Short Bridge Site Research Articles

Adsorption of SO 2 on Ni(110) studied by S K-edge XAFS

Surface structure and electronic properties of submonolayer SO 2 molecularly adsorbed on Ni(110) have been investigated by means of S K-edge XAFS. It is found that the SO 2 molecule is adsorbed with the molecular plane parallel to the surface and with an elongated SO bond distance compared to that of a free molecule. The S atom is located on long- and short-bridge sites with the almost equal probabilities. A significant amount of charge transfer occurs from the substrate to the SO 2 π ∗ orbital. Such structural and electronic properties are essentially similar to those of SO 2 on Ni(100) and Ni(111).

Quantitative structure determination of an NH x species adsorbed on Cu(110)

Using scanned energy mode photoelectron diffraction we have recently investigated the local structure of NH 3 and NH x ( x = 1 or 2) on Cu(110). NH 3 was found to occupy the atop site on the closed-packed Cu rows in the first layer. The NH x species, discussed in more detail in the present paper, occupies the short bridge site on the outermost 〈110〉 rows, at a height of 1.40 ± 0.02 A ̊ above the surface, resulting in a CuN nearest neighbour distance of 1.89 Å.

Adsorption site determination with scanning tunnelling microscopy

By using a very stable, high resolution scanning tunnelling microscope we have imaged the molecules CO, C2H2, C2H4 and C6H6 adsorbed at 4 K on a Cu{110} surface. By simultaneously determining the substrate corrugation and the position of the molecule under conditions of weak tip–surface interaction, it was found that CO occupies the atop site and that C2H2 is probably adsorbed in the short bridge site. For C2H2, a stable (probably chemisorbed) species could only be imaged after increasing the tunnel voltage above 1 V. Energy transfer is obviously required before the molecule ‘locks’ into the short-bridge site. As expected, C6H6 appears to adsorb with its molecular plane parallel to the surface but only an indirect determination of the adsorption site was possible. These experiments demonstrate the potential, and some of the limitations, of adsorption-site determination with the scanning tunnelling microscope.

Ethene adsorbed on Cu(110): a combined photoemission and photoelectron diffraction study

The absorption geometry of ethene on Cu(110) has been investigated using photoelectron diffraction (PhD) in the scanned energy mode as well as angle-resolved valence level photoemission. Two best fit Phf geometries have been found, both of which are compatible with the photoemission data: the molecule is a adsorbed either in an atop site on the close-packed Cu rows at a perpendicular height of 2.08 ± 0.02 Å with a CC bond length of 1.32 ± 0.09 Å, or in a short bridge site on the Cu rows at a perpendicular height of 2.09 ± 0.02 Å with a CC bond length of 1.53 ± 0.13 Å. In each case the molecular plane is parallel to the surface, but the CC axis can be twisted azimuthally out of the [1 1 − 0] direction by as much as 24.

Surface structure of SO 2 adsorbed on Ni(110) studied by S K-edge X-ray absorption fine structure spectroscopy

Surface structure and electronic properties of submonolayer sulfur dioxide SO 2 molecularly absorbed on Ni(110) have been investigated by means of S K-edge surface-extended X-ray absorption fine structure (SEXAFS), near-edge X-ray absorption fine structure (NEXAFS) and S 1s X-ray photoemission spectroscopy (XPS) measurements. The SO 2 molecule was found by SEXAFS and NEXAFS to adsorb with the molecular plane parallel to the surface and an elongated SO bond distance compared to that of a free molecule. The S atom was revealed to locate on the long- and short-bridge sites with almost equal probabilities. No azimuthal-angle dependence was detected as for the SO bond direction. A significant amount of charge transfer from the substrate to the SO 2 π∗ orbital was found by NEXAFS and XPS. Such structural and electronic properties are essentially similar to those of SO 2 on Ni(100) and Ni(111).

Full structure determination of an alkali-metal/CO coadsorption phase for Co{101-bar0}-c(2 x 2)-(K+CO).

A structural analysis of a c(2\ifmmode\times\else\texttimes\fi{}2)-(K+CO) coadsorption phase on Co{101\ifmmode\bar\else\textasciimacron\fi{}0} by low-energy electron diffraction has yielded a full picture of the geometry of a model alkali-metal CO coadsorption system. Potassium atoms are located in fourfold hollow sites above second-layer Co atoms, and CO molecules are displaced by 0.5\ifmmode\pm\else\textpm\fi{}0.2 \AA{} from the high-symmetry Co short bridge sites near the center of gravity of the three neighboring K adatoms. The CO causes a substantial increase, by 0.4 \AA{}, in the K-Co nearest-neighbor bond length, and K adatoms are located with their centers above the O atoms in the neighboring CO. This has important implications for the bonding interactions within the adlayer.

