An XPS investigation of the chemisorption of oxygen on the iridium (111) surface

Using X-ray photoelectron spectroscopy (XPS) to monitor the concentration of adsorbed oxygen on Ir(111), it was found that at temperatures well below the onset of thermal desorption, the adsorbed oxygen disappears from the surface. A pressure of approximately 10−7 Torrof O2 over the crystal surface ensures that the adsorbed oxygen is not removed via reaction with background CO. The implication is that oxygen is diffusing into the bulk of the crystal. Support for this idea comes from the observation of a small, broad peak in the X-ray photoelectron spectrum of the “clean” surface at the same binding energy as the oxygen signal and the difficulty in removing the oxygen which contributes to this peak. Evidently, this peak derives from oxygen dissolved in the Ir lattice. The saturation coverage of oxygen is estimated to be 7.9 × 1014atoms/cm2 by comparison with XPS data for a CO saturated surface. This coverage is consistent with the interpretation that the (2×2) oxygen LEED pattern arises from three independent domains of (2×1) oxygen superstructures rotated 120° with respect to each other.