The reaction of H2S with surface oxygen on Ni(110)

The reactions of H₂S with predosed surface oxygen on Ni(110) surfaces were studied for a variety of coverage conditions. The primary reaction product is H₂O, but the details of the water formation and desorption depends on the coverage of both O and H₂S.

For high coverages of oxygen (p(2 × 1)−O; 0.5 ML), the reaction to form water is quantitative. The loss of oxygen from the surface (as measured by AES) is equal to the increase in sulfur coverage. XPS and HREELS measurements indicate the presence of chemisorbed H₂O immediately following large exposures of H₂S on the oxygen predosed surface at 110 K. Deuterium incorporation results suggest that the primary mechanism for these coverage conditions involves direct transfer of hydrogen from SH or H₂S moieties to the oxygen.

A second mechanism involving reaction of surface hydroxyl groups with surface hydrogen was also identified. This mechanism is particularly important for high coverages of oxygen (0.5 ML) and low coverages of H₂S (0.15 ML), where water desorption was observed at 235 K, but was not observed spectroscopically at 110 K. The sequential addition of two surface hydrogen atoms to surface oxygen is not an important mechanism in this system.

These reactions were modeled using a bond-order conservation method, and the model successfully reproduced the important mechanistic conclusions.     

Atomic adsorption of oxygen on Cu(111) and Cu(110)

We have studied angle-resolved inverse photoemission ( = 9.7 eV) after room temperature adsorption of oxygen on Cu(111) and Cu(110). On Cu(111) exposure to 500 L induces a band (3.0 eV aboveE _F at) which shows clear dispersion (1.0 eV) to higher energies for off normal incidence. Since no LEED superstructure is seen for that system, our results present strong evidence for the presence of short-range surface order. Two adsorbate bands are identified (2.8 eV and 6.3 eV at) on Cu(110)p(2×1)-O. Our results are in good agreement with a long-bridge adsorption site.     

Model pseudopotential for the Cu(110) surface

A method has been proposed for constructing the two-dimensional pseudopotential describing the electronic structure of the Cu(110) surface. This method can also be applied to construct the corresponding pseudopotentials for the (110) surface of a number of other face-centered cubic metals, such as Ag, Au, Al, Pd, and Pt. The electronic structure obtained can be used for fast calculations of single-particle and collective electron excitations both on the pure Cu(110) surface and on the surface covered with adatoms or ultrathin films of other metals.     

Sulfur dioxide adsorption and reaction with atomic oxygen on the Ag(110) surface

The adsorption of SO₂ on Ag(110) and the reaction of SO₂ with oxygen adatoms have been studied under ultrahigh vacuum conditions using low energy electron diffraction, temperature programmed reaction spectroscopy and photoelectron spectroscopy. Below 300 K, SO₂ adsorbs molecularly giving p(1×2) and c(4×2) LEED patterns at coverages of one half and three quarter monolayers. respectively. At intermediate coverages, streaked diffraction patterns, similar to those reported for noble gas and alkali metal adsorption on the (110) face of face-centered cubic metals were observed, indicating adsorption out of registry with the surface. A feature at low binding energy in the ultraviolet photoemission spectrum appeared which was assigned to a weak chemisorption bond to the surface via the sulfur, analogous to bonding observed in SO₂-amine charge transfer complexes and in transition metal complexes. SO₂ exhibited three binding states on Ag(110) with binding energies of 41, 53 and 64 kJ mol^?1; no decomposition on clean Ag(110) was observed. On oxygen pretreated Ag(110), SO₂ reacted with oxygen adatoms to form SO_3(a), as determined by X-ray photoelectron spectroscopy. Reacting preadsorbed atomic oxygen in a p(2 × 1) structure with SO₂ resulted in a c(6 × 2) pattern for SO_3(a). The adsorbed SO_3(a) decomposed and disproportionated upon heating to 500 K to yield SO_2(g), SO_4(a) and subsurface oxygen.     

Thermal behavior of the roughened Cu(110) surface

The roughened Cu(110) surface was prepared by annealing the clean surface at various temperatures ranging from 700 to ∼1000 K. A significant drop in intensity of reflection anisotropy spectroscopy (RAS) peak at 2.1 eV photon energy as a function of increasing sample temperature was found for annealing above the roughening transition at 900 K. The observed change of 2.1 eV peak in RAS spectra is because of the surface state Fermi level shift due to temperature change. The RAS result is in good agreement with an unoccupied surface state energy using inverse photoemission spectroscopy (IPES). New IPES results indicate that the unoccupied surface state intensity decreases with increasing annealing temperature. It was also found that the unoccupied surface state was shifted. IPES results provide that the contributions of the surface state to surface optical properties at 2.1 eV are relevant for the RAS technique.     

