Electron dynamics at polyacene/Au(111) interfaces

Two-photon photoemission (2PPE) spectroscopy is used to examine the excited electronic structure and dynamics at polyacene/Au(111) interfaces. Image resonances are observed in all cases (benzene, naphthalene, anthrathene, tetracene, and pentacene), as evidenced by the free-electron like dispersions in the surface plane and the dependences of these resonances on the adsorption of nonane overlayers. The binding energies and lifetimes of these resonances are similar for the five interfaces. Adsorption of nonane on top of these films pushes the electron density in the image resonance away from the metal surface, resulting in a decrease in the binding energy (-0.3 eV) and an increase in the lifetime (from < 20 to similar to 110 fs). The insensitivity of the image resonances to the size of polyacene molecules and the absence of photoinduced electron transfer from the metal substrate to molecular states both suggest that the unoccupied molecular orbitals are not strongly coupled to the delocalized metal states or image potential resonances.