  Optical measurements on doped C$_{60}$ films provide evidence for a
transition from a high temperature conducting phase to a low temperature
insulating phase in quenched Rb$_1$C$_{60}$ and K$_1$C$_{60}$ compounds, while
Cs$_1$C$_{60}$ did not exhibit this behavior. For slow cooled samples our study
confirms earlier results indicating phase separation of K$_1$C$_{60}$ to
K$_3$C$_{60}$ and C$_{60}$, and the formation of a new Rb$_1$C$_{60}$ phase at
low temperatures. Upon slow cooling Cs$_1$C$_{60}$ behaves similar to
Rb$_1$C$_{60}$.
