Adsorptions de H2 et de O2 sur des alliages NiCu divisés supportés sur SiO2, etudiées par mesure d'aimantation á saturation

The linear combination of surface atom orbitals to which an adatom orbital will couple plays the important role of being the essential link between the adsorbate and the semi-infinite substrate. The group orbital (as it is called) is a function of the substrate character, the local adsorption geometry, and the adatom orbital. We present the spectral densities of states appropriate to the adsorption of a hydrogen or transition metal atom on a transition metal substrate for simple geometries, and comment upon some of the implications of the results for both past and future assumptions in chemisorption theory.     

Imaging and tunneling spectroscopy of individual iron adsorbates at room temperature

We have imaged for the first time individual atoms and small clusters of metal species on a metal substrate at room temperature by means of a scanning tunneling microscope (STM) operated under ultrahigh vacuum conditions. The system we have studied is Fe on W(110), for which a carbon-induced (15×3)-reconstruction of the W(110) substrate has been found to prevent surface diffusion of Fe atoms at 300 K. Upon positioning the STM-tip above individual Fe adsorbates, local tunneling spectra could be obtained. A pronounced empty-state peak at 0.5 eV above the Fermi level has been found, characteristic for individual Fe adsorbates. This peak can serve as a fingerprint for the identification of Fe adsorbate species.     

Imaging and tunneling spectroscopy of individual iron adsorbates at room temperature

We have imaged for the first time individual atoms and small clusters of metal species on a metal substrate at room temperature by means of a scanning tunneling microscope (STM) operated under ultrahigh vacuum conditions. The system we have studied is Fe on W(110), for which a carbon-induced (15×3)-reconstruction of the W(110) substrate has been found to prevent surface diffusion of Fe atoms at 300 K. Upon positioning the STM-tip above individual Fe adsorbates, local tunneling spectra could be obtained. A pronounced empty-state peak at 0.5 eV above the Fermi level has been found, characteristic for individual Fe adsorbates. This peak can serve as a fingerprint for the identification of Fe adsorbate species.     

Electron correlations in the chemisorption theory: Self-consistent calculations of the adatom charge

The single hydrogen-like adatom chemisorption on transition metal surfaces is studied by using the generalized model Hamiltonian. This Hamiltonian includes the possibility of influence of the adatom orbital occupancy on the charge transfer between the adatom and the substrate metal. The correlation effects were included up to second order in V (the single particle coupling strength). The numerical calculations of the charge transfer between an adatom and a substrate metal, as well as the comparison with results obtained for the standard Newns-Anderson model indicate that this generalized Hamiltonian can be more efficient in describing the chemisorption process.     

The kinetic MC modelling of reversible pattern formation in initial stages of thin metallic film growth on crystalline substrates

The results of kinetic MC simulations of the reversible pattern formation during the adsorption of mobile metal atoms on crystalline substrates are discussed. Pattern formation, simulated for submonolayer metal coverage, is characterized in terms of the joint correlation functions for a spatial distribution of adsorbed atoms. A wide range of situations, from the almost irreversible to strongly reversible regimes, is simulated. We demonstrate that the patterns obtained are defined by a key dimensionless parameter: the ratio of the mutual attraction energy between atoms to the substrate temperature. Our ab initio calculations for the nearest Ag-Ag adsorbate atom interaction on an MgO substrate give an attraction energy as large as 1.6 eV, close to that in a free molecule. This is in contrast to the small Ag adhesion and migration energies (0.23 and 0.05 eV, respectively) on a defect-free MgO substrate.     

