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 crysta lline C-60 is presented. Highly structured low-energy bands are observ ed 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 c rystalline C-60, C-12(60), and C-13(60) are found to be identical, apa rt 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 uncha nged as the amount of intercalated O-2 was reduced, as monitored by th e singlet O-2 photoluminescence intensity, in sharp contrast to a prev ious report that the libron scattering was activated by the presence o f oxygen. Spectra of crystals cooled below the glassy transition at si milar to 90 K at different rates are also presented. The libron energi es and intensities are seen to be dependent on the cooling rate, refle cting the differing amounts of residual orientational disorder present in the lowest-temperature phase.