Appearance and disappearance of the second Born effects in the (e,3e) reaction on He

The relative contribution of the first and second Born processes leading to the double ionization of He by electron impact at 500 eV was investigated. In the first Born processes, the projectile interacts with the target only once and ejection of the two target electrons is possible solely due to electron correlations in the target before and after collision. In contrast, in the second Born process, ejection of the two target electrons happens sequentially as a result of two subsequent knock-outs of the projectile on the target. In general, both the first and second Born processes contribute to double ionization of He at an incident energy of 500 eV. However, the second Born contribution is insignificant for ionizing collisions with a large momentum transfer.