Photosensitized Decompositions by Excited Mercury Atoms

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The writers wish to express their indebtedness to the Cleveland Fund of the American Association for the Advancement of Science for a grant of $500.00, which has been used in meeting the expenses of this work. They wish to thank C. N. Shah, who is working on the law of force adjacent to an atom nucleus, for much assistance in connection with this work. 1 Harkins and Ryan, J. Amer. Chem. Soc., 45, 2095 (1923). Rutherford, Phil. Mag., 37, 538 (1919). 3Blackett, Proc. Roy. Soc., 107, 349 (1925). 4 Petterson and Kirsch, Atomzertrammerung (1926), p. 138. 6 Harkins and Wilson, these PROCXnDINGS, 1, 276 (1915); J. Amer. Chem. Soc., 37, 1367 (1915).; Phil. Mag., 30, 723 (1915); and later papers. 2

PHOTOSENSITIZED DECOMPOSITIONS B YEXCITED MERCURY A TOMS By HUGH STOTT TAYLOR AND JOHN REGINALD BAThS LABORATORY OF PHYSICL CE1NaSTRY, PtCNCaTON UNIVERSITY Communicated November 13, 1926

A considerable photochemical literaturel on the subject of hydrogen atoms produced by collisions between excited mercury atoms and molecular hydrogen has served to indicate the potentialities of excited atoms for the purposes of photosensitization. The subject may be expanded in two directions, either by the use of other excited atoms or by the study of other collisions with the excited mercury. Of this latter type is the investigation by Dickinson and Sherrill,2 arising out of their studies of the interaction of hydrogen and oxygen in presence of mercury. They have shown that the production of ozone may be secured by the action of excited mercury and oxygen even though the excitation is only that in the region X = 2536.7 A and, therefore, presumably inadequate for the dissociation of oxygen. They suggest, therefore, that the ozone must arise by collisions between excited oxygen molecules and normal molecules. From our earlier work on the interaction of hydrogen and ethylene3 we have been led to investigate the efficiency of the excited mercury atom in a wide variety of photo-decompositions and have been able to show how efficient an agent it is for such photosensitizations. It has been possible to show that water, ammonia, ethylene, methyl and ethyl alcohols, hexane, benzene, acetone, formic acid and ethyl amine are all made sensitive to decomposition by light of wave-length 2536.7 A if the vapor exposed to the resonance radiation contains a sufficient concentration of mercury vapor. The decompositions thereby secured are, in several of the cases, many hundred-fold those secured by exposure in quartz apparatus to the whole ultra-violet spectrum of the cooled mercury arc and, in one case, the increased rate of decomposition is one-thousand fold.

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CHEMISTR Y.- TA YLOR A ND BA TES

The experimental arrangement is simple, as may be seen from the accompanying diagram. The material under study evaporates, at a known low vapor pressure from the reservoir A, is saturated with mercury at a given temperature in the saturator B, and then flows rapidly through the quartz vessel C, round which a cooled mercury arc of the type previously described by one of us' is operated. The products of reaction are then passed through two liquid air traps in which the condensible constituents are removed and the non-condensible gases (in the cases chosen for study, mainly hydrogen, nitrogen, oxygen, carbon monoxide or methane) were collected in a McLeod gauge. The rate of reaction could be measured by the rate of increase in pressure in the gauge. The gases could be analyzed in the gauge system with the aid of a platinum spiral which could be made incandescent, a copper oxide tube which could be electrically heated and a side tube which could be cooled in solid carbon dioxide or liquid air. To ascertain whether any photosensitized decomposition occurred, the

Afat_

_
g

vapors in question were passed through the arc system both with and without mercury vapor., The difference in rate is attributed to the action of the mercury. A resume of the experimental data is presented in table 1. The second and fourth columns record the pressures in mm. of mercury of the gases entering the McLeod gauge system in 10 minutes' operation. The data suffice to demonstrate the extraordinary efficiency of the excited mercury atom in the disruption of a variety of molecules in which the linkages involved include, C-H, N-H, 0-H. The data must be regarded mainly as a qualitative index of decomposition. Nevertheless, there is a marked difference in the ease of decomposition among the examples cited. Thus, water is much less easily decomposed than methyl alcohol. Ethyl alcohol is more readily decomposed than either. Benzene is surprisingly stable. Water is more stable than ammonia. We have some evidence that the lower hydrocarbons such as methane and ethane are more stable than water. From the energy of the excited mercury atom (112 kg. cal. per gram atom) we can conclude that the energy of the

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liukages N-X, O-i, C-H are all less than 112 kg. cal. in the simple hydrides of these elements. This is in accord with available thermal data, Furthermore, from the thermal data with respect to ammonia and hydrogen, N2 + '3H2 = 2NHs + 22 kg. cal. and H + H = H2 + 100 TABLE 1 RESIDUAL GAS WITHOUT Ho

SUBSTANCE

H20

NHa C2H4 CH3OH

C2H,OH C6H14 C6H6

(CH3)2CO

GAS

VAPOR MM.

