| Summary: | This thesis is concerned with the development and application of a new method of performing multiple scattering calculations of X-ray Photoelectron Diffraction (XPD) from periodic surfaces. The technique is based on the dynamical theory of Reflection High Energy Electron Diffraction (RHEED), allowing more efficient access to high photoelectron energies (1000eV and above) than is available using present calculational schemes. Convergence tests have been performed to identify the limitations of the present method. The method is applied to two overlayer systems for which XPD experimental data is available, c(2 x 2)S and c(2 x 2)O on Ni(001). For the c(2 x 2)S system, which is relatively well understood and therefore a good test system, data at four emission (take-off) angles is fitted, giving optimum agreement with the S atoms occupying hollow sites at a vertical adsorption height of between 1.30A and 1.35A. The agreement is improved slightly for the lowest two take-off angles by an outward relaxation in the top Ni-Ni layer spacing. Comparison is made with single scattering calculations for the system and the present method shown to be far superior at the lowest take-off angle. This clearly demonstrates the importance of multiple scattering at grazing emission angles. For the c(2 x 2)O system, data at two take-off angles is fitted giving optimum agreement with the O atoms in hollow sites at an adsorption height of 0.75A. By allowing a small outward relaxation in the top Ni-Ni layer spacing, an almost equally good fit to the data is found at an adsorption height of 0.80A. An offcentre bonding site which has been proposed, in which the O atoms are displaced from their hollow sites by 0.3A in the [110] direction has been considered and ruled out.
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