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 La³⁺, Ce³⁺, Ce⁴⁺, Nd³⁺, Pr³⁺, Eu³⁺, Dy³⁺, and Tm³⁺ at concentrations of 50, 100, and 200 mM, is presented. The presence of anomalous peaks, appearing in the range from 5 to 7 A^-1 and superimposed to the main single-frequency oscillatory signal, has been explained as due to double-electron transitions 2p4d{→}5d² in the case of L₃ and L₂ edges, and 2s4d{→}6p5d for the L₁ 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 Tm³⁺ 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 {} for La³⁺ down to 2.32 {} for Tm³⁺. 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.