Scanning tunneling microscopy shows that large two-dimensional Ag clusters on Ag(100) can diffuse. The value of the diffusion coefficient at room temperature is of order 10-17 cm2s-1 and varies little, if at all, with cluster size in the range studied, 100 to 720 atoms per cluster. This weak variation rules out periphery diffusion as the main mechanism of cluster diffusion, suggesting instead two-dimensional evaporationcondensation. This conclusion is compatible with the energetics of atomic-scale events within the cluster and with the dissolution of small clusters observed at low coverages. Keywords Ames Laboratory, Mathematics Disciplines Mathematics | Physical Chemistry CommentsThis article is from Physical Review Letters 73, no. 19 (1994) .- (a) . (

The atomic structure of the fivefold symmetric quasicrystal surface of icosahedral AlPdMn has been investigated by means of a dynamical low-energy-electron diffraction (LEED) analysis. Approximations were developed to make the structure of an aperiodic, quasicrystalline surface region accessible to LEED theory. A mix of several closely similar, relaxed, bulklike lattice terminations is favored, all of which have a dense Al-rich layer on top followed by a layer with a composition of about 50% Al and 50% Pd. The interlayer spacing between these two topmost layers is contracted from the bulk value by 0.1 Å, to a final value of 0.38 Å, and the lateral density of the two topmost layers taken together is similar to that of an Al(111) surface. The LEED structural result is qualitatively consistent with data from ion scattering spectroscopy, which supports an Alrich termination. Keywords Ames Laboratory, Physics and Astronomy Disciplines Biological and Chemical Physics | Physical Chemistry CommentsThis article is from Physical Review B 57, no. 13 (1998) The atomic structure of the fivefold symmetric quasicrystal surface of icosahedral AlPdMn has been investigated by means of a dynamical low-energy-electron diffraction ͑LEED͒ analysis. Approximations were developed to make the structure of an aperiodic, quasicrystalline surface region accessible to LEED theory. A mix of several closely similar, relaxed, bulklike lattice terminations is favored, all of which have a dense Al-rich layer on top followed by a layer with a composition of about 50% Al and 50% Pd. The interlayer spacing between these two topmost layers is contracted from the bulk value by 0.1 Å, to a final value of 0.38 Å, and the lateral density of the two topmost layers taken together is similar to that of an Al͑111͒ surface. The LEED structural result is qualitatively consistent with data from ion scattering spectroscopy, which supports an Al-rich termination. ͓S0163-1829͑98͒03713-8͔

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We have investigated the temperature and coverage-dependent transformations of the ordered structures of oxygen on Pd(100). The four ordered structures are the chemisorbed c(2×2) and p(2×2) lattices, and the reconstructed p(5×5) and ((5)1/2×(5)1/2)R27° lattices. We present evidence that the p(5×5) reconstruction forms in an activated stepfrom c(2×2) regions. The onset of p(5×5) formation is associated with an increase in oxygen sticking coefficient. In thermal desorption, there are three states which can be correlated directly with the structure of the adsorbed phase during desorption: At lowest coverage, the α state shows the traits of second-order kinetics and is due to desorption from a disordered adlayer. At higher coverage, the β state appears and is due to desorption from a layer with c(2×2) order. There is a lower barrier to desorption in the β state than in the α state because of the repulsive second-nearest-neighbor interactions in the c(2×2). At highest coverage, the sharp and narrow γ state emerges. This is accompanied by decomposition of the ((5)1/2×(5)1/2)R27° reconstruction, in which cooperative stabilization of the reconstruction by oxygen atoms effectively creates strong quasiattractive oxygen–oxygen interactions. There are interesting similarities between the oxygen-stabilized reconstructions of Pd(100) and the initial stages of oxidation of Ni(100), as well as the oxygen-stabilized reconstructions of Pt(100). The data are obtained from low-energy electron diffraction coupled with a computer-interfaced Video camera, Auger electron spectroscopy, and thermal desorption spectroscopy.

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A metastable c(2x2) structure is formed by atomic oxygen on Pd(100) under conditions of slow diffusion combined with rapid adsorption.We propose that formation of the c(2x2) under these conditions must be controlled by characteristics of the dissociative adsorption event itself.