Covered Metal Surfaces Research Articles

Hydrogen plasma induced photoelectron emission from low work function cesium covered metal surfaces

Experimental results of hydrogen plasma induced photoelectron emission from cesium covered metal surfaces under ion source relevant conditions are reported. The transient photoelectron current during the Cs deposition process is measured from Mo, Al, Cu, Ta, Y, Ni, and stainless steel (SAE 304) surfaces. The photoelectron emission is 2–3.5 times higher at optimal Cs layer thickness in comparison to the clean substrate material. Emission from the thick layer of Cs is found to be 60%–80% lower than the emission from clean substrates.

Nonequilibrium Green's Function Theory of Scanning Tunneling Microscope-Induced Light Emission from Molecule Covered Metal Surfaces: Effects of Coupling between Exciton and Plasmon Modes

Nonequilibrium Green's Function Theory of Scanning Tunneling Microscope-Induced Light Emission from Molecule Covered Metal Surfaces: Effects of Coupling between Exciton and Plasmon Modes

Critical capture distances for highly charged ions above dielectric covered metal surfaces

We model the first stage of the electronic interaction between an ion and a metal surface covered with a thin dielectric layer. Specifically, we seek to answer two questions. (i) As an ion approaches the surface from far away, does the first electron that it captures originate from the exposed dielectric layer or the metal underneath it? (ii) What is the ion’s distance from the metal when the first electron is captured? To answer these questions, the classical potential that an electron is subject to during the interaction is calculated. The dielectric film is treated as a continuum with simple band structure. We input the parameters from recent experiments (Co with 1.5 nm thick Al 2 O 3 film) and found that (i) the first capture proceeds from the metal, and (ii) the dielectric film extends the distance threshold for first capture compared to a metal with no film.

Dynamics of O 2 photodesorption from metal clusters: A significant difference from bulk behaviour

Photodesorption of O 2 from size-selected Ag n O 2 - cluster anions with n = 2, 3, 4 and 8 was studied using time-resolved photoelectron spectroscopy (TR-PES). The spectra indicate that relaxations of photo-excited Ag n O 2 - clusters with n = even numbers accompany ultrafast direct O 2 photodesorption. For the odd-numbered cluster Ag 3 O 2 - , in contrast, a long-lived excited state is observed, since O 2 might be dissociatively chemisorbed, suppressing direct photodesorption of oxygen. Both, direct desorption and long-lived excited states, have not been observed from adsorbate covered metal surfaces, suggesting unique photochemical properties of such small clusters.

High resolution core level photoemission of clean and adsorbate covered metal surfaces

A brief review is given of some fundamental aspects of high-resolution core-level photoemission and of its application to studies of pure and adsorbate covered metal surfaces. We first give a short description of the basic theory and then turn to a number of examples. The first group of examples is aimed at demonstrating the use of core-level binding energy shifts for deriving the surface and interface geometrical structure whereas the second group deals with the core-level lineshape and, in particular, the influence of vibrational and other excitations.

Dielectric response of covered metal surfaces: Oxidation of Al(111)

Dielectric response of covered metal surfaces: Oxidation of Al(111)

Reactions at surfaces studied by ab initio dynamics calculations

Owing to the development of efficient algorithms and the improvement of computer power it is now possible to map out potential energy surfaces (PESs) of reactions at surfaces in great detail. This achievement has been accompanied by an increased effort in the dynamical simulation of processes on surfaces. The paradigm for simple reactions at surfaces — the dissociation of hydrogen on metal surfaces — can now be treated fully quantum dynamically in the molecular degrees of freedom from first principles, i.e., without invoking any adjustable parameters. This relatively new field of ab initio dynamics simulations of reactions at surfaces will be reviewed. Mainly the dissociation of hydrogen on clean and adsorbate covered metal surfaces and on semiconductor surfaces will be discussed. In addition, the ab initio molecular dynamics treatment of reactions of hydrogen atoms with hydrogen-passivated semiconductor surfaces and recent achievements in the ab initio description of laser-induced desorption and further developments will be addressed.

