Interaction between two hydrogen atoms approaching a metal surface

Two regimes of HH interaction outside a metal surface are considered: (i) Beyond the spill-out region of the metal electrons (physisorption) (ii) Embedded protons in the electron spill-out. Using an extension of the Heitier-London model to treat (i), the H₂ binding energy for the molecule parallel to the surface is reduced, whereas for the perpendicular configuration stronger binding obtains. In (ii), the asymptotic form of the screening charge round a single proton is considered, in a linear response framework, and for an infinite barrier model of the surface. If the screened potential has no singularities in k space, then the interaction energy ΔE(X), X being the distance between the protons, falls off as X^?5 times an oscillatory function, for H₂ parallel to the planar metal surface. The effect of self-consistency on this result is then examined and it is concluded that the asymptotic interaction energy is unchanged in form, though the amplitude is altered.     

THz near-field measurements of metal structures

We report on electro-optic measurements of THz electric fields in the near-field of metal structures lying on GaP electro-optic sampling crystals. With (110) oriented crystals, the x or y component of the THz electric field is measured, whereas with a (001) oriented crystal, the z-component is measured. With an estimated spatial resolution of 20 μm, the technique allows us to map the field on a scale, orders of magnitude smaller than the spatial dimensions of these structures. We illustrate the technique by showing results of measurements of the THz electric field in a plane underneath a metal sphere and a single square hole in a metal foil. The technique provides us with a wealth of information on the time-evolution of light fields near metal structures. To cite this article: A.J.L. Adam et al., C. R. Physique 9 (2008).     

Analytical solutions of the Smith model for the neutral and charged metal surface

Smith jellium model for an s-p metal surface is retreated. Exact analytical solution is obtained for the case of neutral metal. Extension of the model on the case of a charged surface is reconsidered to obtain analytical expressions for the lability coefficients, characterizing the response of the surface electronic profile to moderate charging.     

Influence of the promotion of terms on the emission of auger electrons upon collisions of multicharged ions with a metal surface

Upon the collision of multicharged ions with a metal surface there occurs the formation of a dipole, generated from an ion and an induced charge in the metal. Strong rearrangement of electron shells takes place in the dipole field, which leads to the “promotion” of terms generated from states with a principal quantum number n = 2, 3. When the promoted terms cross with the levels of metal atoms, an electron is captured into these states. This effect makes it possible to explain the observed Auger electron spectra.     

Why does hydrogen destroy superconductivity in niobium and lanthanum?

Starting from a theoretical study of the electronic properties of the corresponding stoichiometric metal hydrides by means of an augmented plane wave band structure calculation, we present an evaluation of the electron-phonon coupling parameter λ, using the McMillan formalism. The electronic term η is calculated from first principles using the rigid ion approximation while experimental data are used to estimate the phonon contribution. From a detailed analysis of both the electron-acoustic phonon coupling and of the electron-optic phonon coupling we discuss the reasons why hydrogen kills superconductivity in the high T_c metals Nb and La. These results in conjunction with our studies of a series of metal hydrides are used to draw general conclusions and make some predictions about the possible occurrence of superconductivity in transition metal hydrides.     

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.     

Double-exchange model for molecule-based magnets

We report a detailed study of a double-exchange model proposed for the molecule-based magnets. The model is applied to a two-dimensional periodic complex made of a transition metal and an organic molecule in which the electronic structure is described by effective d orbitals of the transition metal ion at infinite Hund's coupling limit and the lowest unoccupied molecular orbital of the organic molecule, π^∗. Depending on the average electron density of the organic molecules and various superexchange couplings between metal ions' core spins, magnetic states of the complex are investigated. Performing Monte Carlo calculations on a model Hamiltonian for various electron densities of the organic molecule, the average magnetization and critical magnetic ordering temperatures are determined.     

Adatom dipole moments on metals and their interactions

An expression is derived for the dipole moment μ, associated with an arbitrary adatom on an arbitrary metal surface, in terms of the differential work done in moving the adatom from the interior of the metal to its actual position in the presence and absence, respectively, of small, asymptotically uniform electric field on the exterior of the metal. With the aid of this expression it is shown that the non-oscillatory part of the interaction energy of two dipoles μ_A and μ_B, situated on the surface and separated by a large distance v, is given by U_AB = 2μ_Aμ_Bv^?3. This exceeds by a factor of 2 the interaction energy of two dipoles in the same relative configuration but in a vacuum.     

