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 {μ}s and approximately 4 {μ}s, 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 ³Σᵤ⁺ state of the STE.