Effects of restructuring in adsorption and reaction dynamics at metal surfaces

After more than 30 years of research in surface science, the atomic details of the structural diversity of clean and adsorbate covered metal surfaces are now revealed by diffraction and by microscopic techniques. The phenomenon of adsorbate-induced restructuring is found to be pervasive, and proves to lie behind many characteristic processes in the field of heterogeneous catalysis, such as bistability and kinetic oscillations.

A simple theory of two-photon photoemission from clean and adsorbate covered metal surfaces

A simple theory is presented to study the elementary processes of two-photon photoemission (2PPE) from clean metals and adsorbates on metal surfaces. The model can be applied to metals having the occupied surface state and unoccupied image-potential state and also to adsorbates whose unoccupied states are populated from the occupied valence levels or the substrate by photoexcitation. The 2PPE spectrum is calculated on the basis of a simple energy diagram. It is demonstrated that one-photon photoemission from an initially unoccupied state (a fixed intermediate state which is populated by photon-driven electron excitation from an occupied metal state), is accompanied by two-photon photoemission from the occupied initial state via the unoccupied state as intermediate. When an electron is nonresonantly excited from narrow metal states such as d-bands or surface states, the lineshape of the corresponding 2PPE spectrum exhibits a characteristic asymmetry due to the underlying direct 2PPE from the occupied states. At resonant excitation from the narrow initial state to the intermediate state the 2PPE spectra exhibit a narrowing of the linewidth. It is shown that the lifetime of the unoccupied state cannot be deduced from the linewidth of the 2PPE spectra if a width of the initial state is comparable or narrower than the unoccupied state. No resonance enhancement and narrowing effect in the 2PPE spectra appear when a broad bulk continuum serves as an initial state of the 2PPE process.

Adsorption of single lithium atoms on metal surfaces

Nuclear spin relaxation experiments are described which determine the density of states at the Fermi energy times the probability to find an electron at the nucleus for singly adsorbed 6Li and 7Li atoms on clean and oxygen covered metal surfaces of high temperatures.

Scanning tunneling microscopy on clean and adsorbate covered metal surfaces

The use of scanning tunneling microscopy (STM) for atomic scale characterization of clean and adsorbate covered (single-crystalline) metal surfaces is discussed. Topographic images reveal details on their periodic structure and on the atomic arrangement in the surface layer, and in particular on surface defects. The observation and characterization of individual adsorbate species gives access to the local electronic structure of the adsorption complex and to details of the chemical bond between substrate and adsorbate. Atomic resolution imaging opens new perspectives for the investigation of various surface processes such as surface diffusion, thin film growth or surface reactions.

Surface Phonons on Clean and Adsorbate Covered Metal Surfaces

Recent progress in surface phonon spectroscopy using inelastic electron scattering is reviewed. For a number of adsorbate systems the dispersion curves of the adsorbate modes have been measured. It is demonstrated that important structure parameters can be deduced from the local symmetry of the adsorbate revealed by the absence of a disperison branch under particular scattering conditions. Among the adsorbates hydrogen is a particularly interesting case, since there the classical picture of vibrational motion must be replaced by a quantum mechanical description. For some adsorbate covered or metastable clean surfaces the dispersion of the Rayleigh wave indicates a compressive or tensile surface stress. Finally the significance of an experimental determination of the dispersion of surface phonons with odd symmetry will be pointed out.

Theory of scanning tunneling microscopy

The present theoretical understanding of imaging clean and adsorbate covered metal surfaces in scanning tunneling microscopy is examined with special emphasis on a possible theoretical foundation for the observed unexpectedly large corrugation on close-packed metal surfaces. Several suggestions for explaining these experimental findings are investigated. Resonance tunneling via tip d-orbitals might be a possible mechanism of amplifying small lateral structure of electronic or elastic origin. Two complementary theoretical methods are applied. The first one concentrates on a realistic description of the potential and wave functions of the sample surface whereas the second one attempts to model a more realistic transition metal tip. In the first approach the tip is represented by a Gaussian protrusion on an otherwise planar free-electron metal surface. The sample surface is built from muffin-tin potentials accounting for the atomic structure and the d-electrons. The spatial current distribution near the tip region is obtained by summing the contributions of all scattered waves. The method has been applied to study the current to Al(111) and Pd(100) surfaces. The corrugation obtained is rather small and cannot explain the experimental observations. The second approach studies two transition metal tips consisting of a single tungsten atom adsorbed on a flat W(110) surface and on a group of four other W atoms. The cluster of four W atoms is coupled to a flat W(110) surface by using an embedding method. The basis set on the W atoms includes 6s-, 6p-, and 5d-orbitals. The electronic structure of the tip exhibits a 5d2-resonance near the Fermi level. The effects of tip d-orbitals and resonance tunneling on the lateral contrast in STM are analyzed.