Theoretical modeling of experimental HREEL spectra for supported graphene

We present a simple model for the resonant feature designated as the π plasmon in single-layer graphene supported by a metal substrate, which is excited via high-resolution electron energy loss spectroscopy (HREELS) in the off-specular scattering geometry. Using a two-dimensional, two-fluid hydrodynamic model for interband transitions of graphene's π and σ electrons and an empirical Drude–Lorentz model for the metal in the local approximation enables us to reproduce, at the qualitative and semi-quantitative levels, the typical experimental features of the HREEL spectra in the visible to the ultraviolet frequency range.     

Exact saturation and degeneracy of isospin bounds and zeros trajectories in pion-nucleon scattering: D. B. Ion. Joint Institute for Nuclear Research,Dubna, Moscow,P. O. Box 79, U.S.S.R.

By exploiting the analogy between the classical equation of heat conduction and the Bloch equation, the canonical density matrix is calculated exactly for noninteracting electrons in a finite step model of a metal surface. This result is then used to calculate the spatial variation of the electron density ?(z) as a function of distance z from the planar metal surface. In particular an expansion in inverse powers of $\text{V}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and V, V being the step height, is derived. The low-order terms are used to compute the electron density at the surface of Al metal: There is good agreement with a full numerical evaluation. Finally, in the limit $\text{V}\text{E}{}_{\mathrm{f}}\text{→ ∞}$, E_f being the Fermi energy, the change in the shape and size of the exchange (Fermi) hole surrounding an electron is studied as the electron moves into the vicinity of a metal surface. The anisotropy of the Fermi hole is large and the implications for one-body potential theory are briefly discussed.     

XPS studies of carbon supported films formed by the resistive deposition of manganese

The surface and subsurface speciation of carbon-supported vapor-deposited films, prepared by resistively heating manganese chips in the sample chamber of an HP 5950A ESCA spectrometer, have been studied using XPS and argon ion etching. It is shown that the oxide formed in poor vacuum is MnO. Under improved experimental conditions, a mixture of Mn metal and MnO is obtained, but the metal is very reactive to oxygen. For very thick films formed in poor vacuum, the oxide covers a layer of manganese metal. The oxide is no longer evident after 60 h of etching. For the layer of manganese remaining, we have found that the Mn3s multiplet splitting is smaller than that reported in the literature. The present result is shown to be consistent with magnetic susceptibility measurements on α-Mn. For discontinuous MnO films, argon ion etching causes the Mn2p binding energy to shift to higher energy. This is inconsistent with reduction to Mn metal, as the binding energy would shift to a lower value for this case.     

Kinetic and theoretical studies of metal ion adsorption in KDP solution

Metal ion adsorption in saturated aqueous potassium dihydrogen phosphate solution was analyzed using kinetic, equilibrium model and computational chemistry approaches. The isotherm constants (K_F and n) in the Freundlich model and the first order Lagergren kinetic model parameter k assist with a general understanding of the fundamental adsorption behavior of trivalent and divalent metal ions. The electrostatic force based on electrostatic potential distribution was found to be an essential feature for metal ion adsorption via a correlation between the ESP values of each metal ion and these experimental parameters.     

Ultrasonic wave propagation in metals in the hydrodynamic limit: Implications of including the bragg reflection force

We have included the Bragg reflection force, which is expressed in terms of the effective mass $\text{m}{}^1$ of an electron, in calculating the non-resonant changes in the velocity and attenuation of an ultrasonic wave propagating through a metal in the presence of an external magnetic field in the limit kl, ω_cτ ? 1. The results obtained show significantly different magnetic field dependence from that predicted under the free electron approximation. Our results also suggest a way of measuring the effective mass of an electron in a metal.     

