Multiple Ionisation of Atoms in Intense Laser Fields

Abstract

Interaction of strong laser beams with atoms results in multiphoton excitation and ionisation (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 approximation¹. 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 ionisation in MPI, recently discovered for both alkaline-earth atoms² and rare gases³ is an obvious manifestation of the multi-electron nature of these atoms. This collision-free process has now been reported in wide ranges of intensity (10¹¹ W.cm⁻²-10¹⁷ W.cm⁻²) and of wavelengths (193 nm-10000 nm). It can require the absorption of hundreds of photons (triple ionisation of Xe at 10000 nm⁴) and, at very high intensity and short wavelength, it can go along with inner-shell excitations (Auger lines have been reported in photoelectron spectra⁵) Such processses 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.