HIGH RESOLUTION ELECTRON ENERGY LOSS SPECTROSCOPY AND PHOTOELECTRON DIFFRACTION STUDIES OF THE GEOMETRIC STRUCTURE OF ADSORBATES ON SINGLE CRYSTAL METAL SURFACES (OXYGEN, COPPER, NICKEL, SULFUR)

Two techniques which have made important contributions to the understanding of surface phenomena are high resolution electron energy loss spectroscopy (EELS) and photoelectron diffraction (PD). EELS is capable of directly measuring the vibrational modes of clean and adsorbate covered metal surfaces. In this work, the design, construction, and performance of a new EELS spectrometer are described. The initial experiment carried out with the spectrometer was a study of submonolayer coverages of oxygen on Cu(001). A complex evolution of the O-Cu stretching vibration peak was observed as a function of coverage. These results are discussed in terms of possible structures of the O-Cu(001) system. Recommendations for improvements in this EELS spectrometer and guidelines for future spectrometers are given.; PD experiments provide accurate quantitative information about the geometry of atoms and molecules adsorbed on metal surfaces. In an energy-dependent PD experiment, the angle-resolved photoemission cross-section of a core level on an adsorbate atom is measured as a function of photon energy. The technique has advantages when used to study disordered overlayers, molecular overlayers, multiple site systems, and adsorbates which are weak electron scatterers. Four experiments were carried out which exploit these advantages. In three of the experiments, data were collected normal to the sample face. First, the structure of the c(2 x 2) oxygen and sulfur overlayers on Ni(001) was determined. R-factor analysis was used to verify the precision of the results. Fourier-transform analysis was also carried out to obtain certain structural parameters. Second, the structure of selenium on Ni(111) and on Ni(011) was ascertained for the first time. The third normal emission study yielded the structure of carbon monoxide adsorbed on Ni(001). Finally, a study of c(2 x 2) Se on Ni(001) was done at emission angles away from the normal direction. The derived structure was the same as the result of an earlier study at normal emission. A comparison between PD and other techniques is made and a discussion of very recent developments in PD is given.