Angular Dependence of23SExcitation of He by High-Energy Electron Impact

Abstract

Electron impact excitation of metastable He atoms is formulated in the Coulomb-wave approximation. At high energy, both the direct and the exchange scattering amplitudes are approximated in closed analytic forms, which are identical to the Born-Oppenheimer approximation apart from the core potential contribution. From these expressions, the validity of the Ochkur approximation and its limitations are discussed. Also obtained is the forward-peaking differential cross section for $\mathrm{He}\left(2^{3}S\right)$ excitation, which is in qualitative agreement with the recent experiment by Vriens, Simpson, and Mielczarek. Thus an explanation for the observed angular behavior of the cross section is offered for the first time.