Time-dependent Kohn-Sham approach to multiple ionization

Multiphoton processes induced by strong laser pulses acting on atoms or molecules can lead to the production of highly ionized states. We present a formalism based on time-dependent density functional theory that allows one to calculate in a simple manner approximations for the probabilities to find an N-electron system in one of its N + 1 possible charge states. The method is derived starting from an exact definition of multiple ionization probabilities based on the full many-body wave function. Then, some clear-cut approximation steps are performed leading to explicit formulas for the ion probabilities in terms of time-dependent Kohn-Sham single-particle orbitals. The formalism is shown to become rigorous in certain limiting cases, and we also discuss how a numerical implementation is realized in practice. Recent applications to helium atoms in strong laser fields are reviewed. As an illustration of the method, we perform numerical simulations of multiple ionization of a Na-9(+) cluster irradiated with strong, 25-femtosecond laser pulses. (C) 2000 Elsevier Science B.V. All rights reserved.