GAS WITH .Ho VAPOR

ANALYSIS

MM.

1.2 7.8 None 24.1 .......... 0.03 18.0 .......... 38.0 0.7 17.0 0.016 0.3 0.01 4.7 100% CO + CH4 9.6 24.4 0.06 .......... 20.4 0.34 4% N2; 96% H2 None

0.04

RATIO OP. PHOTOSENSITIZXD TO

RESIDUAL

......... 96% H2;4%N2

HCOOH" C2H5NH2 a 6 minutes' operation.

..........

..........

..........

GAS ANALYSIS

PHOTOCHEMICAL RATS

73% H2; 27%O2 200:1 89% H2; 11% N2 .... 88% H2; 12% CH4, etc. 58% H2; 42% CH4 + CO 600:1 50:1 46% H2; 50% CO + CH4 1000:1 96% H2; 4% CH4 30:1 60% H2; 40% CH4 2:1 100% CO + CH4 400:1 76% CO; 24% H2 96% H2; 3.7% CH4; 0.3%N2 60:1

kg. cal., we can by setting the energy of a N-H linkage as 2 112 kg. cal. set an upper limit for the dissociation energy of the nitrogen molecule. This upper limit must be 6(112) - 3(100) - 22 = 350 kg. cal. = 15 volts. Even this upper limit is considerably greater than the value of 11.4 volts based on the most recent spectroscopic data. A detailed discussion of the products of decomposition will be given in a subsequent more extensive publication. It may be pointed out, however, that, contrary to the results of Senftleben,4 we have identified oxygen in addition to hydrogen gas in the products of decomposition of water. The hydrogen is in excess as would be anticipated from the known formation of mercuric oxide in such systems. From ethyl alcohol we have identified acetaldehyde and also its decomposition products methane and carbon monoxide, in addition to hydrogen. With benzene we have detected diphenyl, but the gaseous residue contains some hydrocarbons as well as hydrogen. With hexane the gas was nearly pure hydrogen. Formic acid yielded a larger proportion of carbon monoxide (76 per cent) than was obtained by Ramsperger and Porter5 with a hot arc (64 per cent CO). From ethylene we have identified acetylene, hydrogen and some saturated hydrocarbons. We desire especially to call attention to the results of the gas analyses in both the photochemical and the photosensitized decompositions of ammonia. It will be noted in each case that the percentage of hydrogen in the residue is greatly in excess of that (75 per cent) required by stoichiometric decomposition to the elements. From this excess of hydrogen we conclude that the decomposition of ammonia occurs in stages in the

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first of which hydrogen is eliminated from the ammonia molecule. One such stage would be the formation of hydrazine and hydrogen from two molecules of ammonia. Our condensate,. after evaporation of the -ammonia, gave a, residue of characteristic, somewhat irritating odor. A solution of the residue readily reduced silver nitrate solutions to metallic silver. The absence of nitrogen in the residual gas from ethylamine is also another aspect of the problem. The studies are therefore being further developed. Summary.-The r6le of excited mercury atoms in a number of photosensitized decomposition processes has been studied. It has been shown that water, ammonia, ethylene, methyl and ethyl alcohols, hexane and benzene, acetone, formic acid and ethylamine are all sensitized to the wave-length X = 2536.7 A through the agency of excited mercury. In all cases, decomposition of the molecule occurs. Especial attention is drawn to the decomposition of ammonia, the gases from which show marked excess of hydrogen, pointing to decomposition by stages, probably through hydrazine. See Taylor, J. Am. Chem. Soc., 48, 2840 (1926), for references. 2Proc. Acad. Nat. Sci., 12, 175 (1926). ' J. Phys. Chem., 29, 842 (1925). Z. Physik, 37, 529, 539 (1926).

I

6J. Am. Chem. Soc., 48, 1267 (1926). SOME APPLICA TIONS OF THE IONIZA TION FORMULA BY CECILIA H. PAYNI1 HARVARD COLI1GE OBSERVATORY, CAMRMDGS, MASS.

Communicated November 5, 1926 The earlier astrophysical applications of the theory of ionization were made with the assumption that the same value of the partial electron pressure could be used throughout the atmospheres of all stars. Fowler and Milne2 early pointed' out that far lower pressures might at times be involved for certain relatively abundant atoms, and a rough estimate of the partial pressures appropriate to certain atoms in known energy states was made by the present writer, from observational data,3 fully supporting the suggestion. The preliminary work thus outlined, and various other considerations, indicated that conditions of great complexity govern the origin of stellar absorption lines such as we observe. Each line in a given spectrum is to be regarded as arising from an effective level that is determined by the constants of the corresponding state of the atom, and by the temperature of