Multichannel theory of two-photon single and double ionization of helium

We report on nb initio calculations of two-photon, single- and double-electron ejection cross sections in helium, using multichannel wavefunctions constructed within the framework of discretized bases involving linear combinations of B-splines. Within the lowest non-vanishing order of perturbation theory, two-photon double ionization, for photon energies from 42.5 to 45.5 eV, is found to be about 10% that of single ionization. It is also found that electron correlation in the final state tends to lower the amount of double ionization in comparison to predictions involving incomplete or no correlation. The potential of the approach in the context of further open problems is also discussed.