Raman spectroscopic study of excited states and photo-polymerisation of C60 from solution

The Raman scattering signal of C-60 in solution was monitored as a function of the excitation intensity at 514.5 nm. Whereas at low intensities the A(2g) pentagonal pinch mode was positioned at 1469 cm(-1), at intensities where an inverse saturable absorption is observable, it was positioned at 1466 cm(-1). This mode positioning is ascribed to the molecular triplet state. When measured at the cuvette interface, the ground-state Raman mode, at 1469 cm(-1) in solution was seen to degrade quickly under illumination to a lower Raman frequency of 1458 cm(-1) where an insoluble deposit formed, which has been identified as a photochemical product of C-60 analogous to the solid-state C-60 polymer formed by a 2 + 2 cycle-addition across pi bonds of neighbouring molecules. The process is seen to be reversible under high-intensity illumination, via an intermediate state with mode positioning at 1463 cm(-1). This state is associated with a high-intensity excited state previously reported in the solid state of C-60. (C) 1999 Elsevier Science B.V. All rights reserved.