Search

An approach based on He scattering is used to develop an atomic-level structural model for an epitaxially grown disordered sub-monolayer of Ag on Pt(111) at 38 K. Quantum scattering calculations are used to fit structural models to the measured angular intensity distribution of He atoms scattered from this system. The structure obtained corresponds to narrowly size-dispersed compact clusters with a modest translational disorder, and not to fractals, which might be expected due to the low surface temperature. The clusters are up to two layers in height, the lower one having only a few defects. The relations between specific features of the angular scattering distribution, and properties such as the cluster sizes and shapes, the inter-cluster distance distribution, etc. are discussed. The results demonstrate the usefulness of He scattering as a tool for unraveling new complex surface phases. © 1998 Elsevier Science B.V. All rights reserved.

A theoretical study is made on He scattering from three basic classes of disordered adlayers: (a) translationally random adsorbates, (b) disordered compact islands, and (c) fractal submonolayers. The implications of the results to experimental studies of He scattering from disordered surfaces are discussed, and a combined experimental-theoretical study is made for Ag submonolayers on Pt(111). Some of the main theoretical findings are: (1) The scattering intensities from the three disorder classes differ significantly, and can be used to distinguish between them. (2) Structural aspects of the calculated intensities from translationally random clusters were found to be strongly correlated with those of individual clusters. (3) For fractal islands, just as for all surfaces considered here, the off-specular intensity depends on the parameters of the He/Ag interaction, and does not follow a universal power law as previously proposed in the literature. In the experimental-theoretical study of Ag on Pt(111), we use experimental He scattering data from low-coverage (single adsorbate) systems to determine an empirical He/Ag-Pt potential of good quality. Then, we carry out He scattering calculations for high coverage and compare with experiments for these systems. The conclusion is that the actual experimental phase corresponds to small compact Ag clusters of narrow size distribution, with partial translational disorder. © 1997 American Institute of Physics.

The diffraction of thermal He atoms from mixed Xe+Kr monolayers on Pt(111) was measured, and the results were compared with theoretical studies of these systems. The results shed light on the structural properties of these disordered systems, and on their relation to the He diffraction intensities. Experimentally, the specular (0,0), the (1,0), and the (2,0) Bragg peak intensities were measured for monolayers of different Kr:Xe concentration ratios. The theoretical calculations included Monte Carlo simulations of the mixed disordered monolayers, and quantum calculations in the Sudden approximation of the scattering intensities from the simulated disordered structures. The following main results were obtained: (1) Both experiment and the Monte Carlo simulations suggest that the mixed Xe+Kr monolayers are periodic for all Xe:Kr concentration ratios, the lattice constant varies linearly with the Xe:Kr ratio. The domain size of the 2D crystals, from experiment and theory, is found to be larger than 100 Å. (2) The Monte Carlo simulations suggest that the Xe+Kr monolayers form an almost ideal substitutionally disordered lattice. (3) Using a semiempirical Debye–Waller factor, reasonable agreement is found between the theoretical and the measured diffraction intensities, thus supporting the calculated structural model for the disordered surface. (4) The theoretical scattering calculations show that in addition to the diffraction peaks, there are also intensity maxima at non-Bragg positions. These are entirely due to the lattice disorder, and are identified as a recently found new type of Rainbow effect that can furnish important information on disordered surfaces. The results demonstrate the power of He scattering as a tool for exploring substitutionally disordered surfaces. [ABSTRACT FROM AUTHOR]

We have used variable temperature, ultrahigh vacuum scanning tunneling microscopy (STM), in both static and time-dependent experiments, to study the chemistry of the ethylene/Pt(111) system. Images of ethylene which exhibit long-range order have been obtained at a sample temperature of 160 K. The conversion of ethylene to ethylidyne has been observed directly in STM images. This conversion reaction is observed to occur in a ‘‘patchy’’ manner across the surface at saturation coverage. As the reaction proceeds, well-defined islands of unreacted ethylene continue to be clearly observed. Further dehydrogenation of the ethylidyne formed from ethylene leads to carbon containing particles dispersed randomly across the sample. After annealing the ethylidyne covered sample to 500 K, the surface is uniformly covered with carbon containing particles which exhibit a bimodal distribution of heights (one and two atomic layers) consisting of an average of ten and twenty carbon atoms, respectively. Further annealing to 700 K results in the formation of larger particles which appear to be a single atomic layer high and 10–15 Å in diameter in the STM images. These particles contain an average of thirty-four carbon atoms. Pt steps do not show any preferential reactivity for these reactions. Annealing the carbon particle covered surface to higher temperatures results in the formation of monolayer thick graphite islands, which eventually accumulate at the Pt steps. [ABSTRACT FROM AUTHOR]