Electron transfer from optically prepared states: He+ + Na(32P) → He(2 1,3P) + Na+ angular differential scattering

Abstract

This paper reports experimental and theoretical scattering angle-dependent differential cross sections (DCS) for the electron transfer processes He+ + Na(3s or 3p) → He(2s or 2p) + Na+ at an impact energy of 1 keV. The He(2p) channels are the dominant contributors to electron transfer from the Na(3p) states. Beam experiments with optically prepared Na(3p) states are analysed using time-of-flight spectroscopy without resolution of the final He 21P and 23P states. The theoretical approach is based on a one-electron two-centre atomic orbital expansion of the electronic singlet and triplet scattering states, using the coupled-channel impact parameter method and the eikonal Bessel transformation to obtain quantal DCS predictions. The theoretical results show distinct differences between singlet and triplet channels. In spite of the smaller statistical weight, the singlet contribution about equals the triplet contribution due to the smaller singlet energy defect. For Na(3s) → He(2s) transfer DCS the theoretical results predict a Fraunhofer-type diffraction pattern as seen previously in Na(3s) → Li(2s) transfer, however, the rings are too narrow to be resolved in the present experiment. The spatial pattern of the scattered neutrals is strongly forwardly peaked with typical scattering angles of less than 0.05°. The results reveal pronounced spatial anisotropies, strongly dependent on the initial target preparation. In particular, when a Na(3p) orbital is prepared aligned along the direction of the initial ion beam, electron transfer is much more efficient than for the perpendicular orbital geometry. Furthermore, if the orbital is tilted by 45° with respect to the incident beam direction a strong left–right transfer scattering asymmetry is observed, with more neutrals being scattered to the side to which the p orbital is pointing than to the opposite side. The experimental observations compare well with the theoretical predictions for the spin-averaged electron transfer. Similarities and differences with earlier electron transfer studies for H+ and Li+ on Na(3p) at the same impact energy are pointed out.