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

Conduction band electrons, valence holes and subsequently self-trapped exci tons (STEs) are created in CaF2 and SrF2 by two-photon absorption of 5.9 eV photons. Transient absorption after femtosecond pulse excitation is measur ed at probe wavelengths of 3.0 eV and 4.4 eV, respectively. Transient absor ption of 3.0 eV photons yields a fast initial response in SrF2 due to absor ption by conduction band electrons, while in CaF2 we sample the F centre pa rt 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 sampl ed nanoseconds after STE creation that is attributed to a forced recombinat ion of the nearest-neighbour defect pair. Probing with 4.4 eV photons yield s much weaker absorption but confirms results for 3.0 eV photons. Additiona lly we observe stronger absorption due to the formation of long living defe cts.