ISOTOPE EFFECTS ON THE RAMAN-SPECTRA OF CUCL

We report on changes of the LO and TO Raman phonons of CuCl with isoto pic substitution of copper (Cu-63, Cu-65). While the LO phonon peak sh ifts according to changes of the reduced ion mass, isotope effects on the frequencies of peaks belonging to the anomalous TO phonon structur e are much larger, actually close to those expected for the Cu masses. We attribute this paradoxical result to a strong anharmonic interacti on of the TO phonon with an LA/TA combination band reflecting differen t isotope effects on optic and acoustic modes interacting via a Fermi resonance.