Fluorescence, photoconductivity and polarization properties of divalent rare earth ions in fluoride crystals

The “anomalous” emission usually observed in CaF 2 and SrF 2 doped with Yb 2+ and BaF 2:Eu 2+ is due to an impurity trapped exciton which is the lowest excited state of these systems. The excited “d” electron of RE 2+ ion is delocalized in the “s” orbitals of next neighbor metal ions from where it can either autoionize or recombine with the RE 3+ ion giving rise to red shifted luminescence. When the electron is excited into the lower energy bands it relaxes into the excitation state and the emission is polarized but when it is excited into the higher energy bands it autoionizes without any prior relaxation and the fluorescence is unpolarized. The profile of the fluorescence decay curves shows that the exciton level is populated via relaxation and tunnel effects. The temperature dependence of the photoconductivity spectra enhances the difference between autoionization and thermal ionization.