Hydration of Cm³⁺ in Aqueous Solution from 20 to 200 °C. A Time-Resolved Laser Fluorescence Spectroscopy Study

Abstract

Time-resolved laser fluorescence spectroscopy (TRLFS) is used to study the hydration of the Cm3+ ion in acidified (0.1 M perchloric acid) H2O and D2O from 20 to 200 degrees C. Strong temperature dependency is found for several of the spectroscopic quantities associated with the 6D'(7/2) --> 8S'(7/2) photoemission spectra, with similar relative changes in both solvents. The emission band shifts to lower energy with increasing temperature, which is attributed to an equilibrium between hydrated Cm3+ ions with different numbers of water molecules in the first coordination sphere, namely [Cm(H2O)9]3+ and [Cm(H2O)8]3+. Comparison with crystalline reference compounds and the analysis of hot bands corroborates the assignment of these species. The molar fraction of the octahydrated species increases from approximately 10% at room temperature to approximately 40% at 200 degrees C, indicating an entropy driven reaction. The corresponding thermodynamic parameters are obtained as Delta H degrees = + 13.1 +/- 0.4 kJ mol(-1), Delta S degrees = + 25.4 +/- 1.2 J mol(-1) K(-1), and Delta G298 = + 5.5 +/- 0.6 kJ mol(-1). Both the emission intensity and lifetime decrease with increasing temperature. The temperature dependency of the nonradiative decay rate of the emitting 6D'(7/2) level follows an Arrhenius equation with the activation energy 26.5 kJ mol(-1) (2250 cm(-1)) in both H2O and D2O, which is somewhat lower than the energy gap between 6D'(7/2) and 6P'(5/2) exited state levels.