Ion-scattering structure studies of UO2 surfaces

Planar (111) and stepped (553) surfaces of single crystalline UO₂ have been studied by low-energy He⁺ ion scattering spectroscopy, (ISS). Angular dependent ISS and comparison data from oxide-coated, polycrystalline U indicate that the outermost layer of UO₂(111) is oxygen occupying, to a first approximation, the lattice position expected from a simple termination of bulk structure. A similar study of UO₂(553) indicates that ledge U is covered with excess oxygen.     

Flash desorption and equilibration of H2 and D2 on single crystal surfaces of platinum

The adsorption and flash desorption of hydrogen and the equilibration of H₂ and D₂ has been studied on the (110), (211), (111) and (100) planes of platinum. Desorption from Pt (211), a stepped surface composed of (111) and (100) ledges, yields a desorption spectrum which apparently is a composite of desorption from the individual ledges. Pt (110) is quite similar to the tungsten structural analog, W (211), in that both yield two-peak desorption spectra, and on both planes adsorption kinetics are dramatically different for filling of the two states. On all four planes adsorption kinetics are apparently proportional to (1 ? θ)², and estimates of the initial sticking probabilities show them to decrease in the order: (110) > (211) > (100) > (111). Equilibration activity follows approximately the same order [(110) > (211) > (111) > (100)] with a factor of ~ 5 difference between the most and least active planes; no extraordinary activity is observed for the stepped surface, Pt(211). Below ~ 570 K equilibration of H₂ and D₂ is activated by less than 2 kcal/mole with the magnitude dependent on the specific face, and above this temperature the reaction is nonactivated. The non-activated case apparently results from absorption followed by statistical mixing on the surface. Calculated rates for HD production per cm² based on this model are in excellent agreement with the experimental values for Pt(110) and Pt(211), and in somewhat poorer agreement in the case of Pt (111) and Pt (100). This latter is probably due to the greater inaccuracy in the values of the sticking coefficients on these planes.     

Reconstruction of heterogeneous single-channel neutralization kinetics by the method of fast-ion grazing scattering from metal surfaces

A method is suggested for the unambiguous reconstruction of the heterogeneous slow-ion neutralization kinetics near the surface of a conductor. The method is based on the special features of fast ion grazing scattering with above-thermal energies of translational motion along the normal. It is shown that the angular distributions of fast particles reflected from the surface are related to the slow-ion neutralization rate by a simple algebraic expression. The method allows the reconstruction of the coordinate dependence for the neutralization rate and the interaction potentials. Its possibilities are demonstrated by the example of neutralizing fast He⁺ ions (ion energies E₁≈2 keV and glancing angles θ₀≈0.5°–0.8°) scattered from the A1(111) surface.     

Characterization of oxygen,carbon, and sulfur adlayers on W(211)

Adlayers of oxygen, carbon, and sulfur on W(211) have been characterized by LEED, AES, TPD, and CO adsorption. Oxygen initially adsorbs on the W(211) surface forming p(2 × 1)O and p(1 × 1)O structures. Atomic oxygen is the only desorption product from these surfaces. This initial adsorption selectively inhibits CO dissociation in the CO(β₁) state. Increased oxidation leads to a p(1 × 1)O structure which totally inhibits CO dissociation. Volatile metal oxides desorb from the p(1 × 1)O surface at 1850 K. Oxidation of W(211) at 1200 K leads to reconstruction of the surface and formation of p(1 × n)O LEED patterns, 3 ? n ? 7. The reconstructed surface also inhibits CO dissociation and volatile metal oxides are observed to desorb at 1700 K, as well as at 1850 K. Carburization of the W(211) surface below 1000 K produced no ordered structures. Above 1000 K carburization produces a c(6 × 4)C which is suggested to result from a hexagonal tungsten carbide overlayer. CO dissociation is inhibited on the W(211)?c(6×4)C surface. Sulfur initially orders into a c(2 × 2)S structure on W(211). Increased coverage leads to a c(2×6)S structure and then a complex structure. Adsorbed sulfur reduces CO dissociation on W(211), but even at the highest sulfur coverages CO dissociation was observed. Sulfur was found to desorb as atomic S at 1850 K for sulfur coverages less than $\text{7}\text{6}$ monolayers. At higher sulfur coverages the dimer, S₂, was observed to desorb at 1700 K in addition to atomic sulfur desorption.     

