Resonant states play a governing role in many charge-changing processes in atoms; the photoionization of atoms in gas phase and the corresponding time-reversed process of dielectronic recombinations being the two most prominent and well-known examples. This is so thanks to their strongly localized character, which leads to large transition matrix elements and to the unusually long timescale at which their dynamics unfolds, if compared to free-electron wave packets. Many resonances correspond also to highly correlated electronic states. Hence, on the one hand, high-resolution extreme ultraviolet (XUV) and soft x-ray photoelectron spectroscopy, which give access to electronic resonances, provide unique insight into the effects of electronic correlation in the energy domain; on the other hand, new laser techniques able to produce light pulses of sub-femtosecond duration give a complementary, much needed, view of electronic correlation resolved in time. Here, we discuss resonances in atoms and atomic ions in all these respects. First, we survey how resonances can be accurately localized and characterized with complex scaling, standard scattering techniques and, for heavy ions, with relativistic many-body theory. The need for high precision is particularly stringent in the latter case since resonances located just above the ionization threshold, and thus whose parameters are most affected by numerical uncertainties, are often the most important for recombination. Second, we examine how the spectral properties of resonances translate in the time domain and how the course of the ionization process can be steered with external ultra-short driving laser pulses.
Elsevier, 2012. 247-308 p.