  We demonstrate a direct measurement of the energy levels of impurity-trapped
excitons in CaF$_2$:Yb$^{2+}$. The radically different radiative decay rates of
the lowest exciton state and higher excited states enable the generation of a
transient photoluminescence enhancement measured via a two-step excitation
process. We observe sharp transitions arising from changes of state of
localized electrons, broad bands associated with changes of state of
delocalized electrons, and broad bands arising from trap liberation.
