Photoluminescence of solid state fullerenes

Time resolved photoluminescence studies have been carried out in films, powders and single crystals of C₆₀. The observed spectra for C₆₀ in all forms are consistent with those reported for steady state conditions and may be associated with an intramolecular process. The spectrum is largely temperature independent with the exception of a broadening of the high energy edge with increasing temperature. At low temperatures, the observed C₆₀ luminescence decays rapidly with a lifetime of 1.2 ns and can be interpreted as a low efficiency singlet decay or a decay from the molecules on the surface of the microcrystallites. No long lived decay from the triplet is observable at these intensities. In the case of the films, both the emission spectrum and lifetime are independent of excitation intensity. In the case of the powder and crystal a dramatic increase in the lifetime and broadening of the spectrum is observed at higher intensities. This broadened spectrum shows no fine structure and the emission maximum is red shifted by ∼0.4 eV. Above a 'threshold' intensity, the luminescence intensity is seen to increase with the cube of the input intensity. This behaviour is interpreted as an abrupt onset of emission from the highly populated triplet state. The restrictions of the transition selection rules are relaxed by a banding of the intramolecular triplet states, a process which is dependent on a critical excited state population density. Thus a transition from intramolecular processes to an extended state, band like behaviour is observable at high excitation densities.