Time-resolved spectroscopy of self-trapped excitons in fluorite crystals

We report time-resolved measurements of optical emission and absorption in crystals of ${\mathrm{CaF}}_2$, ${\mathrm{SrF}}_2$, and ${\mathrm{BaF}}_2$ following excitation by pulsed ionizing radiation. Prominent transient absorption bands in the visible and ultraviolet are observed, and their decay is resolved into several time components which correspond to those of the emission. The absorption is shown to originate in the same excited electronic states as the intrinsic recombination luminescence. These states have been identified previously as self-trapped excitons, largely on the basis of polarization measurements and optically detected EPR in the excited states. These results, augmented by kinetic data from the present experiments and by new polarization data, suggest a fairly detailed picture of the various localized electronic states involved in the process of electron-hole recombination in fluorites. The ionic relaxation associated with the self-trapped exciton is evidently more complex than in, e.g., the alkali halides; in the fluorites the lowest triplet states of the self-trapped exciton have been found to resemble a metastable nearest-neighbor fluoride vacancy-interstitial pair, i.e., an $F$-$H$-center pair, with probable point symmetry $C_{1h}$.