G.J. Jackson

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.

Characterization of thiolate species formation on Cu(111) using soft x-ray photoelectron spectroscopy

Soft x-ray photoelectron spectroscopy of the S 2p levels at relatively high spectral resolution has been used to characterize the interaction of methanethiol, , ethanethiol, and dimethyl disulphide, with a Cu(111) surface at temperatures from approximately 130 K to 500 K. The results are consistent with previous reports of the formation of a surface thiolate species at intermediate temperatures, but also provide clear evidence for two distinct surface intermediates in addition to the intact molecules and chemisorbed atomic sulphur reported previously for this surface. These two intermediates appear to be similar to the two thiolate species reported in studies on Ni(111). Prior structural studies of the surface at room temperature show the surface to be reconstructed, and the lower temperature species identified here is assigned to a thiolate species on an unreconstructed surface, reconstruction being hindered at low temperatures. Additional evidence is found for two different atomic sulphur states in a narrow temperature range.

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.

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.

X-ray standing waves at surfaces

Adatoms immersed in an x-ray standing wave at a surface betray their position within the wave by the way they absorb the x-rays; feebly when positioned at the nodes, strongly when positioned at the antinodes. The elemental (and chemical) identity of the adatoms are easily monitored using the binding energies of the photoelectron or Auger electron emissions, while the intensities of these emissions provide the information needed to determine the atomic positions relative to the crystalline substrate which formed the standing wave. By using normal incidence Bragg diffraction to generate the standing wave, the technique is applicable to the rather imperfect crystalline samples and standard manipulators used in most surface science studies. Examples of structural studies from a range of systems will be drawn from recent work carried out at the SRS in Daresbury to illustrate the strengths, and weaknesses, of this structural technique. Specifically, the structure of reactive intermediates (SiHx) formed by chemical reaction of silane on Cu(111); the structure of a physisorbed molecule (ClCH2CH2F) on Cu(111); an example of how chemically shifted Auger peaks may be useful for chemical shift XSW (chloroform on a chlorinated copper surface), and a system which presents many difficulties when studied by this technique, methyl thiolate on Au(111).

Initial stages of oxidation of Mg(0001) and the role of co-adsorbed alkali metals

Abstract Soft X-ray Mg 2p photoemission using synchrotron radiation at moderate spectral resolution has been used to investigate the early stages of oxidation of Mg(0001) with and without preadsorbed Na or K. On the alkali-free surface, some evidence is found for a state characterized by a chemical shift intermediate between that of the metallic and oxidic states, but the limited resolution and overlapping spin-orbit split surface and bulk metallic components precludes more thorough characterization. Preadsorbed Na or K enhance the initial rate of oxidation by approximately a factor of two. It is suggested that the mechanism is an enhancement of the rate of oxygen dissociation.

The structure of PF3 adsorbed on Cu(111)

The structure of PF3 adsorbed on Cu(111) at 110 K has been determined using both near edge X-ray absorption fine structure (NEXAFS) and normal incidence X-ray standing wave (NIXSW) methods. Two X-ray reflection conditions were used, the (111) and (111), to determine the atomic positions of the phosphorus and fluorine atoms by triangulation using NIXSW, whereas NEXAFS was used to determine the alignment of the molecule. PF3 adsorbs at atop sites with the fluorine atoms away from the surface and its C3 axis aligned along the surface normal. The Cu–P distance is 2.25±0.04 A, and the distance between the copper and the fluorine layers along the C3 axis is 3.05±0.04 A. The adsorbed molecule maintains its gas-phase geometry and undergoes either free rotation about the C3 axis, or is azimuthally orientated such that the projections of the P–F bonds on to the surface point towards the next nearest neighbour copper atoms. The NIXSW analysis for the fluorine photoemission data was carried out using a backward/forward asymmetry parameter to compensate for a breakdown in the dipole approximation for photoemission that occurs for fluorine 1s photoemission under the NIXSW conditions used here.

Structural study of Rb and Cl coadsorption on Cu(111) : a case of overlayer compound formation

A structural study of the adsorption of Rb on a - Cl surface has been conducted using normal-incidence x-ray standing-wavefield absorption at both the Cl and Rb surface atoms. The results indicate reaction of the coadsorbates to form an overlayer of approximately 1.2 ML of RbCl(100) which is essentially incommensurate with the substrate. The RbCl - Cu outermost layer spacing is similar to that seen for pure Rb adsorption on Cu(111). This RbCl layer is spread over the surface and not agglomerated into thicker islands. The results are consistent with prior characterization of the system by Auger electron spectroscopy and temperature-programmed desorption, and with STM studies of NaCl deposition on Al(111).