Studies of Chemical Adsorption Using Low-Energy Electron Diffraction

Citation

Williams, Ellen D. (1982) Studies of Chemical Adsorption Using Low-Energy Electron Diffraction. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/DYY3-MQ42. https://resolver.caltech.edu/CaltechETD:etd-09302005-132833

Abstract

Both experimental and computational studies based on low-energy electron diffraction (LEED) have been performed to determine the nature of order in chemically adsorbed overlayers. These studies have been directed towards obtaining a better understanding of adatom-adatom interactions by measurement of their most obvious manifestations; change in overlayer order during adsorption and co-adsorption, island formation, and order-disorder phenomena.

The effect of the co-adsorption of hydrogen on the ordering of CO on Rh(111) has been studied using LEED and thermal desorption mass spectrometry. The results indicate that the adsorption of CO proceeds via a physically adsorbed intermediate. In addition, there is a strong repulsive interaction between CO molecules and hydrogen atoms co-adsorbed on Rh(111). This interaction is apparent at distances up to 2.7-3.1 Å indicating that it is a through-metal effect.

A series of LEED patterns has been observed during the adsorption of sulfur on the reconstructed IR(110)-(1x2) surface. The structure observed at lowest coverages has a p2mg symmetry. This allows a determination of the absolute coverage, and indicates a probable binding site for the sulfur atoms.

A Monte Carlo simulation of the order-disorder behavior of oxygen on W(110) has been performed. General expressions relating the values of the interaction energies to the transition temperatures for a lattice gas with first, second and third neighbor interactions have been determined. Symmetry considerations in selecting a model for the interaction energies are discussed.

The effect of the ordering of adsorbed molecules into small islands on the LEED beam profile has been determined. In the limit of a random distribution of island positions the overall intensity is shown to be the weighted sum of the intensities from the individual islands. Computer simulations of island-containing overlayers have been used to determine the effect on the beam profiles of deviations from a random distribution of islands.

Experimental studies of island formation for CO on Ru(001) have been performed. The finite size of the ordered islands has a strong effect on the order-disorder behavior. Quantitative measurements of this effect have allowed a determination of the island size distribution and thus, the mean island size as a function of coverage.

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