Triple Coincidence Experiment to Explore the Two-Electron Continuum States of the Projectile Resulting from Mutual Ionization in 100-keV He⁰+He Collisions

The existence of the two-electron cusp in atomic collisions, i.e., the enhanced emission of two electrons in the forward direction with velocities equal to that of the projectile, has been investigated experimentally. Using a time-of-flight technique, the energies of the two electrons resulting from the simultaneous target and projectile ionization in 100-keV He⁰+He collisions have been measured by detecting triple coincidence between the electrons and the outgoing He⁺ ion. The coincidence yield clearly shows a peak as a function of the electron energies at the expected cusp position. Furthermore, a strong correlation was found between the energies of the two electrons, which is traced back to an angular correlation of 180° in the projectile-centered reference system.