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J. Phys. Chem. 1994,98, 7945-1957 7945 FEATURE ARTICLE

Summary: J. Phys. Chem. 1994,98, 7945-1957 7945
Cage Exit versus Cage-Induced Reaction upon Photodissociation of Matrix-IsolatedH2S:
Experiment and Statistical Theory
J. Zoval and V. A. Apkarian'
Department of Chemistry, University of California, Irvine, California 92717
Received: April 4, 1994; In Final Form: June 13, 1994'
In its first absorption continuum, a variety of channels are available to the permanent dissociation of matrix-
isolated H2S, the two main ones being cage exit to yield SH +H and the cage-induced bimolecular reaction
SH +H -S +H2. Laser-induced fluorescence (LIF) from SH(A-X) and S(lS-JD) is used to probe each
of these channels. LIFof S atoms is a site-specific probe that candistinguishamong interstitialand substitutional
sites and sites at which the S and H2 products are crowded together. In addition to substitutionally trapped
S atoms, interstitial trapping is observed. The latter implies cage exit of S(lD). In the crowded sites, the LIF
signal from S is stable, implying that there is an activation barrier to reaction of S(lD) with H2. In Kr, the
SH:S branching ratio increases monotonically from 0.7 to 3 for excess energies between 1 and 2.5 eV. The
quantum yield of the S channel is nearly independent of photodissociation energy, while the SH channel yield
increasesmonotonically with excessenergy. A statisticalmodel,based onconesof reactivity,isused torationalize
the branching of the cage-induced reaction H +SH to yield H2S or S +H2. A statistical theory for sudden
cage exit, which incorporates barrier height distributions due to zero-point and thermal fluctuations of the
lattice, is presented and used to characterize the nature of cage exit dynamics. At large excess energies sudden


Source: Apkarian, V. Ara - Department of Chemistry, University of California, Irvine


Collections: Chemistry