SYMMETRY FOR LATTICE MODES IN C-60 AND ALKALI-METAL-DOPED C-60

We present a group-theoretical analysis relevant to the interpretation of recent experimental observations of Raman-scattering spectroscopy in C60 and alkali-metal-doped C60. The group-theoretical analysis makes use of two space groups, both of which reduce the icosahedral point-group symmetry of the isolated C60 molecule to the cubic group T(h). The space group for the fcc arrangement of fullerene is T(h)6 and corresponds to the low-temperature solid phase of pristine C60. The bcc arrangement for solid M6C60, where M is an alkali metal, is consistent with the space group T(h)5 and is the symmetry considered for the fully doped alkali-metal material. The experimental Raman spectra show line splittings which are consistent with a symmetry lowering to T(h) symmetry for the fullerene molecule. However, the polarization selection rules observed experimentally indicate a further lowering of the symmetry, beyond cubic symmetry.