Excitation of the3D1,3and4F1,3Levels of Helium by Direct Electron Impact, and4P1→4F1,3Collisional Transfer

Abstract

Excitation of the $3{}_{}{}^{1,3}{}_{}{}^{}D$ and $4{}_{}{}^{1,3}{}_{}{}^{}F$ levels of helium by electron impact has been studied by time-resolved spectroscopy of the $3{}_{}{}^1{}_{}{}^{}D\to 2{}_{}{}^1{}_{}{}^{}P\left(\lambda 6678\AA \right)$ and $3{}_{}{}^3{}_{}{}^{}D\to 2{}_{}{}^3{}_{}{}^{}P\left(\lambda 5876\AA \right)$ transitions. (Excitation of the $4{}_{}{}^1{}_{}{}^{}F$ and $4{}_{}{}^3{}_{}{}^{}F$ levels are observed as cascade components under time resolution.) Cross sections are estimated for the direct excitation of these levels by 38-, 50-, and 100-eV electron impact. Excitation of the $4F$ levels by direct electron impact are much larger than predicted by the Born approximation but much smaller than implied by previous experimental measurements. Direct excitation of the $3{}_{}{}^1{}_{}{}^{}D$ and $4{}_{}{}^1{}_{}{}^{}F$ by 100-eV electron impact is in excellent agreement and satisfactory agreement, respectively, with preliminary close-coupling approximation calculations. The collisional transfer $4{}_{}{}^1{}_{}{}^{}P\to 4{}_{}{}^3{}_{}{}^{}F$ is much smaller relative to the $4{}_{}{}^1{}_{}{}^{}P\to 4{}_{}{}^1{}_{}{}^{}F$ transfer than has previously been suggested.