Two-fold Bridge Site Research Articles

Electronic structure and orientation of NO on Ni(111) studied by arups using synchrotron radiation

The electronic structure and the orientation of NO adsorbed on Ni(111) at 120 K was studied by angle resolved UPS using linearly polarized synchrotron radiation and a multichannel angle resolving electron analyzer. The layer investigated corresponds to a coverage of 0.5 ML and exhibits a c(4 × 2) LEED pattern with only twofold bridging sites being occupied. The binding energies for 4 \\ ̃ gs, 5 \\ ̃ gs and 1 \\ ̃ gP at the ḡG point are 15.1, 9.3 and ~7.5 eV, respectively; the 1 \\ ̃ gP level is very broad which may be due to splitting. The 2D band structure shows a dispersion of 0.3 eV for the 4 \\ ̃ gs and 0.6 eV for the 5 \\ ̃ gs level, indicative of lateral interactions within the adsorbed NO layer. The photon energy dependence of the photoionization cross sections of the 4 \\ ̃ gs, 5 \\ ̃ gs and 1 \\ ̃ gP NO molecular orbitais has been studied in the energy range 26 eV ⩽ ℏ gw ⩽ 60 eV. The 4 \\ ̃ gs level shows a pronounced maximum at ℏ ω = 36 eV ( E kin = 15 eV) that is peaked in the direction of the surface normal and is interpreted as a shape resonance. For the 5 \\ ̃ gs and 1 \\ ̃ gP levels no shape resonances are observed. Using dipole selection rules, it is demonstrated independently by initial state and final state arguments that in a c(4 × 2) layer the NO molecules are adsorbed with their molecular axis perpendicular to the surface.

Surface-bonding geometry of (2 x 1)S/Ge(001) by the normal-emission angle-resolved photoemission extended-fine-structure technique.

The surface structure of (2\ifmmode\times\else\texttimes\fi{}1)S/Ge(001) was determined using the angle-resolved photoemission extended-fine-structure technique in the normal-emission direction. By comparison of the experimental data with curved-wave, multiple-scattering calculations, quantitative information about the local adsorption geometry was obtained. In particular, adsorption in a twofold bridge site, with a S-Ge bond length of 2.36\ifmmode\pm\else\textpm\fi{}0.05 A\r{}, was found. The twofold S bridge appears most likely to occur between two partially intact symmetric Ge-Ge dimers, with the Ge dimer laterally displaced by 0.10\ifmmode\pm\else\textpm\fi{}0.05 A\r{} from the bulk position. This result therefore provides evidence for S bonding to strong dangling bonds in the original dimers of the clean Ge(001) surface. There is, however, no evidence of significant surface contraction or expansion in the substrate layers, in contrast to the (2\ifmmode\times\else\texttimes\fi{}2)S/Ge(111) case.

Surface structure of (22) S/Ge(111) determined by angle-resolved photoemission fine structure.

Measurements of the extended fine structure in the photoemission intensity from the S(1s) core level were performed for a (2\ifmmode\times\else\texttimes\fi{}2) overlayer of S on Ge(111). This is the first application of angle-resolved photoemission fine structure (ARPEFS) to study an adsorbate on a semiconductor substrate to our knowledge. The adsorption site and local geometry were determined from the ARPEFS with use of comparisons to multiple-scattering calculations. The results of this analysis indicate adsorption in a twofold bridge site 1.03\ifmmode\pm\else\textpm\fi{}0.05 A\r{} above the Ge surface. The separation between the first and second Ge layers is contracted by (9\ifmmode\pm\else\textpm\fi{}6)%, and some Ge---Ge bond lengths between the Ge bilayers are expanded by (8\ifmmode\pm\else\textpm\fi{}3)%. This adsorption site is different from that determined for another chalcogenide, Te, on the Ge(111) surface on the basis of surface extended x-ray-absorption fine-structure measurements, but it is the same as those found for Te/Si(111) and Se/Si(111).

ChemInform Abstract: Electron Energy Loss Characterization of NO on Rh(111). Part 1. NO Coordination and Dissociation.

AbstractIn contrast to its behaviour on other platinum group metal surfaces NO adsorbs on Rh(111) in twofold bridge sites at all coverages.

ChemInform Abstract: Electron Energy Loss Characterization of NO on Rh(111). Part 2. Coadsorption with Oxygen and CO.

