Effect ofC13isotopic substitution on the Raman spectrum ofC60

The Raman spectra of ${\mathrm{C}}_{60}$ crystals with different ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ concentrations are compared in order to determine the contribution of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ isotopic substitution to activating Raman-forbidden vibrational modes of the ${\mathrm{C}}_{60}$ molecule. Due to the natural abundance of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, several authors have suggested that such isotopic substitution is responsible for the unexpectedly large number of modes in the measured Raman and infrared (IR) spectra of solid ${\mathrm{C}}_{60}$. In agreement with a recent IR study, it is found that ${}_{}{}^{13}{}_{}{}^{}\mathrm{induced}$ symmetry breaking does not contribute substantially to the number and strengths of the modes observed in the Raman spectra. Alternate symmetry-breaking mechanisms are suggested to explain the observed weak modes.