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.     

Elastic diffuse and inelastic electron scattering from surfaces with disordered overlayers

The elastic diffuse scattering of electrons can be used for structure analysis of disordered species on surfaces. Effective data acquisition is achieved with instruments which do not resolve between elastic diffuse scattering and inelastic scattering due to phonons. With a newly designed high resolution spectrometer it is shown that the inelastic intensity is of the same magnitude or larger than the elastic diffuse scattering. It is also shown however, that the total inelastic intensity changes little when a lattice gas of adsorbates is present on the surface, whereas the elastic diffuse intensity increases linearly with coverage. Thus instruments with low energy resolution may be used for the analysis of local structure of disordered adsorbates provided one measures the difference between the total diffuse scattering of the adsorbate covered and the clean surface.     

Photoemission from p-GaAs(001) with nonequilibrium cesium overlayers

The dependences of the photoemission current and effective electron affinity on the submonolayer cesium coverage at the adsorption of Cs on a GaAs(001) surface, as well as the kinetics of the photocurrent and affinity after the termination of Cs deposition, which is caused by the relaxation of the structure of a nonequilibrium adsorption layer, have been experimentally studied. The revealed features in the dependence of the photocurrent on the Cs coverage are attributed to a nonmonotonic behavior of the surface band bending in the Cs/GaAs(001) system. It has been established that a relaxation decrease in the photocurrent in the case of coverages smaller than half a monolayer is due to the relaxation of the band bending, whereas an increase in the photocurrent at larger coverages is caused by the relaxation of the electron affinity.     

X-ray-induced polymerization of furan overlayers on Ru(001)

The effects of Mg K X-rays on furan overlayers on the Ru(001) surface have been investigated using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). It was found that X-ray beams can polymerize furan multilayers condensed at 80 K, resulting in the appearance of new emission features at 532.8 eV in the O 1s XPS spectra and at 3 eV in the UPS spectra. In contrast, monolayer furan on Ru(001) at 80 K shows no signs of polymerization under the same conditions.     

Low temperature ordering of sodium overlayers on RU(001)

The adsorption of alkali metals on transition metals can produce several technologically important effects, but only limited results have been reported on the geometrical structure of such adlayers, especially for adsorption temperatures below 300 K. We have examined the adsorption of Na on Ru(001) as a function of coverage and temperature using LEED to determine the adlayer structure and thermal desorption spectroscopy to characterize binding kinetics and relative Na coverages. The only Na LEED pattern observed following adsorption at 300 K was that of $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)}$ structure which occurred near saturation of the first layer. However, Na adsorbed at 80 K produces a progression of distinct, ordered LEED patterns with increasing coverage which does not include the $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)}$ pattern. These patterns result from increasingly compressed, hexagonal arrangements of adsorbate atoms which are uniformly spaced due to mutually repulsive interactions. The order-disorder transition temperature for each structure was also determined by LEED and used to develop a 2D phase diagram for Na on Ru(001). Ordered structures were observed only when Na thermally induced motion was sufficiently limited and the repulsive Na-Na interaction could force the uniform spacing of Na atoms. Thus, low coverage structures only developed where Na mobility was limited by low temperature. High coverage structures were stable to much higher temperatures since motion was inhibited by the high Na density.     

Core level binding energy shifts of K and Cs adlayers on Ru(001)

Using synchrotron radiation from the VUV ring at NSLS and vacuum ultraviolet radiation from a HeI resonance lamp, we have recorded high resolution photoemission spectra of K and Cs overlayers on Ru(001). It is found that for “thin” multilayer coverages ( 3 ML) the K3p and Cs5p core levels exhibit three sets of core levels which can be assigned to interface, “bulkrd and surface emission in increasing binding energy. The results are discussed in terms of the nature of electronic interaction and a thermodynamic model. The K3p core level spin-orbit splitting is also resolved in these measurements for K in the condensed phase, for the first time with photoemission spectroscopy.     

Effects of adsorbates on electron spin polarization in low energy electron diffraction from tungsten (001): I. Disordered CO and O2 overlayers

Electron spin polarization and intensity profiles have been measured in low energy electron diffraction (LEED) for the (00) beam at θ = 13δ and ø = 0δ from W(001) as a function of surface exposure to CO and O₂. Significant changes have been observed in the profiles upon exposure to both adsorbate gases, and the implications of these results are discussed.     

Plasmons in monolayer Na,K and Rb films adsorbed on Ni(100)

Electronic excitations in denser monolayer Na, K and Rb films and Na duolayer films adsorbed on a Ni(100) surface have been investigated using Electron Energy Loss Spectroscopy (EELS). Lateral adatom distributions were monitored by LEED. Angular integrated EEL spectra from the ordered c(2 × 2)Na, coverage θ = 0.5, and the ordered hexagonal structures of K and Rb, θ = 0.29, show prominent losses at 3.1, 1.9 and 1.7 eV, of presumably collective nature. The loss energies shift with coverage as ∝ θ^0.4 and as ∝ θ^0.8 for the Na and K, Rb respectively. Angular resolved EEL spectra indicate an only weak dependence of the loss energies on the momentum transfer, Q. In particular the K and Rb losses seem to pass through shallow energy minima, which is predicted by the “box model”. Low energy losses observed at ?1.3 and ?1.0 eV for the c(2 × 2)Na and the hexagonal K and Rb, respectively, are tentatively identified with interband excitations. The observed interband energies yield, when introduced in the “box model”, 3.1., 2.3 and 2.4 eV for the Na, K and Rb, Q = 0 plasmon energies, which is in fair agreement with the observed plasmon loss energies.     

