B.C.C. Cowie

A structural study of the interaction of SO2 with Cu(111)

Abstract The technique of chemical-shift normal-incidence X-ray standing waves (CS-NIXSW) has been applied to a study of the interaction of SO 2 with Cu(111), yielding quantitative information on the local adsorption geometry of adsorbed SO 2 at low temperature and coadsorbed atomic sulfur with an SO x species, identified on the basis of near-edge X-ray absorption fine structure (NEXAFS) as SO 3 . Atomic sulfur appears to occupy a mixture of face-centred cubic and hexagonal close-packed hollow sites, while the SO 3 species adsorbs with its C 3v axis perpendicular to the surface atop a surface copper atom with the SO bonds out of plane such that the oxygen atoms are closer to the surface; there appears to be some local distortion of the outermost copper layers around this species. While SO 2 is found to adsorb with its molecular plane essentially perpendicular to the surface, and the data are most readily interpreted in terms of a bridging geometry bonding through the oxygen atoms, there are marked inconsistencies between these results and those of an earlier surface extended XAFS (SEXAFS) investigation of this species on Cu(111), and alternative interpretations are discussed.

Surface adsorption structures in 1-octanethiol self-assembled on Cu(111)

A normal incidence X-ray standing wave (NIXSW) and near-edge X-ray absorption fine structure (NEXAFS) study was performed on saturated monolayers of 1-octanethiol on Cu(111). NEXAFS around the S K-edge taken in normal and grazing incidence revealed that the S-C bond was oriented with respect to the Cu(111) surface to give a calculated orientation of the pendant hydrocarbon which was almost normal to the surface, similar to self-assembled alkanethiols on gold surfaces. Modulation in the S(1s) photoemission with variation in the energy of the X-ray standing wave about the (111) and (1¯11) Bragg energies was modelled to yield values of structural parameters: the coherent position, dhkl, and coherent fraction, fhkl, for the adsorbate with respect to the bulk Cu lattice planes. For the (111) reflection, d111 = 1.19 ± 0.10A˚, f111 = 0.70 ± 0.03, but for the (1¯11) reflection, f1¯11 was close to zero (f111 = 0.10 ± 0.05) and d1¯11 was indeterminate. The coherent fraction for the Cu substrate was high for both reflections studied (f111 = 0.95 ± 0.03;1¯11 = 0.92 ± 0.02). The structural parameters are rationalised in terms of penetration of the Cu by thiolate sulfur atoms to cause a surface layer reconstruction. The resultant adsorbate structure is either incommensurate or else consists of a large-mesh commensurate superlattice in which the S atoms exist at a number of different positions with respect to the (1¯11) planes of the bulk.

Structural determination of the Cu(111) -(√3 × √3) R30°-ClBr surface using the normal incidence X-ray standing wave method

Abstract A full structural determination has been made of the Cu (111)(√3 × √3)- R 30°- Cl Br surface, which comprises a mixed Cl Br overlayer with a nominal coverage of one third of a monolayer and a 1:1 stoichiometry, using the normal incidence X-ray standing wave (NIXSW) method. Two Bragg reflections were used, the (111) which uses reflecting planes parallel to, and the (111) which are tilted at an angle of 70.5° to, the (111) surface. Using the (111) data, adsorbate-substrate outermost layer spacings were found to be 1.81 ± 0.05 A for chlorine and 2.00 ± 0.05 A for bromine assuming zero substrate relaxation. These results, coupled with the (111) data, are consistent with a model in which both halogens, each of which has a coverage of 1 6 monolayer, occupy three-fold coordinated hollow sites on the surface. The adsorbates occupy primarily fcc hollows (directly above third-layer Cu atoms) but some occupation of hcp hollows (above second-layer Cu atoms) is also observed. For chlorine the fcc:hcp population ratio is 75%:25% (± 10%), while for bromine the rato is 80%:20% (± 10%). Possible origins of the minor species hcp hollow site occupation are discussed.

The structure of sodium adsorption phases on Al(111)

Abstract The structures formed by the adsorption of Na and Al(111) have been studied by normal incidence standing X-ray wavefield absorption using both the (111) and (111) Bragg reflections in order to obtain not only the Na-Al layer spacings, but also the adsorption sites. In the case of the ordered (√3 × √3)R30°-Na phase, at a nominal coverage of 0.33 ML, and at lower coverages down to approximately 0.12 ML, Na is found to adsorb in a coverage independent site which involves a substitution of some of the top layer Al atoms; if the substrate layer spacings remain unchanged by this reconstruction, the effective radius of the adsorbed Na species is found to be 1.67 A. The higher coverage (2 × 2)-Na phase is found to involve (at least) two distinct Na adsorption sites, but each of these has a very similar layer spacing relative to the extended (111) substrate scatterer lattice planes. A specific model of this phase, involving two reconstructed layers each of stoichiometry NaAl 2 is proposed, which accounts in a quantitative way for the present X-ray standing wave data and previous X-ray absorption data, and in a qualitative way for published high resolution soft X-ray photoelectron spectroscopy results. As Al and Na are essentially immiscible in the bulk, this two-layer structure appears to be an entirely new kind of surface phase.

