UBC Theses and Dissertations
Studies in LEED crystallography Hengrasmee, Sunantha
This thesis is involved with the use of low-energy electron diffraction (LEED) for determining the geometrical structures of well-characterized surfaces of single crystals. Specific applications are to surfaces of rhodium, both clean and when containing adsorbed species. A preliminary problem concerned discrepancies reported previously in the details of the geometrical structures for the clean (100) and (111) surfaces when using rhodium potentials from either a band structure calculation or from the linear superposition of charge density procedure for a metal cluster. A correction has now been made in the calculation of phase shifts for the band structure potential, and reinvestigations of the (100), (110) and (111) surface of rhodium with this potential resolve the discrepancies. These results now support the suggestion, as shown previously in this laboratory for Cu(lll), that the superposition potential provides a good approximation to a band structure potential for the purpose of LEED crystallography. In the structural determinations made here, the degree of correspondence between intensity versus energy curves for different beams from experiment and from multiple-scattering calculations were assessed with the reliability-index r[sub= r] proposed by Zanazzi and Jona. A new aspect considered involved the use of this index for determining the non-structural parameters required in the multiple-scattering calculations. Included in the latter for Rh(lll) are variations of the imaginary part of the constant potential (V[sub= oi]) between the muffin-tin spheres and the surface Debye temperature (θ[sub= D,surf]). Structural conclusions from r[sub r] are compared with visual analyses wherever possible, and this work generally supports the use of the Zanazzi-Jona index in LEED crystallography. The experimental part of this study involved the (100) and (110) surfaces of rhodium. A series of diffraction patterns were observed for the chemi-sorption of 0₂ and H₂S. Intensity versus energy curves were measured for the available diffracted beams for the surface structures designated Rh(100)-(3xl)-0, Rh(100)-p(2x2)-S and Rh(110)-c(2x2)-S. The latter two systems were analyzed by multiple-scattering calculations (using the renormalized forward scattering and layer-doubling methods) and surface structures determined. In each case S atoms adsorb on the centre sites; on Rh(100) S bonds to four neighbouring Rh atoms at a distance of 2.30 Å (very close to the Pauling single-bond value 2.29 Å), and on Rh(110) each S atom is 2.12 Å from the Rh atom directly below in the second layer and 2.45 Å from the four neighbouring Rh atoms in the top metallic layer. An investigation was also made for the use in LEED crystallography of the quasidynamical method recently proposed by Van Hove and Tong. This scheme includes interlayer multiple-scattering properly, but neglects multiple-scattering within individual layers, and has the potential for considerable savings in computing time and core storage. This method was investigated for the clean and sulphur-adsorbed (100) and (110) surfaces, and results compared with the more-complete multiple-scattering methods. The quasi-dynamical method appears to have some promise for making initial selections of the most significant trial structures prior to the more-detailed testing with full multiple-scattering calculations.
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