Resonant nonlinear optical response of the fullerenes<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mrow><mml:mn>60</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mrow><mml:mn>70</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Time-resolved degenerate four-wave mixing experiments were conducted on films of pure ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ using a picosecond tunable dye laser. The fullerenes exhibit a large third-order optical response, and their dynamics show wavelength and fluence dependence. The temporal response is characterized by a long-lived component attributed to the triplet excited state and an early decay associated with population of the singlet excited state. This fast response is considerably shortened at higher laser fluences and an excitonic singlet-singlet annihilation mechanism is proposed.

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