Up conversion of U4+ in ThBr4

Abstract

Up to now, most of the experimental examples involving the anti-Stokes fluorescence called up-conversion were dealing with rare earth ions [1]. For the first time we have found that tetravalent uranium as a doping ion in single crystal as well as in microcrystalline powder form of thorium tetrahalides acts as an up-converter. In the case of ThBr4 : U4+, infrared excitation with λ > 0.82 µ at room temperature gives an emission band in the red between 0.67 and 0.72 µm |2|. The power law of the red emission with respect to the infrared excitation is found to be quadratic ; this is the evidence that excitation is obtained by a two-photon process. Furthermore, we found that the 0.69 µm emission follows a linear law with the U4+ concentration for a concentration range from 0.006 % up to 0.05 % in weight. From this result, we can conclude that the up-conversion process takes place in one uranium ion. For higher concentration a quenching effect is observed. We found that the efficiency was optimized using microcrystalline powder with grain size of about 0.2-0.3 µm and a concentration of 0.05 %. In this case the efficiency is multiplied by a factor of 20 in comparison with the same sample single crystal. This is believed to be due to photon trapping as demonstrated by life time variations with sample shapes. The red emission from 3P0 is obtained by the absorption of photons at 0.95 µm 3H4 → 3H6 and 1.17 µm 3H4 → 3F6 as shown by the double beam [1] excitation spectra of figure 1. The level diagram (fig. 2) of U4+ shows that the 3P0 fluorescence can be obtained by different ways such as summation of two photons (0.95 µm plus 1.17 µm or 2 × 0.95 µm or 2 × 1.17 µm) with phonon assistance, followed by a multiphonon relaxation.