Surface and subsurface oxygen on Pd(111)

The interaction of O2 with Pd(111) in the temperature range from 300 K to 1000 K was studied by molecular beam adsorption, thermal desorption (TDS), low energy electron diffraction (LEED), high-resolution X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). Using a capillary array doser and high effective oxygen pressures, evidence was found for the formation of a densely packed chemisorbed oxygen adlayer saturating at ΘO close to 1 and separately for subsurface migration of oxygen at elevated temperatures, but not at room temperature and below. Up to completion of a p(2×2) oxygen adlayer at 0.25 ML surface coverage, the dissociative sticking probability of oxygen into the chemisorbed state is high and masks the much slower diffusion into the bulk. Beyond 0.25 ML surface coverage, the adsorption rate into the chemisorbed state becomes small and the influence of bulk migration detectable. Exposure of the sample to high oxygen dosages at 1000 K fills up the subsurface reservoir and subsequent sticking measurements are no longer influenced by oxygen loss to the bulk. The subsurface oxygen could be distinguished in both XPS and off-specular HREELS. These latter techniques revealed that considerable concentrations of oxygen in the near-surface region can build up, even at lower temperatures (523 K) and oxygen exposures (40 L). In contrast to chemisorbed oxygen atoms on Pd(111), the subsurface species cannot be removed by reaction with CO.