Coadsorption of cesium and oxygen on Ni(100): I. Cesium probing of NiO bonding

Low energy electron diffraction (LEED) and work function measurements have been used to study the interaction of Cs and O on (100) surfaces of Ni. Deposition of Cs on oxygen-chemisorbed Ni(100) changed the structure of the O array. The work function of this surface showed a minimum at 0.14 monolayer of Cs coverage and a maximum at 0.29 monolayer. On the other hand, Cs on oxidized Ni(100) could not change the oxygen arrangement at all, and the work function remained constant after reaching a minimum value. These two types of behavior of Cs can be used to probe the bonding states of O on metal surfaces. Our results suggest that the c(2×2) structure of O on Ni(100) is simple chemisorption rather than reconstruction and that the c(2×2) coexists with regions of disordered NiO on Ni(100). While LEED intensity variations detect primarily the chemisorbed c(2×2), other measurements like work function, ion neutralization and sticking coefficient should detect both kinds of NiO bonding.     

Adsorption of oxygen on W(110): I. The p(2 × 1) structure

The adsorption of oxygen on W(110) for ccverages below 0.5 monolayer has been studied with a number of complementary techniques. Data on adsorption kinetics, LEED intensities, work function changes and desorption kinetics have been used to propose a model combining statistical adsorption and island growth for the formation of the p(2 × 1) structure. On the basis of the measurements it is concluded that the surface is reconstructed for θ < 0.3 monolayer after heating to T > 2000 K, and for θ < 0.1 monolayer for adsorption at 300 K.     

Coadsorption of cesium and oxygen on Ni(100): II. Cesium enhanced chemisorption and oxidation

The effects of adsorbed Cs atoms on the chemisorption and oxidation of Ni(100) surfaces have been studied with low energy electron diffraction and work function measurements. In addition to the c(2×2) structure of O on clean Ni(100), the preadsorption of Cs caused the formation of a (3×3) and a c(4×2) structure. The experimental results suggest that these new structures were due to ordered arrays of chemisorbed O atoms underneath the Cs layer, with O densities higher than that of c(2×2). It is found that a Cs overlayer increased drastically the rate of O chemisorption and NiO formation. Depending on the initial Cs coverage, the NiO formed in the (100) and (111) crystallographic rientations. During the enhanced oxidation the Cs layer remained on top of the oxide.     

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.     

The adsorption of oxygen on Pd1/W(110)

The behavior of oxygen on Pd₁/W(110) has been investigated from 25 to 200 K by thermal desorption, UPS, XPS, and work function measurements. At 25 K only dioxygen species are present. A weakly bound O₂layer, containing O₂/Pd = 0.31 or 4.4 × 10¹⁴ O₂ molecules/cm² is desorbed at 35 K, leaving a coverage of O₂/Pd = 0.35 or 5 × 10¹⁴ O₂ molecules/cm². Heating to 200 K results in desorption of molecular O₂ as well as conversion to O, with O/Pd = 0.3. The molecular states, except the very weakly bound one, have high dipole moments with electron transfer to O₂, and thus correspond to Superoxide and peroxide species. These have UPS spectra quite different from physisorbed O₂. At 90 K adsorption is still mostly molecular with a sticking coefficient s near unity. At 200 K, adsorption is atomic with an initial s₀ = 0.8. This must be contrasted with Cu₁/W(110) where s₀ is unity even at 300 K. The difference can be explained by the much better size match of Pd and W, than Cu and W which makes it easier for Cu to take up momentum of impinging O₂ molecules.

The behavior of oxygen on Pd₁/W(110) is very similar to that on bulk Pd(111), suggesting that for oxygen the former surface resembles bulk Pd. This is not so for CO adsorption which is much weaker on Pd₁/W(110) than on bulk Pd. The reasons for this difference are not presently understood.     

Vibrational modes of oxygen on W(110)

Electron energy loss measurements of the vibrational modes of oxygen on W(110) as a function of coverage up to 0.5 monolayer are presented and analyzed. A single loss at 67 meV is observed initially; with increasing exposure this loss shifts to 72 meV and another loss appears at 47 meV. These data indicate coexistence of two species on the surface with a coverage-dependent conversion. Angular profiles of the specular elastic beam show a dramatic increase in width with initial oxygen coverage; this is possibly due to an oxygen-induced static disordering of the W surface layer.     

The adsorption entropy of H on W(110)

Recently, Nahm and Gomer have measured the increase in entropy associated with the adsorption of hydrogen on the W(110) surface. We discuss the implications of this data, and address it within the framework of a model introduced recently [Phys. Rev. Lett. 68 (1992) 2846; Surf. Sci. 287/288 (19930 837] to describe hydrogen dynamics on this surface.     

A LEED investigation of Xe adsorption on W(110)

Four ordered LEED patterns are observed for Xe adsorption on W(110) for temperatures between 77 and 90 K. A (2 × 2) structure with an area per Xe atom of 28.3 Ų is transformed into two coincidence structures which correspond to a disordered (100) Xe layer. The area per Xe atom in these structures is 17.6 and 20.2 Ų. Xe adsorption on oxygen covered W(110) leads to one-dimensional disorder in the structures observed on clean W(110) without the formation of new structures.     

