Photodissociation of the methane–argon complex. II. Vibrational predissociation dynamics, spectral linewidths and fragment state distributions

Abstract

We calculated the cross sections for vibrational predissociation of methane–Ar induced by excitation of the methane ν3 mode. We used the ab initio CH4–Ar potential depending explicitly on the ν3 and ν1 normal coordinates of the CH4 monomer that is presented in the preceding paper. It was found that dissociation into CH4 fragments excited in the ν1 mode—a V→V′ process with very low kinetic energy release—strongly dominates over direct dissociation into Ar and ground state CH4, and is responsible for the line broadening observed experimentally. The strong variation of the linewidths, observed as well as calculated, for the van der Waals levels excited in combination with the ν3 mode is related to the opening up of appropriate ν1 dissociation channels and the occurrence of rotational resonances in the ν1 continuum in the energy range of the quasibound ν3 levels. The rotational state distributions of the emerging ν1 excited methane fragment are predicted.