Electron-impact excitation of the(2p2)1Dand(2s2p)1Poautoionizing states of helium

Abstract

An experimental study of the excitation of the ($2{p}^{2}$) ${}^{1}D$ and ($2s2p$) ${}^{1}{P}^{o}$ autoionizing states of helium by 250-eV electron impact is presented. The ejected-electron angular distributions and energy spectra are measured in coincidence with the corresponding scattered electrons for a scattering angle of $\ensuremath{-}{13}^{\ensuremath{\circ}}$ and for a range of ejected-electron angles in both the forward and backward directions. Resonance profiles are analyzed in terms of the Shore-Balashov parametrization to obtain the resonance asymmetry ${A}_{\ensuremath{\mu}}$ and yield ${B}_{\ensuremath{\mu}}$ parameters and the direct ionization cross section $f$. The spectra and their parameters are compared to the previous measurements of Lower and Weigold [J. Phys. B. 23, 2819 (1990)] and McDonald and Crowe [J. Phys. B 26, 2887 (1993)]. Comparison is also made with the recent theoretical triply differential cross-section calculations based on the first and second Born approximations. In general, good qualitative agreement is found between the experimental results. Some differences are found at the forward and backward directions. These differences in the shape and magnitude of the cross sections are attributed to the different incoming electron energies used in the experiments. The second Born approximation with inclusion of the three-body Coulomb interaction in the final state agrees reasonably well with experiments in the binary region. However, the ${}^{1}{P}^{o}$ resonance yield parameter ${B}_{\ensuremath{\mu}}$ is significantly overestimated at the recoil region, giving a relatively large recoil peak, in contradiction to the experiment. There is also a discrepancy between the two theories available for the ${}^{1}D$ resonance yield parameter ${B}_{\ensuremath{\mu}}$ in this region. Remaining discrepancies between theories and experiments are also discussed.