RING-BIPOLARON EXCITON IN NEUTRAL FULLERENE C-60

Neutral fullerene C60 is studied by the electron-lattice-coupling tight-binding model. When one or two electrons are excited from the highest occupied molecular orbital to the lowest unoccupied molecular orbital of a C60 molecule, the lattice relaxes and the lowest-energy state of C60 is discovered to be a ring-bipolaron exciton which is qualitatively consistent with the self-trapped polaron exciton observed in the luminescence experiment of neutral C60. The symmetry of optical excitation C60 is reduced froM I(h) to D5d and it is proposed that four NMR lines would be observed in this material.