The absorption spectrum of the uranyl ion in perchlorate media: Part I. Mathematical resolution of the Overlapping band structure and studies of the environmental effects

Abstract

The complicated absorption spectrum that is observed for the UO 2 ++ ion between 3300 and 5000 Å arises from one or more vibrationally perturbed electronic transitions; some fourteen transitions are observed in this wavelength interval. Each band overlaps the adjacent bands on both the high- and lowfrequency side of the band center and results in the rather ill-defined (in some areas) summed spectrum that is observed. This overlapping, coupled with the fact that the individual transitions react in different ways to changes with respect to temperature or solution parameters, complicates interpretation of the spectra and related spectral studies. With this overlapping, it was heretofore difficult, if not impossible, to obtain accurate values for the parameters (position, intensity, and half-bandwidth) of the individual absorption bands or transitions. Computer techniques that we have developed for the mathematical resolution of complex overlapping spectra have been applied to a study of the fundamental parameters of the absorption spectra of the UO 2 ++ ion and related systems. The present results show that some prior inferences concerning the character and distribution of the bands in the UO 2 ++ spectrum are in error. The parameters of the bands obtained from our recent resolution of the absorption spectrum of uranyl and measurements of the intensity distribution and parameters of the resolved fluorescence emission spectrum of uranyl in perchlorate media are interpreted with respect to some theoretical models recently proposed for the vibronic level structure. By the use of very accurate data, with the perchlorate system, we have found an additional band in the uv region of the visible, which has been found to be part of the same electronic transition as represented by the bands in the 3325–5000 Å region. The parameters of the band distribution used for the previous model will not describe the experimental spectrum. We have attempted to resolve, by computer techniques, the uranyl absorption spectrum according to the previously proposed model, but the band parameters for this model were converted by the nonlinear leastsquares computer method to the same resolved absorption bands that we have found. A close examination of the profile and structure of experimental and resolved spectra has led us to a different assignment of bands into the various levels of a proposed triplet, consisting of bands 1-3, 4-8, and 9-13, as triplet regions 1, 2, and 3, having centers at 21 329, 24 107, and 27 731 cm −1, respectively.