Quantum mechanical calculations of rotational-vibrational scattering in homonuclear diatom-atom collisions

Most calculations of the vibrational scattering of diatom-atom collisions use the breathing sphere approximation (BSA) of orientation averaging the intermolecular potential. The resulting angularly symmetric potential can not cause rotational scattering. We determine the error introduced by the BSA into observables of the vibrational scattering of low-energy homonuclear diatom-atom collisions by comparing two quantum mechanical calculations, one with the BSA and the other with the full angularly asymmetric intermolecular potential. For ·reasons of economy the rotational scattering of the second calculation is restricted by the use of special incomplete channel sets in the expansion of the scattering wavefunction. Three representative collision systems are studied: H_2-Ar, O_2-He, and I_2-He. From our calculations, we reach two conclusions. First, the BSA can be used to analyze accurately experimental measurements of vibrational scattering. Second, measurements most sensitive to the symmetric part of the intermolecular potential are, in order, elastic cross sections, inelastic cross sections, and inelastic differential cross sections. Elastic differential cross sections are sensitive to the potential only if the collision is "sticky," with scattering over a wide range of angles; I_2-He is such a collision. Otherwise the potential sensitivity of elastic differential cross sections is concentrated in the experimentally difficult region of very small angle scattering.