Observation and assignment of silent and higher-order vibrations in the infrared transmission of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mrow><mml:mn>60</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>crystals

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 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ 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 (\ensuremath{\sim}300 lines total), using these 46 modes and their group-theory-allowed second-order combinations.

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