Collisional ionization of helium by 250 eV electrons: experiments with completely determined kinetics

Abstract

Measurements of angular correlations between scattered and ejected electrons are presented for electron impact ionization of helium for the impact energy E₀=256.5 eV. The two outgoing electrons are measured in coincidence. The fast electrons are detected at fixed angles (<or=8 degrees ). The angular distribution of the corresponding slow electrons consists of two maxima (approximately at -60 degrees and +120 degrees ) in opposite directions and zeros in between. Intensity ratios of the two maxima and their angular positions are determined for different values of the energy and the scattering angle of the fast electrons. The experimental results are compared with several approximations, in which the free electrons are described by plane and/or coulomb waves. The influence of exchange scattering and of the electron-nucleus interaction are investigated.