Highlights and Challenges in Strong-Field Atomic and Molecular Processes

Abstract

The interaction of short and intense laser pulses with atoms has revealed a variety of interesting effects (see Chap. 24). Among them, accurate measurements of the multiple ionization yield of atoms have been shown to be orders of magnitude higher than predicted by the single active electron approximation [Fittingho. 1992, Walker 1994]. The “knee” structure in the ion yield curve is interpreted as a nonsequential process, where the energy absorbed from the field is shared by the two electrons and considered as the most distinctive manifestation of electron correlations. Theoretical models based on simplified two-electron interaction [Watson 1994] or perturbative S-matrix approaches [Becker 1996] have approximately reproduced the experimental data. However, the importance of the time-dependent electronelectron correlation was emphasized recently through the numerical solution of the time-dependent Schrödinger equation of a one-dimensional twoelectron model atom [Lappas 1998]. In principle, a direct comparison with the experimental data requires the solution of the three-dimensional timedependent Schrödinger equation for the atom, but these calculations are numerically too demanding and results have been obtained only for a limited range of parameters in the case of helium [Parker 1998, Parker 2001]. Several approaches using time-dependent extended Hartree-Fock wave functions have been proposed in this framework [Pindzola 1995, Dahlen 2001]. However, TDDFT, which in principle allows one to incorporate correlation effects due to electron-electron interaction, appears to be the most valuable tool to study ionization of atoms or molecules in strong laser pulses. But, for the time being, it was shown that these calculations fail to reproduce the correct ionization dynamics in the low frequency regime, where the experiment was performed [Petersilka 1999].