Microspot two-photon photoemission spectroscopy for CuPc film on HOPG

Microspot two-photon photoemission (micro-2PPE) spectroscopy has been applied to measure the lateral distribution of unoccupied levels on copper phthalocyanine (CuPc) film on HOPG. In addition to the LUMO-derived level and the image potential state (IPS) on the film, we identified the modified IPS which is stabilized by the hole localized in a molecule. We show that modified IPS is observed only on bilayer area, reflecting the localization of the hole in a molecule. The modified IPS is absent on monolayer area, because the hole strongly interacts with substrate.