NONADIABATIC BENDING DISSOCIATION IN 16-VALENCE ELECTRON-SYSTEM OCS

The speed, angular, and alignment distributions of S(D-1(2)) atoms fro m the ultraviolet photodissociation of OCS have been measured by a pho tofragment imaging technique. From the excitation wavelength dependenc e of the scattering distribution of S(D-1(2)), the excited slates acce ssed by photoabsorption were assigned to the A' Renner-Teller componen t of the (1)Delta and the A''((1)Sigma(-)) states. It was found that t he dissociation from the A' state gives rise to high- and low-speed fr agments, while the A'' slate only provides the high-speed fragment. In order to elucidate the dissociation dynamics, in particular the bimod al speed distribution of S atoms, two-dimensional potential energy sur faces of OCS were calculated fur the C-S stretch and bending coordinat es by nb initio molecular orbital (MO) configuration interaction (CI) method. Conical intersections of (1)Delta and (1)Sigma(-) with (1)Pi w ere found as adiabatic dissociation pathways. Wave packet calculations on these adiabatic surfaces, however, did not reproduce the low-speed component of S(D-1(2)) fragments. The discrepancy regarding the slow S atoms was attributed to the dissociation induced by nonadiabatic tra nsition from A'((1)Delta) to A'((1)Sigma(+)) in the bending coordinate . This hypothesis was confirmed by wave packet calculations including nonadiabatic transitions. The slow recoil speed of S atoms in the nona diabatic dissociation channel is due to more efficient conversion of b ending energy into CO rotation than the adiabatic dissociation on the upper slate surface. By analyzing the experimental data, taking into a ccount the alignment of S(D-1(2)) atoms, we determined the yield of th e nonadiabatic transition from the A'((1)Delta) to the ground states t o be 0.31 in the dissociation at 223 nm. Our theoretical model has pre dicted a prominent structure in the absorption spectrum due to a Feshb ach resonance in dissociation, while an action spectrum of jet-cooled OCS measured by monitoring S(D-1(2)) exhibited only broad structure, i ndicating the limitation of our model calculations. (C) 1998 American Institute of Physics.