Formation of C-60 dimers: A theoretical study of electronic structure and optical absorption

The polymerization of solid C-60 is studied theoretically within a semi-empirical quantum chemical framework. 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 geometries and electronic structures of the systems are obtained from semiempirical. AM1, calculations. It is found that the charged systems have a substantially lower energy barrier towards formation of C-60 dimers, a result in agreement with experiment. This effect is explained in terms of the occupation of bonding intermolecular orbitals as extra charge is added. The reduction of the energy barrier in the case of photopolymerization is motivated in a similar way by promotion of electrons for antibonding, or to bonding, orbitals in the excited state. Furthermore, optical absorption spectra of the neutral and doubly charged C-60 dimers are calculated from the spectroscopic parameterization of the semiempirical intermediate neglect of differential overlap (INDO) Hamiltonian combined with single excited configuration interaction. The absorption spectrum of the neutral dimer displays distinct peaks with energies and oscillator strengths in qualitative agreement with experiment. Charging of the C-60 dimer leads to characteristic polarized transitions at low energies.