Double photoionization and ionization excitation of the metastable helium S states

Theoretical double photoionization cross sections are predicted for He initially in the 1s2s ¹S or ³S state. The probability for double photoionization is a factor of 6 higher for the ¹S state at 20 eV above the threshold for double ionization and a factor of 3 larger at 80 eV above threshold. We compare these results with each other and with predictions for the high-energy limit to assess the influence of exchange on the double ejection process. Similar comparisons are also made for the process of photoionization-induced excitation of He⁺(n).