Luminescence spectra of uranium(VI) – why we observe a spectral shift in cryospectra


Luminescence spectra of uranium(VI) – why we observe a spectral shift in cryospectra

Geipel, G.; Stumpf, T.

It is well known, that uranyl carbonates emits at room temperature only phosphorescence with very short lifetimes {Ca2UO2(CO3)3 ~ 50 ns}[1, 2]. The lifetime of UO2(CO3)34+ in aqueous solution at room temperature was determined to be 9.2 ± 0.05 ns. Freezing these solutions results normally in a strong increase of the luminescence lifetime as well as in a strong increase in the measured intensity. This effect is usually explained by the dynamic quench effect of the carbonate ion as well as the dynamic quench effect of the water molecules in the solvation shell. In addition all spectra of the uranyl carbonates show a hypsochromic shift.
The emission of the uranyl ion is assigned to a triplet state. It is also known that the non-complexed uranyl ion has two phosphorescence emitting levels (20500 and 21270 cm-1)[3,4]. The first one is assigned to the direct transition to the lowest vibronic level in the ground state, the second is assigned to the transition of a exited vibronic level to the ground state. The emission peaks at lower wavenumbers correspond to vibronic levels in the ground state and are usually assigned to the transition from the lowest triplet state[5].

We observe for the uranyltricarbonate at room temperature an emission at around 21420 cm-1 and at 20610 cm-1 which are the two emitting levels in this system. From the higher level also the vibronic levels in the ground state were populated. Under cryogenic conditions we observe that the emission from the higher level disappears. This induces a slight blue shift of the spectrum of about 65 cm-1.

Keywords: Uranium; luminescence; spectral shift; cryogenic

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