FLUORESCENCE, PHOTOCONDUCTIVITY AND POLARIZATION PROPERTIES OF DIVALENT RARE-EARTH IONS IN FLUORIDE-CRYSTALS

The "anomalous" emission usually observed in CaF2 and SrF2 doped with Yb2+ and BaF2:Eu2+ is due to an impurity trapped exciton which is the lowest excited state of these systems. The excited "d" electron of RE2+ ion is delocalized in the "s" orbitals of next neighbor metal ions from where it can either autoionize or recombine with the RE3+ 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. © 1990.