In situ infrared transmission study of Rb- and K-doped fullerenes

We have measured the four IR-active C60 molecular vibrations in MxC60 (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-cm-1 and 576-cm-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-cm-1 IR mode.