Oxygen adsorption on silver (110): Dispersion, bonding and precursor state

Abstract

Angle-resolved photoemission, both in the laboratory and with synchrotron radiation, has been used to investigate three states of adsorbed oxygen on Ag(110): chemisorbed atomic oxygen, undissociated chemisorbed dioxygen and physisorbed molecular oxygen. For atomic oxygen, dispersion of adsorbate-induced levels was observed indicating strong oxygen-oxygen interaction in the [001] direction. Polarised light combined with selection rules was used to determine the symmetry of the adsorbate-derived bands. The adsorption geometry and symmetry of the oxygen-induced levels for the chemisorbed dioxygen species were also investigated with the selection rules. The molecule appears to lie parallel to the surface with the OO axis oriented in the [110] direction. The adsorbate-induced feature of lowest ionisation potential at 1.1 eV relative to E_F is 1 π_g-derived. The very low frequency reported for the OO stretch and the analogy with coordination chemistry also suggest that the chemisorbed dioxygen species is lying down on the surface. At sufficiently low temperature, oxygen was found to physisorb on the bare metal also with its OO axis parallel to the surface. We identify physisorbed oxygen as an intrinsic precursor state for chemisorption.