The values of cross-section ratio Rk1 of direct k-fold ionization cross section (σ k) to direct single ionization cross section (σ 1) of Ne impacted by Cq+ (q = 1–3) ions in an energy range of 10 keV/u–500 keV/u are measured in this work. The experimental data are compared with the results from our multi-electron classical over-barrier ionization (ME-COBI) model, showing that the model can give a good estimate to the experimental data.

We measure the multiple ionization cross-section ratios 𝑅 𝑘1 of Ar impacted by C 𝑞+ (𝑞 = 1-3) ions in the energy range of 20-500 keV/u. The measured ratios 𝑅 𝑘1 increase with the projectile energy at lower energies, and reach the maximum at energies of 50-150 keV/u, then decrease for higher energies. We also use the classical over barrier ionization model to calculate the ratios 𝑅 𝑘1 , and the calculation results are in good agreement with the data, which suggest that the multiple ionization process is described by the sequential over-barrier ionization process.