Dynamic charge-changing processes in highly charged ions colliding with Ar atoms near the Bohr velocity

The average charge states of $^{20}\mathrm{Ne}^{7+}$ and $^{131}\mathrm{Xe}^{20+}$ ions penetrating through the gaseous argon targets with different areal densities are measured, and the velocities of projectile is close to the Bohr velocity regime ($v\ensuremath{\sim}1$ a.u.). The results show that, with the increase of target density, the dynamic evolution of average charge occurs in three different stages. The dependence of the average charge state on the charge-changing cross section from the nonequilibrium stage to the equilibrium stage is revealed by the theoretical predictions that consider single-electron and additional double-electron processes. It is found that the multi-electron charge-changing process needs to be properly considered in the equilibrium stage. In addition, an analysis model is proposed to predict the average charge state of the projectile in the event of significant energy loss and to quantitatively evaluate the dependence of energy loss on the charge-changing cross section.