VERY HIGH ATOMIC OXYGEN COVERAGES ON RU(001)

Very high oxygen coverages-up to theta(O) almost-equal-to 40 (forty)-were obtained by dissociative adsorption of NO2 on Ru(001) at elevated temperatures. O2 desorption data show three desorption peaks at almost-equal-to 1100, almost-equal-to 1000, and almost-equal-to 900 K, and suggest strong oxygen-oxygen repulsions in the overlayer. An Arrhenius plot of the leading edge desorption rates of the 900 K peak yields E(A) = 53 kcal/mol, implying zero-order desorption kinetics. 02 is the only desorption product for theta(O) < 3; additional RuOx+ (x = 0-3) species were detected at higher coverages. However, neither the O 1s and Ru 3d core levels, nor the valence band spectra showed any evidence for chemical changes, i.e., an oxide formation. The process of an underlayer formation as a distinct stage in the oxidation of the Ru metal will be discussed in terms of a model for subsurface penetration of O on transition metals.