The temperature dependence of Cu(2)O formation on a Cu(110) surface with an energetic O(2) molecular beam

We report a study of the surface temperature (T(s)) dependence of Cu(2)O formation on a Cu(110) surface induced by a hyperthermal O(2) molecular beam (HOMB), using x-ray photoemission spectroscopy in conjunction with synchrotron radiation. From the T(s) dependence of the O uptake curves, the direct dissociative adsorption process mainly contributes to the formation of the p(2?×?1)-O structure with an O coverage (Θ) of 0.5?ML for 2.2?eV HOMB incidence. On the other hand, the rate of oxidation at Θ?>?0.5?ML, particularly in Cu(2)O formation, strongly depends on the T(s). Thicker Cu(2)O islands were found inhomogeneously at 400 and 500?K, suggesting the dominant role of the migration of Cu atoms in the Cu(2)O formations on the Cu(110) surface.     

Chemisorption of oxygen on the silver (110) surface

In the present work the interaction between oxygen and the silver (110) surface is investigated, mainly using LEED-Auger techniques and thermal desorption spectra. The formation and stability of adsorption layers is studied after exposures at pressures from 10^?3 to 1 torr. At maximum coverage, a (2 × 1) superstructure is formed which is stable up to the desorption temperature. At lower coverage, (3 × 1) and (4 × 1) superstructures are also observed. On the basis of the experimental evidence, tentative models for these structures are presented and discussed.     

Investigation of oxygen adsorption on Cu(110) by low-energy ion bombardment

The interaction of molecular oxygen with a Cu(110) surface is investigated by means of low energy ion scattering (LEIS) and secondary ion emission. The position of chemisorbed oxygen relative to the matrix atoms of the Cu(110) surface could be determined using a shadow cone model, from measurements of Ne⁺ ions scattered by adsorbed oxygen atoms. The adsorbed oxygen atoms are situated 0.6 ± 0.1 Å below the midpoint between two adjacent atoms in a 〈100〉 surface row. The results of the measurements of the ion impact desorption of adsorbed oxygen suggest a dominating contribution of sputtering processes. Ion focussing effects also contributes to the oxygen desorption. The ion induced and the spontaneous oxygen adsorption processes are studied using different experimental methods. Sticking probability values obtained during ion bombardment show a strong increase due to the ion bombardment.     

The adsorption of CO and oxygen on Cu layers on a W(110) surface

The adsorption of CO, and to a lesser extent that of oxygen on Cu layers deposited on a W(110) surface has been investigated by thermal desorption. Auger, and XPS measurements. For CO the amount adsorbed decreases monotonically with Cu thickness from 1–5 layers. For O there is a slight increase for 1 layer, followed by a steep decrease up to 4 Cu layers where the amount adsorbed levels off. CO adsorption shifts the core levels of Cu (observed for 1 layer of Cu) to higher binding energy by 0.4 eV; the O 1s level of CO is also shifted to higher binding energy by 1.5 eV, relative to CO/W(110) suggesting that electron transfer from CO occurs but is passed on to the underlying W. For O adsorption there is very little shift in the Cu core levels or in the O 1s level, relative to O/W(110). Thermal desorption of CO at saturation coverage from Cu/W(110) shows desorption peaks at 195, 227 and 266 K, as well as small peaks associated with CO desorption from clean W, namely a peak at 363 K and β-desorption peaks at 1080 and 1180 K. As CO coverage is decreased the 195 and 227 K peaks disappear successively; the W-like peaks remain unchanged in intensity. It is argued that the latter may be due to adsorption on bare W at domain boundaries of the Cu overlayer, while the 190–266 K peaks are associated with adsorption on Cu, but probably involve reconstruction of the Cu layer. For n = 2–8 a single but composite peak is seen, shifting from 180 to 150 K as Cu thickness increases as well as a minor peak at 278 K, which virtually vanishes on annealing the Cu deposit at 850 K. The effect of tungsten electronic structure on the behavior of adsorbates on the Cu overlayers, as well as similar effects in other snadwich systems are discussed.     

