In situinfrared transmission study of Rb- and K-doped fullerenes

We have measured the four IR-active ${\mathrm{C}}_{60}$ molecular vibrations in ${\mathit{M}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ (M= K,Rb) as a function of doping x. We observe discontinuous changes in the vibrational spectra showing four distinct phases (presumably x=0,3,4, and 6). The 1427-${\mathrm{cm}}^{\mathrm{-}1}$ and 576-${\mathrm{cm}}^{\mathrm{-}1}$ modes show the largest changes shifting downward in frequency in four steps as the doping increases. Several very weak modes are visible in the x=6 phase and are possibly Raman modes becoming weakly optically active. We present quantitative fits of data and calculate the electron-phonon coupling of the 1427-${\mathrm{cm}}^{\mathrm{-}1}$ IR mode.