Electron-atom collisions

These difficulties recede for an energetic electrons with the incident energy significantly larger than the ionization potential. In this case the single knock-out collision is sufficient to eject the initially bound target electron into continuum. Multiple collisions of the projectile with the target can be safely neglected in this case. This justifies a theoretical description within the lowest order perturbation theory known as the first Born approximation.

In our recent work, we use this approximation to analyse the recoil-to-binary peak ratio in an energetic (e,2e) reaction on the valence ns  sub-shell of noble gas atoms. Dramatic qualitative change in this ratio dependence on the ejected electron energy can be explained by variation of  reflectivity of the short-range target potential. The reflectivity increases profoundly from lighter (He) to heavier (Ne and Ar) noble gas atoms because of modification of the scattering phases due to occupation of the target np orbitals (Levinson theorem). This effect is further modified due to strong inter-shell correlations in Ar. These theoretical predictions are confirmed experimentally.

This effect can be illustrated using a simple billiard-ball analogy as is shown in the figure. Binary (e,2e) reaction satisfies the momentum conservation also known as the Bethe ridge condition (left diagram). Accordingly, the ejected electron is well confined within a narrow angular range in the forward direction (binary peak). If the Bethe ridge condition is no longer satisfied, there is a need for a momentum exchange with the nucleus (right diagram). This results in an additional recoil peak roughly in the direction opposite to the binary peak.

Conventionally, it was believed that the relative recoil peak intensity would fall with increase of the projectile energy which indeed the case in light atoms like H and He. This happens because the bare nucleus Coulomb barrier (“billiard wall”) is a poor reflector of energetic electrons. This, however, changes in many-electron noble gases. In these systems, electron reflection from the target core potential is increasing with energy because of occupied target orbitals which change profoundly the scattering phases of the ejected electron.

References

A Kheifets, A Naja, E M Staicu Casagrande and A Lahmam-Bennani
Energetic (e,2e) reaction away from Bethe ridge: Recoil versus binary
J. Phys. B 42, 165204, 2009

A Kheifets, A Naja, E M Staicu Casagrande and A Lahmam-Bennani
DWBA-G calculations of electron impact ionization of noble gas atoms
J. Phys. B 41, 145201, 2008

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Kheifets, Anatoli profile
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