Reversible photochemical processes in fullerenes. A Raman study

We report the photo-induced depolymerization of the product of the photochemical degradation of C₆₀ via a metastable excited state intermediate. The Raman scattering signal of C₆₀ crystallites is monitored as a function of excitation intensity at 514.5 nm. At low intensities, the characteristic 1468 cm^-1 C₆₀ A_g "pentagonal pinch" mode degrades quickly to the Raman line at 1459 cm^-1 which has been identified as characteristic of an irreversible photochemical product of C₆₀ The feature at 1459 cm^-1 is seen to be stable below an apparent threshold intensity, which is somewhat sample dependent, above which it shifts abruptyly to 1463 cm^-1. The shift of the line to 1463 cm^-1 is accompanied by a sharp increase in the Raman intensity and the background luminescence. With increasing intensities, no further shift of the Raman line is seen. Upon a return to low intensities, the 468 cm^-1 C⁶⁰ A_g "pentagonal pinch" mode is seen to reappear. The process is interpreted as a photo-dissociation of the C₆₀ photo-prod The positioning of the Raman line at 1463 cm^-1 is associated with an excited state intermediate, observable only at high excitation densities.