Formation of subsurface oxygen at Ru(0001)

The conditions for formation of subsurface oxygen on the Ru(0001) surface h ave been studied using thermal desorption spectroscopy, low energy electron diffraction (LEED) and specular helium scattering. The incorporation of ox ygen has been performed via dissociative chemisorption of three molecular c arriers of atomic oxygen: NO2, O-2 and N2O. The rates for oxygen dissolutio n can be related to the initial dissociative sticking probability of the mo lecules on the bare Ru surface. For sample temperatures below 800 K, oxygen penetration into the subsurface region starts only when oxygen molecules i mpinge on the saturated adsorbed layer characterized by the 1X1 O LEED patt ern, indicative of 1 monolayer. A thermally induced transformation of this chemisorbed 1X1 O phase into subsurface oxygen could not be caused even at temperatures close to the onset of oxygen desorption. Oxygen incorporation into the subsurface region by passing through the 1X1 O adsorbed layer, how ever, shows Arrhenius-type behavior. For impinging O-2 molecules, the onset of subsurface oxygen formation appears at 550 K and the entire process is characterized by a rather low activation energy of about 0.5 eV. Deposition of alkali metals on the Ru(0001) surface does not enhance the probability for oxygen dissolution. The resulting oxygen content is substantially reduc ed and this effect strongly depends on the coverage of alkali-metal atoms. In contrast structural defects on the Ru surface, as generated by Ar 1 sput tering and characterized by specular He scattering, act as promoters for ox ygen accommodation. The onset for oxygen penetration on a rough surface alr eady begins at about 350 K and the resulting oxygen content can be directly related to the surface roughness. (C) 1999 American Institute of Physics. [S0021-9606(99)71105-0].