Recent theoretical advances in small-angle electron scattering

Two recently developed methodologies for small-angle, including zero, electron scattering are presented briefly and applied to various atomic and molecular transitions. The first is the momentum dispersion method of Haffad et al. (1996) which is based on Regge pole theory; it uses the analytical continuation of the generalized oscillator strength (GOS) function to obtain the small angle, including zero, data from their more reliably determined larger angular measurements. The second method is the forward scattering function of Avdonina et al. (1997) which employs only the optical oscillator strength as input; it describes a unique path to reach the Lassettre limit without traversing the nonphysical region and provides a stringent test of both theory and measurement at zero scattering angles. Combined, these methods are also effective in normalizing the measured relative electron differential 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 power and versatility of the methods to analyze small-angle electron impact data and identify spurious behavior. The results cover the energy range from 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 forward scattering function. (C) 1998 Elsevier Science B.V. All rights reserved.