OBSERVATION AND ASSIGNMENT OF SILENT AND HIGHER-ORDER VIBRATIONS IN THE INFRARED TRANSMISSION OF C-60 CRYSTALS

We report the measurement of infrared transmission of large C-60 singl e crystals. The spectra exhibit a very rich structure with over 180 vi brational absorptions visible in the 100-4000 cm-1 range. Many silent modes are observed to have become weakly IR active. We also observe a large number of higher order combination modes. The temperature (77-30 0 K) and pressure (0-25 kbar) dependences of these modes were measured and are presented. Careful analysis of the IR spectra, in conjunction with Raman scattering data showing second-order modes and neutron sca ttering data, allows the selection of the 46 vibrational modes of C-60 . We are able to fit all of the first- and second-order data seen in t he present IR spectra and the previously published Raman data (approxi mately 300 lines total), using these 46 modes and their group-theory-a llowed second-order combinations.