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

Small barium-oxide clusters Ba _m O ⁺ _n 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 Ba ₂ O ⁺ ₂ and Ba ₂ O ⁺ ₃ 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 lessnegatively-charged oxygen atoms without change in the charge state of barium.