Spin–orbit branching in the photodissociation of HBr: Time-independent, time-dependent, and semiclassical calculations

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.