V. Cháb

An X-ray absorption and photoelectron diffraction study of the Cu{100} c(2 × 2) CO structure

Abstract The structure of CO adsorbed on Cu{100} has been investigated using near edge X-ray absorption fine structure at the oxygen K-edge to determine the molecular orientation, and normal emission photoelectron energy-intensity spectra for the C 1s and O 1s core levels to determine the adsorption site. The photoelectron diffraction data have been interpreted by comparison with the results of single scattering model calculations. Good agreement is found for CO molecules adsorbed normal to the surface on on-top sites with a CuC distance of 1.92±0.05 A , in accordance with a previous LEED study. Calculations show that intramolecular scattering can be rather unimportant for this technique and that the individual atomic sites may then be obtained from the photoelectron diffraction spectrum by studying the emission from the relevant atom and scattering from the substrate. The method should therefore be applicable to more complex molecular adsorbates.

Adsorption of Histidine and Histidine-Containing Peptides on Au(111)

The adsorption of histidine (His) and three His-derived peptides on Au(111) has been studied by soft X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the nitrogen and oxygen K edges. The peptides were glycyl-histidine (Gly-His), glycyl-histidine-glycine (Gly-His-Gly), and glycyl-glycyl-histidine (Gly-Gly-His) and were adsorbed at saturated coverage on the Au(111) surface from aqueous solution. Coverages of 1 and 0.5 monolayers (ML) of His were adsorbed by evaporation in vacuum and compared with 1 ML deposited from solution. There were no significant chemical differences between the monolayers deposited in vacuum or from solution. The Au 4f core level shift indicates that a chemisorption rather than a physisorption bond is formed. In both deposited phases, His bonds to the gold surface in anionic form via the imino nitrogen atom of the imidazole ring and the oxygen atoms of the carboxylate group. N and O K-edge NEXAFS indicate that the ring and carboxylate triangle of adsorbed His are tilted at approximately 35 degrees and approximately 27 degrees, respectively, with respect to the Au(111) surface. The peptides bond to the gold surface in a mode similar to the single His molecule, via the imino and carboxylate groups, while the peptide group is at a steep angle to the surface. However, the peptides adsorb with a higher atomic density, consistent with the peptide groups being above the surface. There are also differences between Gly-His-Gly and Gly-Gly-His, implying that the sequence within the peptide has a significant influence on the bonding geometry.

Simultaneous current, force and dissipation measurements on the Si(111) 7×7 surface with an optimized qPlus AFM/STM technique.

Summary We present the results of simultaneous scanning-tunneling and frequency-modulated dynamic atomic force microscopy measurements with a qPlus setup. The qPlus sensor is a purely electrical sensor based on a quartz tuning fork. If both the tunneling current and the force signal are to be measured at the tip, a cross-talk of the tunneling current with the force signal can easily occur. The origin and general features of the capacitive cross-talk will be discussed in detail in this contribution. Furthermore, we describe an experimental setup that improves the level of decoupling between the tunneling-current and the deflection signal. The efficiency of this experimental setup is demonstrated through topography and site-specific force/tunneling-spectroscopy measurements on the Si(111) 7×7 surface. The results show an excellent agreement with previously reported data measured by optical interferometric deflection.

The electronic structure and adsorption geometry of L-histidine on Cu(110).

The adsorption of L-histidine on clean and oxygen-covered Cu(110) surfaces has been studied by soft X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The valence band spectra, carbon, nitrogen and oxygen 1 s XPS and N K edge absorption spectra were measured for submonolayer, monolayer, and multilayer films. The spectra provide a detailed picture of the electronic structure and adsorption geometry at each coverage. In the monolayer, the histidine molecules are randomly oriented, in contrast to the submonolayer regime, where the molecules are coordinated to the copper surface with the imidazole functional group nearly parallel to, and strongly interacting with, the surface. The pi*/sigma* intensity ratio in NEXAFS spectra at the nitrogen edge varies strongly with angle, showing the imidazole ring is oriented. Adsorption models are proposed.

