Characterization of oxygen phases created during oxidation of Ru(0001)

Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, lo w energy electron diffraction (LEED), and the reactive scattering of a CO m olecular beam have been applied to determine the relationship between the f ormation of the subsurface oxygen phase and the growth of oxides during oxi dation of Ru(0001). Emission of RuOx (x < 4) molecules observed in the ther mal desorption spectra during the heating of the oxygen-rich sample has bee n used as a simple measure for the presence of bulk oxides. When performing the oxygen exposure at a temperature lower than the onset for oxygen desor ption (T-p < 850 K) a mobile atomic oxygen species is predominantly formed in the subsurface region. The conversion of these subsurface oxygen atoms i nto a regular RuxOy phase takes place within the temperature region of 900- 1150 K. The growth of oxide films becomes the dominating reaction channel w hen performing the oxidation at temperatures higher than the onset for oxyg en desorption. The oxide formation is strongly reduced when conducting the oxidation at temperatures higher than 1250 K. In this case only a relativel y low amount of oxygen atoms adsorbed on the bare Ru surface can be achieve d, neither oxides nor subsurface oxygen have been found. The presence of a RuO2 coating layer manifests itself by LEED patterns characteristic for a p articular RuO2 single crystal face as well as by additional features in the valence ultraviolet photoelectron spectra. The oxidation of CO molecules r eactively scattered at these oxygen-rich surfaces proceeds as long as mobil e oxygen atoms are present in the subsurface region. The reaction is entire ly quenched when the subsurface oxygen is replaced by an uniform film of Ru O2. (C) 2000 American Institute of Physics. [S0021-9606(00)71010-5].