Optical Spectra and Crystal-Field Levels of [Cm(H₂O)₉]³⁺ Ions with <i>C</i>_3<i>h</i> Symmetry in Isotypic Rare-Earth Triflate and Ethyl Sulfate Salts

Fluorescence emission and excitation spectra of [Cm(H2O)9]3+ ions with regular and distorted tricapped trigonal prismatic coordination geometries are reported at temperatures of 293 and 20 K. The Cm3+ impurities are incorporated into the hexagonal crystal lattices of the isotypic [M(H2O)9](CF3SO3)3 (M = La (1), Y (2)), and [Y(H2O)9](C2H5SO4)3 (3) salts, and into the low symmetry [La(H2O)9]Cl3·15-crown-5·H2O (4) salt. Small but significant structural differences in the MO9 polyhedra influence the crystal-field levels of the 8S′7/2 ground-state and the 6D′7/2 excited-state multiplets. Thus, the total 6D′7/2 splitting is smaller in 1−3 (376−393 cm−1) than in 4 (430 cm−1), which explains the marked blue shifts of the emission spectra of 1−3 to those of 4 and Cm3+(aq) at 293 K. The transitions between the ground state and the two lowest crystal-field levels of the 6D′7/2 multiplets in 1−3 give rise to narrow fluorescence lines at the emitting level at 20 K, resolving the crystal-field levels of the ground state as sharp and narrowly spaced lines. The total ground-state splittings in 1 (8.0 cm−1), 2 (6.0 cm−1), and 3 (7.5 cm−1) are about three to four times larger than those for Cm3+ in LaCl3 (2.0 cm−1), but four to six times smaller than those for Cm3+ in [Y(H2O)8]Cl3·15-crown-5 (35 cm−1). Inhomogeneous line broadening prevents resolving the ground multiplet levels in 4. Vibronic side bands associated with the 8S′7/2−6D′7/2 transition are observed in the low temperature emission and excitation spectra. The intensities of these side bands are <1% of the parent electronic transition, the stronger ones being located below ∼400 cm−1 and correspond to various CmO9 skeletal modes. The luminescence lifetimes in 1−4 are between 63 and 74 μs.