Femtosecond laser pulse excitation of electrons and excitons in CaF2 and SrF2

Conduction band electrons, valence holes and subsequently self-trapped excitons (STEs) are created in CaF2 and SrF2 by two-photon absorption of 5.9 eV photons. Transient absorption after femtosecond pulse excitation is measured at probe wavelengths of 3.0 eV and 4.4 eV, respectively. Transient absorption of 3.0 eV photons yields a fast initial response in SrF2 due to absorption by conduction band electrons, while in CaF2 we sample the F centre part of the STE at 3.0 eV and determine its formation time constant to be 690 fs. Excitation at 3.0 eV also yields a reduction in STE luminescence sampled nanoseconds after STE creation that is attributed to a forced recombination of the nearest-neighbour defect pair. Probing with 4.4 eV photons yields much weaker absorption but confirms results for 3.0 eV photons. Additionally we observe stronger absorption due to the formation of long living defects.