EFFECT OF ISOTOPIC DISORDER ON THE F-U MODES IN CRYSTALLINE C-60

The reflectance of undoped single-crystals of natural C-60 and isotopi cally pure C-12(60) and C-13(60) has been measured from 400 to 1600 cm (-1) at high resolution (0.2 cm(-1)) above and below the cm orientatio nal-ordering transition. In natural C-60, the F-u(1) mode shows eviden ce for isotopic splitting at 295 K; all the other modes are singlets. Below the orientational-ordering transition the F-u(1) mode splits int o five resolvable components, the F-u(3) and F-u(4) modes split into q uartets, and the F-u(2) mode appears to remain a singlet. In the isoto pically pure materials all the modes are singlets at 295 K; below the orientational-ordering transition, the F-u(1) mode splits into only fo ur components, the F-u(3) and F-u(4) modes split into triplets; the F- u(2) mode shows only weak evidence for splitting. In those modes which display crystal-field splitting, isotopic disorder generates only a w eak side band. The F-u(1) mode displays fine structure which may be re lated to the intrinsic merohedral disorder in the ground state. It is proposed that the anomalous increase in the oscillator strength of the F-u(4) mode is due to a modulation of the charge density along the [1 10] intermolecular bonding directions.