used to predict the dependence of damage threshold on the laser pulse width[1]. The data of the damage threshold only can be obtained by the heat-conduction model including the heat source term. But the heat source term is not included in this heat-conduction model, so the damage thresholds can not be given by the model. However, our unified model includes the heat source term and also describes the phenomenon of two temperature in the electron and lattice for ultrashort pulse duration. From Fig.1, we can find the calculated data of damage threshold from our unified model increase with the increment of laser pulse width. Especially for ultrashort pulse duration, our unified model can give the evolvement of damage threshold along with the laser pulse width. By assuming the damage starts when the maximum lattice temperature reaches the melting point, 1337.33K for gold [20] in the simulation, the value of a can be determined as 0.67 through fitting the experimental results with computed results. The absorptance A = 1 − R of a pure metal consists of two components, A = AIN T R + ASR . AIN T R is the intrinsic absorptance and ASR is the contribution due to surface roughness. For an optically smooth metal surface, the order of ASR is about 1 − 2% of AIN T R while the role of ASR enhances as the surface roughness increases. In Fig.9, each data point originates from typically 600 shots on one spot. However, for multipulse ablation, only the first laser pulse interacts with an undamaged surface. All the subsequent laser pulses interact with a structurally modified surface and their absorption is determined by both AIN T R and ASR . For low pulse duration (i.e., less than 10 ps), the modified surface is obscure, the value of ASR is very small. On the other hand, the modified surface is obvious for larger pulse duration, so the value of ASR becomes bigger accordingly when pulse width is longer than 10 ps [30]. In our simulation, we only take into account the intrinsic absorptance AIN T R without considering ASR . So for pulse duration longer than 10 ps, the theoretical data are a little bit higher than the experimental ones. For low pulse duration (i.e., less than 10 ps), the electron temperature is much higher within the ultrashort pulse duration. Within the duration of a single ultrashort pulse, the change of lattice temperature is generally negligible. Both Ce and ke are functions of the electron temperature, Ce = Ae Te and ke = Ke,0 Te /Tl. The constants Ae = 71Jm−3 K −2 and Ke,0 = 318W m−1K −1 is for the gold target [10]. In our simulation, it is assumed that Ce and ke are independent of the electron temperature. Due to the constant electron heat capacity and electron heat conductivity, the electron temperature in the surface region of the irradiated target can be transiently brought to higher values [60]. Therefore, the theoretical data are always below the experimental ones. 4.2.2. The evolvement of vaporization threshold fluence with laser pulse width As shown in Fig.10, it presents the relations of vaporization threshold fluence and laser pulse width. In the experiment, the threshold for vaporization was studied across the pulse range from 150f s to 7ns using laser radiation at 770 nm. Such a laser (770 nm) was used in the experiment where the oscillator is pumped with an argon-ion laser and the amplifier with a doubled Nd:YAG operating at 10 Hz repetition rate. The energy of each pulse is measured

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Figure 10. Vaporization threshold fluence versus pulse width for the gold target irradiated by 770nm pulse(experimental and computed modeling results). by the photodiode and is stored along with the integrated optical emission from that pulse. Experimental vaporization threshold, as a function of pulse width, is determined by the extrapolation technique. The extrapolation technique produces a threshold that is defined as the fluence needed to bring the system up to, but not actually at, the onset of electronically excited vapor as observed with a photomultiplier tube (PMT) [5]. The vaporization will start when the maximum lattice temperature reaches the vaporization temperature ( 2873K for gold in the simulations). The calculated data of vaporization threshold on the laser pulse width are presented by calculation of the one-dimension heat-conduction model including the heat source term [5]. Nonequilibrium process between electrons and lattice is already significant for ultrashort pulses, in which the electron temperature can be much higher than the one of the lattice [20]. The corresponding thermophysical effects must be described by TTM. The data calculated by the one-dimension heat-conduction model are not change with the pulse width for ultrashort pulses. However, the experimental data vary with the increment of the pulse width. The calculated data from our unified model increase with the pulse width. It is shown that the theoretical results accord with the experimental data approximately. It can be proved that the unified thermal model is available to describe the thermal phenomenon of laser ablation with laser pulse width ranges from nanosecond to femtosecond. In section IV, a unified thermal model which can describe the thermophysical phenomenon with laser pulse width ranges from nanosecond to femtosecond is presented in this section. As an example of gold target, the numerical solutions are obtained by solving the heat flow equations using a finite difference method. The parameter of the unified

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thermal model is determined through comparing theoretical and experimental value of damage threshold fluence with laser pulse width ranges from nanosecond to femtosecond time regime. The temperature of electron and lattice which is dependent on space and time in target at a certain laser fluence is presented. The time-dependence of lattice and electron temperature for different laser fluence is performed. The satisfactory agreement between our numerical results and experimental results of vaporization threshold fluence indicates that the unified thermal model is reasonable.

5.

Conclusion

We have reviewed our recent research work on Thermophysical effects of femtosecond laser ablation of metal target. The effect of pulse width and fluence of femtosecond laser on the electron-phonon relaxation time is studied depending on TTM. In order to describe femtosecond laser ablation of metal target accurately, an improved thermal model considering electron temperature-dependent heat capacity, thermal conductivity of the electron, absorption coefficient and absorptivity has been developed in this chapter. This chapter also presents a unified thermal model, which can describe the thermophysical effects with laser pulse width ranges from nanosecond to femtosecond.

6.

Ackowledgments

This work was supported by the National Natural Science Foundation of China through Grant Nos.10904177,11004257,10804132, Natural Science Foundation Project of CQ CSTC 2010BB9404, and by the Scientific Research Foundation for Doctor of Chongqing University of Posts and Telecommunications No. A2009-15.

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[28] C.K. Sun, F. Vallee, L. Acioli, E.P. Ippen and J.G. Fujimoto, Phys.Rev.B 48 12365 (1993). [29] C.K. Sun, F. Vallee, L. Acioli, E.P. Ippen and J.G. Fujimoto, Phys.Rev.B 50 15337 (1994). [30] A.Y. Vorobyev and C.L. Guo, Phys.Rev.B 72, 195422 (2005). [31] Arun Inam, X.D. Wu, T. Venkatesan, S.B. Ogale, C.C. Chang, and D. Dijkkanp, Appl.Phys. Lett. 51,1112 (1987). [32] D.M. Zhang, L. Li, Z.H. Li, L. Guan, S.P. Hou and X.Y. Tan, Acta Phys. Sin. 54 1283 (2005). [33] Y.C. Lan, D.V. Tran, and H.Y. Zheng, Laser Part. Beams 25, 155 (2007). [34] D.M. Zhang, L. Li, Z.H. Li, Eur. Phys.J-Appl. Phys. 33, 91 (2006). [35] S. Amoruso, G. Ausanio, R. Bruzzese, L. Gragnaniello, L. Lanotte, M. Vitiello, X. Wang, Appl. Surf. Sci. 252, 4863 (2006). [36] X. Zeng, X.L. Mao, R. Greif, R.E. Russo, Appl. Phys. A 7180, 237 (2005). [37] C.H. Crouch, J.E. Carey, J.M. Warrender, M.J. Aziz, E. Mazur, Appl. Phys. Lett. 84, 1850 (2004). [38] Q.M. Lu, Phys. Rev. E 67, 016410 (2003). [39] Q.M. Lu, S.S. Mao, X.L. Mao, R.E. Russo, Appl. Phys. Lett. 80, 3072 (2002). [40] Q.M. Lu, S.S. Mao, X.L. Mao, R.E. Russo, Proceedings of the SPIE - The International Society for Optical Engineering, 4760, 959 (2002). [41] J.G. Lunney, R. Jordan, Appl. Surf. Sci. 127-129, 941 (1998). [42] S. Amoruso, M. Armenante, R. Bruzzese, N. Spinelli, R. Velotta, and X. Wang, Appl. Phys. Lett. 75, 7 (1999). [43] V. Craciun and D. Craciun, Appl. Surf. Sci. 138-139, 218 (1999). [44] D.M. Zhang, D. Liu, Z.H. Li, Appl. Surf. Sci. 253, 6144 (2007). [45] D.M. Zhang, X.Y. Tan, Z.H. Li, Physica B 357, 348 (2005). [46] R.R. Fang, D.M. Zhang, Z.H. Li, phys.stat.sol(a) 204,(12) 4241 (2007). [47] G.L. Eesley, Phys. Rev. B 33, 2144 (1986). [48] S. Nolte, C. Momma, H. Jacobs, A. T¨ unnermann, B.N. Chichkov, B. Wellegehausen and H. Welling, J. Opt. Soc. B 14, 2716 (1997). [49] K. Furusawa, K. Takahashi, H. Kumagai, K. Midorikawa and M. Obara, Appl. Phys. A 69, S359 (1999).

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[50] N.N. Nedialkov, S.E. Imamova and P.A. Atanasov, J. Phys. D: Appl. Phys. 37, 638 (2004). [51] J.G. Fujimoto, J.M. Liu, E.P. Ippen, and N. Bloembergen, Phys. Rev. Lett. 53, 1837 (1984). [52] T. Hertel, E. Knoesel, M. Wolf, and G. Ertl, Phys. Rev. Lett.76, 535 (1996). [53] J.C. Wang and C.L. Guo, Appl. Phys. Lett. 87, 251914 (2005). [54] P. Andrea, M. Antonio and K. Roger, Phys. Rev. E 50 4716 (1994). [55] S. Guo, Electric Dynamic, Higher Education Publishing Company.(1997). [56] C.K. Sun, The Effect of Laser Irradiation, National Defence Industry Press, Beijing.(2002). [57] B.H. Billings, H.P.R. Frederikse, American Institute of Physics Handbook. 3rd ed., McGraw-Hill, New York.(1972). [58] S.I. Anisimov, B.L. Kapeliovich, and T.L. Perelman, Sov. Phys. JETP 39:375 (1974). [59] H.E. Elsayed-ali, T.B.Norris, M.A. Pessot and G.A. Mourou, Phys.Rev.Lett. 58,1212 (1987). [60] Zhibin Lin, Leonid V. Zhigilei, Appl. Surf. Sci. 253, 6295 (2007).

In: Laser Ablation: Effects and Applications Editor: Sharon E. Black

ISBN: 978-1-61122-466-5 © 2011 Nova Science Publishers, Inc.

Chapter 8

FORMATION OF NANOPARTICLES UNDER LASER ABLATION OF SOLIDS IN LIQUIDS G. A. Shafeev Wave Research Center, Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, Russia

ABSTRACT The process of nanoparticle formation under laser ablation of solids in liquids is described. Critical parameters are discussed that govern the properties of nanoparticles ejected into the surrounding liquid. These parameters are laser wavelength, pulse duration, interaction of individual nanoparticles with laser beam inside the liquid. A review of previous results is presented on the properties of nanoparticles of noble metals. Recent data on laser-assisted generation of other metals is given, including the formation of alloyed nanoparticles. Micro- and nanostructuring of the target upon its laser ablation in liquid environment is discussed. Examples are given of the influence of the surrounding liquid on the chemical composition of generated nanoparticles. The laser control over the size distribution of nanoparticles in liquids is demonstrated either by spatial profiling of laser beam intensity or proper tuning of laser wavelength into plasmon resonance of nanoparticles. Recent results are given on excitation of high energy levels of media under laser exposure to laser pulses of picosecond range of duration.

INTRODUCTION The advent of lasers opened a new branch of research of interaction of radiation with matter. The primary “eye-visible” effect of laser action on a solid target is removing of some material from the target surface within the laser spot. This process was called “laser ablation” from a Latin word ablatio, which means removal. The process of laser ablation of solids in liquids has attracted much attention of researchers during the last decade. This is due mainly to the simplicity of the experimental

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setup. Many modern laboratories (and not only physical ones) are equipped with lasers, and synthesis of nanoparticles is a strong temptation. The process proceeds in one step and results in immediate formation of nanoparticles in the liquid in which the target is immersed. The main feature of the process is that ideally the liquid contains only nanoparticles made of the target material and the liquid. There are no counter-ions or residuals of reducing agents left in the solution. For this reason laser ablation of solids in liquids can be considered as a method of nanoparticle synthesis, which is an alternative to chemical methods. Commercially available laser sources are characterized by a number of parameters, such as peak power, average power, wavelength of emission, pulse repetition rate, etc. If the final purpose of ablation of a target immersed into a liquid is the synthesis of nanoparticles with desired properties, such as their chemical composition, size distribution, concentration, etc., then the above mentioned laser parameters are of different importance to the properties of desired nanoparticles. Also, the nature of the liquid plays a significant role in the final properties of nanoparticles generated under laser ablation. The objective of this text is to outline the relative importance of various experimental parameters to the properties of nanoparticles synthesized by laser ablation of a solid target immersed into a liquid.

General Setup of Laser Ablation in Liquids Figure 1 shows a generalized setup of experiments on laser ablation of solids in a liquid environment. This setup may vary from one research group to another, though their common features are the same. Laser radiation is focused onto a solid target immersed in a liquid. It is assumed that the liquid is transparent at the laser wavelength, otherwise the focalization of the beam would be problematic due to absorption of laser radiation in it. The simplest way is working with the free surface of the liquid, which allows avoiding additional reflection at the interface “covering glass/air”. However, the use of volatile liquids, such as acetone, ethanol, etc., requires covering the liquid with a window that is transparent at the laser wavelength.

Figure 1. Experimental setup for laser ablation of solids in a liquid environment

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EXPERIMENTAL TECHNIQUE The experimental setup for laser ablation in a liquid environment is perfectly simple. It is usually assumed that the liquid that surrounds the solid target is transparent at laser wavelength while the laser radiation is absorbed by the target. A solid target is placed under a thin (several millimeters) layer of liquid and is exposed to laser radiation through this layer. Different pulsed laser sources can be used, e.g., a Nd:YAG laser at 1.06 µm output and its harmonics, a Cu vapor laser, a Ti:sapphire laser, etc. UV excimer lasers are less common, since most liquids and NP absorb in the UV region. The only necessary requirement is that the laser beam is sufficiently powerful to induce local melting of the target. Usually the laser beam is focused onto the target using an appropriate optics to a certain size of laser spot (see Figure 1). In some experiments the target is rotated under the laser beam to avoid the exposure of the same area. Some research uses a sealed-off cell to avoid oxidation of NP by air oxygen, but basically the setup is the same. It is the density of the laser energy (in J/cm2), or so-called fluence, that determines the temperature of the target and the possibility to produce surface melting and eventual generation of NP. The irradiation of the metal surface results in fast removal of the material that is confined to the laser spot. The ejected nanoparticles remain in the liquid that surrounds the target, resulting in formation of a socalled colloidal solution. Unlike real solutions that contain ionic or molecular species, colloidal solutions also contain particles, e.g., NP and clusters. Due to accumulation of NP in the surrounding liquid, their prolonged interaction with laser radiation is possible. Therefore, the thickness of the liquid layer above the target is also an important experimental parameter that may influence the properties of generated NP. Different laser parameters are of different importance to the efficiency of NP generation.

1. Pulse Duration Usually, pulsed laser sources are used for generation of NP in liquids with pulse duration from hundreds of femtoseconds to hundreds of nanoseconds. The NP under laser ablation in liquids are formed owing to sputtering of the molten layer by the recoil pressure of the liquid that surrounds the target. Therefore, the necessary condition of NP synthesis is melting of the target material. In general, the temperature distribution under laser exposure of solids can be found solving a heat conduction equation with corresponding boundary conditions. However, in the case of short laser pulses of the duration mentioned above, the complicated problem of temperature calculation can be significantly simplified. This simplification is based on the fact that the heat diffusion length ld from laser-exposed areas of the target during the duration of the laser pulse tp is small compared to laser spot size d. Indeed, in a typical experiment on pulsed laser ablation d ~ 10 µm, while the heat diffusion length is much less. Assuming that the laser beam has a flat-top profile (typical of excimer lasers, metal vapor lasers, etc.) one may suggest that the absorbed laser energy is spent for heating of the target layer whose thickness is of order of (atp)1/2, where a stands for the heat diffusion coefficient of the target material, while the area of this layer coincides with the laser spot. Unlike to what one might believe, the presence of liquid around the target does not alter noticeably the temperature inside the laser spot. The reason is that the heat diffusion coefficient for liquids is even

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smaller than that of solids. Presence of the liquid may affect only the average temperature rise of the target if the repetition rate of laser pulses is elevated, e.g., of order of 1 kHz and more. Simple heat balance equation leads for the following expression of the temperature T within the laser spot:

T≈

Aj , cρh

where A stands for absorptivity of the target at the laser wavelength, A = 1- R, where R is the reflectivity coefficient at laser wavelength, c stands for heat capacity of the target material, is density of the target material, and h is the heat diffusion length inside the target. One can see that in a quite natural way the temperature is proportional to the energy density of the laser beam, or so called fluence j. The heat diffusion length h depends on the heat diffusivity of the target material:

h ∝ at p , where in turn a = k/c , where k is the heat conduction coefficient of the target, and tp stands for laser pulse duration. The longer is the laser pulse tp, the thicker is the layer of the material which is heated by absorbed laser energy. The above-made estimation of the temperature rise T is made assuming superficial absorption of laser radiation. If α is the coefficient of absorption of laser radiation, then this condition can be written as follows: α-1 << h

Of course some energy is consumed for heating and evaporation of a liquid adjacent to the laser spot, but this energy is small compared to the absorbed one due to low thermal conductivity of liquids. In case of metal targets laser radiation is absorbed by free electrons that transfer their energy to the metal lattice within 3- 5 picoseconds. Virtually no heat exchange with the bulk of the solid target occurs, and the absorbed laser energy is spent for heating of the layer within the absorption depth α-1 of laser radiation. The absorptivity A of the target surface is a complex parameter. For a smooth metallic surface it can be calculated using reference data on both real and imaginary part of a complex dielectric function of the material. However, as soon as the surface of the target is not flat and is characterized by a certain relief, the absorptivity of the target may largely deviate from its theoretical value. This is due to the dependence of absorptivity on the angle of incidence of radiation. Since the target material is dispersed into surrounding liquid as nanoparticles during laser ablation in liquids, the formation of the relief indeed occurs.

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2. Laser Wavelength As soon as laser ablation of metals is considered, any wavelength is appropriate. However, laser radiation can be absorbed by NP that are generated under ablation of the target. The majority of NP absorb in UV region, which imposes certain drawback on the use of excimer UV lasers for generation of NP.

3. Repetition Rate Nanoparticles are ejected from the solid target at each pulse provided that the absorbed laser energy is sufficient to melt it. Therefore, the higher is the repetition rate of laser pulses, the higher is the rate of NP generation. However at high repetition rate the target may be screened from the beam by gas bubbles that have remained from previous pulses. This can be avoided using either a flow cell or elevated velocity of scanning (rotation) of the target.

HISTORICAL REVIEW Formation of nanoparticles under laser ablation of solids either in gas or in vacuum has been extensively explored during the last decade. Understanding of the mechanisms of cluster formation is needed to control the process of Pulsed Laser Deposition (PLD) that is widely used now for deposition of a large variety of compounds. Formation of nanoparticles under laser ablation of solids in liquid environment has been studied to a lower extent. A number of wet chemical methods of nanoparticles (NP) preparation are known up to date. Laser ablation of solids in liquid environment emerged as an alternative technique that is capable of producing “pure” NP without counter-ions and surface-active substances. Primary motivation for generation of NP by laser exposure of solid targets in liquids was the hope that pure NP would be more efficient in the process of Surface Enhanced Raman Scattering (SERS), since no other compounds can obscure the surface of a metallic NP [1-5]. Formation of both large variety of NP under laser ablation of corresponding solids has been reported during the last decade [6-17]. Further studies showed that the laser ablation in liquids is a complex process with a number of experimental parameters. In particular, chemical interaction of ejected nanosized species with hot liquid vapors at high transient pressure results in large diversity of final particles that remain in the liquid. Also, surface tension at the interfaces is dominant force that governs the dynamics of interaction of molten target material with vapors of surrounding liquid on a nanometer scale. Under pulsed laser exposure the liquid also undergoes chemical changes that may be stipulated by ejected NP that act in this case as a catalyst. Initially liquid substance may become supercritical in the vicinity of the target, which alters its reactivity and opens new channels of chemical reactions especially in case of aqueous solutions. Interaction of metal nanoparticles with laser light proceeds via its absorption by free electrons. Free electrons exhibit plasmon resonance, whose position is determined by both their concentration, their effective mass (that depends on the solid) and particle size. NP of noble metals, e.g., Au or Ag, show a strong selective absorption in the visible, so the suspensions of these NP in liquids are colored. Plasmon resonances of other metallic NP are

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situated in the UV region [18, 19]. Though if the liquid is transparent at the onset of laser exposure of the target, the appearance of NP may change this situation, and the laser radiation is absorbed by the colloidal solution. This may lead to poor control of the laser spot size on the target due to either thermal lensing inside the solution or different non-linear phenomena induced by laser beam in NP [20-22]. So, the thickness of the liquid layer above the target is also one of the experimental parameters that are essential during ablation in liquids. Depending on the dwell time of the laser beam on the target, a more or less significant amount of the target material is transferred into the surrounding liquid as NP. The target relief is therefore modified, and a crater is formed in the target in case of its exposure to a stationary laser beam. To avoid the formation of the crater the target is either rotated or is scanned under the laser beam. However, a periodic relief arises on the target surface at sufficiently high number of laser shots of order of 104. The structures are densely packed micro-cones, and their period linearly varies with the size of laser spot on the target as shown in Figure 2 [23]. These micro-cones are separated by long channels with high aspect ratio that run deep inside the target body. The formation of these micro-structures alters the properties of NP generated by laser ablation, since the conditions of vapor expansion inside the channels are different from those on a flat solid-liquid interface. In other words, the molten areas of the target are no longer on its surface but are situated mostly in the channels. Indeed, the microcones are oriented in a way to reflect the incident laser beam. Therefore, the properties of NP generated from a smooth target surface at the onset of laser exposure are different from those at later stages when the micro-relief has been formed. The cross section of a brass target subjected to radiation of a Cu vapor laser in ethanol is presented in Figure 3. One can see deep channels that separate the microstructures formed in the target upon laser ablation. The melt is formed predominantly on the bottom of channels due to both increased angle of incidence of laser radiation compared to a flat surface and to reflection of laser light from side surface of micro-cones. The molten layer is pushed along the cone surface and is partially dispersed into surrounding liquid as NP. Other part of melt solidifies on cone surface forming in some cases typical tips on their tops as can be seen in Figure 2.

