Resonant two-photon photoemission study of electronically excited states at the lead phthalocyanine/graphite interface

Occupied and unoccupied electronic states of lead phthalocyanine (PbPc) films of 1 ML thickness formed on graphite substrates have been measured by two-photon photoemission (2PPE) spectroscopy. Highly reproducible and well-resolved 2PPE spectra were obtained by selecting sample positions with submicrometer light spot. We have identified the molecule-derived states due to the HOMO-1, HOMO, LUMO, LUMO+1, LUMO+2 as well as the first image-potential state (IS). The occupied states are in good agreement with one-photon photoemission results. Resonant excitations between the HOMO and LUMO+2-related levels as well as those between the HOMO level and IS were observed in consistency with energies of relevant levels determined from off-resonant conditions.