J. Lopez

Interaction energy between two identical atoms chemisorbed on a normal metal

Abstract The interaction energy between two identical monovalent adatoms is studied and its dependence on the interatomic bond length R is investigated. This interaction essentially involves three processes (direct coupling, indirect coupling and dipole-dipole interaction). When R is large enough some approximations allow us to evaluate their contribution in an algebraic way. In the short distance region this algebraical study is completed by a computation carried out in the case of the chemisorption of Na on Cu. In the range of R values in which the hopping parameter between the atomic states on the adsorbate is large compared with 1 (2πR) the chemisorption weakens the interatomic bond. In the neutral chemisorption case, this weakening arises from the filling of the antibonding virtual level of the adsorbate to the detriment of the bonding one. In the ionic chemisorption case, since the valence orbitals are nearly empty (cathionic chemisorption) or nearly filled (anionic chemisorption), the bond between the adatoms is broken. Then the interaction between each dipole consisted of an adatom and its screening charge laying inside the substrate subsists alone. Conversely, when the hopping parameter between the adsorbate valence states is small compared with 1 (2πR) , the interaction energy between the adatoms reduces to the sum of the indirect energy and the dipole-dipole one. The relative importance of these energies depends on the more or less ionic character of the chemisorption.

Mirror electron microscopy applied to the determination of the total electron reflection coefficient at a metallic surface

It is shown that the intensity of the electronic current backscattered from the surface is the convolution product of the total reflection coefficient by the energy distribution of the incident beam. A deconvolution method has been used to obtain this coefficient and this method is based on a rigorous inversion of the convolution integral operator. Numerical tests show that this method is not very sensitive to the experimental random noise. Results are given for W(100), Cu(100), and O/Cu(100) surfaces, and these are correlated with earlier measurements.

Subthreshold effects in VLEED and the non-isotropy of absorption potential in metal

Abstract Sharp structures observed in LEED specular intensities on the (111) surfaces of fcc metals appear close to the emergence of new diffracted beams inside the crystal. It is shown that they issue from the energy band structure of the metal. A LEED calculation of these structures leads to a good agreement with experiment only if we suppose that the damping of a plane wave in the metal depends on the direction of its wave vector. In this way, the absorption potential is no more isotropic. In our case, the new emerging waves which propagates in directions parallel to the densely packed (111) surfaces are less damped than waves which propagate in other directions.

Interaction energy between two atoms chemisorbed at a transition metal surface

A detailed investigation of the interaction energy between two identical monovalent atoms chemisorbed at a transition metal surface is presented in terms of a tight-binding model. The direct interaction between the adatoms is explicitly included in the model. Information on the diadatom chemisorption process is obtained from an algebraical investigation of the limiting cases corresponding to weak and strong adatom-metal couplings. The intermediate coupling domain is studied through a computation carried out in the case of the (100) surface of an s-band simple cubic crystal with or without steps.

Statistical aspects of the diffuse LEED problem

Diffuse low-energy electron diffraction (LEED) intensities can be observed in all situations where the surface cannot be represented by a perfect biperiodic array of atoms or molecules. For instance, this occurs when binding sites are either occupied or not by adatoms, when several atoms or molecules are co-adsorbed, when one absorbed species can be located at several possible binding sites, etc. So far, the statistical aspects of this problem have been disregarded. In fact, under certain circumstances, it is possible to extract from diffuse intensities a quantity that only depends on the local atomic arrangement near a binding site. A method based upon this possibility has been systematically used in all diffuse LEED studies and consequently these studies have been totally devoted to the determination of this local atomic arrangement. The authors show in this paper that this method only works if one binding site is occupied or not (binary chemisorption). In the other cases, investigation of diffuse LEED intensities requires knowledge of the statistical distribution of occupied sites and thus the statistical aspects of this problem can no longer be bypassed. For this reason and also because this problem is intrinsically interesting, the authors particularly focus on it here. Diffuse LEED intensity can be approximated by a sesquilinear form of the form factors for each adsorbed species. The coefficients of this form are the Fourier transforms on the two-dimensional surface lattice of the site-occupancy pair correlation functions. A self-consistent molecular-field approximation of these correlation functions is given in this paper. Particularly, the validity of this approximation is discussed in detail for the binary chemisorption case. Diffuse LEED intensities are provided in some other cases: (i) two kinds of atom are distributed at the surface of a binary metallic alloy; (ii) two adsorbed species coexist at a single crystal surface. The authors arrive at the conclusion that investigation of diffuse LEED intensities generally requires the direct comparison of measured and calculated intensities.

