Charge-state analysis of small barium-oxide clusters by x-ray absorption spectroscopy.

Small barium-oxide clusters [Formula: see text] are studied by mass spectrometry and x-ray absorption spectroscopy (XAS) to discuss stability of the clusters and oxidation state of constituent atoms. It is found that clusters with bulk composition, n  =  m, are stable, which can accommodate one or two excess oxygen atoms additionally as manifested by n  =  m  +  1 and m  +  2 species in the mass spectrum. XAS spectra of [Formula: see text] and [Formula: see text] reveal that the oxidation state of barium atoms stays at  +2 (the bulk BaO value) even after binding excess oxygen, whereas spectral features originating from oxygen exhibit composition dependence. The present finding suggests that stoichiometric small barium-oxide clusters bind less-negatively-charged oxygen atoms without change in the charge state of barium.