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Using first-principles total energy calculations within density functional theory, we investigated adsorption and diffusion of various elements (S, Ge, Pd, Ag and Pt) on the Ru(0 0 0 1) surface. It was found that the hcp hollow site is the preferred adsorption site for all atoms considered (S, Ge, Pd and Pt) except Ag which has equal probability to be adsorbed at the hcp hollow site, fcc hollow site, and the bridge site. Furthermore, S and Ge addlayers become unstable when the coverage goes beyond 0.25 monolayer (ML) due to the repulsive interaction between adsorbates. In contrary, the binding energies of the metal adsorbates Pd, Ag and Pt increase with coverage, all the way up to full coverage (1 ML). The diffusion barriers of the five adsorbates on Ru(0 0 0 1) are investigated and their relationships with the covalent radii of the adsorbates are discussed.

Structural and electronic properties of self-assembled monolayer with 4-(4-amino-phenylazo) benzoic acid (APABA) on the Si(001)(4 x 2) surface are investigated by ab initio calculation based on density functional theory. For the APABA chemisorption on the silicon surface, we have assumed two different binding sites: (i) amino group of molecule and (ii) carboxyl group of molecule. Considering amino-site, we have assumed two possible models for the chemisorption of molecules on the Si(001)-(4 x 2) surface: (i) an intrarow position between two neighboring Si dimers in the same dimer row (Model 1), (ii) on-dimer position (Model 11). We have found that Model 11 is 1.10 eV energetically more favorable than Model I. The Si-N bond length was calculated as 1.85 angstrom which is in excellent agreement with the sum of the corresponding covalent radii of 1.87 angstrom. Considering carboxyl-site, we have assumed exactly the same model as mentioned above. Again we have found that Model 11 is energetically favorable than Model I. The calculated bond lengths for Si-O and O-C are 1.76 and 1.35 angstrom, respectively. (c) 2007 Elsevier B.V. All rights reserved.

The negative H, O and matrix secondary ion emissions resulting from Ar+, Cs+ and O+2 bombardment of Sc, Ti, V, Co, Ni and Cu foils were found to exhibit a wide range of energy distribution shapes and intensities. These energy distributions could be reconstructed using two velocity dependent functions. Characteristic velocity values extracted from these velocity dependent functions were found to fall into groups according to the primary and secondary ions studied. Characteristic velocity values from within a particular group were found to correlate with work function and electron affinity arguments consistent with the Auger and resonance charge transfer processes (the Auger process predominating at lower emission energies ( 40 eV)). All secondary ion groups (H, O and the matrix secondary ion) resulting from a particular primary ion species could then be related on introducing a function relating the covalent radius of the secondary ion-surface atom pair. This, in turn, allowed for an empirical description of the negative secondary ion yield from metal surfaces to be defined.

The local geometrical structures of the p(2 × 2) and c(2 × 2) phases of S Pd (100) as well as of the p(2 × 2) structure of atomic oxygen on the same surface were determined by LEED- IV analysis using the energy range between 35 and 350 eV. The total energy range available for analysis varied between 2900 eV for the p(2 × 2) structures and 1750 eV for the c(2 × 2) structure. Pendry R -factors between 0.15 and 0.19 were obtained for the best fit geometries. In all cases the adsorbate atoms were found to stay above the first substrate layer and to occupy the fourfold coordinated hollow site. Bond lengths to the first substrate neighbors are close to the sum of covalent radii for both S structures, but significantly longer (by 0.10 A) for oxygen. Whereas lateral shifts and rotations of groups of atoms in the substrate layers turn out to be below the detection limits, significant buckling of the second substrate layer was found for all three systems, being largest for oxygen (0.12 A). The unusually extended first to second layer distance found already for the clean Pd(100) surface remains virtually unchanged in all structures and is not compensated by a contraction between the second and third layers.

The structures of oxygen-adsorbed and deuterium-adsorbed NbC(111) surfaces have been studied by impact-collision ion-scattering spectroscopy (ICISS) reflection high-energy electron diffraction (RHEED) and by computer simulation. At room temperature, both oxygen and deuterium are dissociatively adsorbed on the clean surface without long-range order, and an adsorbate atom goes into the threefold hollow site. The saturation coverage is more than 0.9 ML (monolayer) in both cases. The height of an adsorbate from the first layer is 1.10 A for oxygen and 1.00 A for deuterium, which corresponds to 2.13 A for the ONb distance and 2.08 A for the DNb distance. The ONb distance a little larger than the sum of the covalent radii (2.00 A) and the DNb distance is much larger than the sum of the covalent radii (1.64 A). Judging from the bond length and electronegativity, O and D atoms are supposed to have some negative charge.