The angular dependence of flux,mean energy and speed ratio for D2 molecules desorbing from a Ni(111) surface

Experimental data are presented for the angular dependence of the relative flux, the mean energy and the speed ratio of deuterium molecules desorbing from a Ni(111) crystal surface at a surface temperature of T_s = 1143 K and at sulphur coverages ranging between 30% and less than 2% of a monolayer.The angular flux distribution is sharply peaked in the forward direction (cos^dθwith 3 ? d ? 5) and the mean energy 〈E〉 of the desorbate depends strongly on the desorption angle θ. For normal desorption $\text{(θ = 0°)}\text{〈E〉}\text{2k}$ is about 700 K higher than T_s and for glancing angles (θ = 80°) it decreases to about 400 K below T_s The results obtained on sulphur free and sulphur covered Ni(111) surfaces are compared with our former data on polycrystalline nickel. The main differences in the kinetic features can be ascribed to the surface roughness. Accordingly, the angular distributions of flux, mean energy, and speed ratio, which deviate strongly from the Knudson and Maxwellian law, do not seem to depend considerably on sulphur coverage and surface structure. A qualitative explanation for these deviations is presented using the principle of detailed balancing.     

The adsorption of xenon by W(111), and its interaction with preadsorbed oxygen

The physisorption of Xe on W(111) and of Xe on partial layers of oxygen chemisorbed on W(111) has been studied using flash desorption and work function methods. It has been found that xenon adsorbs up to monolayer coverages at 104K. Xenon desorbs from W(111) as a single binding state following first order kinetics. At low coverages (θ_Xe < 0.07) the binding energy decreases with increasing coverage possibly because of the presence of high energy adsorption sites due to crystal imperfections and edge effects. For θ_Xe > 0.07 the desorption data fit a first order rate expression with a desorption energy of 9.3 kcal/mol and preexponential ν = 10¹⁵s^?1. The observed work function change of ?1.1 ± 0.1 eV is consistent with monolayer estimates reported in field emission studies of physisorbed xenon on tungsten. The effect of preadsorbed oxygen layers on the physisorption of xenon on this surface is very striking. The energy of desorption shifts as much as 50% higher for a moderate exposure of oxygen. Several physisorption models are explored along with estimates of dispersion and electrostatic interaction contributions.     

Silver sulphide growth on Ag(111): A medium energy ion scattering study

The interaction of S₂ with Ag(111) under ultra-high vacuum conditions has been investigated by medium energy ion scattering (MEIS). 100 keV He⁺ MEIS measurements provide a direct confirmation of a previous report, based on thermal desorption, that the growth of multilayer films of Ag₂S occurs through a continuous corrosion process. These films show a commensurate (√7 × √7)R19° unit mesh in low energy electron diffraction, consistent with the epitaxial growth of (111) layers of the high-temperature F-cubic phase of Ag₂S. The substantial range of co-existing film thicknesses found indicates that the growth must be in the form of variable-thickness islands. The use of 100 keV H⁺ incident ions leads to a very rapid decrease in the sulphide film thickness with increasing exposure that we attribute to an unusual chemical leaching, with implanted H atoms interacting with S atoms and desorption of H₂S from the surface.     

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.     

A single crystal study of the initial stages of silver sulphidation: The chemisorption and reactivity of molecular sulphur (S2) on Ag(111)

