The Role of Charge-Charge Correlations and Covalent Bonding in the Electronic Structure of Adsorbed C-60: C-60/A1

Aromatic molecules are central components of model systems for molecular electronics, with C-60 one of the most studied. Upon adsorption on (metallic) substrates a splitting of the frontier orbitals is commonly observed, with a strong dependence on substrate material, but little dependence on substrate structure. We report the detailed photoelectron angle dependence of C-60/A1(110) over a wide range of energy, finding a strong remnant molecular character. In particular, certain HOMO-derived suborbitals couple strongly, and others weakly, with the metal, which results in final state charging for those weakly coupled. C Is data correlate well with the assignments made on this basis, as does the comparison of ground state partial densities-of-states (PDOS) to photoelectron spectra. Detailed analysis of the PDOS supports a rough division into surface-near and surface-far components, in agreement with the molecular picture. The component spectral widths are attributed to intramolecular vibrational coupling, which is suggested to aid in the electronic decoupling of certain suborbitals from the substrate, facilitating the observed final state charging.