Interaction of C-60 with Ge(111) in the 3 root 3X3 root R30 degrees phase: A (2X2) model

The adsorption of a close-packed C-60 Single layer on the clean c(2 x 8)-Ge(111) surface leads to the formation of a 3 root 3 x 3 root 3R30 degrees low-energy electron diffraction pattern upon annealing at 450-500 degrees C. We have investigated this phase using synchrotron-radiation photoemission. The Ge 3d core-level spectra show the quenching of the surface features related to the adatoms/rest-atoms complex of the original Ge(lll)-c(2 x 8) reconstruction, suggesting that the observed 3 root 3 x 3 root 3R30 degrees ordering is not due to a Ge(111) reconstruction induced by the C-60 adsorption, as instead previously argued on the basis of scanning tunneling microscopy measurements. Moreover, no filling of the lowest unoccupied molecular orbital derived bands has been observed. The C Is core level and the valence-band photoemission spectra point to the presence of a localized covalent bond between the C-60 molecules and the Ge atoms. This bond induces the splitting of the C-60 highest occupied molecular orbital with a feature appearing at about 2.3 eV.