X-ray absorption spectroscopy of Ce₂O₃ ⁺ and Ce₂O₅ ⁺ near Ce M-edge

X-ray absorption spectroscopy (XAS) of size-selected free cerium-oxide cluster ions has been carried out by employing an ion trap. X-ray absorption spectra of Ce₂O₃⁺ and Ce₂O₅⁺ were obtained in the energy range of the Ce M-edge. The spectra consisted of two prominent peaks corresponding to Ce M₅^- and M₄^- white line peaks. The photon energies of the peaks showed that the charge state of Ce in Ce₂O₃⁺ was comparable with a formal valence number. On the other hand, the estimated charge state of Ce in Ce₂O₅⁺ was much smaller than a formal valence number, which was consistently understood by assuming that Ce₂O₅⁺ has a Ce₂O₃-O₂ configuration with a molecular O2 adsorbed. It was thus demonstrated that an ion trap facilitates XAS measurement for determining charge states of atoms in a free cluster.