Focusing effects by one and two Coulomb centers in the autoionization of He

S. MARTÍNEZ; S. OTRANTO; S. SUAREZ; C. R. GARIBOTTI

Kalamazoo, Michigan

Conferencia; XXVI INTERNATIONAL CONFERENCE ON PHOTONIC, ELECTRONIC AND ATOMIC COLLISIONS; 2009

Synopsis In this work we consider the autoionization of He following double electron capture in He2+ + H2 collisions at an impact energy of 14 keV/amu. The post-collisional interaction with the two Coulomb centers is treated within the Barrachina-Macek model by employing the F2 correlated wave function introduced by Gasaneo et al to represent the continuum of the emitted electron in the field of two Coulomb centers. We compare the angular profiles in the electron spectrum with those obtained following double electron capture for the collision system He2+ + He. Clear differences are observed in the spectra obtained for the atomic and molecular targets. In recent years, the scattering of an electron previously emitted from a doubly excited atomic state by two Coulomb centers has been studied theoretically as well as experimentally. The two-slit problem imagined by Feynman in 1963 in which a single electron collides with two atomic-size slits was brought to reality by employing a double-center system instead (see [1] and references therein). In this work, we study the collision system He2+ + H2 at an impact energy of 14 keV/amu (v=0.73 au). At impact velocities below 1 au, double capture to doubly excited states of He is very likely. These autoionizing states decay by emitting an electron with sharply defined energy. Therefore, the backward emitted electrons collide with the receding H2 2+ ions, providing the conditions for a Coulomb postcollisional interaction. We use the Barrachina-Macek model [2] together with the F2 correlated continuum wave function [3]. This function provides a correlated picture of the emitted electron in the field of two Coulomb centers. We restrict the analysis to the 2s2, 2s2p and 2p2 doubly excited states of He since they have the higher population rates in this energy regime. In addition, their lifetimes suggest a strong influence of the postcollision interaction in the emission spectra. Since the H2 molecule suffers a Coulomb explosion due to the double capture process, the calculations were performed including a timedependent internuclear distance. Several orientations of the internuclear-axis relative to the projectile direction were also considered. In Figure 1, the angular distribution for Auger electron emission from the 2s2 state is shown for a perpendicular molecular orientation. The present profile is also compared to that obtained for He targets. We clearly see that the focusing peak obtained for the molecular target is narrower than that obtained for He target. Furthermore, differences in the angular profiles can be inferred from the ratio shown in the inset. The present calculations are in reasonable agreement with preliminary data measured at the Centro Atómico Bariloche [4]. -30 -20 -10 0 10 20 30 0 2000 4000 6000 8000 10000 12000 14000 -30 -20 -10 0 10 20 30 0.0 0.2 0.4 0.6 0.8 1.0 H2/He Ratio q (deg) F2-2s2 q=90º He2++He V=0.73 s(q) (Arbitrary units) q (deg) He2++He He2++H2 Fig. 1. Autoionized He 2s2 electron angular spectrum in the projectile frame. The molecular orientation is taken perpendicular to the incidence direction. Work at UNS supported by PGI 24/F038, PICT- 2007-00887 (ANPCyT) and PIP 112-200801- 02760 (CONICET). References [1] Frémont F et al 2009 Nuclear Instruments and Methods in Physics Research B 267 206. [2] Barrachina R O and Macek J H 1989 J. Phys. B 22 2151. [3] Gasaneo G et al 1997 Phys. Rev. A 55 2809. [4] Suárez S, Bernardi G, Fregenal D, and Frémont F, private communication. 1 E-mail: smartine@criba.edu.ar 2 E-mail: sotranto@uns.edu.ar XXVI International Conference on Photonic, Electronic and Atomic Collisions IOP Publishing Journal of Physics: Conference Series 194 (2009) 082045 doi:10.1088/1742-6596/194/8/082045 c 2009