Vibrational properties of the graphite (0001) surface

The temperature dependence of low-energy electron diffraction intensity-energy spectra from the graphite (0001) surface has been measured from 100 to 600 K. The perpendicular and parallel effective surface Debye temperatures and the effective mean-square vibration amplitudes as a function of electron energy have been determined from these data. The values show surface enhancement and approach the bulk values with increasing electron energy. At energies below ≈ 100 eV, however, there is little anisotropy observed between the effective Debye temperatures parallel and perpendicular to the graphite surface.     

Mean square displacement of xenon atoms in an epitaxial monolayer on (0001) graphite

The thermal variation of the mean square displacement of xenon atoms adsorbed in epitaxy on (0001) graphite is measured by low energy electron diffraction in the 48–73°K temperature range. Lattice dynamics calculation is in good agreement with experimental results.     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

Adsorption of Ag and Au atoms on wurtzite ZnO (0001) surface

We perform first-principles calculations to investigate various surface structures in the absorption of Ag and Au atoms on wurtzite ZnO (0001) surface. The results show that both Ag and Au atoms prefer to be absorbed on the H₃ sites (the center of Zn–O ring) of the surface, and the most favorable monolayer (ML) coverage is 1. The calculated electron structure shows that the Ag- and Au-adsorbed ZnO (0001) surfaces exhibit metallic characteristics even the ML coverage of the adatoms is very low. Finally, the work functions of Ag- and Au-adsorbed ZnO (0001) surfaces are calculated and discussed for the first time in the present work.     

Ru(0001)表面氮分子和钡原子的相互作用

本文采用密度泛函理论方法研究了Ru(0001)表面氮分子和钡原子的相互作用.计算结果表明,钡原子的作用弱化了氮分子键.氮分子键长从Ru(001)-N2表面的0.113 nm伸长互Ru(001)-N2/Ba表面的0.120 nm;分子的拉伸振动频率从2221 cm-1减小到1746 cm-1;氮分子得到的电荷数从清洁表面的0.3e增加到1.1 e.电荷从钡原子6s轨道向钌原子4d轨道转移,转移电荷增强了氮分子2π空轨道和钌原子4d轨道间的杂化作用,导致5σ分子轨道和dπ杂化轨道发生极化.轨道极化使分子电偶极矩增加了约-0.136 e(A).金属钡在Ru(0001)表面氮分子活化过程中具备电子型助催剂的特征.     

Adsorption of D(H) atoms on Ar ion bombarded (0 0 0 1) graphite surfaces

Adsorption of thermal (2000 K) D (H) atoms on HOPG surfaces prior to and after bombardment with 500 eV Ar ions was studied with thermal desorption and vibrational spectroscopies. Ion bombardment of HOPG generates vacancy (VD, displaced surface C atoms) and interstitial (ID, Ar captured between 1st and 2nd C plane) defects. These defects remove the ability of the surface to adsorb D like on virgin HOPG surfaces and to form C_gr–D bonds. After a dose of 0.1 Ar per C surface atom, D adsorption is markedly suppressed. Annealing of bombarded surfaces at 1350 K, connected with desorption of trapped Ar and removal of ID, recovers a large fraction of the adsorption capacity for D. Therefore, the long range stress in the surface plane introduced by ID must be responsible for a significant fraction of D adsorption blocking. It is suggested that ID prevent reconstruction of the C surface which is required for the formation of C_gr–D bonds. For ion doses above 0.5 Ar/C, adsorption of D on the surface is negligible. After annealing at 1350 K, D can be adsorbed in quantities comparable to the virgin HOPG surface, however forming C–D bonds which are similar to those observed in hydrogenated amorphous carbon instead of those which are normally formed on HOPG. Instationary etching via release of deuterocarbon species occurs primarily in the C₁ and C₂ channels. It is only observed at bombarded HOPG prior to annealing and probably due to the presence of isolated C₁ and C₂ species on the surface generated upon VD formation.     

Metastable structures and recombination pathways for atomic hydrogen on the graphite (0001) surface

We present scanning tunneling microscopy results which reveal the existence of two distinct hydrogen dimer states on graphite basal planes. Density functional theory calculations allow us to identify the atomic structure of these states and to determine their recombination and desorption pathways. Direct recombination is only possible from one of the two dimer states. This results in increased stability of one dimer species and explains the puzzling double peak structure observed in temperature programmed desorption spectra for hydrogen on graphite.     

Nuclear level mixing (NLM) experiments on polarized Na atoms chemisorbed on a W(110) surface

Nuclear level mixing (NLM) experiments were performed on polarized Na atoms chemisorbed on a clean and oxygen covered W(110) surface. Components of the electric field gradient tensor generated by the electronic environment of the nucleus were determined. Advantages and disadvantages of this method if compared to NMR measurements are listed briefly. The results are discussed in the framework of simple models.     

Binding energies and K-edge spectra of NO chemisorbed on Ru(0001)

The electronic structures, adsorption energies and equilibrium geometries of chemisorbed NO on the Ru(0001) surface for several adsorption sites were determined using cluster models and employing the first principles, local density theory. Ground state one-electron eigenvalues and wave functions were obtained using the discrete variational method with numerical atomic bases. K-edge X-ray absorption spectra of both nitrogen and oxygen for each geometry were obtained using a scattered wave method. The effects of the surface on these spectra were studied by comparison with results from an isolated NO molecule and with the XPS and UPS experiments of Umbach et al. Both the NO and the NO-substrate vibration frequencies were calculated. Comparison between these theoretical results, recent electron energy loss spectra of Conrad et al. and early experiments of Thomas et al. suggests that NO is adsorbed only at threefold and top sites.     

