Adsorption of O-2 on polycrystalline rhenium metal at room temperature studied by synchrotron X-ray photoemission spectroscopy

Clean rhenium (Re) surfaces were made by sputtering and annealing in UHV. T he clean surfaces were then exposed to O-2 in the pressure range of 10(-8) (1 L) to 5 Torr (similar to1.5X10(7) L) at room temperature. The surface th at was exposed at 5 Torr O-2 was also annealed at several temperatures. Cor e level spectra of the Re 4f and O ls were acquired at photon energies of 2 00 and 790 eV, respectively, from clean, O-2 exposed, and annealed surfaces . At least two components, approximately 1.4 eV apart, can be identified in the O Is spectra. One is attributed to atomic oxygen, and the other to mol ecular oxygen. At least two new features appear in the Re 4f spectra after the clean surfaces were exposed to O-2, and they may correspond to surface Re atoms surrounded by different numbers of oxygen. Annealing the O-2 expos ed surfaces results in rearrangement of surface oxygen. Although equilibriu m thermodynamic calculations predict that Re,O, is the most stable phase in the O-2 pressure and temperature range used in the exposure experiments, n o appreciable Re oxide was observed, suggesting that the oxidation of Re me tal is controlled by kinetics rather than thermodynamics. (C) 2001 Elsevier Science BN. All rights reserved.