Observation of higher-order infrared modes in solid C60 films.

Abstract

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-\ensuremath{\mu}m-thick ${\mathrm{C}}_{60}$ film on KBr in the range 400\char21{}4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. 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.