Optical properties of Cm(III) in crystals and solutions and their application to Cm(III) speciation

Cm(III), atomic number 96, has the electronic configuration [Rn core, 5f7] and is the actinide analog of the lanthanide ion Gd3+ [Xe core, 4f7]. The ground term has only ∼78% 8S7/2 parentage due to the large spin-orbit interaction as compared to the almost 100% 8S7/2 parentage for Gd3+. The electrostatic interaction between equivalent electrons in the f7 shell results in a large gap between the octet state (8S7/2) of the ground term and the sextet states (6D7/2, 6P7/2, 6P5/2, …) of the excited multiplets. For Gd3+ this splitting results in a gap greater than 30,000 cm−1. The greater spatial extent of the 5f electron shell results in a smaller electrostatic interaction between equivalent electrons in the 5f shell than in the 4f shell. Thus for Cm(III), this splitting is on the order of 16,800 cm−1. The ground term multiplet splitting is small (∼2-50 cm−1) because the largest component of the ground multiplet has zero angular momentum. The large gap between the first excited multiplet and the ground term results in strong fluorescence. The lifetime of this fluorescence is sensitive to the number and nature of the ligands in the first coordination shell in solution and solid complexes as is the energy of the transition. From measurements of this lifetime the number of H2O molecules in this coordination shell can be determined. In this paper the theories necessary to analyze the energy level spectra of the Cm(III) in crystals and Cm(III) solution intensities are briefly discussed. Applications of Cm(III) fluorescence measurements for the determination of solution and solid species are reviewed.