过渡金属原子在离子晶体上的化学吸附

本文研究了一种描写过渡金属原子在离子晶体上的化学吸附模型。其中离子晶体用半无限A—B交替原子链代表,而吸附原子d轨道电子间的库仑排斥作用则用Anderson-Newns方法表述。对d轨道电子与晶体表面相互作用的几种不同耦合常数,用自洽格林函数方法计算了化学吸附能和吸附原子的电荷转移量。讨论了各种自洽解的存在条件和性质,得到了一些有趣的定性结论。 关键词：     

Atomic chemisorption on simple metals: Chemical trends and core-hole relaxation effects

We have obtained essentially exact numerical solutions for a simple model of atomic chemisorption on simple metals. The approximations constituting the model are the semi-infinite jellium simulation of the metal substrate and the self-consistent local density theory of exchange and correlation. The solutions provide a detailed picture of the electronic charge making up the chemisorption bond. The variation of this picture with the valence of the adatom exhibits in a direct and microscopic way the roles of electronegativity, charge transfer, and covalency. Predicted bond energies, bond lengths, and dipole moments are consistent with measurements and independent theoretical considerations.     

The theoretical and experimental study of the ionization processes during the low energy ion sputtering

The process by which atoms are ionized as they are sputtered from a metal surface has been analyzed both theoretically and experimentally. In the theoretical part the expressions for ionization coefficient R⁺ of atoms having the ionization energy much larger than the metal work function have been derived using a molecular orbital method. The effect of the level crossing was estimated in an approximate way. In the experimental part the SIMS experiments on clean Ni and Al surfaces and on Ni surface covered with a submonolayer of adsorbed K, Na and Al are reported. It has been found and it is for the first time reported that the energy distribution of ions sputtered from a submonolayer of adatoms is independent of energy (200–2500 eV) and mass (Ar⁺ Xe⁺ of incident ions and depends only upon the adsorption energy of the adatom. The energy distribution of ions sputtered from bulk samples has been found dependent on the primary ion energy. The measurement of the absolute value of R⁺ has shown that there is a strong correlation between the number of the adatom valence d-electrons and the value of R⁺, the value of R⁺ being smaller for atoms with more d-electrons. These experimental data have been compared with the theoretical expressions and the important role of the mechanism which takes into account the bending of the adatom energy level has been assessed.     

H,O原子在过渡金属表面吸附能的规则性

用Allan的简化d带模型描叙过渡金属的表面电子态,用广义相移法计算吸附原子在表面的吸附能,结果表明,不仅很好地描绘了H,O在一个周期的过渡金属表面吸附能的变化趋势,而且所算得的吸附原子感应的分离能级也同紫外光发射谱的实验符合得很好;同时还指出,简单气体在过渡金属表面的吸附能呈现规则性变化主要决定于费密能级E_F与吸附原子的有效能级ε_a之差(E_F-ε_a),其次是转移矩阵元ν和能带宽度w_b。 关键词：     

Molecular cluster theory for chemisorption of first row atoms on nickel (100) surfaces

Self-consistent Hartree-Fock-Slater molecular cluster models for the chemisorption of first row atoms on Ni(100) surfaces are presented. Energy levels and ground state charge distributions for XNi_S clusters with the adatom X = H, C, N, O located in C_{4 V} symmetry at a fixed height of h = 2.0 au above the surface are given. The variation of properties with h was studied in detail for the case of oxygen. Theoretical results compare rather well with experimental photoelectron and energy loss data. Local-densities-of-states diagrams are used to clarify the interaction between adsorbate levels and metal conduction bands.     

DFT study of NH3 dissociation on Si(1 1 1)-7 × 7. The role of intermolecular interactions

The adsorption of NH₃ molecule on the Si(1 1 1)-7 × 7 surface modelled with a cluster has been studied using density functional theory (DFT). The results indicate the existence of a precursor state for the non-dissociative chemisorption. The active site for the molecular chemisorption is the adatom; while the NH₃ molecule adsorbs on the Si restatom via this preadsorbed state, the adsorption on the Si adatom is produced practically without an energy barrier. The ammonia adsorption on the adatom induces an electron transfer from the dangling bond of this atom to the dangling bond of the adjacent Si restatom, hindering this site for the adsorption of a second NH₃ incoming molecule. However, this second molecule links strongly by means of two H-bonds. The dissociative chemisorption process was studied considering one and two ammonia molecules. For the dissociation of a lonely NH₃ molecule an energy barrier of ∼0.3 eV was calculated, yielding NH₂ on the adatom and H on the restatom. When two molecules are adsorbed, the NH₃-NH₃ interaction yields the weakening of a N-H bond of the ammonia molecule adsorbed closer the Si surface. As a consequence, the dissociation barrier practically disappears. Thus, the presence of a second NH₃ molecule at the adatom-restatom pair of the Si(1 1 1)-7 × 7 surface makes the dissociative reaction self-assisted, the total adsorption process elapsing with a negligible activation barrier (less than 0.01 eV).     

