Super-Intense Laser-Atom Physics

The study of atomic systems exposed to super-intense laser fields de­ fines an important area in atomic, molecular and optical physics. Although the concept of super-intense field has no absolute meaning, it is now usual to call an electromagnetic field super-intense when it exceeds the atomic binding field. In the case of the simplest atomic system, hydrogen in its 16 2 ground state, this occurs above an intensity of 3. 5 x 10 Wattfcm which is the atomic unit of intensity. Presently at the laboratory scale and in ex­ tremely short and tightly focussed laser pulses, the electric field strength 16 18 2 reaches peak values which are of the order of 10 - 10 Wattfcm in the infrared frequency regime, the prospect being that such peak intensities may be reached within a few years in a regime of much higher frequencies (XUV or even X). The interaction of such electromagnetic fields with an atomic system has a highly non-linear character which has led to the observation of to­ tally unexpected phenomena. There are three fundamental processes which have marked the beginning of an intensive research in the field of super­ intense laser-atom physics (SILAP). These processes which only involve one atomic electron are (i) the so-called above-threshold ionisation i. e.