A measurement of the electron impact ionization cross section of helium atoms in metastable states

Abstract

Cross sections for the ionization of metastable helium atoms, predominantly in the 2³S state, at electron energies from the threshold to 1000 eV have been measured using the crossed electron and fast atom beam technique. Corrections are made for the presence of ground-state atoms and a very small concentration of highly excited atoms. The semiquantal binary encounter data agree satisfactorily up to 20 eV, but less well at higher energies. From 200 to 1000 eV, the measured data agree well with Bethe-Born cross sections of Briggs and Kim (1971) for He(2³S) provided a correction is made to take account of a spurious contribution to the ionization signal which is only significant at the higher energies. All the various Born cross sections, at high energies, are substantially lower than the present measurements and also the Bethe-Born data, but they are either comparable with or smaller than the measured cross section at its peak.