Multielectron excitations at theLedges in rare-earth ionic aqueous solutions

An extensive investigation of the extended x-ray-absorption fine structure (EXAFS) at the L edges of the rare-earth atoms of aqueous ionic solutions of ${\mathrm{La}}^{3+}$, ${\mathrm{Ce}}^{3+}$, ${\mathrm{Ce}}^{4+}$, ${\mathrm{Nd}}^{3+}$, ${\mathrm{Pr}}^{3+}$, ${\mathrm{Eu}}^{3+}$, ${\mathrm{Dy}}^{3+}$, and ${\mathrm{Tm}}^{3+}$ at concentrations of 50, 100, and 200 mM, is presented. The presence of anomalous peaks, appearing in the range from 5 to 7 A${\mathrm{̊}}^{\mathrm{-}1}$ and superimposed to the main single-frequency oscillatory signal, has been explained as due to double-electron transitions 2p4d→5${\mathit{d}}^2$ in the case of ${\mathit{L}}_3$ and ${\mathit{L}}_2$ edges, and 2s4d→6p5d for the ${\mathit{L}}_1$ spectra. The energy of the double-excitation absorption edge increases as the atomic number of the element of the rare-earth series is increased and is in fair agreement with previous theoretical bound-to-bound calculations. The intensity of the anomalous feature decreases for increasing Z numbers, as expected from theory, but the intensity values, calculated from comparison with the main single-electron absorption line, are lower than those calculated by other authors and the double-excitation peak disappears in the ${\mathrm{Tm}}^{3+}$ spectrum. A structural analysis of the EXAFS spectra was also carried out with the twofold aim of characterizing rare-earth water solutions and quantifying the errors introduced in the structural parameters by the mixing of single- and double-electron phenomena. The results show the rare-earth ions are always surrounded by 12 water molecules and the rare-earth–O distance decreases with Z number, varying from 2.56 Å for ${\mathrm{La}}^{3+}$ down to 2.32 Å for ${\mathrm{Tm}}^{3+}$. The presence of the anomalous peaks introduces small errors in the bond-length’s determination, the effect being proportional to the magnitude of the double-excitation peak.