Abstract

The technique of time-of-flight scattering and recoiling spectrometry (TOF-SARS) with detection of both neutrals and ions is applied to structural analysis of oxygen adsorbed on a W(211) surface. The site position for oxygen in the high-dose [\ensuremath{\Theta}=1.5 monolayers (ML), saturation coverage] p(1\ifmmode\times\else\texttimes\fi{}2) low energy electron diffraction (LEED) pattern is determined; a preliminary study of oxygen in the low-dose (\ensuremath{\Theta}=0.5 ML) p(2\ifmmode\times\else\texttimes\fi{}1) LEED pattern is also presented. Both Ar and Ne backscattering (BS) and oxygen recoiling spectra, induced by pulsed 4-keV ${\mathrm{Ar}}^{+}$ and 5-keV ${\mathrm{Ne}}^{+}$ primary-ion beams, are monitored as a function of polar beam incident angle \ensuremath{\alpha}, surface azimuthal angle \ensuremath{\delta}, scattering angle \ensuremath{\theta}, and recoiling angle \ensuremath{\varphi}. Plots of BS (or recoil) intensities in (\ensuremath{\alpha},\ensuremath{\delta}) space provide scattering (or recoiling) structural contour maps and three-dimensional scattering (or recoiling) structural plots which are representative of the adsorption sites of oxygen on the W(211) surface; the symmetry of the adsorption sites is determined from these plots.Measurements of BS and recoil intensities as a function of \ensuremath{\alpha} along different azimuths \ensuremath{\delta} provide experimental values of the critical incident angles ${\ensuremath{\alpha}}_{c}$,sh${}^{\mathrm{i}}$ for shadowing and the critical ejection angles ${\ensuremath{\beta}}_{c}$ (or ${\ensuremath{\alpha}}_{c}$,bl) for blocking by neighboring atoms. Trajectory simulations and calculations of the shadowing and blocking cones obtained from potentials calibrated for this system are used to determine the oxygen-adsorption-site coordinates. The results show that oxygen is dissociatively adsorbed within the troughs along the [1 1\ifmmode\bar\else\textasciimacron\fi{} 1\ifmmode\bar\else\textasciimacron\fi{}] direction. For the saturation p(1\ifmmode\times\else\texttimes\fi{}2) structure, the oxygen is in threefold sites formed by two first- and one second-layer W atoms; these are the only sites which are consistent with all of the experimental data. The coordinates of these sites relative to the W coordinates are determined to an accuracy of \ifmmode\pm\else\textpm\fi{}0.1 A\r{}. The oxygen-tungsten chemisorption bond length is determined as 1.83 A\r{} to the two first-layer W atoms and 2.17 \AA{} to the second-layer W atom; this reflects the unsaturated and fully saturated valencies of the first- and second-layer W atoms, respectively. These trough sites are occupied for coverages up to more than one monolayer of oxygen. For the low-dose p(2\ifmmode\times\else\texttimes\fi{}1) structure, the data indicate occupancy of the same threefold sites, but with different coordinates. The data are consistent with shifting of the relaxed clean W surface structure to the bulk truncated structure upon ${\mathrm{O}}_{2}$ chemisorption. The sensitivity of TOF-SARS to details of adsorbate structure, the use of both BS and recoil data in a complementary manner, and the ability to extract adsorbate structural information with only simple calculations are presented.

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