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

Absolute apparent cross sections for electron-impact excitation out of the 2³S metastable level of He to the 3³S, 4³S, 3³P, 3³D, 4³D, 5³D, and 6³D 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 3³P, 3³S, and 3³D levels are obtained.