Ab initio chemisorption studies of H on Fe(110)

Ab initio configuration interaction calculations are performed to study the chemisorption of atomic H on a Fe(110) surface. The lattice is modeled as an embedded three-layer, 40-atom cluster with the Fe atoms fixed at the bulk position. Fe 3d orbitals are explicitly included on five Fe atoms on the surface. Hydrogen strongly binds to the Fe(110) surface at the long-bridge, short-bridge, and quasi three-fold sites. The calculated adsorption energies are 2.76, 2.73, and 2.71 eV, respectively. H-surface bonding at the on-top Fe site is more than 0.4 eV weaker. The calculated H-surface distances are 0.89, 1.03, and 0.87Åfor H at the long-bridge, short-bridge, and quasi three-fold sites, respectively, which agrees well with the LEED value of 0.9 ± 0.1Å. The H-surface stretching vibrational frequencies are calculated to be 1070, 1066, and 1073 cm −1, at the long-bridge, short-bridge, and quasi three-fold sites, respectively. The work function of Fe(110) decreases on H adsorption. The present calculations indicate that H diffusion into the bulk through the short-bridge site will have a much higher activation barrier than via the long-bridge and quasi three-fold sites.

STM study of the Ni(110)-(2 × 1)-2CO system: structure and bonding-site determination

The structure of the Ni(110)-(2 × 1)-2CO system has been studied with scanning tunneling microscopy (STM) at room temperature. Contrary to many previous studies, it is found that atomically-resolved STM imaging of CO molecules is attainable. By comparing the apparent CO adlayer registry with coadsorbed low concentrations of O and S, it is found that the CO molecules bond in short-bridge sites along the Ni [11̄0] rows and alternately tilt away from the normal direction within the (001) plane.

Self-diffusion of adatoms on fcc(110) surfaces

We investigate the mechanisms and associated energies of diffusion for the Ni Ni(110) , Cu Cu(110) , Pd Pd(110) and Ag Ag(110) systems using the corrected effective medium (CEM) method. On the (110) surface, cross-channel diffusion can occur via atomic exchange of the adsorbate atom with an atom of the channel wall or by hopping over the short-bridge site located in the channel wall. We find that the barrier for cross-channel diffusion is much smaller for the atomic exchange path. In-channel diffusion occurs via hopping over the long-bridge site. We find that the activation barrier associated with in-channel diffusion is smaller than the activation barrier associated with cross-channel diffusion by 0.17, 0.01, 0.03 and 0.08 eV for Ni, Cu, Pd and Ag, respectively. We also investigate convergence of the activation barriers by varying the number of active atoms in the simulations. Decreases of 0.3–0.6 eV are found as the number of active atoms is increased from nearest neighbors to a (converged) large number of active atoms; this is due to relaxation of the strain. The decreases are nearly identical for the in-channel and cross-channel mechanisms.

Phase diagram and overlayer structures for CO on Rh(110)

Low energy electron diffraction (LEED) patterns have been recorded for CO on Rh(110) and analyzed as a function of sample temperature or CO dose using a video-LEED system. The results provide a diagram of boundaries and existence regions for overlayer phases. The overlayer is a disordered lattice gas at absolute CO coverages (θ CO) below about 0.08 for all temperatures. A pattern characterized by splitting of c(2 × 2) spots appears most intense for θ CO ≈ 0.3–0.6 below 180 K. The “split” c(2 × 2) pattern converts to a pattern with diffuse, broad c(2 × 2) spots above 180 K. Overlapping diffraction features from “(3 × 2)”, “(4 × 2)”, and “(5 × 2)” symmetries are observed for θ CO above about 0.62 and below 240 K. The “(5 × 2)” predominates by θ CO ≈ 0.8. With further increase in θ CO, (2 × 1) LEED spots appear with the “(5 × 2)” features. A pure (2 × 1) pattern appears at monolayer θ CO. Overlayer structural models with CO molecules in short-bridge sites (off-top models) or tilted from on-top adsorption sites (tilt models) are proposed to account for features in the LEED patterns. Off-top models giving rise to pseudo-hexagonal ordering are favored for the “split” c(2 × 2) overlayer. The “c(2 × 2)” overlayer contains uniformly sized domains with ideal c(2 × 2) ordering and disordered domain walls. Increasing 3-, 4-, and 5-fold symmetry in off-top adlayers arises from uniform, commensurate compression along [110]. In tilt overlayer models by comparison, the “( n × 2)” structures contain CO molecules in rows with local (2 × 1) symmetry separated by 3, 4, or 5 unit cell spacings. A zig-zag tilt model for reconstruction in the (2 × 1) CO phase is proposed.

