Direct observation of subsurface oxygen on Rh(111)

A Rh(111) surface has been exposed to oxygen under conditions that favour the formation of a subsurface oxygen species. After exposure to 105 L O2 at a surface temperature of 470 K X-ray photoelectron diffraction data of O 1s emission reveal that (5±2)% of a monolayer of oxygen atoms occupy octahedral interstitial sites just underneath fcc adsorption sites. The occupation of such a site forces the oxygen atoms adsorbed in its close vicinity to switch from the stable fcc site to the hcp adsorption site. Implications of this finding for the interpretation of temperature-programmed desorption spectra, as well as for the catalytic behaviour of the oxidized Rh(111) surface, are discussed.