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.     

Adsorption and dissociation kinetics of alkanes on CaO(100)

The adsorption kinetics of ethane, butane, pentane, and hexane on CaO(100) have been studied by multi-mass thermal desorption (TDS) spectroscopy. The sample cleanliness was checked by Auger electron spectroscopy. A molecular and dissociative adsorption pathway was evident for the alkanes, except for ethane, which does not undergo bond activation. Two TDS peaks appeared when recording the parent mass, which are assigned to different adsorption sites/configurations of the molecularly adsorbed alkanes. Bond activation leads to desorption of hydrogen and several alkane fragments assigned to methane and ethylene formation. Only one TDS feature is seen in this case. Formation of carbon residuals was absent.     

The chemisorption of mercury on tungsten (100): Adsorption and desorption kinetics,equilibrium properties and surface structure

The adsorption of mercury on W(100) has been studied by Auger spectroscopy, LEED and by work function and thermal desorption measurements. Mercury adsorbs at room temperature to form a (1 × 1) monolayer, with a sticking probability of unity and a heat of adsorption of 208 ± 12kJ mol^?1. The coverage dependence of the work function change is interpreted according to an island growth mechanism which is shown to be consistent with the LEED observations. At higher temperatures, the equilibrium isotherms show evidence for attractive adsorbate-adsorbate lateral interactions. The isotherms were simulated using a localised adlayer model within the quasi-chemical approximation. This gave a nearest-neighbour pairwise interaction energy of 5.85 kJ mol^?1. The attractive interactions are shown to be consistent with the mechanism invoked to explain the desorption kinetics, which are zero order.     

Adsorption of oxygen on Si(100) steps: a study at semiempirical level

In this study oxygen adsorption on steps of the Si(100) surface is investigated with quantum mechanical detail. A cluster model represents the step structure and the oxygen impurity is deposited in this cluster above the dimer rows. A map of the total energy sensed by the adsorbate is constructed by displacing the oxygen atom on the terraces and down the step and allowing the system to relax to the total-energy-optimized configuration by a steepest-descent method. A self-consistent semiempirical molecular orbital method is applied to the evaluation of the total energy. Though the adsorption geometries are reminiscent of the ones observed for the flat surface, the effects of the step geometry are noticeable. They influence both the preferred adsorption sites and the properties of the electronic configuration of the adsorbed system.Received: 24 July 2003, Published online: 22 September 2003PACS: 68.43.Fg Adsorbate structure (binding sites, geometry) - 68.90.+g Other topics in structure, and nonelectronic properties of surfaces and interfaces; thin films and low-dimensional structures (restricted to new topics in section 68) - 07.05.Tp Computer modeling and simulation     

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.     

Distorted surface oxygen structure on UO2(100)

Low-energy electron diffraction (LEED) has been combined with ion-scattering spectroscopy (ISS) measurements of He⁺ at 500 eV to characterize experimentally the surface structure formed by oxygen atoms on UO₂(100). Insight into the surface geometry required to generate the LEED features was gained via laser transform simulation and kinematical diffraction analysis of two-dimensional arrays. Integrating the above approaches leads to a UO₂(100) surface model consisting of a monolayer of oxygen atoms arranged in distorted bridge-bond, zig-zag chains along 〈100〉 directions. Configurational energies were calculated which support the distorted UO₂(100) zig-zag structure.     

Surface lattice-dynamical approach to the reconstruction of W(100) and W(100): H

We review the surface reconstruction of the bcc (100) transition metal surface, particularly W(100) and some of the results obtained, with the method of the effective surface lattice dynamics.     

Electron stimulated desorption ion energy distribution (ESDIED) and surface structure: O on W(100)

The yield and energy distributions of O⁺ ions ejected by electron stimulation into selected angular regions is studied as a function of oxygen exposure and annealing temperature of the adsorbate on W(100). Yield and energy distributions are related to coverage and structure via AES, LEED and ISS which indicates that ESDIED measurements supply useful information on adsorbate bonding.     