AbstractWhen annealed to 275 K in the presence of coadsorbed oxygen NO is converted from twofold bridge sites to linear adsorption sites.

Hydrogen-induced reconstruction on W(100) and Mo(100) by surface infrared spectroscopy

Infrared absorption and low-energy electron-diffraction measurements of H adsorbed on W(100) and Mo(100) show that on each surface, distinct wavenumbers characterize the H-substrate stretching modes associated with the different long-range structures of the complicated T-θ phase diagram. Hydrogen is bonded at a two-fold bridge site at all temperatures and coverages investigated and the wavenumber of the symmetric stretch mode, v 1, is determined by the local geometry, i.e. the substrate dimer length. Analysis of the coverage dependence of the v 1 wavenumber shows that, at low coverages (θ ≲0.3), the effective H-H interactions are very different for the two substrates, leading to a uniform H layer on W(100) and to island formation on Mo(100). In general, the phase transitions are continuous on W(100), with regions of intermediate structures, and first order on Mo(100), with regions of coexisting phases.

Electron energy loss characterization of NO on Rh(111). I. NO coordination and dissociation

Electron energy loss spectroscopy (EELS) has been used to study the adsorption of nitric oxide on Rh(111). NO adsorbs in twofold bridge sites at all coverages. The dissociation rate of NO on this surface at low coverage has been measured directly using EELS. The first order dissociation rate parameters are Ea =19.2±0.3 kcal/mol and ν(1)0 =1×1014±0.3 s−1 at θNO ≊0.2. Ordered low energy electron diffraction (LEED) patterns are observed only for partially dissociated NO adlayers and not for NO adsorbed molecularly at 95 K. Because NO adsorbs in twofold bridge sites at all coverages, the N–O stretching frequency exhibits a 150 cm−1 shift from zero to saturation coverage.

Electron energy loss characterization of NO on Rh(111). II. Coadsorption with oxygen and CO

High resolution electron energy loss spectroscopy has been used to examine the effects of coadsorbed oxygen and carbon monoxide on the bonding of nitric oxide adsorbed on the Rh (111) surface. NO is converted from twofold bridge sites (1500–1600 cm−1) to linear sites (1840 cm−1) when annealed to 275 K in the presence of coadsorbed oxygen. Terminally bound linear NO does not dissociate, but desorbs molecularly, a finding consistent with the commonly observed oxygen inhibition of NO dissociation over noble metals. Coadsorbed CO does not significantly inhibit or promote the dissociation of a low coverage of nitric oxide compared to the clean Rh (111) surface. CO and NO appear to segregate and form islands of each species at 95 K if CO is adsorbed first. Annealing this adlayer above 250 K disperses the islands and mixes the adsorbate layer at temperatures below where NO dissociation and reaction with CO are significant. The Rh–CO bond is weakened in well-mixed CO+NO adlayers, indicating that the NO–CO repulsive interaction acts through the Rh substrate rather than directly between the adsorbed molecules.

Nitric oxide chemisorption on nickel(111): coverage effects, site preferences, and adsorption geometry

Several possible rationales for the different NO adsorption states observed on a Ni(111) surface are discussed, based on tight binding calculations of the extended Hueckel type. In the two fold bridge site, as the coverage of NO increases, for perpendicular adsorbed NO the N-O bonding increases and N-Ni bonding decreases. For slightly bent NO at low coverage and perpendicular NO at high coverage the same trends are obtained. These trends are consistent with the observed N-O, N-Ni stretching frequency change. At coverage 1/3, and in the perpendicular geometry, both N-O bonding and N-Ni bonding increase when NO changes adsorption sites from threefold to twofold or from two fold to on-top. In the on-top site it is calculated that both N-O bonding and N-Ni bonding increase when NO changes geometry from bent to perpendicular. In the twofold bridge site the effect of bending is to produce similar but smaller trends. 30 references, 7 figures, 7 tables.

Vibrational analysis of water adsorbed on Pd(100): Sensitivity of the isotope shifts of bending modes to the bonding site.

A harmonic picture of the vibrations of water adsorbed on Pd(100) is presented. The shift of the water bending mode (against the surface plane) upon deuteration is well described by this purely harmonic picture. Normal-mode calculations in which the Pd(100) substrate is described by a finite cluster of 66 atoms have been used to study the sensitivity of the isotope shift of the bending-mode frequency to the bonding site. The on-top and twofold bridge sites are consistent with experimental results whereas the fourfold hollow site is not.