Surface carboxylate species on Cu(100) studied by angle-scanned photoelectron diffraction and LCAO-LDF calculations

Chemisorption site geometries of formate and acetate species on Cu(100) have been studied by means of angle-scanned X-ray photoelectron diffraction (XPD) and first-principle quantum-mechanical calculations on a Cu 60-formate cluster in the framework of local density functional theory (LDF). According to our LDF calculations, the short bridge site results to be more stable than the cross bridge one by about 20 kcal mol . The reasons for the more effective interaction of the adsorbate in the short bridge site have been clearly outlined on the basis of the analysis of the LDP wavefunctions. LDF-optimized structural parameters have been used as a starting input in a series of single-scattering cluster spherical wave (SSC-SW) simulations of the O 1s XPD curves, which have experimentally confirmed the occupation of the short bridge site proposed by LDF calculations. The SSC-SW simulations have also outlined the role of the low-frequency-hindered rotational modes of the adsorbate (librations) and have furnished an estimate of the frequency (∼ 70 cm −1) of the hindered rotation on the axis perpendicular to the surface. Furthermore, as regards the formate species, the experimental data give evidence of a Cu-O distance of 1.95 Å, in agreement with the LDF calculations and with angle-resolved photoemission fine structure (ARPEFS) literature results. An estimate of the OCO angle (129° ± 5°) has been obtained from polar scans for both formate and acetate species.

Adsorption site determination by means of surface core level shift high energy photoelectron diffraction: Pd{110}(2 x 1)p2mg-CO.

We propose a novel application of the photoelectron diffraction method for determining the adsorption site of an adsorbate, based on the diffraction of high kinetic energy photoelectrons originating from the topmost layer of the substrate. Sensitivity to the topmost layer is achieved by working at grazing emission angle and by means of the adsorbate induced surface core level shift. A simple single scattering theory is shown to be adequate to interpret the data. We apply the method to the $\mathrm{Pd}{110}(2\ifmmode\times\else\texttimes\fi{}1)p2mg\ensuremath{-}\mathrm{CO}$ system and provide clear evidence that CO adsorbs in the short bridge sites.

A tensor LEED analysis of the Rh(110)-p2mg(2 × 1)-2CO structure

The structural geometries of both the clean and CO covered Rh(110) surfaces have been determined by a LEED intensity analysis. The clean surface exhibits a top layer contraction of 0.09 ± 0.02 Å and a second layer expansion of 0.03 ± 0.03 Å relative to the bulk interlayer spacing. Upon the adsorption of 1 ML of CO the molecules are arranged in a zig-zag fashion along the [ 110] direction to produce the observed glide plane symmetry in the LEED pattern. The clean surface relaxations are removed and each CO bonds near a short bridge site with the CO molecular axis oriented approximately 24° from the surface normal. This bonding geometry results in a RhC bond length of 1.97 ± 0.09 Å and a CO bond length of 1.13 ± 0.09 Å.

Anisotropic amplitudes of adsorbate bending vibrations from x-ray photoelectron diffraction: A multiple-scattering evaluation.

The C 1s photoelectron angular emission from a disordered CO layer on Ni(110) at 300 K (coverage approximately 0.7 monolayer) was studied for polar angles up to 80\ifmmode^\circ\else\textdegree\fi{} and several azimuths. The experimental intensity distributions are compared to multiple-scattering cluster calculations which include the possibility of anisotropic bending vibrations of the adsorbed molecule based on a new formalism for thermal averaging in the diffraction theory. From the azimuth-dependent width of the forward scattering peak we have determined bending vibraional amplitudes of 12\ifmmode^\circ\else\textdegree\fi{} and 17\ifmmode^\circ\else\textdegree\fi{} for the [11\ifmmode\bar\else\textasciimacron\fi{}0] and [001] azimuths, respectively. The optimum agreement between experiment and theory over the whole range of polar angles is obtained for a CO layer with short-range order domains and equal occupation of on-top and short-bridge sites.

A reinvestigation of the surface structure of Ni(110)−(2×1)2CO by LEED

New LEED measurements using improved apparatus and new calculations by a more accurate method than previously lead to the same conclusion by LEED that CO adsorbs on Ni(110) at the short bridge site at full coverage.

