DISORDER, IMPURITY, AND ISOTOPE EFFECTS IN THE RAMAN-ACTIVE LIBRON SPECTRUM OF CRYSTALLINE C-60

A Raman-scattering study of the intermolecular mode spectrum of crystalline C-60 is presented. Highly structured low-energy bands are observed and unambiguously attributed to librations of the C-60 molecules by comparing the spectra of C-60 single crystals prepared from 99.95% C-12 and 99.7% C-13 Starting material. The low-energy Raman spectra of crystalline C-60, C-12(60), and C-13(60) are found to be identical, apart from the expected mass shift of the mode energies. The influence of impurities on the libron spectrum has been investigated by preparing crystals without any atmospheric exposure. The spectrum remained unchanged as the amount of intercalated O-2 was reduced, as monitored by the singlet O-2 photoluminescence intensity, in sharp contrast to a previous report that the libron scattering was activated by the presence of oxygen. Spectra of crystals cooled below the glassy transition at similar to 90 K at different rates are also presented. The libron energies and intensities are seen to be dependent on the cooling rate, reflecting the differing amounts of residual orientational disorder present in the lowest-temperature phase.