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 ${\mathrm{BaF}}_2,$ ${\mathrm{SrF}}_2,$ and ${\mathrm{CaF}}_2$ excited at room temperature by two-photon absorption of femtosecond laser pulses by recording time-resolved triplet-luminescence spectra. For ${\mathrm{BaF}}_2$ we detected significant spectral changes as a function of decay time but this effect was much less pronounced for ${\mathrm{SrF}}_2$ and barely to be identified in ${\mathrm{CaF}}_2.$ A careful analysis of ${\mathrm{BaF}}_2$ data revealed two principal decay times of 0.6 μs and approximately 4 μs, 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.