Spin–orbit branching in the photofragmentation of HCl

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Abstract
The dynamics of the photofragmentation of HCl and DCl, subsequent to A 1Π←X 1Σ+ 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(2 P 1/2) and Cl(2 P 3/2)] 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 1Π state and the Ω=1 components of the a 3Π and 1 3Σ+ states.