Photophysics of the excited uranyl ion in aqueous solutions. Part 2.—Acidity effects between pH 0.5 and 4.0

Abstract

Fluorescence quantum yields and decays of excited uranyl ion in aqueous solutions have been studied over the pH range from 0.5 to 4.0. The fluorescence yield decreases between pH 0.5 and 2, is constant up to pH 2.5 and then varies again with a maximum at pH 3.5. The pH dependence of the emission decay is complex but when analysed in terms of a reversible-crossing mechanism reveals that the rate of reversible crossing has a maximum at pH 3 whereas the rate of irreversible decay has a maximum at pH 2 and a minimum at pH 3. All these effects have been interpreted in terms of several aquo, hydroxo–aquo and polynuclear uranyl cations (UO²⁺₂)^*, [UO₂(OH)⁺]^*, [UO₂(OH)₂]^* and [(UO₂)₂(OH)^–₅]^*, with a pH-dependent distribution curve and with different rates of fluorescence decay. Good agreement between the decay and the stationary-fluorescence data is found within a reversible-crossing kinetic scheme between two uranyl states that are energetically very close, a higher U^* state and a lower X^* state. Increasing uranyl concentration increases only the rate of decay of state U^*, except at pH 1 where some effect is detected in the X^* decay. The acidity effect on the autoquenching rate constant is not very pronounced. The laser intensity does not affect the rates of irreversible decay of both states, but affects the rate for the reversible transition between U^* and X^*. This effect depends on the pH.