Observation and assignment of silent and higher-order vibrations in the infrared transmission ofC60crystals

We report the measurement of infrared transmission of large ${\mathrm{C}}_{60}$ single crystals. The spectra exhibit a very rich structure with over 180 vibratonal absorptions visible in the 100–4000 ${\mathrm{cm}}^{\mathrm{-}1}$ range. Many silent modes are observed to have become weakly IR active. We also observe a large number of higher-order combination modes. The temperature (77–300 K) and pressure (0–25 kbar) depedences of these modes were measured and are presented. Careful analysis of the IR spectra, in conjuction with Raman scattering data showing second-order modes and neutron scattering data, allows the selection of the 46 vibrational modes of ${\mathrm{C}}_{60}$. We are able to fit all of the first- and second-order data seen in the present IR spectra and the previously published Raman data (∼300 lines total), using these 46 modes and their group-theory-allowed second-order combinations.