Neutral fullerene ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$ molecule, the lattice relaxes and the lowest-energy state of ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$. The symmetry of optical excitation ${\mathrm{C}}_{60}$ is reduced from ${\mathit{I}}_{\mathit{h}}$ to ${\mathit{D}}_{5\mathit{d}}$ and it is proposed that four NMR lines would be observed in this material.

• Received 14 June 1993

DOI:https://doi.org/10.1103/PhysRevB.48.17615