Experimental evidence for anticorrugating effects in He-metal interactions at surfaces.

We show that systematic differences occur in the corrugations of the systems Ni(110)c(2×4)H, and Rh(110)(1×3)H and (1×2)H, when determined from He and Ne diffraction: Whereas the hills along the H-free close-packed metal rows correspond to underlying metal atoms in all corrugations deduced from Ne diffraction, they are shifted to short bridge sites in all He-derived corrugations. This assignment is possible, as the H adatoms mark threefold hollow sites on both (110) substrates. These observations provide strong experimental support for anticorrugating effects in He-metal surface interactions as proposed by Annett and Haydock [Phys. Rev. Lett. 53, 838 (1984)]

Bridge-bonded CO in Ni(110)-p2mg(2× 1)-CO: a SEXAFS study

Surface-extended X-ray-absorption fine-structure measurements have been performed to determine the adsorption site of CO in the densely-packed system Ni(110)-p2mg(2 × 1)-CO. The analysis shows that CO adsorbs in a tilted configuration on short-bridge sites yielding a nearest-neighbour O-Ni distance of 2.84 ± 0.05 Å. Tilted CO molecules in on-top positions can be definitely ruled out.

Bridge-bonded CO in Ni(110)−p2mg(2×1)-CO: a SEXAFS study

Surface-extended X-ray-absorption fine-structure measurements have been performed to determine the adsorption site of CO in the densely-packed system Ni(110)−p2mg(2×1)-CO. The analysis shows that CO adsorbs in a tilted configuration on short-bridge sites yielding a nearest-neighbour ONi distance of 2.84±0.05 Å. Tilted CO molecules in on-top positions can be definitely ruled out.

Structural determination of p2mg(2 x 1)CO/Ni(110) with the use of angle-resolved photoemission extended fine structure.

The technique of angle-resolved photoemission extended fine structure has been used to study the chemisorption geometry of the dense p2mg(2\ifmmode\times\else\texttimes\fi{}1)CO/Ni(110) overlayer at low temperatures. Photoemission intensities from the carbon 1s core level were measured in three directions as a function of photoelectron kinetic energy in the range 60--400 eV. Using multiple-scattering spherical-wave (MSSW) modeling, we confirmed that the CO molecules form a zigzag-chain geometry on the Ni(110) surface at saturation coverage, with adjacent CO molecules along the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction displaced alternatively in opposite directions towards the [001] and the [001\ifmmode\bar\else\textasciimacron\fi{}] azimuths. We further determined that the CO molecules are adsorbed on the short-bridge sites, the tilt angle for the direction linking the carbon atom and the center of the nickel bridge being 16\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{} from the surface normal. The carbon-nickel interatomic distance was determined to be 1.94\ifmmode\pm\else\textpm\fi{}0.02 \AA{}. The first- to second-layer spacing of nickel is 1.27\ifmmode\pm\else\textpm\fi{}0.04 \AA{}, up from 1.10 \AA{} for the clean Ni(110) surface, but close to the 1.25-\AA{} Ni interlayer spacing in the bulk. Using the findings of earlier studies of this system, the C-O bond length and tilt angle were varied within small ranges (1.10--1.20 \AA{} and 15\ifmmode^\circ\else\textdegree\fi{}--23\ifmmode^\circ\else\textdegree\fi{}, respectively) in our MSSW simulations. At 1.16 \AA{} and 19\ifmmode^\circ\else\textdegree\fi{} the best agreement between the experimental data and the theoretical simulations was achieved. The above results yield an O-O distance of 2.95 \AA{} for the two nearest CO molecules, close to twice the van der Waals radius (\ensuremath{\sim}1.5 \AA{}) for oxygen.

Identification of two tilted adsorbed μ 2-methoxy species on Ni{110} using RAIRS

Two different adsorbed methoxy species on Ni{110} are identified using reflection absorption infrared spectroscopy both of which are twofold coordinated and tilted with respect to the surface normal. One methoxy species is adsorbed onto short bridge sites on the close-packed rows, with a large tilt angle in the [001] direction. The other methoxy species is adsorbed at a long bridge site across the close-packed rows and tilted with a smaller angle in the [1 1 0] direction. We also report a remarkable similarity between the infrared data for methoxy adsorption on the same single crystal faces of Ni and Cu, indicating that the nature of the chemisorbed species is determined by the site symmetries on the different planes, and not by the metal itself.