In this paper, we present a rich (4 strong ${\mathit{F}}_{1\mathit{u}}$ first-order and an additional 98 weak lines) ir spectrum for a 4-μm-thick ${\mathrm{C}}_{60}$ film on KBr in the range 400–4000 ${\mathrm{cm}}^{\mathrm{-}1}$. Most of the weak lines are identified with second-order combination modes. More accurate frequencies of some optically inactive modes in ${\mathrm{C}}_{60}$ have been obtained by simultaneously analyzing the higher-order ir and Raman spectra. Isotope-induced symmetry lowering in ${\mathrm{C}}_{60}$ molecules has also been observed and is responsible for activating several fundamental modes which would otherwise be ir inactive.

• Received 1 June 1993

DOI:https://doi.org/10.1103/PhysRevB.48.11375