Jacques Jupille

Ionic oxygen species formed in the presence of lithium, potassium and cesium

Abstract Various thick oxidized Li, K and Cs layers have been analysed. The O 2− , O 2 2− and O 2 − ions have been characterized through the multiplet structures of their valence-band spectra. Furthermore, the energy separations between the O 1s core levels and the O 2p valence levels were seen to take some given values for each oxygen species. This provides a new way to unambiguously identify these species. In addition, the above energy separations compare rather well to the free ion values, thus allowing discussion on the representation of the oxygen ions which are formed in the presence of alkali metals in terms of a point-charge model.

High-resolution electron-energy-loss spectroscopy of isolated hydroxyl groups on α-Al2O3(0001)

Abstract Hydroxylated (0001) faces of α-Al 2 O 3 bulk crystals have been examined by high-resolution electron-energy loss spectroscopy (HREELS). Surface charging being eliminated by means of a defocused electron beam, it is shown that the electron fluence can be low enough to avoid significant damages. A resonance scattering from chemisorbed OH groups is evidenced, at impact energies lower than 12 eV, by strong enhancements of the cross sections for excitation of both the fundamental (3720 cm −1 ) and the first overtone (7270 cm −1 ) of the stretching mode and by the angular distribution of the inelastically scattered electrons.

Self-inhibition of water dissociation on magnesium oxide surfaces

Abstract Hydroxylated MgO surfaces have been prepared by exposure to water vapour of MgO crystals at room temperature. High hydroxyl coverages were achieved on freshly cleaved surfaces. However, upon adsorption–desorption cycles of the hydroxyl adlayer, the ability of the MgO surfaces to dissociate water was seen to be dramatically inhibited. Reduced reactivities have also been observed on both air- and water-exposed MgO surfaces. This reactive behaviour is discussed in relation to the theoretical prediction that the MgO(100) face is not expected to dissociate water molecules.

The van der Waals contribution to the adhesion energy at metal-oxide interfaces

Abstract The van der Waals contribution to the adhesion energy between a metal and an insulating oxide is expressed in terms of a dielectric continuum model. Values derived from the model are shown to give a good qualitative account for experimental data. Furthermore, they are seen to reproduce commonly reported trends in adhesion energy at metal-oxide interfaces since they tend to increase as the plasmon energy of the metal increases and/or as the bandgap of the insulator narrows. Such a model does not account for kinetic energy and electrostatic contributions; its relevancy to describe the adhesion energy in the metal-oxide case is discussed, and shown to be plausible provided the oxide energy gap is wide enough and the metal is sufficiently nonreactive.

Surface chemistry of weathered zircons

We have investigated the surface morphology and chemistry of zircon in sediments and soils of the Amazon basin (Brazil). The extent of metamictization in the near-surface region of these zircons was previously determined by Raman micro-probe. Scanning electron microscopy (SEM) reveals scarce dissolution features and numerous marks related to the sedimentary transport of the zircon grains, which attest to the high resistance of these zircons to tropical weathering. Rutherford backscattering (RBS) micro-analysis and X-ray photoelectron spectroscopy (XPS) failed to reveal any strong chemical change at the surface of the zircon grains. In particular no Zr enrichment was detected, which excludes the occurrence of a protecting layer of Zr oxide or hydroxide at the zircon surface. This result suggests that the slow weathering rate of zircon could be the principal factor limiting the Zr mobility in the investigated weathering environments.

Layer-by-layer growth mode of silver on magnesium oxide (100)

Abstract The growth mode of silver thin films on MgO(100) is shown to strongly depend on the chemical purity of the oxide surface. On a carbon- and defect-free surface the silver deposit grows layer-by-layer, while the presence of any of these chemical defects prevents such a growth mode.

Interaction of H2O with basal and stepped faces of rhenium: An ESDIAD study

The adsorption and dissociation of water has been examined between 80 and 500 K on two different Re surfaces — Re(0001) and a stepped vicinal Re[16(0001) × 2(1101)] — using TDS, LEED, AES and ESDIAD (Electron Stimulated Desorption Ion Angular Distribution). Above 250 K, the adsorption is dissociative on both surfaces but the extent of dissociation is larger on a stepped than on a basal face. During adsorption, evolution of hydrogen is observed after an induction period. This phenomenon is related to the formation of OH radicals. The adsorption on Re(0001) is molecular at 80 K. Several steps could be observed: adsorption of isolated molecules, formation of bilayer clusters ordered in two (3 × 3)R30° structures and finally the formation of a multilayer. Situations of non-uniform coverage of the surface have been found in which multilayer patches, bilayer clusters and uncovered areas are simultaneously present. Three molecular water desorption peaks can be observed. Peaks A (180 K) and B (150 K) are produced by desorption from the first bilayer and peak C (140–150 K) is related to the desorption from the multilayer. Furthermore, molecules in the first bilayer are partly dissociated as shown by desorption of hydrogen between 250 and 450 K. ESDIAD and LEED experiments have shown that desorption, manifested by peak A, occurs after a reconstruction of the bilayer, ordered in two (2 × 2) structures, resulting from a displacement of water molecules from the on-top sites to three fold sites. The latter molecules dissociate between 190 and 240 K giving OHads + Hads. Dissociation of OH would occur only above 360 K. On stepped Re dissociative adsorption occurs on steps, even at 80 K, while adsorption remains molecular on terraces. This might be accounted for by the lowering of the activation barrier of the dissociation on steps.

Ethylene monolayer and multilayer on Pt(111) below 52 K: determination of bond lengths by near-edge X-ray fine structure

Abstract At temperatures lower than 52 K, ethylene condenses on Pt(111). Valence-band analysis shows that the first layer and the condensed phase exhibit almost the same positions of the molecular orbitals. Carbon K-shell excitation leads to the appearance of π∗ and σ∗ resonances from which it is deduced that the low temperature ethylene monolayer is π-bonded to the substrate with minor C-C elongation, namely about 0.004 nm. A comparison is made with C-C bond lengths of ethylene monolayers and multilayers measured using NEXAFS on various substrates and given in the literature. It shows that they correlate nicely with the values of the πσ parameter deduced from HREELS results for the same monolayers.

The adsorption of oxygen on a cesiated Ni(111) surface: Evidence for the formation of molecularly chemisorbed oxygen species

Abstract Oxygen adsorption is investigated by means of UPS, XPS and work function measurements on a cesiated Ni(111) surface at 100 K (precovered with one cesium monolayer). Upon oxygen exposure of the cesiated surface, the work function decreases, passes through a minimum and then increases as to reach a plateau. It is shown that up to the work function minimum a peroxo-like species (O 2− 2 ) is formed, meanwhile after the work function minimum a superoxo-like species (O − 2 ) is identified too.

Numerical modeling of the optical response of supported metallic particles

The present work reports a general method for the calculation of t he polarizability of a truncated sphere on a substrate. A multipole ex pansion is used, where the multipoles are not necessarily localized in the center of the sphere but can freely move on the revolution axis. From the weak formulation of the boundary conditions, an infinite set of linear equations for the multipole coefficients is derived. To obta in this set, the interaction between the island and the substrate is t aken into account by the technique of image multipoles. For numerical implementation, this set is truncated at an arbitrary mu ltipole order. The accuracy of the method is jugded through the stabil ity of the truncated sphere polarizability and the the fulfilment of t he boundary conditions which are demonstrated to be satisfied in large regions of the parameter space. This method brings an improvement wit h respect to the Bedeauxs case \cite{Wind87a,Wind87b} where the multi poles are located in the center of the sphere.