Trajectory surface-hopping study of methane photodissociation dynamics

We use the fewest switches nonadiabatic trajectory surface hopping approach to study the photodissociation of methane on its lowest singlet excited state potential surface (1 ¹T₂) at 122 nm, with emphasis on product state branching and energy partitioning. The trajectories and couplings are based on CASSCF(8,9) calculations with an aug-cc-pvdz basis set. We demonstrate that nonadiabatic dynamics is important to describe the dissociation processes. We find that CH₃(X̃²A²″) +H and CH₂(ã¹A₁)+H₂ are the major dissociation channels, as have been observed experimentally. CH ₃+H is mostly formed by direct dissociation that is accompanied by hopping to the ground state. CH₂+H₂ can either be formed by hopping to the ground state to give CH₂(ã¹A ₁)+H₂ or by adiabatic dissociation to CH ₂(b̃¹B₁)+H₂. In the latter case, the CH₂(b̃¹B₁) can then undergo internal conversion to the ground singlet state by Renner-Teller induced hopping. Less important dissociation mechanisms lead to CH₂+H+H and to CH+H₂+H. Intersystem crossing effects, which are not included, do not seem essential to describe the experimentally observed branching behavior. About 5% of trajectories involve a roaming atom mechanism which can eventually lead to formation of products in any of the dissociation channels. Branching fractions to give H and H₂ are in good agreement with experiment, and the H atom translational energy distribution shows bimodal character which also matches observations.