Chemisorption of hydrogen on the Ag(111) surface

Interaction of water with a Pd(110) surface, studied by thermal desorption spectroscopy, LEED and Δφ

Hydrogen adsorption on a Pt(110)−(1 × 2) surface

Interaction of Cl 2 with the Au(111) surface in the temperature range of 120 to 1000 K

The technique of time-of-flight scattering and recoiling spectrometry (TOF-SARS) with detection of both neutrals and ions is applied to structural analysis of oxygen adsorbed on a W(211) surface. The site position for oxygen in the high-dose [\ensuremath{\Theta}=1.5 monolayers (ML), saturation coverage] p(1\ifmmode\times\else\texttimes\fi{}2) low energy electron diffraction (LEED) pattern is determined; a preliminary study of oxygen in the low-dose (\ensuremath{\Theta}=0.5 ML) p(2\ifmmode\times\else\texttimes\fi{}1) LEED pattern is also presented. Both Ar and Ne backscattering (BS) and oxygen recoiling spectra, induced by pulsed 4-keV ${\mathrm{Ar}}^{+}$ and 5-keV ${\mathrm{Ne}}^{+}$ primary-ion beams, are monitored as a function of polar beam incident angle \ensuremath{\alpha}, surface azimuthal angle \ensuremath{\delta}, scattering angle \ensuremath{\theta}, and recoiling angle \ensuremath{\varphi}. Plots of BS (or recoil) intensities in (\ensuremath{\alpha},\ensuremath{\delta}) space provide scattering (or recoiling) structural contour maps and three-dimensional scattering (or recoiling) structural plots which are representative of the adsorption sites of oxygen on the W(211) surface; the symmetry of the adsorption sites is determined from these plots.Measurements of BS and recoil intensities as a function of \ensuremath{\alpha} along different azimuths \ensuremath{\delta} provide experimental values of the critical incident angles ${\ensuremath{\alpha}}_{c}$,sh${}^{\mathrm{i}}$ for shadowing and the critical ejection angles ${\ensuremath{\beta}}_{c}$ (or ${\ensuremath{\alpha}}_{c}$,bl) for blocking by neighboring atoms. Trajectory simulations and calculations of the shadowing and blocking cones obtained from potentials calibrated for this system are used to determine the oxygen-adsorption-site coordinates. The results show that oxygen is dissociatively adsorbed within the troughs along the [1 1\ifmmode\bar\else\textasciimacron\fi{} 1\ifmmode\bar\else\textasciimacron\fi{}] direction. For the saturation p(1\ifmmode\times\else\texttimes\fi{}2) structure, the oxygen is in threefold sites formed by two first- and one second-layer W atoms; these are the only sites which are consistent with all of the experimental data. The coordinates of these sites relative to the W coordinates are determined to an accuracy of \ifmmode\pm\else\textpm\fi{}0.1 A\r{}. The oxygen-tungsten chemisorption bond length is determined as 1.83 A\r{} to the two first-layer W atoms and 2.17 \AA{} to the second-layer W atom; this reflects the unsaturated and fully saturated valencies of the first- and second-layer W atoms, respectively. These trough sites are occupied for coverages up to more than one monolayer of oxygen. For the low-dose p(2\ifmmode\times\else\texttimes\fi{}1) structure, the data indicate occupancy of the same threefold sites, but with different coordinates. The data are consistent with shifting of the relaxed clean W surface structure to the bulk truncated structure upon ${\mathrm{O}}_{2}$ chemisorption. The sensitivity of TOF-SARS to details of adsorbate structure, the use of both BS and recoil data in a complementary manner, and the ability to extract adsorbate structural information with only simple calculations are presented.

