Surface vibrations of Na(001) and K(001) surfaces

We present the results of a theoretical study of the dynamics of the atom motion of Na(001) and K(001) surfaces. The total electronic energy is calculated using a pseudopotential approach with a confined electron gas as unperturbed system. With this theory the dynamical matrix can he derived without resorting to empirical parametrizations. Surface phonon dispersion curves are reported for the high symmetry directions of the two-dimensional Brillouin zone for ideal and relaxed configurations. The calculated spectra are compared with the results of semi-empirical force constant calculations. The effects of single and multilayer relaxations on the location and the nature of the main surface bands are examined.     

The geometry of CO on Ru(001): Evidence for bending vibrations in adsorbed molecules

As a test of the utility of the ESDIAD method (Electron Stimulated Desorption Ion Angular Distributions) in studies of the geometry of adsorbed molecules, the chemisorption of CO on Ru(001) has been examined. Data previously reported using UPS (ultraviolet photoemission spectroscopy) and EELS (electron energy loss spectroscopy) have indicated that CO is terminally bonded to the Ru surface through the C atom, with the CO axis perpendicular to the surface. The ESDIAD results for CO confirm this orientation; for all CO coverages in the temperature range 90 K to ~ 350 K, the angular distributions of O⁺ and CO⁺ ESD ions are centered about the surface normal. The widths of the ion beams are temperature dependent; for both O⁺ and CO⁺, the half widths at half maximum, α, of the ion cones are ~16° at 300 K, and ~12° at 90 K. This temperature dependence, coupled with a simple model calculation, indicates that the dominant factors contributing to the width of the ESD ion beams are the CO surface bending vibrations, i.e., initial state effects. Thus, the data suggest that both the directions and widths of ESDIAD beams are determined largely by the structure and dynamics of the initial adsorbed state.     

Surface charge-density wave phase transitions on Mo(001) and W(001)

The recently reported phase transitions of Mo(001) and W(001) are discussed in terms of surface charge-density waves. Band structure, surface lattice distortion and possible accompanying surface antiferromagnetism are considered. The expected two-dimensional critical behaviour is also investigated.     

Comparison between Rydberg surface resonances on W(001) and W(110)

Comparison is made between high-resolution LEED measurements of electronic surface resonances in the (00) beam from W(001) and W(110). The study of W(110) extended to several polar angles of incidence provides new information that helps to explain the data obtained on W(001) at a fixed angle.     

Surface vibrations of CO on W(100)

High resolution electron-energy-loss spectroscopy has been used to study the surface vibrations of CO on a W(100) surface at 300 K. For small exposures (β-CO) two losses at ～68 meV and ～78 meV are observed. This vibrational spectrum of β-CO is a clear indication of dissociative adsorption with the carbon and oxygen atoms in fourfold coordination sites each. With further exposure to CO two additional losses at 45 meV and 258 meV are observed, which represent the vibration of undissociated α-CO in upright position on top of a W atom. Furtheron results of coadsorption of H₂/CO and O₂/CO on W(100) are reported.     

Electron reflection measurements on W(001) and W(110) between 0 and 40 eV

High resolution low energy electron reflection measurements from W(001) and W(110) in the range 0–40 eV are reported. Spectra recorded for a variety of angles of incidence in several different azimuths are presented. In particular, some fine structure features associated with beam thresholds are observed for all incidence conditions. However, it is found that other threshold effects are either absent or very weak. It is shown that this phenomenon is probably a consequence of the limited azimuthal resolution of the spectrometer used. Because of the variety of incidence conditions investigated, the measurements comprise a suitable data base for testing LEED calculation schemes in the very low energy range.     

Surface vibrations on clean and hydrogen saturated W(100)

Electron energy loss Spectroscopy (EELS) and lattice dynamical calculations are used to investigate the vibrational properties of surface phonons and adsorbate phonons on clean and hydrogen-saturated W(100). Two distinct intrinsic surface vibrations are observed in specular scattering geometry. One of the surface modes is attributed to a surface resonance of a bulk longitudinal phonon at the hydrogen-stabilized (1×1) surface. The second surface mode occurs only on the low-temperature-stabilized c(2×2) [011] displacement surface. Detailed experimental studies and lattice dynamical analysis of the hydrogen-saturated W(100) surface account for a new hydrogen-derived loss peak at 118 meV in terms of an optic mode of the adsorbed layer. Experimental evidence of impact scattering resonances under certain kinematic conditions are observed.     

A comment on the W(001) surface stability and reconstruction

Arguments are presented in favour of an additional softening which those changes of surface force constants that lead to a pronounced “soft” surface phonon mode (e.g. M₅) can invoke in the global surface phonon spectrum. This result is illustrated for the W, Mo and Cr (001) surface. Analysis of phonons of an isolated layer is used together with a “perturbation theorem”.     

Long-wavelength surface phonon spectrum of W(001)

We calculate eigenvectors and local densities of long-wavelength phonon states for a (001) semi-infinite crystal of W, by a microscopic force constant approach. The results are discussed in relation to surface structure (relaxation) and to surface-sensitive spectroscopies.     

Bonding of nitrogen atoms on Cu(001) surfaces: A cluster approach

A study of the chemisorption of nitrogen atoms on a copper surface has been performed, based on an analysis of the electronic structure of the Cu₅N cluster obtained from self-consistent-field Xα scattered-wave calculations. Our calculations show that the chemisorption of nitrogen on Cu(001) surfaces induces peaks below and above the Cu d-band region in the total density of states curve. The bonding orbitals formed between the N 2p and the Cu valence orbitals are generally found near the bottom of the Cu d-band region, while the antibonding orbitals formed between the N 2p and Cu orbitals are found to lie above the Cu d-band region. These hybridized orbitals involving the N 2p orbital gave a satisfactory interpretation of the adsorbate-induced structure reported in N/Cu(001) ultraviolet photoemission (UPS) studies. In addition, the separate contributions of the N 2p⊥ and 2p∥ states to the total density of states curve of the Cu₅N cluster are given. This information may be useful in interpreting angleresolved UPS data.