Distorted-Wave Approximation and Its Application to the Differential and Integrated Cross Sections for Electron-Impact Excitation of the2P1State of Helium

Abstract

Theoretical results are given for the application of the distorted-wave approximation to electron-atom impact excitation for transitions from an $L\ensuremath{-}S$ coupled initial state to an arbitrarily coupled final state. Expressions for the differential cross section and spin polarization of the emitted electrons are given for unpolarized electron beams incident upon unpolarized atoms. These results are applied to excitation of helium from its ground state to the $1s2p^{1}P_{1}$ excited state for incident-electron energies between 26.5 and 300 eV. The results are compared with previous theoretical and experimental works. It is found that the distorted-wave calculation is superior to previous calculations in fitting the absolute magnitude and angular distribution of the experimental data. The improvement over the plane-wave calculations is greater at large angles, where the plane-wave approximations fail by several orders of magnitude.