X-ray absorption spectroscopy of small copper-oxide cluster ions for analyses of Cu oxidation state and Ar complexation: CuOAr⁺ and Cu₂O₂ ⁺

Abstract Small copper-oxide cluster cations, namely, CuOAr+ and Cu2O2 +, are studied by mass spectrometry and X-ray absorption spectroscopy (XAS) to investigate the oxidation state of copper atoms. The XAS in Cu L3-region revealed that the oxidation number of Cu is almost +2.1 and +2.3 for CuOAr+ and Cu2O2 +, respectively, which are consistent with the natural charges obtained by quantum chemical calculation. It was also revealed that the binding energy of Ar to CuO+ is unexpectedly high to form CuOAr+ dominantly as a result of a significant amount of charge transfer to the Ar atom.