Triple differential cross section measurements for the electron impact ionization of helium: Comparison with a second Born approximation

Triple differential cross sections for the ionization of helium by electrons of 500 eV impact energy have been measured in a coplanar, asymmetric geometry and for energies of the ejected electrons of 2.5, 5, 10 and 20 eV. A comparison is made between experimental data and a second Born treatment of Byron, Joachain and Piraux [1] for atomic hydrogen but for corresponding momenta of the incoming and the two outgoing electrons and for corresponding momentum transfer during the collision. The second Born describes quite well the shifts of the binary peak and the recoil peak to larger angles, the asymmetry of the recoil peak and the ratios of the intensities of binary to recoil peaks for different scattering parameters. — A quantitative estimate is made (for high impact energy) of the contributions to the total cross section of those ionizing collisions, in which the recoil peak intensity represents a considerable quantity. Probably most of these events can not be described by a single collision approximation.