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 $2<E<\mathrm{}9\mathrm{}$ eV, polar angles $15°<\theta <45\mathrm{}°$, (01) azimuth]. The intensity structure $I\left(E\right)\mathrm{}$ on the low-energy side of the threshold is found to depend on the spin polarization of the incident electrons. For $\theta >25\mathrm{}°$, corresponding peaks in $I^{\uparrow }\mathrm{}\left(E\right)\mathrm{}$ and $I^{\downarrow }\mathrm{}\left(E\right)\mathrm{}$ are split in proportion to their width [symbols ↑ (↓) designate spin up (down) with respect to the scattering plane]. For $\theta <25\mathrm{}°$ the splitting-to-width ratio increases, and a shoulder grows up on the low-energy side of the lowest-energy peak of $I^{\uparrow }\mathrm{}\left(E\right)\mathrm{}$. 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 $\theta >25\mathrm{}°$ the differences between $I^{\uparrow }\mathrm{}\left(E\right)\mathrm{}$ and $I^{\downarrow }\mathrm{}\left(E\right)\mathrm{}$ 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 $\theta <25\mathrm{}°$ there are additional important contributions from higher-order indirect terms. These terms add coherently to produce a resonance perturbation of the line shape of $I^{\uparrow }\mathrm{}\left(E\right)\mathrm{}$. 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.