The Interaction of Oxidizing Adsorbates with the NICKEL(111) Surface.

The interaction of two oxidizing adsorbates, nitric oxide and oxygen, with the Ni(111) surface has been investigated. High resolution electron energy loss spectroscopy (HREELS) was used to identify and characterize the vibrations of the molecularly adsorbed NO molecule in a c(4 x 2) ordered overlayer on this surface. The first experimental observation of an NO vibrational mode polarized parallel to a surface was made using HREELS, and the dispersion parallel to the surface of this mode and of the NO intramolecular stretch mode was measured using off-specular HREELS. The frustrated translation was found to be dispersionless across the surface Brillouin zone of the substrate, at an energy of about 11 meV. The dispersion of the NO stretch was fit with a dipole-dipole coupling model, from which were extracted the vibrational and electronic polarizabilities of the adsorbed species. Oxygen adsorbs dissociatively on the Ni(111) surface, and at higher exposures over a certain temperature range, will oxidize the surface to a saturation oxide thickness of three layers of NiO. This thesis demonstrates that at low temperatures, ({<~150} K), oxidation of this surface proceeds extremely slowly if at all, in contrast to accepted belief. The discrepancy with previously published results is attributed to the use of electron beam based spectroscopies as a probe of oxidation extent in the earlier studies. The electron beam effect on the oxidation is quantified, including the measurement of the relevant cross section. The temperature dependence of the oxidation of this surface without the presence of perturbing electrons is reinvestigated, and two kinetic models of the oxidation process are used which help to clarify the mechanism and temperature dependence of electron stimulated oxidation. It is shown that the electron beam creates nucleation centers in the presence of adsorbed oxygen, that the number of nucleation sites created by the electron beam is an increasing function of decreasing temperature, and that these sites promote oxidation easier than intrinsic oxide nucleation sites.