The adsorption and decomposition of carbon monoxide on Ni(100) and the oxidation of the surface carbide by oxygen

The interaction of carbon monoxide with Ni(100) has been studied by ellipsometry and Auger electron spectroscopy. Bombardment by electrons of a relatively high energy (2500 eV) leads to the disproportionation of the adsorbed CO (2 COad → Cad + CO2g ). The rate of oxidation of this surface carbide is , where hc is the carbon 272 eV Auger peak height, n=0.5 and the apparent activation energy Eact =13.3 kcal/mole. This relation is valid at 200–400°C and at oxygen pressures of 5 × 10−9−8 × 10−7 Torr

Adsorbate-induced substrate relaxation and the adsorbate-adsorbate interaction

We formulate the theory of the perturbation caused by an adsorbate upon the substrate lattice in terms of a local modification of the interatomic potential energy around the adsorption site, which leads to the relaxation of substrate atoms. We apply the approach to CO chemisorption on close-packed metal surfaces, and show that the adsorbate-adsorbate interaction and a variety of other properties can be well described by a simple model.Comment: 4 pages, 2 eps figures, RevTex, submitted to PR

Segregation in thin films: the effect of mass balance and bonding to the support

The effect of mass balance on segregation in thin films is studied by minimizing the free energy of the film with respect to the composition of each layer for a fixed overall mole fraction. It is shown that the Bragg-Williams and quasi-chemical approximationsgive almost identical results. The effect of a difference in bond strength of the two components with the support is shown to lead to large variations in the composition of the outer film surface, even for thick (more than about 30 layers) films

Gas physisorption studied with density functional theory: hysteresis and the Kelvin equation

We study nitrogen adsorption on oxidic surfaces within the framework of density functional theory. On the basis of a simple free energy functional, with which we calculate nitrogen density profiles and adsorption isotherms, we argue that some experimental hysteresis effects can directly be related to metastable phases: the jumps in the adsorption occur at the spinodal of the metastable branches. For sufficiently large slits or cylinders this occurs at a “universal” relative pressure of 0:42 in the desorption branch. The agreement of the classical Kelvin theory, used to describe hysteresis by the effect of curved menisci, with our present approach is shown to be approximate only

Ellipsometry of clean surfaces, submonolayer and monolayer films

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