Multiple ionization of the rare gases by successive electron impacts (0–250 eV). II. 1S–2S transition in 3He+

The tertiary collision sequence of electrons with helium (He → He+ → He+m(2S) → He2+) has been observed with a trapped-ion mass spectrometer. The variation of He2+ ion current with electron energy shows a threshold at 40.8 eV, and structure, which results from excitation to autoionizing states of the atom below the n = 3 threshold of He+, is observed in the range 45–49 eV. Estimates of the cross section for the 1S–2S transition in He+ lie slightly below the values from close-coupling calculations.