Cross sections for electron-impact excitation out of metastable helium levels.

Abstract

Absolute apparent cross sections for electron-impact excitation out of the ${2}^{3}$S metastable level of He to the ${3}^{3}$S, ${4}^{3}$S, ${3}^{3}$P, ${3}^{3}$D, ${4}^{3}$D, ${5}^{3}$D, and ${6}^{3}$D levels have been measured by a novel method. A beam of metastable atoms from a hollow-cathode discharge is crossed by an electron beam of energy from 3 to 16 eV, and the emission from the excited states is detected. Absolute calibration is made by measuring the ratio of electron excitation cross section to the known optical-absorption cross section and using knowledge of the pertinent branching ratios. Direct electron-impact excitation cross sections for the ${3}^{3}$P, ${3}^{3}$S, and ${3}^{3}$D levels are obtained.