Optically Induced Inter- and Intrafacial Electron Transfer Probed by Two-Photon Photoemission: Electronic States of Sexithiophene on Au(111)

Using two-photon photoemission spectroscopy we investigated the electronic structure of the organic semiconductor a sexithiophene (6T) adsorbed on Au(111). Beside the quantitative determination of the energetic position of electronic states originating from the highest occupied molecular orbitals (HOMO and HOMO-1) and the lowest unoccupied molecular orbitals (LUMO and LUMO+1), a localized exciton state that possesses a binding energy of 0.9 eV has been identified. Whereas the creation of the exciton is the result of an intramolecular excitation involving a HOMO-LUMO transition, the transient population of the LUMO and LUMO+1 follow from an optically induced charge transfer from the metallic substrate to the molecule. The present study provides important parameters such as the energetic position of the transport level and the exciton binding energy, which are needed to understand the physics in organic-molecules-based optoelectronic devices.