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.

• Received 27 January 1989

DOI:https://doi.org/10.1103/PhysRevLett.62.2253