Physics Research Works
Permanent URI for this collectionhttp://hdl.handle.net/1903/1597
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Item Critical Exponents of a Four-State Potts Chemisorbed Overlayer: p(2x2) Oxygen on Ni(111)(American Physical Society, 1981) Roelofs, L. D.; Kortan, A. R.; Einstein, Theodore L.; Park, Robert L.We report the first determination of critical exponents for a chemisorbed overlayer, using low-energy electron diffraction. We examine the order-disorder transition of p(2x2) oxygen on the (111) surface of nickel. This study is the first of critical behavior of a two-dimensional system in the four-state Potts universality class. Discussion of disparity between our results and predicted exponents considers several possibilities, including logarithmic corrections.Item Extended Appearance-Potential Fine-Structure Analysis: Oxygen on Al(100)(American Physical Society, 1980) Laramore, G.E.; Einstein, Theodore L.; Roelofs, L. D.; Park, Robert L.To measure O-Al separation at Al(100) surfaces disordered (low-energy electron-diffraction beams extinguished) by reaction with oxygen, the extended appearance-potential fine structure was analyzed above the threshold for electron-bombardment excitation of the O 1s core. Calculation shows that the outgoing electron has angular momentum l=0, allowing simple Fourier inversion of the fine structure. The separation, 1.9+-0.05 A, suggests that oxygen lies under the top layer, a result undetectable in extended-x-ray-absorption fine-structure measurements on thicker films.Item Effect of the central atom potential on the extended fine structure above appearance potential thresholds(American Physical Society, 1980) Laramore, G.E.; Einstein, Theodore L.; Roelofs, L. D.; Park, Robert L.The formalism previously given for describing the extended fine structure above appearance-potential-spectroscopy (APS) thresholds is extended by incorporating the effects of the excited "central" atom potential in an exact manner. The excitation-matrix elements are expressed in terms of the exact wave functions of the central atom potential. This introduces a "phase renormalization" into the excitation-matrix elements and eliminates a previously noted "phase difference" between single- and multiple-scattering calculations employing a plane-wave basis set. A series of approximations is then made which leads to an expression for the APS extended fine structure in terms of sinusoidal functions and hence provides a rationale for a Fourier-transform analysis. Simple model calculations assuming a constant "bare" excitation-matrix element, a spherically symmetric electronic density of states, and only S-wave scattering from the atomic cores are performed for a cluster of atoms having the atomic geometry of bulk vanadium. These calculations display the major predictions of the formalism and indicate that for a given system there may be some optimal energy range for data analysis. The problem of electron characteristic losses is considered, and it is pointed out that in the small-momentum-transfer limit the simple dipole selection rules appropriate to a photon-excitation process again apply. This may obviate many of the problems introduced by multiple angular momentum final states in the APS process.Item Two-dimensional ordering of chlorine on Ag(100)(American Physical Society, 1985) Taylor, David E.; Williams, Ellen D.; Park, Robert L.; Bartelt, N. C.; Einstein, Theodore L.Chlorine adsorption on Ag(100) saturates at (1/2) monolayer (ML) in a c(2x2) structure. Desorption occurs above 650 K without disordering. The sticking coefficient remains constant to within 0.05 ML of saturation, consistent with the existence of a highly mobile precursor to adsorption. The c(2x2) structure forms at a critical coverage of 0.394+-0.007 ML at room temperature. This is slightly higher than the critical coverage of the hard-square model (0.368 ML), suggesting the influence of further-neighbor interactions.