Screening and antiscreening by projectile electrons in high-velocity atomic collisions

The scattering amplitude for a projectile of nuclear charge Z₁ carrying N electrons colliding with an atomic target with charge Z₂, evaluated in the first Born approximation using hydrogenic wave functions, is compared with recent experimental results. In the present approximation where the minimum momentum transfer tₘᵢₙ is considered to be approximately independent of the final state of the projectile, the differential cross sections separate into a product of one term that depends only on the target times the square Z₁²(t) of an effective projectile charge. Here an analytic expression for Z₁²(t) is given for 2, 1, and 0 electrons. Some total ionization cross-section ratios are also given.