Absorption spectrum of the uranyl ion in perchlorate media III. Resolution of the ultraviolet band structure; some conclusions concerning the excited state of UO 22+

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The ultraviolet spectrum of the aqueous uranyl ion is a complicated network of absorption bands which overlap so seriously that a very careful investigation is essential for the observation of the band structure. This feature has heretofore prevented the observation of the band structure, and as a result has handicapped the development of a suitable theoretical model for the uranyl ion. This report explains how the ultraviolet spectrum of the aqueous uranyl ionin a perchlorate medium has been resolved into twelve Gaussian shaped absorption bands; this is a continuation of the band analysis for the visible spectrum. The visible and uv absorption spectra (1795–5000 Å; 55 710–20 000 cm −1) of the uranyl ion, which under the experimental conditions is complexed only with water ligands, is composed of 24 detectable bands. A study of the relationships between the bands has led to a breakdown of the 24 bands into 7 major absorption bands (or band groups) which have an average spacing of 6137 cm −1. A linear relationship between the band energies and the logarithms of the oscillator strengths of the 7 major bands is noted. The first two major bands are in agreement with two triplet excited states aspreviously proposed from theoretical considerations. The lower-lying, vibrationally perturbed triplet at 24 107 cm −1 is composed of 12 bands. The second triplet at 31 367 cm −1 which is also vibrationally perturbed, and has not been previously observed experimentally, is composed of 7 bands. It is proposed that the 5 remaining very broad bands which show no vibrational structure represent electron transitions from the ground state to five excited states having regular energy increments. The fluorescence emission spectrum is shown to result from two excited levels to five vibrational levels in the ground state, but only 4.66 percent of the emitting energy is from the higher of the two excited levels.