Influence of the crystal field on the Raman intensity of C${}_{60}$ fullerites

The Raman-scattering intensity of C${}_{60}$ intramolecular modes in fullerite, excited at 2.41 eV, strongly increases (up to one order of magnitude) upon cooling below room temperature. The effect has different strength for the $A_g$ and $H_g$ modes as well as for predominantly radial (low-frequency) and tangential (high-frequency) modes. The temperature dependence of the Raman intensity suggests that the effect is connected with the orientational ordering of C${}_{60}$ in solids. At low temperature, in the sc phase, the orientational ordering and relatively low site symmetry ${(S}_6)$ of C${}_{60}$ split the highly degenerate electronic bands. The Raman intensity of the asymmetric $H_g$ modes at resonance turns out to be very sensitive to the band-structure changes due to the vibrational mixing of the electronic bands. We attribute the relatively strong increase of Raman intensity of the radial modes in the sc phase to their more effective modulation of the Coulomb interaction between oriented neighboring C${}_{60}$ molecules.