Surface diffusion of K on Pd{111}: Coverage dependence of the diffusion coefficient determined with the Boltzmann–Matano method

The surface diffusion of potassium on Pd{111} has been studied with photoelectron emission microscopy (PEEM) for coverages up to one monolayer. The coverage dependence of the chemical diffusion coefficient is determined by analysis of the concentration profiles obtained from the PEEM images with the Boltzmann–Matano method. The diffusion coefficient, D, decreases with increasing coverage but a local maximum is found at a coverage of Θ≈0.5 ML. The values of D at low coverages (Θ<0.3 ML) agree well with those obtained in a previous investigation for Θ≈0.12 ML. The maximum in D is interpreted in terms of an order–disorder phase transition in the adsorbed layer.

Thermodynamics of adatoms diffusing on a surface with two different sites: a new type of phase transition

Abstract The chemical surface diffusion coefficient D has been determined for a lattice-gas model where diffusion proceeds through two non-equivalent lattice sites with different activation energies, E1 and E2. By calculating the thermodynamic factor as well as jump diffusion rate we obtain the coverage dependence of D as a function of the energy difference E1 − E2. No lateral interactions are considered. Depending on the sign of E1 − E2, the diffusion coefficient shows either a maximum or a step-like increase at a particular coverage, both of which develop into discontinuities in the limits of either E1 − E2 → ± ∞. In the case of the step-like behaviour of D, the variation of D can be classified as being due to a kind of diffuse phase transition between a low and a high diffusivity state, which in the limits is shown to be of first order.

A model description of surface diffusion in the presence of two non-equivalent lattice sites

Abstract A dynamical description of surface diffusion is presented for a surface containing two non-equivalent bonding sites with different binding energies. Diffusion proceeds via single hopping events between adjacent sites which can be of both kinds. Dynamical coupling is introduced by assuming that the hopping rates into and out of one type of site depend on whether the adjacent site of the other kind is occupied or empty. The derivation starts out from the microscopic difference equations accounting for the single jump events for all possible configurations. These are then transformed into differential equations with continuous functions and solved by the method of finite differences. The activation energies for individual jumps serve as adjustable parameters. It is shown that the widely observed dependence of the diffusion coefficient on coverage also follows naturally from this model, although direct particle-particle interactions in the usual sense are specifically excluded. The influence of the substrate-adsorbate interaction and of the surface geometry on the diffusion process is also discussed.

Temperature dependence of the surface diffusion coefficient of K atoms on Pd{111} measured with PEEM

The surface diffusion of potassium on Pd(111) has been studied in real time as a function of temperature between 100 and 190 K in the limit of zero concentration (Θ < 0.05 ML) using photoelectron emission microscopy (PEEM). Very fast migration of K atoms was observed on flat, quasi-one-dimensional regions of the surface between the step bunches. The temperature dependence of the diffusion coefficient indicates an activated process with an energy barrier of 66 ± 11 meV.

Coverage-dependent surface diffusion of K on Pd{111} studied with photoelectron emission microscopy

Abstract The surface diffusion of K on Pd〈111〉 has been studied with photoelectron emission microscopy (PEEM). It is shown that the method can be used to investigate diffusion in the full monolayer coverage region by calibrating brightness versus coverage in the PEEM image. In addition to a very fast diffusion stage at the lowest concentrations (

Surface core level shifts of A II–VI compound: CdTe

Abstract The surface core level shifts (SCLS) of the Cd and Te 4d levels in the compound semiconductor CdTe have been measured. Values of 0.24 ± 0.05 eV for Cd and −0.26 ± 0.05 eV for Te were obtained. These results cannot be explained solely by the reduction in the Madelung potential at the surface; other factors are at least as important. We also show that SCLS can be related to heats of segregation of n-type impurities via a Born-Haber cycle.