LOW-FREQUENCY RAMAN-SCATTERING INVESTIGATION OF RBC60

We present a low-frequency Raman study of RbC60 in its orthorombic pol ymer phase. The H-g(1) mode splits in three components at the polymer- orientationally-disordered-monomer phase transition (T(p/d)similar or equal to 400 K) We use this split as a probe of the state of the sampl e to study the temperature dependence of the low-frequency (about 30 c m(-1)) peak characteristic of the polymer phase. In the whole polymer- phase temperature range, the behavior of the mode is that expected for regular anharmonicity. No significant change is observed at the metal -insulator transition temperature (T(m/i)similar or equal to 50 K). By contrast, the peak vanishes at T-p/d in agreement with recent inelast ic neutron scattering, confirming its assignment to a twist mode of th e chains.