SPIN-ORBIT BRANCHING IN THE PHOTOFRAGMENTATION OF HCI

The dynamics of the photofragmentation of HCl and DCl, subsequent to A 1PI <-- X 1SIGMA+ electronic excitation, is treated exactly based on new multireference, configuration-interaction ab initio calculations of the relevant electronic potential energy curves and off-diagonal matrix elements. The calculated total cross section agrees well with both earlier calculations and experiment. By contrast, the relative cross sections for formation of the two accessible fine-structure channels [Cl(2P1/2) and Cl(2P3/2)] disagree with the most recent experimental results, and, more dramatically, with the results of prior theoretical predictions. Analysis of the redistribution of the photofragment flux, as a function of the H-Cl separation, reveals that the product branching is determined at relatively large HCl distances, considerably beyond the Franck-Condon region, and is governed by the spin-orbit coupling between the initially excited A 1PI state and the OMEGA = 1 components of the a 3PI and 1 3SIGMA+ states.