Multiple ionization of atoms in intense laser fields

Interaction of strong laser beams with atoms results in multiphoton excitation and ionization (MPI), i.e. bound-bound and bound-free transitions involving the simultaneous absorption of several photons. Such processes have been quite well described within the framework of time-dependent perturbation theory and single-electron approximation. The validity of the latter is expected to be very good for alkali atoms, but is clearly questionable for alkaline-earth atoms, in which the two outermost electrons can be strongly correlated so that both are expected to participate to the interaction. Of course, it is even more questionable for rare gases, in spite of several MPI experimental results which do not contradict the prediction of the one-electron approximation. Multiple ionization in MPI, recently discovered for both alkaline-earth atoms and rare gases is an obvious manifestation of the multi-electron nature of these atoms. Such processes are obviously far from being completely understood and the theoretical approaches are still unpolished. The aim of this paper is to briefly review the most significant experiments and the available theoretical methods in this field. Experiments can be classified into two main groups: those performed at moderate intensity (up to 1013 W.cm-2) with tunable lasers and electron spectroscopy on alkaline-earths (Sec. I) and those performed at different fixed laser wavelengths and very high intensity chiefly on rare gases (Sec. II). Theories range from statistical models (when large numbers of photons are absorbed) to many-body, multiphoton perturbative approaches (two-photon double-ionization of Helium), tunneling, and more speculative mechanisms (Sec. III). 32 references, 3 figures