Figure 2. SEM top view of a brass target produced by scanning beam of a Cu vapor laser. Inset shows the enlarged view of the same surface tilted for 35°. Scale bar denotes 20 µm and 10 µm in inset

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Figure 3. SEM view of a cross section of a brass target subjected to laser beam of a Cu vapor laser under the layer of ethanol. Space bar denotes 20 µm

Formation of periodic micro-relief on the target surface inexorably alters the character of temperature distribution along its surface during the laser pulse. The temperature distribution is smooth on initial flat target surface, and in case of a flat top profile of laser beam intensity the temperature profile is also almost flat except for side effects. As soon as periodic microrelief is formed, the temperature profile consists of numerous “hot spots” located in the channels between micro-cones. The energy density (fluence) inside these spots exceeds by far the fluence on the initial surface without micro-relief. Since the size of NP depends on the fluence, the size distribution of NP generated via laser ablation of the micro-structured target should be different from that generated at the first stages of laser ablation when the target is still flat. Unlike laser ablation in vacuum where synthesized NP rapidly leave the laser beam and never come back due to sticking on the chamber walls, NP synthesized by ablation in liquids remain in it and may return into the laser beam during their motion. The efficiency of coupling of radiation to nanoparticles depends on the proximity of laser wavelength λ to plasmon resonance of charge carriers. The energy from electrons to the lattice is transferred within 3-5 ps [24], and the temperature T of a nanoparticle of radius R can be estimated on the basis of conventional heat diffusion equation [25]. For 8πkR/l <<1 one obtains T ~ 2πI0kR2/λkliq, that is the temperature of the nanoparticle in the laser beam is proportional to its geometric cross-section. Here kliq stands for the thermal conductivity of the surrounding liquid which is assumed to be constant during its evaporation for the sake of simplicity. Note that due to the small size of NP their temperature is proportional to the peak power density of the laser beam I0 (Watts per square centimeter). The extinction coefficient κ under large detuning from the plasmon resonance is close to that of the bulk metal. However, in the vicinity of plasmon resonance κ=κ (λ) shows resonant behavior, and the temperature T of the particle strongly depends on the laser wavelength. The main difference between laser ablation in vacuum and in liquid environment is a short free path of ablated species in the latter case. Indeed, the formation of NP under laser irradiation of a metal target immersed into a liquid proceeds via local melting of the metal. The adjacent liquid layer is heated to almost the same temperature owing to heat transfer from the metal. The thickness h of this layer can be estimated using the heat diffusion coefficient of a liquid a = 10–3 cm2/s and the diffusion time equal to duration of the laser pulse τp = 20 ns.

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Then the thickness of the liquid layer h ~ (aτp)1/2 = 0.3 µm. This thin layer has a temperature much higher than boiling point of the liquid at normal pressure and is hence in the vapor phase. The pressure in this vapor layer can be roughly estimated as the vapor pressure of the liquid at temperature of the substrate. This means that in typical experimental conditions all liquids are in vapor phase at pressure of the order of hundreds atmospheres. The thickness of the molten layer on the target is about the length of heat diffusion into the metal, hm ~ (amτp)1/2 . am = 1.7 and 0.13 cm2/s for Ag and Au, respectively, which gives the heat diffusion length of 1.9 µm for Ag and 0.5 µm for Au. This is the maximal thickness of the reservoir for generation of nanoparticles that is gained at fluence much higher than the melting threshold. That is why the size of nanoparticles is almost independent on the pulse width, from hundreds of nanosecond through hundreds of femtosecond. At lower fluence the thickness of the melt is smaller than hm. Expanding vapors of the liquid splash this reservoir resulting in the removal of the molten layer. Note that formation of nanoparticles via evaporation of the metal is unlikely, since the pressure of metal vapor at a temperature close to melting is too low compared to vapor pressure of the surrounding liquid. Surface tension stabilizes the molten drop of the metal, while the pressure of surrounding vapor of the liquid tends to split it. As a first approximation one may suggest that the size of a stable drop of metal can be found from the following relation: 2σ/R ~ pliq, where σ is the surface tension coefficient of the metal, R is the radius of the drop, and pliq is the pressure of the vapor of the liquid surrounding the target. Substitution of these parameters for Ag ablated in water for R = 30 nm and T = 1000° C gives pliq ~108 Pa, which is a reasonable value. The melting temperature decreases with the size of the nanoparticles, so at sufficiently small R the particle melts under the laser pulse, but the vapor pressure of surrounding liquid is not high enough to cause its further splitting.

LASER ABLATION OF AN AG TARGET IN LIQUID ENVIRONMENT Historically Ag NP were the first ones synthesized by laser ablation of a silver plate in water. Various laser sources have been successfully used for this synthesis ranging from a nanosecond Nd:YAG through Ti:sapphire femtosecond lasers [21,26,27]. Ablation of a metallic Ag immersed into water by radiation of a Cu vapor laser at wavelength of 510.6 nm and pulse duration of 20 ns results in visible coloration of the liquid; it takes on a yellowish color. The optical spectra of Ag NP generated by laser ablation in different liquids are presented in Figure 3. The maximum of absorption is at about 400 nm, which is typical for so called plasmon absorption of Ag nanoparticles in water [18,19]. The intensity of the plasmon band increases with irradiation time, while its position does not vary significantly. Ag NP are slowly oxidized by air oxygen dissolved in the liquid, and the position of plasmon resonance shifts to the red with time due to formation of oxide layer with high refractive index [22]. The Transmission Electron Microscope (TEM) view of Ag nanoparticles produced by ablation of an Ag target in acetone is shown in Figure 5. The average size of particles is around 15 nm. Moreover, they are flat disks, and their thickness is of order of few nanometers [15]. This is clearly seen in those areas where the TEM image of intersection of two or more Ag particles appears darker than each separate particle.

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  0,20

Optical density

0,15

acetone

0,10

ethanol

0,05

water 0,00 400

500

600

700

800

Wavelength, nm Figure 4. Absorption spectrum of Ag nanoparticles obtained by ablation of Ag in different liquids. The plasmon peak of Ag is well pronounced at ca 400 nm

Figure 5. TEM view of Ag nanoparticles obtained by ablation of a bulk Ag in acetone. Scale bar denotes 50 nm

Smaller Ag NP are synthesized using sodium dodecyl sulphate (SDS) as a surface active substance [26]. Laser pulses of various durations have been used for synthesis of Ag NP in water ranging from ns to fs domain [21, 27].

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LASER ABLATION OF AN AU TARGET IN LIQUID ENVIRONMENT Gold NP exhibit a well-pronounced dependence of their absorption spectrum on their shape and size. In general, a metallic ellipsoid is characterized by three different plasmon resonances. In case of a spherical particle all three of them are degenerated, and Au NP show the maximum of absorption around 520 nm independent on the way of their preparation. Elongated Au NP (nanorods) are characterized by two absorption bands that correspond to so called transverse and longitudinal plasmon resonances. The position of the latter varies with aspect ratio of Au nanorods, and for sufficiently high aspect ratio the longitudinal resonance moves from the visible to the near IR range of the spectrum [28]. The optical spectra of water after laser exposure of metallic Au show well-pronounced absorption band at ca 520 nm that is characteristic of transverse plasmon resonance [18, 19]. At higher laser fluence elongated Au NP are also synthesized, which is accompanied by the appearance of a red wing. Au NP prepared in water in absence of surface-active agents are metastable against precipitation, and their significant precipitation is observed within several days after preparation. Precipitation of NP is accompanied by appearance of a wide red wing that indicates the presence of elongated Au NP with various aspect ratios. Synthesis of a stable colloidal solution of Au NP in water has been reported by several groups [29-32]. The coupling of laser radiation to the colloidal solution can lead to the modification of the size distribution of Au NP. This is confirmed by TEM view of Au particles as obtained after the ablation of Au in water. Laser radiation modifies not only the size of the particles, but also their shape. As-obtained particles are elongated, while those exposed to laser radiation are the nano-disks similar to Ag nanoparticles. Average size of Au as-obtained after ablation by a Cu vapor laser in H2O in absence of surface-active substances is 20 nm, while with further laser exposure of the solution alone their size diminishes to ca 10 nm. Au NP produced at high laser fluence on the target are characterized by elongated shape independent of the nature of the surrounding liquid. This effect has been observed for laser ablation in alkanes [33] as well as in water. Figure 6 shows the TEM view of Au NP synthesized by laser ablation of a gold target in water. The majority of NP has a spherical shape, while some NP have aspect ratio ranging from 2 to 10. Accordingly, the absorption spectrum of this solution is characterized by two maxima, one at ca 540-560 nm and the second having a wide red wing centered at longer wavelengths. The rate of generation of Ag and Au NP depends on laser fluence. For instance, it is 0.6 and 1.2 mg/hour, for respective metals, with a laser spot of 30 μm and fluence of 30 J/cm2. Nanoparticle size can be drastically reduced by the use of aqueous solutions of surfactants, which cover the particles just after their ablation and thus prevent them from further agglomeration. Sodium dodecyl sulphate (SDS) is one of the most popular agents capable of reducing the mean size of Au nanoparticles down to 5 nm during nanosecond laser ablation of gold [29-31]. Ablation of a gold target with shorter laser pulses, e.g., femtosecond ones, results in synthesis of bimodal distribution function of nanoparticle size at elevated laser fluence and almost mono-dispersed Au NP at lower fluence of 50 J/cm2 [32]. Another stabilizing agent for Au NP are cyclodextrins [34]. Their addition provides perfect stability of Au NP as small as 2-2.5 nm synthesized by laser ablation with femtosecond pulses. NP of another noble metal Pt can also be synthesized by laser ablation of a metallic Pt target. In presence of SDS their size can be as small as 5 nm [35].

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Figure 6. TEM view of Au NP obtained by ablation of a gold target immersed into water at elevated laser fluence of 90 J/cm2 at 1.06 µm, pulse width of 130 ns

INTERACTION OF NANOPARTICLES WITH LASER BEAM Coupling of laser radiation to isolated NP proceeds via its absorption by free electrons. Then the absorbed energy is thermalized and transferred to NP material. Since these electrons are characterized by some plasmon resonance, the efficiency of this coupling strongly depends on the detuning of laser radiation from maximum of absorption. This interaction is not important in case of laser generation of NP in vacuum, since NP do not come back to the laser beam. In liquid environment NP remain in the liquid and do come back inside the beam due to convective flows. Repeated interaction of NP with laser beam leads to observation of some interesting phenomena described below.

FRAGMENTATION OF NP UNDER LASER EXPOSURE IN LIQUIDS Fragmentation of NP under their laser exposure in liquids has been reported as early as the synthesis of NP itself [2, 6, 30]. The process manifests itself as a gradual decrease of the size of NP upon laser irradiation. As a result, the distribution function of particle size shifts to lower dimensions. The dependence of the NP size on the laser wavelength has been reported [36, 37] indicating the spectral dependence of the fragmentation process. Optical constants of

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metallic targets do not vary largely in the wavelength range of lasers used for NP synthesis, so fragmentation of NP should be ascribed to interaction of synthesized NP themselves with the laser beam inside the liquid. The physical processes that lead to fragmentation of NP in the laser beam are still under discussion. The most probable reason leading to reduction of NP size are the instabilities that develop at the interface of molten NP with the vapors of surrounding liquid [25]. Indeed, a NP may absorb enough energy from the laser beam to undergo the phase transition into a liquid state. The temperature of NP exceeds at this moment the boiling temperature of surrounding liquid. The latter is therefore vaporized and forms a shell around the NP. The pressure of vapors in this shell is around 108 Pa, and its possible asymmetry would result in the break of molten NP inside it into smaller parts. The most probable process is splitting of the molten NP into two equal particles, since, first, this corresponds to lowest value of total surface energy, and second, is the lowest mode of perturbation of the surface of molten NP. This hypothesis is in qualitative agreement with the fact that the average size of NP exposed to laser radiation in various liquids decreases with the decrease of boiling temperature of the liquid. The balance is gained when the pressure inside the NP of radius R is of order of the pressure of the surrounding liquid pliq: 2σ/R ~ pliq, where σ stands for the surface tension of molten NP. Fragmentation of the NP under their laser exposure introduces feedbacks into the system “laser radiation – NP”. Indeed, the average size of NP under their laser exposure is inversely proportional to the peak intensity of the laser beam inside the liquid. The size of NP decreases upon exposure till NP become so small that the energy absorbed from the laser pulse is not sufficient for melting. The system therefore demonstrates a negative feedback and autostabilization. On the other hand, the melting temperature of a NP decreases with its size, which is a positive feedback. Finally, small NP formed via fragmentation from bigger ones do not absorb enough energy from the laser beam to be molten, and the ensemble of NP acquires its upper limit of NP size.

SHAPE-SELECTIVE FRAGMENTATION The fragmentation of NP under laser exposure of their suspension in a liquid can be shape-selective, if the spectrum of plasmon resonance depends on their shape. This can be illustrated by shape-selective fragmentation of Au NP having elongated shape (nanorods). Au nanorods are characterized by two absorption peaks, one of them corresponds to so called transverse resonance (TR), while the second is called longitudinal one (LR). These resonances correspond to oscillations of free electrons in the directions across the nanorods axis and along it, respectively [28]. The position of TR near 560-570 nm is common for Au nanorods with any aspect ratio, while the position of their LR is a linear function of the aspect ratio and moves to the red with its increase [38-41]. If laser radiation is tuned into TR, then all nanorods are molten and fragmented into spherical NP at sufficiently high peak power of the laser beam [40]. However, tuning the laser wavelength into LR allows selective fragmentation of Au nanorods with aspect ratios which are in resonance with laser radiation [42].

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This fragmentation (splitting) is illustrated in Figure 7 in which the evolution of absorption spectrum of suspension of Au NP with wide distribution of aspect ratio is presented. Au NP are especially convenient for this type of experiments since their absorption spectrum depends on their shape. The radiation of a green line of a Cu vapor laser fits well the transverse plasmon resonance of Au NP, which is common for all Au NP and does not

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depend on their aspect ratio. On the contrary, the yellow line of a Cu vapor laser is absorbed by non-spherical (elongated) Au NP. Therefore, the evolution of the absorption spectrum is different if various laser outputs are used for NP exposure. One can see gradual decrease of the optical density of the suspension in the red that corresponds to fragmentation of Au nanorods irrespective to their aspect ratio (Figure 7, 1). On the contrary, exposure of the suspension to radiation that is preferentially absorbed by Au nanorods with the aspect ratio 2 – 3 results in the decrease of the optical density just at the laser wavelength (Figure 7, 2). This decrease indicates the depopulation of the suspension with nanorods that are in resonance with laser radiation. Of course the laser fluence should not be too high otherwise nanorods with “non-resonant” aspect ratio will also be fragmented, since their absorption wings are very wide. Au nanorods with aspect ratio about 10 absorb in the near IR region. Exposure of their suspension to radiation of a Nd:YAG laser operating at its fundamental output at 1.06 µm results in self-stabilization of fragmentation: no changes of absorption spectrum of the solution occur as soon as long nanorods are fragmented [42]. Similar fragmentation of Au NP is also observed in ethanol and acetone. Their size in these liquids is somewhat smaller than in water and is of 7 nm under the same laser fluence. As a consequence, these colloidal solutions are stable against sedimentation. Finally, the addition of a surface-active substance, e.g., poly(vinyl pyrrolidone) (PVP) with molecular mass of 104, to ethanol in which the ablation is carried out further decreases the average size of Au nanoparticles down to ca 4 nm at otherwise equal conditions.

FORMATION OF THE AU-AG ALLOY UNDER LASER IRRADIATION OF NANOPARTICLES Direct interaction of NP with laser beam inside the liquid may lead to their melting during each laser pulse at sufficiently high laser peak intensity. If the colloidal solution contains NP of various materials, their absorption of laser radiation may be different. Laser exposure of a mixture of different NP may lead to formation of alloyed NP, and at least one kind of NP should be molten by laser beam. Recent publications [43-45] have reported the formation of gold-silver alloy nanoparticles in liquid environment under laser exposure of core-shell particles obtained by a chemical method. The spectrum of core shell (non-alloyed) nanoparticles is not a linear combination of the spectra of monometallic colloids [45]. On the contrary, the spectrum of a mixture of individual colloids is composed of spectra of monometallic particles. Upon laser exposure, these peaks disappear, and the single alloy peak arises. Laser exposure of Ag-Au core-shell NP may lead to both Ag shell removal from Au NP and their alloying depending on laser wavelength and its fluence [46]. Individual NP of various materials have to come into contact to form an alloy, and therefore the laser alloying of NP is concentration-dependent process. The position of plasmon resonance of alloyed NP is a linear combination of plasmon frequencies of individual NP. Formation of alloyed AuAg NP is accompanied by peculiar modification of the absorption spectrum indicating an intermediate phase of the Au-Ag alloy formation [47-49]. The temporal evolution of the optical density of the mixture is presented in Figure 8.

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Fiigure 8. Evolutiion of the absorrption spectrum m of the mixture of both Au andd Ag NP synthesized by laaser ablation of respective metaals in ethanol unnder laser expossure at 55 J/cm2 of a Cu vapor laser raadiation (waveleength of 510.6 nm) n

Fiigure 9. TEM view v of Au-Ag nanoparticles n affter 2 hours of exposure e (a). Cooncentration off PVP is 0.1 g//l, Cu vapor laseer, laser fluencee of 9 J/cm2. Brrighter particles are Ag while thhe darker ones are a Au. Sccale bar denotess 40 nm. Alloyeed Au-Ag nanopparticles obtainned by exposuree of the mixturee of inndividual colloid ds in ethanol duuring 4 hours (bb). Scale bar dennotes 100 nm. Average A size off alloyed A Au-Ag particles is 7 nm

The formaation of an intermediate i p phase of the alloy is obsserved only in i case of suufficiently largge initial NP of o each metal (50 -70 nm). Smaller NP of o 10-20 nm in i diameter allso show the deviation off the maximuum of plasmoon resonance from its stoiichiometric poosition but this effect is weaak. on of plasmonn frequency of an Au-Ag alloy a varies coontinuously with w relative The positio cooncentration of o metals and is always in between the maxima m of plasmon resonaance at 400 nm m and 520 nm m of Ag and Au, respectivvely. In case of individuall particles, hoowever, the

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maximum of absorption falls out of this interval after certain exposure time and is significantly red-shifted after 1-2 hours of exposure. With further exposure the maximum returns to the interval corresponding to an Au-Ag alloy and remains there for even longer exposures indicating the formation of the alloy. The TEM view of NP at the stage of anomalous spectrum (Figure 8, spectrum 2) reveals multiple contacts between Au and Ag nanoparticles that are well observed due to different atomic mass of both metals. With further exposure these hybrid particles are alloyed, and the final position of the absorption maximum corresponds to the Ag-Au alloy (Figure 8, spectrum 4). The size of alloyed nanoparticles is about 5 nm, which is much smaller than initial ones. Formation of alloyed nanoparticles is sensitive to the presence of surface-active substances in the liquid. The rate of alloy formation increases under addition of PVP (10-5 M). However, the alloying is inhibited at given laser intensity at elevated concentrations of PVP exceeding 5×10-5 M [48]. The rate of alloying decreases with dilution of the mixture, presumably due to the increase of the average distance between the nanoparticles. It is pertinent to note that the surface-active substance, e.g., PVP, apparently remains untouched under laser exposure of NP in spite of their elevated temperature. Raman analysis shows no variation in the glassy carbon content before and after laser exposure of suspensions of NP with PVP, though this should be so if the pyrolysis of the surfactant on hot NP took place. This might be explained by the preferential localization of PVP at the interface of a vapor bubble that is formed around the NP during their laser exposure. The absorption spectrum of the Au-Ag alloyed NP is closer to the wavelength of a Cu vapor laser (510.6 nm) than the plasmon maxima of individual NP. Therefore, the efficiency of coupling of laser radiation increases upon formation of the alloy. As a result, the average size of alloyed Au/Ag NP is much smaller than that of individual ones [48, 49]. This is illustrated in Figure 9 where TEM images of NP are presented at different stages of Au-Ag alloy formation. Similar behavior is observed under exposure of the mixture of colloidal solutions of Cu/Ag and Cu/Au. The alloyed NP are formed under sufficiently long laser exposure of the mixture, and the final position of the plasmon resonance depends on ratio of individual colloids in it.

NANOPARTICLES OF CU, BRASS, AND BRONZE Copper is chemically active and easily reacts with vapors of surrounding liquid. The chemical interaction may take place both during the ablation and after it due to contact with air oxygen dissolved in the colloidal solution. Typically, the plasmon resonance of Cu nanoparticles lies in the visible. Cu NP synthesized by chemical means in anaerobic conditions show the absorption peaks from 570 to 590 nm [49, 50]. Laser ablation of Cu target in various liquids also leads to formation of NP that are characterized by absorption in the visible [51]. The peak position is around 590 nm, which agrees with previously reported data for Cu NP (see Figure 10). Ablation of a Cu target in water leads to formation of compounds containing only ionic species Cu+ without any plasmon peak. The stability of generated Cu NP to slow oxidation is different. Namely, Cu NP produced by ablation in ethanol are slowly oxidized. This is manifested by gradual

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disappearance of the peak of plasmon resonance (Figure 10). Remarkably, the initial plasmon peak can be restored by exposure of aged colloidal solution to the radiation of a Cu vapor laser at sufficiently high fluence of several tens of J/cm2. On the contrary, ablation of a Cu target in acetone results in a stable spectrum of the colloidal solution of NP. The difference in behaviour of Cu nanoparticles in these two liquids stems from their images obtained with a transmission electron microscope. Cu NP generated by ablation in ethanol are low-contrast entities with average size of 5 nm. Cu NP in acetone are metallic as one can conclude from their high contrast and well pronounced peak of plasmon resonance. However, in this instance they are embedded into some amorphous cloud [51]. Raman analysis of the dried suspension indicates that this cloud consists of glassy carbon. The layer of glassy carbon protects Cu NP from oxidation with air oxygen and preserves their plasmon resonance in the visible. Note that decomposition of acetone on Cu NP that occurs during laser ablation of a Cu target is highly chemically selective - it takes place on Cu nanoparticles but not on Ag ones in similar experimental conditions. Cu NP can also be synthesized via exposure of a suspension of micro-particles of CuO to laser radiation [52, 53]. At first stages of the exposure the micro-particles are broken into smaller entities, and then their reduction occurs by surrounding 2-propanol. It is worth mentioning that Ag NP can also be produced by exposure of suspended Ag2O micro-particles in an appropriate hydrocarbon liquid to laser radiation. In both cases, fragmentation of microparticles into smaller particles occurs due to absorption of laser energy by inter-band transition of corresponding oxide, while the subsequent formation of metallic NP takes place due to chemical reduction of oxide by surrounding liquid that acts like a reducing agent. 1,4

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Brass NP can be synthesized in a similar way. Unlike Cu NP they are stable to oxidation for at least several months. The position of the plasma resonance of brass nanoparticles depends slightly on the type of laser source used for their synthesis and lies near 510-515 nm. The presence of brass NP in the liquid is corroborated by the X-ray diffractometry of the evaporated suspension. Bronze NP synthesized by laser ablation of a bronze target do not show any well-defined absorption maximum, instead a wide plateau in the green range of spectrum is observed in the absorption spectrum of their colloidal solution. However the absorption spectrum is merely different from that of Cu NP. The mean size of NP of these Cu alloys is around 20-30 nm.

