Time- and angle-resolved two-photon photoemission (2PPE) spectroscopy has been employed to investigated the electronic structure, the electron dynamics and localization at the interface between tetra-tert-butyl imine (TBI) and Au(111). At a TBI coverage of one monolayer (ML), the two highest occupied molecular orbitals, HOMO and HOMO-1, are observed at an energy of -1.9 eV and -2.6 eV below the Fermi level (EF), respectively, and coincide with the d-band features of the Au substrate. In the unoccupied electronic structure the lowest unoccupied molecular orbital (LUMO) located at 1.6 eV with respect to EF has been observed. In addition, two delocalized states which arise from the modified image potential at the TBI/metal interface have been identified. Their binding energies depend strongly on the adsorption structure of the TBI adlayer, which is coverage dependent in the submonolayer (·1ML) regime. Thereby the binding energy of the lower interface state shifts from 3.5 eV at 1.0 ML to 4.0 eV at 0.5 ML, which is accompanied by a pronounced decrease of its lifetime from 100 fs to below 10 fs. This is a result of di®erences in the wave function overlap with electronic states of the Au(111) substrate at different binding energies. This study shows that not only adsorbate- and substrate-induced electronic states have to be considered in order to fully understand the electronic structure of organic adsorbates at metal surfaces but also interface states, which are the result of a potential formed by the interaction between the adsorbate and substrate.
The original publication is available by link DOI: 10.1088/1367-2630/12/12/125022