Determination of a Cu(110) surface contraction by leed intensity analysis

Normal incidence LEED intensity spectra from six symmetrically inequivalent diffraction beams from a clean Cu(110) surface are presented and analyzed. The analysis has employed the results of two distinct sets of dynamical LEED calculations, which differ only in the choice of the electron scattering potential. It is indicated by the analysis that the Cu(110) surface region is in lateral registry with the truncated bulk, the first atomic layer is contracted toward the second by 10.0 ± 2.5% relative to the bulk spacing, and the second to third layer spacing is within 2.5% of the bulk value. A discrepancy of unknown origin has been found between the experimental and calculated I–V proflies for three beams in the 60 eV region; however, this discrepancy does not appear to affect the crystallographic conclusions of the analysis.     

Structural analysis of oxygen segregated Nb(110) surface by photoelectron diffraction

The atomic structure of the Nb(110) surface with oxygen segregated from bulk by annealing at 1500K in UHV has been analyzed by electron and photoelectron diffraction techniques. The observed LEED patterns indicate the modulation of topmost Nb layer in [11ˉ0] direction. Relation of unit cells of substrate and overlayer was determined. The surface component in the Nb 3d photoelectron spectra has a chemical shift of 1.6eV corresponding to that of NbO. The thickness of Nb oxide overlayer is estimated to be about one or two atomic layers. The Nb 3d spectra collected at 4945 different directions are fitted with bulk and surface components. Photoelectron diffraction patterns from Nb atoms within overlayer as well as bulk are successfully extracted. Topmost oxygen atoms are found to be located at about 1.2 Å above the Nb plane. Presented at the X-th Symposium on Suface Physics, Prague, Czech Republic, July 11–15, 2005.     

The azimuthal dependent oxidation process on Cu(110) by energetic oxygen molecules

We report a study on the oxidation process induced by a hyperthermal oxygen molecular beam (HOMB) on Cu(110) using X-ray photoemission spectroscopy in conjunction with a synchrotron radiation source. The oxidation process induced by energetic O₂ beams on Cu(110), depending on the azimuthal angle of incidence, suggests that the –Cu–O– added row structure has a role in inhibiting adsorption as a steric obstacle for incident O₂ molecules.     

Effect of oxygen gas pressure on orientations of Cu2O nuclei during the initial oxidation of Cu(100), (110) and (111)

The orientations of oxide nuclei during the oxidation of Cu(100), (110) and (111) surfaces have been examined by in situ transmission electron microscopy. Our results indicate that the epitaxial nucleation of oxide islands on these surfaces cannot be maintained for a whole range of oxygen gas pressure varying from 10^? 5 Torr to 750 Torr. The critical oxygen gas pressure, pO₂, leading to the transition from nucleating epitaxial to non-epitaxial oxide nuclei shows a dependence on the crystallographic orientations of the Cu substrates with pO₂⁽¹⁰⁰⁾ > pO₂⁽¹¹¹⁾ > pO₂⁽¹¹⁰⁾. By fitting the experimentally determined critical oxygen pressures to a kinetic model, we find that such dependence can be attributed to the effect of surface orientations of the Cu substrates on the oxygen surface adsorption and diffusion, which dominate the kinetic processes of oxide nucleation.     

Initial growth of Cu films on oxygen covered W(110)

The growth of Cu films on the p(2 × 1)O-W(110) and (1 × 1)O-W(110) surfaces has been monitored using specular He scattering. Deposition at temperatures below 200 K leads to rough films on both surfaces. Deposition at T > 300 K leads to phase separation of Cu and O on the p(2 × 1)O-W(110) surface. Deposition at 300 K on the higher oxygen coverage surface leads to films which are nearly as smooth as these deposited on clean W(110). Deposition at 800 K leads to needle-like crystallite formation without an initial continuous film.     

Hydrogen-induced self-organized nanostructuring of the Ir(100) surface

We show that the Ir(100) surface forms a new nanostructure in a self-organized way when its reconstructed equilibrium surface is exposed to hydrogen. Scanning tunneling microscopy and quantitative low-energy electron diffraction retrieve that a long-range ordered superlattice of defect-free Ir chains with average lateral spacing of 1.36 nm and micrometer lengths develops. This can be used as a template for the formation of other nanostructures as is demonstrated.