FOCUSING OF AN IONIC PROJECTILE BY A SINGLE CONTINUUM ELECTRON

V D RODRÍGUEZ; P. A. MACRI; R. O. BARRACHINA

Freiburg, Germany

Congreso; 25th International Conference on Photonic, electronic and Atomic Collisions; 2007

ICPEAC

As in the David vs. Goliath story, the trajectory of an ionic projectile impinging on an atomic target can be affected by its interaction with the intervening electrons. This effect would usually be small, namely of the order of the ratio of the masses of the electron and the ion, but still prone to experimental observation. The deflection experienced by the projectile when capturing an electron from the target, as predicted by Llewellyn Hilleth Thomas in 1927, is oneof the most well known examples of this kind of phenomena. In this communication we investigate the appearance of similar effects in ion-atom ionization collisions. More precisely, we analyze whether the projectile scattering can be affected by the interaction with the emitted electron or not, and to what extent. To this end, we employ a Continuum Distorted Wave - Eikonal Initial State (CDW-EIS) theory and a Classical Trajectory Monte Carlo (CTMC) method. We show that both models predict the same striking result: under certain conditions the positive ion might be focused in the forward direction by its postcollision interaction (PCI) with the emitted electron. This effect was first studied by Schulz and co-workers in 50 to 150 keV proton-helium ionization collisions. They observed a changeof slope in the half-width at half-maximum of the projectile angular distribution when the velocityve of the emitted electron matched the projectile’s velocity. In this communication we demonstrate that a clearcut fingerprint of this surprising feature can be attained when the attention is focused on the recoil-ion rather than the emitted electron. To this end, we calculate the ionization double differential cross section (DDCS) in the projectile scattering angle èP and the longitudinal recoil-ion momentum PRk. This DDCS has the advantage of allowing the study of the projectile scattering when the electron capture to the continuum (ECC) process is dominant.Actually, PRk approaches its threshold when we matches the projectile velocity. We propose that a prospective experiment analyzing this DDCS might serve to throw new light on this effect.For instance, in spite of some quantitative differences, we see in the figure that both calculations agree in two important features: the mean proton scattering angle decreases as the kinetic threshold is approached. On the contrary, a clear defocusing effect is observed in the case of antiproton impact.