The influence of charge-transfer and rydberg states on the luminescence properties of lanthanides and actinides

Abstract

The influence of charge-transfer and rydberg (f n−1 d) states on the luminescence of lanthanides and actinides (more specifically hexavalent uranium) is demonstrated. Optical transitions between the ground state and these excited states, and their dependence on electron configuration and crystallographic surroundings of the complex involved, are discussed first. The influence of charge-transfer and f n−1 d states on emission spectra is dealt with. It is shown that the characteristics of emission spectra are often very sensitive to the energetic position of these states. Even more drastic is their influence on the temperature quenching of these emissions. Especially the case of the Eu3+ ion in oxysulfides is reviewed extensively. Direct feeding of the 5D levels of Eu3+ by charge-transfer states occurs, but the reverse process is also possible. The spectral position of transitons to charge-transfer and f n−1 d states can influence energy transfer probabilities. Metal ion-metal ion charge transfer states are also of great importance in this field. Finally some luminescence properties of hexavalent uranium are discussed. It is shown that often this emission is due to an octahedral UO 6 6− group and not to the well-known uranyl (UO 2 2+ ) group. Charge-transfer states involving 5/ and possibly 6d levels determine the dependence of the emission characteristics on the host lattice.