The effect of metal particle size on the stoichiometry of adsorption

As an example of similar calculations for any gas/metal combination, a geometrical upper limit for the stoichiometry of adsorption θ_m (adsorbed molecules per metal surface atom) of CO on small fcc Pd crystallites of various regular shapes is calculated. The value of θ_m, normalized with respect to the corresponding value for the particle plane faces, is obtained as a function of an average particle diameter D_av up to 6 nm. The normalized value is found to increase rapidly above 1 as D_av decreases below ~ 2.5 nm, in agreement with previously reported experimental data.     

On the propagation characteristics of metal clad waveguides with a quantum well

The electromagnetic modes of planar metal clad dielectric waveguides containing an n-doped quantum well (QW) are studied theoretically. Special attention is paid on the coupling between metal surface plasmons and intersubband plasmons and the manifestation of this coupling in the propagation characteristics of metal/QW/dielectric and multimode metal/QW/dielectric/metal waveguide structures. The results obtained indicate that the modification of the propagation characteristic induced by the above-mentioned coupling is substantial only in the case of metal/QW/dielectric waveguide structures.     

Metallic nonlinear magnetooptical nonreciprocal isolator

In advanced optical fiber communication systems where reflection light might cause degradation of the signal quality, optical waveguide isolators play an important role. The effect of a metal layer with negative permittivity on the behavior of nonlinear-magnetooptic isolator is studied. The isolator consists of metal film, nonlinear cladding, and magnetooptic substrate. It is found that difference between forward and backward propagation for TM₀ mode increases with increasing the absolute value of the tuning parameter which is the permittivity of the metal film, ?_f. It is also found that the maximum cut-off thickness of the isolator occurs in self-defocusing case around η = 0.65 and at the highest assumed value of ?_f = −8. These results are helpful in fabricating an isolator with high performance.     

On the role of tunneling in metal-semiconductor nanocontacts

The shape of the contact potential that arises at the interface between semiconductor and a metal nanoparticle is calculated in the approximation of complete depletion. The particle represents a sphere of radius a?S, where S is the thickness of the depletion layer in the semiconductor in the case of an infinite plane contact with the metal. A WKB approximation is applied to develop a theory of thermal-field current transfer through such a contact. It is shown that, as the radius of the metal nanoparticle decreases, the component of thermal field emission current plays an increasing role in the current transfer, while the backward current density increases and may become comparable to the density of forward current. In this case, the current-voltage characteristics (CVCs) become more symmetric.     

The effect of reflectance coefficients on the interaction of an H wave with a thin metal film

The interaction between an H wave and a thin metal film is calculated in the case of different values of the angle of incidence θ of the wave and reflectance coefficients q ₁ and q ₂ upon reflection of electrons from the surface of a thin metal layer. The behavior of the reflection, transmission, and absorption coefficients is analyzed as a function of the dimensionless frequency of bulk collisions of electrons X and dimensionless frequency of the external field y. The obtained results are compared with experimental data.     

Heterogeneous relaxation of molecule vibrational energy on metals

A mechanism of heterogeneous relaxation of a molecule vibrational energy on metal via conduction electrons is proposed. The probability of vibrational transition W of a molecule adsorbed on metal surface is calculated. The not very sharp dependence of W on the distance between the molecule and the metal surface, on metal parameters and on the molecular vibrational quantum provides a satisfactory explanation of the experimental data available.     

Light induced molecular magnetic polaron in EuO

Stoechiometric EuO samples do not present insulator metal transition below Curie temperature (T_c = 70 K). Experimental data presented in this paper show that when these samples are illuminated, it appears: an insulator metal transition below T_c, and, at room temperature, a decrease of activation energy, as well as a small increase in conductivity. Oxygen deficient samples exhibit usually this insulator metal transition. When they are illuminated room temperature activation energy remains unchanged, and the insulator metal transition is more important. These results cannot be explained by means of a free electron model. Electrons are photoexcitated from 4? levels, or donor levels, to conduction band. Then, these electrons polarise Eu²⁺ spins and make magnetic polarons. These polarons cannot be “Bound Magnetic Polarons” because samples do not contain oxygen vacancies. In addition these polarons would be stable until room temperature. Thus, they are expected to be “Molecular Magnetic Polarons”. Our results confirm then this model proposed by Kusuya.