Recent theoretical advances in small-angle electron scattering

Two recently developed methodologies for small-angle, including zero, elect ron scattering are presented briefly and applied to various atomic and mole cular transitions. The first is the momentum dispersion method of Haffad et al. (1996) which is based on Regge pole theory; it uses the analytical con tinuation of the generalized oscillator strength (GOS) function to obtain t he small angle, including zero, data from their more reliably determined la rger angular measurements. The second method is the forward scattering func tion of Avdonina et al. (1997) which employs only the optical oscillator st rength as input; it describes a unique path to reach the Lassettre limit wi thout traversing the nonphysical region and provides a stringent test of bo th theory and measurement at zero scattering angles. Combined, these method s are also effective in normalizing the measured relative electron differen tial cross-sections (DCSs). Electron impact excitation DCSs for H, He, Na, Xe, F-2, SF6 and N-2 are investigated through the GOSs to demonstrate the p ower and versatility of the methods to analyze small-angle electron impact data and identify spurious behavior. The results cover the energy range fro m near threshold to the first Born Approximation limit; in particular, for He 1 S-1-2 P-1(0) the most recent measurements are compatible with the forw ard scattering function. (C) 1998 Elsevier Science B.V. All rights reserved .