Excitons trapped at impurity centers in highly ionic crystals

It is shown that there are several examples of impurity ions in ionic host crystals all of whose localized excited states lie above the ionization energy of the impurity in the crystal (SrF₂:Yb²⁺, BaF₂:Eu²⁺, and others). Excitation of the localized states followed by relaxation leads to a lowest excited state in which the electron resides in the host lattice while the hole remains trapped on the impurity ion. Heretofore unexplained luminescence bands result from the radiative decay of this ``impuirty trapped exciton.''