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 o
f the relevant electronic potential energy curves and off-diagonal mat
rix 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 re
sults, and, more dramatically, with the results of prior theoretical p
redictions. Analysis of the redistribution of the photofragment flux,
as a function of the H-Cl separation, reveals that the product branchi
ng is determined at relatively large HCl distances, considerably beyon
d the Franck-Condon region, and is governed by the spin-orbit coupling
between the initially excited A 1PI state and the OMEGA = 1 component
s of the a 3PI and 1 3SIGMA+ states.