INTRAMOLECULAR ENERGY TRANSFER AND SENSITIZED LUMINESCENCE IN ACTINIDE(3) BETA-DIKETONE CHELATES

Survey experiments were performed to determine whether uv-excited sharp-line sensitized luminescence (SLSL) could be detected and studied in some α-active actinide (An3+) β-diketones, namely, the hexafluoroacetylacetonate chelates CsAn(HFA)4·nH2O with the An3+ ions 241Am3+, 243Am3+, 244Cm3+, 249Bk3+, 249Cf3+, and 253Es3+. The relatively heavy HFA- ligand was chosen as the chelating agent so the work could be performed effectively at room temperature. Measurements were made on the pure crystals CsAn(HFA)4·H2O only in the Am3+ and Cm3+ cases, on the chelates in anhydrous ethanol solutions in all cases, and with An3+ as a dopant in the CsGd(HFA)4 crystal matrix in all cases. SLSL was detected only in the case of Cm3+; it is highly efficient in all three media, resembling Eu3+ in red SLSL color and in high quantum efficiency. The results indicate that laser emission could be demonstrated in certain Cm3+-chelate solutions. The absence of SLSL over the 4000-10,200-Å experimental range in the case of Es3+ suggests that the first excited state of this 5f10 electronic configuration is below 9800 cm-1. Measurements of self-excited luminescence of the crystalline Cs244Cm(HFA)4·H2O show that it is essentially the same as the uv-excited luminescence and that the radiolytic decomposition is linear with a total decomposition time of 6 hr. This is approximately the time required for the time-integrated molar radioheat to equal the sum of the molar bond energies and crystal binding energy. Evidently the individual ligands are excited by the α activity and the energy is transferred via the ligand singlet and/or triplet state to the Cm3+, as it is with uv excitation. Estimates are made of the radiolysis lifetime for each sample studied, assuming the radiolytic decomposition is in each case linear in time and that the decomposition rate is proportional to the specific radioheat of the isotope under consideration. These results indicate that radiolytic decomposition over the time required to do the experiments is not responsible for the absence of SLSL in the Am3+, Bk3+, Cf3+, and Es3+ cases.