$\mathrm{CO}$ adsorption on $\mathrm{Ce}\text{\ensuremath{-}}\mathrm{Pt}(111)$ has been studied by temperature programmed desorption (TPD), low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS). Thin layers (1 to 3 ML) of Ce on $\mathrm{Pt}(111)$ form upon annealing to $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ a surface alloy which most likely is Pt terminated. The $\mathrm{CO}$ overlayer structure on $\mathrm{Ce}\text{\ensuremath{-}}\mathrm{Pt}(111)$ at saturation coverage is argued to be an intermediate between $(2\ifmmode\times\else\texttimes\fi{}2)$ and $c(4\ifmmode\times\else\texttimes\fi{}2)$ superstructures. This intermediate structure exhibits a $c(4\ifmmode\times\else\texttimes\fi{}2)$ LEED pattern in which some of the spots appear together as ordered triangles. The thermal desorption spectra for the $\mathrm{Ce}\text{\ensuremath{-}}\mathrm{Pt}(111)$ surface alloy are shifted down by a temperature of about $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ compared to the $\mathrm{CO}$ desorption of pure $\mathrm{Pt}(111)$, most likely due to a change in the $d$-band due to hybridization between Ce and Pt states, which results in a weakening of the resonant interaction between $\mathrm{CO}$ $5\ensuremath{\sigma}$ and Pt $5d$ states.

We investigated the structure and chemisorption properties of a Ti/Pt(100) surface alloy using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), x-ray photoelectron diffraction (XPD), low-energy alkali ion scattering (ALISS), and temperature programed desorption (TPD). Samples were prepared by evaporating Ti onto a clean hex-Pt(100) reconstructed surface at 300 K. After annealing the sample to 800 K, a $c(2\ifmmode\times\else\texttimes\fi{}2)$ LEED pattern was observed that sharpened as the temperature was increased to 920 K. Further annealing to 1000 K caused this $(2\ifmmode\times\else\texttimes\fi{}2)$ LEED pattern to become diffuse because of the onset of disorder in the surface layers resulting from Ti diffusion into the bulk. Using XPD and ALISS, we determined that this LEED pattern is due to an ordered alloy structure that has Ti atoms present in the second layer of a $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{T}\mathrm{i}/\mathrm{P}\mathrm{t}(100)$ surface alloy, and not in the topmost layer. Thus, the surface layer of this alloy is pure Pt. XPS showed that the Ti $2p$ peak from the surface alloy is shifted by 1.4 eV to higher binding energy than that of a thick Ti film, and the Pt $4f$ peak is shifted by 0.1 eV higher from that of the clean hex-Pt(100) reconstructed surface, consistent with the formation of strong intermetallic bonds upon alloying. The chemisorption properties of the surface alloy were probed using CO and ${\mathrm{H}}_{2}$ adsorption. CO adsorbed reversibly on the alloy, desorbing in TPD in a broad peak with a maximum at 376 K. This is lower by 132 K from the CO desorption peak from a clean Pt(100) surface. Thermal desorption of ${\mathrm{H}}_{2}$ showed a similar peak shift to lower temperature, and much less hydrogen (20%) adsorbed on the $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{T}\mathrm{i}/\mathrm{P}\mathrm{t}(100)$ alloy than on Pt(100). These results show that second layer Ti atoms exert a strong ``ligand'' or electronic effect on Pt atoms at the surface. Thus, the $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{T}\mathrm{i}/\mathrm{P}\mathrm{t}(100)$ surface alloy represents an interesting model system for studying ligand effects on the chemistry of a bimetallic alloy surface.

This study involves the adsorption of Cs on Si (111)-(7 × 7) surfaces at room and elevated temperatures, with increasing coverage of Cs to saturation, in ultrahigh vacuum (UHV) conditions. The techniques of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and work function (WF) measurements were utilized. A WF change at the minimum, found to be Δϕ=-3.2 eV at an approximate 0.23 ML Cs coverage, corresponds to half the saturation coverage. We propose that, at that coverage, the adatom dangling bonds of the Si (111)-(7 × 7) substrate are completely filled. At saturation coverage (0.47 ML), Cs forms a single saturation layer in a near-metallic surface state, where the atomic radius of the Cs adatoms is likely to be 2.2× 10-8 cm. The surface exhibits a high degree of disorder with 0.47 ML of adsorbed Cs, and it is likely that the adsorbate remains in cluster-like domains within the center of the disordered 7×7 unit cells, instead of evenly covering the Si surface. Saturation coverage of Cs on Si (111)-(7 × 7) surfaces exhibits a WF value 0.5 eV less than that of pure metallic Cs. Cesium adatoms form a stronger bond to the Si (111)-(7 × 7) surface than to Ni(100), with a calculated energy of 1.64 eV/atom. The induced surface disorder and strong binding energy are evidence of the strong Cs–Si interaction, which most likely prevents the formation of a purely metallic Cs overlayer.

