Uranyl formate monohydrate spectroscopy

We report the fluorescence excitation and emission spectra of normal and isotopically labeled uranyl formate monohydrate powder, UO2(HCOO)2.H2O, at 4.20K. Dual fluorescence from the two lowest excited states, I and II, occurs in this compound giving rise to a temperature-dependent lifetime. The lowest vibronic levels are assigned on the basis of the observed oxygen and uranium isotope shifts. The unusual activity and relative frequencies of the symmetric and asymmetric O-U-O stretches in the excited state and can be successfully predicted with the addition of a bond-bond interaction term in addition to the usual valence bond potential. The kinetics of nonresonant energy transfer between isotopic /sup x/OU/sup y/O+2 antitraps and the U16O2+2 lattice were measured over the range from 1.7 to 4.20K. The observed rates are in agreement with a quadrupole-quadrupole coupling mechanism accompanied by one and two phonon processes which compensate for the energy defect. The asymmetric lineshapes, broader linewidths and tenfold reduction of absorption intensity for the U16O2+2 lattice compared to the isolated isotopically doped uranyl ions are attributed to the collective nature of the excited state for the majority species