  We have measured the four IR active $C_{60}$ molecular vibrations in
$M_{x}C_{60}$ $(M = K, Rb)$ 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 $1427cm^{-1}$ and $576cm^{-1}$ modes
show the largest changes shifting downward in frequency in four steps as the
doping increases. Several new very weak modes are visible in the $x=6$ phase
and are possibly Raman modes becoming weakly optically active. We present
quantitative fits of the data and calculate the electron-phonon coupling of the
$1427cm^{-1}$ IR mode.
