X-ray absorption spectroscopy of size-selected cerium oxide cluster ions, Ce 3 O 4–7 + , has been carried out by fragment-ion-yield measurement. X-ray absorption spectra measured in the Ce M 4 -edge and the O K-edge regions provided novel experimental data for chemical analysis of the constituent atoms and for discussion of geometric structures. The spectra near the Ce M 4 -edge exhibited a clear main peak and subtle substructures. Composition dependence of the spectra indicated that the oxidation state of Ce atoms evolves from + 3 to + 4 as an O atom is introduced one by one. On the other hand, the O K-edge spectra consisted of a main peak at ca. 532 eV and two additional peaks in the pre-edge region, where the intensities of the pre-edge peaks were significantly dependent on Ce:O composition. These pre-edge peaks suggested that the geometric structures of Ce 3 O 5–7 + retain the “framework” of Ce 3 O 4 + with “peripheral” O atoms bound on-top to the Ce atoms; coexistence of a structural isomer was suggested for Ce 3 O 6 + , indicating presence of a molecular oxygen with Ce 3 O 4 + −O 2 geometry.
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