THE ROLE OF COVALENCY AND BRIDGING LIGANDS IN 2-ION COOPERATIVE OPTICAL-TRANSITIONS IN LANTHANIDE ION-COUPLED SYSTEMS

The influence of covalency and bridging ligands on cooperative optical effects in dimers with closely coupled lanthanide ions in insulating crystals and binuclear complexes is theoretically treated. It is shown that there is a specific interaction between Ln3+ ions (which may be called covalent coupling) in tight ion pairs originating from virtual electron hoppings between lanthanide ions via common bridging ligands. This interaction can be regarded as a generalization of the superexch ange interaction mechanism. A general microscopic model and the corres ponding formalism for the covalent coupling mechanism are developed. A general expression to estimate the intensity of electric dipole two-i on cooperative optical transitions is derived. Both the nature of brid ging ligands and the local geometry of the Ln3+(A)-ligand Ln3+(B) dime r are found to affect strongly the intensities of cooperative transiti ons. Our model predicts rapid increase in the contribution of this mec hanism to the intensities of two-ion transitions when going from small ''ionic'' ligands (F- and O2-) to large ''covalent'' ligands (Br-, I- ). Possible manifestations of the covalent coupling mechanism in fast energy transfer processes and unusually strong up-conversion fluoresce nce observed in CsCdBr3:Ln3+ systems and as well as in electronic spec tra of the U2Cl10, binuclear compound are discussed.