Optical response of fullerites: exciton and lattice-fluctuation effects

Summary form only given. The fullerene family shows many, interesting optical properties related with delocalized /spl pi/-electrons. We develop a theory of optical excitations by using a tight binding model with long-range Coulomb interactions. The model is applied to mutually-interarting C/sub 60/ (and C/sub 70/) clusters, and is treated by the Hartree-Fock approximation followed by a configuration inter action method. Lattice fluctuations are taken into account by a bond disorder model [1]. We first examine what strength of Coulomb interaction is appropriate to describe the electronic structures observed by photo-electron and optical absorption spectroscopy. Then, we demonstrate that the photo-excited states are mainly intramolecular (i.e. Frenkel) excitons with small charge-transfer components. We pay attention to the dipole forbidden transitions of a single C/sub 60/ molecule around 2.8eV, and examine how they become dipole-allowed by solid state effects and lattice fluctuations. Furthermore, we calculate nonlinear optical response of fullerites, for example, dispersions of electroabsorption and third harmonic generation. A comparion with recent experiments is made, and relations among various optical response functions axe discussed.