SEXAFS Data Research Articles

The structure of sulfur adlayers at Cu(110) surfaces: an STM and XPS study

A detailed understanding of the sulfur phases formed by the dissociation of hydrogen sulfide and methyl mercaptan at Cu(110) surfaces has been obtained in a combined STM/X-ray photoelectron spectroscopy study. At low concentrations adsorbed sulfur atoms are mobile at room temperature, forming the ordered c(2×2) structures only when the concentration of sulfur reaches approximately 3.8×10 14 cm −2. Surface oxygen inhibits sulfur diffusion and ordered c(2×2) sulfur structures are formed at much lower sulfur concentrations in its presence, the sulfur also influences the oxygen structure inhibiting the formation of large oxygen islands. Sulfur concentration ( σ s) limits for the c(2×2) ( σ s<4.4×10 14 cm −2), p(5×2) (4.4×10 14 cm −2< σ s<6.6 ×10 14 cm −2) and the p(3×2) ( σ s=7.1×10 14 cm −2) sulfur phases have been established. Models, involving surface buckling, have been suggested for these structures which reconcile the previous LEED, STM and SEXAFS data. The adsorption of methyl mercaptan at 295 K results in mercaptide formation but no ordered structures; a herringbone type structure is, however, observed when methyl mercaptan is adsorbed at 70 K and then warmed to room temperature. On heating to 450 K mercaptide dissociates with the complete desorption of carbon as hydrocarbons, providing an alternative route to high chemisorbed sulfur concentration structures. On the basis of XP spectra of methyl mercaptan and a correlation between the XP data and the STM images, the total photoionisation cross-section of the S 2p orbital reported in the literature has been shown to be in error and a new value (2.10, relative to the C 1s cross-section) derived.

New Opportunities in the Soft X-Ray Absorption to Characterize the Adsorbate Bonding

One of the important applications of XAS in the soft X-ray regime is the ability to measure the EXAFS of light elements, like carbon, nitrogen, etc. up to sulfur and chlorine. These elements, adsorbed on metal surfaces, are the most important constituents for the investigation of gas-metal interactions. One decade ago, the surface EXAFS world was harmonic, the nearest neighbor distances were determined within the harmonic approximation, R did not change with temperature, and the coordination number N was determined from measurements at a single temperature. Recently, SEXAFS data were taken as a function of T and at various angles of X-ray incidence. This advance, in combination with multiple scattering analysis and full simulation computer codes, allows to routinely determine the pair distribution function up to the third moment. We relate in general terms the Einstein temperature, the local static disorder and thermal expansion to the ionic versus covalent bonding, shape of effective pair potentials and adsorbate induced surface reconstruction. Furthermore, these low Z systems are ideal candidates to detect unambiguously the photoelectron backscattering by interstitial charge densities (AXAFS).

The structure of sodium adsorbed c(2 × 8)Ge(111) surface

We performed SEXAFS and core level photoemission studies of Na adsorbed on c(2 × 8)-Ge(111) surface at different coverage regimes. The c(2 × 8) LEED pattern of the clean Ge(111) surface changes to a (1 × 1) with higher background after Na adsorption. A plasmon feature appears on the low energy side of Na 2p core level which indicates metallization of the Na overlayer. We determined the NaGe bond length from the SEXAFS data using the theoretical phase shift of NaGe, calculated by FEFF program. SEXAFS model calculations show that Na atoms are adsorbed on threefold-coordinated hollow sites with a NaGe bond length of 2.9 Å and a second nearest neighbour distance of 3.4 Å to the Ge atoms in the second layer.

Poisons and promoters on TiO2 surfaces

Abstract Synchrotron radiation measurements have investigated the interaction of SO 2 and K with rutile TiO 2 surfaces. NEXAFS spectra from TiO 2 (110)SO 2 indicate that SO 2 bonds to Ti at 105 K, converting to a sulphate-like species at higher temperatures via a sulphite-like intermediate. SEXAFS data from TiO 2 (100)c(2×2) K and the corresponding resonance photoemission spectra show that K occupies a bridge site, donating a quarter of an electron to Ti via O atoms.

A SEXAFS study of the p(2 × 2)-O/Ni(111) system

Oxygen K-edge SEXAFS studies on the p(2 × 2)-O/Ni(111) system show that the O atoms are located in threefold coordinated fcc sites with a nearest-neighbour O-Ni bond length of 1.85 ± 0.03 A ̊ . The SEXAFS data are compatible with outward shifts of the three nearest-neighbour Ni atoms by up to 0.2 Å. It is shown that a possible outward shift of the three nearest-neighbour Ni atoms must be coupled with an inward shift of the next nearest-neighbour Ni atoms by roughly the same amount.

SEELFS study of Ni(001)(2 × 2)C p4g structure

The energy loss spectrum above the carbon K edge for the Ni(001)(2 × 2)C p4g structure is reported. SEELFS results are compared with recent SEXAFS measurements for the same system. The radial distribution function obtained by SEELFS shows a good agreement with SEXAFS data.

Face and coverage-dependent sulphur coordination on the (110) and (111) faces of Ni using polarization-dependent SEXAFS and NEXAFS

We have studied the polarization-dependent S K-edge SEXAFS and NEXAFS of Ni(110)c(2 × 2)S and the SEXAFS spectra of the p(2 × 2)S and p(5 × 5)S overlayers on Ni(111). For Ni(110)c(2 × 2)S, we find good agreement with the LEED and ion scattering results, assigning the sulphur to the two-fold hollow site and observing a first-nickel-layer relaxation of ca 12% from the bulk value. We also observe an unusually strong azimuthal dependence of the near edge structure. Using a multiple scattering model, we find that this is due to long-range scattering and not scattering solely from the five nickel atom near-neighbours, in contrast to the NEXAFS observed for molecular adsorbates, which is dominated by the local structure. The SEXAFS data from the Ni(111) S overlayers indicate bonding to the three-fold hollow sittes in the p(2 × 2) phase, with Ni atoms occupying the surface layer in the p(5 × 5) phase. For the three overlayer phases, the S-Ni nearest-neighbour bond distance increases with coordination number.

Some applications of statistical tests in analysis of EXAFS and SEXAFS data

A simple formula is developed, based on standard statistical methods, that can be used to examine the physical significance of adding a new shell of neighbors to an EXAFS analysis. This can readily be applied in conjunction with the widely used data analysis program EXCURVE. The formula is appropriate for fitting primary data only, but a variation is offered for use in fitting data that have been forward and reverse Fourier transformed. The problem of error estimation where correlated parameters are involved is also considered.

The interaction between noble gases and the basal plane surface of graphite

Calculations are presented of the laterally averaged interaction between a noble gas atom and the basal plane surface of graphite. The proposed model is seiempirical, based in part on a previous successful treatment of the He/graphite case. The calculated potentials have minima which are 5–10% closer to the surface than earlier values. The results are consistent with LEED and SEXAFS data.