Alignment of helium excited by thin carbon foils

We have measured the linear polarization of the 5016-{}, 2s¹S-3p¹P transition of Hei excited by a thin carbon foil perpendicular to the incident He⁺ beam at ion energies 50-500 keV. We find that the alignment of the 3p¹P term depends upon the beam current density within the range (2-150 {μ}A cm^-2) measured. Oscillations in the dependence of both M/I (the linear polarization fraction) upon energy and the current density variation of M/I with energy are observed. We discuss possible origins of these variations.