We report the measurement of infrared transmission of large C60 single crystals. The spectra exhibit a very rich structure with over 180 vibrational absorptions visible in the 100 -4000 cm 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 -300 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 scattering data, allows the selection of the 46 vibrational modes of C60. We are able to fit all of the Srst-and second-order data seen in the present IR spectra and the previously published Raman data ( 300 lines total), using these 46 modes and their group-theory-allowed second-order combinations.
Optical measurements were performed on thin films of RbxC60, identified by X-ray diffraction as mostly x = 1 material. The samples were subjected to various heat treatments, including quenching and slow cooling from 400K. The dramatic increase in the transmission of the quenched samples, and the relaxation towards the transmission observed in slow cooled samples provides direct evidence for the existence of a metastable insulating phase. Slow cooling results in a phase transition between two electrically conducting phases.
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