INTERNAL SEGREGATION OF BRASS NP Apart from fragmentation described above brass NP undergo drastic modification under their laser exposure in a liquid. This is an internal segregation of a nanoparticle that is repulsion of one of its components to its periphery [54]. The material of the nanoparticle is subjected to additional pressure due to its small radius. For a liquid nanoparticle, the capillary pressure is given by the expression p=2 /R, where is the surface tension coefficient and R is the particle radius, which is equal for Cu NP to 120 MPa for R=20 nm. In addition to this pressure, the nanoparticle during the laser pulse is subjected to the vapor pressure of the surrounding liquid, which reaches a maximum simultaneously with the particle temperature and decreases rapidly upon expansion of the vapor shell. The pressure of ethanol vapors surrounding the particle with a temperature of 1500 K is estimated at about 20 MPa from the equation of state of a real gas. Note that the pressure of nanoparticle-substance vapors at the melting temperature for most metals is negligibly low compared to the above values. As synthesized Cu NP are not stable towards oxidation by air oxygen. The oxidation is accompanied by disappearance of the peak of plasmon resonance of Cu NP as shown in Figure 11. It is pertinent to note that the spectra of NP of Cu alloys are more stable with time. This is due to the formation of a shell around them as described below. Laser radiation acting on the ensemble of brass NP in the liquid changes their absorption spectrum. The absorption spectrum of brass NP evolves to that of Cu NP (Figure 12). With laser exposure the peak corresponding to Cu NP becomes noticeable. Upon further action on the colloidal solution, the peak corresponding to brass nanoparticles disappears, and only the plasmon resonance peak of Cu NP remains. Qualitative evidence of the effect is the absence of small brass nanoparticles (with a radius smaller than 5 nm) upon the laser ablation of a brass target in ethanol. Estimates show that the time of diffusion of zinc atoms to a distance of about the diameter of a nanoparticle in the liquid state is of order of several ns, which is comparable with duration of the laser pulses commonly used in experiments [54]. Small nanoparticles rapidly lose zinc upon laser irradiation and are transformed to copper NP oxidized by air oxygen. A similar shell is observed under laser exposure of bronze NP. In this case the shell thickness is only few nm due to lower content of the component with low melting temperature (Sn) (Figure 13). Plasmon resonance of bronze NP is less pronounced than that of brass NP though is merely different from both Cu and brass NP.

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Figure 12. Evolution of the absorption spectrum of brass NP upon exposure of their colloidal solution in ethanol to radiation of a 130 ns Nd:YAG laser. As synthesized brass NP (1), 10 (2), and 90 min of exposure (3)

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Figure 13. TEM view of core-shell bronze NP synthesized by laser ablation of corresponding targets (8% Sn, 92%Cu) and then subjected to laser radiation in suspension. The diameter of big NP is about 40 nm

Small radius of NP is responsible for decrease of their melting temperature compared to bulk material. Also, it has been theoretically demonstrated that the melting temperature is different for different shapes of NP composed of the same material [55]. Internal segregation of NP under laser exposure leads to repulsion of the components with lower melting temperature. In this sense one may speak about purification of the core material under laser exposure.

SELF-INFLUENCE OF A FEMTOSECOND LASER BEAM Remarkably, the initial average size of nanoparticles produced by ablation of metallic targets is almost independent on the duration of laser pulse from 100 fs to 100 ns and is of order of 10 nm. This implies that the size is determined rather by thermal properties of the target. Indeed, the layer of metal molten by a laser pulse is the only source from which the nanoparticles are formed. The difference in size becomes pronounced under sufficiently long laser exposures when the majority of generated nanoparticles pass through the laser beam in the liquid. In this case the key parameter that determines the efficiency of coupling is detuning of laser wavelength from the plasmon resonance. In certain cases a good coupling of laser radiation to the ensemble of NP is achieved via non-linear transformation of laser radiation by NP themselves. Indeed, the wavelength of a femtosecond Ti:sapphire laser (810 nm) is too far from the plasmon resonances of both Au and Ag NP. However, prolonged laser exposure of the colloidal solution leads to drastic difference in their average size (14 nm vs 4 nm) [21]. Also, there is a significant amount of even smaller Ag nanoparticles whose size is below the measuring limits of the microscope (< 2 nm). The ablation is accompanied by intense generation of the second harmonics of laser radiation at 405 nm. This wavelength

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exactly fits the plasmon resonance of Ag NP, while for Au nanoparticles it is significantly detuned. The effect of second harmonics generation (SHG) on small metal clusters is well documented in the literature [56, 57]. Despite too small size of metal clusters compared to the laser wavelength, SHG can be very effective due to their high density, so that the conditions of spatial synchronism are easily fulfilled. Several models account for high value of the second order non-linear susceptibility χ(2) and for efficient generation of the second harmonics. This is assigned to quantum size effect for wave functions of electrons confined to small metal spheres, which gives rise to quadruple oscillations and, consequently, to non-zero χ(2) even for nanoparticles with a center of symmetry. The observed difference in particle size is attributed to the self-influence of laser radiation upon generation of nanoparticles via laser ablation in liquids. Indeed, appearing nanoparticles double the frequency of laser radiation. Owing to exact matching to plasmon resonance of Ag NP, their temperature can be much higher than under exposure to initial laser radiation. This causes fragmentation of nanoparticles that are in resonance with the second harmonics that is Ag ones. The third order non-linear susceptibility of NP χ(3) is responsible for non-linear lensing of laser radiation inside the colloidal solution. As a result, the focusing conditions change during the accumulation of NP in the beam path, and the ablation rate decreases even though laser radiation is largely detuned from plasmon resonance of NP. In general, the limiting factor of interaction of NP immersed in a liquid with a femtosecond laser beam is generation of a white continuum. This continuum consumes a significant part of laser energy and reduces the efficiency of interaction with NP. This is well pronounced for pulse durations less than 60 fs.

INFLUENCE OF THE NATURE OF THE LIQUID Ablation of a Ti Target The composition of NP produced by laser ablation of Ti was found to be dependent on the nature of the liquid [16] (see Figure 14). Namely, laser ablation of Ti in ethanol results in formation of Ti nanoparticles having the cubic structure. It should be reminded that the initial polycrystalline Ti plate has tetragonal structure. It is known that the cubic phase of Ti is metastable and exists only at elevated temperature > 600° C. At the same density of laser energy laser ablation of Ti target immersed into dichlorethane leads to the formation of nanoparticles of titanium carbide TiC. Four the most intense peaks of TiC can be distinguished on the X-ray diffractogram of the evaporated suspension. Finally, laser ablation of Ti in water results in the formation of nanoparticles having the composition of nonstoichiometric oxide TiOx, where x = 1.04. The width of the peaks indicates the small size of the nanoparticles. All the diffractograms contains several unidentified peaks. Formation of TiC via ablation of Ti in dichlorethane may be due to catalytic action of Ti nanoparticles, since no carbon-containing particles have been observed during ablation of either Si or Au in this liquid under otherwise equal experimental conditions. It is worthwhile to mention that the surface of Ti target after laser ablation in dichlorethane does not contain any detectable by X-ray diffraction amount of TiC. This indicates the formation of TiC nanoparticles via chemical reaction of ejected metal nanoparticles with liquid vapors.

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Fiigure 14. Difracctograms of NP P synthesized byy laser ablation of metallic Ti target in differennt liquids. Etthanol (upper cu urve), dichloretthane (middle), and water (botttom curve). Thee size of nanopaarticles caalculated from broadening b of diffraction d peaks is 25, 35, and 25 nm, respecttively. Cu vaporr laser, fluuence of 4 J/cm m2

Unlike to what is beliieved, NP geenerated by laser l ablationn in liquids are a always crrystalline, and d the fraction of o amorphous component is usually very low. l Oxidation state of NP material m is stronngly influenceed by the natuure of surrounnding liquid evven in case nooble metals, e.g., Au. It wass confirmed byy ablation of Au A in water, etthanol, and chhloroform [58 8]. X-ray Phottoelectron Speectroscopy (XP PS) analysis of o dried colloiids showed diifferent degreee of oxidationn of Au. The foormation of a gold–chlorinee compound iss suggested inn case of ablation of Au in chloroform c witth 5 ns laser shhots at waveleength of 532 nm. n Another exxample of thee influence of o the liquid on the chemiical composittion of NP geenerated via laaser ablation of o a solid targget is copper. In I this case finnal NP are meetallic only unnder ablation in i acetone, whhile water andd ethanol lead to t non-metalliic NP.

A Ablation of Sn S Tin is a meetal with low melting point, and laser fluuence requiredd for its ablatioon is much loower than for other metaals mentionedd above. A certain c depenndence of the particles coomposition onn the concentraation of a surfface-active subbstance is founnd in this casee [59]. Non-stoichhiometric tin oxide o SnO2-x is formed unnder laser abllation of a tinn target in aqqueous solutio ons containingg SDS [60]. Ablation A of Sn in either wateer or ethanol at a moderate laaser fluence (less than 0.2 J/cm J 2 at nanossecond pulse duration) d leadss to formationn of Sn NP. This is corrobborated both by X-ray difffraction of the t dried susppension and its optical abbsorption specctra. Since meltting temperatuure of bulk Snn is low, Sn NP N are liquid at room tempperature. In geeneral, this is due to high fraction f of surrface atoms inn NP compareed to their tottal number. H However, evaporation of their colloidal soolution in abseence of surfacee-active substaances leads too aggregation, and X-ray diffraction shhows narrow peaks of Snn NP about 100 1 nm in diiameter. A golld plate immersed into collooidal solution is rapidly covvered by numeerous layers off Sn. Once fix xed on a large substrate, Sn NP are solidiified and form m a kind of nanno-contacts too the substratee. Mixing Sn NP N and Au NP P in ethanol leeads to wettingg of Au NP byy liquid Sn annd therefore to o specific coree-shell NP. Thheir TEM view w is presented in Figure 15.

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Fiigure 15. TEM view of Au NP P wetting by Sn NP. Both colloids were syntheesized by ablation of coorresponding metallic m targets inn ethanol using radiation of a Cu C vapor laser. Scale bar denotes 200 nm

The thickn ness of the shhell depends on the volum me of the Snn NP. Large Sn S NP are soolidified just after a the contaact with Au NP, N and no sheell is formed. Smaller Sn NP N envelop A NP resultin Au ng in a shelll with homoggeneous thickkness. Diffusee entities are tentatively asssigned to tin oxide. At picoseccond pulse duuration the reesulting NP consist mostlyy of tin oxidee, which is coonfirmed by thheir characteriistic absorptioon peak at 360 nm.

W and Mo NP N Other metaals that might interact with surrounding liquid l during ablation remaain metallic annd are oxidizeed very slowlyy with air oxygen that is disssolved in liquuid. This conccerns NP of eiither W or Moo upon their abblation in watter. These mettals with high melting tempperature are diispersed into surrounding liquid as meetallic NP, which w is corrooborated by both b X-ray diiffraction and optical spectrra of corresponnding colloidaal solutions.

Fiigure 16. TEM view of W NP synthesized by ablation of a W target in ethannol by radiationn of a Cu vaapor laser. Scalee bar denotes 500 nm

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Their morphology alters during several months of storage at room temperature due to oxidation, which results in the formation of core-shell NP (see Figure 16). Like many other metals, the oxide layer prevents further oxidation of the metallic core. Similar result has been reported for W NP obtained by ablation of a W target in ethanol using high-repetition-rate copper bromide laser radiation [61]. The authors deduce the formation of non-stoichiometric oxide WO3–x around W NP on the basis of absorption spectra of the colloidal solution. Carbon NP can also be obtained by laser ablation of a graphite target immersed into liquid [62]. A peculiar feature of laser ablation in water is enhanced solubility of some compounds in it under high temperature and high transient pressure that are established during the laser pulse. This leads to so called hydrothermal synthesis of NP of unusual morphology, e.g., platelets or disks of Pb(Zr,Ti)O3 [63]. Aqueous solutions at pressure and temperature that exceed its critical value take on the ability to dissolve solids that are not soluble in normal conditions. Synthesis of compounds via their dissolution in supercritical compounds is called the hydrothermal one. In nature this type of synthesis occurs in the vicinity of underwater volcano, which results in formation of various minerals. Hydrothermal synthesis is typical for non-metallic compounds, such as oxides of more complex ceramics. The material of the target is dispersed and dissolved in a supercritical medium during the laser pulse. Upon cooling the solubility drops, and the formation of NP occurs from oversaturated solution. Apparently, similar process may take place under laser ablation of single crystal rutile in an aqueous environment [64]. The specific surface of NP is very high, for instance, 1 ml of Au colloid prepared by laser ablation of a bulk target at typical density has the surface of 30 m2. High surface favors catalytic reactions in liquids. Not only NP material but also the liquid itself may undergo chemical changes during laser ablation. In some cases these changes are catalyst-specific that is occur with NP of some specific metals. Laser exposure of most metallic targets (Sn, brass, etc.) in ethanol leads to its noticeable modification towards formation of products with higher molecular mass. Indeed, a wide absorption band centered at 350 nm is observed that correspond to a viscous non-volatile liquid that remains after evaporation of the colloidal solution synthesized by laser ablation. This product might be a low-molecular mass polyethylene, which may also form a kind of shell on the synthesized NP. Formation of this product occurs mostly on the stage of exposure of NP to laser radiation, and catalytic effects of its formation mediated by NP of various compositions are not excluded.

Modeling of Distribution Function Modeling of distribution function of nanoparticles size can be performed by a numerical solution of kinetic equation on the basis of first principles. The following processes should be taken into account: generation of nanoparticles by laser ablation of a solid target immersed into liquid, aggregation of nanoparticles, their fragmentation and escape of NP adsorbed on the wall of a reactor [25, 65]. In the specific conditions of laser ablation in liquids, the last term can be negligibly small unlike laser ablation in vacuum. On the contrary, fragmentation of the particles in the laser beam is the main channel of modification of their distribution function. The peculiar feature of ablation in liquids is simultaneous action of all 4 processes,

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since the generated NP remain in the liquid and, in particular, on the optical path of laser beam. Formation of NP due to coagulation of individual metal atoms ejected from the target is unlikely, since the necessary condition of their synthesis is melting of the target. NP are ejected from the target at the very beginning of ablation. Melting of NP and their subsequent fragmentation may occur only in those areas of the liquid where the peak power of laser radiation is sufficiently high. The model takes into account a factor proportional to the ratio of the volume of laser beam waist to total volume of the liquid. In typical experimental conditions this factor is about 10-4, which explains the need of prolonged laser exposures required to alter the distribution function at measurable level. Laser exposure of NP in liquid in absence of the metal target results in their fragmentation due to absorption of laser energy by individual particles. Under laser fluence typically used NP are molten during the laser pulse, and their splitting is due to hydrodynamic instabilities of the molten metal embedded into a vapor pocket. According to the model derived above, the size of the particles goes to 0 under long exposure. In reality for each value of laser fluence there is a certain minimal size of particles that are too small to absorb the energy sufficient for their melting from laser beam. The model that takes into account all above-mentioned considerations shows good qualitative agreement with the experimentally measured distribution function [25].

INFLUENCE OF INTENSITY DISTRIBUTION OF THE LASER BEAM ON THE SHAPE OF NANOPARTICLES The influence of the laser beam profile onto the morphology of NP generated via laser ablation of solids in liquids is not studied so far and is often ignored in current scientific literature. The origin of this influence is a certain value of the energy density of the laser beam, which is needed to melt the target material. The size and shape of NP generated via laser ablation are sensitive to the laser fluence. Typically higher fluence leads to higher average size of generated NP. Also, in case of laser ablation of Au at elevated laser fluence the generated NP have elongated shape (see Figure 6). The term “elevated” should be understood in term of laser fluence needed to melt the target surface. One may suggest that this is due to specific way of interaction of the melt bath within the laser spot with surrounding medium. Indeed, sputtering of the melt is due to splashing of the molten layer on the target surface by high pressure vapors of surrounding liquid. However, the ejection of the melt in the central areas of the laser spot is different from that at its periphery. The melt ejected in the spot center has high probability to return to the target surface being pushed by the expanding vapor cloud. On the contrary, ejection of the melt from the periphery of the melt bath very likely results in the formation of NP, since the lateral dimensions of the expanding vapor pocket above the target are quite close to the laser spot size. This means that only the edges of the molten area contribute to NP formation, while the central part of the laser spot provides relatively small fraction of NP. These considerations suggest the following experiment on NP generation in which the laser beam that causes ablation of a target immersed into liquid has another type of symmetry of its intensity distribution different from that of a flat-top or Gaussian profile. A good possibility is formation of a periodic intensity distribution in the plane of the target. Such

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periodic distribution may occur spontaneously due to, for example, interference of the laser beam with Surface Electromagnetic Wave excited in the solid by this beam. Another possibility is exposure of the target to the interference pattern of two coherent laser beams. In this case the laser fluence is a periodic function along one of coordinate, so does the temperature distribution as schematically shown in Figure 17. If the fluence in the maxima of interference pattern are sufficient to melt the target, then the melt baths will have elongated shape on the target, which in turn may affect the shape of ejected NP. The experimental setup is shown in Figure 18. A Cu vapor laser emits at two laser lines, either green (510.6 nm) or yellow (578.2 nm). In this particular experiment only one output was used, namely, the green one. The green laser beam was split into two beams of approximately equal intensities using dividing glass cube. These two beams were superimposed on the surface of a gold target placed into liquid ethanol. The angle between two beams corresponded to the interference pattern on the target surface with period of 4 μm. The cell with gold sample was moved under the laser beams in the direction parallel to the maxima of interference pattern.

Figure 17. Temperature distribution on a target exposed to two coherent laser beams. Solid line T(x)=Tm indicates melting temperature of the target 3

2 1

4

5 6

7

Figure 18. Experimental setup on laser-assisted synthesis of elongated Au NP. 1 – laser beam (Cu vapor laser, 510.6 nm laser output), 2 – splitting cube, 3 – dielectric mirror, 4 – focusing lens, 5 – cell, 6 – Au target, 7 – liquid ethanol

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Figure 19. TEM view of Au nanoparticles obtained by ablation of an Au target in ethanol by two interfering laser beams of a Cu vapor laser. Scale bar denotes 50 nm

Ablation of the Au target in these conditions results in visible coloration of the liquid, and the color of the solution has bluish tint typical of elongated Au NP. TEM examination of generated Au colloid confirms the presence of elongated Au NP, as demonstrated in Figure 19. Some Au NP are indeed elongated, and their aspect ratio (ratio of length to diameter) exceeds 10.

NANOSTRUCTURING OF SOLIDS UNDER THEIR LASER ABLATION IN LIQUIDS The total laser fluence absorbed by a target should be sufficient to melt it in order to produce NP in the liquids. Ideally smooth surface is characterized by definite threshold laser fluence needed for melting. In practice the surface of a target has certain micro-roughness, and this surface may not melt simultaneously. Indeed, any protrusion which is weakly thermally coupled to the bulk of the target is molten at lower fluence than the smooth target. Therefore, at certain fluence a rough surface will not be molten only in some protruding areas. The vapors of surrounding liquid that expands from the target will then carry away the molten fragments thus forming spikes in its surface. The rate of NP generation in the liquid is very low, so at threshold laser fluence the material tends to leave the target in the form of NP but remains on it solidified as nano-spikes or nano-bumps. Period of nano-spikes does not depend on the laser wavelength and is determined mostly by characteristics of the target material. Formation of NS on the target is accompanied by modification of its absorption spectrum. Indeed, the electrons that oscillate within single NP are also confined, and therefore the spike edges act like the edge of NP as soon as their dimensions are compared with mean free path of electrons at Fermi level (see Figure 20).

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Figure 20. Origin of the surface coloration. Schematic view of oscillations of free electrons in metallic nanostructures. Plasmon resonance of these electrons is close to that of nanoparticles of the same lateral dimensions 1,0

0,8

A

0,6

0,4

0,2

0,0 300

400

500

600

700

800

Wavelength, nm

Figure 21. Absorption of an Ag target ablated under water layer with 5 ps laser pulses at wavelength of 248 nm. Black curve corresponds to initial Ag target while the red one to Ag with NS

This confinement causes the coloration of some metals with formed NS. For instance, Ag plate with NS looks yellow in appearance [66]. A macro view of Ag surface with NS is shown in Figure , a. Yellowish coloration in Ag is typical of silver NP. Absorption spectrum of both initial Ag target and of Ag with NS is shown in Figure 21. Initial spectrum corresponds to plasmon resonance of electrons in bulk Ag. Formation of NS manifests itself in broadening of this resonance. A new maximum appears near 360 nm. This maximum shifts to red with oxidation of Ag upon storage in air during several days. Similar modifications of absorption have also been observed in large variety of metals, e.g., Ti, W, Cu, etc., subjected to laser ablation in liquids at threshold laser fluence. Typical morphology of both Ag and Ta target with NS formed by laser ablation in water is presented in Figure 16. One can see that the period of NS is about 50 nm for Ag and 300 nm for Ta, which is much smaller than laser spot. In this sense NS are self-organized. NS on Ta are characterized by high aspect ratio, while on Ag they are rather nano-bumps. Formation of NS on metallic targets ablated in liquids requires sufficiently short laser pulses. Indeed, no NS are formed with nanosecond pulse duration. The longest tested pulses that produce NS are 350 ps delivered by a Nd:YAG laser at wavelength of 1.06 µm. Another

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necessary condition for formation of NS is certain initial roughness of the target surface. The local roughness required for NS formation should exceed 30-50 nm. These two conditions allow suggesting local melting of the target in the micro-protrusions of the target relief, which develops with further increase of the number of laser shots into an array of self-organized nano-spikes. The process of NS formation under laser ablation of solids immersed into liquids requires further studies. However the experimental data available so far indicate relation of NS to some instability that develops at the interface “melt-vapor of liquid” within the laser spot. In typical conditions the pressure of vapor of the liquid exceed by at least one order of magnitude the capillary pressure stipulated by curvature of the melt surface, so the recoil pressure of vapor that expands from the target plays dominant role.

