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

Adsorption of CO on a Ni(100) surface has been investigated using secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). At room temperature the positive ion spectra show Ni⁺, Ni⁺₂, Ni⁺₃, NiCO⁺ and Ni₂CO⁺. At saturation coverage, small amounts of Ni₃CO⁺ and Ni⁺₄ appear. All ions show a smooth increase in intensity with CO adsorption and reach saturation after 2 L CO or half monolayer coverage as measured by XPS. The NiCO⁺/Ni⁺ ratio shows a plateau at 2 L and a rise at 4-6 L that reflects the change from the c(2 × 2) to a hexagonal LEED structure observed by Tracy. The measured Ni₂CO⁺ ratio is higher at very low coverages than at saturation. Based on ejection-yield calculations this observation suggests that a high percentage of bridged bonded CO may be present in the early stages of adsorption. Ni(2p _{3 2}), O(1s) and C(1s) XPS spectra from the clean and CO saturated surface are presented. CO adsorption on a partly C covered Ni(100) surface caused a strong increase in the C(1s) intensity but no increase in the O(1s) intensity and is taken to indicate the presence of a disproportionation reaction. Exposure to CO at 523 K showed no NiCO⁺ or Ni₂CO⁺, but appearance of ions like C⁺, C⁺₂, NiC⁺ Ni₂C⁺, and Ni₂O⁺ indicated dissociative adsorption. A sputtered Ni(100) surface had a much larger capacity for CO uptake as indicated by increased ion yields.