Optical properties of solid C60

The optical properties of materials are strongly related to the nature of the photoexcitations in the solid. Fullerene C60 in the solid state is a typical molecular material in which the molecules are weakly held together by van der Waals forces and the electronic excitations are very close in energy and nature to those of the free molecule. The lowest electronic states in solid C60 are dipole forbidden and can be described as tight bound (Frenkel) excitons. Optical emission of C60 single crystals can be explained in terms of photoluminescence sub-spectra built on vibronically allowed false origins. Further to these neutral excitations which determine the optical gap there are charged excitations corresponding to the promotion of electrons from ball to ball and from ball to the free electron continuum. We observe intermolecular charge-transfer excitons by measuring the electroabsorption of thin films at liquid helium temperature. The energy necessary to generate directly the electron—hole (e—h) pairs is set at least 0.6 eV higher than the optical gap.