In the interaction of intense laser fields with atoms, even subtle details of single ionization and high-harmonic generation can be well described in the so-called single-active-electron approximation where just one electron reacts to the laser field. Therefore, the surprisingly high efficiency of laser-induced nonsequential multiple ionization (where two or more electrons are removed by the field in one coherent process) as opposed to sequential ionization (where they are dislodged one at a time) has raised great interest. Indeed, this premium for cooperation, that is, the ratio of nonsequential over sequential ionization, has been found to be as high as six orders of magnitude in the case of double ionization of helium at near-infrared wavelengths. Ever since these observations were made, the question of the physical mechanism behind this cooperation has been intensely debated, but no final conclusions have been reached.

Up to the past year, the available experimental information was limited to total yields of the various ionic charge states. Then, two experimental groups applying the COLTRIMS (cold-target recoil-ion-momentum spectroscopy) technique were able to measure momenta of the particles involved, first the vector-momentum distribution of the doubly charged ion (which equals the total momentum of the two electrons, in so far as the momentum transferred by the laser photons can be ignored) and later the momentum of one electron in coincidence with the former, thus obtaining a complete kinematical characterization of the process. Currently, it seems these data have lent some support to the “recollision mechanism”. This intuitively appealing scenario assumes that in a first step the most lightly bound electron is set free by tunneling ionization. When the laser field drives it back to a recollision with the ion, this electron can, via inelastic scattering, detach the second electron (or more). This mechanism is the cause of the observed hot electrons in single ionization (through elastic scattering) as well as of the plateau in high-harmonic generation (through recombination). Some of the data, however, do seem to imply that this is not yet the whole story.

The present focus issue aims at presenting the experimental state of the art and at reflecting the various theoretical approaches to this problem whose accurate numerical solution for helium is now coming within reach. It consists of 13 invited papers, three experimental, including the two groups mentioned above, and ten theoretical papers. The latter have been selected to represent the possible routes: solution of the time-dependent two-electron Schrödinger equation in one or in three dimensions, density-functional methods, and many-electron S-matrix calculations that attempt to identify and evaluate the dominant Feynman diagrams. We hope that these papers will allow the reader a synopsis of the status of this rapidly advancing field.

Wilhelm Becker, Max-Born-Institute, Berlin

Mikhail V. Fedorov, General Physics Institute, Moscow