Mechanisms of Photo Double Ionization of Helium by 530 eV Photons

Abstract

We have measured fully differential cross sections for photo double ionization of helium 450 eV above the threshold using the COLTRIMS technique. We have found an extremely asymmetric energy sharing between the two electrons and an angular asymmetry parameter β ≃ 2 for the fast electrons and β ≃ 0 for the slow electrons. The asymmetric energy sharing together with the asymmetry parameter β ≃ 2 for the fast electron indicate that the fast electron absorbs not only most of the photon energy but also its angular momentum. The electron angular distributions show that the very slow electrons (about 2 eV) are emitted isotropically with a slight backward emission to the momentum of the fast electron which is expected from the shake-off mechanism. At the same time, the slow electrons at higher energies are mostly emitted at 90 degree to the fast electron indicating an (e,2e) like collision between the two electrons. All our data are in good agreement to CCC calculations.