Adsorbate core-level azimuthal photoelectron diffraction at intermediate energies of 230-900 eV: Grazing emission with polarization dependence

Synchrotron radiation in the energy range 2700-3400 eV is used to study adsorbate core-level azimuthal x-ray photoelectron diffraction (XPD) in the as-yet-unexplored kinetic-energy range from 230 to 900 eV. The well-defined $c(2\mathrm{}\times 2)\mathrm{S}$ overlayer on Ni(001) is studied at 10° grazing electron emission and with two radiation orientations: $s$ polarization and a specially selected $p$ polarization maximally emphasizing substrate Ni scattering relative to the primary wave. Pronounced XPD effects of 27-47% are observed in both cases. The $s$-polarization results are well described by a simple single-scattering model over the full energy range, although sensitivity to adsorbate vertical position is predicted to be low unless the adsorbate is within ≲1.0 Å of the Ni surface. Corresponding $p$-polarization results at 230 eV are markedly different in features and exhibit higher anisotropies. In order to be semiquantitatively described by theory, the $p$-polarization data require a reduction in scattering amplitudes, possibly due to inelastic effects, and the inclusion of double-scattering events. In $p$ polarization, a higher sensitivity to adsorbate vertical positions is found. Both sets of data are consistent with the known structure of this overlayer.