Vibrational motion of hydrogen atoms chemisorbed on Ni(100)

The vibrational motion of hydrogen and deuterium atoms adsorbed, at different coverages, on the Ni(100) surface has been investigated using high-resolution electron-energy-loss spectroscopy. In specular scattering the vibrational spectra show only one prominent fundamental excitation, which can be reconciled with the perpendicular motion of a localized adatom in a fairly harmonic potential well. Substantial coverage induced vibrational energy shifts are observed. These are caused by static and dynamic adsorbate-adsorbate interaction effects, as revealed by spectra for a substitutionally disordered p(1 × 1)(H + D) structure and a model calculation based upon the effective medium theory.     

C60分子与石墨(0001)、硅(111)面的碰撞

用分子动力学模拟的方法和Tersoff多体势函数对以一定能量入射的C 60在石墨(0001)表面以及硅(111)表面碰撞的过程进行模拟研究.结果发现:碰撞过程是高度非弹性的,在弹回过程中,C60分子质心的运动可被看作是在准谐势下的运动 .C60以240 eV初始能量入射到石墨表面时,C60分子有严重的扭曲,最终将平铺在石墨表面形成薄膜;C60分子以30 eV初始动能入射到石墨表面时,将保持完好球形沉积在石墨表面;C60分子以60 eV的初始动能碰撞硅(111)表面时,C60分子最终沉积在硅表面,碰撞过程中C60分子有形变.     

Phase transitions in physisorbed films on cadmium (0001): comparison with graphite (0001)

Physical adsorption of simple molecules has been studied experimentally on cadmium (0001). Adsorption isotherms of Kr and CF₄ at 77.3 K show a first-order phase transition in the first layer, characteristic of adsorption on uniform substrates.

The nature of the phase transition on cadmium (0001) in the first layer is investigated for Kr and CF₄ by a thermodynamic approach in comparison with results obtained earlier on graphite (0001).

From the comparison between the two uniform substrates, we show that (1) cadmium (0001) has a less attractive force field than graphite (0001); and (2) like graphite (0001), cadmium (0001) is a very uniform surface suitable for studying two-dimensional phases according to the Gibbsian formulation.     

Weak 2D hole localization on the (0001)Te surface under hydrostatic pressure

Effect of hydrostatic pressure on the anomalous magnetoresistance (AMR) of the two-dimensional (2D) hole layer on (0001) surface of Te has been studied. Application of hydrostatic pressure was found to reverse the sign of AMR of the 2D hole system from positive to negative. The results are interpreted in terms of weak localization theory for noninteracting particles, modified by taking into account the specific features of tellurium valence-band energy spectrum, namely, lifted spin degeneracy, many-valley nature, and trigonal spectrum distortion on the (0001) plane. The pressure dependences of the characteristic relaxation times representing the parameters of the theory have been found. It is shown that hydrostatic pressure symmetrizes the 2D hole spectrum at the (0001)Te surface, which manifests itself, in particular, in a reduced trigonal distortion of the spectrum. The pressure dependence of the spin-orbit splitting between the midpoint of the gap separating the two uppermost valence subbands and the lowest, spin-degenerate subband, Δ₁(P), has been determined. Fiz. Tverd. Tela (St. Petersburg) 39, 568–572 (March 1997)     

Structural characterization of a (krypton-methane) film adsorbed on (0001) graphite

Krypton-methane films adsorbed on (0001) graphite have been studied by neutron diffraction, between 45 and 98 K. A two-dimensional solid solution has been observed. Its melting temperature has been determined for two compositions of the layer. The melting appears to be a first-order phase transition.     

On the order-disorder transition of chemisorbed H on Ni (111)

The order-disorder transition of chemisorbed H on Ni (111) has been analyzed by Kittler and Bennemann in terms of the lattice gas model. The main purpose of this paper is to refine the above calculation. The expression with regard to adlayer domains is given for the half-order LEED spot intensity, from which the dependence of the transition temperature and of the LEED spot intensity on H-coverages is calculated. In particular, the H-coverage dependence of the transition-temperature T_c and the LEED intensity I for θ? 0.25 is restudied. We present results obtained by using only pair interactions between the H-atoms and no 3H-interactions. It is shown that the observed asymmetric phase diagram with respect to H-coverages can result from the two kinds of pair interactions between chemisorbed H atoms.     

Thermodynamic characterization of a dichloromethane-xenon film adsorbed on (0001) graphite

Xenon adsorption between 95 and 115 K on graphite pre-plated with a dichloromethane monolayer results in a partial displacement of the pre-adsorbed layer and in the formation of a two-dimensional (dichloromethane-xenon) solution. When the dichloromethane pre-plating is in the sub-monolayer range, a xenon monolayer is condensed on the bare part of the surface before the composite layer organization. For dichloromethane coverages greater than a monolayer, kinetic problems arise for the formation of xenon bilayer patches, which implies in this case its separation from the two-dimensional solution.