Spectroscopic investigation of unoccupied states in nano-and macroscopic scale: Naphthalene overlayers on highly oriented pyrolytic graphite studied by combination of scanning tunneling microscopy and two-photon photoemission

We have clarified the correlation between geometric and electronic structures of naphthalene ultrathin films on graphite using a combination of scanning tunneling microscopy (STM) and two-photon photoemission (2PPE) spectroscopy. Depending on the geometrical superstructure, as characterized by STM, shifts in the first image potential states are observed by STM-based local spectroscopy on the nanometer scale, which is consistent with coverage-dependent 2PPE spectra measured on the macroscale. An adsorption-induced unoccupied feature, which is specific to the (2√3×2√3) R30 superstructure, is detected at submonolayer coverages and is assigned to the lowest unoccupied molecular orbital (LUMO) derived level. It is interesting that the LUMO feature disappears for multilayer films. These behaviors indicate that a drastic change in electronic states occurs at the organic/metal interface associated with the change in the geometric structure.