Target element dependent spin-orbit coupling in polarized 4He+ ion scattering

We studied low-energy (1.57 keV) electron-spin polarized 4He+ ion scattering on various 5d transition metal targets. The scattered ion intensity generally differed between the incident He+ ions with up and down spins. This spin dependent ion scattering is attributed to the spin-orbit coupling (SOC) that acts transiently on the He+ 1s electron spin in the He+-target binary collision. We observed that the amplitude of the spin dependence in ion scattering, i.e., the spin asymmetry, differs between 5d transition metal targets. This target element dependence of the spin asymmetry is discussed in terms of re-ionization of He0, which originates from the neutralization of the He+ ion during the He+-target collision. Since the re-ionization is spin independent process, it degrades the effective spin polarization of the He+ ion beam. This explains smaller spin asymmetry with the target on which He0 is re-ionized with higher rate.