Electronic structure of 1ML NTCDA/Ag(111) studied by photoemission spectroscopy

We performed high-resolution photoemission experiments at different photon energies to investigate the valence band structure of 1ML of 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) on Ag(111). Besides the known occupied molecular orbitals HOMO and HOMO−1 we observe a new state close to the Fermi level, which results from the interaction between NTCDA and the substrate, partially filling the lowest unoccupied orbital of the free molecule (LUMO) in the monolayer system. By tuning the photon energy through the carbon K-edge, a resonance like change of the spectral intensity at the HOMO and HOMO−1 energies is clearly revealed, which we use for an assignment of the individual spectral features to a predominant localization either at the naphthalene core or the anhydride group.