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

We calculated the cross sections for vibrational predissociation of methane -Ar induced by excitation of the methane nu (3) mode with the aid of an ab initio CH4-Ar potential depending explicitly on the nu (3) and nu (1) norma l coordinates of the CH4 monomer. We found that dissociation into CH4 fragm ents excited in the nu (1) mode, a V --> V' process with very low kinetic e nergy release, strongly dominates over direct dissociation into Ar and grou nd state CH4, and is responsible for the line broadening observed experimen tally. The (observed and calculated) strong variation of the line widths fo r the Van der Waals levels excited in combination with the nu (3) mode (giv ing states of A, F and E symmetry) is related to the opening up of appropri ate nu (1) dissociation channels and the occurrence of rotational resonance s in the nu (1) continuum in the energy range of the quasi-bound nu (3) lev els.