Formation and regulation of unoccupied hybridized band with image potential states at perylene/graphite interface

Occupied and unoccupied electronic structures of submonolayer perylene (C₂₀H₁₂) on a graphite surface have been investigated using two-photon photoemission (2PPE) spectroscopy for two phases at room and low temperatures. Low energy electron diffraction measurements indicated that the molecules are disordered at room temperature and form a well-ordered superstructure below 180 K. In 2PPE, a specific unoccupied peak (Lx) was observed at around room temperature (>180 K) but not at low temperature (<180 K). The temperature-dependence of the excitation probability was attributed to a contribution of a diffuse unoccupied state, which is characterized by the molecular orbital extending outside the perylene molecular framework. At around room temperature, perylene adopts a flat-lying molecular orientation so that the diffuse state can hybridize with a free-electron-like unoccupied surface state, image potential states (IPS). As a result, the hybridized Lx state can be excited from the occupied bulk band through the IPS-mediated process. In contrast, hybridization is not efficient in the low-temperature phase due to the standing molecular orientation, which decouples the molecule away from the image plane of the substrate. The size of molecular islands also affects hybridization between the diffuse states and IPS because the two states encounter each other at the edge part of molecular aggregates. The temperature-dependent 2PPE results indicate that the molecular orientation and island size of perylene are directly linked to the formation of hybridized states, and thus, the excitation probability at the interface can be regulated by the morphology on the surface.