Molecular sulphur undergoes rapid dissociative chemisorption on Ag(111) with an essentially constant sticking probability of unity up to the completion of the first layer of S atoms. At this stage a $\text{(√39 R 16.1° × √39}\text{R}\text{?}\text{16.1°)}$ structure is formed in which the S atom arrangement and spacing is similar to that in the (100) plane of γ-Ag₂S (the high temperature form of silver sulphide). Further dosing with S₂ leads to continued rapid uptake of sulphur and the appearance of a (√7 × √7) R 10.9° structure, the Auger, Δφ and thermal desorption data all indicate that fast formation of Ag₂S now occurs. Very well-ordered growth of γ-Ag₂S(111) is now observed, and low-temperature S₂ desorption spectra appear which show that the activation energy for S₂ desorption is ~175 kJ mol^?1 ; this value is in excellent agreement with that observed for the enthalpy of decomposition of bulk Ag₂S (2 Ag₂S(s) → 4 Ag(s) + S₂(g), ΔH = +179 kJmol^?1). All the properties of the Ag(111)-S system imply that the material characterised by the √39 structure (i.e. the first adsorbed layer of S) is very different from bulk Ag₂S. This is discussed and compared with the results of other studies on metal-sulphur systems.     

Velocity dependence of azimuthal anisotropies in ion scattering from rhodium {111}

The scattering of He⁺, Ne⁺ and Ar⁺ ions from Rh {111} is measured as a function of the azimuthal angle of the primary ion for an incident polar angle of 70° from the surface normal and an inplane collection angle of 60°. In this case anisotropy is defined as the ratio of the yield of ions scattered having the azimuth of 〈110〉 to the yield of those having the azimuth of 〈211〉. The yield ratio for all particle types correlates with particle velocity. The ratio is ～ 1 at low velocities, decreases to ～ 0.2 at 8 × 10⁶$\text{cm}\text{s}$ and then increases to a value of 1.4 at 25 × 10⁶$\text{cm}\text{s}$. Molecular dynamics calculations have been performed for Ne⁺ ion scattering from Rh{111} in order to understand the changes in anisotropy with particle velocity. Qualitative agreement with the experimental results is obtained without having to account for neutralization. A neutralization probability that depends on the collision time improves the agreement between the calculated and experimental yield ratios. A velocity dependent probability will not affect the ratio of yields in two different azimuthal directions.     

Adsorption of hydrogen on a Pt(111) surface

The H₂/Pt(111) system has been studied with LEED, ELS, thermal desorption spectroscopy and contact potential measurements. At 150 K H₂ was found to adsorb with an initial sticking coefficient of about 0.1, yielding an atomic H:Pt ratio of about 0.8:1 at saturation. H₂/D₂ exchange experiments gave evidence that adsorption is completely dissociative. No exrea LEED spots due to adsorbed hydrogen were observed, but the adsorbate was found to strongly damp the secondary Bragg maxima in the I/V spectrum of the specular beam. The primary Bragg maxima were slightly increased in intensity and shifted to somewhat lower energy. A new characteristic electron energy loss at ?15.4 eV was recorded upon hydrogen adsorption. The thermal desorption spectra were characterized by a high temperature (β₂-) state desorbing with second order kinetics below 400 K and a low temperature (β₂-) state that fills up, in the main, after the first peak saturates. The β₂-state is associated with an activation energy for desorption E1 of 9.5 kcal/mole. The decrease E1 with increasing coverage and the formation of the β₁-state are interpreted in terms of a lateral interaction model. The anomalous structure in the thermal desorption spectra is attributed to domains of non-equilibrium configuration. The work function change Δ? was found to have a small positive maximum (～ 2 mV) at very low hydrogen doses (attributed to structural imperfections) and then to decrease continuously to a value of ?230 mV at saturation. The variation of Δ? with coverage is stronger than linear. The isosteric heats of adsorption as derived from adsorption isotherms recorded via Δ? compared well with the results of the analysis of the thermal desorption spectra.     

Desorption of halogens from some Nb,Mo, Ta and W single crystal surfaces

A systematic investigation of the thermal desorption of halogens from well characterized (111), (100) and (110) 4d (Nb, Mo) and 5d (Ta, W) transition metal surfaces has been carried out under low coverage conditions (θ < 10^?2 of a monolayer). Characterization of the surfaces was achieved by LEED, AES and work function determinations while the desorption kinetics were recorded in a large temperature range (1700–2300 K) using a pulsed ionic beam method. The new data concerning some Ta and W surfaces are presented and the results of this systematic study are discussed. It is shown that the halogen desorption parameters, e.g., desorption energies and preexponential factors, are independent of both surface structure and d bond filling of the substrate; E(F) ～4.75 eV, E(Cl) ～4.1 eV, E(Br) ～3.7 eV and τ₀ ～10^?13 ?10^?14 S. The halogen behaviour is compared with that of other adsorbates and with the predictions of a general chemisorption model.     

