OPTICAL-PROPERTIES OF SOLID C-60

The optical properties of materials are strongly related to the nature of the photoexcitations in the solid. Fullerene C-60 in the solid sta te is a typical molecular material in which the molecules are weakly h eld together by van der Waals forces and the electronic excitations ar e very close in energy and nature to those of the free molecule. The l owest electronic states in solid C-60 are dipole forbidden and can be described as tight bound (Frenkel) excitons. Optical emission of C-60 single crystals can be explained in terms of photoluminescence sub-spe ctra built on vibronically allowed false origins. Further to these neu tral excitations which determine the optical gap there are charged exc itations corresponding to the promotion of electrons from ball to ball and from ball to the free electron continuum. We observe intermolecul ar 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 t han the optical gap.