Figure 22. AFM view of NS on Ag (top) and Ta (bottom). NS were generated by ablation of corresponding metal target in water, pulse width of 350 ps at wavelength of 1.06 μm

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Figure 23. Schematic picture of detachment of nanodrops from the melt on the surface of a solid target

Nanostructures on Ag generated by its laser ablation in water show the SERS effect of adsorbed organic molecules [67]. The estimated enhancement factor is about 105. Unlike Ag colloid in a liquid, Ag plate with nanostructures can be used for SERS measurements many times, since the NS do not degrade upon measurements. Different stages of NS development are schematically shown in the Figure 23. These structures correspond to morphology that can be found within the laser spot on the target. At sufficiently high laser fluence the nano-drop of the melt is detached from the target surface into surrounding liquid as nanoparticles. It is also clear that probe microscopes fail to image correctly the mushroom-like structures. This type of nanostructures is imaged by an AFM as a cone. Small size of NS implies another mechanism of their formation, which should not involve capillary waves. Indeed, capillary waves with period of hundreds of nanometers would be suppressed since capillary pressure at this scale tends to smooth the melt. Thus, formation of nanostructures under laser ablation of metals in liquid environment is a general conformity. It is characterized by several features. First, nanostructures are observed only in case of sufficiently short laser pulses. Second. Their period foes not depend on the laser wavelength. Nanostructures are merely different from periodic ripples that are generated owing to interference of a Surface Electromagnetic Wave with laser beam. The reviewed results on laser-assisted generation of nanoparticles via laser ablation in liquids represent only one point of view. Recent review [69] and recent book [70] consider another processes and approaches that have been developed during last decade.

EXCITATION OF HIGH ENERGY LEVELS In typical conditions of experiments on laser ablation of solids in liquid environment the nanoparticles (NP) sputtered into liquid are optically thin at the wavelength of most common lasers. Despite their small size (about 10 nm), the NP can efficiently absorb the laser radiation. The efficiency of this interaction is a function of numerous experimental parameters, such as particle size, detuning of laser frequency from the position of the plasmon resonance of NP, etc. However in most cases, NP are optically thin, that is almost transparent at the laser wavelength. Hence, the time required to reach their temperature under laser exposure is shorter than the duration of laser pulse as soon as the latter exceeds the characteristic time of electron-phonon relaxation. The temperature of NP inside the laser beam is proportional to the peak power of the laser radiation. Estimations show that the temperature of Au nanoparticles inside Cu vapor laser beam with peak intensity of 108 W/cm2 is several kK, and this temperature scales linearly with laser intensity. Therefore, exposure of NP suspension in liquid environment is a novel approach that allows excitation of high energy levels of both the NP material and its close environment. The liquid surrounding the

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NP in this case turns into a vapor having high temperature and pressure, and eventually into plasma that perfectly surrounds it. The present paper describes several proofs that laser exposure of NP suspension in liquid may lead to excitation of high energy levels including those of nuclei and neutron release. Laser initiation of neutron emission may open new possibilities for transformation of elements, synthesis of desired isotopes, disposal of nuclear waste, etc. The suspension of Hg in D2O is a model system for laser-assisted transmutation of Hg into Au since the neutron binding energy in D has the lowest value [71,72]. Two kinds of Hg were used for a model system of transmutation of Hg into Au: (i) Hg of analytical purity with natural isotopic composition or (ii) enriched Hg containing 55.6% of 196 Hg and 41.4 % of 199Hg, with much lower content of other isotopes, obtained by selective photochemical reaction of Hg with oxygen [73]. Exposure of the colloidal solution of Au NP in water to radiation of a 350 ps Nd:YAG laser is accompanied by visible emission from the solution. The spectrum of this emission contains several spectral features. The first one corresponds to anti-Stokes Raman scattering of H2O vapors of laser radiation excited at 1060 nm. The second broad peak coincides with the emission of atomic Au(I) at 627.8 nm during transition from the resonant to the metastable level. The peak at 530 nm coincides with the position of plasmon resonance of Au NP in H2O. One can also see that the laser-produced plasma is characterized by intense UV radiation at least up to 200 nm where water itself and Au NP have strong absorption [74]. If the laser spark is placed in the vicinity of a Be window of the cell one can get an image of a metallic grid on Be using an X-ray photo film, though the exposure time has to be at least 2 hours. Therefore, both deep UV and X-ray radiation is produced during exposure of Au NP colloid in H2O to laser radiation with rather moderate intensity. This result is in agreement with previous observations of X-ray emission under exposure of aqueous solutions to femtosecond laser radiation [75,76]. The energy of X-ray photons is sufficient to cause the modification of the NP environment. This has been confirmed in a model experiment on exposure of Au NP in D2O to radiation of a 350 ps Nd:YAG laser at peak intensity of 1010 W/cm2. Raman spectrum of D2O with Au NP subjected to 2 hours laser exposure indicates the formation of HDO molecules. Suspension of Hg in D2O is characterized by an absorption peak in the range 270-290 nm even prior to laser exposure. This peak is assigned to the plasmon resonance of Hg NP, which is close to the theoretical position of 290 nm reported for 10 nm NP of Hg in H2O [18]. Therefore, in the initial stages, exposure of Hg suspensions to laser radiation leads to the formation of Hg NP (size of the order of 10 nm). Using either a Cu vapor laser (8 hours of exposure) or a 100 femtosecond Ti:sapphire laser (2 hours of exposure), no Au content was detected within the accuracy of measurements. After exposure to a 90 ps Nd:YAG laser radiation, the suspension precipitates very slowly, unlike a freshly prepared suspension in ultrasonic bath. After 1 day of sedimentation the liquid separated into two parts, the first close to the bottom and the other remaining in suspension. Analytic data presented in Table 1 show the formation of Au. Experiments with a 350 ps Nd:YAG laser were carried out with two types of Hg samples, either of natural isotopic composition or enriched with 196Hg. Results of analysis of Hg suspension of natural isotope composition in D2O are similar to those obtained with a 90 ps laser. The formation of Au in exposed suspensions of Hg in D2O is confirmed by analysis, though in general the Au/Hg ratio is lower.

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Hg in D2O initial 35 0.009 0.00026

Hg in D2O sample 1 23 0.1 0.0043

Hg in D2O sample 2 8 0.024 0.003

Table 2. Exposure of suspension of 196Hg in D2O to radiation of a 350 ps Nd :YAG laser. The results are averaged over 4 different exposures Sample

Au, mg/l Hg, mg/l Au/Hg

196

Hg/D2O initial 0.0073 20 0.00036

196 Hg/D2O exposed Hg drop

0.38 8.94 0.0425

196 Hg/D2O 4 hours of exposure (averaged over 4 samples) 0.23 2.31 0.10

196

Hg/D2O after sedimentation 0.17 12.6 0.0135

The results of analysis for Hg enriched with 196Hg are presented in Table 2. Note that Au content is found under laser exposure of a Hg drop immersed into D2O for about 10 min in order to disperse the metal. Four hours of laser exposure increases the Au content to almost 10%. The mechanism of transmutation is still under discussion. The effect is observed only under sufficiently long laser pulses of picosecond range. One may suggest that the beginning of a laser pulse ionizes the nanoparticles within the laser beam while the remaining laser energy is spent for acceleration of electrons as it is observed in the case of interaction of strong laser beams with plasma [77]. These electrons provide X-ray photons needed for release of a neutron from Deuterium.

CONCLUSION The research on the processes of laser ablation in liquids is still in progress. There is a certain hope that the simplicity of the experimental setup on laser-matter interaction will allow developing new cost-effective technologies. Several specific features, such as the absence of a vacuum, may provide wide use of liquid-assisted generation of nanoparticles in medicine, catalysis, etc. The novel process of laser-assisted nanostructuring of solids via their laser ablation in a liquid environment is also of high interest for various applications in medicine, biology, aero- and hydrodynamics.

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[65] Bozon-Verduraz, F; Brayner, R; Voronov, VV; Kirichenko, NA; Simakin, AV; Shafeev, GA. Quantum Electronics, 2003, 33(8), 714-720. [66] Zavedeev, EV; Petrovskaya, AV; Simakin, AV; Shafeev, GA. Quantum Electronics, 2006, 36(10), 978-980. [67] Lau Truong, S; Levi, G; Bozon-Verduraz, F; Petrovskaya, AV; Simakin, AV; Shafeev, GA. Appl.Phys., 2007, A89 (2), 373-376. [68] Lau Truong, S; Levi, G; Bozon-Verduraz, F; Petrovskaya, AV; Simakin, AV; Shafeev, GA. Applied Surface Science, 2007, 254, 1236-1239. [69] Yang, GW. Progress in Materials Science, 2007, 52, 648-698. [70] Kruusing, A. Handbook of Liquids-assisted laser processing, Elsevier, 2008, 454. [71] Shafeev, GA; Bozon-Verduraz, F; Robert, M. Preprint of GPI, No45, Moscow, April 12, 2006. [72] Shafeev, GA; Bozon-Verduraz, F; Robert, M. Physics of Wave Phenomena, 2007, 15(3), 131-136. [73] Viazovetsky, Yu.V; Senchenkov, AP. Journal of Technical Physics, 1998, 68(1), 67. [74] Shafeev, GA; Simakin, AV; Bozon-Verduraz, F; Robert, M. Applied Surface Science 2007, 254 1022-1026. [75] Hatanaka, K; Miura, T; Fukumura, H. Appl.Phys.Lett., 2002, 80(21), 3925. [76] Hatanaka, K; Fukumura, H. X-ray generation from optical transparent materials by focusing ultra-short laser pulses, Chapter 9 in: Three-Dimensional Laser Microfabrication: Principles and Applications, H; Misawa, S. Juodkazis, Eds. WileyVCH, 2006, (199-238). [77] Pukhov, A; Meyer-ter-Vehn, J. Appl. Phys., 2002, B74, 355.

In: Laser Ablation: Effects and Applications Editor: Sharon E. Black

ISBN: 978-1-61122-466-5 © 2011 Nova Science Publishers, Inc.

Chapter 9

NANODIAMONDS FROM LASER ABLATION IN LIQUID G. W. Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, School of Physics Science and Engineering, Zhongshan University, Guangzhou, P.R. China

ABSTRACT Laser ablation in liquid, i.e. pulsed-laser induced liquid-solid interface reaction (PLIIR) has been developed to synthesize diamond nanocrystals. Chemical and physical mechanisms of the nanodiamonds synthesis upon PLIIR are addressed based on the nucleation thermodynamics and growth kinetics. Our studies showed that PLIIR could be expected to be a general route to synthesize the nanocrystals with the metastable phases.

1. INTRODUCTION Pulsed-laser ablation of solid materials has been studied intensively in recent years, because it has shown the great potential in laser material processing including thin solid film preparation, nanocrystals synthesis, laser surface cleaning, and device fabrication. Since laser ablations of solid materials are easily carried out in conventional deposition chambers with vacuum or filled gases, most of researchers have focused their attention on pulsed laser ablation of solid target in vacuum and diluted gas, i.e. pulsed laser ablation at gas-solid interface, aiming at various applications above [1]. Compared with applications of pulsed laser ablation at gas-solid interface, however, the application of pulsed laser ablation of solid target in a confined liquid are really comparatively limited in the field of interactions between pulsed laser and materials [4,5]. Recently, Yang et al. and Singh et al. have clarified that *

Corresponding author: E-mail: [email protected], Tel. & Fax: +86-20-8411-3692.

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pulsed-laser ablation in a confined liquid is an effective method to syntheses metastable phase nanocrystals and nanostructures and named the novel method to be the novel method is named to be pulsed-laser induced liquid-solid interface reaction (PLIIR) [30-37]. Typically, diamond and c-BN nanocrystals and immiscible alloying nanocrystals and nanorodes have been synthesized by PLIIR. Following them, many researchers started to express their interesting in synthesis of a variety of nanocrystals by PLIIR [38-58]. For example, nanocrystals of element metals and alloys such as Ti, Au, Ag, Fe-Ni, and TiC, quantum dots of ZnS and US, ultrafine oxide nanocrystals such as SnO2-x and CeO2, and Ag2Se nanocrystals. To further extend the application of PLIIR in synthesis of nanocrystals and have a clear insight into laser ablation in a confined liquid, in this review chapter, we therefore introduce the fundament physical and chemical aspects, and applications of PLIIR in fabrications of nanostructures.

2. LASER ABLATION IN LIQUID 2.1. Fundamental Processes in Laser Ablation In Liquid The pioneer work of pulsed laser ablation in liquid for materials synthesis was reported in 1987, in which authors first time synthesized the iron oxides with metastable phase by pulsed laser ablation of a iron target in water [11]. Since then, few researchers started to use the novel pulsed laser ablation to prepare new materials such as metallic oxides and nitrides [8587], diamond and diamond-like carbon [12-14,88-90], and carbon nanotubes [91]. Generally, for laser ablation in liquid, three basic processes of the plasma plume from laser ablation of solid target in liquid, i.e. generating, transforming, and condensing, will play important roles in materials synthesis. Basically, a plasma plume from the solid target will generate on the interface when the front part of the incident laser pulse irradiate the interface between the solid target and the confined liquid through the liquid schematically illustrated in Figure 1 (a), and we call the plasma to be the laser-induced plasma. Different from free expansion of the plasma plume from pulsed laser ablation in vacuum and dilution gas, the expansion of the laser-induced plasma is confined by the liquid once generation. Following Fabbro [7-10,1620], there is the formation of the shock wave by the laser-induced plasma. The laser-induced plasma adiabatically expands at a supersonic velocity to create a shock wave in front when it absorbed the later part irradiation of the laser pulse and got the continual complement of the vaporizing species from the solid target. Then, the shock wave will induce an extra pressure called plasma-induced pressure in the laser-induced plasma. Further, the plasma-induced pressure will lead to an additional temperature increasing of the laser-induced plasma. Therefore, the shock wave induced by the laser-induced plasma pushes the laser-induced plasma into the state of higher temperature, higher pressure, and higher density (HTHPHD). For example, Berthe et al. reported that the plasma-induced pressure levels reach to 2-2.5 GPa when the 0.308 μm XeCl excimer laser with pulse duration of 50 ns and power of 1-2 GWcm-2 was applied to ablate the Al target in water [18]. Then, Peyre et al. reported that the short laser pulse such as 3 ns allows the generation of higher plasma-induced pressure than longer pulse like as 30 ns (10 GPa versus 5 GPa) [19]. In fact, the wavelength and power of the laser pulse can influence on the value of the plasma-induced pressure [17]. Moreover,

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Sakka’s measurements showed that the density of the ablation species is 1022-1023 cm-3, the temperature of laser-induced plasma is 4000-5000 K, and the pressure of laser-induced plasma is about 10 GPa, when the 532 nm Nd:YAG laser with pulse duration of 10 ns and power of 1010 Wcm-2 was used to ablate a isotropic graphite target in water [117].

(a)

(b)

(c)

(d1)

(d2)

Figure 1. Three typical stages of the laser-induced plasma evolution in vacuum or dilution gas. (a) The generation of the laser-induced plasma on the surface when the front part of the laser pulse ablating the target. (b) The free expansion of the laser-induced plasma in vacuum or gas when the plasma absorbed the later part of the laser pulse. (c) The ejection of the plasma plume from the target. (d) Two different condensations of the laser-induced plasma: one is used to deposit thin films on substrates, and another one is used to synthesize nanoparticles in gases

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Figure 2. The evolution of the laser-induced plasma in liquid. (a) The generation of the laser-induced plasma due to the irradiation of the front part of the laser pulse on the target. (b) The expansion of the plasma plume in liquid due to the absorbing the later laser pulse and the plasma-induced pressure created by the shock wave. (c) Four kinds of chemical reactions taking place inside the plasma and liquid, and the interface between the plasma and liquid. (d) Two condensations of the plasma plume in liquid: one is used to prepare surface coatings on the target surface, and another one is used to fabricate nanoparticles in liquid

Accordingly, the special laser-induced plasma with HTHPHD generates at the liquidsolid interface when the laser pulse irradiated the surface of the solid target through the liquid, due to the confined effect of the liquid, shown in Figure 2 (b). Noted that the thermodynamic state with HTHPHD is obviously favorable for the formation of the metastable phases that are in the high-temperature and high-pressure region in their thermodynamic equilibrium phase diagram. Four kinds of chemical reactions would take place in the laser-induced plasma and at the interface between the laser-induced plasma and the liquid during the transformation of the laser-induced plasma. The first kind of the chemical reaction occurs inside the laser-induced plasma. Owing to the laser-induced plasma being in the state with HTHPHD, the new phase, especially metastable phase, could form by high-temperature chemical reacting between the ablations from the target. The second kind of chemical reaction also takes place inside the laser-induced plasma, then, the reactant species are from the target and the liquid, respectively. The high temperature and high pressure in front of the laser-induced plasma will result in the excitation of the liquid molecules at the interface between the laser-induced plasma and the liquid to create the new plasma of the liquid molecules, called the plasmainduced plasma. Naturally, the plasma-induced plasma could rapid dissolve into the laserinduced plasma once generation, then, the chemical reactions between the species from the

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laser ablating target and the species from the liquid molecules excitation would appear inside the laser-induced plasma. The third chemical reactions occur at the interface between the laser-induced plasma and the liquid, because the state with HTHPHD of the laser-induced plasma provides a good opportunity to the high-temperature chemical reactions between the ablation species from the target and the molecules of the liquid, which would directly take place at the plasma-liquid interface. The fourth kind of chemical reaction is inside the liquid. The extremely pressure in front of the laser-induced plasma will impinge the ablation species from the solid target at the plasma-liquid interface into the liquid, then, chemical reactions between the ablation species and the liquid molecules would form inside the liquid. It is noticed that three of four kinds of chemical reactions are simultaneously involved in two species that are from the solid target and the confined liquid, respectively. ×103 40 ×5 30 B+

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Figure 3. The emission spectra and their time dependence of pulsed laser ablation in liquid. (a) BO molecules were detected by emission spectra when the Nd:YAG laser with wavelength of 1064 nm, pulse duration of 20 ns, and the energy fluence of 8-9 J/cm2 was used to ablate a hexagonal boron nitride target in water. (b) CN molecules were synthesized by the same pulsed laser ablating a graphite target in benzene solution

Therefore, these chemical reactions provide occasions to fabricate new materials combining of the elements of the target and the liquid. Importantly, the reported experimental

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data supported the deductions about the chemical reactions above. [11,31,23]. Figure 2 (c) schematically illustrates these chemical reactions mentioned above. The last stage of the plasma plume generated by the pulsed laser ablation in liquid is cooling down and condensation in the confined liquid shown in Figure 2 (d). Clearly, just like as the pulsed laser ablation in vacuum and dilution gas, the different condensation will result in different application in materials preparations. A part of the plasma plume would condense and deposit back on the surface of the solid target during the plasma quenching in the liquid, due to the confined pressure from the liquid. Naturally, the condensation and deposition of the plasma plume lead to thin films formation. Another part of plasma plume will condense and be dispersed into the liquid during the plasma quenching, due to the cooling down of the confined liquid. Therefore, the condensation of the plasma results in small particles synthesized in the liquid. Generally, these small particles float on the surface of the liquid, as they have a large surface tension. Now, more and more researchers, who are interested in nanocrystals synthesis, have stated to pay attention on the later [30-58]. Accordingly, it is a very interesting scene: the productions of pulsed laser ablation in a confined liquid are dived into two parts, one part at the liquid’s bottom in thin films, and another part at the liquid’s top in particles, respectively. There are unique kinetic characters of pulsed laser ablation in a liquid. Firstly, the higher ablating rate to the solid target can be created in pulsed laser ablation in a confined liquid than that in vacuum and dilution gas. Clearly, the plasma with HTHPHD etches the solid target at the plasma-solid interface to promote the total ablating rate of the laser ablating the target [28 29,118,119]. Zhu et al. reported the laser ablation rate of Si varies with the thickness of the water layer above the Si target when a 248 nm KrF excimer with pulse duration of 23 ns was used to ablate a single Si substrate, and found that the laser ablation rate is the most highly enhanced with a water layer of 1.1 mm shown in Figure 4, as the optimal water layer thickness can induced the strongest pressure in the laser-induced plasma. These results thus show that the high producing can appear in the pulsed laser ablation in the confined liquid for both of thin films deposition and small particles synthesis. Secondly, the shorter quenching time of the laser-induced plasma can be achieved in the confined liquid. An experimental comparison of the plasma durations is shown in Figure 5 [117]. Definitely, we can see that the plasma duration of pulsed laser ablation in air is ten times than that of pulsed laser ablation in liquid in the case. Accordingly, the plasma plume created by pulsed ablation in a liquid will rapidly quench in the confined liquid. Noted that the novel kinetics opens a new door to synthesize nanocrystals and nanostructures by pulsed laser ablation. Based on the three basic processes of the plasma plume evolution in a confined liquid above, the production of laser ablation generates in the duration of the plasma, in sequence, cluster forming, nucleating, and crystals growing. Remarkably, the short quenching times of the plasma plume can effectively limit the size of the crystals growing. For instance, the size of the synthesized nanocrystals may be at the nanometer scale, when the quenching time of the plasma plume is the order of magnitude of ns. In fact, the size distribution of all the small particles is at the nanoscale when the laser pulses with pulse duration of less than 20 ns are used to synthesize nanocrystals by laser ablation in a liquid [30-58]. Additionally, the cooling effect of the confined liquid on the laser-induced plasma should enhance the formation of the metastable structures generating in the plasma transformation. In other words, some metastable phases can be frozen in the duration of the transition from metastable to stable, due to the instantaneous cooling time (quenching time) of the plasma plume in the confined liquid. For

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example, Yang et al. observed the intermediate phase from graphite to cubic diamond in the synthesis of diamond nanocrystals by the pulsed laser ablating the graphite target in water [30]. Apparently, the kinetics provides an advantage for the metastable phase preparation by pulsed laser ablation in liquid.

