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The photodouble ionization of water at about 32 eV excess energy has been investigated both experimentally and theoretically. In an energy and angular resolved photoelectron–photoelectron coincidence experiment, the two photoelectrons in unequal energy sharing (25 and 7 eV) condition, have been detected in a plane perpendicular to the propagation direction of the linearly polarized radiation. The measured angular distributions have been compared with, molecular orientation averaged, triple differential cross sections calculated with a recently developed theoretical model (Randazzo et al 2020 Phys. Rev. A 101 033407). The model uses separable products of orbitals as initial electronic state of the water molecule taken as a two-electron target and describes accurately the correlated two electron continuum. The combination of calculated cross sections corresponding to different dication states capture most of the measured features in terms of the evolution of both the shape and the intensity as a function of the faster electron direction which is set at 0°, 30° and 60° with respect to the polarization vector ε ⃗ of the incident radiation. A detailed analysis in terms of the different dication states as well as of the partial wave contributions to the electron pair wave function sheds light on the origin of such features.

The double photoionization of Mg has been studied experimentally and theoretically in a kinematic where the two photoelectrons equally share the excess energy. The observation of a symmetrized gerade amplitude, which strongly deviates from the Gaussian ansatz, is explained by a two-electron interference predicted theoretically, but never before observed experimentally. Similar to the Cooper minima in the single photoionization cross section, the effect finds its origin in the radial extent and oscillation of the target wave function.

New experimental techniques have been applied to the study of single-photon double ionization of atoms and molecules. A coincidence technique is described in which the spectra of nearly zero energy photoelectrons are recorded in two identical photoelectron energy analysers. The results are discussed in terms of the available theoretical calculations. New techniques for the measurement of double differential cross sections (DDCS) and triple differential cross sections (TDCS) are discussed. New results using these techniques are presented, in which DDCS and TDCS are measured as continuous functions of photon energy.

The Coulomb explosion of the hydrogen molecule, after absorption of a 76 eV photon, has been studied by momentum imaging the two electrons and the two protons. Absolute fully differential cross sections (FDCS) are compared with Time Dependent Close Coupling (TDCC) calculations and the first-order helium-like model in the coplanar geometry for equal electron energy sharing. While the helium-like model gives a consistent agreement in shape and magnitude with experimental data, the comparison with the TDCC calculations highlights the limit of this model when the molecular axis' orientation is along the polarization direction. New evidence of changes in the FDCS with internuclear separation is reported for the coplanar geometry.

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.