Electron-molecule scattering: a generalisation of the fixed-nuclei approximation to include nuclear relaxation

Electron-molecule collision processes involving vibro-rotational and, possibly, electronic excitations are analysed in a fixed-nuclei picture, and the influence of the nuclear motion isolated in the total scattering TAU matrix. Corrections due to the nuclear relaxation are in the form of an exponential operator, involving both space and energy differentiations of a fixed-nuclei electronic TAU matrix. The theory suggests that this amplitude should be evaluated off-shell, at the correct 'electronic' energies of the total system, in the entrance and exit channels, respectively. It is finally shown that, under the hypothesis of a short-lived resonance, Herzenberg's boomerang model follows naturally from the present theory. (author)