Geometrical and optical properties of fullerene polymers

The polymerization of solid C-60 is studied theoretically using the semiempirical AM1 Hamiltonian. Model systems consisting of two interacting C-60 molecules are used in order to model polymerization of neutral C-60 as well as of alkali metal doped C-60 The fact that C-60 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.