The dipole-dipole interaction between two adatoms viewed as a surface plasmon mediated interaction

Abstract The semi-classical dipole-dipole interaction law previously derived from the classical image force theory is rederived from a model hamutonian in which the dipole-dipole interaction between two adatoms is mediated by surface plasmons. From this approach it emerges that the semi-classical law is only valid in the adiabatic limit where image charges can quasi-instantaneously follow the charge fluctuations on the adsorbate. Connection with the second order perturbation theory of dipole-dipole interaction between two adatoms is also analysed.

A method for comparing measured and calculated VLEED fine structures

Abstract In this paper, we present a method for comparing measured and calculated VLEED fine structures for the determination of surface barrier shapes. This method amounts to compare — via a metric distance — a discrete set of experimental data with the corresponding set obtained from calculation. For this comparison we choose the location of maxima and minima of the rapid intensity oscillations associated with threshold effects. The distance between experiment and theory essentially depends on the height, the coulomb tail location and the shape of the surface barrier. On the other hand, this distance is not sensitive to the absorption potential inside the metal nor to its decrease into vacuum. Contrarily to other works devoted to this problem, where the comparison between experiment and theory was carried out profile by profile, we must emphasize that in our method this comparison simultaneously involves several profiles obtained at various incidence conditions. Our method is tested out in the case of a data set including five profiles obtained on W(110). We adopt here the modified image barrier model of Jennings and Read. Although a good overal agreement is obtained with a barrier height of 15 eV and a barrier image origin of −1.41 A, small discrepancies subsist when we examine each profile separately. The presumed origin of these discrepancies is discussed and some improvements are proposed in the description of surface barrier shapes.

Effect of the variation of the inner potential of the crystal surface in VLEED

In very low-energy electron diffraction (VLEED), the fine structures appearing below the emergence thresholds in I00(E) are usually used to test surface barrier models. The authors reinvestigate this problem, in particular for the case of a loosely packed surface. Two surface barrier models are tested in the case of a very-low-energy intensity profile (E <or approximately=5 eV) obtained on the W(001) surface for theta =45 degrees along the (10) azimuth: the modified image barrier introduced by Jennings and Read (1974) and a new saturated image barrier. Neither of these models yields a good fit of experimental data as long as one assumes that the barrier joins the bulk inner potential at the top layer edge continuously or with a potential step. Without challenging the validity of the muffin-tin approximation inside the first layer, it can be suspected that the top-layer potential is less attractive than any bulk-layer one. it is found that for both barrier models, shifting the top-layer inner potential upwards leads to a significant improvement of the fit. It is also found that the modified image barrier tends to exhibit a non-physical region of constant potential potential near the top layer and as a consequence the saturated image barrier appears to be more reliable. Finally this work illustrates the fact, that, especially for loosely packed surfaces, the surface barrier concept must not be restricted to the vacuum region but has to be extended inside the crystal region where the bulk potential is modified by the presence of the surface.

Dynamical effects of the surface potential barrier in very low energy electron diffraction

Abstract On the basis of algebraical results examplified with exact computations, we investigate the dynamical effects associated with the presence of the surface potential barrier in very low energy electron diffraction (VLEED). These effects essentially have three origins: (i) the diverging phase of the reflection amplitude from metal back to metal leads to rapid oscillations of intensity located below emergence thresholds, the maxima of which are arranged in a Rydberg-like series. This effect, wrongly attributed to a resonance process, is actually due to a simple interference mechanism, (ii) The singularity near zero of the transmission amplitude of the barrier from metal to vacuum leads to a discontinuity in the current of non-specular beams at their emergence threshold, (iii) The interference between the waves directly reflected at the surface barrier and at the atomic planes generally induces large modifications of the intensity profiles spreading out on the whole VLEED energy range. Thus the approximation consisting in neglecting the reflection at the surface barrier is inadequate. In conclusion, the above barrier effects must be properly included in any quantitative analysis of VLEED spectra.

Surface extended energy-loss fine structure spectroscopy: extraction of structural information from spectra recorded in the second derivative mode of the electron yield

The authors reinvestigate the relation between the Fourier transform of SEELFS (surface extended energy-loss fine structure spectroscopy) data recorded in the second derivative mode of the electron yield, and the radial distribution function F(R) of atoms around the emitter. This relation is not obvious since the Fourier transform is done over wavevector modulus and the derivation is done with respect to energy. They show that, providing a k3 weighting factor is introduced, an EXAFS-like (extended X-ray absorption fine structure) analysis leads to R2F(R) with a good accuracy. This gives an a posteriori justification of the treatment currently used in the literature. The reasoning is easily extended to the case of data recorded in the first derivative mode, for which RF(R) is obtained with a k2 weighting factor. It is also shown that the R2 (respectively R) (respectively R) factor must be eliminated to obtain shell positions with good accuracy, which is generally not achieved in previous work. All these conclusions are supported by a numerical analysis performed in the case of a model spectrum associated with the Ni(111)M23 loss spectra.