RAMAN STUDIES OF NONLINEAR PHENOMENA IN FULLERENE CRYSTALLITES

The resonant Raman scattering of C60 crystallites is monitored as a function of excitation intensity at 514.5 nm. At low intensities, a strong line at 1468 cm-1 is observed. No feature at 1459 cm-1 is observable. With increasing intensities, the 1468 cm-1 line shifts continuously and reversibly to lower frequencies. The mode softening is nonlinearly dependent on the input intensity and is accompanied by a nonlinear increase in the Raman intensity. The spectral changes are discussed in terms of a nonlinear reduction in force constant and increase in bond polarisability as a result of an increased intermolecular delocalisation of the pi-electron cloud. The nonlinear changes are associated with the nonlinear luminescence and photoconductive response observed in fullerenes and are contrasted to the irreversible phototransformation recently discussed.