Computer simulations of electron stimulated desorption patterns from oxygen on W(100) and (111) surfaces

Computer simulations of Electron-Stimulated-Desorption lon-Angular-Distribution patterns (ESDIAD) from oxygen covered W(100) and (111) surfaces were performed, using classcial dynamical formulas for the calculation of the O₊ ion trajectories. A model for the reconstruction of the O covered W(100) surface in the temperature range of 700–900 K is presented. The simulated ESD patterns have been brought into agreement with experimental results from literature by the proper choice: first of the repulsive atomic potentials acting on the ions, second of the rms amplitudes of the O atoms and third of the constants in the formula for Auger neutralization. The angular widths of the ESD spots were fitted by introducing bending vibrations of the chemisorbed O atoms and also by distributing the directions of the chemical bonds in limited angular areas. Assuming Franck-Condon type transitions and neglecting intermediate states, the final ion energies led to distances between the starting positions of the ions and the neighboring W atoms in the range of the known chemical bond lengths. The repulsive atomic potentials, obtained from Hartree-Fock-Slater self-consistent field calculations for W and O atoms in different electronic states and with different electronic charges, were compared with the potentials giving best agreement with the experimental ESDIAD results. In this way, qualitative conclusions concerning the electronic charge at the surfaces were derived.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

Adsorption and decomposition of ammonia on W(100); XPS and UPS studies

The adsorption and decomposition of ammonia on a clean and c(2 × 2)-N ordered W(100) surface has been studied by photoemission spectroscopy (XPS and UPS). At 120 K molecularly adsorbed ammonia was identified by N(1s) core level emission at 400.9 eV and the valence emissions at 7.6 and 11.7 eV. By heating the sample stepwise the N(1s) core level shifted to lower binding energy. In the valence region, the corresponding spectral changes were obtained, where the dependence of the peak intensity on photon energy was observed. These observations were interpreted to demonstrate that adsorbed ammonia dissociates its hydrogen successively to form NH_x(a) and finally to atomic nitrogen. On the other hand, ammonia was molecularly adsorbed on a c(2 × 2)-N ordered surface even at temperatures as high as 300 K, although the spectra at 400 K or above were very similar to those under a steady state flow condition, where the tungsten surface was mostly covered by atomic nitrogen. At higher ammonia pressure up to about 100 Pa thicker nitride layers were formed at 700 K, which were characterized by the N(1s) core level at 397.3 eV and a broad emission around 6 eV in the valence level.     

Adsorption of NO on SNi(100)

Nitric oxide has been studied on sulfur-covered Ni(100) using thermal desorption spectroscopy (TDS), ultraviolet (UPS) and X-ray (XPS) photoelectron spectroscopy, and work function change (Δφ) measurements. Low sulfur coverages reduce the saturation amount of adsorbed NO; each sulfur blocks ≈ 2 NO. The initial sticking coefficient is altered according to S₀ = (1−2θ_s). Adsorbed sulfur also inhibits the dissociation of NO but does not change the desorption energetics of N₂. Above θ_s ≈ 0.38, NO does not dissociate. Changes in the O(1s) binding energy and in Δφ indicate sulfur-induced changes in NiNO bonding consistent with decreased interaction between NO and the metal in the presence of adsorbed sulfur.     

Simultaneous SIMS and EID investigation on the interaction of oxygen with a W (100) surface

A UHV system, containing a beatable tungsten ribbon target (showing [100] planes), an ion source (Ar⁺, 2 keV) with mass separator, an electron source (300 eV), a quadrupole secondary ion mass filter, and a quadrupole gas analyzer is used for the study of the interaction of O₂ with W (100) by simultaneous, i.e. fast interchanging, “static” SIMS (secondary [ion-induced] ion mass spectrometry) and EID (electron-induced [ion] desorption). Two different adsorptive binding states can be distinguished: β₂ and β₁. The O⁺ emission cross section under electron bombardment from the β₂ state is smaller by a factor of about 10³ than from β₁ and is found to be temperature-dependent. After the state β₂ has been saturated and before the occupation of β₁ begins, an oxide formation process starts. This oxidation can be interpreted by a two-stage model.     

Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.