Non-Adiabatic Effects in Low-Energy Electron-Molecule Scattering

Abstract

The most sophisticated methods presently available for tackling the low-energy electron-molecule scattering problem are usually implemented within the framework of the adiabatic-nuclei approximation (Golden, et al. 1971). Indeed, much of the theoretical literature on electron-molecule scattering has focused on the development of ab initio methods for solving the electronic fixed-nuclei problem. The assumption that underlies the adiabatic-nuclei approximation is that the target molecule is effectively fixed during the course of the collision; this approximation consequently provides an accurate description of electron-molecule scattering only when the collision time is short compared with the rotational and vibrational periods of the molecule. This assumption of course breaks down in near-threshold regions where cross sections computed with the adiabatic-nuclei approximation do not vanish at the appropriate thresholds (Morrison 1988). Consequently, cross sections computed using the adiabatic-nuclei approximation near threshold will be suspect, no matter how sophisticated the treatment of the electronic portion of the scattering problem. The breakdown of the adiabatic-nuclei approximation can be particularly severe for light molecules with large vibrational energy spacings (eg. ∼.5 eV in H2).