Signature of a chemical bond in the conductance between two metal surfaces

Conductance in monatomic metal contacts is quantized; it increases in discrete steps of one conductance quantum 2e(2)/h. By contrast, in a vacuum barrier between two metal surfaces we find that conductance increases linearly and continuously with the interaction energy between individual atoms. This behavior shows unambiguously that current flow between single atoms is a measure for their chemical interaction. In the controlled environment of a scanning tunneling microscope it should allow us to study the formation of covalent bonds up to the point where these atoms finally jump into contact.     

Rates of desorption from solid surfaces: Coverage dependence

The recently developed Classical Stochastic Diffusion Theory is applied to obtain the coverage dependence of desorption rates for Xe on W(111). Using the attractive Xe-Xe potential from gas phase experiments, we find a strong coverage dependence in the desorption rates and calculate Temperature Programmed Desorption Spectra (for a potential with reduced attractiveness) that are in excellent qualitative agreement with experimental results. We also investigated the effect of purely repulsive adsorbate-adsorbate interactions where we find, for some coverage ranges, that two different adsorption configurations can be stable (at different temperatures) leading to a marked change in the corresponding desorption rates and to distinct non-Arrhenius behavior.     

Simulation of adsorbate-induced faceting on curved surfaces

A simple solid-on-solid model, proposed earlier to describe overlayer-induced faceting of bcc(1 1 1) surface, is applied to faceting of curved surfaces covered by an adsorbate monolayer. Surfaces studied in this paper are formed by a part of sphere around the [1 1 1] pole. Results of Monte Carlo simulations show that the morphology of a faceted surface depends on the annealing temperature. At an initial stage the surface around the [1 1 1] pole consists of 3-sided pyramids and step-like facets, then step-like facets dominate and their number decreases with temperature, finally a single big pyramid is formed. It is shown that there is a reversible phase transition at which a faceted surface transforms to an almost spherical one. It is found that the temperature of this phase transition is an increasing function of the surface curvature. Simulation results show that measurements of high temperature properties performed directly and after fast cooling down to a low temperature lead to different results.     

Band structure and chemical bond in alkali metal carbonates

The band structure, state density, optical functions, and distribution of valence and difference density in alkali-metal carbonates are calculated within the local electron-density functional theory using the method of pseudopotential in the basis of numerical pseudoorbitals. When passing from a lithium cation to a potassium one, the character of hybridization between the crystal sublattices changes to result in an increase in the valence-band width, a decrease in the forbidden-band width, a complication of the structure of state-density spectrum, and a shift of the maxima of optical functions to the low-energy range. It is found that the electron overflow between the σ-and π-orbitals of crystallographically nonequivalent oxygen atoms can occur in different ways, hence their interaction force with the surrounding atoms is different. The role of cations in stabilization of anion chains resulting from the electron-cloud overlapping in lithium and sodium carbonates is shown. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 61–65, October, 2006.     

Secondary-ion emission from oxygen and hydrogen-covered beryllium surfaces: I. Coverage dependence

Secondary ion emission from a beryllium surface is studied in the presence of hydrogen and oxygen. For submonolayer coverages of oxygen, the adsorption follows site-exclusion statistics. On this basis, the Be⁺ secondary ion yield and energy distribution can be separated into oxygen-dependent and oxygen-independent processes governed by the local properties of the surface. Ionic molecular species with hydrogen or oxygen present include Be₂O⁺, Be₂O^?, BeO^?, and BeH⁺ but not BeO⁺ or BeOH⁺. This result is inconsistent with emission in which the oxide molecule is formed in the near-surface vacuum region by agglomeration of individually sputtered surface atoms. For BeH⁺emission the opposite conclusion is reached.     

On the bond lengths reported for chemisorption on metal surfaces

The bond lengths and geometrical arrangements reported with multiple-scattering analyses of LEED intensities for chemisorption on metal surfaces are assessed with Pauling's bond length-bond order relation. The approach here is partly empirical and it depends on information on the atomic valencies; the hybridisation model proposed for metals by Altmann, Coulson and Hume-Rothery is also used to guide the allocation of bonding electrons for the purposes of estimating the orders of surface bonds. It is shown that this framework allows estimates of surface bond lengths to within 0.1 Å of the values reported with LEED, and often the correspondence is substantially closer. Other factors that are likely to influence surface bond lengths have been recognised, but refinements to the present analysis should probably await clarifications of uncertainties in the structures determined with LEED including, for some chemisorption systems, consideration of possible displacements of metal atoms.     

Electronic excitations by chemical reactions on metal surfaces

Dissipation of chemical energy released in exothermic reactions at metal surfaces may happen adiabatically by creation of phonons or non-adiabatically by excitation of the electronic system of the metal or the reactants. In the past decades, the only direct experimental evidence for such non-adiabatic reactions has been exoelectron emission into vacuum and surface chemiluminescence which are observed in a special class of very exothermic reactions. The creation of e–h pairs in the metal has been discussed in many theoretical models but it was only recently that a novel experimental approach using Schottky diodes with ultrathin metal films makes direct measurement of reaction-induced hot electrons and holes possible. The chemical reaction creates hot charge carriers which travel ballistically from the metal film surface toward the Schottky interface and are detected as a chemicurrent in the diode. By now, such currents have been observed during adsorption of atomic hydrogen and deuterium on Ag, Cu and Fe surfaces as well as chemisorption of atomic and molecular oxygen, of NO and NO₂ molecules and of certain hydrocarbons on Ag. This paper reviews briefly exoelectron and chemiluminescence experiments and the concept of the Nørskov–Newns–Lundqvist model. The major part is devoted to the detection of chemically induced e–h pairs with thin metal film Si Schottky diodes by discussing the different influences on the chemicurrent magnitude and presenting experimental results predominantly with hydrogen and deuterium atoms. The experiments introduce a new method to investigate surface reaction kinetics and dynamics by use of an electronic device. In addition, the diodes may be used as selective reactive gas sensors.     

