Orientation of a molecular precursor: a NEXAFS study of O sub(2)/Ag(110)

We present the results of a near-edge X-ray absorption fine structure (NEXAFS) study of physisorbed and chemisorbed molecular oxygen on Ag(110). The physisorbed species was found to lie down on the surface for the monolayer case (with a mean tilt of 29 degree plus or minus 5 degree away from the surface) and with the molecular axis laying along the (001) direction; this is perpendicular to the conventionally held azimuthal orientation of the chemisorbed molecule. For the chemisorbed phase, our spectra show features not seen in earlier studies; these cannot be interpreted in terms of the orbitals of the free molecules and imply both the pi and pi super(*) oxygen molecular orbitals are involved in bonding with the substrate.