Surface and subsurface oxygen on Pd(111)

The interaction of O-2 with Pd(lll) 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 oxyg en adlayer saturating at Theta(O) close to 1 and separately for subsurface migration of oxygen at elevated temperatures, but not at room temperature a nd below. Up to completion of a p(2 x 2) oxygen adlayer at 0.25 ML surface coverage, the dissociative sticking probability of oxygen into the chemisor bed 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 b ecomes small and the influence of bulk migration detectable. Exposure of th e sample to high oxygen dosages at 1000 K fills up the subsurface reservoir and subsequent sticking measurements are no longer influenced by oxygen lo ss to the bulk. The subsurface oxygen could be distinguished in both XPS an d off-specular HREELS. These latter techniques revealed that considerable c oncentrations of oxygen in the near-surface region can build up, even at lo wer temperatures (523 K) and oxygen exposures (40 L). In contrast to chemis orbed oxygen atoms on Pd(111), the subsurface species cannot be removed by reaction with CO. (C) 2000 Elsevier Science B.V. All rights reserved.