PHOTOCONDUCTIVITY AND FLUORESCENCE PROPERTIES OF DIVALENT YTTERBIUM IONS IN FLUORIDE-CRYSTALS

Photoconductivity measurements were crucial in establishing that the "anomalous" emission usually observed in CaF2 and SrF2 doped with Yb2+ and in BaF2:Eu2+ is due to an impurity trapped exciton which is the lowest excited state of these systems. Especially in the SrF2:Yb2+ photoconductivity spectrum, two bands corresponding to the first absorption band, are well resolved. At low temperature these two bands are assigned to the autoionization of the T1u and exciton levels. As the temperature increases the signal due to thermal ionization of the exciton level becomes stronger and induces a drastic change of the relative intensity of the two photoconductivity bands. The temperature dependence of the intensity of the photoconductivity signal allowed us to estimate a bonding energy of the exciton in SrF2:Yb2+ of about 1500 cm-1. A three level scheme taking into account the non-radiative transitions is used to explain the temperature dependences of the lifetime and fluorescence intensity of these systems.