Direct observation of subsurface oxygen on Rh(111)

A Rh(111) surface has been exposed to oxygen under conditions that favour t he formation of a subsurface oxygen species. After exposure to 10(5) L O-2 at a surface temperature of 470 K X-ray photoelectron diffraction data of O Is emission reveal that (5+/-2)% of a monolayer of oxygen atoms occupy oct ahedral interstitial sites just underneath fcc adsorption sites. The occupa tion 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 desorptio n spectra, as well as for the catalytic behaviour of the oxidized Rh(lll) s urface, are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.