The geometric and electronic structure of the surface region of a crystal is often different from the bulk structure and therefore the optical properties differ in principle also. Theories for the optical properties of (sub)monolayer films are compared, with special attention to anisotropic layers. The review of experimental studies in ultra high vacuum systems mostly concerns ellipsometric measurements of chemisorption of oxygen on metal surfaces (Ag, Cu, Ni). With the combination ellipsometry-AES-LEED linear relationships have been observed between the change in Δ and the coverage of oxygen chemisorbed on Ag(110), Cu(100), Cu(111) and Cu(111). At room temperature the δ Δ values per oxygen atom on the different Cu planes are equal, and are larger than the a δ Δ value per atom taken up in a later stage of oxidation. For the anisotropic Cu(110) plane the δ Δ and δ ψ values per adsorbed oxygen atom depend on the crystal temperature and the azimuthal orientation of the plane of incidence of the light beam. In order to explain the observed changes in Δ and ψ, models are discussed in which changes in the substrate, caused by chemisorption, are taken into account
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