We have measured the pump-probe spectrum for CaF2:Sm2+ over a wide (360–1020 nm) spectral range. We observed gain throughout most of the 730-nm emission band, although there is some evidence of excited-state absorption (ESA) in this region. A much stronger ESA was observed near 500 nm. The temperature dependence of the ESA signal clearly indicates that the absorption is associated with divalent samarium. Because of the width and strength of this transition, we assign the ESA to a 4f55d → conduction band transition. We calculated the oscillator strength for this ESA band to be 0.10 and rationalized its magnitude based on an intensity-borrowing mechanism that involves the transition from the valence band to the conduction band.
The spectral dependence of the photoionization energy of Eu2+:CaF2, Ce3+:YAG, and Sm2+:CaF2 systems have been measured and thresholds experimentally determined and compared with theoretical values calculated from electrostatic models. It is shown that the excited state absorption transitions or the persistent hole burning observed by other authors occur above the threshold energy of photoionization of the impurities and that the states of the crystal which form the bottom of the conduction band may play an important role in the strong probability of these processes. A review of thresholds now known is also given.
A crystal of Sr
F2:
Sm2+ was operated as a pulsed optical maser at temperatures close to 4.2\ifmmode^\circ\else\textdegree\fi{}K. The 6969 \AA{} output corresponds to the wavelength of an exceedingly sharp line ($\ensuremath{\Delta}\ensuremath{\nu}<0.6$ ${\mathrm{cm}}^{\ensuremath{-}1}$) in which almost all of the low-temperature fluorescence is concentrated. In contrast to what prevails in Ca
F2:
Sm+2 masers, the output is characterized by strong relaxation oscillations. This may be explained by the large difference in the fluorescent lifetimes in the two systems. While in Ca
F2:
Sm2+ the value of $\ensuremath{\tau}$ is approximately 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ sec, in Sr
F2:
Sm2+ it has a value of 15 msec. The reason for the large difference in $\ensuremath{\tau}$ values, in turn, depends upon the relative proximity of the long-wavelength edge of the red band to the position of the metastable
5D0 level from which fluorescence occurs. Absorption measurements taken at helium temperatures on Sr
F2:
Sm2+ show that the lowest vibrational level of the lowest lying $4f\ensuremath{-}5d$ band lies \ensuremath{\sim}600 ${\mathrm{cm}}^{\ensuremath{-}1}$ above the position of the
5D0 level while in Ca
F2:
Sm2+ there is near coincidence of the corresponding levels and consequently strong configurational mixing. Crystals of Sr
Cl2:
Sm2+ were also studied in this regard and display an expected close similarity in spectral characteristics to Sr
F2:
Sm2+. The vibrational structure on the $4f\ensuremath{-}5d$ bands is much more pronounced, however, than in Ca
F2:
Sm2+ and Sr
F2:
Sm2+, indicating weaker coupling to the lattice in the excited
5d state. A tentative interpretation for the splitting of the red band seen in the absorption spectra of
Sm2+ ions is given.