Mechanism for Electron Transfer in Fast Ion-Atomic Collisions

We discuss a mechanism for electron capture in fast ion-atom collisions which has not been considered so far. Similarly as in the radiative capture, where the electron transfer occurs due to photon emission, within the mechanism under consideration the electron capture takes place due to the emission of an additional electron. This first-order capture mechanism leads to the so-called transfer ionization and has a number of interesting features; in particular, in the target frame it results in electron emission mainly into the backward semisphere.

Figure 1

Momentum spectrum of the recoil electron emitted via the $e\mathrm{\text{−}}e$ capture in $p^{+}+\mathrm{He}(1s^2)\to \mathrm{H}(1s)+{\mathrm{He}}^2+e^{-}$ collisions ($v=15\mathrm{a}.\mathrm{u}.$). The spectrum is represented by the cross section $d^2\sigma /(d{k}_{\mathrm{tr}}d{k}_{\lg })$ and is given in the rest frame of the target atom.

Figure 2

Longitudinal momentum spectrum of the electrons and ${\mathrm{He}}^{2+}$ in the transfer ionization of $\mathrm{He}(1s^2)$ by 4 MeV $p^{+}$. (a) Solid curve: The calculated spectrum of ${\mathrm{He}}^{2+}$. Dashed and dotted curves: The contributions of the $e\mathrm{\text{−}}e$ and the two-step and shake-off channels, respectively. Symbols: Experimental data from 19 (normalized to the total cross section from Table I of 19). (b) The recoil electron and ion spectra produced via the $e\mathrm{\text{−}}e$ mechanism.