Electronic structure of the thiophene/Au(111) interface probed by two-photon photoemission

The electronic structure of thiophene adsorbed on Au(111) has been investigated by two-photon photoemission (2PPE) spectroscopy and density functional theory (DFT) calculations. The dominant interfacial feature observed in the 2PPE spectra is a nondispersive unoccupied state whose width and energy (referenced to the Fermi level) decrease with increasing coverage. We assign this feature to a thiophene LUMO-derived state of mixed sulfur and carbon p-π character. DFT calculations indicate that the experimentally observed decrease in width of this state with increasing coverage is a result of weakening of the thiophene-Au(111) interaction. Increasing the molecular density forces the thiophene plane to tilt away from the surface, rotating the molecular orbitals away from an orientation more favorable for S-Au and π-Au hybridization. We attribute the ∼0.2 eV shift of the LUMO toward the Fermi level to stabilization of the transient anion due to the increasing effect of charge-induced polarization of the neighboring thiophene molecules with increasing coverage.