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.     

The adsorption of nitric oxide on W(110) at 300 K: An electron spectroscopic study

Nitric oxide adsorbs dissociatively onto W(110) at 300 K. The simplest interpretation requires adsorption into three states. In the first of these N and O penetrate the surface. In the second state N and O form an overlay er on the W(110) surface. The third state also involves surface penetration but this is deeper than for state 1. The adsorbate stoichiometry was 1:1 throughout adsorption at 300 K. The occupancy of each state was similar after a 300 L exposure when the total amount adsorbed exceeded 17 (±2) × 10¹⁸ atoms m^?2.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

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     

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.     

Photoemission study of the adsorption of O2, CO and H2 on GaAs(110)

Ultraviolet photoemission spectroscopy with hv < 12 eV has been used to study O₂, CO, and H₂ adsorption on the cleaved GaAs(110) face. It was found that O₂ exposures above 10⁵ L(1LM = 10^?6 Torr sec) were required to produce changes in the energy distribution curves. At O₂ exposures of 10⁶ L on p-type and 10⁸ L on n-type an oxide peak is observed in the EDC's located 4 eV below the valence band maximum. On p-type GaAs, O₂ exposures cause the Fermi level at the surface to move up to a point 0.5 eV above the valence band maximum, while on n-type GaAs O₂ exposures do not remove the Fermi level pinning caused by empty surface states on the clean GaAs. CO was found to stick to GaAs, but to desorb over a period of hours, and not to change the surface Fermi level position. H₂ did not affect the EDC's, but atomic H lowered the electron affinity and raised the surface position of the Fermi level on p-type GaAs. A correlation is found in which gases which stick to the GaAs cause an upward movement of the Fermi level at the surface on p-type GaAs, while gases which stick only temporarily do not change the surface position of the Fermi level.     

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 ferromagnetic monolayer Fe(110) on W(110)

Ferromagnetic order in the pseudomorphic monolayer Fe(110) on W(110) was analyzed experimentally using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). The monolayer is thermodynamically stable, crystallizes to large monolayer patches at elevated temperatures and therefore forms an excellent approximation to the ideal monolayer structure. It is ferromagnetic below a Curie-temperatureT _c,mono, which is given by (282±3) K for the Ag-coated layer, (290±10) K for coating by Cu, Ag or Au and ≈210 K for the free monolayer. For the Ag-coated monolayer, ground state hyperfine fieldB _hf (0)=(11.9±0.3) T and magnetic moment per atom μ=2.53 μ_B could be determined, in fair agreement with theoretical predictions. Unusual properties of the phase transition are detected by the combination of both experimental techniques. Strong magnetic anisotropies, which are essential for ferromagnetic order, are determined by CEMS.     

Surface crystallography of W(110) and W(110) p (2×1)-O: The position of W atoms

The position of W atoms in the surface layers of clean W (110) and W (110) p (2 × 1)-O is studied using Constant-Momentum-Transfer Averaging of LEED intensities. It is shown that the clean surface is not relaxed to an uncertainty of < 0.06 Å. Analysis of superstructure beam intensity averages from W(110) p (2 × 1)-O indicates that oxygen does not reconstruct W(110) at these coverages and below 1000 ° K. An upper limit of 0.05 Å can be put on the out-of-plane displacement of W atoms by the oxygen. Substrate beam averages from W (110) p (2 × 1)-O verify the non-reconstruction. The use of CMTA for adsorbed-layer crystallography in general is briefly 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.     

The reactive adsorption of CCl4 on GaAs(100) surfaces at 300 K

Auger electron spectroscopy has been used to monitor the adsorption of CCl₄ on an As-rich GaAs(100) surface at 300 K. Intensities of the Ga (55 eV), As (34 eV), C (270 eV) and Cl (181 eV) transitions have been used to estimate surface number densities at saturation and relative C : Cl stoichiometry of the surface species. Number densities of (4.3 ± 0.2) × 10¹⁴ and (2.0 ± 0.2) × 10¹⁴ cm ⁻² are obtained for carbon and chlorine respectively, suggesting that coverage saturates near one theoretical monolayer and that the C : Cl stoichiometry is approximately 2:1. These data are discussed in terms of a reactive adsorption mechanism.     

Theoretical investigation of the influence of isotope mass on chemicurrents during adsorption of H on K(110)

Using our recently developed method for calculation of electron–hole (e–h) spectra in adsorption on metal surfaces [Phys. Rev. B 19:165407, 2009], we investigate the system H/K(110). Comparing to our previous results for H/Al(111), we show that the narrower conduction band of K in contrast to Al leads to notable differences in the excitation spectra of electrons and holes. We also find that our results do not obey the scaling of the number of excited charge carriers above a certain energy barrier with the particle's velocity, which is in our case mainly depending on the isotope mass. Instead, we find a different (approximately m^1/6 rather than m^1/2) scaling. Extrapolating our results to adsorbates with large masses, we expect larger electronic excitations than from the “forced oscillator” approach. This makes the electronic dissipation channel for energy more important even for heavy adsorbates. Our results are in qualitative agreement with other theoretical and experimental results.     

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.