ABSTRACT
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.
ABSTRACT
The interaction of a molecular orbital with a surface state is important to understand the spatial distribution of the wave function at the molecule/substrate interface. In this study, we focus on hybridization of an unoccupied state of lead phthalocyanine (PbPc) with the image potential state (IPS) on a graphite surface. The hybridization modifies the energy-momentum dispersions of the IPS on PbPc films as observed by angle-resolved two-photon photoemission. On the PbPc 1 monolayer film, the IPS band forms a band gap and back-folding appears at the first Brillouin zone boundary due to the periodic potential by the adsorbate lattice. The modification of the dispersion is accompanied by the intensity enhancement of the IPS. We attributed the origin of the modified dispersion and intensity enhancement to a hybridization of the IPS with a molecule-derived unoccupied level. From the photon energy-dependent measurement on multilayer films, we have found the diffuse unoccupied molecular level in the vicinity of the IPS. The tail part of the IPS wave function in the substrate is enhanced by the hybridization with the unoccupied state, and thus strengthens the transition from the occupied substrate band to the hybridized IPS.
