Electroabsorption in C60 and C70. Third-order nonlinearity in molecular and solid states

We have studied electroabsorption (EA) in both films and disperssed molecules of C60 and C70. In contrast to a recent model for the third harmonic generation (THG) spectrum, we find that the two-photon states important to the nonresonant χ(3)(0) in molecular C60 are above the 3.6 eV T1u state, not below it. We present a model consistent with spectra of EA, THG and degenerate four-wave mixing (DFWM). The lowest-energy resonances in χ(3)(ω) are dominated by ‘forbidden’ transitions made allowed by vibronic coupling in the molecule and intermolecular coupling in the solid.