Tunneling spectroscopy of isolated C 60 molecules in the presence of charging effects

The discrete molecular-level spectrum of an isolated ${\mathrm{C}}_{60}$ molecule is resolved and its interplay with single-electron charging effects is studied using room-temperature and cryogenic scanning tunneling spectroscopy. The tunneling current-voltage spectra of these molecules exhibit rich structures resulting from both resonant tunneling through the discrete levels and charging effects. In particular, we observe degeneracy lifting within the molecular orbitals, probably due to the Jahn-Teller effect and local electric fields. Theoretical fits account well for our experimental data.