Chemisorption of CO on tungsten (100): Combined flash desorption and electron stimulated desorption study. II

The chemisorption of CO on W(100) at ~ 100K has been studied using a combination of flash desorption and electron stimulated desorption (ESD) techniques. This is an extension of a similar study made for CO adsorption on W(100) at temperatures in the range 200–300K. As in the 200–300 K CO layer, both α₁-CO and α₂-CO are formed in addition to more strongly bound CO species upon adsorption at ~ 100K; the α-CO states yield CO⁺ and O⁺ respectively upon ESD. At low CO coverages, the α₁ and α₂-CO states are postulated to convert to β-CO or other strongly bound CO species upon heating. At higher CO coverages, α₁-CO converts to α₂-CO upon thermal desorption or electron stimulated desorption. There is evidence for the presence of other weakly-bound states in the low temperature CO layer having low surface concentration at saturation. The ESD behavior of the CO layer coadsorbed with hydrogen on W(100) is reported, and it is found that H(ads) suppresses either the concentration or the ionic cross section for α₁ and α₂-CO states.     

Fast-ion channeling in fullerene crystals

The motion of H⁺ and He⁺ ions with energies of 230 and 2000 keV in C₆₀ and K₃C₆₀ crystals was calculated by the Monte Carlo method. Ion channeling was shown to occur in the 〈100〉 and 〈111〉 directions. The main parameters characterizing channeling were determined. Medium-energy ions were found to be preferable in the detection of channeling in C₆₀ films.     

Adsorption-desorption properties,coadsorption, and surface structural chemistry of chlorine and oxygen on Ag(331)

At 300 K oxygen chemisorbs on Ag(331) with a low sticking probability, and the surface eventually facets to form a (110)?(2 × 1) O structure with ΔΦ = +0.7 eV. This facetting is completely reversible upon O₂ desorption at ~570 K. The electron impact properties of the adlayer, together with the LEED and desorption data, suggest that the transition from the (110) facetted surface to the (331) surface occurs at an oxygen coverage of about two-thirds the saturation value. Chemisorbed oxygen reacts rapidly with gaseous CO at 300 K, the reaction probability per impinging CO molecule being ~0.1. At 300 K chlorine adsorbs via a mobile precursor state and with a sticking probability of unity. The surface saturates to form a (6 × 1) structure with ΔΦ = +1.6 eV. This is interpreted in terms of a buckled close-packed layer of Cl atoms whose interatomic spacing is similar to those for Cl overlayers on Ag(111) and Ag(100). Desorption occurs exclusively as Cl atoms with E_d ~ 213 kJ mol^?1; a comparison of the Auger, ΔΦ, and desorption data suggests that the Cl adlayer undergoes significant depolarisation at high coverages. The interaction of chlorine with the oxygen predosed surface, and the converse oxygen-chlorine reaction are examined.     

Chlorine chemisorption and surface chloride formation on Au(111)

The adsorption-desorption properties of the gold(111)-chlorine system have been investigated. Thermal desorption experiments following chlorine adsorption at 298 K indicated two desorption processes: the high temperature peak (ΔH = 217 kJmol^?1) showed desorption of equal numbers of molecukr and atomic chlorine species, while the lower temperature peak (ΔH = 140 kJmol^?1) was due to the desorption of Cl₂ only. Chlorine adsorption led to a maximum work function change of +0.5 V and electron-stimulated desorption proceeded with constant cross-section (1.4 × 10¹⁸ cm²), until all chlorine had been removed from the surface These observations are consistent with the immediate formation of a surface chloride (AuCl₃) during chlorine adsorption on Au(111) at 298 K without the intervention of an initial adsorbed overlayer.     

