Structural analysis of Pt(1 1 1)c(√3 × 5)rect.–CO using photoelectron diffraction

Core level shift scanned-energy mode photoelectron diffraction using the two distinct components of the C 1s emission has been used to determine the structure of the Pt(1 1 1)c(√3 × 5)rect.–CO phase formed by 0.6 ML of adsorbed CO. The results confirm earlier assignments of these components to CO in atop and bridging sites, further confirm that the best structural model involves a 2:1 occupation ratio of these two sites, and provides quantitative structural parameter values. In particular the Pt–C chemisorption bondlengths for the atop and bridging sites are, respectively, 1.86 ± 0.02 Å and 2.02 ± 0.04 Å. These values are closely similar to those found in the 0.5 ML coverage c(4 × 2) phase, involving an atop:bridge occupation ratio of 1:1, obtained in earlier quantitative low energy electron diffraction studies. The results also indicate a clear tilt of the molecular axis of atop CO species in this compression phase, consistent with the finding of an earlier electron-stimulated desorption ion angular distribution investigatio

The local structure of OH species on the V2O3(0 0 0 1) surface: a scanned-energy mode photoelectron diffraction study

Scanned-energy mode photoelectron diffraction (PhD), using O 1s photoemission, together with multiple-scattering simulations, have been used to investigate the structure of the hydroxyl species, OH, adsorbed on a V2O3(0 0 0 1) surface. Surface OH species were obtained by two alternative methods; reaction with molecular water and exposure to atomic H resulted in closely similar PhD spectra. Both qualitative assessment and the results of multiple-scattering calculations are consistent with a model in which only the O atoms of outermost layer of the oxide surface are hydroxylated. These results specifically exclude significant coverage of OH species atop the outermost V atoms, i.e. in vanadyl O atom sites. Ab initio density-functional theory cluster calculations provide partial rationalisation of this result, which is discussed the context of the general understanding of this system

Local structure determination of a chiral adsorbate: Alanine on Cu(110)

N 1s and O 1s scanned-energy mode photoelectron diffraction (PhD) has been used to investigate the local structure of a single enantiomer of deprotonated alanine, alaninate, NH2CH3CHCOO–, on Cu(1 1 0) in the (3 × 2) phase. The local site is found to be similar to that of glycinate on Cu(1 1 0), with the N atoms in near-atop sites and the O atoms sites consistent with bonding to single surface Cu atoms but substantially off-atop. Unlike the Cu(1 1 0)(3 × 2)pg-glycinate phase, however, in which the two molecular species per unit mesh are mirror images of one another in identical local sites, the intrinsic chirality of l-alaninate means that the two molecules per unit mesh of the (3 × 2) surface phase occupy slightly different local sites. However, an excellent fit to the PhD data can be achieved by a minor modification of the structure found in DFT calculations by R.B. Rankin and D.S. Sholl [Surf. Sci. 574 (2005) L1] in which the heights of the N and O atoms above the surface are reduced by approximately 0.1 Å. The resulting average N–Cu and O–Cu values are 2.02 and 1.98 Å, respectively, with an estimated precision of ±0.03 Å. These bondlengths are shorter than those obtained from DFT by 0.08 and 0.10 Å, respectively

Quantitative structural determination of the high coverage phase of the benzoate species on Cu(110)

The local adsorption site and molecular orientation of the benzoate species adsorbed on Cu(1 1 0) at 330 K have been determined using O1s scanned-energy mode photoelectron diffraction. The molecule was found to interact with the surface via the carboxylate group, giving a Cu–O bond length of 1.91 Å. The molecules occupy the short bridge sites (the O atoms being slightly displaced from atop sites) and are aligned along the [ 1 0] azimuth with the molecular plane oriented perpendicular to the surfac

Methyl on Cu(111)-structural determination including influence of co-adsorbed iodine

