(e,2e)and(e,3−1e)studies on double processes of He at large momentum transfer

Abstract

The double processes of He in electron-impact ionization, single ionization with simultaneous excitation and double ionization, have been studied at large momentum transfer using an energy- and momentum-dispersive binary $(e,2e)$ spectrometer. The experiment has been performed at an impact energy of $2080\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the symmetric noncoplanar geometry. In this way we have achieved a large momentum transfer of $9\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.}$, a value that has never been realized so far for the study on double ionization. The measured $(e,2e)$ and $(e,3\ensuremath{-}1e)$ cross sections for transitions to the $n=2$ excited state of ${\mathrm{He}}^{+}$ and to doubly ionized ${\mathrm{He}}^{2+}$ are presented as normalized intensities relative to that to the $n=1$ ground state of ${\mathrm{He}}^{+}$. The results are compared with first-order plane-wave impulse approximation (PWIA) calculations using various He ground-state wave functions. It is shown that shapes of the momentum-dependent $(e,2e)$ and $(e,3\ensuremath{-}1e)$ cross sections are well reproduced by the PWIA calculations only when highly correlated wave functions are employed. However, noticeable discrepancies between experiment and theory remain in magnitude for both the double processes, suggesting the importance of higher-order effects under the experimental conditions examined as well as of acquiring more complete knowledge of electron correlation in the target.