"The acoustic branch of the translational modes with Tu symmetry has a) = 0 at q = 0 and is not listed. The Au and Eu modes are neither Raman active nor infrared-active. The Tu mode is IR active and all gerade modes (i.e., Ag, Eg, and Tg) are Raman-active.

fcThe conversion between wave numbers and meV is 1 meV = 8.0668 cm~'.

cLi et al. , Ref. [11.60], dYildirim and Harris, Ref. [11.11], cLipin and Mavrin, Ref. [11.61].

^Inelastic neutron scattering [11.13]. Estimated from dispersion curves.

'Inelastic neutron scattering [11.12]

■'Inelastic neutron scattering [11.52],

*If the calculated librational modes in Ref. [11.63] are multiplied by 2, then good agreement is obtained with experiment. 'Raman spectra at 10 K [11.57], softening of the orientational potential (see §7.1.3), since single-crystal inelastic neutron scattering [11.12] does not show a broadening of individual modes in this temperature range.

Axe et al. [11.51] have recently reviewed the structure and dynamics of solid C60, emphasizing the contributions from neutron scattering studies to our understanding of the vibrational spectra and structural properties of fullerenes. One difficulty with interpreting many of the early Raman and neutron measurements relevant to the intermolecular vibrations is the paucity of polarization and mode identification information [11.12,57]. Single-crystal data for the dispersion relations are now becoming available for low frequency intermolecular translational modes in C60 (see Fig. 11.7) from inelastic neutron scattering studies [11.12].

Since the C60 molecules rotate rapidly about their fee equilibrium positions above the orientational ordering temperature T01 ~ 261 K [11.59], an orientational restoring force is ineffective above T0l, and the rotational intermolecular modes (i.e., librational modes or "librons") are lost from the phonon spectrum. Above Tm, solid C60 exhibits an fee lattice which has only one molecule per primitive cell. Therefore, only three acoustic branches (all with Tu symmetry for the cubic group at the T-point) are expected, and these involve approximately rigid axial and radial displacements of the molecules. In Fig. 11.7(a), we display the phonon dispersion data (closed symbols) of Pintschovius and co-workers for solid C60 in the orientation-ally disordered (fee) state [11.13], obtained by inelastic neutron scattering along the [100], [110], and [111] directions of a small 6 mm3 single crystal at T = 295 K. The solid curves in the figure are the result of a two-parameter

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