Double ionization of helium in collision with 20–500-keV/amu C^q+ and O^q+ (q=1–3) ions

Target ionization and projectile charge changing were investigated for 20–500 keV/u C^q+, O^q++He (q=1–3) collisions. Double- to single-ionization ratios R₂₁ of helium associated with no projectile charge change (direct ionization), single-electron capture, and single-electron loss were measured. The cross-section ratio R₂₁ depends strongly on the collision velocity v, the projectile charge state q, and the outgoing reaction channel. Meanwhile, a model extended from our previous work [ J. X. Shao, X. M. Chen and B. W. Ding Phys. Rev. A 75 012701 (2007)] is presented to interpret the above-mentioned dependences. Good agreement is found between the model and the experimental data.