Figure 4. The ablated depth of the hole left on the target surface vs laser pulse number at a laser fluence of 4.5 J/cm2 in air and water

Figure 5. A comparison of the plasma durations between pulsed laser ablation in air and liquid. A series of images of the visible plasma emission generated by the ablation of a pulsed Nd:YAG laser with wavelength of 1064 nm, pulse duration of 20 ns, and the energy fluence of 10 J/cm2 to a graphite target in air (a) and water (b)

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2.2. Synthesis of Nanocrystals Using Laser Ablation in Liquid As the descriptions of the basic processes of the laser-induced plasma evolution in a confined liquid above, two condensations of the plasma in the liquid can find their applications in materials synthesis, respectively, one for nanocrystals fabrications, and another one for thin films preparation. Actually, most of reports of materials researches concerning pulsed laser ablation in liquid have focused on the two issues. Additionally, a few of applications of pulsed laser ablation in liquid are involved in laser-based materials processing such as drilling and welding, and Fabbro [120] and Kruusing [116] have devoted two reviews for these special applications. To our knowledge, there are three kinds of nanomaterails synthesized, i.e. nanocrystals of diamond and related materials, nanoparticles of element metals, and nanostructures of metallic alloys and oxides, and two kinds of thin films prepared, i.e. metallic oxides and nitrides, and diamond-like carbon and related materials, by pulsed laser ablation in liquid in the literature. Accordingly, these applications of pulsed laser ablation in liquid in materials preparations are briefly summarized as follows. Researchers first selected diamond nanoparticles as the synthesized object by using pulsed laser ablation in liquid, because one side that the properties of diamond are quite extreme compared with that of other materials, and another side that the synthesis of diamonds under conditions of normal temperature and pressure is not predicted by the equilibrium thermodynamic phase diagram of carbon. Ogale et al. reported that diamond particulates was detected from the productions of pulsed ruby laser irradiation of the pyrolytic graphite target in the benzene solution by Raman spectroscopy, scanning electron microscope (SEM) and small angle x-ray diffraction (XRD) [13]. Yang et al. [30-32] and Zheng et al. [90] studied the formation of diamond nanocrystals from the Nd:YAG pulsed laser ablation of a isotopic graphite target in water, acetone and alcohol, and characterized the structure and crystalline morphology of diamond nanocrystals by Raman spectroscopy and transmission electronic microscope (TEM). Importantly, a relatively complete Raman spectra [32] and nice single crystal morphology and diffraction images of TEM [30] of diamond nanocrystals were reported. Further, Pearce et al. [48] reported diamond nanocrystals were synthesized by the Nd:YAG pulsed laser ablation of a pure graphite target in the cyclohexane, and provided the selected area electron diffraction (SAD) of TEM and Raman spectra of diamond nanocrystals. Notably, they pointed out that their study first time reproduced the findings of Yang et al. [30]. Besides diamond nanocrystals, nanocrystals of diamond related materials were also synthesized by pulsed laser ablation in liquid. α-, β-, cubic C3N4 and cubic boron nitride nanocrystals were prepared by the pulsed laser ablation of a hexagonal boron nitride target in water and the ammonia solution, respectively [31,121], and the Fourier transform infrared spectroscopy, XRD, energy dispersive x-ray spectroscopy (EDS) and TEM were used to characterize the structure, composition, and crystalline morphology of the production. Secondly, very recently, noble metals nanocrystals synthesis was intensively studied upon pulsed laser ablation in liquid, due to their applications as catalyst. Simakin et al. synthesized nanodisks of Au and Ag by the Cu vapor laser (wavelength 510.5 nm, pulse duration 20 ns) ablation of Au and Ag target in water, in which the diameter and thickness of nanodisks are in the of 20-60 nm range and few nanometers [38]. Meanwhile, nanoparticles of Au, Ag, Ti and Si were prepared by the same pulse laser ablation of Au, Ag, Ti and Si targets in the liquid environments (H2O, C2H5OH, C2H4C12) [39,41]. Comagnini et al. reported that nanocrystals of Au, Ag, and Pt were synthesized with average sizes of 5-30 nm

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by the Nd:YAG pulsed laser (wavelength 532 nm, pulse duration 5 ns) ablation of metallic targets in n-alkanes generates metallic solutions [43-45]. Further, Ni and Co nanocrystals were prepared by laser ablation of their target in the flowing ethanol [46]. Differently, Kondow et al. synthesized ultrafine Au, Ag, and Pt particles (less than 5 nm in diameter) by the 532 nm pulsed laser ablation of metal targets in an aqueous solution of sodium dodecyl sulfate, and found that the surfactant concentration responding to controlling of the size of the synthesized nanoparticles [122-125] On the other hand, Jeon et al. and Tsuji et al. produced silver and copper colloids by synthesized their nanoparticles in various solvents, e.g. Ag colloids from pulsed laser ablation of an Ag target in water, methanol, and iopropanol, and Cu colloids by laser ablation of CuO powders in 2-propanol [54,55,126-128], and showed that the wavelength of the pulsed laser influence greatly on the size of prepared nanoparticales. Thirdly, alloying and oxides nanocrystals and nanostructures can be fabricated by pulsed laser ablation in liquid. Singh et al. synthesized metastable silver-nickel alloying nanoparticles by a continuous wave CO2 and Nd:YAG laser ablation in the nitrate and acetate precursors of silver and nickel [36,37], in the same way, they also prepared silver and nickel oxides nanoparticles [129]. Moreover, the synthesis of Au-Ag alloying nanoparticles was studied by pulsed laser ablation of the mixture of these nanoparticles in water or ethanol [42], and Fe-Ni alloying nanoparticles were successively produced by pulsed laser ablation of an iron-nickel target in the flowing ethanol [47]. Addition to, metal alloy nanoparticles were fabricated by pulsed laser irradiation of metal powder suspensions in either aqueous or organic solutions [130]. Remarkably, Yang et al. reported that one-dimensional silver-nickel alloying nanostructures were fabricated by a Nd:YAG pulsed laser (wavelength 532 nm, pulse duration 10 ns) ablation of a nickel target in a silver nitrate solution, and typical diameters and lengths of these nanorods are in the range of 30-50 nm and 300-500 nm, respectively [35]. Anikin et al. fabricated ZnSe and US quantum dots with sizes of 10-20 nm by a Cu vapor laser irradiation of corresponding bulk semiconductors in diethyleneglycol and ethanol [40]. In oxides nanocrystals synthesis, Liang et al. synthesized ultrafine (3-5 nm) tin oxide nanocrystals by pulsed laser ablation of a tin plate target in water and aqueous solutions of sodium dodecyl sulfate [49]. Chen et al. prepared CeO2 nanoparticles by a Nd:YAG pulsed laser ablation of a CeO2 target in water [52]. Additionally, nanocrystals of aluminum hydroxides were fabricated by nanosecond pulse laser ablation of an Al rod immersed in water, and these nanoparticles show three different shapes: triangular, rectangular, and fibrous [56], and nanoparticles of magnesium and zinc hydroxides were fabricated by pulsed laser ablation of a Mg and Zn plate targets immersed in deionized water or aqueous solutions of sodium dodecyl sulfate surfactant [50,51]. More recently, Nath et al. synthesized novel inorganic fullerene-like nanostructures of the layered hafnium sulfide Hf2S and quasispherical nanoparticles of HfS by the Nd:YAG pulsed laser (wavelength 532 nm, pulse duration 10 ns) ablation of HfS3 powders in tert-butyl disulfide solvent [131], and the size of these nanoparticles is in the range of 20-80 ns. Accordingly, these results mentioned above indicate that these parameters such as wavelength and pulse duration of the laser pulse and kinds and concentrations of liquid environments can effectively control the size of synthesized nanoparticales by pulsed laser ablation in liquid. Metastable iron oxides synthesized by a pulsed Ruby laser ablation of a iron target in water stimulated Ogale et al. to develop a new surface oxidation and nitridation of metals by pulsed laser irradiating the metals targets in liquid environments, and surface coatings of iron oxides and nitrides, and GaAs oxides were prepared by this method [85-87]. Further, Sharma

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et al. deposited diamond-like carbon films on the substrate surface by pulsed laser irradiation of a tungsten substrate immersed cyclohexane and decalin liquid, and glancing XRD and Raman spectra of the as-deposited films indicated that the deposited films consists mainly a mixture of hexagonal polytypes of diamond [132]. Meanwhile, Singh et al. reported diamondlike carbon films were synthesized on the copper surface by pulsed laser irradiating a copper substrate immersed into liquid benzene [88]. Noted that more experimental data late show the prepared films from pulsed laser ablation of organic liquid on the metal surface are actually diamond-like carbons not diamonds [99-103], although the authors mentioned above claimed diamond films were synthesized. Following these studies, Lu et al. reproduced the Sharma’s findings by using a excimer pulsed laser irradiation of a single crystalline silicon surface in cyclohexane liquid [92], and prepared a series of diamond-like carbon coatings by pulsed laser irradiating various organic liquid [94,97,98]. Lyalin et al. reported that amorphous diamond films were deposited on the substrate surface by a copper vapor pulsed laser irradiating the transparent substrates (glass, fused silica, sapphire, and CaF) immersed in liquid aromatic hydrocarbons [93,95,96,99]. Moreover, Simakin et al. prepared carbon films on the Si surface by pulsed laser irradiating a Si substrate immersed into liquid benzene [100]. For the surface coatings preparations of diamond related materials by pulsed laser ablation in liquid, Sharma et al. synthesized tetrahedrally coordinated crystalline carbon nitride films on the substrate surface by pulsed laser ablation of hexamethylenetetramine or hexamine (C6H12N4) thin layer on the tungsten substrate immersed into liquid ammonia [14]. In fact, the ablation of the substrates simultaneity takes place when the laser pulse irradiated the liquid-substrate interface in these experimental cases. It is noticed that, these thin films preparations above by pulsed laser ablation in liquid are distinctly different from those nanocrystals by the same method. Clearly, one can see that compositions of nanocrystals synthesized by pulsed laser ablation in liquid are mainly from the ablation of solid targets, then, the as-deposited thin films are usually from the ablations of the liquid at the liquid-solid interface. However, both of the nanocrystals synthesis and the surface coatings preparations are within the frame of the laser-induced plasma evolution from pulsed laser ablation in liquid, which is detailed described above.

3. PULSED-LASER INDUCED LIQUID-SOLID INTERFACE REACTION 3.1. PLIIR Apparatus Based on the characters of the laser-induced plasma evolution from laser ablation in liquid, we named the novel method of pulsed laser ablation in liquid for materials preparation to be pulsed-laser induced liquid-solid interface reaction (PLIIR). A basic experimental apparatus is schematically illustrated in Figure 6. A solid target or substrate is fixed on the rotatable holder at the bottom of the chamber, and the target or substrate is immersed into flowing liquid environments, and the chamber is maintained at room temperature by watercooling. The rotatable holder ensures homogeneous of the laser ablation of targets when the pulsed laser irradiated the target surface. Since nanoparticles synthesized by laser ablating solid targets usually float on the surface of liquids due to their large surface tension, flowing

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liquids result in the successive producing of nanocrystals. The thickness of the liquid layer between gas and surface of target or substrate is the order of magnitude of millimeter. Inert gases such as Ar are guided to eject oxygen gas filled with the chamber before, because the volatiles of organic liquids, regarded as liquid environments, easily fire induced the laser pulse when they meet oxygen gas. The incident pulsed laser is focused on the liquid-solid interface by a lens during materials preparations. Noted that the laser beam need scan the surface of substrates for the application of pulsed laser ablation in liquid in surface coatings preparations.

3.2. Thermodynamic and Kinetic Factors of PLIIR Distinctly, thermodynamic and kinetic factors of PLIIR can greatly influence on the nanocrystals formation based on the understandings of the evolution of the laser-induced plasma generated by PLIIR. Taking the pulsed laser ablation of graphite and aluminum targets in water as typical examples, we contribute a basic description of thermodynamic and kinetic factors of PLIIR by detailed characterizations of three important thermodynamic parameters, i.e. density of species, temperature, and pressure, of the laser-induced plasma.

Laser 1

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Figure 6. The schematic illustration of PLIIR apparatus. 1. Laser. 2. Lens. 3. Reflected mirror. 4. Chamber. 5 and 6. Cooling water tubes. 7. Guide tubes of gas. 8 and 9. Guide tubes of liquid for input and output. 10. Holder. 11. Target. 12. Liquid. 13. Quartz window

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Intensity (au)

The density of species in the laser-induced plasma plume from pulsed laser ablation in liquid can be estimated based on the measurement of the expansion volume of the plasma plume and the calculation of the amount of the ablated species from the volume of the hole left on the target surface after ablating. The expansion volume of the plasma plume is measured from images of the light emitting region on the target surface produced the laser pulse ablating the target. Figure 7 shows the image and the intensity distribution of the light emitting region, and the volume of the plasma plume can be estimated to be 9.9×107 cm-3 by assuming that the plume is hemisphere with the diameter of the FWHM intensity. Figure 8 displays the vertical section profile of the hole left on the target surface after laser ablation. Considering volume linearly increasing with number of pulses increasing, the ablating volume by a single laser pulse is determined to be 7.4×10-8 cm3 from Figure 7b. Therefore, the density of the ablated species in the plasma plume generated by the Nd:YAG pulsed laser ablation of a graphite target in water is calculated to be 6.7×10-21 cm-3. The optical emission spectra of the ablated species from the laser-induced plasma confined in liquid are effective method to determine the temperature of the plasma plume [21,23-26,133-135]. For instance, Sakka et al. obtained the temperature of the laser-induced plasma plume of about 5000 K on the basis of the measurements of the emission spectra of C2 molecules fabricated by the ablation of a pulsed Nd:YAG laser with wavelength of 1064 nm, pulse duration of 20 ns, and the energy fluence of 10 J/cm2 to a graphite target in water [24].

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Figure 7. The image and intensity distribution of the laser-induced plasma plume from the ablation of a pulsed Nd:YAG laser with wavelength of 1064 nm, pulse duration of 20 ns, and the energy fluence of 10 J/cm2 to a graphite target in water

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8

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ACOUSTIC WAVE

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TARGET SHOCK WAVE

Figure 9. The schematic illustration of pressure measurements of the laser-induced plasma in the confined region by characterizations of the shock wave in the target and the acoustic wave in water

The high pressure in the laser-induced plasma originates from the shock wave generation when pulsed laser ablation in a confined liquid. Fabbro et al. therefore developed a series of experimental techniques to measure the pressure by characterizations of the shock wave in Figure 9a, and built the theoretical model of the laser-induced pressure generation in pulsed laser ablation in water. Similarly, Lu et al. measured the pressure in the laser-induced plasma by recording the acoustic wave in water induced by the plasma in Figure 9b. Berthe et al. reported that the maximum pressure in the laser-induced plasma was obtained as high as 5.5GPa with a pulse duration of about 50 ns when a pulsed laser with wavelength of 1064 nm, power density of 10 GW/cm-2, and pulse duration of 20 ns irradiating a aluminum plate in

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water. Based on a accepted analytical model, the maximum pressure generated by the laserinduced plasma in water is given by the following relation:

P(GPa ) = 0.01

α α +3

(

Z gcm −2 s −1

) I (GWcm ) , −2

0

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where a is the fraction of internal energy devoted to thermal energy (typically a ~ 0.25),

I0

the incident power intensity, and Z the reduced shock impedance between target and the confining water defined by the relation:

2 1 1 = + , Z Z water Z t arg et

(3.2)

where Zwater and Ztarget are the shock impedances of the water and the target respectively. For example, for aluminum target, Zwater = 0.165×106 g cm-2s-1 and Ztarget = 1.5×106 g cm-2 s-1, for silicon target, Ztarget = 2.1×106 g cm-2 s-1. Noted that the relationship between temperature and pressure in the laser-induced plasma is not completely consistent with the predictions from the idea gas state equation of P = nNAkT/V (n: gas density, NA: Avogadoro constant, k: Boltzmann constant, V: gas volume), as the formation of the laser-induced plasma in pulsed laser ablation in liquid is a far from thermodynamic equilibrium process, and the plasma is usually not regarded as an idea gas. For instance, assuming the plasma as an idea gas, the estimated pressure by the equation mentioned above is much lower than that measured by experimental techniques.

4. DIAMOND NANOCRYSTALS SYNTHESIS BY PLIIR 4.1. Synthesis of Nanodiamonds by PLIIR The interest in preparation of diamond is motivated by its unique combinations of physical hardness, high thermal conductivity and optical transparency and others. Many methods have been developed to prepare diamond since the 1950s when diamond was synthesized first with a high temperature and high pressure method [136]. PLD has been proved to be an efficient method for the preparation of a variety of functional thin films [2], [3]. Particularly, the deposition of amorphous carbon films with diamond-like characters has been widely reported by laser ablation of graphite in vacuum [137-150], in which some researchers reported the growth of crystalline particles, but the lack of a diamond peak in Raman spectra was a common result in all reports. It was not until in 1995 that Polo et al first showed the Raman spectroscopy analysis of the sample that is confirmed to have diamond cubic structure of the crystals by the presence of a sharp peak at 1332 cm-1 [151]. Ogale et al. first reported that diamond particulates with cubic structure were detected from the productions of pulsed ruby laser irradiation of the pyrolytic graphite target in the benzene solution [13]. Zheng et al. synthesized diamond nanoparticles by the Nd:YAG pulsed laser

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ablation of a isotopic graphite target in water [90]. Yang et al. [30-32] studied the formation of nanodiamonds upon the Nd:YAG pulsed laser ablation of a graphite target in water, acetone and alcohol, and obtained a relatively complete Raman spectra [32] and nice single crystal morphology and diffraction images of TEM [30] of nanodiamonds. Taking Yang’s experiments as an example, here we devote a summary of nanodiamonds synthesis by pulsed laser ablation in liquid. The synthesis setup of nanodiamonds is shown in Figure 6. The second harmonic is produced by a Q-switched Nd:YAG laser with wavelength of 532 nm, pulse duration of 10 ns, repetition frequency of 5 Hz, power density of 1010 Wcm-2. The targets are spectroscopically pure polycrystalline graphite. Water, acetone, and alcohol are regarded as reaction liquids in our case. The duration of the pulsed laser ablation of the graphite target in liquid is in the range of 30-60 min. Finally, the powders floating on the surface of the liquid are collected as the sample to be analyzed. The typical TEM morphologies of nanodiamonds are shown in Figure 10. Sizes of most nanodiamonds are in the range of 50-100 nm, and these nanocrystals display spherical (Figure 10 (c)). However, few large diamond particles were found in the production shown in Figure 10 (a) and (b).

Figure 10. TEM morphologic images of nanodiamonds synthesized by PLIIR. (a) and (b) Large particles. (c) Nanocrystals

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Figure 11. Corresponding SAD (a) and indexing (b) (BC = [112], BG = [1010]) patterns of nanodiamonds. One sees that the hexagonal diamond and cubic diamond are intermixed in the crystallites investigated here, and the orientation between these two phases is [110]c||[1210]h, [111]c ||[0001]h and [112]c||[1010]h

Clearly, we can see the crystalline planes and very regular morphology clearly, which is a common morphology in diamond. The corresponding SAD and indexing are shown in Figure 11. These nanocrystals are identified to be of a mixture of cubic and hexagonal diamond structures on basis of these results. Accordingly, one can see that the cubic and hexagonal diamond phases of the resulted sample were intermixed in the crystallites here, and the oriented relationship between these two phases is [110]c ⁄⁄ [1210]h, [111]c ⁄⁄ [0001]h and [112]c ⁄⁄ [1010]h. For the lattice parameter of the cubic phase, we obtain value of 3.56 ± 0.01 Å, which is in good agreement with values found in the literature [152], and for the lattice parameters of the hexagonal phase, we obtain values of 2.53 ± 0.01 Å and 4.11 ± 0.01 Å, which are in excellent agreement with values found in the literature for the Wurzite-like hexagonal diamond structure often referred to as Lonsdaleite [153]. To our knowledge, the Raman spectrum of nanodiamonds has been not yet understood both in experiment and theory in detail. Figure 12 shows the Raman spectra of the synthesized sample. A Raman line at 1579 cm-1 of single crystal graphite, two broad bands centered at 1352 and 1100 cm-1, and other two peaks centered at 926 and 623 cm-1, are observed in the Raman spectra in Figure 12 (a). The Raman lines of nanodiamonds usually become broader and weaker, and the peak frequency shifts to lower frequency [154]. Therefore, the Raman spectra of the resulted sample in our study need to be analyzed by deconvolution method. Although the 1332 cm-1 Raman line is not observed from Figure 12, the Raman peak at 1307 cm-1 is deconvoluted from the broad band centered at 1352 cm-1, which is usually attributed to cubic-diamond [155]. The detailed analysis of the Raman spectra in the range of 1000-2000 cm-1 is displayed in Figure 12 (b). The detailed analysis of the Raman spectra in the range of 1000-2000 cm-1 is displayed in Figure 12 (b). One can clearly see from Figure 12 (b), four peaks considering Gaussian line shapes are best fitted with the positions of 1152.5, 1125.5, 1090.9, and 1005.8 cm-1, respectively. In the early studies, Nemanich et al. [156] postulated that microcrystalline hexagonal diamond should give its strongest vibration Raman frequency at 1175 cm-1. In the report by Maruyama et al. [157] on hexagonal diamond, a Raman response was shown at

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1150 cm-1. Accordingly, we think that observed Raman peak at 1152.5 cm-1 is attributed to the microcrystalline hexagonal diamond. The observed band centered at 1090.9 cm-1 is consistent with the proposed for nanocrystalline diamond [158], and its position was close to the main peak of the vibration density of states (VDOS) of the cluster model of diamond calculated by Beeman et al. [159]. For the 623 cm-1 Raman peak, the theoretical calculation and experimental result have proved that it is attributed to diamond. Mao and Hemerly [160] have measured the Raman spectra of diamonds under ultrahigh pressure and found a broad band around 590 cm-1. Yoshikawa et al. [154] also have observed the broad band around 600 cm-1. However, the strongest peak centered at 926 cm-1 of the Raman spectrum has not yet known to us. We carefully studied the Raman spectra of the graphite target used in our study, just two graphite Raman peaks were observed (Figure 12 (c)). The Raman line at 926 cm-1 is thus not original from graphite. Based on references [156-159], we deduce that the Raman line around 926 cm-1 would be one of the series Raman lines of nanodiamonds. Accordingly, these experimental data definitely indicate that nanodiamonds are synthesized by pulse laser ablation of graphite in various liquids.

