Propensity for distinguishing two free electrons with equal energies in electron-impact ionization of helium

Abstract

We report a combined experimental and theoretical study on the electron-impact ionization of helium at ${E}_{0}\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}70.6\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ and equal energy sharing of the two outgoing electrons (${E}_{1}={E}_{2}=23\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$), where a double-peak or dip structure in the binary region of the triple differential cross section is observed. The experimental cross sections are compared with results from convergent close-coupling (CCC), $B$-spline R-matrix-with-pseudostates (BSR), and time-dependent close-coupling (TDCC) calculations, as well as predictions from the dynamic screening three-Coulomb (DS3C) theory. Excellent agreement is obtained between experiment and the nonperturbative CCC, BSR, and TDCC theories, and good agreement is also found for the DS3C model. The data are further analyzed regarding contributions in particular coupling schemes for the spins of either the two outgoing electrons or one of the outgoing electrons and the $1s$ electron remaining in the residual ion. While both coupling schemes can be used to explain the observed double-peak structure in the cross section, the second one allows for the isolation of the exchange contribution between the incident projectile and the target. For different observation angles of the two outgoing electrons, we interpret the results as a propensity for distinguishing these two electrons---one being more likely the incident projectile and the other one being more likely ejected from the target.