| Abstract: |
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{uarr} (E) and I{darr} (E) are split in proportion to their width [symbols [{uarr} ({darr}) 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{uarr}(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{uarr}(E) and I{darr}(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{uarr} (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. |
