Lawrence Berkeley National Laboratory

We present a non-empirical potential model for studying threshold vibrational excitation of polar moleculesby electron impact. This work builds on the zero-range potential virtual state model of Gauyacq and Herzenberg (J.P. Gauyacq and A. Herzenberg, Phys. Rev. A 25, 2959 (1982)), using known analytic properties of the S-matrix for a dipole potential to predict the analytic continuation of the negative ion potential curve into the continuum. We derive an equation that determines the nuclear dynamics which can be solved without the need for an expansion in target vibrational states. The model is applied to e{sup -} - HCl and is found to capture the essential features of the observed excitation cross sections, including both the threshold peaks as well as oscillatory structures at energies above threshold.