Overbarrier model with electron backcapture

We present an extension of the classical overbarrier model [F. Sattin, Phys. Rev. A 62, 042711 (2000)] to include the effect of electron backcapture. Backcapture is the process by which an electron that has already been captured by the projectile ion is recaptured by the target atom. Backcapture reduces the electron capture cross section at low impact velocities when the projectile ionization energy is less than that of the target. This creates a cross-section peak. We alter the location of this peak to correspond to that predicted by an adiabatic criterion by using a free parameter of the model. These extensions bring the overbarrier model more in line with experimental data, especially at low impact velocity.

Figure 1

Charge-exchange cross sections between H and He calculated with the overbarrier model with backcapture using various values of the free parameter $f_T$.

Figure 2

The velocity $v_m$ at which the cross-section peaks as a function of the energy defect $\Delta E$ calculated from the overbarrier model with backcapture. The straight Massey-Hasted line is the prediction of $v_m$ from the adiabatic criterion.

Figure 3

Charge-exchange cross sections. The black diamonds are experimental data from Refs. [12, 13]. The solid blue lines are overbarrier model with backcapture calculations that use the value of $f_T$ indicated on the bottom of each figure, which is calculated from the procedure using the adiabatic criterion. The green dashed lines are overbarrier model with backcapture calculations using $f_T=1$, and the short-dashed red lines are overbarrier model without backcapture calculations using $f_T=1$.