Final-state distributions in charge transfer from sodium Rydberg atoms near the matching velocity

Abstract

Reports measurements of the distribution of final n states that are populated in charge transfer collisions of Na+ ions with Na in highly-excited Rydberg states. Relative cross sections for final principal quantum numbers n'=18-42 are shown for laser-excited Na initial states 24d, 25s, 28d, 29s, 33d and 34s at kinetic energies 550-2000 eV. These results span the velocity-matching region in which projectile and orbital speeds are comparable. At the matching velocity the final-state distribution exhibits a maximum close to the n' that corresponds to the smallest change of internal energy relative to the initial state. The distribution shifts toward lower n' and becomes broader as collision velocity is increased. Initial s and d states yield similar distributions above but not at the matching velocity. The results are consistent with excitation of only mod m1 mod <or=1 sublevels referred to the incident beam direction, but several l' orbital angular momentum values appear to be present. Results for Ne+ and Ar+ incident on Na(24d) are similar to Na+ at equal velocity. The range of orbital speeds in a final-state distribution is approximately proportional to the projectile speed irrespective of initial state. The electronic orbital speed that corresponds to the centre of the distribution is a linear function of the projectile speed and initial orbital speed. The distributions disagree with predictions of the Brinkman-Kramers approximation.