Theory of single and double ionization of helium through two-photon excitation of the 1SOe(1) doubly excited resonance

We present a theory and calculations of two-photon-resonant three-photon ionization of He via the lowest even parity doubly excited state 1S0e(1). We assess the importance of double ionization relative to single ionization into excited ionic states. Although double ionization is found to be quite small in the present context, our calculations reveal the importance of autoionizing doubly excited states as virtual intermediate states and suggest contexts in which double ionization may be relatively more efficient. (orig.)