Occupied and unoccupied electronic states in tetra-tert-butyl-azobenzene (TBA) absorbed of Ag(111) have been investigated by one-photon and two-photon photoemission spectroscopy. These measurements allow the quantitative determination of energetic positions of the highest occupied (HOMO) and the lowest unoccupied molecular orbital (LUMO) as well as the n = 1 image potential state. The assignment of the electronic states are supported by quantum chemical calculations. Experimentally a HOMO-LUMO gap of 2.85 eV is observed, whereas the gap obtained from the calculated molecular orbital energies is 0.92 eV larger. This discrepancy can be explained by image charge screening. Furthermore, two unoccupied final states located 0.18 eV and 0.43 eV above the vacuum level, respectively, have been identified.
The publication is available at link DOI: 10.1007/s00339-007-4047-0