LIBRATIONAL MODES IN SOLID C-60 AND DOPED SOLID C-60

We present a theoretical study of librational modes in solid C60 and d oped solid C60. General results are derived by utilizing the space gro up symmetries of the solids. At the Brillouin-zone center GAMMA, in so lid C60 there should be in general five distinct libron frequencies in the simple-cubic phase, and ten in the so-called 2a0-fcc phase; in M3 C60, where M=K,Rb, a single libron frequency is expected; in M6C60, on e also expects a single libron frequency. The case of M3C60 is of part icular interest since in the first approximation, the potential govern ing the libron motion contains a single undetermined parameter, and is thus completely determined by a measurement of the libron frequency.