Vibronic states in single molecules: C60 and C70 on ultrathin Al2O3 films.

Vibronic states are observed in single C(60) and C(70) molecules by scanning tunneling microscopy. When single fullerene molecules are adsorbed on a thin layer of Al(2)O(3) grown on a NiAl(110) substrate, equally spaced features are observed in the differential conductance (dI/dV), which are clearly resolved in d(2)I/dV(2) spectra. These features are attributed to the vibronic states of the molecule. The vibronic progressions are sensitive to the molecular orientations and can have different spacings in different electronic bands of the same molecule. For C(60,) these vibronic states are associated with the intramolecular A(g) and H(g) vibrational modes. Vibronic states are not resolved in molecules adsorbed on the metal surface. However, inelastic electron tunneling spectroscopy exhibits a vibrational mode at 64 meV for C(60) and 61 meV for C(70) adsorbed on NiAl(110).