Nonlinear optical properties of the C₆₀ molecule: Theoretical and experimental studies

Frequency-dependent molecular linear polarizability, α, and the second hyperpolarizability, γ, of the C₆₀ molecule have been calculated by the time-dependent coupled-perturbed Hartree-Fock method within extended basis intermediate neglect of diatomic differential overlap approach. The orientationally averaged values of α and γ for C₆₀ are respectively about 13 and 30-34 times larger than those for benzene. The α and γ tensors appear to be isotropic, indicating a spherically symmetric charge distribution in C₆₀. Experimental observations of third-order optical nonlinearity of C₆₀ by the degenerate four-wave mixing technique at the wavelength of 602 nm and by optical Kerr gate measurements at the wavelength of 620 nm indicate its ultrafast response time. The experimentally measured optical second hyperpolarizability is a complex number for which γ^r = -(5 ± 2) × 10^-33 esu and γⁱ = (9 ± 1) × 10^-33 esu.