The role of rotation in the vibrational relaxation of diatomic molecules

Abstract

Listen

Translate article

The mechanism by which molecular rotation enhances vibrational relaxation of a diatomic molecule is examined. Three factors are discussed, lowering by rotational transitions of the vibrational energy defect, coupling due to potential matrix elements off-diagonal in rotational state, and additional phase shifts introduced into the incoming and outgoing waves by anisotropy in the intermolecular potential. The distorted wave solution of the coupled states scattering equations elucidates, for H 2(ν = 1)-He and CO(ν = 1)-H 2, the importance of each factor. For H 2He, rotation plays both an energetic and a dynamical role. In the energetic role, rotation of the H 2 lowers the vibrational energy defect and thereby enhances vibrational relaxation. The dynamic contribution takes the form of additional phase shifts introduced into the incoming and outgoing waves by the potential anisotropy. For the relaxation of CO(ν = 1) by H 2, rotational enhancement by H 2 is almost entirely due to lowering of the vibrational energy defect. Our results suggest that molecular anisotropy, particularly in the region of the classical turning point, is an important factor in the rotational enhancement observed in recent comparisons of CSA and IOSA vibrational relaxation rate constants for the N 2He and COHe systems. We further discuss vibrational energy transfer from H 2(ν = 1) to CO(ν = 0, 1, 2). In this case, the role played by H 2 rotation changes markedly as a function of the final CO vibrational state.