Electrochemically initiated (co)polymerization of acrylamide and acrylonitrile on a steel cathode?electrochemical and impedance study

Investigations were carried out into the processes of electrochemically initiated (co)polymerization (EIP) of acrylamide (AA) and/or acrylonitrile (AN) from acidic aqueous solutions on a steel cathode. The aim was to obtain (co)polymer films with given electroinsulating and corrosion protection properties. The kinetics of (co)polymer film formation were investigated using various electrochemical methods (constant current and constant potential electrolysis, cyclic voltammetry, double-layer capacity measurements) and ellipsometry. The electrical properties of the films formed were studied (ex situ) by measuring their impedance (Z) and phase angle (θ) as a function of frequency. It was shown that equivalent circuit (EC), proposed for covered metal surfaces in solutions, can be satis-factorily applied. The values of the EC elements obtained by curve fitting to experimental results show the influence of the thermal treatments and/or moisture concentration on the impedance characteristics of polymer films. The mechanism of (co)polymer film formation as well as the electrical and corrosion protection properties are discussed with regard to the experimental parameters and some characteristics of the films obtained.

Vibrational structure in theN2−(Πg2)electron resonance ofN2films

The formation of a temporary $\mathrm{N}_{2}^{}{}_{}{}^{\ensuremath{-}}$ state around 2 eV in electron scattering from ${\mathrm{N}}_{2}$-covered metal surfaces is established by resolving the vibrational structure of the anion in the $v=1,2,3,\mathrm{and} 4$ decay channels of ground state ${\mathrm{N}}_{2}$ and by the demonstration of the presence of a "boomerang" effect. The average vibrational energy (0.29 meV) and the lifetime (\ensuremath{\sim} 3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ sec), which are close to the gas-phase values, indicate that the solid-phase anion has a symmetry similar to the $^{2}\ensuremath{\Pi}_{g}$ configuration.

Scattering of metastable rare gas atoms from solid surfaces

This work reports on measurements of the survival probabilities (SP) and angular distributions of metastable He atoms backscattered from clean and adsorbate covered metal surfaces, as well as on data for He ∗, Ne ∗ and Ar ∗ reflected from LiF and NaCl surfaces. The SP for He ∗ are always below ∼ 10 −3, they increase for the clean metal surfaces in the order Cu<Pd<W, as well as upon adsorption of CO or oxidation. Equally small values (∼10−4) were found for He ∗ and Ar ∗ reflected from the ionic crystals, while a considerably higher value (∼ 10 −2) was determined for Ne ∗ in this case. The angular distributions vary considerably from those of the respective ground state atoms which has to be attributed to the marked change of the interaction potentials.

Studies of the desorption mechanism of O − by electron impact from oxygen covered metal surfaces

Electron-stimulated desorption of negative ions from oxygen covered tungsten and molybdenum has been investigated. The O − desorption current as a function of incident electron energy was measured. The following results were observed: (1) the threshold of negative ion desorption is much lower than that of positive ions (20–25 eV for O +; 4–5 eV for O −); (2) some resonant-like structures appear in the curves of desorption current as a function of electron energy in the low energy regions; these structures do not appear in the high energy regions; (3) these resonant-like structures are basically the same for O 2-W and O 2-Mo systems. Experimental results seem to indicate that the desorption of negative ions are essentially a feature of the absorbate and can be most readily explained based on potential energy curves for some electronic states of oxygen molecules and ions, and within the framework of the Franck-Condon principle.

Studies of the desorption mechanism of O - by electron impact from oxygen covered metal surfaces

Studies of the desorption mechanism of O - by electron impact from oxygen covered metal surfaces

Origin of Secondary Electrons Ejected from Gas-Covered Surfaces by Fast Neutral Beams

Secondary electron emission from gas covered metal surfaces, discussing results of neutral beam investigations