Measurement of the second-order molecular hyperpolarizability of fullerene anions by CARS spectroscopy

We have determined the second-order molecular hyperpolarizability, γ, for the fullerene anions C602− and C603− by use of coherent anti-Stokes Raman spectroscopy (CARS) vibrational lineshape analysis. We observe values that are only 2–3-times greater than the corresponding value for the singly charged anion C60−. The behavior of γ with the addition of charge is explained by symmetry considerations. Comparison with similar results for rare earth-containing endohedral fullerenes suggests that increased charge on the cage, and not asymmetric charge distribution due to metal–cage interactions, is the primary reason for increased nonlinearity compared to neutral fullerenes.