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UBC Theses and Dissertations

LEED crystallographic studies of sulfur chemisorbed on the (111) surfaces of nickel and rhodium Wu, Yuk Kuen


The work in this thesis includes investigations of adsorbate-induced metal relaxations with low-energy electron diffraction (LEED) for the surface structures designated Ni(111)-(2x2)-S and Rh(111)-(√3x√3)30'-S; a set of LEED intensity-versus-energy (I(E)) curves have also been measured for normal incidence on the Ni(111)-(2x2)-O surface. In the investigations of adsorbate-induced metal relaxations, 1(E) curves measured for a set of diffracted beams were compared with the corresponding curves calculated by multiple scattering methods for various structural models. The objective is to find the structure that gives the best correspondence between experiment and calculation. Levels of correspondence between experimental and calculated I(E) curves were assessed with the reliability indices proposed by Zanazzi-Jona and Pendry as well as with visual comparison. In the LEED intensity analysis for the Ni(111)-(2x2)-S surface structure, the S-Ni interlayer spacing (d₀₁), the lateral displacement (Δ) of the first nickel layer and vertical relaxations of the first two Ni-Ni interlayer spacings (d₁₂, d₂₃) were investigated. The best geometrical parameters were determined to be d₀₁=1.50 Å, Δ =+0.02 Å (1.4% expansion), d₁₂=2.09 Å (2.8% expansion) and d₂₃=2.08 Å (2.3% expansion) with the S atoms adsorbed on the "expected" 3f sites of the Ni(111) surface. The present study leads to a nearest-neighbour S-Ni bond distance of 2.10 Å, which is a little shorter than those obtained by low energy ion scattering (2.16 Å) and the surface extended X-ray adsorption fine structure technique (2.20 Å) but agrees rather well with the value 2.12 Å predicted by a bond length-bond order analysis. In the investigation of adsorbate-induced metal relaxations for the Rh(111)-(√3x√3)30°-S surface, theoretical I(E) curves were calculated for a range of models involving either unreconstructed or reconstructed rhodium structure at the surface. These calculations were done over appropriate ranges of the S-Rh interlayer spacing (d₀₁), the lateral displacement (Δ) for the first Rh layer, and the first Rh-Rh interlayer spacing (d₁₂). The best level of correspondence between calculation and experiment appears for the 3F1 model with d₀₁=1.57 Å, Δ =M).025 A (1.6 % contraction) and d₁₂=2.18 Å (0.5 % contraction). In this model the nearest-neighbor S-Rh bond distance is found to be 2.19 Å.

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