Anomalous optical response ofC60andC70in toluene

Degenerate-four-wave-mixing experiments were performed on ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ samples in toluene using 20-ps pulses at 532 nm for several pump-beam crossing angles. In the ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ samples, an anomalously large periodic scattering was observed with a peak at least an order of magnitude larger than the ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ peak. Compared to toluene, the probe scattering due to ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ at zero delay was lowered in the presence of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$. The behaviors were interpreted using an index grating model and the periodic signal was attributed to laser-induced phonons.