The Coulomb Four-Body Problem: Electron Impact Double Ionization of Helium in the Threshold Regime

Double ionization of helium by electron impact represents one of the most fundamental four-body problems. While various experiments exist for fast collisions the threshold region where theories predict a strongly correlated and fully symmetric emission with 120$^{\circ}$ relative angles of the three electrons is unexplored concerning differential measurements. We present kinematical complete experiments for impact energies 27 eV and 5 eV above the threshold. At the lower energy clear signatures for the threshold region being reached and the symmetric electron emission are observed.