The excited state properties of C_(60) revisited: A Raman study

摘要

The excited state properties of C_(60) thin films have been probed in the temperature range 77-273K using Raman spectroscopy. The change in the Raman pentagonal pinch mode of C_(60) (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 C_(60) below the phase transition, as similar measurements in solution show no Raman evolution beyond 1466cm '. This species observed at 1463cm~(-1) has previously been reported in the depolymerisation of solid state C_(60), as well as in reversible processes in C_(60) 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 C_(60) The data presented highlights the existence of a highly non-linear delocalised excited state species al low temperatures which is intrinsic to solid state C_(60).