S-Matrix Theory of Nonsequential Double Ionization

Laser-induced double and multiple ionization of atoms may proceed either sequentially or nonsequentially in one coherent process. For near-infrared lasers of moderate intensity (1014 to 1015 W/cm2), the physical mechanism of the coherent process is inelastic rescattering of a first-ionized electron off its parent ion. The quantum-mechanical S-matrix description of this process is reviewed. Momentum distributions of the ejected electrons and the doubly charged ion are calculated and compared with the experimental data. Their shape is found to be determined by the effective electron-electron interaction, by which the recolliding first electron ejects the up to this time bound second electron. The significance of the finalstate electron-electron interaction is assessed. The underlying classical dynamics are elucidated. Recent experiments with ultra-short phase-stabilized laser pulses are discussed. Nonsequential multiple ionization is modeled by assuming that the returning electron thermalizes with a certain number of bound electrons, and the corresponding thermalization time is estimated.