Cluster photofragmentation dynamics: Quasiclassical trajectory studies of Ar-n-H2S and Ar-n-SH (n=1,2)

Quasiclassical trajectory calculations with model potential energy surfaces have been used to elucidate the formation dynamics of open-shell radical c lusters by "gentle-recoil" photolysis of closed-shell hydride clusters. Spe cifically, model surfaces for Ar-H2S and Ar-2-H2S have been constructed and used to explore photofragmentation dynamics at 193 and 248 nm for comparis on with previous experimental results. A remarkable efficiency (as high as 25%) for forming highly excited radical Ar-SH and Ar-2-SH clusters is calcu lated, despite photolysis recoil energies more than 100-fold in excess of t he dissociation limit. This surprisingly high survival probability is trace d to two dynamical sources. First, ejection of the light H atom from Ar-n-H 2S effectively removes all but a small fraction of the excess photolysis en ergy from the nascent radical cluster in the center-of-mass frame. Second, although trajectory calculations indicate that nearly 50% of the surviving clusters contain energies up to two-fold higher than the dissociation limit , these clusters are classically bound due to novel angular momentum barrie rs predicted by Pollak [J. Chem. Phys. 86, 1645 (1987)] for a polyatomic sy stem. Finally, an analysis is presented that indicates the "gentle-recoil" photolysis mechanism may permit efficient formation of highly internally ex cited, chemically reactive radical clusters of OH and SH with light species such as H-2 and D-2. (C) 2000 American Institute of Physics. [S0021- 9606( 00)00348-2].