(a)

(b) Figure 12. Continued

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(c) Figure 12. The Raman spectra of nanodiamonds and graphite target. (a) and (b) The Raman spectrum and its deconvolution. (c) The Raman spectrum of the graphite target

4.2. Thermodynamic Nucleation of Diamonds upon PLIIR PLIIR is expected to be advantageous in the preparation of metastable nanocrystals that prefer a state of high pressure and high temperature. Importantly, PLIIR is a relatively new laser-based material processing method, and the mechanisms involved in the nucleation and phase transition of nanocrystals upon PLIIR are not well understood. It is therefore important to provide theoretical tools to investigate the physical and chemical phenomena involved in this processing method. More recently, taking the surface tension induced by nanosize curvature of crystalline nuclei into account, we developed a thermodynamic nucleation on the nanoscale to elucidate the nucleation of nanodiamonds and related materials [163]. To date, the thermodynamic nucleation theory has been used in the studies of cubic boron nitride nucleation in high-pressure and high-temperature superfluid systems and chemical vapor deposition, the homogenous and heterogeneous nucleation of diamond in chemical vapor deposition, and the nucleation of diamond nanowires inside carbon nanotubes [164]-[169]. Accordingly, to gain a better understanding of the formation of diamond nanocrystals upon PLIIR from the point of the view of thermodynamics, in this section, we perform a thermodynamic analysis, with respect to the effect of nanosize-induced additional pressure on the Gibbs free energy of diamond nuclei, to have a clear insight into the microscopic process of diamond formation on the nanoscale during PLIIR, based on the thermodynamic nucleation mentioned above. Basically, from the previous work [30,32,36], the formation of nanodiamonds during PLIIR can be outlined as follows. Compared with the laser ablation at gas or vacuum-solid interfaces, a special plasma plume with HPHTHD could be created at the liquid-solid interface when a pulsed-laser irradiated a solid target immersed in liquid. Thus, nanocrystals could form during the plasma rapidly quenching confined in liquid. Accordingly, in our cases [30-32], diamond nanocrystals form in plasma condensation confined in liquid, as described

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below, upon pulsed-laser ablation at the graphite–water interface. Generally, the laserinduced plasma contains some species, e.g. atomic clusters with sp2 bonding, and their ions, from the laser ablated solid. Because of the laser-induced pressure mentioned above, the laser-induced plasma is driven into the HPHTHD state. For example, in the case of pulsedlaser ablation at the graphite-water interface, the pressure-temperature region is determined to be in the range of 10-15 GPa and 4000-5000 K [30,32], which belongs to the stable region of diamond in the carbon phase diagram shown in Figure 13. Sequentially, diamond nucleation and the phase transition from graphite to diamond could take place during the plasma quenching. Since the diamond phase with sp3 bonding is a stable phase and the graphite phase with sp2 bonding is a metastable phase in the region created by PLIIR, the phase transition from graphite to diamond can take place. Thus, the formation of diamond nuclei is preferable to that of graphite in the plasma [170]. Moreover, the diameter of the grown crystals is usually on the nanometer scale, as the growth time (plasma quenching time) of diamond nuclei is very short. It is noticed that the performance of the phase transition proposed in this section is limited to the case of the diamond nucleation mentioned above. Meanwhile, there is another possibility of diamond nucleating directly from the graphite bulk upon PLIIR. It is well known that the laser-induced plasma includes particulate contaminants and droplets from the solid target [171], such as graphite particulates or droplets in the case of pulsed-laser ablation at the graphite-water interface. Naturally, diamonds could nucleate directly from these graphite fragments in the plasma, because of the higher pressure and temperature, and the shock wave generated by PLIIR. Since diamond nucleation directly from graphite was not discussed in previous studies [170], we will not consider it. 50 G 40

Pressure (GPa)

DIAMOND F

30

20 C 10

LIQ

A GRAPHITE

0

0

1000

2000

3000

4000

5000

6000

Temperature (K)

Figure 13. P, T phase and transition diagram of carbon established by Bundy. Solid lines represent equilibrium phase boundaries. A: commercial synthesis of diamond from graphite by catalysis, B: P = T threshold of very fast (less than 1 ms) solid–solid transformation of graphite to diamond, C: P, T region of synthesis of diamond by PLIIR, D: single crystal hexagonal graphite transforms to retrievable hexagonal-type diamond, B,F,G: threshold of fast P=T cycles, however generated, that convert either type of graphite or hexagonal diamond into cubic-type diamond

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Based on the understanding from experimental observations until 1994, Bundy provided a new pressure–temperature phase diagram of carbon [172] (Figure 13). The solid lines represent equilibrium phase boundaries, and the solid line from low temperature and low pressure to the triple point (12 GPa-5000 K) of diamond, graphite and liquid carbon is the socalled Berman-Simon line (B-S line) in the phase diagram. From the phase diagram one can see that diamond is in the metastable state in the region above the B-S line. The A region is the pressure-temperature region utilized for the HTHP commercial preparation of diamond from graphite. The dashed line, B-F-G, marks the threshold of very fast (i.e. ms-μs) transition of highly compressed graphite, or its low temperature derivatives, to cubic-type diamond. The B region on the dashed line B-F-G marks the temperature–pressure threshold of very fast (<1 ms) and complete solid-solid transformation of graphite to diamond. This transition always yields cubic-type diamond. Experimentally, it is done by pressurizing graphite above 12 GPa and heating the sample with a pulse of electric current or laser radiation. Therefore, our deductions and calculations are focused on the C region. Moreover, this theory is formulated based on the following assumptions: (i) nanonuclei are perfectly spherical with no deformation of the internal structure from the bulk, (ii) nanonuclei are mutually noninteractive. Generally, the Gibbs free energy is an adaptable measure of the energy of a state in the phase transformation among competing phases. At the given thermodynamic conditions, both diamond and graphite phases could co-exist. However, only one of the two phases, with the minimum free energy, is stable. The other must be metastable and may transform into the stable state. Thermodynamically, the phase transformation could be promoted by the difference of the free energies. In detail, the Gibbs free energy of a phase can be expressed as a function of the pressure-temperature condition, and is determined by a general coordinate or reactive coordinate [173]. Therefore, the Gibbs free energy difference arising from the formation of spherical carbon clusters, with the diamond structure, in the gas phase is expressed as a function of radius r, pressure P, and temperature T

4 ΔG(r ) = πr 3 × Δg Vm + 4πr 2γ , 3 where Δg is the mole volume Gibbs free energy difference,

(4.1)

γ and Vm are the surface free

energy and the mole volume of diamond, respectively. Furthermore, we have P ⎛ ∂ΔT ,P ⎞ ⎟⎟ = ΔV , in which Δ V is the molar volume difference Δg T , P = Δg T0 + ∫ ΔVdP from ⎜⎜ P ∂ ⎠T ⎝ 0 between graphite and diamond, and Δg0T is the molar Gibbs free energy difference at zero pressure. Bundy’s experimental results showed that Δ V usually remains approximately −6

constant with pressure-temperature condition, i.e. Δ V = 1.77 × 10 m3mol-1 [172]. Under the assumption of spherical and isotropic nanocrystalline diamond nuclei above, the sizeinduced additional pressure Δ P of diamond nuclei is given by the Laplace-Young equation: Δ P = 2γ/r. On the other hand, the equilibrium phase boundary between graphite and diamond in the carbon phase diagram is expressed by Pe = 2.73 × 10 T + 7.23 × 10 [172]. Due to the additional pressure Δ P, the external pressure Pe that is necessary for the transition from 6

8

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graphite to diamond will decrease by the same amount [164]-[168]. Thus, we can obtain the size-dependent equilibrium phase boundary

P e = 2.73 × 10 6 T + 7.23 × 108 − 2γ r .

(4.2)

When the pressure-temperature conditions are on the equilibrium line given by Eq. (2), we have Δg T, P = 0. Thus, we can obtain the molar volume Gibbs free energy difference of the phase transition from graphite to diamond:

(

)

Δg Td, P = ΔV × P − 2.73 × 10 6 T − 7.23 × 108 + 2γ r . Then, considering the nano-size effect, the Gibbs free energy difference of the phase transition from graphite to diamond is expressed as

(

)

4 ΔG(r ) = πr 3 ΔV × P − 2.73 × 10 6 T − 7.23 × 108 + 2γ r Vm + 4πr 2γ . 3

(4.3)

When ∂ΔG (r ) = 0 , the critical size of diamond nuclei is deduced as follows ∂r

⎛2 V ⎞ r * = 2γ ⎜ + m ⎟ ⎝ 3 ΔV ⎠

(2.73 ×10 T + 7.23 ×10 6

8

)

−P .

(4.4)

Substituting Eq. (4) into Eq. (3), we have the critical free energy of diamond nuclei as follows

( )

(

)

4 ΔG r * = πr *3 ΔV × P − 2.73 × 10 6 T − 7.23 × 10 8 + 2γ r * Vm + 4πr *2γ . 3

(4.5)

According to Eq. (4), Vm = 3.417 × 10 −6 m3/mol [6], and γ = 3.7 Jm-2 [13,8], the dependence of the radius r* of diamond critical nuclei on the temperature is illustrated in Figure 14(a) under the conditions of the given pressures and temperature in the range of 10-15 GPa and 4000-5000 K, respectively. Naturally, corresponding to the radius r* of diamond critical nuclei, the ΔG(r*)-T curves are illustrated in Figure 14(b) under the same conditions. Accordingly, from Figure 1(a) and (b), one can clearly see that these diamond nuclei with small radius r* and low formation energy ΔG(r*)could be obtained upon PLIIR. For example, the radius r* of diamond nuclei can reach a value of 3 nm, while the formation energy is less than 1 × 10 −15 J, when P = 10 GPa and T = 5000 K. Therefore, these results indicate that diamond nucleation is favorable upon PLIIR. Furthermore, both radius r* and formation energy ΔG(r*)of diamond critical nuclei decrease with increasing temperature, and increase with increasing pressure. The theoretical results could therefore be regarded as an important reference to control the size of nanodiamonds by choosing the relevant experimental parameters of PLIIR. In fact, the size distribution of diamond nanocrystals

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synthesized by PLIIR just falls in the range expected by the deduction and calculation above [30], [32].

4.3. Graphite-Diamond Phase Transition upon PLIIR The synthesis of nanodiamonds by PLIIR has been recognized to be an advance in diamond synthesis in recent years [48]. However, compared with the experimental studies of diamond nanocrystals prepared by PLIIR, few theoretical studies involved in the thermodynamic nucleation of diamond nanocrystals are found in the literature [32,48]. Wang and Yang proposed a thermodynamic phase transition model to calculate the phase transition probability from graphite to diamond upon PLIIR [174], however the model didn’t take into account the effect of nanosize induced additional pressure of diamond nuclei on the phase transition probability. Considering the capillary effect induced by nanosized curvature of diamond nuclei on the phase transition from graphite to diamond, we therefore calculate the phase transition probability from graphite to diamond taking place in PLIIR, based on the thermodynamic nucleation mentioned above.

Figure 14. The dependence of the radius r* and the Gibbs free energy ΔG(r*) of diamond critical nuclei on the temperature upon PLIIR under the conditions of various pressure, (a) for r*, and (b) for ΔG(r*)

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The Gibbs free energy is an adaptable measure of the energy of a state in phase transformation among the competing phases. For the given pressure-temperature condition, both diamond and graphite phases can co-exist, but only one of the two phases is stable, with minimal free energy, and the other must be metastable and may transform into the stable state. Thermodynamically, the phase transformation is promoted by the difference of the free energies, the phase transformation is therefore determined quantitatively by the probability of the carbon atoms crossing a potential barrier of intermediate state [175]. Generally, the Gibbs free energy GT,P of a phase can be expressed as a function of the pressure-temperature condition, and determined by a general coordinate or reaction coordinate r schematically illustrated in Figure 15. The probability of the phase transformation from the metastable phase to the stable phase is determined not only by the Gibbs free energy difference ΔGT,P , but also by an activation energy (Ea – ΔGT,P ), which is necessary for the transition. When the two phases are at the equilibrium condition, i.e. ΔGT,P = 0, Ea is the maximum potential energy for both sides with respect to the general coordinate r. The general expression of the probability f of the phase transformation from the initial state to the final state is [174] f= exp [-(Ea-ΔGT, P) / RT] – exp [- (Ea/RT) ],

(4.6)

where R is the gas constant. For the phase transformation from graphite to diamond, f = fd and ΔGdT, P = GgT, P (graphite) - GdT, P (diamond). Meanwhile, (Ea-ΔGdT, P) is the activation energy of the phase transition from graphite to diamond. Therefore, fd can be given by Eq. (6). Instead, for the phase transformation from diamond to graphite, f = fg and ΔGgT, P = GdT, P (graphite) – GgT, P (diamond), fg should be expressed as fg = exp [- (Ea/RT) ] – exp [-(Ea-ΔGgT, P) / RT]. It is noted that, for the phase transition from diamond to graphite, Δ V = - 1.77 × 10 mol-1, and then the molar volume Gibbs free energy difference ΔGgT, P can be given by ΔGgT, P = 1.77 × 10

−6

(2.73 × 10 T + 7.23 × 10 - 2γ/r – P). 6

8

Ea-ΔG c-BN ΔG

h-BN

Coordinate

Figure 15. Schematic diagram of Gibbs free energy vs coordinate

(4.7) −6

m3

(4-8)

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Figure 16. The phase transition probabilities between graphite and diamond, in which the probabilities from graphite to diamond transition are above the B-S line, the probabilities from diamond to graphite transition are below the B-S line

Substituting Eq. (4) into Eq. (6), Eq. (8) into Eq. (7), and having Ea = 120 kJmol-1 [174], we calculate the phase transition probability between graphite and diamond upon PLLIR. The P-T curves with the radius r = 5.0 nm are shown in Figure 16. Clearly, comparing with Wang’s calculation without the nanosize effect [174], one can see that the probability distribution of the phase transition from graphite to diamond moves downwards along the B-S line in the carbon phase diagram, from Figure 16. This distribution change is caused by the nano-size-induced surface tension. In other words, considering the nano-size-induced surface tension of diamond nuclei, the phase transition probability from graphite to diamond goes up at the same pressure-temperature condition. More importantly, we can see that the probability of the phase transition is rather high, up to 10-3-10-2, in the pressure-temperature region generated by PLIIR in Figure 16. Actually, these results imply that PLIIR would be a very efficient way to synthesize nanodiamonds. From Eq.(6) and (8), the dependence of the phase transition probability from graphite to diamond on the radius of crystalline grains is shown in Figure 3 under the conditions of P = 13 GPa and T = 4500 K. The phase transition probability markedly decreased with the increasing radius of crystalline grains in Figure 17. Moreover, the probability increases much more when the size of crystalline grains is less 5 nm. Accordingly, from the results, we deduce that diamond nanocrystals with a radius of 2-4 nm would be easily synthesized by PLIIR compared to grains of size 10-20 nm. Furthermore, the comparison of the dependence of the phase transition probability on the pressure is shown for various grain sizes and T = 4500 K in Figure 18. One can see that the probability curves are parallel to each other at the different grain sizes, and almost linearly increase with increasing pressure under the conditions of the temperatures used in Figure 18. In fact, Winter and Ree calculated the phase stability of carbon particles as a function of the size by first-principles and semiempirical molecular orbital calculations [176]. Their results showed that the diamond phase is more stable than the graphite phase when the carbon particle includes less 104-105 carbon atoms (a cluster size of 4-5 nm). However, the clusters

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with diamond structure larger than 105 carbon atoms become less stable than threedimensional graphite clusters with the same size. Therefore, our theoretical results are consistent with Winter and Ree’s calculations. Accordingly, the thermodynamic analysis above implies that PLIIR could be expected to be advantageous in the synthesis of ultrananocrystalline diamonds.

Figure 17. The dependence of the phase transition probability from graphite to diamond on the size of the crystalline grain under the conditions of P = 13 GPa and T = 4500 K

Figure 18. The dependence of the phase transition probability from graphite to diamond on the pressure under the conditions of T = 4500 K and various sizes of the crystalline grain

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4.4. Structural Transformation of Diamonds upon PLIIR The hexagonal diamond (hex-d) so-called Lonsdaleite, as the metastable phase with respect to cubic diamond (cub-d), was first prepared by shock-wave (SW) [177]. Then, Bundy also obtained the hex-d by using the static high-pressure pressing the hex-g along the c axis at the high temperature [178]. However, recent attempts to reproduce the conversion of graphite to hex-d under static pressure failed [179,180]. It is therefore so far not completely confirmed that the conversion of graphite to hex-d could be achieved under static high-pressure. For the theoretical side, the highly symmetric transformation path from rhombohedral graphite (rh-g) to cub-d was first considered, i.e. rh-g as an intermediate phase of hex-g to cub-d transformation [181,182], and the transformation path would lead a final [111] diamond orientation parallel to the original graphite c axis. These deductions are in agreement with the catalyst-aided conversion in HTHP [178], in which catalyst was suggested to aid the mutual orientation formation of hex-g and cub-d [183]. The orthorhombic graphite (or-g) was also considered to be the transformation path of hex-g to cub-d and hex-d [184,185], and this path would lead to the final [112] diamond orientation parallel to the c axis of the hex-g, similar to the shock-wave experimental results [186]. Considering a large activation barrier from graphite to diamond, we would like to know whether the intermediate phase with metastable structure as the transformation path would exist in the conversion of graphite to diamond. However, the existence of the hypothesized intermediate phase, i.e. rh-g and or-g, in the conversion of graphite to diamond has never been truly substantiated by the direct experiments [181,187,188]. On the other hand, PLIIR is a very fast and far from equilibrium process, so that all metastable and stable phases forming at the initial, intermediate and final stages of the conversion could be reserved in the final products, especially, for the metastable intermediate phase. In other words, the quenching times of PLIIR are so short that the metastable phase forming at the intermediate stage of the conversion could be frozen in the obtained final products. However, HTHP and SW methods Hardly have the two features (the quenching times of SW are much longer than those of PLIIR) simultaneously. It is therefore very unlikely that these possible metastable intermediate phases in the conversion of graphite to diamond could remain in the two methods, because these phases maybe transform into the stable phase at the final stage. Therefore, PLIIR provides a good experimental method to detect the intermediate phase forming in the conversion of graphite to diamond, if these phases are existing. Using PLIIR, here we show the observation of an rh-g phase, the intermediate phase as the transformation path of graphite to diamond in experimental [170]. The basic experimental results have been described in the section of nanodiamonds synthesized by PLIIR. Figure 11 shows the indexing of the SAD. The cubic and hexagonal phases of diamond are intermixed in the crystallites investigated here, and the oriented relationship between these two phases was [110]c||[1210]h, [111]c||[0001]h, and [112]c||[1010]h. Noted that the ratio of diamonds to whole powders synthesized by PLIIR increases with the pulsed laser energy increasing, which means the transition probability of graphite to diamond would increase with the used pulsed laser energy increasing. In addition, the conversion of graphite to diamond was achieved by various radiated pulsed laser energies from 250 mJ to 350 mJ. To detect the possible metastable intermediate phase, the pulsed lasers having energy values of 280 mJ, 300 mJ, and 330 mJ were used to produce the conversion of graphite to diamond, respectively. Figure 19 shows the corresponding X-ray

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CPS

diffraction spectra (XRD). In the XRD of the graphite target in Figre 19, one can see that that all the diffraction peaks belong to the hex-g, and the diffraction lines of the rh-g are not found. However, with the used laser energy increasing, the remarkable diffraction peaks of the rh-g were clearly observed in the XRD of the sample prepared by using the 330 mJ laser pulse. The strongest (003) diffraction line, 2θ = 26.6, of the rh-g overlaps the strongest (005) diffraction line of the hex-g, and the second strongest (101) diffraction peak of the rh-g, 2θ = 43.44, was in between the hex-g (101) and hex-g (102) lines. These experimental results only show the experimental evidence that the metastable rh-g phase indeed formed in the conversion of graphite to diamond upon PLIIR. To gain further insight into the transformation of the intermediate phase rh-g, the second strongest (101) diffraction of the rhg was selected as the mark line to study the dependence of intensity of the mark line on the used laser energy. The results are shown in Figure 20. One clearly sees that the intensity of the (101) diffraction peak of the rh-g rapidly increases with the used laser energy increasing. This observation means that the transformation probability of the hex-g to rh-g increases with the used pulsed laser energy values increasing. Therefore, the above experimental results indicate two important evidences.

b 8K

a 25

50

100

2θ

Figure 19. XRD of the prepared samples by PLIIR. (a) The XRD of the original solid graphite target. (b) The XDS of the powders obtained by using 330 mJ pulsed laser

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CPS

3270

2247 2242

280

300 Pulsed-laser energy (mJ)

330

Figure 20. The dependence of the (101) diffraction peak of the rhombohedral graphite on various used pulsed laser energies

First, excluding the metal catalyst effect, the conversion of graphite to diamond was strikingly confirmed upon PLIIR. Second, the metastable rh-g phase as the intermediate phase in the hex-g to cub-d conversion was firmly obtained in this experiment. Accordingly, the formation mechanism of two structural diamonds are suggested that hex-g would be directly transformed to hex-d, however, cub-d was obtained by an indirectly transforming, i.e. from hex-g first to an intermediate phase rh-g, then to the final state cub-d by a solid–solid transformation manner [30]. Figure 21 shows this structural evolution. In other words, the basal planes of the hex-g could become boat-buckled and di-intermediate phase rh-g could be remained. Obviously, the deductions are well in agreement with the experimental result. A thermodynamic analysis of the graphite-to-diamond transition could aid the understanding of the various transformation paths. Based on Figure 13, one can see that diamond is in the metastable state in the region below the B-S line, and the A region (5–12 GPa, 2000–3000K) is the pressure–temperature region utilized for HTHP commercial preparation of diamond from graphite, and the conversion of graphite to diamond via the rh-g intermediate phase as a transformation path seems to exist in this region [178].

