Material-dependent variations of alignment in beam-foil excitation

We have measured the linear polarization fractions of the 2s¹S-3p¹P He I transition at 5 016 Å excited by the passage of helium ions through thin perpendicular foils at beam energies of 40 to 220 keV. Metallic foils such as Au and Ag show less polarization than carbon, while non-conducting SiO₂ foils produce increased linear polarization. Time variations of the polarization show that the build-up of impurities on the final surface of the foils yields carbon-foil like polarizations. At higher beam energies of 0.6 to 1.0 MeV, all foils show increased polarization for neon transitions, but decreased polarization for He II transitions with respect to the carbon foil values.