Simple models of vibrational excitation in energy transfer molecular collisions

A simple impulsive model of vibrational excitation by energy transfer collisions is described and applied to T–V, E–V and photodissociation processes. Intermolecular potentials are classified according to the sign of the model parameter, q, which is a decreasing function of energy. When q=0 the model reduces to the spectator model. Positive q potentials (e.g. potentials of the Landau-Teller type) correspond to elongation of the BC bond as the atom A departs, while negative q potentials result in its compression. Negative q potentials should be typical of systems which are potentially reactive, i.e. where the ‘reaction path’ bends upwards. Negative q potentials lead to energy transfer in excess of the spectator limit. Near collinear orientations are found to favour T-V transfer whereas bent ones favour T-R transfer. A prescription for determining the distribution of final vibrational states is given. The model is used to rationalize the energy transfer in ballistic T–V processes (i.e. T–V processes which exceed the Mahan spectator limit) and in the photodissociation of ICN and to predict the relative transfer for isotopic variants of K⁺+H₂. The detailed vibrational distributions for the E–V process Hg^⋆+M(υ≈0)→Hg+M(υ>0), where M=HF and CO, are analysed in terms of the model. It is shown for HF that bent orientations lead to efficient energy transfer, and for CO that both models that favour bent orientations (cf. Gonzalez et al.) and near-collinear orientations (cf. Levine and Bernstein) are in reasonable agreement with experiment.