[111]

Figure 21. Geometric path for graphite-to-diamond transformation: hexagonal graphite → rhombohedral graphite → cubic diamond, the different stackings of the hexagonal planes are viewed along the sideways

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The B region (15 GPa, 1000 K) represents the conversion of graphite to diamond via an or-g intermediate phase as the transformation path found in SW. Furthermore, we mainly paid attention to the C region (10–15 GPa, 4000–5000K) in the P–T phase diagram, in which the pure conversion of graphite to diamond via a rh-g transformation path is achieved upon PLIIR. Our previous studies showed that the transformation probabilities of the carbon atoms over a potential barrier to transform diamond structure in these regions above were different [174]. For A, B, and C regions, the transiting probabilities of C region are the highest, and in turn the transiting probabilities of B region are lowest. Based on the analysis above, it is suggested that two conversions of graphite to diamond, i.e. or-g and rh-g transformation paths, could exist in B and C regions in the carbon phase diagram, respectively. Since the parallelism of the [111] orientation of the final cub-d with the c axis of the initial hex-g was found in the catalyst-aided conversion of graphite to diamond by HTHP [189], the model of the rh-g transformation path seems to find the experimental evidence. However, the use of the metal catalysts is known to drastically decrease the transiting pressure and temperature, and is therefore expected to alter significantly the conversion mechanism (possibly through a metalinduced buckling of the graphite basal planes) [183]. However, in our results, the graphite-todiamond conversion was more pure, i.e. there were not any metal catalysts, the observations in this study were therefore the true experimental evidences of the above model. Considering the conversion of graphite to diamond upon PLIIR taking place in C region of the carbon phase diagram, it is suggested that the hex-g to rh-g to cub-d phase transformation may not be achieved in A region if the transition metal catalysts are not used. The parallelism of the [112] orientation of the final cub-d with the c axis of the initial hex-g was found in SW-induced graphite-to-diamond conversion, which takes place in B region. It is therefore believed to be the experimental proof of the theoretical model of the graphite-to-diamond conversion through an or-g path, but the or-g phase had never been obtained in the corresponding experiment [183]. Interestingly, using the constant-pressure ab initio molecular dynamic simulations recently confirmed this model [190]. From these results, again, it is suggested that the different graphite-to-diamond conversions may exist in the different pressure–temperature regions in the carbon phase diagram [172], for example, the two mentioned above. However, it is not comprehensible why the transforming by the rh-g path never appeared in Scandolo et al. simulation [190]. According to HTHP, SW, and PLIIR, a reasonable deduction was that the important factor about the conversion of graphite to diamond, i.e. the temperature, was not considered in their simulations. In other words, the temperature did not play a relevant role due to the overwhelming effect of the pressure in the molecular dynamic simulations. In fact, the pressure of SW-induced conversion (B region) is not much larger than that of the pressures of HTHP (A region) and PLIIR (C region), but the temperatures of the conversions upon HTHP and PLIIR are much higher than that of SW. Therefore, for the conversion of graphite to diamond, the pressure favorably induces an or-g transformation path, then the temperature favorably produces a rh-g transformation path.

4.5. Stability of Nanodiamonds Synthesized by PLIIR Compare diamond with the cubic structure, it is well known that hexagonal diamond is a metastable phase. Naturally, the formation of the cubic structure of diamond would be prior to

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that of the hexagonal structure in competing growth of cubic and hexagonal phases from the point of the view of thermodynamics [190]. Interestingly, nanocrystalline diamonds with cubic and hexagonal structures were synthesized simultaneously by PLIIR [30]. Similarly, nanocrystalline boron nitrides with cubic and explosion phase that is a high-temperature and high-pressure phase were also prepared simultaneously by PLIIR [32]. Then, it is noted that the explosion phase boron nitride is a metastable phase compared with cubic structure boron nitride. To gain a better understanding to the stability of nanodiamonds with different phases upon PLIIR from the point of view of structure energy, first-principles calculation using the CASTEP code (version 2.2 Ab Initio Total Energy Program) [192] was therefore employed to perform the phase stability of the allotropes of nanodiamonds upon PLIIR [193]. The CASTEP code is based on the density-functional theory to describe the electron–electron interaction and a pseudopotential description of the electron-core interaction, and has been publicized as a Cambrigde Serial Total Energy Package (CASTEP). Usually, it can give the sum of electronic energy of a large system, as well as its band structure. Transferability and robustness of the assumed pseudopotentials of each element seem to be confirmed by success in reproducing the physical properties such as lattice parameters of many compounds. Therefore, it can be expected to give the relative stability of crystalline allotrope. The detail experimental procedure was described in the section of nanodiamonds prepared by PLIIR. We calculated the total energy of cubic diamond, hexagonal diamond, and graphite versus the various volumes (V) by ab initio calculation. The curves of the total energy versus various atomic volumes (V) with regard to cubic diamond, hexagonal diamond, and graphite using the CASTEP code are shown in Figure 22. Clearly, one can see that graphite phase is equilibrium phase. The result is in agreement with our case that the ratio of graphite to the whole prepared powders was more 95%. In addition, we can see that cubic and hexagonal are far from the equilibrium phase and the energy difference between cubic diamond and hexagonal diamond is very small, with a magnitude of about 30 meV/atom. Moreover, the total energy of hexagonal diamond is slightly higher than that of cubic diamond. As the expected, these results indicated that hexagonal diamond is less stable than the cubic one. Generally, the origin of the small total energy difference of the cubic and hexagonal diamond is attributed to their similar structural properties. The crystal structures of the cubic and the hexagonal diamond were shown in Figure 23. Distinctly, one can see that the atomic arrangement is very similar between cubic diamond and hexagonal diamond. Both the two structures have covalent tetrahedral bonds and contain only six-atom rings of bonds. Moreover, the two structures have the same environments of the first and the second nearest neighbors, and but different environments of the third nearest neighbors. In detail, along <111> atomic planes direction of cubic diamond and the c direction of the hexagonal diamond, the difference between the two structures is better understanding by considering the crystals to be constructed. In both structures, atomic layers are stacked on top of one another with successive layers displaced sidewise. In cubic diamond, every fourth layer atoms are stacked on top without any sidewise displacement, giving rise to the stacking sequence ABC ABC.... as shown in Figure 23 (a). On the other hand, the stacking sequence is the type of AB AB... in hexagonal diamond, as shown in Figure 23 (b). Moreover, in cubic diamond, all six atoms form the ring, in which four atoms lie on a plane and the remaining two atoms occur on the opposite sides of this plane. Then, it forms the so-called ‘‘chair’’ shapes, such as the shade part shown in Figure 23 (a). However, in the hexagonal diamond, it has both the chair shapes as well as the

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‘‘boat’’ shapes. In contrast to the chair shapes, the boat shapes have four atoms on a plane and the remaining two atoms occur on the same side of this plane, as shown in the shade part Figure 23 (b). Whereas the space groups of the cubic diamond and hexagonal diamond are the Oh7 and O6h4 group, respectively. The structure of hexagonal diamond has lower symmetry than the cubic diamond. However, why could cubic diamond and hexagonal diamond coform and coexist in our synthesis system? It was proposed that PLIIR would be a very fast and far from equilibrium process, so all metastable and stable phases forming at the initial, intermediate, and final stages of the conversion could be reserved in the final products, especially, for the metastable intermediate phase [170] In other words, the quenching times of PLIIR are so short that those metastable phases forming at the intermediate stage of the conversion could be frozen in the final products. On the other hand, owing to the similarities of cubic and hexagonal diamond in structures as the mentioned earlier, and the total energy difference of the two structures is very small. Accordingly, the hexagonal diamond could not only form at the intermediate stage of the conversion, but also could exist in the final products in a far-from equilibrium process, such as PLLIR. Importantly, the energy, pressure, and temperature are always shifting periodically with the repetition-frequency operation of the laser beam in the pulselaser ablation system, even though the shift is very small. It is therefore possible that the cubic and hexagonal diamond nanocrystals were synthesized one after the other under the difference energy, pressure, and temperature conditions. More importantly, Wu et al. [194] pointed out that the formation of hexagonal diamond is usually favored under the conditions of the high carbon supersaturation, whereas the cubic diamond grows at much lower supersaturation levels. In the PLIIR, at the initial stage of pulsed-laser ablating graphite target, the density of the enabled carbonelement supersaturation is always much high, that time, hexagonal phase formation should be favored. With the pulse over, the density of carbon supersaturation will greatly decrease. Thus, cubic phase growth will be favored at the stage. Therefore, hexagonal and cubic phases could form by turns when the density of carbon supersaturation shifts periodically with the repetition-frequency operation of the laser beam.

-155.2

Etot (eV/atom)

-155.4 -155.6 -155.8 -156.0

… hexagonal diamond --- cubic diamond graphite

-156.2 4

5

6

7

8 3 (Å /atom) Volume

9

10

Figure 22. Total energies of cubic diamond, hexagonal diamond, and graphite calculated for various volumes

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Figure 23. Crystal structures of the cubic and hexagonal diamond. (a) Cubic diamond. (b) Hexagonal diamond

CONCLUSION There are the concluding remarks of pulsed laser ablation in liquid on the basis of experimental and theoretical studies above. (i) The plasma plume with HTHPHD is created at the liquid-solid interface when the laser pulse ablated the solid target in the confined liquid. Meanwhile, the higher ablating rate to the solid target and shorter quenching time of the plasma plume in liquid can be achieved. These traits offer a potential to synthesize

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nanomaterials and nanostructures. (ii) PLIIR has been verified to be an effective and general route to synthesize nanostructures, especially, metastable nanostructures that prefer high temperature and high pressure. New phase formation of nanostructures may involve in both liquid and solid. (iii) Diamonds nanocrystals have been synthesized by PLIIR in our studies. The relevant thermodynamics and kinetics of the nanostructures formation upon PLIIR are suggested in theoretical. (iv) PLIIR could allow researchers to choose and combine interesting solid targets and liquid to fabricate nanostructures of new compounds for purpose of fundamental research and potential applications.

ACKNOWLEDGMENTS The National Science Foundation of China and the Ministry of Education of China funded this work. The author is grateful to Dr. J. B. Wang of Xiangtan University, Dr. Q. X. Liu of Guangdong University of Technology, and Dr. C. X. Wang of Tokyo Institute of Technology, who ever worked in the author’s group and made the important contributions to the research field covered by this review chapter.

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INDEX A  absorption spectra, 212, 214  accounting, 33, 34  acetic acid, 54  acetone, 192, 198, 199, 204, 207, 212, 234, 241  acid, 53, 54, 145  activation energy, 137, 249  aerosols, 5, 20  AFM, 219, 220  aggregation, 128, 137, 212, 214  aging process, 129, 131  algorithm, 107, 114  alloyed nanoparticles, x, 191, 206  alters, 195, 196, 197, 214  aluminium, 2, 3, 6, 7, 9, 13, 15, 24, 26, 31  ambient air, 5, 32  ambient gas, vii, 1, 3, 5, 8, 11, 13, 17, 19, 23, 25, 26,  27, 29, 31, 32, 33, 35, 39, 100, 107  ammonia, 234, 236  amplitude, 120  aqueous solutions, 195, 200, 212, 221, 235  archaeological skeletal materials, vii  argon, 17, 20  aromatic hydrocarbons, 236  aspiration, 54, 55  asymmetry, 202  atmosphere, 4, 33, 55, 128, 129  atmospheric pressure, 11, 18, 31  atomic positions, 102, 155  atoms, 5, 8, 17, 22, 26, 32, 33, 34, 107, 108, 109,  110, 112, 137, 154, 155, 156, 208, 212, 215, 251,  256  attachment, 137  Au nanoparticles, 200, 204, 211, 217, 220 

B  background gas, ix, 32, 99, 103, 107, 109, 110, 111,  112  background noise, 25  band gap, 139  bandgap, 118  barium, 84, 85  baths, 216  beams, vii, 1, 4, 5, 25, 28, 37, 216, 217, 222  bending, 146, 147  benefits, 15  benzene, 231, 234, 236, 240  bias, 52  bicarbonate, 25  binding energy, 221  biomedical applications, 146  BMI, 224  BN nanocrystals, 228  Boltzmann constant, 8, 114, 240  bonding, 155, 245  bonds, 155, 256  bone, 46, 50, 96  bones, 52, 93, 96  boreal forest, 47  brass, 9, 11, 12, 17, 20, 24, 31, 196, 197, 208, 209,  214  breakdown, 3, 6, 14, 32, 37, 39, 42, 102, 119, 120,  147 

C  calcification, 51, 52  calcium, viii, 46, 51, 52, 55, 82, 83, 85, 91, 93, 144  calculus, 143, 150 

268

Index

caliber, 143, 147  calibration, 24, 54, 55  Cambrian, 48  candidates, 52  capillary, 208, 219, 220, 248  carbon, 95, 206, 207, 211, 228, 234, 236, 240, 244,  245, 246, 249, 250, 255, 257  carbon atoms, 249, 250, 255  carbon film, 236, 240  carbon nanotubes, 228, 244  case study, 94, 96  catalysis, 101, 222, 245  catalyst, 195, 214, 234, 252, 254, 255  catalytic effect, 214  cation, 54  cattle, 95  cesium, 84  chaos, 110  chemical, vii, viii, x, 1, 8, 45, 47, 51, 92, 103, 107,  128, 131, 148, 150, 191, 192, 195, 204, 206, 207,  211, 212, 214, 228, 230, 231, 244  Chicago, 94  China, 227, 259  chlorine, 212  chloroform, 212  circadian rhythm, 50  cladding, 147  classification, 4  cleaning, 100, 101, 227  climate, 47  cluster model, 243  clustering, 81  clusters, 15, 20, 101, 103, 115, 193, 211, 245, 246,  250  CO2, 2, 28, 235  coatings, 101, 230, 235, 236, 237  collateral, 100  collinear geometry, vii, 1, 10, 37  collisions, 107, 109, 110, 113, 119  combined effect, 17  compatibility, 131  complement, 228  complexity, 19, 47, 92, 103  complications, 35, 148  composites, 133  composition, viii, x, 3, 8, 15, 17, 20, 45, 47, 52, 53,  56, 82, 150, 191, 192, 211, 212, 221, 222, 234  compound semiconductors, 101  compounds, 195, 206, 214, 256, 259  compression, 103, 110, 113 

computing, 155  condensation, 20, 103, 115, 232, 244  conduction, 33, 117, 118, 119, 121, 154, 158, 193,  194  conductivity, 24, 101, 102, 122, 159, 194, 197, 240  configuration, vii, viii, 1, 2, 4, 5, 6, 7, 8, 10, 11, 12,  13, 14, 16, 17, 18, 19, 20, 22, 24, 25, 26, 27, 28,  30, 31, 33, 35, 36, 37, 39, 145  confinement, ix, 13, 25, 26, 35, 127, 128, 134, 144,  218  conformity, 220  consent, 128  conservation, 146, 154  construction, 154  contour, 110, 112  cooling, 115, 121, 214, 232, 236  copper, 9, 13, 17, 24, 49, 208, 212, 214, 235, 236  correction factors, 79  correlation, 16, 83  correlations, 85, 88  cost, 14, 128, 132, 147, 149, 222  Coulomb interaction, 157  covering, 47, 192  critical density, 119, 159  critical temperature, viii, 2, 31, 106, 113  critical value, 32, 33, 159, 214  crown, 49, 51, 55  crystal structure, 51, 134, 155, 256  crystalline, 101, 129, 131, 133, 134, 136, 212, 234,  236, 240, 242, 244, 250, 251, 256  crystallites, 242, 252  crystallization, 105  crystals, 51, 232, 240, 245, 256  cubic boron nitride, 234, 244  culture, 49  cyanide, 145, 148, 150, 152  cycles, 245  cyclodextrins, 200  cysteine, 145  cystine, 144 

D  database, 56  decay, 4, 11, 13, 24, 25, 26, 35, 38, 115, 116, 122,  133  decomposition, ix, 99, 103, 113, 114, 115, 116, 117,  122, 150, 207  deconvolution, 242, 244  deduction, 248, 255 

Index defects, 133  deformation, 246  degradation, 146, 148, 151  density values, 6, 8  deposition, 14, 28, 50, 101, 113, 140, 195, 227, 232,  240, 244, 261  deposits, 48  derivatives, 159, 246  desorption, 100, 105, 107  detachment, 220  detection, 6, 7, 26  detonation, 29, 32  deviation, 92, 115, 205  diamonds, 234, 236, 243, 245, 251, 252, 254, 256  dichotomy, viii, 46, 84  dielectrics, x, 103, 153, 154, 155, 157  diet, 51, 81, 82, 96  dietary intake, 50  differential equations, 117  diffraction, 133, 212, 234, 241, 253, 254  diffusion, ix, 28, 36, 37, 38, 99, 102, 110, 154, 157,  158, 159, 193, 194, 197, 208  diffusion time, 28, 37, 38, 197  diffusivity, 16, 36, 194  dilute gas, 110  dimensionality, 107  direct measure, 15  discontinuity, 31, 32, 104  discs, 49  dispersion, 120, 130, 133  displacement, 13, 31, 36, 39, 256  dissociation, 26, 107, 138  distribution function, 154, 156, 200, 201, 214, 215  divergence, 52, 92, 147  diversity, 195  doping, 132, 134  Double Pulse (DP) Laser ablation, viii, 2  double‐pulse laser ablation, vii  drainage, 47  drawing, 92  drying, 133  dynamic evolution, vii, 2, 22 

E  Early Bronze Age, viii, 45, 49  electric current, 246  electrolyte, 130  electron, ix, 6, 8, 10, 11, 22, 25, 26, 30, 32, 33, 37,  87, 100, 102, 108, 109, 113, 115, 117, 118, 120, 

269

121, 122, 133, 139, 149, 154, 155, 156, 157, 207,  220, 234, 256  electron diffraction, 133  electronic materials, 140  electron‐phonon coupling, 102  electrons, 27, 32, 101, 102, 103, 108, 109, 110, 113,  117, 118, 119, 120, 132, 156, 157, 194, 195, 197,  201, 202, 211, 217, 218, 222  elephants, 96  elongation, 24  emission, 3, 4, 5, 8, 9, 10, 11, 12, 13, 14, 15, 17, 18,  20, 22, 24, 27, 32, 36, 37, 38, 133, 135, 139, 150,  154, 192, 221, 231, 233, 238  employment, 131  enamel, viii, 45, 50, 51, 53, 55, 79, 82, 85, 86, 87, 93,  95, 96  enamel matrix, viii, 45, 50, 51, 79, 82  endoscopic laser lithotripsy, ix, 143  energy, vii, viii, ix, xi, 2, 3, 6, 7, 8, 9, 10, 11, 13, 14,  15, 16, 18, 19, 20, 25, 26, 27, 28, 29, 31, 33, 34,  36, 37, 38, 55, 99, 102, 105, 108, 109, 111, 113,  115, 117, 137, 139, 143, 144, 146, 147, 148, 150,  154, 155, 156, 158, 159, 191, 193, 194, 195,  197,201, 202, 207, 211, 215, 220, 221, 222, 231,  233, 234, 238, 240, 246, 247, 249, 252, 256  environmental characteristics, 39  environmental conditions, 19  epithelium, 148  equilibrium, 28, 37, 101, 102, 106, 107, 115, 234,  245, 246, 247, 249, 252, 256, 257  equipment, 79  etching, 128, 130, 134  ethanol, 130, 131, 136, 139, 192, 196, 197, 204, 205,  206, 207, 208, 209, 211, 212, 213, 214, 216, 217,  235  evaporation, 27, 28, 100, 101, 104, 105, 107, 113,  194, 197, 198, 212, 214  excimer lasers, 193  excitation, ix, xi, 2, 9, 10, 26, 33, 100, 102, 107, 117,  118, 122, 131, 135, 155, 191, 220, 230  exclusion, 37  experimental condition, viii, 2, 3, 4, 6, 18, 25, 30, 33,  37, 100, 103, 122, 198, 207, 211, 215  exploitation, 14  exponential functions, 155  exposure, xi, 11, 97, 191, 193, 195, 196, 200, 201,  202, 203, 204, 205, 206, 207, 208, 209, 210, 214,  215, 216, 220, 221, 222  expulsion, 31  extinction, 197 

270

Index

F  fabrication, ix, 101, 127, 128, 129, 131, 138, 139,  227  faith, 92  faunal samples, viii, 45, 49  femtosecond, ix, x, 3, 4, 99, 100, 101, 102, 113, 121,  122, 149, 153, 154, 157, 163, 164, 165, 166, 168,  169, 170, 171, 175, 178, 180, 181, 182, 185, 186,  198, 200, 210, 211, 221  femur, 49  Fermi level, 217  fiber, 143, 144, 145, 146, 147, 148, 149  fibers, 146, 147, 148, 151  film formation, 132  films, 101, 102, 130, 139, 140, 234, 236  filters, 54  fish, 47  fishing, 47  flexibility, 131, 146, 147  flight, 106, 107, 137  fluid, 102, 108  Fokker‐Planck approach, x, 153, 154  formula, 131, 132  fragmentation processing, ix, 127, 128, 134, 135  fragments, 53, 132, 144, 145, 146, 149, 217, 245  France, 99  free energy, 139, 244, 246, 247, 248, 249  freedom, 107  fullerene, 128, 235  functionalization, 136 

G  gel, 51  geochemical composition, viii, 45  geochemical data, viii, 46, 82  geography, 51, 81, 82, 84  geometry, vii, x, 1, 3, 10, 15, 28, 146, 153, 160  Germany, 95  Gibbs energy, 137  glasses, 3  graphite, 214, 229, 231, 233, 234, 237, 238, 239,  240, 241, 242, 243, 244, 245, 246, 247, 248, 249,  250, 251, 252, 253, 254, 255, 256, 257  Greece, 93  growth time, 245  Guangdong, 259  Guangzhou, 227 

guidance, 148  guidelines, 51 

H  hafnium, 235  hardness, 240  heat capacity, 102, 194  heat conductivity, 113  heat transfer, 33, 34, 197  heavy particle, 108, 113, 114  helium, 17  hemisphere, 238  histology, 96  holmium, 150, 151  hot spots, 25, 197  HRTEM, 133, 134  Hunter, 45, 47, 94, 95  hunter‐gatherers, viii, 46, 84  hybrid, 107, 206  hydrodynamic model, ix, 99, 102, 113  hydrothermal synthesis, 214  hydroxyapatite, 82, 85  hydroxyl, 146  hydroxyl groups, 146  hypothesis, 15, 20, 26, 27, 28, 32, 36, 106, 202 

