Approximate phases in electron scattering

During the last five years considerable attention has been devoted to the scattering of electrons by atoms and molecules of a gas or vapour. On the experimental side a large number of scattering curves have been obtained showing the angular distribution of the electrons scattered from a primary beam by atoms and molecules of many different gases and vapours. The most characteristic feature of these curves is the presence of diffraction maxima and minima. On the theoretical side expressions, valid under different conditions, have been obtained showing the variation of scattered intensity with angle. Where a comparison between theoretical and experimental curves has been made good agreement is obtained, and the theory is able to account for the observed diffraction effects. However, assuming the atomic field is accurately known, the calculations required to obtain an accurate theoretical curve for even the elastically scattered electrons are exceedingly laborious and consequently approximations have generally been made. These consist in using approximate solutions of the wave equation, from the asymptotic solution of which the phases are determined.