Direct ionization of helium by low and intermediate Cq+ and Oq+ (q = 1-4) ions

Cross-section ratios σDDI/σDSI of direct double ionization (DDI) to direct single ionization (DSI) of Cq+- and Oq+-He (q = 1-4) collisions in the energy range of 15-480 keV/u (0.8 a.u. ≤ vp ≤ 4.4 a.u.) are experimentally determined. For a fixed projectile charge the ratios depend strongly on the projectile energy and there is a maximum at about 100q1/2 keV/u. Comparison is made with other available experimental data. The present results are in agreement with the previously published data. On the whole, the ratios appear in two groups according to the different projectile ions. Combining the Bohr-Lindhard model and statistical model, a theoretical estimate is presented. The estimate shows the trend in good agreement with experimental data.