Experimental results on Raman-scattering spectra of ${\mathrm{C}}_{60}$ and ${\mathit{M}}_6$${\mathrm{C}}_{60}$ (M=K, Rb, and Cs) taken at room temperature are presented and discussed. The spectra for the ${\mathit{M}}_6$${\mathrm{C}}_{60}$ compounds are all quite similar, and show only weak dependence on the alkali-metal mass or radius. Several of the fivefold degenerate ${\mathit{H}}_{\mathit{g}}$ symmetry ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$, whereas ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$ cluster.

• Received 7 November 1991

DOI:https://doi.org/10.1103/PhysRevB.46.2595