[en] The fraction of metastable-state Ar₂⁺(3p₄₁D) ions in Ar₂⁺ beam has been determined by an optical attenuation method (OAM) combined with the conventional beam attenuation method. The present OAM is based on observation of spatial decay of specified emission line intensities arising from charge-changed ions, along the beam axis in a target gas cell. The validity of the OAM is discussed in detail. The cross sections for one-electron capture by the ground-state Ar₂⁺(₃P) ions, σ²¹, and by the metastable-state Ar₂⁺(₁D) ions, σ²¹ *, from Na have been measured independently by the OAM. Both the cross sections are of the order of 10^-₁₄ cm₂ and σ²¹ * is about 1.3 times as large as σ²¹ at the collision energy of 1.5 keV. (author)