Bridge/atop site conversion of CO on Ni(111): Determination of the binding energy difference

A rapid site exchange process is observed in the equilibrated chemisorbed layer of CO on Ni(111). Following adsorption at 298 K, the relative populations of CO adsorbed on atop sites and twofold bridge sites are monitored by the high resolution electron energy loss intensities of the respective CO vibrational modes as a function of surface temperature. Since equilibrium is established, the binding energy difference between the terminal and bridge adsorption sites is determined. The bridge site is more stable than the atop site by 0.94±0.15 kcal/mol at a coverage of 0.13. As the coverage is increased to 0.42, the difference in binding energies decreases to 0.44±0.07 kcal/mol. At saturation coverage, 0.5, the binding energy difference effectively becomes very large, resulting in CO occupation of the twofold bridge sites exclusively.

LCGTO-Xα model cluster study for the chemisorption of CO on twofold sites of Ni surfaces

The cluster Ni 2CO is studied as a simplified model for the chemisorption of CO on twofold bridging sites of transition metal surfaces. Using the LCGTO-Xα method we have calculated the potential energy surface for the totally symmetric stretching motion keeping the Ni-Ni distance fixed at the bulk value. The minimum energy is found at a Ni-C distance of 1.72 Å and a C-O bond length of 1.19 Å. The vibrational frequency for the CO bond (1850 cm -1) shows reasonable agreement with EELS data (1810, 1870 cm -1), whereas the (Ni 2)-C frequency of 495 cm -1 is remarkably higher than the experimental values (380, 400 cm -1) indicating an overestimation of the chemisorption bond strength in this simple cluster model. The bonding between CO and Ni is analyzed using orbital correlations, ionization energies and Mulliken population analysis. Important bonding contributions from π backdonation are identified while the a 1 orbital manifold exhibits strong antibonding effects.

LCGTO-Xα model cluster study for the chemisorption of CO on twofold sites of Ni surfaces

The cluster Ni 2CO is studied as a simplified model for the chemisorption of CO on twofold bridging sites of transition metal surfaces. Using the LCGTO-Xα method we have calculated the potential energy surface for the totally symmetric stretching motion keeping the NiNi distance fixed at the bulk value. The minimum energy is found at a NiC distance of 1.72 Å and a CO bond length of 1.19 Å. The vibrational frequency for the CO bond (1850 cm −1) shows reasonable agreement with EELS data (1810, 1870 cm −1), whereas the (Ni 2)C frequency of 495 cm −1 is remarkably higher than the experimental values (380, 400 cm −1) indicating an overestimation of the chemisorption bond strength in this simple cluster model. The bonding between CO and Ni is analyzed using orbital correlations, ionization energies and Mulliken population analysis. Important bonding contributions from π backdonation are identified while the a 1orbital manifold exhibits strong antibonding effects.

An infrared reflection-absorption study of CO chemisorbed on clean and sulfided Ni(111) — Evidence for local surface interactions

Infrared Reflection-Absorption Spectroscopy (IRAS) has been used to characterize CO chemisorption on a clean and sulfur-modified Ni(111) surface under both ultrahigh vacuum and high CO pressure conditions. On clean Ni(111) correlations between the observed LEED patterns and the infrared spectra show that for the c(4 × 2) structure with a CO coverage of θ co = 0.50, all CO molecules occupy two-fold bridge sites as indicated by a single infrared band and 1910 cm −1. For θ co = 0.57, a ( 7 2 × 7 2 )R19.1° structure forms with infrared bands at 2058 cm −1 corresponding to terminally bound CO and at 1925 cm −1 for the bridge-bonded CO. The change in the overlayer structures is accompanied by an abrupt change in the coverage dependence of the bridge-bonded CO frequency for adsorption at 80 K. In studies involving known sulfur coverages on Ni(111). a new sulfur-induced CO state is observed with an infrared band which shifts from 2109 cm −1 to 2103 cm −1 corresponding to increasing sulfur coverage. The sulfur-induced CO state is only populated at 300 K when a high ambient CO pressure is present. The infrared data clearly indicate that the CO and S interact on the Ni surface through a local short-range mechanism: this short range COS interaction model is supported by kinetic studies of CO desorption from the sulfided Ni(111) surface.

