PHOTOIONIZATION OF HE ABOVE THE N = 2 THRESHOLD

We report a theoretical study of He(1s2) + hv --> He+ (1s, 2s, 2p) + e- photoionization processes, for photon energies greater than 65.4 eV. We pay special attention to the energy region 69.0-73.0 eV, where recent synchrotron experiments exhibit clearly resonant structure associated to 3lnl' doubly excited states of He. Our method is based on a Feshbach partitioning of the total wave function that includes explicitly resonant structure. Total and partial cross sections do not depend on parametrization, although an obvious one can be obtained in a straightforward manner in the vicinity of isolated resonances; this is very useful for the analysis of most of the resonance peaks observed experimentally. An appealing feature of our approach is the use of L2-integrable basis sets to describe the scattering wave functions. Our discretization method provides coupled continuum states with the proper delta-function normalization and with the correct asymptotic behavior. With this method, we have calculated partial photoionization cross sections for leaving the ion in the 1s, 2s, and 2p levels, and the results are in good agreement with recently published experimental data. A complete set of parameters describing the first twelve resonances in partial cross sections is also provided.