Tests of Born Approximations: Differential and Total2S3,2P1, and2S1Cross Sections for Excitation of He by 100- to 400-eV Electrons

Abstract

The angular dependence of $2^{3}S$, $2^{1}P$, and $2^{1}S$ excitation of He for incident electron energies from 100 to 225 eV ($2^{3}S$) and 400 eV ($2^{1}P$ and $2^{1}S$) has been measured. Apparent generalized oscillator strengths $f(K)$ and differential cross sections for the transition $1^{1}S\ensuremath{\rightarrow}2^{1}P$ are obtained by normalizing to Schiff and Pekeris's optical oscillator strength. From the experimental intensity ratios $\frac{2^{3}S}{2^{1}P}$ and $\frac{2^{1}S}{2^{1}P}$ we then calculate differential $1^{1}S\ensuremath{\rightarrow}2^{3}S$ and $1^{1}S\ensuremath{\rightarrow}2^{1}S$ cross sections. The differential cross sections are integrated to get total cross sections. The $f(K)$ found here for $2^{1}P$ and $2^{1}S$ excitation decrease faster with increasing momentum transfer $K\ensuremath{\hbar}$ than in earlier studies. Departures from the Born approximation appear only below 200 eV for $2^{1}P$ excitation, but occur at higher energy and are larger (especially for large momentum transfers) for $2^{1}S$ excitation. The angular dependence found for $2^{3}S$ excitation disagrees strongly with the Ochkur (Bonham) approximation. Our total $2^{3}S$ cross sections are much lower than all other existing theoretical and experimental data.