Raman-scattering study of isotopically engineered crystalline C-60

A high resolution Raman scattering study of the intramolecular phonons of crystalline C-60 is presented. Many of the Raman-active vibrations are richly structured, revealing a crystal field splitting both above and below the orientational ordering transition at 260 K. Much of the vibrational fine structure is also shown to be strongly influenced by the small amount of orientational, or merohedral, disorder which persists in the low-temperature phase. The impact of isotopic disorder is examined by the comparison of Raman spectra of single crystals made from naturally abundant carbon, 99.95% C-12, and 99.7% C-13, as well as a range of intermediate C-13 concentrations. The C-12(60) and C-13(60) spectra are identical, apart from a uniform softening of the C-13(60) vibrational energies by the factor (12/13)(1/2). As the C-13 concentration is increased from approximately zero, the bulk of the vibrational modes display only a softening in energy and a broadening in width. In sharp contrast to this expected behavior is the A(g)(2)-derived band which shows both a softening in energy and additional splittings with increasing C-13 content.