Image Potential State Mediated Excitation at Rubrene/Graphite Interface

Abstract

We report an electronic excitation of adsorbed molecules mediated by the image potential state (IPS) on the substrate. Two-photon photoemission (2PPE) spectroscopy for rubrene films formed on a highly oriented pyrolytic graphite (HOPG) surface reveals a prominently enhanced peak due to an unoccupied molecular level (denoted as Ln), which is resonantly excited from the highest occupied molecular orbital (HOMO) derived level. The enhancement of the Ln peak becomes less significant at the coverage higher than 1 monolayer (ML), where the IPS peak on the substrate disappears. The resonance enhancement is moderate with s-polarization, by which the transition to IPS is completely suppressed. We ascribe that the excitation of the Ln level is mediated by the IPS on HOPG. Though the IPS wave function extends outside the molecules, it interacts with the unoccupied molecular orbital at the edges of molecular islands, causing the strong resonance enhancement of the Ln level.