Analysis of the absorption and emission spectra of U3+ in CsCdBr3 single crystal

Laser selective excitation and emission spectra as well as sigma- and pi-polarized absorption spectra have been recorded for U3+ ions diluted in a CsCdBr3 host crystal. Three distinct U3+ sites were observed. For the principal site, which was attributed to a symmetric dimer center, 45 crystal-field energy levels in the 0-13250 cm(-1) energy range were assigned. These experimentally determined levels were fitted to thirteen parameters of a semiempirical Hamiltonian representing the combined atomic, one-electron crystal field as well as two-particle correlation crystal-field (CCF) operators, with an rms deviation of 41 cm(-1). Above 14000 cm(-1) strong electric-dipole allowed 5f(3)-5f(2)6d(1) transitions were observed. The lifetime of the (4)G(7/2) fluorescing level has been measured for each of the three U3+ sites at liquid-helium temperatures and are 2.5, 4.3, and 7.6 mus. The emission is strongly influenced by temperature due to strong phonon coupling of the 5f(3) states with the nearby 5f(2)6d(1) states. Visible upconversion fluorescence observed when pumping the I-4(9/2) - (F-4(7/2) + I-4(15/2)) absorption transitions of the principal site A is attributed to excited-state absorption (ESA).