Cu/ZnO(0001&#x0304;) and ZnOx/Cu(111): Model catalysts for methanol synthesis

In an attempt to understand the relative importance of the various constituents of copper-zinc oxide catalysts for methanol synthesis (2H₂ + CO → H₃COH), we have prepared and characterized a number of single-crystal surface structures of Cu-ZnO. The model catalysts have also been tested in terms of their activity for methanol synthesis. The growth of vapor-deposited Cu overlayers on a ZnO(0001&#x0304;) (O face) single crystal has been investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), He⁺ ion scattering spectroscopy (ISS) and low-energy electron diffraction (LEED). The results are consistent with a growth model in which, at room temperature, the first monolayer spreads uniformly across the surface in a p(1 × 1) structure. As more Cu is added, thick Cu(111) islands grow and these are separated by large regions of the p(1 × 1)-Cu monolayer. The Cu(111) islands are rotationally aligned with the ZnO substrate, and at high enough coverages grow together to cover the ZnO. Increasing temperature favors more agglomeration. A clean Cu(111) crystal and one containing a ZnO_x (x ⋍ 3) monolayer were also studied. None of these model catalysts gave rates of methanol production which were measurable in our present experimental limits (TOF < 2 × 10⁻³ molecules site⁻¹ s⁻¹) at 500–600 K and CO + H₂ pressures up to 1500 Torr. Under these “reaction” conditions, the Cu in direct contact with ZnO may be slightly oxidized; all the other Cu is completely metallic. The Cu does not change its character between deposition and reaction conditions, even if heavily oxidized to CuO in between. The addition of CO₂ at very high levels under reaction conditions does not change the character of a Cu(111) model catalyst surface, and no surface oxygen is ever observable after treatment under reaction conditions.     

Two-spectator quasifree processes

Two-spectator quasifree processes (graphs with two spectator particles) for the ³He(³He, dd)pp reaction at the symmetric angle pairs θ = 30.1° at E_³He = 50 MeV and θ = 37° at E_³He = 78 MeV are investigated. The theoretical cross sections are calculated with the PWIA model. The measured cross sections are smaller but have shapes which are in good agreement with the calculation. The ratio N = experiment/theory is approximately 0.05 at 50 MeV and 0.1 at 78 MeV. The reaction ²H(d, pp)nn also was studied at the symmetric angle pair θ = 34.8° and E_²H = 34.7 MeV and the ratio is about 0.14.     

Partial virial theorems for atomic-molecular systems

New virial relations for three-and four-particle atomic-molecular systems are proposed. Using operators of extension or squeezing of interparticle distances, it is shown that, for all pairs of j and k particles in S states of these systems, the following partial virial relations are valid: 〈2T _jk 〉+〈 V _jk 〉=0, where 〈V _jk 〉 is the average Coulomb interaction energy for a pair of particles and 〈T _jk 〉 is a part of the average kinetic energy of the system. There are three and six such relations for three-and four-particle systems, respectively. The conventional virial theorem (〈 2T〉+〈V〉=0) for the average total kinetic and potential energies of the system (〈 T〉 and 〈V〉, respectively) corresponds to the summation of partial virial relations over all pairs of particles. It is shown by an example of variational calculations of the helium atom ⁴He²⁺ e ^? e ^? and the helium muon-electron mesoatom ⁴He²⁺μ^? e ^? that partial virial relations are a highly sensitive indicator of the accuracy of wave functions.     

Surface chemistry of the metal-halogen interface: Bromine chemisorption and dibromide formation on palladium (111)

At 300 K and in the coverage regime ($\text{0<θ<}\text{1}\text{3}$) bromine chemisorbs rapidly on Pd(111); the sticking probability and dipole moment per adatom remain constant at 0.8 ± 0.2 and 1.2 D, respectively. This stage is marked by the appearance of a √3 structure: desorption occurs exclusively as atomic Br. At higher coverages, desorption of molecular Br₂ begins (desorption energy ～130 kJ mol^?1) as does the nucleation and growth of PdBr₂ on the surface. This latter stage is signalled by the appearance of a √2 LEED pattern and the observation of PdBr₂ as a desorption product (desorption energy ～37 kJ mol^?1). Some PdBr₂ is also lost by surface decomposition and subsequent evaporation of atomic Br. The data indicate that the transition state to Br adatom desorption is localised and that PdBr₂(a) ? Br(a) interconversion occurs; these surface species do not appear to be in thermodynamic equilibrium during the desorption process.