Polarized-low-energy-electron-diffraction study of the mechanism of electron reflection from W(001) at low energies

Polarized-low-energy-electron-diffraction (PLEED) measurements on W(001) are reported for incidence conditions close to the (01) beam threshold [energies $2<E<9$ eV, polar angles 15^∘<θ<45^∘, (01) azimuth]. The intensity structure $I(E)$ on the low-energy side of the threshold is found to depend on the spin polarization of the incident electrons. For θ>25^∘, corresponding peaks in I^↑(E) and I^↓(E) are split in proportion to their width [symbols {↑} ({↓}) designate spin up (down) with respect to the scattering plane]. For θ<25^∘ the splitting-to-width ratio increases, and a shoulder grows up on the low-energy side of the lowest-energy peak of I^↑(E). The observations are explained by the superposition of reflection-amplitude contributions from "direct" or single scattering at the substrate and "indirect" processes in which the (01) beam is multiply reflected between the substrate and the surface-potential barrier. For θ>25^∘ the differences between I^↑(E) and I^↓(E) derive from the spin dependence of the phase of superposition of direct and indirect amplitude terms. The main effects come from the first indirect term, which corresponds to a single reflection at the surface potential barrier. For θ<25^∘ there are additional important contributions from higher-order indirect terms. These terms add coherently to produce a resonance perturbation of the line shape of I^↑(E). The present results, taken together with earlier LEED results, indicate that the threshold interference mechanism is the dominant mechanism of very low-energy (10 eV) electron reflection at the W(001) surface.