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

The surface structures of the (2 x 2)-O phase and the (O + CO) coadsorbate system on Pd(lll) 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(lll) system, we resort to state-of-the-art DFT calculations. In the (2 x 2)-O phase the oxygen atoms reside in fee sites. The coadsorption of CO induces a compression of the (2 x 2)-O into the (root3 x root3)R30 degrees -O overlayer as a consequence of the large difference in adsorption energy of CO in on-top and fee sites (0.5 eV). The local adsorption geometry of oxygen in the (root3 x root3)R30 degrees -O phase is identical to that found for the (2 x 2)-O phase. DFT calculations give evidence that the CO molecules form a separate (root3 x root3)R30 degrees 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. (C) 2000 Elsevier Science B.V. All rights reserved.