Low-energy electron scattering by metastable helium

Abstract

All significant partial cross sections for elastic and inelastic scattering of electrons by helium in the singlet and triplet metastable states have been calculated in the close-coupling approximation for collision energies up to 7.5 ev. In the doublet total spin system cross sections have been obtained both with and without inclusion of coupling to the ground state of helium. In the quartet total spin system cross sections for the elastic scattering of electrons by triplet metastable helium have been calculated in the static exchange approximation and with inclusion of coupling to He (1S)(3 3S) above the threshold for this state. The calculated total cross section for the superelastic process converting the singlet metastable to the triplet state by collision with thermal electrons at 300 °K is less by a factor of 2 than the experimental value given by Phelps. The estimated total cross sections for the scattering of electrons by triplet metastable helium are less by a factor of 3 than the experimental results of Neynaber et al. for 1 eV electrons. However, the first order partial cross sections are found to yield, in general, a dominant contribution to the total cross sections. It is concluded that the effect of coupling to the nearby 2P helium states, omitted in this calculation, will be important.