Non-dipole photoemission effects in x-ray standing wavefield determination of surface structure

Non-dipolar effects in the angular distribution of core level photoemission are shown to have a substantial influence on the interpretation of x-ray standing wavefield determinations of surface adsorption structures when the x-ray absorption is monitored by photoemission, even at photon energies below 3 keV. Results for I adsorption on Cu(111) are shown to be compatible with theoretical calculations for atomic Xe.

A structural study of methanethiolate adsorbed on Cu(100)

The interaction of methanethiol, CH3 SH, with Cu(100) has been studied by S 1s photoemission, S K-edge near-edge x-ray absorption fine structure and normal-incidence standing x-ray wavefield absorption at both (200) and (111) reflections. The results indicate that a single methanethiolate species, CH3 S-, is formed and this is bonded to the unreconstructed surface via the S atoms which adopt the fourfold symmetric hollow site. This conclusion confirms the results of a recent near-edge and surface extended x-ray absorption fine structure study, but contrasts with the established adsorbate-induced reconstruction produced by this species on Cu(111).

A NIXSW and NEXAFS investigation of thiophene on Cu(111)

The local registry and geometry of thiophene at sub-monolayer coverages on Cu(111) has been probed with normal incidence X-ray standing wavefield absorption (NIXSW) and near edge X-ray absorption fine structure (NEXAFS). Thiophene was found to adopt a flat geometry on the surface with the sulphur atom in an atop position. The relatively short data collection times required for the NIXSW technique were found to be beneficial when looking at thiophene overlayers which are sensitive to X-ray induced damage.

The structure of sulphur adsorption phases on Ni(111) studied by X-ray standing wavefield absorption

Abstract The technique of normal-incidence X-ray standing wavefield (NIXSW) absorption has been applied to an investigation of several structural phases of S on Ni(111), specifically the (2 × 2), (√3 × √3)R30° and (5√3 × 2)rect. phases, with a view in particular to clarifying recent controversy over the structure of the last of these. Absolute adsorbate site determination has been effected through real-space triangulation using both (111) and (111) Bragg reflections. For the (2 × 2) phase the results are consistent with a simple overlayer structure with S atoms occupying the “fcc” hollow sites (directly above third-layer Ni atoms), in agreement with earlier determinations by LEED, ion scattering and SEXAFS. The same local site is found to be occupied in the (√3 × √3)R30° phase, although the NISXW data indicate poorer local order. For the (5√3 × 2)rect. phase, a simple undistorted missing-row model is found to be inconsistent with the data, as is any ideal “coincidence lattice” structure, including a simple undistorted pseudo-(100) surface reconstruction. A modification of this latter model, in which some S atoms penetrate the pseudo-(100) layer to bond to the underlying (111) substrate does, however, contain the main ingredients needed to fit the data. By using not only (111) and (111) NIXSW but also (111) and (111) absorption profiles, recorded from a near single-domain structure formed on a miscut crystal surface, specific information on some of the structural parameters of this modified reconstruction model is obtained and compared with complementary data from other published studies.

The structure of the surface phase: a new normal-incidence X-ray standing wave study

Abstract The local geometry of the adsorbed sulphur atoms in the Cu(111)( 7 × 7 )R19°-S surface phase has been investigated with normal-incidence X-ray standing wavefield (NIXSW) absorption using the (111), (111) and (200) Bragg reflections. The results are compared with the predictions for previously proposed models based on earlier investigations using NIXSW [but only the (111) reflection], surface extended X-ray absorption fine structure, quantitative low energy electron diffraction, surface X-ray diffraction (SXRD) and scanning tunneling microscopy. The best agreement is found for the model deduced from SXRD based on Cu 4 S adclusters, but for the specific overlayer substrate registry in which these clusters are centred atop an outermost substrate layer Cu atom. All of the other models show significant inconsistencies with these new data.

The local adsorption structure of SO2 on Ni(111) : a normal incidence X-ray standing wavefield determination

Abstract A normal incidence X-ray standing wavefield study of the structure of molecular SO 2 on Ni(111) has been conducted, using photoabsorption at both the O and S atoms and real-space site triangulation using {111} scatterer planes both parallel to, and at 70° to, the surface plane. Both O and S atoms are found to be in the vicinity of atop sites, although the S atoms are displaced significantly further from these high symmetry sites. S K-edge NEXAFS confirms an earlier finding that the molecule lies with its molecular plane parallel to the surface. The detailed sites can only be reconciled with a model in which the SO 2 molecules are centred close to hollow sites (with equal occupation of both types of hollow) and the internal conformation of the molecule, especially the OSO bond angle, is significantly different from that of the gas-phase molecule. Specifically, the OSO bond angle is estimated to be no more than 100°, while the data indicate an SO bond-length expansion of 5% or more. This change is attributed to the unusual π-bonding (for which there appears to be no analogue in coordination compounds) and thus partial occupation of the 3b 1 π∗ LUMO of the molecule.