Time-dependent Kohn-Sham approach to multiple ionization

Multiphoton processes induced by strong laser pulses acting on atoms or mol ecules can lead to the production of highly ionized states. We present a fo rmalism based on time-dependent density functional theory that allows one t o 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 metho d is derived starting from an exact definition of multiple ionization proba bilities based on the full many-body wave function. Then, some clear-cut ap proximation steps are performed leading to explicit formulas for the ion pr obabilities 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. Recen t applications to helium atoms in strong laser fields are reviewed. As an i llustration of the method, we perform numerical simulations of multiple ion ization of a Na-9(+) cluster irradiated with strong, 25-femtosecond laser p ulses. (C) 2000 Elsevier Science B.V. All rights reserved.