M.D. Crapper

Determination of the adsorption structure for formate on Cu(110) using SEXAFS and NEXAFS

Abstract Near-edge X-ray absorption fine structure (NEXAFS) is used to establish the CO bond length (1.25±0.05 A ) , the OCO angle (124±15°) and the azimuthal orientation (along [1 1 0]) of the molecular plane of the formate catalytic intermediate adsorbed on Cu(110). Extended X-ray absorption fine structure (EXAFS) is then used to determine the CuO bond length (1.98±0.07 A ) and the adsorption site (atop copper atoms in the top (ridge) rows, with oxygen atoms near short bridge sites). The problems of obtaining reliable quantitative results from NEXAFS are discussed. The adsorption site and bond length, which agrees well with that for anhydrous copper formate, is contrasted with the much longer bond length found for this species adsorbed on Cu(100) by Stohr et al. [Phys. Rev. Letters 54 (1985) 1256].

A simple X-ray standing wave technique for surface structure determination - theory and an application

Abstract A modification of the usual X-ray standing wave technique for adsorbate structure determination is described which is applicable not only to highly perfect semiconductor surfaces but also to normal metal crystals having substantial mosaicity. The method, which relies on normal incidence Bragg reflection, also places weak demands on the energy resolution and degree of collimation of the X-ray beam and is appropriate for existing synchroton radiation beam lines designed for surface EXAFS and photoelectron diffraction. Experimental data show the Auger detection mode is to be preferred to photoelectron or total yield, and an analysis of the Cu(111)(√3 × √3) R30°-Cl structure yields a Cl to bulk Cu layer spacing of 1.81 ± 0.05 A, consistent with a previous SEXAFS and photoelectron diffraction study of this surface.

Investigation of the Cu(111) (√3 × √3)R30°-Cl structure using sexafs and photoelectron diffraction

Abstract SEXAFS measurements above the Cl K-edge reveal that the Cu-Cl distance is the Cu(111) (√3 × √3)R30°-Cl structure is 2.39 ± 0.02 A, with adsorption in three-fold symmetric hollow sites. These measurements are not, however, able to distinguish the two inequivalent hollow sites on this surface, even with the aid of multi-shell simulations. Photoelectron diffraction data for the Cl1s state, taken at kinetic energies between 100 and 400 eV are interpreted with the aid of single scattering calculations and shown to resolve this question, demonstrating that the Cl atoms adopt the same sites as would be occupied by the next fcc metal atom layer.

The structure of formate on Cu(100) and Cu(110) surfaces

A reanalysis of SEXAFS data for formate adsorbed on Cu(110) and Cu(100) using a multishell simulation procedure shows that substantial interference of similar but different Cu−O bond lengths can occur due to the low adsorption site symmetry of the oxygen atoms caused by the mismatch of the O−O and Cu−Cu distances. The results show that the large differences between the apparent adsorption sites and bond lengths previously reported are incorrect and that Cu−O nearest neighbour distances of 1.94±0.10 A and 1.99±0.10 A are found on Cu(110) and (100), respectively, similar to that found in many other Cu−O compounds including copper formate. Moreover, the best fit sites for both surfaces are with formate atop a copper atom, although the orientation differs for the two surfaces. The role of intramolecular (O−O) scattering in the EXAFS is also shown to be important. The results highlight the need for extreme caution in the analysis of SEXAFS data, particularly from molecular adsorbates using single shell Fourier transform methods.

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.

Evidence of roughness distributions and interface smoothing in Co/Cu multilayers deposited under energetic particle bombardment

The interfacial structure of Co/Cu multilayers deposited under energetic particle bombardment is investigated using x-ray reflectivity. The energetic bombardment is varied by controlling the ion bombardment of the growing film. Specially modified unbalanced magnetron sources are used in collaboration with bias sputtering techniques in order to independently vary both the ion flux and average ion energy of the bombardment. Quantitative analysis of the specular reflected intensity reveals the existence of variations of the interface roughness in multilayers deposited with high applied negative biases of 200 V, such that the interfaces become smoother towards the free surface. A maximum smoothing effect is observed for films deposited with the highest available levels of ion flux. A static interface roughness is observed when applied biases of are applied.

Complete Adsorption Site Information for Cl on Cu(111) Using X-Ray Absorption Fine Structure and Photoelectron Diffraction

Surface extended X-ray absorption fine-structure (SEXAFS) studies of the Cu(111) (?3 ? ?3)R 30?-Cl phase yield a Cu-Cl nearest-neighbour distance of (2.39???0.02) ? and show the Cl atoms to be chemisorbed in threefold hollows. Very simple analysis of photoelectron diffraction data from the same surface distinguishes the two inequivalent threefold hollows (not distinguished in this or previous SEXAFS studies) by establishing the direction of the nearest neighbours. The adsorption site is found to be that which is threefold co-ordinated with respect to both top and second-substrate atom layers.

A mercaptide intermediate on Cu(111)

Angle resolved ultraviolet photoemission (ARUPS) and sulphur K-edge near edge X-ray absorption fine structure (NEXAFS) have been used to study the interaction of dimethyl disulphide ((CH 3 S) 2 ) and methyl mercaptan (CH 3 SH) on Cu(111) Both species are adsorbed molecularly at low temperature while on warming to, or exposing at, room temperature, a single common species is found which we identify as methyl mercaptide, CH 3 S-. ARUPS from this species shows only three peaks in the HeI spectra which were ascribe to an increase in the symmetry to C 3v . Assinging the first stron gpeak in the NEXAFS spectrum to a σ CS shape resonance, we find that the polarisation dependence of this peak indicates a tilt in the C-S bond away from the surface normal of approximately 30°. This degree of tilt is apparently insufficient to introduce significant loss of energy degeneracy in the valence orbitals probed by ARUPS.

A surface EXAFS study of a surface iodide phase on Ni{100}

Surface EXAFS data from a surface iodide phase formed on Ni{100} are compared with data from bulk NiI2. Two independent analyses are presented. In the first, the nearest neighbour Ni-I bond length change is extracted by Fourier filtering using methods previously used for SEXAFS analysis. The second method uses a comparison of calculated and experimental EXAFS based on calculated scattering phase shifts. Both analyses show the surface phase to involve a contraction of the NiI bond length by 0.04 ± 0.02 A, while the second analysis also shows a similar contraction of the II bond length.

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.