The 3s-3p excitation of Na atoms by ${\mathrm{He}}^{+}$ ions in the impact energy range 0.6–10 keV has been studied both experimentally and theoretically. In the experiment, the total excitation cross section, the alignment of the Na 3p state, and the collision-induced change of spin polarization of the Na valence electron were investigated. For the latter purpose, the Na atoms were polarized by a hexapole state selector, and a magnetic field was applied to the collision region, resulting in a spin polarization of the Na ground-state atoms of $P_S$≊0.74. The spin polarization of the final 3p excited state was derived from the measured circular polarization of the fluorescence light. The experimental results are compared to published calculations in the Coulomb approach and to our own calculations in the molecular approach using a 12-state molecular expansion and employing a common translation factor. The molecular approach gives satisfying agreement with all experimental data. In particular, the measured increase of spin depolarization with decreasing collision energy is well reproduced. The depolarization originates from the exchange interaction between the 1s electron of ${\mathrm{He}}^{+}$ and the valence electron of Na during the collision.

• Received 17 April 1989

DOI:https://doi.org/10.1103/PhysRevA.40.2958