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UBC Theses and Dissertations

Studies in LEED crystallography Moore, William Thomas


The work in this thesis involved assessments and applications of procedures used for determining surface geometrical structures with low energy electron diffraction (LEED), a subject that is now often referred to as LEED crystallography. Specific studies were made for the (311) surfaces of copper and nickel, and for the (0001) surface of zirconium. These surfaces were cut from high purity single crystal samples, studied under conditions of ultra-high vacuum, and characterized by LEED and Auger electron spectroscopy. The LEED crystallographic studies of the (311) surfaces of face-centered cubic metals represent the first investigations of stepped surfaces which employed multiple scattering calculations. In general, in this thesis, the multiple scattering calculations used the renormalized forward scattering and the layer doubling methods, and the experimental intensity versus energy curves for the diffracted electron beams were measured with our combined vidicon-photographic method. In these studies, many of the procedures currently employed in LEED crystallography were investigated, particularly in regard to their effects on surface geometry determinations. These procedures included smoothing raw experimental data, averaging beams that are expected to be equivalent from symmetry considerations, correcting measured intensities for effects of non-uniform grid transparency, and setting appropriate values for the suppressor grid voltage. In addition, consideration was given to investigating ways of dealing with numerical instabilities, which sometimes occur in the perturbative methods used for calculating LEED intensities, and assessments were made of the reliability index recently proposed by Pendry. The latter gives numerical values for the 'goodness of fit' between experimental and calculated intensity curves, and was compared especially with the index proposed previously by Zanazzi and Jona. Some consideration was also given to the question of assessing uncertainties in structural analyses based on the use of reliability indices. The topmost interlayer spacing in the (311) surfaces of copper and nickel are indicated to be contracted by about 5% and 14.5% respectively from the bulk spacings. Possible reasons for this difference are discussed in the thesis. By contrast, the (0001) surface of zirconium has a topmost spacing which is very close to the bulk value. Experimental intensity versus energy curves have also been measured for three surfaces involving oxygen adsorption on Zr(0001); one of these surfaces is designated Zr(0001)-(2x2)-O while the other two are both designated Zr(0001)-(1x1)-O. For the latter, one is believed to involve a monolayer coverage of oxygen atoms while the other probably involves oxygen incorporation into the surface region. The LEED intensity data for these oxygen structures, once they have been analyzed with multiple scattering calculations, should provide a basis for detailing structural aspects of the initial stages of the oxidation of Zr(0001).

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