The far-infrared transmission spectrum of solid ${\mathrm{C}}_{60}$ is presented in the 7.5–420-${\mathrm{cm}}^1$ range at 2 K. It is found that the cubic environment of ${\mathrm{C}}_{60}$ molecules in the solid results in the activation and splitting of silent molecular modes. The two infrared-active translational modes propagating in the ${\mathrm{C}}_{60}$ solid are observed below 60 ${\mathrm{cm}}^{\mathrm{-}1}$. Their energies are found to be significantly higher than predicted by molecular-dynamics simulations. This suggests a harder intermolecular potential for solid ${\mathrm{C}}_{60}$ than presently used in these simulations.

• Received 7 August 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.2666