RAMAN-SCATTERING IN C-60 AND ALKALI-METAL-SATURATED C-60

Experimental results on Raman-scattering spectra of C60 and M6C60 (M = K, Rb, and Cs) taken at room temperature are presented and discussed. The spectra for the M6C60 compounds are all quite similar, and show only weak dependence on the alkali-metal mass or radius. Several of the fivefold degenerate H(g) symmetry C60 lines are observed to split upon doping. Furthermore, the high-frequency lines associated with primarily tangential carbon atom motions are observed to down-shift relative to their position in C60, whereas C60 modes that exhibit primarily radial character down-shift to a lesser degree, and in some cases even up-shift slightly upon doping. These observations are explained as a consequence of the interplay between (1) the charge-transfer dependence of the bond stretching and bond-bending force constants and (2) the electrostatic effects associated with the net negative charge on the C60 cluster.