Alignment and orientation of the 21P state of helium by electron impact at 29.6 and 51.2 eV

Abstract

The electron-photon coincidence technique has been used to deduce the lambda and chi parameters for excitation of the 21P state of helium by electron impact at 29.6 and 51.2 eV incident energies and for electron scattering angles in the range 15 to 130 degrees . The lambda parameter shows a minimum at small and large scattering angles. When combined with the 1979 data of Hollywood and co-workers at 81.2 eV, it can be seen that as the incident energy decreases, the depth of the low-angle minimum decreases and its position moves to larger scattering angles. In contrast, the depth of the second minimum increases with decreasing incident electron energy. The experimental values of lambda and chi are compared with values predicted from the first Born approximation, a distorted-wave polarised-orbital approximation, a first-order many-body theory and an R-matrix theory.