Isotope effect on the Raman spectrum of the pentagonal-pinch mode inC60

We present a detailed experimental and theoretical study of the $A_g$(2) Raman spectrum of pure and isotopically mixed C${}_{60}$ molecules. The spectrum shows a striking correlation with the mass spectrum of the molecule, consistent with a simple nondegenerate perturbation analysis. Comparisons with nonperturbative calculations using several first-principles and phenomenological dynamical models show that the study of this phenomenon can provide a stringent test of vibrational models for C${}_{60}.$