J . Phys. Chem. 1984, 88, 3930-3932
3930
OH(A2z+-+X2n) Photofragment Emission from Two-Photon Photolysis of Hydrogen Peroxide at 193.3 nm C. B. McKendrick, E. A. Kerr, and J. P. T. Wilkinson* Department of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ, Scotland (Received: May 21, 1984)
The photolysis of hydrogen peroxide using focused 193-nm radiation from an excimer laser gives fluorescencefrom OH(A2Z+). Emission from the (0,O) and (1,O) bands is detected. This emission is attributed to the products of two-photon absorption by H 2 0 2at 193.3 nm. The rate of quenching by H 2 0 2of the nascent OH(A2Z+)produced by two-photon dissociation of cm3 molecule-’ s-l. H202was found to be (2.93 & 0.09) X
Introduction The photochemistry of the hydrogen peroxide molecule is of interest for a number of reasons. One aspect is that the existence of hydrogen peroxide in the Earth’s atmosphere has been predicted for a number of years.’S2 The amount present in the atmosphere will depend strongly upon both the photochemical behavior of hydrogen peroxide in the radiation field present in the atmosphere as well as on the chemical reactions which lead to its formation. These include the disproportionation reaction between hydroperoxyl radicals HO2
+ HO2
+
H202
+0 2
(1)
a subject of much study r e ~ e n t l y . ~ - ~ A second reason for interest in the photodissociation of hydrogen peroxide is that a knowledge of the energy partitioning in the photofragments which are formed is of great value in the study of the detailed dynamics of the photodissociation process. The results obtained from high-resolution studies of photofragment quantum state distributions constitute some of the most stringent tests of the current theories of photodissociation.6 Experimental studies of hydrogen peroxide single-photon photodissociation in the ultraviolet have been relatively sparse until very r e ~ e n t l yparticularly ~,~ in the vacuum u l t r a v i ~ l e t . ~How,~~ ever, two recent publications have clarified the field considerably. A study of the OH(X211) product state distribution from both 248- and 193-nm laser single-photon photodissociation of hydrogen peroxide” has led to a very detailed description of the energy disposal in this process. No nascent OH(A28+)was detected from photolysis at either wavelength. The major fraction of the available energy was found to reside in the translational degrees of freedom of the photofragments. The study by Suto and Leet2using synchrotron radiation has shown both the detailed dependence of the absorption cross section upon wavelength in the region from 106 to 193 nm as well as the onset of significant fluorescence from OH(A2Z+) at wavelengths at or below 172.2 nm. (1) P. J. Crutzen and J. Fishman, Geophys. Res. Lett., 4, 321 (1977). (2) J. W. Waters, J. C. Hardy, R. F. Jarnot, and H. M. Pickett, Science, 214,(1981). (3) B. A. Thrush and G. S . Tyndall, J . Chem. Soc., Faraday Trans. 2,78, 1469 (1982’1. (4)’s. P.’Sander, M.Peterson, R. T. Watson, and R. Patrick, J . Phys. Chem., 86, 1236 (1982). (5) R. Patrick and M. J. Pilling, Chem. Phys. Lett., 91, 343 (1982). (6) K. F. Freed and Y. B. Band in “Excited States“, Vol. 3,E. C. Lim, Ed., Academic Press, New York, 1977. (7) H. Okabe, ‘Photochemistry of Small Molecules”, Wiley, New York, 1978. (8) L. T. Molina, S . D. Schinke, and M. J. Molina, Geophys. . . Res. Lett., 5, ii3 (1978). (9) K. H.Becker, W. Groth, and D. Kley, Z . Naturforsch. A , 20, 748 (1965). (10) L. J. Stief and W. J. DeCarlo, J . Chem. Phys., 50, 1234 (1969). (1 1) G. Ondrey, N.van Veen, and R. Bersohn, J . Chem. Phys., 78, 3732 (1983). (12) M. Suto and L. C. Lee, Chem. Phys. Lett., 98, 152 (1983).
0022-365418412088-3930$01.50/0
The experiments reported here investigate the nonlinear photochemistry of hydrogen peroxide by studying the wavelengthresolved fluorescence from OH(A2Z) following the photodissociation of hydrogen peroxide using focused 193-nm laser radiation. The quenching of the OH(A22+) by hydrogen peroxide is also investigated.
Experimental Section The apparatus used in these experiments has been described previ~usly’~ and only a brief description will be given here. The beam from an excimer laser (Lambda Physik EMGSOO), operating on the ArF 193-nm laser transition, was focused into a glass cell with a 15-cm focal length lens. The cell, which was fitted with quartz windows, was evacuated with a conventional greaseless vacuum line. The hydrogen peroxide sample was contained in a small reservoir fitted to the cell. In order to avoid any complications due to decomposition of hydrogen peroxide during an experiment a flow was set up by carefully adjusting the reservoir and vacuum pump taps in order to obtain a steady pressure (
