Time-dependent luminescence of self-trapped excitons in alkaline-earth fluorides excited by femtosecond laser pulses

We studied the decay of self-trapped excitons (STE's) in BaF2, SrF2, and CaF2 excited at room temperature by two-photon absorption of femtosecond laser pulses by recording time-resolved triplet-luminescence spectra. For BaF2 we detected significant spectral changes as a function of decay time but this effect was much less pronounced for SrF2 and barely to be identified in CaF2. A careful analysis of BaF2 data revealed two principal decay times of 0.6 mus and approximately 4 mus, respectively, where the latter is actually a range of decay times correlating with photon energy within the luminescence band. We attribute this spectral characteristic to a superposition of contributions from several relaxed STE configurations while the two principal decay times are believed to result from the zero-field splitting of the (3)Sigma (+)(u) state of the STE.