Controlled modification of individual adsorbate electronic structure

Modification of the electronic structure of a single Mn adsorbate placed within a geometrical array of adatoms on Ag(111) is observed using local spectroscopy with the scanning tunneling microscope. The changes result from coupling between the adsorbate level and surface electronic states of the substrate. These surface states are scattered coherently within the adatom array, mediating the presence and shape of the array to the adsorbate within. The dimension and geometry of the adatom array thus provide a degree of control over the induced changes.     

A phenomenological explanation for the buckling of surface alloys

A phenomenological model, in which the interactions between the nearest-neighbor (NN) atoms are described as bondings but not hard sphere contacts, is proposed to explain the unexpected reduced buckling in surface alloy systems. In the model, the forces acting on an adsorbate atom from its NN substrate atoms in different layers may be either repulsive or attractive, depending on whether the bond between the adsorbate atom and its NN substrate atoms is compressed or stretched. It is found that the forces on the adsorbate atom from its NN substrate atoms in the sub-surface layer play a more important role for the buckling of surface alloy than those from its NN substrate atoms in the surface layer do. The bucklings expected by the model are significantly smaller than those predicted by the simple hard sphere model and are in good agreement with the experiments when the equilibrium bond length of the NN adsorbate-substrate atom pairs is taken as the sum of the corresponding metal radii.     

Lithium adsorption on armchair graphene nanoribbons

Lithium adsorption on two dimensional graphene and armchair graphene nanoribbons is studied using advanced density functional theory calculations. The relative stability of different adsorption sites is investigated taking into account different ribbon widths, adsorbate densities, and spin states. We find the singlet spin state to be the true ground state of the systems considered. For this spin state, the binding energy increases with decreasing adatom density due to lower Coulomb repulsion between the partially charged Li atoms. At low adsorbate densities the favorable adsorption sites on the nanoribbons are found to be the hollow sites near the edges of the ribbon, whereas at higher densities, Li atoms tend to couple on next-nearest neighboring hexagons close to the ribbon's edge. Adsorption of the metal atoms is found to significantly decrease the bandgaps of all systems studied, turning them metallic for sufficiently large adatom densities. This suggests lithium doping as a possible route for bandgap engineering of graphitic systems.     

Theory of surface chemical reactivity

Fundamental reactivity concepts of relevance to the reactivity of transition metal surfaces are reviewed using elementary quantum-chemical concepts. The Newns-Anderson tight binding model of chemisorption is presented and subsequently used to outline the electronic structure characteristics of weak versus strong chemisorption. Fundamental concepts such as electron localization and surface complex embedding energies are defined and used to help explain surface reactivity. The emphasis here is on establishing an understanding of the surface chemical bond as a function of adatom coordination number, degree of coordinative unsaturation of the surface atoms and electron occupation of the d-type valence electron band. We derive from formal chemisorption theory the important relationships that exist between measured chemisorption properties and the average position of the d-valence electron band. The Newns-Anderson model is also used to show the relationship that exists between the d-band center and the coordinative unsaturation of the metal surface atoms. The general conclusion is that for Group VIII metals the shift of the average energy of the surface local density of states correlates with the strength of the interaction of the surface atoms with the metal atoms next to the surface layer. The same model is then used to analyze the Shustorovich bond order conservation model. The BOC or its modern version UBI-QEP is found to be consistent with a surface interaction potential comprised of a two-body repulsive term along with a constant attractive interaction independent of the number of coordinating atoms. The concepts of weak and strong chemisorption provide a very good basis for the subsequent analysis of the Brønsted-Eyring-Polanyi (BEP) relation. The extreme values of the BEP proportionality constant are related to the concept of loose and tight transition states. This proportionality constant between transition state energy and reaction energy can be expressed in parameters from the Newns-Anderson model by identifying loose transition states with intermediates in which the bond to be activated has not yet been broken, whereas in tight transition states this bond can be considered to be broken. We conclude the paper with an analysis of surface reconstruction. The power of the surface-molecule complex view of chemisorption will be quite apparent. The paper has an extensive introductory section to relate the topics of the four sections that follow with important classical catalytic notions.     

