Uranyl ions, vibrational excitation

The ground state of the uranyl ion has a closed-shell electron configuration. There is a characteristic absorption 25 000 cm (400 nm) which frequently gives uranyl compounds a yellow colour (though other colours like orange and red are not infrequent). This absorption band often exhibits fine structure due to progressions in symmetric 0=U=0 vibrations in the excited state, sometimes very well resolved, sometimes not (Figures 12.1 and 12.2). 

State lifetimes and modes of energy transfer within the structure. Examples of this are photoluminescence of ZnS nanoparticles studied by Wu et al. (1994), and Mn doped ZnS nanoparticles by Bhargava et al. (1994). In the latter study, the doped nanocrystals were found to have higher quantum efficiency for fluorescence emission than bulk material, and a substantially smaller excited state lifetime. In the case of environmental nanoparticles of iron and manganese oxides, photoluminescence due to any activator dopant would be quenched by magnetic coupling and lattice vibrations. This reduces the utility of photoluminescence studies to excited state lifetimes due to particle-dopant coupling of various types. The fluorescence of uranyl ion sorbed onto iron oxides has been studied in this way, but not as a function of particle size. 

Irradiation also affects the course of more conventional separation processes. Visible and ultraviolet light have been found to affect plutonium solvent extraction by photochemical reduction of the plutonium (12). Although the results vary somewhat with the conditions, generally plutonium(VI) can be reduced to pluto-nium(IV), and plutonium(IV) to plutonium(III). The reduction appears to take place more readily if the uranyl ion is also present, possibly as a result of photochemical reduction of the uranyl ion and subsequent reduction of plutonium by uranium(IV). Light has also been found to break up the unextractable plutonium polymer that forms in solvent extraction systems (7b,c). The effect of vibrational excitation resulting from infrared laser irradiation has been studied for a number of heterogeneous processes, including solvent extraction (13). 

Since we had proposed a similar experiment with irradiation in the ultraviolet-visible absorption band of the uranyl ion (15), we tried to reproduce these results, but without success (16). Collisions in the liquid phase occur so rapidly (about 10l2 s x) that vibrational excitation of the uranyl ions would be dissipated long before any significant fraction of excited uranyl ions could reach the interface and therefore change the distribution between the two phases. Rapid loss of vibrational excitation in relation to other processes is a generic problem for infrared laser effects in any system of condensed phases. However, differences between experimental setups may account for the differences in results,... 

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