Monte Carlo simulations of hydrogen adsorption on the W(110) and Mo(110) surfaces

Kinetics of low-temperature hydrogen and deuterium adsorption on W(110) and Mo(110) surfaces have been studied by the real-time Monte Carlo simulations. Recently reported qualitative dependence of the adsorption characteristics on variation of the H₂ flux is described in terms of the dynamical equilibrium between incident and desorption fluxes and improved conditions for accommodation for the hydrogen molecules at high incident fluxes. The role of the intrinsic precursor state in hydrogen dissociative adsorption is analyzed.Received: 16 February 2004, Published online: 28 May 2004PACS: 82.65. + r Surface and interface chemistry; heterogeneous catalysis at surfaces - 02.50.Ng Distribution theory and Monte Carlo studies - 82.20.Wt Computational modeling; simulation     

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.     

Adsorption of oxygen on W(110): II. The high coverage range

The structure and composition of the interaction layer between oxygen and a W(110) surface for oxygen coverages θ above 0.5 monolayers is studied with LEED, AES, thermal desorption and work function change measurements. Oxygen is adsorbed by depositing WO₂ followed by annealing. The results are interpreted in terms of a topmost layer consisting only of oxygen atoms followed by the formation of isolated three-dimensional WO₃ crystals after saturation of the two-dimensional oxidation layer at 15 × 10¹⁴ O atoms cm^?2. All available experimental evidence is compatible with this interpretation.     

The adsorption of molecular oxygen species on Ag(110)

The adsorption of oxygen on the Ag(110) surface was examined at temperatures down to 123 K. In addition to the dissociatively adsorbed state which desorbed at 590 K, a second oxygen state desorbed at 190 K following adsorption at 150 K and below. This high temperature state appeared to form prior to the development of the low temperature state. The ratio of coverages of the two states was a strong function of both exposure and adsorption temperature. Isotopic exchange experiments indicated that the low temperature state was molecularly adsorbed. The desorption of the molecularly adsorbed oxygen exhibited complex kinetics due to interaction with adsorbed oxygen atoms.     

Tetraoxygen on reduced TiO2(110): oxygen adsorption and reactions with bridging oxygen vacancies

Oxygen adsorption on reduced TiO2(110) is investigated using temperature programmed desorption and electron-stimulated desorption. At low temperatures, 2 O(2) molecules can be chemisorbed in each oxygen vacancy. These molecules do not desorb upon annealing to 700 K. Instead, for 200 K相似文献   

The adsorption of water on clean and oxygen covered Cu(110)

The water adsorption on clean and oxygen precovered Cu(110) surfaces is studied by means of UPS, LEED, work function measurements and ELS. At 90 K on the clean surface molecular water adsorption is indicated by UPS. The H₂O molecules are bonded at the oxygen end and the H-O-H angle is increased as compared with the free molecule. In the temperature range between 90 and 300 K distorted H₂O molecules and adsorbed hydroxyl species (OH) are detected, which are desorbed at room temperature. On an oxygen covered surface hydroxyl groups are formed by dissociation of adsorbed water molecules at a lower temperature than on the clean surface. Multilayers of condensed water are found below 140 K in both cases.     

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.     

A statistical model for oxygen adsorption on W (211)

A series of previous experimental investigations revealed that oxygen adsorption on a W (211) surface causes, at $\text{θ =}\text{1}\text{2}$, a 2 × 1 LEED pattern whose half order spots are streaked at lower coverages and become continuously weaker at $\text{θ >}\text{1}\text{2}\text{until at}\text{θ = 1}$ a 1 × 1 structure is formed. A model is proposed wherein the configuration of the adsorbed atoms with respect to each other is governed by a set of anisotropic interactions between neighbouring particles. These (three) interaction energies have magnitudes between kT and 2kT. The equilibrium arrangements for various coverages were simulated by means of the Monte Carlo technique, and the corresponding LEED patterns (angular profiles) were evaluated by using the kinematic approximation. Good agreement with experimental data is obtained for the variation of the relative intensity as well as of the breadth of the half order LEED spots, without the need for any further assumptions on the mechanism of the formation of the ordered adsorbed layers.     

A very low energy electron reflection study of hydrogen adsorption on W(100) and W(110) surfaces

Experimental results of the elastic backscattering of electrons with energies between 0 and 20 eV from the surface systems H/W(100) and H/W(110) are reported and interpreted on the basis of the Darwin model. On W(100) the adatom distance from the reconstructed substrate is found to be different from that on the relaxed surface. Large vertical displacements of the substrate atoms during reconstruction can be excluded. On W(110) the results indicate two different binding states which are occupied sequentially and which differ significantly in their distances from the surface. Work function change data are also reported for both systems.     

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.