Angle- and Time-Resolved Two-Photon Photoemission Spectroscopy for Unoccupied Levels of Lead Phthalocyanine Film

Abstract

We have measured angle- and time-resolved two-photon photoemission (2PPE) from unoccupied levels of graphite (HOPG) and lead phthalocyanine (PbPc) films. Our angle-resolved 2PPE microspot spectroscopy with a lateral resolution of 0.4 μm can effectively select flat and homogeneous regions of the film, allowing measurements of the dispersion of the image potential state (IPS) on the PbPc film. The effective mass of the IPS on the film is 2.2 ± 0.2, which is heavier than that for the bare HOPG, 1.2 ± 0.1. The lifetimes of the states at 30 K are 30 fs for both the HOPG and the PbPc film. These results suggest a hybridization of the IPS with a molecule-derived unoccupied level. The lifetimes of the lowest unoccupied molecular level (LUMO)-derived levels are about 80 fs. A possible decay process to lower states is suggested.