Geometrical and optical properties of fullerene polymers

The polymerization of solid C60 is studied theoretically using the semiempirical AM1 Hamiltonian. Model systems consisting of two interacting C60 molecules are used in order to model polymerization of neutral C60, as well as of alkali metal doped C60. The fact that C60 polymerizes in the charged and in the photoexcited state, but not in the neutral ground state is explained by the Woodward-Hoffmann rules, i.e., the necessary symmetry breaking is introduced by changing the occupancy of the molecular orbital spectrum either by means of electron excitations or by charging of the system. We have also calculated the optical absorption spectrum of both the neutral and the charged system. The charged spectrum is found to give rise to a low lying absorption polarized along the chain direction, a feature which is unique for the polymerized phase.