The angular dependence of flux,mean energy and speed ratio for D2 molecules desorbing from a Ni(111) surface

Experimental data are presented for the angular dependence of the relative flux, the mean energy and the speed ratio of deuterium molecules desorbing from a Ni(111) crystal surface at a surface temperature of T_s = 1143 K and at sulphur coverages ranging between 30% and less than 2% of a monolayer.The angular flux distribution is sharply peaked in the forward direction (cos^dθwith 3 ? d ? 5) and the mean energy 〈E〉 of the desorbate depends strongly on the desorption angle θ. For normal desorption $\text{(θ = 0°)}\text{〈E〉}\text{2k}$ is about 700 K higher than T_s and for glancing angles (θ = 80°) it decreases to about 400 K below T_s The results obtained on sulphur free and sulphur covered Ni(111) surfaces are compared with our former data on polycrystalline nickel. The main differences in the kinetic features can be ascribed to the surface roughness. Accordingly, the angular distributions of flux, mean energy, and speed ratio, which deviate strongly from the Knudson and Maxwellian law, do not seem to depend considerably on sulphur coverage and surface structure. A qualitative explanation for these deviations is presented using the principle of detailed balancing.     

The scattering of thermal He atoms at ordered and disordered surfaces

The use of thermal energy atom scattering (TEAS) as a surface investigation tool is reviewed. Specific properties of thermal He scattering, which are particularly advantageous in surface analysis, are discussed; the main causes which hampered so far the broad application of this technique to the investigation of ordered surfaces are analysed. The recent development of TEAS based on the exploitation of the large cross section for He diffuse scattering of adsorbates and vacancies is reviewed. This development extends the capabilities of TEAS to a straightforward investigation of various aspects of disordered surfaces so far hardly accessible to other methods. Examples concerning the lateral distribution of adsorbates and defects, the mutual attraction and repulsion, 2D-phase transitions, mobility and surface diffusion are given.     

Anomalous preexponential factor for hydrogen desorption from platinum single crystals?

Abnormally low values found by some authors for the preexponential for hydrogen desorption from platinum surfaces have recently been explained by an adsorbate induced change in the dynamical characteristics of the surface itself. This possibility has been investigated in the present work exploiting the Debye-Waller effect in He beam diffraction from a hydrogen covered Pt(997) surface. Essentially no dependence of adsorbate induced attenuation of the diffracted beam intensities on the primary He-beam energy has been observed. Therefore no indications have been found for changes in the perpendicular component of platinum surface vibrations due to adsorbed hydrogen. Likewise, heats of adsorption, obtained from He diffraction experiments, do not depend on the primary energy of the He beam. This result supports the obtained normal value for the preexponential factor of $\text{ca. 10}{}^{\mathrm{?2}}\text{cm}{}^2\text{atom · s}$ (Poelsema et al., Surface Sci. 111 (1981) 519).     

A scanning tunneling microscopy study of the adsorption of Xe on Pt(111) up to one monolayer

The adsorption of Xe on Pt(111) has been investigated from the arrival of the very first atoms up to completion of the monolayer using a variable-temperature Scanning Tunneling Microscope (STM). Surprisingly, in the initial stages of the adsorption Xe preferentially binds to a low coordination site, theupper edge of the platinum steps. The strong binding to these sites leads to a local repulsive interaction with further Xe atoms. Therefore, the Xe atoms located at the upper edge of the steps do not serve as nuclei for 2D Xe islands, which, instead, form on the terraces and at thelower edges of the platinum steps. Only during completion of the monolayer do these islands make contact with the atoms adsorbed at the beginning in the upper-edge positions. The full monolayer exhibits the Hexagonal Incommensurate Rotated (HIR) phase already known from earlier helium-diffraction experiments.     

Structure investigation of the nitrogen induced Cu(110)−(2 × 3) phase with 180° low energy impact collision ion scattering spectroscopy

The 180° low energy impact collision ion scattering spectroscopy with detection of noble gas neutrals (180°-NICISS) has been used to investigate the nitrogen saturated Cu(110) surface, which is known to exhibit a (2 × 3) diffraction pattern. The nitrogen induced (2 × 3) phase appears to be the result of a surface reconstruction of a new missing row type, in which every third 100 row of Cu atoms of the first layer is missing. The 180° NICISS patterns further indicate within an accuracy of 0.1–0.2 Å, that the double periodicity in the [1 0] direction is not due to the reconstruction of the Cu surface. Its origin has to be found in the arrangement of the N atoms.     

Fast deuterium molecules desorbing from metals

The velocity distribution of D₂ desorbing from Pd(100) reveals the existence of two distinct groups of molecules, fast and slow ones. A new mechanism is proposed for the desorption of the molecules belonging to the fast group: after diffusing to the surface, absorbed D atoms recombine directly, without equilibrating in the chemisorption well. The fast molecules observed in the D₂ desorption spectra from Cu and Ni seem to desorb through the same mechanism. In the case of Pd this mechanism is controlled by adsorbed sulphur.     

Bulk dissolved Si as a cause of the “oxide” formation on Pt(111) surfaces

The influence of silicon atoms dissolved in the bulk of a Pt(111) crystal on the high temperature “oxidation” of Pt was studied by Auger spectrometry (AES) and low energy ion scattering (IS). The “oxidation” procedure was applied on clean and Si-prepared crystals. The preparation was performed by depositing a Si layer on the Pt(111) surface and thermally diffusing it into the bulk beyond the AES detectability. The results show that even deeply diffused silicon plays a decisive part in the “oxidation” process. This seems to explain the observations of other authors concerning the unexpected high stability of the “oxide” and the poor reproducibility of its rate of formation as well as of its saturation density. There is evidence that the “oxide” is primarily a silicon-oxygen bond.     

Dynamics and stability of nanostructures on metal surfaces

Ultrathin metal films consisting of two-dimensional clusters are typically unstable: the cluster ensemble has the tendency to reduce its total free energy via Ostwald ripening or dynamic coalescence of mobile clusters. In this paper we give an overview of recent model experiments addressing these coarsening mechanisms. The experiments have been performed using STM on ensembles consisting of adatom or vacancy clusters with typical diameters in the nanometer range on fcc(111)-metal surfaces. Agreement with and deviations from conventional theories are discussed. Received: 29 March 1999 / Accepted: 17 August 1999 / Published online: 30 September 1999