Unoccupied states in C60 thin films probed by two-photon photoemission

Unoccupied electronic states in C-60 thin films on Cu(111) have been measured by laser two-photon photoemission (2PPE) spectroscopy. These measurements allow the quantitative determination of energetic positions for the lowest unoccupied molecular orbital (LUMO), LUMO + 1, LUMO + 2, image potential states, as well as the highest occupied molecular orbital (HOMO). The transiently populated LUMO and LUMO + 1 levels are stabilized by the on-molecule charge correlation energy during the 2PPE process. Compared to previous measurements using one-photon photoemission and inverse photoemission, the HOMO-LUMO gap determined by 2PPE is stabilized by half of the Hubbard U. Both intramolecular excitation and a minor metal-to-molecule electron-transfer excitation channel are observed.