Study of the electron-electron correlation via observing the two-electron cusp

In a recent experiment we found evidence for the existence of the two-electron cusp in atomic collisions, i.e. the enhanced simultaneous emission of two electrons in forward direction with velocities equal to that of the projectile. In the experiment the energies of the two electrons resulting from the mutual target and projectile ionization in 100 keV He0 + He collisions were measured. The strong correlation observed between the energies of the electrons was explained by an angular correlation of 180° in the projectile-centered reference system. For the interpretation of the experimental results we carried out a Monte Carlo simulation based on the Wannier theory for the two-electron break-up process at threshold. In the article we review the details of the simulation and present two extensions of the model: One takes into account the post-collisional interaction (PCI) between the outgoing electrons and the ionized target atom, while the other contains a correction for a two-center effect.