Excitons in C_60 studied by Temperature Dependent Optical Second- Harmonic Generation

The electric dipole forbidden T_1g excitonic state of solid C_60 at omega=1.81 eV can be probed with a Second-Harmonic Generation (SHG) experiment. We show that the SHG line shape depends strongly on the degree of rotational order. We observe a splitting into two peaks below the rotational ordering phase transition temperature of 260 K. The origin of this splitting is discussed in terms of a possible Jahn-Teller effect, a possible Davydov splitting due to the four molecules per unit cell in the low temperature phase, and a mixing of the nearly degenerate T_1g and G_g free molecule states because of the lower symmetry in the solid. The exciton band structure is calculated with a charge transfer mediated propagation mechanism as suggested by Lof et al. and with one-electron (-hole) transfer integrals determined from band structure calculations. Comparison with our experimental SHG data leads to a reasonable agreement and shows that a mixing of T_1g and G_g states may explain the splitting at low temperature.