Photo-double ionisation and the dynamics of electron pairs in the continuum

The study of the emission of two electrons from an atom by absorption of a single energetic photon has become of much current interest because it provides the most detailed information on the interaction of the electrons between themselves. Its investigation in the simplest two-electron system, the He atom, in the last ten years has challenged experimentalists and theorists alike. By using selected examples from electron–electron and electron–recoil ion coincidence experiments the main achievements in the field are reviewed. It is shown that the dynamics of the electron pair is strongly constrained by its own symmetry and the Coulomb repulsion. The further aspects brought in by the experiments in diatomic molecules (H2, HD and D2) as well as in the heavier rare gases are also illustrated. Future perspectives which involve other processes which result in an electron pair in the continuum are considered.