On the interpretation of low-energy electron-HCl scattering phenomena

A general non-adiabatic theory of near-threshold resonant electron-molecule scattering in the presence of a long-range direct scattering potential is developed. As a special case, resonant s-wave scattering in the presence of a dipole potential is worked out in detail. An exactly solvable dynamical model for electron-HCl scattering is proposed and is shown to reproduce the essential features of the vibrational excitation functions of HCl measured by Rohr and Linder (1976). Particular emphasis is given to the analysis of the analytic properties of the S matrix and related quantities in the fixed-nuclei approximation. The passage of a resonance through threshold is shown to be a non-trivial phenomenon for polar molecules. The authors propose a re-interpretation of the HCl^- potential energy curves obtained by the stabilisation method.