The bonding of chalcogen overlayers on Ni (100)

We present new evidence for the character of two forms of bonding by chalcogen adsorbates on Ni(100) for two different coverages, while the adsorbate site remains the same. Comparisons with data from solid state systems, rather than molecular analogs, are used and may be appropriate sources for models in such cases of strongly chemisorbed adatoms. By comparing recent photoemission data on solid chalcogens (S, Se, Te) with that of chalcogen overlayers on Ni(100), we are able, but for different reasons, to support the previous proposal that with the chalcogen atoms adsorbed in a four-fold site that (a) for the ordered c(2×2) one half-monolayer coverage, the chalcogen adatoms on Ni(100) are bonded to two Ni atoms and that (b) for an ordered one-fourth monolayer coverage, the chalcogen is bonded to four Ni atoms. Two chalcogen adsorbate-induced levels, one largely non-bonding and the other made up of bonding electrons, should be found in the valence region when the chalcogen is two-fold coordinated, as it is in the elemental solid. Several methods for testing these proposals have been suggested. The modification of adatom bonding as a function of coverage implies important adatom-adatom interactions which may be involved in the poisoning of Ni catalysts by chalcogen compounds.     

X-ray photoelectron diffraction investigation of the (2 × 2) overlayers of Cs and K on Cu(111)

X-ray photoelectron diffraction measurements have been made for the Cu(111)(2 × 2)-Cs and Cu(111)(2 × 2)-K systems to explore the possibility of using substrate emission XPD for the elucidation of adsorbate-substrate registry in these cases of strongly scattering adsorbate atoms and anticipated atop adsorption sites. Although scattering effects within the substrate, for sub-surface emitters, clearly complicate the interpretation of the data, simple symmetry arguments do give substantial support to the identification of atop adsorption sites in both systems.     

Theory of adsorption: Ordered monolayers from Na to Cl on Si(001) and Ge(001)

First principles calculations of clean and adsorbate-covered surfaces of Si(001) and Ge(001) are reported. Chemical trends in the adsorption of ordered Na, K, Ge, As, Sb, S, Se and Cl overlayers are discussed. The calculations are based on the local-density approximation and employ non-local, norm-conserving pseudopotentials together with Gaussian orbital basis sets. The semi-infinite geometry of the substrate is properly taken into account by employing our scattering theoretical method. From total-energy minimization calculations we obtain optimal surface reconstructions which show asymmetric dimers for Si(001), Ge(001) and Ge:Si(001). For As:Si(001), Sb:Si(001) and Sb:Ge(001), we find symmetric adatom dimers in the equilibrium geometries. S or Se adlayers are found to be adsorbed in bridge positions forming a (1×1) unit cell with a geometry very close to the configuration of a terminated bulk lattice. Cl atoms adsorb on top of the dangling bonds of symmetric Si dimers residing in the first substrate-surface layer. Our calculations for Na:Si(001) and K:Si(001) confirm valley-bridge site adsorption for half monolayer coverage. For full monolayer alkali-metal coverage, adsorption in pedestal and valley-bridge positions is found to be energetically most favourable. The calculated optimal adsorption configurations are in excellent agreement with a whole body of recent experimental data on surface-structure determination. For these structural models, we obtain electronic surface band structures which agree very good with a wealth of data from angle-resolved photoemission spectroscopy investigations.     

Surface magnetic structure analysis of Ni(001) by polarized electron diffraction

For spin-polarized low-energy electrons diffracted from ferromagnetic Ni(001), the exchange-and the spin-orbit-induced scattering asymmetries A_ex and A_so of two diffracted beams have been simultaneously measured at T = 300 K and at T = 520 K for several constant energies as functions of the polar angle of incidence. Corresponding “rocking curves” calculated by dynamical theory are in generally good agreement with the data. Analysis of A_so shows firstly that an energy-dependent exchange approximation is adequate, secondly that the topmost interlayer spacing is essentially bulk-like with a possible slight outward relaxation (1%), and thirdly that 3p → 3d excitation is significant. From A_ex the following magnetic information is obtained: The surface magnetization for T = 300 K and 520 K is such as to imply, via extrapolation by mean field theory, a T = 0 value slightly enhanced (by + 5% ± 5%) with respect to the bulk magnetization. The spin asymmetries of localized and uniform inelastic processes are very small, leading, for incident electron energy 69 and 89 eV, to a combined inverse mean free path asymmetry of less than 0.3%.