Sources of rotational cross section asymmetry σ(J→J + ΔJ)/σ(J→J - ΔJ) in molecule-atom and molecule-molecule systems

Abstract

Asymmetries are investigated for the rotationally inelastic transitions J→J + ΔJ and J→J - ΔJ. Previous cross section expressions are extended to handle the case of two colliding symmetric top molecules. Asymmetries are shown to arise from (1) the previously considered interference terms in the cross sections, (2) angular momentum coupling effects, and (3) R⇄T energetic effects. It is shown that symmetric top molecules with threefold or higher symmetry axes are not likely to exhibit interference asymmetries unless high-order tensor terms are important in the intermolecular potential. However, substantial asymmetries can still arise from angular momentum coupling and energetic effects. The conditions for the dominance of these sources of asymmetry are discussed and investigated numerically for the molecules HCl, HCN, and OCS perturbed by Ar and Xe as well as the systems HCl:HCl, HCN:HCN, OCS:OCS, HCl:OCS, and NH3:NH3. In these latter cases, the first molecule in each pair is the observed species and the second constitutes a thermal bath. Angular momentum coupling asymmetries are found to be important only at small J. Energetic asymmetries are particularly important for systems with large rotational constants.