Abstract
A semiclassical quantal theory of threshold electron impact ionisation is developed. Singular wavefunctions, classical differential cross sections and matching procedures are avoided. The Kohn variational principle is applied perturbatively to evaluate the scattering amplitude. The absolute electron-helium singlet ionisation total Wannier cross section is estimated to be 2.37 (Ea0/e2)1.127a02, in good agreement with the experiment of Pichou et al. (1978). Both helium electrons and the first four partial waves are included. Doubly differential cross sections p0.8(Er,1/2 pi ) at E=0.8 eV are found at 2.58*10-19 cm2 sr-1 eV-1, in good agreement with the experiment of Pichou et al. (1978). Triple differential cross sections are compared favourably with the recent experiment of Fournier-Lagarde et al. (1985).
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