Growth and crystalline structure of Co layers on Cu(0 0 1)

The interaction of molecular water with the GaP(110) surface has been studied under ultrahigh vacuum conditions with a combination of x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD) methods. An adlayer of water at 1 ML coverage (referenced to the number of undercoordinated Ga surface atoms) desorbs from the GaP(110) surface over the temperature range of 250-370K in TPD experiments. This water monolayer exhibited a c(2 × 2) surface structure, observed with both LEED and STM, consisting of alternating OH and H2O surface-bound species, as determined by XPS data. However, at water coverages below 1 ML, the surface concentrations of OH species were higher than that of molecular H2O, and in the 0-0.75 ML surface coverage range, the (OH:H2O) ratio was constant at 2:1. We also observed another stable surface adlayer at 4/3 ML water coverage that exhibited quasi-ordering with a c(3 × 2) LEED pattern. TPD spectra showed that this extra 1/3 ML desorbed near 210K before the onset of the desorption of the 1 ML coverage.

Electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and temperature programmed thermal desorption (TPD) have been used to study the adsorption of CO on Ni(111) in the temperature range 80−300 K. For low coverages, the CO layer is disordered; a c(4×2) pattern appears at coverages ϑ∼0.5, the maximum coverage at 300 K. At temperatures 220–240 K, a well-ordered (√7/2×√7/2)R 19° LEED pattern forms at saturation (ϑ∼0.57). At 80 K, the CO saturation layer is characterized by a ’’complex’’ LEED pattern. Only one binding state is seen in TPD for ϑ≲0.40 (peak temperatures 450–430 K); species having lower desorption temperatures populate at higher coverage. At 300 K adsorption the only ESD ion observed is O+, with desorption centered about the direction perpendicular to the surface. The O+ ion yield shows a maximum at intermediate coverages. CO+ ions are also observed at adsorption temperature < 300 K at higher coverages. The ESDIAD patterns for saturation coverage in the range 80–260 K indicate off-normal ion emission in addition to the normal component. The data suggest that for ϑ≲ 0.5, CO is adsorbed in multiply coordinated sites with the molecular axis perpendicular to the surface. At temperatures < 300 K, a fraction of CO can adsorb in singly coordinated sites as well. Upon heating the CO layer formed at 80 K (ϑ?0.57) the CO+ ion yield maximizes at ∼240 K when the (√7/2×√7/2)R 19° LEED pattern is well defined. The CO+ yield therefore reflects the ordering behavior in the adsorbate layer at temperatures below the onset of desorption and indicates a shift of a fraction of CO molecules from multiply coordinated to singly coordinated sites. Finally, a model is proposed involving resonance charge exchange between adsorbed CO molecules to account for the observed variations of the O+ yield with surface coverage.

LEED and thermal desorption studies of small molecules (H 2, O 2, CO, CO 2, NO, C 2H 4, C 2H 2 and C) chemisorbed on the stepped rhodium (755) and (331) surfaces

Growth and surface structure of vanadium oxide on anatase (0 0 1)

We present a method of producing a long-range ordered pentacene ultra-thin film on a benzenethiol organic buffer layer at saturation coverage, deposited on a cleaned. single crystalline Cu(100) surface, by means of ultra-high-vacuum organic molecular beam epitaxy. The growth morphology is investigated using in-situ low-energy electron-diffraction (LEED) and high-resolution UV photoelectron spectroscopy (HR-UPS). The change in Surface symmetry from c(4x4) at low coverage to c(2 x 6)/c(2 x 2) at saturation coverage of benzenethiol has been examined by LEED. The corresponding valence band modifications have been found by HR-UPS. Pentacene layers Lip to a thickness of 0.6nni possess exactly the same symmetry of the beneath organic buffer layer at saturation coverage, but with more intense and sharper spots as observed in the LEED patterns.

A study on oxygen adsorption and coadsorption with carbonmonoxide on a stepped nickel surface

Structures of sulfur on TiO 2( [formula omitted]) determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction