Strongly Enhanced Backward Emission of Electrons in Transfer and Ionization

We studied three-dimensional angular distributions and longitudinal momentum spectra of electrons ejected in transfer plus ionization (TI), i.e., the ejection of one and the capture of a second target electron, for ion-helium collisions. We observe a pronounced structure strongly focused opposite to the projectile beam direction, which we associate with a new correlated TI mechanism proposed recently. This process contributes significantly to the total cross sections over a broad range of perturbations $\eta$, even at $\eta$ as large as 0.5, where uncorrelated TI mechanisms were thought to be dominant.

Figure 1

Three-dimensional angular distribution of electrons ejected in $1\mathrm{MeV}/\mathrm{amu}$ ${\mathrm{Li}}^{3+}+\mathrm{He}$ collisions. (a) Calculation for electrons ejected in single ionization; (b) calculation for electrons ejected in independent transfer ionization; (c) experimental data for electrons ejected in transfer-ionization; (d) calculation for electrons ejected in the independent and $ee$ transfer-ionization processes.

Figure 2

Longitudinal momentum spectra of electrons ejected in transfer ionization in (a) 630 keV $p+\mathrm{He}$; (b) $630\mathrm{keV}/\mathrm{amu}$ ${\mathrm{He}}^{+}+\mathrm{He}$; (c) $630\mathrm{keV}/\mathrm{amu}$ ${\mathrm{He}}^{2+}+\mathrm{He}$; (d) $1\mathrm{MeV}/\mathrm{amu}$ ${\mathrm{Li}}^{3+}+\mathrm{He}$. The dashed and dash-dotted curves are calculations for the $ee$ and independent transfer-ionization processes, respectively, and the solid curve is the incoherent sum of both contributions.