BENZENE COADSORBED WITH CO ON PD(111) AND RH(111) - DETAILED MOLECULAR DISTORTIONS AND INDUCED SUBSTRATE RELAXATIONS

Two coadsorbed benzene/CO structures on Pd(111) and Rh(111) have been reanalyzed by automated tensor LEED. This computation permits the simultaneous optimization of many more atomic coordinates than before. The results show significant admolecule-induced substrate relaxations, the benzene causing an outward buckling and the CO causing a smaller inward buckling in the topmost metal layer. This analysis also confirms, on Rh(111), the likely presence of Kekule distortions in the benzene carbon ring (i.e. alternating long and short C-C bond lengths around the ring), with a sizable C-ring expansion. Together with improved scattering potentials, the R-factors are reduced by 44% and 28%, respectively. The clean surface structures were also re-examined in terms of multilayer relaxations, yielding a slight contraction of the topmost Rh-Rh interlayer spacing, but a slight expansion for Pd.