Time-dependent density-functional theory for strong-field multiphoton processes: Application to the study of the role of dynamical electron correlation in multiple high-order harmonic generation

We present a self-interaction-free time-dependent density-functional theory (TDDFT) for nonperturbative treatment of multiphoton processes of many-electron atomic systems in intense laser fields. The theory is based on the extension of the time-dependent Kohn-Sham formalism. The time-dependent exchange-correlation potential with proper short-and long-range behavior is constructed by means of the time-dependent optimized effective potential (TDOEP) method and the incorporation of an explicit self-interaction correction (SIC) term. The resulting TDOEP-SIC equations are structurally similar to the time-dependent Wartree-Fock equations, but include the many-body effects through an orbital-independent single-particle local time-dependent exchange-correlation potential. We also introduce a generalized pseudospectral lime-propagation method, allowing optimal spatial grid discretization, for accurate and efficient numerical solution of the TDOEP-SIC equations. The theory is applied to the study of the role of dynamical electron correlation on the multiple high-order harmonic generation (HHG) processes of He atoms in intense laser fields. We also perform a detailed study of the mechanisms responsible for the production of the higher harmonics in He atoms observed in a recent experiment that cannot be explained by the single-active-electron model. We found that both the dynamical electron correlation and the He+ ion are important to the generation of the observed higher harmonics. The present TDDFT is thus capable of providing a unified and self-consistent dynamical picture of the HHG processes. [S1050-2947(98)04601-0].