An (e, 2e) study of helium autoionization experiment and theory

Results are presented for the triple differential (e,2e) cross sections in helium in the region encompassing the (2s2)1S, (2s2p)3P, (2p2)1D and (2s2p)1P resonances at an incident energy of 94.6 eV for a scattered electron angle of 30o and for ejected-electron scattering angles in the range -25 to -135o. The measured coincidence ejected-electron spectra are compared against those calculated in an equivalent local form of the distorted-wave impulse approximation with the resonant scattering amplitudes being evaluated by a six-state momentum-space coupled-channels method. In each case the calculation is folded with the experimental energy resolution. The data are analysed in terms of the Shore-Balashov parametrization to obtain the direct (e, 2e) cross section fr and the resonance parameters ar and br for the (2s2)1S, (2p2)1D and (2s2p)1P resonances as a function of ejected-electron momentum. These derived parameters are compared against the results of a calculation where configuration interaction expansions for the resonances and helium ground state, which employed hydrogenic and multiconfiguration Hartree-Fock orbitals, respectively, were used. Here the distorted-wave Born approximation was employed for the (e, 2e) cross section calculation. The calculated parameters agree quite well with the experimental results with both indicating strong correlations between the resonance amplitudes and the direct ionization amplitudes. Finally we report and discuss our results of an experimental investigation into post-collision interaction effects for the present kinematical conditions. (author)