Coincidence electron impact ionisation of helium: absolute experimental cross sections and comparison with first-order theories

Abstract

Absolute triple differential cross sections (TDCS) for the ionisation of helium by 8 keV electron impact are measured in an asymmetric coplanar electron-electron coincidence experiment (e, 2e method). The angular distributions are made absolute by assuming a cylindrical symmetry about the momentum transfer direction, K, then integrating over all ejection angles and comparing with the corresponding double differential cross section. The latter is independently measured on an absolute scale in a separate experiment. The overall accuracy of the absolute TDCS is about 12%. Systematics in the behaviour of the binary and the recoil intensities as a function of scattering angle and ejection energy are shown. At small K values, the data are discussed in detail in relation to the optical limit (K to 0), and a new qualitative explanation is given for the shift of the recoil lobe axis from the -K direction.