Influence of multi-electron excitation on EXAFS spectroscopy of trivalent rare-earth ions and elucidation of change in hydration number through the series

We have made a detailed study of the extended X-ray absorption fine spectra (EXAFS) at the K edge of aqueous Y ion and at L₃ edges of aqueous lanthanide ions and thereby elucidated the systematic changes in their hydration structures. Anomalous peaks arising from double-electron excitation (2p, 4d → 5d, 5d) appear in the EXAFS signals of La³⁺-Tb³⁺ between 5–7 Å⁻¹. We established a removal process of double-electron excitation from EXAFS spectra. Using that process, we confirmed that the intensity and energy position of the extracted double-electron excitation are comparable to previously reported data. The presence of double-electron excitation engenders a smaller error than the errors estimated in the fitting process. Consequently, double-electron excitation does not seriously affect the determination of the structures of REE³⁺ aquo ions in the first coordination sphere. Subsequent EXAFS analyses of hydrated REE³⁺ ions suggest that the hydration numbers, the interatomic distances, and the Debye-Waller factors decrease from 9.7, 2.55 Å, and 9.0 × 10⁻³ Ų for La³⁺ to 7.9, 2.31 Å, and 5.7 × 10⁻³ Ų for Lu³⁺. These parameters change as a sigmoid curve with increasing atomic number. The hydration structures of REE³⁺ ions are inferred to change from the nonahydrated structure for La³⁺-Nd³⁺ to the octahydrated structure for Tb³⁺-Lu³⁺ through intermediate structures for Sm³⁺, Eu³⁺, and Gd³⁺. In addition, the hydration state of Y³⁺ closely resembles that of Ho³⁺ because the two have almost identical ionic radii.