EFFECTS OF ISOTOPIC COMPOSITION ON THE LATTICE-DYNAMICS OF CUCL

We have investigated the changes in the Raman spectra of CuCl resultin g from isotope substitution. Our samples were made from the stable iso topes Cu-63, Cu-65, Cl-35 and Cl-37 as well as the natural elements. W hile the longitudinal-optic (LO) mode shifts according to the expected reduced mass behavior of a Gamma-point phonon, the transverse-optic ( TO) vibration shows pronounced deviations. Due to the strong anharmoni city of CuCl, the TO mode interacts with a two-phonon combination band of acoustic phonons and one obtains coupled excitations which have be en attributed to a Fermi resonance. We describe this process using a s hell model for the lattice dynamics of CuCl and only one cubic coeffic ient for the anharmonic interaction. The Fermi resonance results in an anomalous line shape of the TO phonon, which reflects properties of t he zone center mode as well as of the two-phonon combination band. Iso tope substitution allows us to discern the two contributions and to ch eck the validity of the model. The off-center model proposed earlier t o explain the TO anomaly cannot account for our findings.