Differential and Integral Cross Sections for the Excitation of the2 S1,2 S3, and2 P3States of He by Electron Impact at 29.6 and 40.1 eV

Abstract

Differential and integral cross sections for the electron-impact excitation of the $2 {}_{}{}^3{}_{}{}^{}S$, $2 {}_{}{}^1{}_{}{}^{}S$, and $2 {}_{}{}^3{}_{}{}^{}P$ states of He relative to the $2 {}_{}{}^1{}_{}{}^{}P$ state have been measured at 29.6- and 40.1-eV energies in the 3° to 138° angular range. The relative cross sections have been normalized to the absolute scale by utilizing the previously determined $2 {}_{}{}^1{}_{}{}^{}P$ cross sections [D. G. Truhlar, S. Trajmar, W. Williams, S. Ormonde, and B. Torres (unpublished)]. The differential cross section for the $2 {}_{}{}^1{}_{}{}^{}S$ state has a deep minimum at 50°; for the $2 {}_{}{}^3{}_{}{}^{}P$ state it is nearly isotropic, and for the $2 {}_{}{}^3{}_{}{}^{}S$ state it shows a complicated structure which has not been previously reported. The experimental cross sections are compared to the results of quantum-mechanical calculations performed by Truhlar, Yates, Tenney, Cartwright, Steelhammer, and Lipsky using the Born, Born-Ochkur-Rudge, and Glauber approximations for the $2 {}_{}{}^1{}_{}{}^{}S$ excitation and the Born-Oppenheimer and the Ochkur-Rudge approximations for the $2 {}_{}{}^3{}_{}{}^{}S$ and the $2 {}_{}{}^3{}_{}{}^{}P$ excitations. None of the first-order plane-wave calculations predict the observed differential cross sections for these optically forbidden transitions. It is interesting to note, however, that the Ochkur-Rudge model predicts the correct magnitude and approximate shape of the $2 {}_{}{}^3{}_{}{}^{}P$ differential cross sections in the whole angular range at these energies. The Glauber approximation shows considerable improvement compared to first-order plane-wave theories in predicting the cross sections for the $2 {}_{}{}^1{}_{}{}^{}S$ state. However, at large scattering angles, calculation and experiment differ significantly. It is clear from the present study that more refined calculations are needed to predict angular distributions for these optically forbidden excitations in the 30-40-eV energy region.