S. Hubert

Crystal field analysis of U3+ : LiYF4 absorption spectrum

Abstract Absorption and emission spectra of a single crystal of U 3+ in LiYF 4 are reported at room and liquid helium temperatures. From the analysis of the polarized absorption spectra, 40 crystal-field levels have been assigned. Assuming that the D 2d symmetry is a good approximation for the S 4 point symmetry site, the electrostatic, spin-orbit, and crystal-field parameters have been obtained. A least-squares adjustment yields a mean error of 50 cm −1 with the following values (in cm −1 ) of the B k q parameters: B 2 0 = 420, B 4 0 = − 2458, B 4 4 = − 2845, B 6 0 = 93 and B 6 6 = − 3280. Using the N v parameter, it is shown that the crystal-field strength for U 3 + in LiYF 4 is a factor of 2.4 larger than for Nd 3 + in the same matrix and the U 3 + ground state splitting in LiYF 4 is about twice as large as that in CaF 2 or SrF 2 .

Reduction by γ-irradiation of tetravalent uranium in LiYF4:U crystals for laser application

Abstract Recently, there has been a considerable interest in obtaining LiYF4 activated with trivalent uranium for IR laser application. It has been shown from the absorption spectra, that using the Czochralski method, the uranium ions in LiYF4 are tetravalent. We show that γ-irradiation can reduce 100% of U4+ in U3+ in LiYF4, and the conversion efficiency measured from the absorption spectra of the samples is studied as a function of the time of exposure and the uranium concentration. Finally the U3+ ions obtained after γ-irradiation are stable over a few years and a temperature of 450°C is needed to oxidize them.

Optical spectra of U3+ and U4+ in LiYF4 crystals

Optical studies on single crystals or U:LiYF 4 grown by the Czochralski method are carried out. Absorption spectra in UV, visible and IR performed at room and low temperature are characteristic or tetravalent uranium. Under reducing conditions, single crystals or U:LiYF 4 are brown and the absorption spectra exhibit a strong large band around 500 nm typical or the r-d transitions or U 3+ (5r 3 configuration) while the bands in the IR between 2000 and 2400 nm correspond to the absorption of the 4 I 11/2 state. From the analysis of the IR emission spectra between 1900 and 3000 nm at 300 and 10 K, the level scheme or 4 I 9/2 and 4 I 11/2 has been deduced

U3+ new activator ion in various fluoride matrices for infrared laser: synthesis and optical properties

Abstract Recently, there has been considerable interest in infrared diode pumped solid state lasers. U3+ in fluoride matrices is a good candidate, however the trivalent state is not the most stable valence state. We show that γ-irradiation can reduce 100% of U4+ in matrices such as LiYF4, LiYbF4 and β-YF3, but it fails mostly in the fluoride glasses where tetravalent sites are present. A dose rate of about 500 kGy is necessary for reducing 1000 ppm of uranium in the single crystals. Then the conversion rate decreases with the increase of uranium concentration. The large crystal-field in LiYF4 and β-YF3 induces a large ground multiplet splitting, allowing a four level scheme laser at room temperature for the 4 I 11 2 → 4 I 9 2 transitions terminating at the highest energy found multiplet. These transitions occur at 2.5, 2.83 and 2.97 μm for LiYF4 and 2.29, 2.5 and 2.66 μm in β-YF3. Moreover, the emission cross sections for these transitions are at least 10 times larger than the ones for 4f—4f transitions such as the emission at 2.7 μm of Er3+ for example in LiF4.

Energy transfer between lanthanide and actinide ions in LiYF4

Abstract In this paper, we investigate the dynamics of the energy transfer process which takes place in LiYF4 codoped with a lanthanide Nd3+(4f3) and an actinide U3+(5f3). Excitation spectra and decay profiles of the Nd3+ provide evidence of resonant non radiative energy transfer from Nd3+ to U3+ ions. From the analysis of the fluorescence decays of Nd3+ for several codoped samples, the microparameter CDD, linked to the energy migration of Nd3+ and the microparameter CDA, related to the direct interaction between Nd3+ and U3+ ions have been obtained using the Yokota model. Moreover, the CDA parameter deduced from the fit is compared to the one obtained with the calculated oscillator strengths.

Electronic structure and 5f-6d energy gap of trivalent actinides in AnO813− , AnCl63− and AnF85− clusters using DV — Xα method

Abstract Relativistic self-consistent calculations using the Dirac-Slater discrete variational method (DVM) have been carried out in order to obtain the electronic structure of AnO 8 13− , AnCl 6 3− and AnF 8 5− clusters (AnU 3+ , Am 3+ , and Cm 3+ ). From the states compositions, the degree of An (5f)-ligand (np) covalency is discussed. From the absorption and excitation spectra of compounds such as ThO 2 , Cs 2 NaYCl 6 and LiYF 4 doped with Am 3+ , Cm 3+ or U 3+ , intense broad bands observed in the UV region are attributed to 5f-6d transitions. The first maximum of these bands is compared to the energy difference between the last occupied 5f orbital and the first unoccupied 6d orbital in the ground state electronic structure of the clusters.