Effect of isotopic disorder on the Fu modes in crystalline C60.

The reflectance of undoped single crystals of natural ${\mathrm{C}}_{60}$ and isotopically pure $^{12}\mathrm{C}_{60}$ and $^{13}\mathrm{C}_{60}$ has been measured from 400 to 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at high resolution (0.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) above and below the orientational-ordering transition. In natural ${\mathrm{C}}_{60}$, the ${\mathit{F}}_{\mathit{u}}$(1) mode shows evidence for isotopic splitting at 295 K; all the other modes are singlets. Below the orientational-ordering transition, the ${\mathit{F}}_{\mathit{u}}$(1) mode splits into five resolvable components, the ${\mathit{F}}_{\mathit{u}}$(3) and ${\mathit{F}}_{\mathit{u}}$(4) modes split into quartets, and the ${\mathit{F}}_{\mathit{u}}$(2) mode appears to remain a singlet. In the isotopically pure materials all the modes are singlets at 295 K; below the orientational-ordering transition, the ${\mathit{F}}_{\mathit{u}}$(1) mode splits into only four components; the ${\mathit{F}}_{\mathit{u}}$(3) and ${\mathit{F}}_{\mathit{u}}$(4) modes split into triplets; the ${\mathit{F}}_{\mathit{u}}$(2) mode shows only weak evidence for splitting. In those modes which display crystal-field splitting, isotopic disorder generates only a weak side band. The ${\mathit{F}}_{\mathit{u}}$(1) mode displays fine structure which may be related to the intrinsic merohedral disorder in the ground state. It is proposed that the anomalous increase in the oscillator strength of the ${\mathit{F}}_{\mathit{u}}$(4) mode is due to a modulation of the charge density along the 〈110〉 intermolecular bonding directions.