Optical absorption spectra and geometric effects in higher fullerenes

The optical excitations in C-70 and higher fullerenes, including isomers of C-76, C-78, and C-84, are theoretically investigated. We use a tight-binding model with long-range Coulomb interactions, treated by the Hartree-Fock and configuration-interaction methods. We find that the optical excitations in the region where the energy is smaller than about 4 eV have most of their amplitudes at the pentagons. The oscillator strengths of projected absorption almost accord with those of the total absorption. When the projection is performed onto each pentagon and pentagon dimers, the resultant spectrum in the low-energy region is quite different from that of the total absorption. The spectral shapes of the total absorption turn out to be determined mainly by the geometrical distributions of the pentagons in the fullerene structures.