Molecular effects in beam-foil collision-induced alignment of He I

We have measured the alignment of beam-foil collision-excited states of He I produced by bombarding carbon foils of various (1.3-110 μg/${\mathrm{cm}}^2$) areal densities with beams of ${\mathrm{He}}^{+}$ and He${\mathrm{H}}^{+}$. In addition, we have measured the total light yield of several transitions in He I, He II, and H as a function of foil thickness using beams of He${\mathrm{H}}^{+}$ ions. Experiments were done with He-foil exit energies of 125, 500, 550, and 650 keV. He I alignment decreases in all cases for the thinnest foils when molecular projectiles are used. Total light intensities generally increase with thin foils (small proton-He emergent internuclear separation), but a few decrease or are independent of foil thickness. We are able to explain several features of the alignment and intensity data in terms of the formation of quasimolecular He${\mathrm{H}}^{+}$ states at or near the foil surface. Alignment reduction results from incoherent Stark mixing of the He I excited states in the field of the close proton. A calculation of the rms emergent H-He internuclear separation as a function of foil thickness and beam energy is presented.