Azimuthal and polar angle dependent sexafs of (2 × 1)O and N 2O on Cu(110)

Oxygen adsorbs at room temperature in a low symmetry bridge site. This position allows the determination of the azimuthal and polar angle dependence of SEXAFS. For normal incidence and the E vector parallel to the [100] and [110] directions we determine the first and second nearest neighbour distances to be 1.84(2) Å and 2.00(5) Å, respectively. This can be explained by a model with the oxygen occupying the two-fold long bridge site in the [100] direction 0.35 Å above the first Cu layer. Similar data are taken for N 2O on this substrate. They confirm the decomposition of the molecule and an adsorption of the remaining atomic O on the same site.

Temperature dependence of the vibrational lineshape of CO chemisorbed on the Ni(111) surface

The lineshape of the carbon-oxygen stretching vibration for CO chemisorbed at the two-fold bridge sites and on top sites of Ni(111) has been measured over the temperature range 80 to 300 K with infrared reflection absorption spectroscopy. The bridge bonded CO undergoes pronounced broadening at higher temperatures while the terminally bonded CO is only slightly broadened. The results are interpreted according to a recent vibrational dephasing model developed for condensed phase molecules. In this model the dephasing is brought about by rapid energy exchange between low frequency modes of the substrate and low frequency modes of the molecule which are anharmonically coupled to the high frequency band being studied.

Surface EXAFS of the (2 × 1) oxygen adlayer on Ag(110)

Azimuthal and polar angular dependent surface extended X-ray absorption fine structure (SEXAFS) studies have been performed on Ag(110)-O(2 × 1) using the synchrotron radiation of the new electron storage ring BESSY. Independent measurements with the electric field vector E parallel to the [100] and [110] directions yield first and second nearest-neighbour distances of 2.06 and 2.17 Å, respectively. They show unambiguously that the adsorbed atomic oxygen is located on the two-fold bridge site in the [100] direction 0.2 Å above the surface Ag atoms. This result is supported by a comparison of SEXAFS amplitudes.

A SIMS and classical dynamics study of the chemisorption of CO on Ni{7 9 11}

The dependence of secondary ion intensities on azimuthal angle of incidence of the primary ion beam in SIMS experiments on a stepped Ni{7 9 11} surface with adsorbed CO has been examined in detail. The surface was examined after various exposures to CO and at temperatures of 231 and 300 K. The angular anisotropies for the Ni+, Ni+2, Ni+3, Ni2CO+, and especially the NiCO+ species were found to be quite sensitive to surface structure changes suggested using other surface science techniques. Ion yield ratios as a function of exposure of CO to the surface, however, were found to be nearly insensitive to these structural changes. Of particular significance was the presence of a sharp feature in the NiCO+ ion yield at an Ar+ ion angle of incidence of φ=110° with the crystal temperature at 231 K and after a CO exposure adequate to populate the step edge sites. At higher exposures or temperatures, this feature was washed out when presumably mostly terrace sites are occupied. Using these ion yield versus azimuthal angle curves, it is also apparent that the saturation coverage structures at 231 and 300 K are different. By introducing 10−7 Torr of CO into the chamber at 300 K the ion yield curves are identical to those for the saturation coverage at 231 K, indicating similar surface structures. Classical dynamics calculations aimed at modeling the structure corresponding to a CO exposure of 0.6 L at 231 K support the idea that the CO molecules reside in twofold bridge sites along the bottom of the step edge.

Azimuthal- and Polar-Angle-Dependent Surface Extended X-Ray—Absorption Fine-Structure Study: (2×1)O on Cu(110)

A low-symmetry twofold bridge site of (2\ifmmode\times\else\texttimes\fi{}1) oxygen on Cu(110) allows the determination of the azimuthal dependence of the surface extended x-ray---absorption fine structure. For normal incidence and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}$ parallel to the [100] and [110] directions we determine the nearest- and second-nearest-neighbor distances to be 1.84(2) and 2.00(5) \AA{}, respectively. We show that the atomic oxygen is occupying the low-symmetry twofold bridge site in the [100] direction 0.35 \AA{} above the first Cu layer.

ChemInform Abstract: INELASTIC NEUTRON SCATTERING STUDY OF HYDROGEN ADSORBED ON IMPURE PALLADIUM BLACK

AbstractDie inkohärenten inelastischen Neutronenstreuspektren (i.n.s.) von fest an Pd‐Schwarz chemisorbiertem Wasserstoff lassen sich bei 77 Kohne Störung durch adsorbierten Wasserstoff vom C‐Typ erhalten.