STEFANJ. RZADAND KRISHANM. BANSAL
2374 of PNDA photoreduction, despite the very great differences in the two techniques.
Conclusions From these considerations of reducing and oxidizing processes a t ZnO-H20 interfaces under uv illumination, it appears that present results on extent and quantum efficiency of photoreduction, on lack of large pH changes and on effects of inhibiting and promoting additives, can be qualitatively accounted for by an overall mechanism combining (IIIa) or (IIIb) with oxidative steps. Further detailed investigation of the dependence of photoreaction and Il.ea upon solution pH and additives
are needed to establish which oxidative processes best correlate with the varying magnitude of the epr signal and with kinetics of photoreaction. Such investigations are in progress in these l a b o r a t ~ r i e s . ~ ~ Acknowledgments. The authors are grateful to J. Kelly, A. Penny, M. Donovan, and M. Nowlan for experimental assistance and to J. Thompson for loan of photolysis equipment. Stimulating discussions with J. Clarke and W. NIehl are also acknowledged. Financial and equipment support from AFOSR greatly assisted the study and H. Z. is grateful to University of Bagdad for a grant.
Radiolysis of Liquid 2,2,4-Trimethylpentane. Kinetics of Scavenging Processes1 by Stefan J. Rzad* and Krishan M. Bansal Radiation Research Laboratories and Center for Special Studies, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 16813 (Received January 27, 1973) Publication costs assisted by the U.S. Atomic Energy Commission
The methyl radical yield observed upon electron scavenging by methyl bromide in r-irradiated 2,2,4-trimethylpentane can be quantitatively described by a n empirical model similar to that proposed for cyclohexane, G(CH3) = Gfi G g i d ~ ~ ~ a ~ r [ C H a B r ] ~/ (cl Y c H ~ B ~ [ C HThis ~ B model ~ ] ) . was also found to be a good description of data from other scavengers-ethyl bromide and sulfur hexafluoride-in 2,2,4-trimethylpentane. The following parameters were obtained by treating the results in this manner: Gf, = 0.3, G,, = 4.54, C Y C H ~ B=~ 24.7 M-l, C Y S F ~= 65 M-l, C Y C ~ B ~=B ~5 1M-l. From the analysis of the scavenging data it is concluded t h a t (1) the characteristic thermalization length in 2,2,4-trimethylpentane is -50% longer than t h a t in cyclohexane and (2) the rate constant of electron scavenging by CH3Br as well as the constant A, representing the recombination rate of geminate ions, are approximately ten times higher in 2,2,4-trimethylpentane than in cyclohexane. The analysis of the nitrogen yields from N20-2,2,4trimethylpentane solutions allows one t o estimate that the reaction of the secondary ion produced upon electron capture by NzO proceeds a t a diffusioncontrolled rate.
+
Introduction
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(free ions). Furthermore, the physical methods also indicated that the yield of ions escaping geminate reAlthough the radiation chemistry of n-alkanes (e.y., combinationst0 increased with increasing branching in n-hexane) and cycloalkanes (e.g., cyclohexane) has been the hydrocarbons. I n addition, the electron mobility studied extensively, much less attention has been focused on branched hydrocarbons.2 Recent charge (1) Supported in part by the U. S. Atomic Energy Commission. scavenging studies indicated that ionic processes play (2) For a review see, for example, T. Galimann and J. Hoigne, Ed., "Aspects of Hydrocarbon Radiolysis," Academic Press, New York, an important part in product formation in cyclohexane N. Y . , 1968. r a d i ~ l y s i s . ~These charge scavenging studies are (3) For a review see J. M. Warman, K.-D. Asmus, and R. H. Schuler, amenable t o quantitative interpretation4+ in terms of Advan. Chem. Ser., No. 82, 25 (1968). an empirical model previously proposed. Physical (4) J. ,M.Warman, K.-D. Asmus, and R. H. Schuler, J . Phys. Chem., substantiated by chemical s t ~ d i e s 4 - ~ , ' ~73, 931 (1969). (5) K.-D. Asmus, J. M. Warman, and R. H. Schuler, ibid., 74, 246 have shown that in n-alkanes and cycloalkanes most (1970). ions (geminate ions) generated by radiation recombine (6) K. M. Bansal and R. H. Schuler, ibid., 74, 3924 (1970). very rapidly (
