LUMINESCENCE SPECTRA OF URANYL IONS ADSORBED ON DISPERSE SIO2 SURFACES

In this paper, the luminescene spectra of UO22+ molecular ions adsorbed on disperse SiO2 surfaces were investigated under photoexcitation. It has been established that luminescence properties of this system are determined by the type and structure of the adsorption complexes (AC's) formed. Various AC's cause many elementary luminescence spectra (about 20) that were detected by means of selective laser excitation. It has been shown that uranyl ions are linked with the surface by water molecules. H2O molecules coordinate uranium in water-uranyl complexes and take part in the formation of adsorption bonds. One can observe the so-called nonuniform broadening of the emisson spectra, which is due to variations of the degree of water-uranyl-complex perturbaton under AC formation. It is shown that there is a correlation between the emission maxima shift and perturbation degree of uranyl ions for each AC. We observe the phenomenon of resonant energy transfer among various AC's in their excited states. The resonant energy transfer is a result of the coincidence of the perturbation energy value and the energy of symmetrical and deformation vibrations in excited states. The rate of nonradiative energy relaxation among these AC's is consistent with the rate of vibrational relaxation in each concrete AC. The effect of heat pretreatment (320-1300 K) on the luminescence properties was also investigated. The water-uranyl-complex bond energies were obtained. We demonstrate the use of uranyl ions for the investigation of adsorption properties of the disperse SiO2 surface.