I  ideal, 33, 34, 108  image, 136, 198, 220, 221, 238  images, 11, 12, 22, 23, 24, 35, 132, 133, 206, 207,  233, 234, 238, 241  immigrants, 56, 84  impurities, 119  incidence, 5, 32, 119, 194, 196  indexing, 242, 252  inequality, 106  inferences, 94  infrared spectroscopy, 234  initial state, 249  initiation, 29, 31, 32, 221  initiation time, 29, 31, 32  insulation, 25  insulators, 16, 36  integration, 131  interface, xi, 33, 105, 128, 147, 192, 196, 202, 206,  219, 227, 228, 230, 232, 236, 244, 258 

Index interference, viii, 46, 52, 53, 54, 85, 86, 92, 94, 216,  220  intracorporeal laser lithotripsy, ix, 143, 146, 149  intrinsic value, 159  ion implantation, 38  Ion Mobility Spectrometry, vii, 1  ionization, ix, 8, 11, 15, 16, 22, 25, 26, 27, 33, 35, 46,  100, 102, 103, 108, 109, 110, 117, 118, 120, 122,  156, 157, 158, 159  ions, 5, 26, 38, 52, 85, 101, 103, 108, 109, 113, 156,  160, 192, 195, 245  iron, 17, 228, 235  irradiation, 2, 101, 103, 108, 115, 116, 130, 133,  134, 135, 137, 138, 139, 144, 145, 146, 148, 193,  197, 198, 201, 208, 228, 230, 234, 235, 236, 240  isobaric interference, viii, 46, 52, 54  isotope, 46, 47, 49, 51, 54, 55, 84, 85, 86, 87, 88, 93,  94, 95, 96, 97, 221, 222  Italy, 162 

J  Japan, 127, 139, 263 

K  kidney, 147, 148  kinetics, xi, 102, 227, 232, 259 

L  LA‐Atomic Fluorescence Spectrometry, vii, 1  Lake Baikal, viii, 45, 47, 48, 49, 94, 95  LA‐Microwave Induced Plasma‐Atomic Emission  Spectrometry, vii, 1  landscape, 50  laser ablation, vii, ix, x, 1, 4, 13, 15, 17, 29, 30, 31,  32, 34, 35, 36, 39, 46, 50, 52, 53, 54, 55, 56, 84,  86, 89, 93, 95, 97, 99, 100, 101, 103, 104, 107,  110, 112, 113, 121, 122, 127, 128, 129, 131, 132,  133, 134, 135, 137, 138, 149, 150, 153, 154, 191,  192, 193, 194, 195, 196, 197, 198, 200, 205, 207,  208, 209, 210, 211, 212, 214, 215, 218, 219, 220,  222, 227, 228, 231, 232, 233, 234, 235, 236, 237,  238, 239, 240, 241, 243, 244, 257, 258  Laser ablation, vii, xi, 54, 55, 57, 78, 90, 93, 96, 97,  100, 128, 140, 195, 206, 227  laser beam, vii, x, 1, 4, 5, 25, 28, 32, 100, 103, 107,  120, 129, 130, 135, 138, 145, 146, 147, 148, 161, 

271

174, 191, 193, 194, 196, 197, 201, 202, 204, 210,  211, 214, 215, 216, 217, 220, 222, 237, 257  laser energy, vii, viii, 2, 18, 29, 31, 32, 33, 36, 108,  111, 113, 115, 117, 143, 144, 148, 154, 164, 165,  168, 169, 179, 180, 193, 194, 195, 207, 211, 215,  222, 252  laser flux, vii  Laser Induced Breakdown Spectroscopy (LIBS), vii, 1  laser radiation, 32, 38, 103, 104, 109, 146, 147, 154,  156, 157, 159, 192, 193, 194, 195, 196, 200, 201,  202, 204, 205, 206, 207, 210, 211, 214, 215, 220,  221, 246  laser wavelength, x, 118, 153, 169, 173, 174, 191,  192, 193, 194, 197, 201, 202, 203, 204, 210, 217,  220  Laser‐Ablation Inductively Coupled Plasma (LA‐ICP),  vii, 1  laser‐ablation quadrupole, viii, 45  laser‐matter interactions, vii, ix, 99, 100  lasers, 3, 19, 20, 28, 40, 100, 101, 129, 130, 132,  139, 143, 144, 145, 147, 149, 151, 153, 191, 192,  193, 195, 198, 202, 220, 252  lattice parameters, 242, 256  leaching, 54  lead, vii, viii, x, 1, 2, 13, 22, 24, 51, 82, 97, 101, 135,  153, 160, 196, 200, 202, 204, 212, 221, 228, 232,  252  leakage, 147  lens, 54, 129, 147, 216, 237  lifetime, 13, 27, 47, 49, 113, 114, 115, 139  light scattering, 133  light transmission, 147  Limit Of Detection (LOD), vii, 1  linear dependence, 38  linear function, 202  liquid phase, 104, 105, 114, 116, 117, 135, 159  liquid‐gas‐mixture, ix, 100  liquids, ix, x, 5, 127, 128, 191, 192, 193, 194, 195,  197, 198, 199, 201, 202, 204, 206, 207, 211, 212,  214, 215, 217, 218, 219, 220, 222, 236, 241, 243  lithium, 25  lithotripsy, ix, 143, 144, 145, 146, 147, 148, 149,  150, 151, 152  localization, 154, 206  lower plasma shielding, viii, 2, 35  LSD, 3, 29, 31, 32, 33, 35, 36  luminescence, ix, 127, 133  Luo, 262  lying, 115 

272

Index

M  magnesium, 235  magnitude, 5, 14, 17, 25, 37, 110, 219, 232, 237, 256  majority, 49, 87, 195, 200, 210  management, 147  manganese, 82  manufacturing, 101  mass spectrometry, 46, 93, 95, 97  matrix, viii, 5, 6, 8, 13, 15, 24, 25, 38, 45, 50, 51, 52,  55, 79, 82, 84, 92, 93, 133  media, ix, xi, 127, 128, 129, 137, 138, 139, 191  Mediterranean, 96  melt, 13, 28, 30, 31, 36, 39, 195, 196, 198, 215, 216,  217, 219, 220  melting, 16, 37, 101, 104, 105, 113, 116, 144, 156,  159, 161, 193, 197, 202, 204, 208, 210, 212, 213,  215, 216, 217, 219  melting temperature, 198, 202, 208, 210, 212, 213,  216  melts, 198  metal nanoparticles, 195, 211  metals, x, 5, 16, 25, 31, 36, 39, 101, 102, 103, 113,  122, 159, 191, 195, 200, 205, 208, 212, 213, 214,  218, 220, 228, 234, 235  metastable liquid region, ix, 100, 116  metastable phases, xi, 227, 230, 232, 257  methanol, 235  methodology, 52, 79, 92  Mexico, 97  microcrystalline, 242  microelectronics, 38  microhabitats, 47  Micro‐sampling, viii, 45  microscope, 207, 210, 234  microscopy, 13  microstructure, 51  microstructures, 101, 128, 196  migration, 49, 95  Ministry of Education, 259  mixing, 23, 51, 93, 96, 112  model system, 221  modelling, 17, 35, 36, 39, 113, 154  models, 33, 93, 101, 102, 103, 104, 106, 113, 122,  154, 157, 160, 211  moderates, 47  molar volume, 106, 246, 247, 249  mole, 246  molecular dynamics, ix, x, 100, 102, 153, 154, 160  molecular mass, 100, 204, 214 

molecules, 26, 53, 107, 110, 112, 137, 220, 221, 230,  231, 238  molten pool, viii, 2, 13, 36  momentum, 29, 146  Mongolia, 47  monolayer, 26  Monte Carlo method, 107, 110  morphology, 11, 13, 15, 31, 35, 82, 214, 215, 218,  220, 234, 241, 242  Moscow, 43, 94, 99, 191, 225  Moses, 144, 145  motivation, 195  MP, 93, 96, 97  MPI, 117, 118  mucosa, 148  multicollector, viii, 45, 97  multi‐photon excitation, ix, 100 

N  nanocrystals, xi, 4, 128, 227, 232, 234, 235, 236,  237, 241, 242, 244, 247, 248, 250, 257, 259  nanomaterials, 259  nanometer, 195, 232, 245  nanometer scale, 195, 232, 245  nanometers, 137, 198, 220, 234  nanoparticle formation, x, 101, 191  nanoparticles, x, 4, 5, 39, 103, 128, 131, 136, 191,  192, 193, 194, 195, 197, 198, 199, 200, 204, 205,  206, 207, 208, 210, 211, 212, 214, 218, 220, 222,  229, 230, 234, 235, 236, 240  nanorods, 200, 202, 203, 204, 235  nanosecond laser ablation, ix, 4, 99, 129, 154, 200  nanostructured materials, 101  nanostructures, ix, 127, 128, 136, 218, 220, 228,  232, 234, 235, 259  nanowires, 244  National Research Council, 1  nebulizer, 54  neglect, 119  nickel, 102, 235  nitrides, 228, 234, 235, 256  nitrogen, 17  noble metals, x, 191, 195, 212, 234  nonequilibrium, 106  non‐metals, 39  nucleation, xi, 4, 114, 227, 244, 245, 247, 248  nuclei, 221, 244, 245, 246, 247, 248, 250  null, 33  numerical aperture, 147 

Index

O  OH, 146  operations, 92  opportunities, 47, 101  optical density, 203, 204  optical fiber, x, 143, 146, 148, 151  optical gain, 139  optical properties, 16, 19, 36, 134  optimization, x, 6, 101, 153  optoelectronic properties, ix, 127  optoelectronics, 38, 128  orthogonal geometry, vii, 1  overlap, 2, 25, 28, 47, 147  oxalate, 144  oxidation, 86, 139, 193, 206, 207, 208, 212, 214,  218, 235  oxygen, 17, 95, 96, 111, 112, 137, 139, 193, 198,  206, 207, 208, 213, 221, 237 

P  parallel, vii, 1, 4, 19, 28, 55, 145, 216, 250, 252  parallelism, 255  partial differential equations, 154, 159  particle mass, 105  partition, 8  passivation, 131, 134, 139  pathways, 51  perforation, 148  permeability, 138  permission, iv  permit, 133  permittivity, 118  phase boundaries, 245, 246  phase diagram, 114, 115, 230, 234, 245, 246, 250,  255  phase explosion mechanism, viii, 2, 36  phase transformation, 246, 249, 255  phase transitions, 103, 113  phosphates, 93  phosphorous, 82, 134, 135  phosphorus, 83, 130, 134  photographs, 2, 12, 131  photoluminescence, 127, 128, 135, 139  photonics, 101  photons, 118, 154, 221, 222  physical mechanisms, viii, xi, 2, 227  physical properties, 256 

273

physics, ix, 101, 127  pilot study, 95  PL spectrum, 133, 134, 135  plants, 47, 50  plasma, vii, viii, ix, 2, 3, 4, 5, 6, 8, 10, 11, 13, 15, 17,  19, 20, 22, 23, 24, 25, 26, 27, 28, 29, 31, 32, 33,  34, 35, 37, 38, 46, 53, 86, 93, 95, 97, 99, 100, 107,  108, 110, 117, 118, 119, 121, 122, 127, 128, 129,  136, 138, 139, 140, 144, 154, 156, 157, 158, 159,  208, 221, 222, 228, 229, 230, 232, 233, 234, 236,  237, 238, 239, 240, 244, 258  plasma plume, ix, 3, 4, 11, 29, 38, 99, 100, 108, 110,  122, 154, 228, 229, 230, 232, 238, 244, 258  platelets, 214  poison, 148  polarization, 101  polymer, ix, 106, 127, 131, 132, 133, 134, 135, 136,  137, 139  polymer materials, 106  polymer matrix, 133  polymer solutions, 131, 139  polymers, 16, 36, 128, 129, 131, 133, 134, 139  population density, 112  positive feedback, 202  potassium, 25  practical knowledge, 149  precedent, 17  precipitation, 200  preparation, iv, 46, 53, 54, 130, 139, 195, 233, 236,  240, 244, 246, 254  prevention, 100  probability, 120, 155, 215, 248, 249, 250, 251, 252  probability distribution, 250  probe, 220  project, 139  propagation, 29, 102, 103, 117, 120, 121, 138  pulp, 95  pulse duration, x, 29, 100, 102, 108, 109, 144, 146,  150, 153, 160, 165, 181, 184, 191, 193, 194, 198,  211, 212, 213, 218, 228, 231, 232, 233, 234, 235,  238, 239, 241  pulsed laser, vii, 101, 107, 122, 127, 128, 130, 137,  139, 143, 146, 164, 172, 193, 195, 227, 228, 231,  232, 233, 234, 235, 236, 237, 238, 239, 240, 241,  252, 253, 254, 258  pulsed‐laser induced liquid‐solid interface reaction  (PLIIR), xi, 227, 228, 236  purification, 210  purity, 128, 131, 221  PVP, 204, 205, 206 

274

Index

pyrolysis, 206  pyrolytic graphite, 234, 240 

Q  quantum confinement, ix, 127, 128, 129, 131, 132,  133, 134, 136, 137, 139  quantum dot, 228, 235  quantum dots, 228, 235 

R  radiation, 3, 33, 110, 119, 146, 147, 154, 157, 159,  191, 192, 193, 194, 196, 197, 198, 200, 201, 202,  203, 204, 207, 208, 209, 210, 211, 213, 220, 221,  222  radius, 23, 24, 33, 34, 35, 107, 111, 112, 161, 197,  198, 202, 208, 210, 246, 247, 248, 250  Raman spectra, 136, 234, 236, 240, 242, 244  Raman spectroscopy, 234, 240  rare earth elements, 46, 84  reactant, 230  reactions, 107, 214, 230, 232  reactivity, 195  reading, 54  reality, 215  recognition, viii, 46, 52, 144  recombination, 27, 107, 109, 110, 118, 131, 134,  139, 156, 157, 158  recombination processes, 110  recommendations, iv  reconstruction, 94  reference frame, 105  reflectivity, 16, 19, 24, 27, 113, 119, 160, 194  refraction index, 119  refractive index, 13, 118, 131, 198  relaxation, 37, 102, 115, 117, 118, 154, 157, 158,  220  relaxation process, 117  relaxation processes, 117  relevance, 38  reliability, 46, 52  relief, 194, 196, 197, 219  renal calculi, 143  replication, 55  researchers, 52, 128, 159, 191, 227, 228, 240, 259  residuals, 192  resolution, 46, 94  Resonant LA, vii, 1 

resources, 47, 92, 107  rhenium, 84  rings, 49, 256  rodents, 50  room temperature, 35, 128, 129, 130, 131, 133, 139,  159, 212, 214, 236  roughness, 217, 219  Rouleau, 140, 259, 261  Royal Society, 97  rubidium, 46, 79, 84  Russia, 99, 191  rutile, 214 

S  sapphire, 193, 198, 210, 221, 236  saturation, 17, 37, 161  scatter, 101  scattering, 38, 133, 221  seasonality, viii, 45  secretion, 50  sedimentation, 204, 221, 222  seed, 120  segregation, 107, 208, 210  selected area electron diffraction, 234  self‐assembly, ix, 127, 128, 136  semiconductor, 102, 136  semiconductors, x, 16, 103, 128, 153, 154, 155, 157,  159, 235  sensitivity, 3, 6, 25, 26, 38  shade, 256  shape, 23, 36, 100, 101, 109, 129, 132, 154, 160,  161, 200, 202, 203, 215, 216  sheep, 96  shock, 11, 13, 23, 24, 26, 27, 29, 33, 34, 35, 106,  107, 110, 115, 121, 129, 132, 138, 139, 144, 145,  146, 228, 230, 239, 240, 245, 252  shock waves, 23, 24, 107, 110, 129, 132, 138, 139,  144, 145  shores, viii, 45, 48  Siberia, viii, 45, 47, 48, 94, 95  side effects, 197  signals, 25, 50, 51, 53, 82  silica, 38, 118, 119, 120, 146, 147, 236  silicon, ix, 127, 128, 129, 130, 131, 132, 133, 134,  135, 136, 137, 138, 139, 156, 236, 240  silicon nanocrystals, ix, 127, 129, 133, 135, 138  silver, 37, 102, 198, 204, 218, 235  simulation, ix, x, 99, 101, 102, 107, 110, 117, 122,  153, 154, 155, 160, 161, 255 

Index simulations, ix, 100, 117, 122, 255  SiO2, 32, 128, 131, 134  skin, 115, 116  sodium, 25, 199, 235  software, 55, 133  solid state, 115  solidification, 130, 132, 133, 134  solubility, 214  solvents, 235  sound speed, 114  South Africa, 96, 97  species, ix, 2, 3, 5, 8, 11, 17, 19, 20, 26, 29, 46, 47,  50, 52, 55, 86, 99, 103, 107, 110, 111, 121, 193,  195, 197, 206, 228, 230, 237, 238, 245  specific heat, 108, 158  specific surface, 135, 214  spectroscopy, 6, 101, 133, 234  speed of light, 118  spin, ix, 127, 128, 132, 139  spin on glass (SOG), ix, 127, 128, 132, 139  stabilization, 136, 138, 202, 204  stages of mineralization, viii, 45  standardization, 24  stars, 117  states, 114, 115, 134, 243  statistics, 158  steel, 6, 7, 10, 14, 15, 18, 25  stoichiometry, 38, 107  storage, 214, 218  striae, 50  strictures, 56  strontium, 46, 47, 48, 49, 50, 51, 52, 54, 55, 79, 82,  85, 86, 89, 92, 94, 95, 96  structuring, 101  subsistence, 47  substitutes, 85  substrates, 229, 236, 237  subtraction, 54, 158  sulfate, 235  sulfur, 145  Sun, 262  supercooling, 105  superfluid, 244  suppression, 137  surface chemistry, 138  surface energy, 202  surface layer, 38  surface properties, 100, 128  surface structure, 49 

275

surface tension, 114, 195, 198, 202, 208, 232, 236,  244, 250  surface treatment, 101  surfactant, 136, 206, 235  surgical technique, 143  susceptibility, 211  suspensions, 130, 195, 206, 221, 235  symmetry, 107, 211, 215, 257  synthesis, xi, 5, 101, 129, 192, 193, 198, 199, 200,  201, 208, 214, 215, 216, 221, 227, 228, 232, 234,  235, 236, 241, 245, 248, 251, 257 

T  technology, 128, 131, 139  teeth, viii, 45, 47, 51, 52, 53, 55, 56, 57, 78, 79, 82,  85, 89, 90, 92, 93, 97  TEM, 198, 199, 200, 201, 205, 206, 210, 212, 213,  217, 234, 241  temperature dependence, 105, 114  tension, 114, 198, 250  territory, 46, 50  thermal decomposition, 116, 122, 145  thermal energy, 38, 108, 111, 240  thermal evaporation, 109, 113  Thermal model, ix, 99  thermal properties, 102, 210  thermalization, 157  thermodynamic, vii, viii, 2, 6, 8, 15, 22, 34, 106, 108,  113, 114, 115, 230, 234, 237, 240, 244, 246, 248,  251, 254  thin films, 101, 107, 132, 139, 229, 232, 234, 236,  240, 261  tin, 212, 213, 235  tissue, 136  titanium, 28, 211  tooth enamel, viii, 45, 51, 93, 95, 96, 97  total energy, vii, 1, 10, 11, 27, 154, 156, 256, 257  total internal reflection, 147  toxicity, 131, 149  trace elements, 53, 85, 94, 95  trade‐off, 52  traits, 258  transformation, 210, 221, 230, 232, 245, 246, 249,  252, 253, 254, 255  transition metal, 255  transmission, 3, 38, 148, 207, 234  transparency, 240  transport, 33, 102, 104, 110, 156  tungsten, 236 

276

Index

tunneling, 102, 117, 118, 120  tunneling effect, 117, 120 

U  UK, 97, 110  ultrashort laser pulses, viii, 2, 14, 37  underlying mechanisms, 37  uniform, 26, 110, 137, 146  ureter, 147, 148, 151  uric acid, 148, 150  urinary calculi, ix, 143, 144, 149, 150  urinary tract, 143  UV, 193, 195, 196, 221 

V  vacancies, 137  vacuum, ix, 19, 28, 29, 99, 101, 103, 106, 107, 114,  118, 121, 128, 195, 197, 201, 214, 222, 227, 228,  229, 232, 240, 244  valence, 118, 156  vapor, 106, 108, 109, 114, 115, 144, 145, 193, 196,  197, 198, 200, 203, 205, 206, 207, 208, 209, 212,  213, 215, 216, 217, 219, 220, 221, 234, 235, 236,  244  variables, 86, 87  variations, 15, 34, 95, 107  varieties, 53  vector, 108, 146  velocity, 33, 34, 105, 106, 108, 109, 110, 111, 112,  131, 137, 195, 228  Vereshchagin, 265  vibration, 242  viscose, 108  visualization, 133  volatilization, 24 

W  waste, 221  water, ix, 3, 48, 50, 51, 54, 85, 127, 128, 129, 130,  131, 132, 133, 134, 135, 136, 137, 139, 144, 148,  198, 199, 200, 201, 203, 204, 206, 207, 211, 212,  214, 218, 220, 221, 228, 232, 233, 234, 235, 237,  238, 239, 240, 241, 245  water evaporation, 136  watershed, 48  wave laser beam, vii  wave propagation, 121  wavelengths, 200  wealth, 107  wear, 53  welding, 234  wetting, 212, 213  wide band gap, ix, 100, 117, 118, 128  windows, 18  workers, 107 

X  XPS, 212  X‐ray diffraction, 211, 212, 213, 253  XRD, 133, 134, 234, 236, 253 

Y  yield, viii, 37, 45, 79, 92  yttrium, 150 

Z  zinc, 81, 84, 208, 235  zirconium, 86, 92