X-ray absorption spectroscopy of Ce sub(2)O sub(3)

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 &${{\rm{Ce}}}_{2}{{{\rm{O}}}_{3}}+}$; and &${{\rm{Ce}}}_{2}{{{\rm{O}}}_{5}}+}$; were obtained in the energy range of the Ce M-edge. The spectra consisted of two prominent peaks corresponding to Ce M sub(5)- and M sub(4)-'white line' peaks. The photon energies of the peaks showed that the charge state of Ce in &${{\rm{Ce}}}_{2}{{{\rm{O}}}_{3}}+}$; was comparable with a formal valence number. On the other hand, the estimated charge state of Ce in &${{\rm{Ce}}}_{2}{{{\rm{O}}}_{5}}+}$; was much smaller than a formal valence number, which was consistently understood by assuming that &${{\rm{Ce}}}_{2}{{{\rm{O}}}_{5}}+}$; has a Ce sub(2)O sub(3)-O sub(2) configuration with a molecular O sub(2) adsorbed. It was thus demonstrated that an ion trap facilitates XAS measurement for determining charge states of atoms in a free cluster.