Classical Electron Capture in Collisions of Ions with H, He Atoms at Low and Intermediate Velocities

The Bohr-Lindhard (B-L) model is used to describe the classical electron-capture process. The impact-parameter dependence of the capture probability is derived by considering the impact-parameter dependence of the collision time between ion and atom. This model limits the impact parameter to be less than the capture radius. In the framework of the B-L model, although the contribution from all impact parameters may be studied through the spatial distribution function of electrons, the multiple numerical integral has to be carried out. In this work, it is proposed that the impact-parameter dependence of the electron-capture probability can be given by a simple exponential decay function based on the (B-L) model. Electron-capture cross sections for A^q+(q=2~6)-H collisions, and double-electron-capture cross sections for A^q+(q=3~6)-He collisions are calculated at low and intermediate velocities. The calculated results are in good agreement with the existing experimental data. The energy and charge-state dependences of the electron-capture process are well described. This work can also be used to calculate the cross sections of electron capture from He and H targets by other ions with different charge states.