Energy transfer during ``orbiting'' collisions

Semiclassical collision theory is used to calculate the temperature dependence of cross sections for exchange of vibrational excitation from HF and DF to CO2. For collisions with kinetic energy less than four-fifths of the intermolecular potential well depth there exists a narrow range of impact parameter for which the scattering angle exceeds 2π (i.e., more than one revolution). That part of the trajectory in excess of a single revolution can be accurately represented by a simple model of a circular orbit with a constant angular velocity replacing the instantaneous classical turning point. Calculations are compared to experimental data and illustrate the significance of multiple revolution collisions.