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 abruptly 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 1468 cm-1 C60 A(g) ''pentagonal pinch'' mode is seen to reappear. The process is interpreted as a photo-dissociation of the C60 photoproduct. The positioning of the Raman line at 1463 cm-1 is associated with an excited state intermediate, observable only at high excitation densities.