Total Cross Sections for Inelastic Scattering of Charged Particles by Atoms and Molecules. VI. Metastable Helium

Sum rules for the asymptotic Born cross sections have been applied to the evaluation of various inelastic-scattering cross sections of metastable He ($2{}_{}{}^1{}_{}{}^{}S$ and $2{}_{}{}^3{}_{}{}^{}S$). The ionization cross sections for the two species are similar in magnitude, and each constitutes only a small part of the total cross section for inelastic scattering. Although the first Born approximation is inappropriate for the scattering of slow charged particles, the energy dependence exhibited in the recent experiment by Long and Geballe for the ionization of He ($2{}_{}{}^3{}_{}{}^{}S$) by low-energy electrons (≤ 16 eV) is compatible with the high-energy behavior predicted by the theory. The exchange correction evaluated from the Mott formula has been included in the ionization cross section by electron impact. Optical oscillator strengths for the first three transitions to doubly excited states [$2{}_{}{}^1{}_{}{}^{}S\to \left(2\ln l^{\prime }\right){}_{}{}^1{}_{}{}^{}P$] are also given.