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(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 1Π state and the Ω=1 components of the a 3Π and 1 3Σ+ states.

It is proposed that the two Λ-doublet levels of linear molecules with nonzero electronic orbital angular momentum be labeled Λ(A′) and Λ(A″), e.g., Π(A′) and Π(A″) for Π states, etc., according to the following prescription: All series of levels in which the electronic wave function at high  J is symmetric with reflection of the spatial coordinates of the electrons in the plane of rotation will be designated Λ(A′) for all values of J, and all those for which the electronic wave function is antisymmetric with respect to reflection will be denoted Λ(A″). It is emphasized that this notation is meant to supplement, and not replace, the accepted spectroscopic e/f labeling and the parity quantum number. The utility of the Λ(A′)/Λ(A″) notation is that it is of most relevance in the mechanistic interpretation of reactive or photodissociative processes involving open-shell molecules.

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The dynamics of the photofragmentation of HBr is treated within time-independent, time-dependent, and semiclassical methods. The calculated relative cross sections for formation of the two accessible fine-structure channels [Br(2P1/2) and Br(2P3/2)] agree well with the experimental results, both in magnitude and in dependence on photon excitation wavelength. For relatively small photon wavelength (λ=193 nm), vertical excitation in the Franck–Condon region populates preferentially the A 1Π state, and only three states (A 1Π, the Ω=1 components of the a 3Π and 1 3Σ+), coupled by the spin–orbit interaction, are invoved in the dissociation process. For larger photon wavelength (λ=243 nm), the product branching is governed by initial excitation in both the A 1Π state and the a 3Π(Ω=0) component. Comparison of the redistribution of the time-independent photofragment fluxes as a function of the H–Br separation with the temporal evolution of the populations within a time-dependent framework shows that the two methods, although based on a different point of view, provide equivalent mechanistic information on the dissociation process.