ELS study on initial oxidation of Ni(100) surfaces

Electron energy loss spectra of clean and oxygen-covered Ni(100) surfaces were observed with concomitant measurements of LEED, work function change, and Auger peak height ratio O(KL_{2, 3}L_{2, 3})/Ni(L_{2, 3}VV). The observed electronic transitions are interpreted on the basis of primary election energy dependence, and of comparison with the loss spectrum for a UHV-cleaved NiO(100) surface and optical data of Ni. The observed loss peaks at 9.1, 14, and 19 eV in the clean surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electrons, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Three oxygen-derived peaks at 6.0, 8.0, and 10.3 eV in the low oxygen exposure region (?4 L) are ascribed to the O 2p(e) → Ni 3d, O 2p(a₁) → Ni 3d, and O 2p → Ni 4s transitions, respectively. In the high oxygen exposure region (?50 L), the spectra become quite similar to that of the UHV-cleaved NiO(100) surface. The oxidation process consistent with LEED, Auger peak height ratio and work function change measurements is discussed.