Local adsorption structure determination of chemically-specific species using normal incidence X-ray standing wavefields

The surface structure determination technique Normal Incidence X-ray Standing Wavefield (NIXSW), has been applied in chemical-state sensitive mode (CS-NIXSW), to three adsorption systems. Structure determinations are presented for each system, which involve adsorption of molecules on the low index {111} plane of single crystal nickel and copper substrates. Non-dipole photoemission effects are included within the NIXSW analysis, together with an investigation that successfully measured the non-dipole (forward/backward) correction factor (Q) for O 1s photoemission at a photon energy around 3 keV to be 0.27 ± 0.02. For the PFx/Ni(111) system CS-NIXSW was successful in determining local adsorption geometries of the photon induced PFx fragments for a warm (300 K) surface together with a proposed model for the adsorption geometries of the fragmented species on a cold (140 K) surface. For the warm surface, PF3 and PF or P species were found to occupy atop and fcc hollow sites respectively with P-Ni bond lengths of 2.04 ± 0.05 A (PF) and 2.34 ± 0.05 A (PF/P). For the low temperature surface, three distinct species were identified and were proposed to be PF3 adsorbed in the atop site, PF2 adsorbed in either bridge or mixed fcc and hcp hollows and a P-PFx species (x unknown) adsorbed in the fcc hollows via a P atom with its molecular axis (P-P) tilted around 50 deg to the surface normal. For the CH3SH/Cu(111) surface, S 1s XPS provided evidence of two thiolate species co-adsorbed with the molecular thiol. CS-NIXSW was successful in determining the local adsorption structure for the thiol, whilst proposing the adsorption structure for the two thiolates. At low temperature (140 K) the thiol was found to occupy the atop site with a S-Cu bond length of 2.38 ± 0.05 A, this was co-adsorbed with a low temperature (LT) thiolate species occupying either mixed fcc and hcp hollows or the bridge site. At room temperature (RT) the surface consisted of co-adsorbed S and the RT thiolate, which produced an adsorbate-induced reconstruction with the RT thiolate embedded in the surface via its S atom. Small traces of the RT thiolate were also evident on the low temperature surface, indicating that the LT thiolate is in a transition state. For the SO2 and SOx on Ni and Cu(111), NEXAFS and CS-NIXSW successfully determined the adsorption geometry for SO2 on Ni(111) whilst proposing the local structure for SO2 on Cu(111). S 1s XPS also identified the SOx species to be SO3 for Cu(111) and the more likely species for Ni(111). In the SO2 on Ni(111) NIXSW study, it was found that both S and O atoms in the molecule were bonded to the nickel surface in just-off atop sites with the molecular plane of SO2 being parallel to the underlying surface. In this case the data indicate a 5% expansion in the S-O bond, however this can be reconciled by an unusual π-type bonding of the molecule. For SO2 and SO3 on Cu(111) and SOx on Ni(111) the combined NIXSW and NEXAFS results indicated that the molecules were oriented with the molecular plane of SO2 and the symmetry axis of SO3 perpendicular to the underlying surface and bonded to the surface with their oxygen atoms below the sulphur atom. (author)