ANOMALOUS SPLITTING OF THE F(1U)(-]3F(U)) VIBRATIONS IN SINGLE-CRYSTAL C-60 BELOW THE ORIENTATIONAL-ORDERING TRANSITION

The reflectance of undoped single-crystal C60 has been measured from 100 to 1600 cm-1 at high resolution above and below the orientational-ordering transition. All of the F1u modes allowed in the free molecule are observed in the solid at almost-equal-to 526, 576, 1183, and 1430 cm-1. Below the orientational-ordering transition, all the lines split into quartets, with the exception of the 576 cm-1 line, which narrows and remains unsplit. The appearance of quartets is attributed to the possibility of triplet splitting of the F1u(--> 3F(u)) modes due to the local crystal field in the low-temperature phase, as well as the perturbation of the local crystal field due to the merohedral disorder, that may lead to further line splitting.