A semiclassical model for vibrational energy relaxation in simple liquids and compressed fluids

Using a simple model intermolecular potential, a density dependence of the vibrational population relaxation time in liquids is obtained which is in qualitative and quantitative agreement with experiment, over the whole range of density and temperature studied. This model is applicable to systems where the energy transfer involved is large with respect to thermal energies and describes the variation of the relaxation time as being essentially related to the variation of the collision frequency on the strongly repulsive part of the intermolecular potential. The formalism employed demonstrates clearly the role of excluded volume at high density and of bound states at low density and temperature.