DOUBLE-IONIZATION OF HE(1S(2)) AND HE(1S2S S-3) BY A SINGLE HIGH-ENERGY PHOTON

We have calculated the energy and angular distributions for double ion ization of He(1s2) and He(1s2s 3S) by one photon, over a range of phot on energies up to a few keV. The calculations were based on using a fa irly accurate initial-state wave function, determined so as to exactly satisfy the Kato cusp conditions, and a final-state wave function whi ch is a product of three Coulomb wave functions modified by- a short-r ange correction term. There are at least three different mechanisms fo r double ionization, and each one leaves a mark on the angular distrib ution. When the energies of the two electrons are equal, the contribut ion of each mechanism to the angular asymmetry parameter can be estima ted on theoretical grounds; we compare these estimates with the calcul ated results to give a further indication of the roles of the various mechanisms. Concerning the shapes of the energy and angular distributi ons, we find significant differences between double ionization of sing let and triplet helium; in particular, the probability for one high-en ergy photon to eject two equal-energy electrons from triplet helium ne arly vanishes owing to the Pauli exclusion principle and to interferen ce effects resulting from antisymmetrization. In two appendixes we pre sent some details of the integration involved in the calculations.