Energy Terms Diagram for Solid C60 Fullerite Deduced from Transmission Spectra

Remarkable variations of the absorption peaks in the optical transmission spectra are observed at a temperature T = 30 K on C-60 polycrystalline films of various thicknesses L from 0.2 similar to 60 mu m with incident photon energies from 1.46 similar to 2.07 eV. To account for these rich variations, a schematic energy term diagram based on the Davydov Splitting C caused by the weak intermolecular van der Waals interactions together with the Herzberg-Teller coupling to the singlet S-1 state and to the triplet T-1 state in this organic C-60 molecular solid is proposed. Then the observed absorption peaks are explained in terms of the transitions from the ground singlet So state to the S-1-derived states and to the T-1-derived states, respectively. Furthermore, this experimental work reveals that for thick C-60 polycrystalline films with L >= 50 mu m the transitions from the ground state to the S-1-derived states are observed to be much stronger than those from the ground state to the T-1-derived states; thus, an optical bandgap is correspondingly defined.