No abstract

Picosecond flash photolysis studies of Cu(dmp)2+ and Cu(bcp)2+ have been carried out, where dmp denotes 2,9-dimethyl-1,10phenanthroline and bcp is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. In each case the excited-state absorption spectrum resembles that of the radical anion of 1,10-phenanthroline, consistent with the charge-transfer nature of the excited state. With this technique we have for the first time been able to measure the excited-state lifetime (2 ± 0.3 ns at 20 °C) of Cu(dmp)2+ in acetonitrile. The quenching kinetics of photoexcited Cu(dmp)2+ by acetonitrile, acetone, and p-dioxane in CH2C12 solution have been determined from studies of the luminescence lifetime, and the results are interpreted in terms of a mechanism in which deactivation follows the formation of a complex denoted as *Cu-Q. The fact that AH* is negative for all three quenchers suggests that *Cu-Q may represent an exciplex in which the quencher (Lewis base) serves as a fifth donor for the copper center. Exciplex emission has not been detected; however, the kinetic data indicate that the steady-state concentration of *Cu-Q is quite small. Whether the five-coordinate adduct represents a kinetic intermediate, a successor complex, or even the transition state of the reaction cannot be determined rigorously from our data, but bond formation has been shown to be an essential part of the quenching process. Finally, we consider reasons why exciplexes are not commonly observed for the excited states of transition-metal complexes.

We have investigated the structures of U(VI) oxides precipitated from room temperature aqueous solutions at low ionic strength as a function of pH. Using the uranium Lm-edge extended Xray absorption fine structure (EXAFS) and infrared (IR) spectroscopies as probes of the local structure around the uranium, a trend is observed whereby the axial oxygen bond lengths from the uranyl groups increase from 1.80 A at pH = 7 to 1.86 A at pH = 11. Shifts in the IR spectral frequencies support this assignment. A concomitant decrease in the equatorial oxygen and nearest-neighbor uranium bond lengths also occurs with increasing pH. Expansion of the linear 0=U=0 group is seen directly at the L m absorption edge where multiple scattering resonances systematically shift in energy. EXAFS curve-fitting analysis on these precipitates and a sample of synthetic schoepite indicate that the structure of the species formed at pH = 7 is similar to the structure of schoepite. At pH = 11, the precipitate structure is similar to that of a uranate.

The effects of hydrostatic pressure on the metal to ligand charge-transfer (MLCT) excited-state emission lifetimes for the copper® complexes Cu(dmp)2+ and Cu(dpp)2+ (dmp = 2,9-dimethyl-1,10-phenanthroline, dpp = 2,9-diphenyl-1,10-phenanthroline) have been determined in CH2C12 solution. In the absence of added quenchers, volumes of activation * for the nonradiative decay pathway have been determined to be -3.4 and -1.6 cm3/mol, respectively. For the previously reported quenching of [Cu(dmp)2+] * by the cosolvents CH3CN and CH3OH, * values of-6.2 and -5.4 cm3 5/mol were measured. These data are consistent with the proposal that an associative mechanism plays a role in the nonradiative deactivation of the MLCT excited state of the less sterically crowded (dmp)2 complex.

Delayed fission following the electron-capture decay of Am was studied. The "Np(a, 7n) ' Am reaction with multiple ' Np targets was used to produce "Am. The fission properties and half-life of Am were measured using a rotating-whee1 system. The half-life of ' Am was determined to be 2.32+0.08 min from measurements of the fission activity. A highly asymmetric mass-yield distribution was observed for the fission activity, and the average total kinetic energy of the fission fragments was found to be 173+5 MeV. Radiochemica1 separations confirmed the elemental assignment of the fissioning species to americium or fission from short-lived excited states in its EC daughter, plutonium. The cross section for ' Am produced by this reaction and decaying by electron capture was determined to be 5.4+1.3 pb by measuring the intensities of the daughter plutonium K x rays in radiochemically separated americium samples. The branching ratio of the 6.46-MeV a peak of" Am was found to be (3.9+1.2) X 10 in on-line measurements.The delayed-fission probability was determined to be (6.6%1.8) X 10 from the measured ratio of fissions to plutonium K x rays. The observed fissions were unambiguously assigned to an ECdelayed fission process by measuring fissions coincident with the K-capture x rays.