Vibrational predissociation dynamics of methane–Ar: an ab initio approach

Abstract

We calculated the cross sections for vibrational predissociation of methane–Ar induced by excitation of the methane ν₃ mode with the aid of an ab initio CH₄–Ar potential depending explicitly on the ν₃ and ν₁ normal coordinates of the CH₄ monomer. We 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 (observed and calculated) strong variation of the line widths for the Van der Waals levels excited in combination with the ν₃ mode (giving states of A, F and E symmetry) 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 quasi-bound ν₃ levels.