Using automated tensor LEED (ATLEED), the structure of I on Rh{111} has been studied in the coverage range from 0.12 to 0.33 monolayers, including the ordered (3 × 3)R30° phase. Jodine adatoms are bonded in fcc three-fold hollow “lattice” sites with an iodine-rhodium interplanar separation of 2.22 ± 0.06 A and a first to second rhodium interlayer contraction of 3.7%. An iodine-to-rhodium nearest neighbour bond length of 2.71 A indicates an iodine “effective radius” of 1.37 A, very close to the covalent radius of 1.35 A assuming a rhodium metallic radius of 1.34 A. Adsorption in hcp hollow sites yielded a minimum Pendry R-factor of 0.40. The possibility of site mixing was examined and an R-factor minimum of 0.27 was obtained for 85 ± 15% fcc site occupation. The structure at lower coverages (0.25 and 0.12 ML), where only a diffuse p(1 × 1) LEED pattern is observed was examined by diffuse I–V analysis. Diffuse I–V spectra were found to be practically indistinguishable from spectra from the (3 × 3)R30° structure for identical parallel momentum transfer. An R-factor analysis indicates no coverage-dependent site switching and a coverage independent RhI bond length in the coverage range studied (0.12 ≤ θ ≤ 0.33 ML).

The surface structure of the clean Co{1010} surface and a c(2 × 2) potassium overlayer have been determined by quantitative low energy electron diffraction. The Co{1010} sample has been shown to be laterally unreconstructed with the surface being uniquely terminated by an outermost closely packed double layer (dz12 = 0.68 A). A damped oscillatory relaxation of the outermost three atomic layers occurs, with relaxations Δ dz12 = − 6.5 ± 2% and Δ dz23 = +1.0 ± 2%. The c(2 × 2) overlayer formed at a coverage of 0.5 ML was subjected to a full I-V analysis. A range of adsorption sites were tested including fourfold hollow, on-top, and both long and short bridge sites in combination with both “long” and “short” cobalt interlayer terminations. A clear preference was found for adsorption in the maximal coordination fourfold hollow site. No switching of surface termination occurs. The potassium adatoms reside in the [1210] surface channels directly above second layer cobalt atoms with a potassium to outermost cobalt interlayer separation of 2.44 ± 0.05 A. Potassium-cobalt bond lengths of 3.40 ± 0.05 and 3.12 ± 0.05 A between the four (one) outermost (second) layer nearest-neighbour substrate atoms suggests a potassium effective radius of 1.87 ± 0.05 A, somewhat smaller than the Pauling covalent radius and considerably larger than the ionic radius (1.38 A). The alkali-surface bonding is thus predominantly “covalent”/“metallic”.

We have used X-ray photoelectron diffraction to investigate the formation of As dimers on the c(2 × 8)(2 × 4) reconstructed surface of GaAs(001). Comparison of experiment with simulations employing single scattering theory suggests that dimers form with a bond length of approximately twice the covalent radius of As. This finding represents the first application of the X-ray photoelectron diffraction technique to the study of surface reconstructions.

Recent work on the structure of ordered overlayers of chalcogens on nickel is reviewed, and discussed as a convincing example of the use of LEED intensity spectra for surface structure determination. Guidelines are suggested for successful application of LEED to surface structure, and the special suitability of the muffin-tin potential for the LEED calculation is brought out. The model of the complex crystal potential used for the calculation, and the systematic determination of the four parameters of the model from the data for clean Ni are described. The application to chalcogen overlayers is illustrated by the case of c(2 × 2)S and c(2 × 2)O on Ni(001) surfaces, and the correspondence between experiment and theory is exhibited for the intensity versus energy spectra of 1 2 1 2 beams. The correspondence is close and detailed; the complex shapes of broad features are reproduced and the average of the magnitude of the deviation between peaks in measured and calculated spectra is less than 2 eV, whereas the accidental correspondence of two spectra is shown to fluctuate around 4 eV. The observed values of the bond lengths of O, S, Se, Te on Ni are shown to be plausible on the basis of the covalent radii and the expected deviations from them. Results for other surfaces and other structures are tabulated. In all cases but one [O/Ni(110)] the chalcogen occupies sites that the next layer of Ni atoms would use.

The saturation monolayer coverage of an adsorbed metal overlayer on a smooth metal surface is largely determined by the adsorbed metal's radius. Experimental maximum packing densities in two-dimensional metal overlayers are compared with predictions based on several different definitions of metal atom radii: atomic radii, covalent radii, minimum bulk interatomic distance (:2) and the Zachariasen [J. Inorg. Nucl. Chem. 35 (1973) 3487] metal radii. Best agreement is found with the last, which is obtained by assuming that the bulk, pure metal density is obtained from an ideal, hexagonal close-packed structure of spheres of that radius.