Photodissociation of the methyl radical: the role of nonadiabatic couplings in enhancing the variety of dissociation mechanisms.

The nonadiabatic photodissociation dynamics of the CH (and CD) radical from the 3p and 3s Rydberg states is investigated by applying a one-dimensional (1D) wave packet model that uses recently calculated 1D electronic potential-energy curves and nonadiabatic couplings. Calculated predissociation lifetimes are found to be too long as compared to the experimental ones. The 1D dynamical model, however, is able to predict qualitatively and explain the fragmentation mechanisms that produce the hydrogen-fragment translational energy distributions (TED) measured experimentally for the ground vibrational resonance of the 3p and 3s Rydberg states (CH( = 0, 3p) and CH( = 0, 3s)). The CH( = 0, 3p) TED found experimentally displays a rather large energy spreading, while the experimental CH( = 0, 3s) TED is remarkably more localized in energy. The present model also predicts a widely spread CH( = 0, 3p) TED, produced by a complex dissociation mechanism which involves predissociation to one dissociative valence state through a nonadiabatic coupling, as well as transfer of population to a second valence state through three conical intersections. Also in agreement with experiment, the model predicts a rather localized CH( = 0, 3s) TED because the conical intersections no longer operate in this photodissociation process, and predissociation occurs only into a single valence state. Another complex dissociation mechanism is predicted by the model for initial CH( > 0, 3s) and CD( > 0, 3s) resonances. In this case the mechanism is gradually activated, as vibrational excitation increases, by the interplay between the two nonadiabatic couplings connecting the 3s and 3p Rydberg states with the dissociative A valence state, and produces complex TEDs with signals from several resonances of both 3s and 3p. Thus the present 1D quantum model reveals a rich photodissociation dynamics of methyl, where a variety of complex fragmentation mechanisms is favored by the presence of different nonadiabatic couplings between the electronic states involved.