Generalized floquet theoretical formulation of time-dependent density functional theory for many-electron systems in multicolor laser fields

We generalize the Floquet formulation of the time-dependent density functional theory (TDDFT) [Telnov and Chu, Chem. Phys. Lett. 264, 466 (1997)] to the case of many-electron systems in multicolor or polychromatic time-dependent fields. It is shown that the time-dependent Kohn-Sham equations can be transformed into an equivalent time-independent infinite-dimensional Floquet Hamiltonian eigenvalue problem. For the case of bound-free transitions, we introduce the notion of complex density and present a non-Hermitian many-mode Floquet formulation of TDDFT. A procedure is presented for the determination of the complex quasi-energies and the calculation of the total and partial ionization rates from individual electron orbitals. The theory is illustrated by a case study of multiphoton ionization of He atoms in the presence of an intense 248 nm laser field and its third harmonic. Novel intensity- and phase-dependent behavior of multiphoton processes in two-color fields is reported. (C) 1998 John Wiley & Sons, Inc.