Model study of vibrational excitation in electron-polyatomic molecule collisions

The effective range approximation, which has been limited to the treatment of electron-diatomic molecule interactions, is generalized to the case of electron-polyatomic molecule collisions. A model study of the vibrational excitation process is presented with the aim of recognizing features specific to the polyatomic targets. The model target molecule is modelled by a two vibrational mode molecule, one of the vibrational frequencies being very small to mimic a quasicontinuum of vibrational levels. It is found that the vibrational modes of the molecule cannot be treated independently, and that a resonance created by the motion along one coordinate can induce a vibrational excitation along another coordinate. This effect is analysed by looking at the time delay and time dependence of the vibrational excitation process. A significant production of low energy electrons is also observed in the resonance region, thus yielding an interpretation of the 'unspecific' vibrational excitation observed by Allan (1984,1988,1989) in large polyatomics.