Comprehensive characterization of the (2×2)-O and the CO-induced (3×3)R30°-O overlayers on Pd(111)

The surface structures of the (2×2)-O phase and the (O+CO) coadsorbate system on Pd(111) are determined by low energy electron diffraction (LEED) and density functional theory (DFT) calculations. To elucidate the energetics and electronic structure of the OPd(111) system, we resort to state-of-the-art DFT calculations. In the (2×2)-O phase the oxygen atoms reside in fcc sites. The coadsorption of CO induces a compression of the (2×2)-O into the (3×3)R30°-O overlayer as a consequence of the large difference in adsorption energy of CO in on-top and fcc sites (0.5 eV). The local adsorption geometry of oxygen in the (3×3)R30°-O phase is identical to that found for the (2×2)-O phase. DFT calculations give evidence that the CO molecules form a separate (3×3)R30° phase rather than a mixed (O+CO) overlayer. The rather weak OPd bonding is discussed in electronic terms by comparison with the related ORu(0001) system.