A RAMAN ANALYSIS OF C-60 AT LOW-TEMPERATURES - A STUDY OF MOLECULAR AND CRYSTAL-FIELD EFFECTS

The Raman spectra in the region of 200-1700 cm(-1) of C-60 oxygen-free crystals and films at low temperatures are found to possess a rich vibrational character. A total of 57 vibrations in the crystal and 47 vibrations in the film are observed. in addition to the strong Raman-active 2A(g) and 8H(g) modes, 6 silent gerade fundamentals and 10 silent ungerade fundamentals in the crystal and 7 gerade and 7 ungerade in the film can be assigned. The activation of silent fundamentals is observed to be largely independent of the solid-state environment and sample morphology and is attributed to the presence of C-13. A further 40 vibrations in the crystal and 35 in the film are assigned to fundamental combinations and overtones. Notably, 14 modes in the crystal and 12 in the film are observed, which cannot be assigned to the above and these are attributed to the splitting of 10 of the fundamental and combination modes. These splittings are attributed to crystal-field effects. Some differences in splitting between crystals and films are observed and are discussed in terms of the inhomogeneous and polycrystalline character of the latter.