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 L, 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 La(3+)-Tb(3+) between 5-7 angstrom(-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 REE(3+) aquo ions in the first coordination sphere. Subsequent EXAFS analyses of hydrated REE(3+) ions suggest that the hydration numbers, the interatomic distances, and the Debye-Waller factors decrease from 9.7, 2.55 angstrom. and 9.0 x 10(-3) angstrom(2) for La 31 to 7.9, 2.31 angstrom, and 5.7 x 10(-3) angstrom(2) for Lu(3+). These parameters change as a sigmoid curve with increasing atomic number. The hydration structures of REE(3+) ions are inferred to change from the nonahydrated structure for La(3+)-Nd(3+) to the octahydrated structure for Tb(3+)-LU(3+) through intermediate structures for Sm(3+), Eu(3+), and Gd(3+). In addition, the hydration state of Y(3+) closely resembles that of Ho(3+) because the two have almost identical ionic radii.