Chemisorption of oxygen on Ni(100) by SIMS and XPS

The interaction of oxygen with a clean Ni(100) surface at room temperature has been studied using the static mode of secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). The O^- yield as a function of coverage shows a fast adsorption at low coverage followed by a slow oxygen uptake between ∼1 and 8 L due to the low sticking coefficient of oxygen in this region. Before the O^- intensity reaches its maximum at ∼300 L, a second plateau between ∼40 L and 50 L appears.The p(2 × 2) and c(2 × 2) LEED structures can be associated with the two plateau regions. At saturation two states of oxygen are apparent since XPS spectra show a doublet O 1s structure with peaks at 529.7 and 531.3 eV (peak height ratio of 3) which are correlated with NiO and Ni₂O₃, "defect oxide", respectively. The Ni 2p _{3 2} line with a binding energy of 852.8 eV is not altered by the adsorbed oxygen species, but strong changes are observed due to the formation of the NiO layer. Beside O^- ions, the following secondary ions have been detected and investigated with SIMS: O₂^-, O⁺, O₂⁺, Ni⁺, Ni₂⁺, Ni₃⁺, NiO^-, NiO₂^-, Ni₂O₃^-, NiO⁺, and Ni₂O⁺. The dependence of Ni₂⁺, and Ni₃⁺, on the oxidation process yields evidence for the importance of the local atomic environment of the surface Ni atoms on the formation of these ions. Relatively high O₂^-, yields at low oxygen coverage suggest that O atoms do not migrate far from each other after chemisorption. NiO₂^-, and Ni₂O⁺, are enhanced relative to NiO^-, and NiO⁺, in the region when c(2 × 2) is formed. Ni₂O₃^-, may be associated with the Ni₂O₃ defect oxide. Comparison of results to recent calculations of secondary particle emission supports both close proximity requirements and formation of the molecular clusters by formation above the surface plane.