The adsorption geometry of the methyl species on Cu(1 1 1) with and without coadsorbed iodine has been determined using scanned energy mode photoelectron diffraction. Under all circumstances the three-fold-coordinated hollow sites are occupied. At saturation coverage of pure methyl species only the ‘fcc' site, directly above a third layer Cu atom is occupied, whereas at half saturation coverage 70% of the methyl species occupy these fcc hollows and 30% occupy the ‘hcp' sites above second layer Cu atoms. Best agreement between theory and experiment corresponded to a methyl group adsorbed with C3v symmetry, but the possibility that the species was tilted on the surface could not be excluded. The height of the C above the surface in a pure methyl layer was 1.66±0.02 Å, but was reduced to 1.62±0.02 Å in the presence of co-adsorbed iodine, suggesting that iodine may increase the strength of adsorption. Iodine was also found to occupy the fcc hollow sites with a Cu–I bondlength of 2.61±0.02 Å

The local adsorption geometry of CO and NH3 on NiO(1 0 0) determined by scanned-energy mode photoelectron diffraction

The local adsorption structures of CO and NH3 on NiO(1 0 0) have been determined by C 1s and N 1s scanned-energy mode photoelectron diffraction. CO adsorbs atop Ni surface atoms through the C atom in an essentially perpendicular geometry (tilt angle 12±12°) with a C–Ni nearest-neighbour distance of 2.07±0.02 Å. NH3 also adsorbs atop Ni surface atoms with a N–Ni distance of 2.06±0.02 Å. These bondlengths are only very slightly longer than the comparable values for adsorption on metallic Ni surfaces. By contrast theoretical values obtained from total energy calculations, which exist for CO adsorption on NiO(1 0 0) (2.46 Å and 2.86 Å) are very much longer than the experimental value. Similar discrepancies exist for the N–Ni nearest-neighbour bondlength for NO adsorbed on NiO(1 0 0). Combined with the published measurements of the desorption energies, which also exceed the calculated bonding energies, these results indicate a significant failure of current theoretical treatments to provide an effective description of molecular adsorbate bonding on NiO(1 0 0)

Molecular Adsorption Bond Lengths at Metal Oxide Surfaces: Failure of Current Theoretical Methods

New experimental structure determinations for molecular adsorbates on NiO(100) reveal much shorter Ni-C and Ni-N bond lengths for adsorbed CO and NH3 as well as NO (2.07, 1.88, 2.07 Å) than previously computed theoretical values, with discrepancies up to 0.79 Å, highlighting a major weakness of current theoretical descriptions of oxide-molecule bonding. Comparisons with experimentally determined bond lengths of the same species adsorbed atop Ni on metallic Ni(111) show values on the oxide surface that are consistently larger (0.1–0.3 Å) than on the metal, indicating somewhat weaker bonding

Structure determination of methanethiolate on unreconstructed Cu(1 1 1) by scanned-energy mode photoelectron diffraction

The local structure of methanethiolate, CH3S–, on an unreconstructed Cu(1 1 1) surface at low temperature, has been investigated by S 2p and C 1s scanned-energy mode photoelectron diffraction, with chemical state sensitivity. 71(+14/−16)% of the methanethiolate was found to occupy bridge sites, 29±14% to occupy fcc hollow sites and 0+19% to occupy hcp hollow sites. In the bridge site the layer spacing of the sulphur atom to the outermost substrate layer is 1.87±0.03 Å giving a Cu–S bondlength of 2.27±0.03 Å. The methanethiolate adsorbed in the fcc hollow site has a Cu–S layer spacing of 1.73±0.04 Å, corresponding to the same bondlength of 2.27±0.04 Å. The S–C bondlength was found to be 1.92±0.10 Å. These conclusions are consistent with the results of previous X-ray standing wave and scanning tunnelling microscopy studies for a common model involving co-occupation of bridge and hollow sites, although differing relative occupations and long-range ordering are thought to arise from different preparation conditions. The new data favour a model in which the S–C bond axis of the bridge-bound thiolate is tilted by 45±12° away from the surface normal in the azimuth directed towards the fcc hollow site