ADIABATICALLY CORRECTED SUDDEN APPROXIMATION FOR ROTATIONALLY INELASTIC-COLLISIONS BETWEEN POLAR-MOLECULES

The sudden approximation is applied to rotationally inelastic collisions between two polar molecules. The relative motion is assumed to be described by a straight-line classical path and only the dipole-dipole interaction is considered. The expression for the degeneracy averaged cross section for the jajb → ja′jb′ transition can be factored into the product of angular coupling terms and j-independent dynamical factors. Introduction of an effective energy gap into the sudden action integral makes it possible to correct for the breakdown of the sudden approximation at large impact parameters, where the collision time is long compared to the rotational dephasing time. Within the resulting adiabatically corrected sudden (ACS) approximation the transition probabilities are related to the pure sudden values by a straightforward nonlinear mapping. Comparison with our earlier quantum close-coupling study of the HF-HF system indicates that the ACS method can provide accurate partial and integral cross sections for rotationally inelastic collisions at hyperthermal collisions with a reduction in computation time of at least three orders of magnitude over that required for comparable quantum or classical trajectory calculations. © 1979 American Chemical Society.