Photodissociation dynamics of HCl in solid Ar: Cage exit, nonadiabatic transitions, and recombination

The photodissociation of HCl in solid Ar is studied by non-adiabatic Molecular Dynamics simulations, based on a surface-hopping treatment of transitions between different electronic states. The relevant 12 potential energy surfaces and the non-adiabatic interactions between them were generated by a Diatomics-in-Molecules ~DIM! approach, which incorporated also spin-orbit coupling. The focus of the study is on the non-adiabatic transitions, and on their role both in the cage-exit of the H atom, and in the recombination process. It is found that non-adiabatic transitions occur very frequently. In some of the trajectories, all the 12 electronic states are visited during the timescale studied. At least one non-adiabatic transition was found to occur even in the fastest cage-exit events. The other main results are: ~1! The total yields for photofragment separation ~by cage exit of the H atom! and for H1Cl recombination onto the ground state are roughly equal in the conditions used. ~2! The cage exit events take place in the time-window between ;70 fs and ;550 fs after the excitation pulse, and are thus all at least somewhat delayed. The recombination events span a much broader time-window, from almost immediately after excitation, and up to ;1100 fs and beyond. ~3! The electronic energy relaxation events during the process depend significantly on symmetry and interactions of the states involved, and not only on the energy gaps between them. ~4! Different electronic states reached in the course of the process exhibit different propensities with regard to the recombination versus cage exit outcome. ~5! Spin-orbit interactions, and spin-forbidden transitions play an important role in the process, especially for recombination events. © 1997 American Institute of Physics. @S0021-9606~97!01916-8#