Spectroscopy
The colour of uranyl compounds is due to LMCT charge transfer transitions at ca. 420 nm, on the blue edge of the visible spectrum. The exact location of the absorption band and NEXAFS bands depends on the nature of the equatorial ligands. Compounds containing the uranyl ion are usually yellow, though some compounds are red, orange or green.
Uranyl compounds also exhibit luminescence. The first study of the green luminescence of uranium glass, by Brewster in 1849, began extensive studies of the spectroscopy of the uranyl ion. Detailed understanding of this spectrum was obtained 130 years later. It is now well-established that the uranyl luminescence is more specifically a phosphorescence, as it is due to a transition from the lowest triplet excited state to the singlet ground state. The luminescence from K2UO2(SO4)2 was involved in the discovery of radioactivity.
The uranyl ion has characteristic ν U-O stretching vibrations at ca. 880 cm−1 (Raman spectrum) and 950 cm−1 (infrared spectrum). These frequencies depend somewhat on which ligands are present in the equatorial plane. Correlations are available between the stretching frequency and U-O bond length. It has also been observed that the stretching frequency correlates with the position of the equatorial ligands in the spectrochemical series.
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