Calculation of geometries and chemisorption energies of adatoms on simple metals

We report results of calculations with a formalism that in principle applies quite generally for chemisorption on a real metallic substrate. Including the substrate structure within perturbation theory on a self-consistent jellium-plus-adatom calculation, we have computed the dependence of the binding energy of an adatom on the surface geometry. Specifically, in the case of hydrogen on Al, our model calculation predicts that the stable positions are bridge configurations on the (100) and (110) surfaces and atop positions on the (111) surface, and that they have almost the same heats of chemisorption (1.8–2.0 eV). For geometrical reasons the bridge configuration seems to be a reasonable result while the atop result for the (111) surface is more uncertain. Thus, chemisorption of H on Al should require predissociation of the H₂ gas. In addition, the predicted values for hydrogen desorption imply that measurements on H on Al surfaces should be performed at low temperatures to avoid desorption. Results for H on a jellium of Na density indicates that hydrogen should be absorbed in rather than adsorbed on Na metal.     

A study of local density of states of atom clusters on the W(110) surface

The kinetic energy distributions of field desorbed He ions from tungsten clusters of one to five atoms on a W(110) surface are measured using a high resolution pulsed-laser time-of-flight atom probe. The He field ion energy distribution from the single W adatom shows an extra peak-like feature centered at 2.7 eV above the Fermi level. It has a full width at half maximum (fwhm) of 2.3 eV. The data from two tungsten adatoms separated by two lattice constants have nearly the same feature with the extra peak located at 2.5 eV above the Fermi level. These peaks arise from resonance tunneling with the adatom local density of states (LDOS). The He ion energy distribution of a tungsten dimer has an extra peak centered at 1.5 eV above the Fermi level. The fwhm is about 4 eV. The spectra from four and five tungsten adatom clusters show only one peak each, similar to that from a flat plane.     

Optical properties and electronic structure of copper atoms adsorbed on a platinum electrode

The optical properties of Cu atoms deposited in the underpotential region from electrolytic solution onto a polycrystalline Pt electrode have been studied by differential reflectance spectroscopy. Spectra of the normalized reflectance change ($\text{ΔR}\text{R}$) have been obtained in the photon energy range between 1.5 and 5.5 eV for p- and s-polarized light as a function of coverage θ and angle of incidence ?₁. Evaluation of the adsorbate optical constants by a simple 3-phasemodel reveals three absorption bands in the range studied, one of them being strongly angular dependent. From an assignment of these transitions the positions of the 3d-, 4s- and 4p-derived levels in the energy level diagram of the Cu adatom is determined. As expected these levels are considerably shifted and broadened in comparison to their unperturbed atomic counterparts. Optical evidence for the occurrence of a structural change in the adsorbate layer at about θ = 0.6 is also presented.     

Chemisorption on a model insulator

The adsorption both of a single atom and a monolayer of atoms on the (001) surface of a model two-band crystal with the CsCI structure is investigated using the Green's function formalism and the phase shift technique. The electronic structure of the surface is described within the Linear Combination of Atomic Orbitals (LCAO) scheme and the Tight Binding (TB) approximation. Each adatom is represented by a single non-degenerate energy level. The adatoms are placed on the surface in both the on-site and the centered fourfold-site configuration. The change in the density of electronic states upon chemisorption is found, and comparison is made with similar results on a metal surface. It is shown that many, but not all, of the qualitative features in chemisorption on metallic surfaces can be transferred to the case of an insulating surface. In addition, it is shown that there are systematic variations in the density of states with adatom coverage which depend upon the absorption site.