Adsorption sites in O and CO coadsorption phases on Rh(111) investigated by high-resolution core-level photoemission

Abstract

High-resolution core-level spectroscopy is used in combination with low-energy electron diffraction (LEED) and photoelectron diffraction to identify the adsorption sites for three different coadsorbed phases consisting of ordered overlayers of oxygen coadsorbed with CO on the Rh(111) single-crystal surface. The three ordered overlayer structures, which may be denoted as 2O+CO/Rh(111), O+CO/Rh(111) and O+2CO/Rh(111), all show (2×2) LEED patterns. In the 2O+CO and O+CO phases the CO molecules are found to occupy only on-top sites while the O+2CO phase shows CO molecules in both on-top and three-fold hollow sites. In all cases the oxygen atoms are found in three-fold hollow sites. For the O+CO and O+2CO phases our results confirm previous determinations by LEED, while the 2O+CO phase has not been observed before on Rh(111). The core-level binding energies of the C 1s and O 1s core levels for both adsorbates are characteristic of the adsorption site and are very close to the binding energies found for the pure cases of only oxygen or CO adsorbed on Rh(111). In the coadsorption phases we find that the interaction between the adsorbates has only a minor influence on the core-level binding energies. For the O+2CO/Rh(111) coadsorption phase we find that a full CO coverage is not obtained; less than 80% of the unit cells contain two CO molecules, in line with previous findings.