Energy expression of the chemical bond between atoms in metal oxides

The chemical bond between atoms in metal oxides is expressed in an energy scale. Total energy is partitioned into the atomic energy densities of constituent elements in the metal oxide, using energy density analysis. The atomization energies, ΔE_M for metal atom and ΔE_O for O atom, are then evaluated by subtracting the atomic energy densities from the energy of the isolated neutral atom, M and O, respectively. In this study, a ΔE_O vs. ΔE_M diagram called atomization energy diagram is first proposed and used for the understanding of the nature of chemical bond in various metal oxides. Both ΔE_M and ΔE_O values reflect the average structure as well as the local structure. For example their values vary depending on the vertex, edge or face sharing of MO₆ octahedron, and also change with the overall density of binary metal oxides. For perovskite-type oxides it is shown that the ΔE_O value tends to increase by the phase transition from cubic to tetragonal phase, regardless of the tilting-type or the 〈1 0 0〉 displacement-type transition. The bond formation in spinel-type oxides is also understood with the aid of the atomization energies. The present approach based on the atomization energy concept will provide us a new clue to the design of metal oxides.     

Orientation dependence in the dissociative scattering of hydrogen molecules from metal surfaces

We investigate the molecular orientation dependence of the dissociation probability of neutral molecular hydrogen scattered from metal surfaces. The probability distributions are calculated in the cases when positive and negative hydrogen ions (ion pair) are formed and when two neutral hydrogen atoms are formed. On the basis of numerical results, it is found that a parallel orientation preference of probability distributions shows a tendency to be suppressed in the case when an ion pair is formed as compared with the case when two neutral hydrogen atoms are formed.     

Temperature dependence of surface tension and capillary waves at liquid metal surfaces

The temperature dependence of the surface tension γ(T) is treated theoretically and experimentally. The theoretical model based on the Gibbs thermodynamics of a one-component fluid relates ∂γ/∂T to the surface excess entropy density −ΔS. All specific surface effects, namely ordering, capillary waves, and double layer influence the surface entropy, which in turn governs the sign and the magnitude of ∂γ/∂T. Experimental data collected at a free Hg surface in the temperature range from 0°C to 30°C show that ∂γ/∂T is negative. Zh. éksp. Teor. Fiz. 114, 2034–2042 (December 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Hydrogen bonds at metal surfaces: Universal scaling and quantification of substrate effects

Simple bio-molecules demonstrate a propensity to form hydrogen-bonded networks. Here, individual hydrogen-bond energies are determined through topological analysis of the theoretical electron density for a variety of systems, demonstrating the existence of a universal relationship between hydrogen-bond length and strength. Comparison of local hydrogen-bond strength to global stabilisation allows the quantification of contributions to intermolecular interactions within surface overlayers in unprecedented detail.     

Electrical conductivity and chemical bond of graphite intercalation compound with fluorine and metal fluoride

Graphite intercalation compounds with fluorine and metal fluoride (MgF₂ or CuF₂) were prepared from petroleum coke and pyrolytic graphite. With progress in the intercalation reaction, the first stage compound with identity period 9.4 Å changed to another structure of identity period 10.7 Å. It was found from ESCA measurements that the chemical interaction between intercalated fluorine and carbon was similar to the covalent bond around the surface and slightly ionic in the bulk. The maximum electrical conductivities in the direction of the ab-axis were (1.9–2.0) × 10⁵ (ωcm)^-1, which were 10–13 times that of the original pyrolytic graphite.     

Chemisorption bonding and bond lengths on transition metal surfaces: Effect of coordination and valency saturation

A smooth correlation between the determined chemisorption bond lengths, for O, S, Se and Te on Ni (001) and for sulphur on Ni (110) and Ni (111) as well, and Pauling's resonating bond ionicity is demonstrated, when the latter are calculated with due regard for the coordination and valency saturation effects. Pauling's bond length-bond number relation is used to provide (i) an independent estimate of the Ni-O bond length on Ni (001), which is found capable of discriminating among the reported values, and (ii) estimates of the bond lengths for O, S, Se and Te on Ni (111) and Ni (110) by using the determined bond lengths on Ni (001). Agreement with the determined values for sulphur is surprisingly good.     

Long range substrate mediated mass transport on metal surfaces induced by adatom clusters

Mass transport at surfaces can proceed either (i) by hopping diffusion of atoms on top of the surface from one site to another or (ii) by propagation of small displacements from one atom to the next within the topmost atomic layer. In the latter case, a long range substrate mediated mass transport has been postulated but never observed explicitly. Experimental and theoretical evidence is shown here for the occurrence of such a mechanism on the reconstructed Au(111) surface, where the movement is shown to be well described by a soliton.