ENERGY-TRANSFER IN CACL2-EU-MN CRYSTALS

An investigation was made of the energy transfer between Eu2+ and Mn2+ ions in CaCl2 single crystals under photoexcitation. The studies performed at several impurity concentrations and the measurements of the excitation spectra and lifetimes of the observed europium and manganese emissions in the temperature range 11-300 K suggest that the Eu2+ --> Mn2+ energy transfer process taking place in our slightly doubly doped crystals of CaCl2:Eu:Mn occurs in small complexes of Eu-Mn which are formed in the lattice. The spectroscopic data obtained substantiate the ionic radius criterion proposed by Rubio et at to predict pairing between two impurity ions in a solid material between which energy transfer is desired. For the complex system studied in the present investigation, it was not possible to identify unambiguously the nature of the ion-ion interaction producing the energy transfer. However, calculations based on the Dexter model of energy transfer indicate that the rate of Eu --> Mn energy transfer, estimated from our experimentally determined data, can only be explained if a short-range interaction mechanism such as dipole-quadrupole interaction or exchange in nature is active. In order to obtain a better understanding of the Eu --> Mn energy transfer process occurring in two-activator CaCl2:Eu:Mn, we also obtained emission, excitation and lifetime data for one-activator CaCl2:Eu and CaCl2:Mn phosphor materials.