Negative ions – Fragile quantum system

The extra electron in a negative ion does not experience the Coulomb force from the nucleus at large distances. Instead, core polarization induced by the extra electron stabilizes the ion. The correlated motion of the electrons require theoretical models that go beyond the independent particle approximation. Hence, experimental investigations of the structure and dynamics of negative ions can lead to an increased understanding of many-electron effects. In this colloquium the general properties of negative ions will be discussed, and various experimental techniques for studies of negative ions will be presented. Experiments at the negative collinear ion beam facility GUNILLA in Sweden, using infra-red, visible and UV lasers will be presented. This has yielded information about bound as well as continuum structures of negative ions. The presentation will include the first results from a novel collinear photoelectron spectrometer. Femtosecond spectroscopy has been performed using a velocity map imaging (VMI) spectrometer in an experiment in which a wave-package of the ground state of the carbon atom is observed. It will also be shown how the 2-dimensional VMI technique can be used to produce a complete 3-dimensional image using a tomographic technique. Further, the first experiment using the newly commissioned cryogenic electrostatic double storage ring DESIREE, in which the life time of an excited state in S- has been measured, will be presented. Finally, future investigations of negative ions of radioactive isotopes at ISOLDE at CERN will be discussed.