The excited state properties of C₆₀ revisited: A Raman study

The excited state properties of C₆₀ thin films have been probed in the temperature range 77-273K using Raman spectroscopy. The change in the Raman pentagonal pinch mode of C₆₀ (whose position is largely independent of temperature) was monitored as a function of the excitation intensity at 514.5nm. This mode, normally positioned at 1469cm^-1, was seen to shift reversibly to a lower Raman frequency with increasing laser intensity. Two excited state species have been identified. The first, at 1466cm^-1 has been associated with the molecular triplet of C ₆₀ as determined from measurements preformed in solution. The second species at 1463cm^-1, has been speculated to be an excited state co-operative involving two or more excited states in the solid and is seen to be intrinsic to solid state C₆₀ below the phase transition, as similar measurements in solution show no Raman evolution beyond 1466cm ^-1. This species observed at 1463cm^-1 has previously been reported in the depolymerisation of solid state C₆₀, as well as in reversible processes in C₆₀ crystals and has been characterised by a nonlinear photo-luminescence and photoconductivity. It is further proposed in this study that this excited state is analogous to a number of highly conducting states recently reported for C₆₀ The data presented highlights the existence of a highly non-linear delocalised excited state species at low temperatures which is intrinsic to solid state C₆₀.