Photoionization of the helium metastable states above the n=2 threshold

Abstract

Differential cross sections for the photoelectric transitions He(2¹S₀,2³S₁)+h(cross) omega to He⁺(1s,2s,2p,3s,3p,3d)+e^- have been calculated for incident photon energies above the n=2 threshold using Hylleraas bound state wavefunctions and close coupling final state wavefunctions. Particular attention has been given to the angular distribution of the photoelectrons ejected in the transitions to the excited states of the residual He⁺ ion. When the partial cross sections are transformed to a representation which diagonalizes the long range r^-2 interactions coupling degenerate final state channels, it is found that some linear combination of the degenerate channels is preferentially excited over a range of incident photon energies from the threshold to several times the threshold value. At higher energies the calculated ( sigma _2s+ sigma _2p)/ sigma _1s and ( sigma _3s+ sigma _3p+ sigma _3d)/ sigma _1s ratios approach the asymptotic values predicted by the theory of Kabir and Salpeter.