THEORY FOR EXCITON EFFECTS ON OPTICAL-ABSORPTION SPECTRA OF C-60 MOLECULE AND C-60 CRYSTAL

Optical absorption spectra of a C60 molecule and a C60 crystal are theoretically studied by using an extended Hubbard model, so as to clarify exciton effects. The Hartree-Fock approximation is adopted for the ground state, while, the first-order perturbation theory is adopted for electron-hole attractions in the excited states. The calculated results show that exciton effects are very important in both cases of the molecule and the crystal. The spectral intensity at the low energy side of the main absorption peak, in the case of crystal, is shown to be enhanced greatly by the effect of inter-molecular charge-transfer exciton. Several new peaks due to the Stark effect of the isolated molecule have also been found. These results are in good qualitative agreement with experiments.