Formation of subsurface oxygen at Ru(0001)

Abstract

The conditions for formation of subsurface oxygen on the Ru(0001) surface have been studied using thermal desorption spectroscopy, low energy electron diffraction (LEED) and specular helium scattering. The incorporation of oxygen has been performed via dissociative chemisorption of three molecular carriers of atomic oxygen: NO2, O2 and N2O. The rates for oxygen dissolution can be related to the initial dissociative sticking probability of the molecules on the bare Ru surface. For sample temperatures below 800 K, oxygen penetration into the subsurface region starts only when oxygen molecules impinge on the saturated adsorbed layer characterized by the 1×1 O LEED pattern, indicative of 1 monolayer. A thermally induced transformation of this chemisorbed 1×1 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 1×1 O adsorbed layer, however, shows Arrhenius-type behavior. For impinging O2 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 reduced and this effect strongly depends on the coverage of alkali–metal atoms. In contrast structural defects on the Ru surface, as generated by Ar+ sputtering and characterized by specular He scattering, act as promoters for oxygen accommodation. The onset for oxygen penetration on a rough surface already begins at about 350 K and the resulting oxygen content can be directly related to the surface roughness.