TEMPERATURE-DEPENDENCE OF THE LATTICE-DYNAMICS OF RB3C60

The phonon density of states of Rb3C60 has been measured above and below the superconducting phase transition temperature of 29 K and in the energy transfer range 1 < DELTA-E < 300 meV at an energy resolution DELTA-E/E = 2%. Marked changes in the molecular vibrational spectrum of C60 are observed upon intercalation with rubidium. In particular the H(g) modes at 54 meV and the T1u mode at 66 meV are strongly quenched and other modes are broadened and shifted. Distinct changes in the H(g), T1u modes near 180 meV and the H(g) modes near 135 meV are observed on passing through the superconducting transition temperature region. These changes strongly suggest the involvement of molecular motions in the superconductivity of these materials.