TIME-DEPENDENT DENSITY-FUNCTIONAL THEORY FOR STRONG-FIELD MULTIPHOTONPROCESSES - 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 t heory (TDDFT) for nonperturbative treatment of multiphoton processes o f 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 lo ng-range behavior is constructed by means of the time-dependent optimi zed effective potential (TDOEP) method and the incorporation of an exp licit 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-indepe ndent single-particle local time-dependent exchange-correlation potent ial. We also introduce a generalized pseudospectral lime-propagation m ethod, 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 ato ms in intense laser fields. We also perform a detailed study of the me chanisms 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 electro n correlation and the He+ ion are important to the generation of the o bserved higher harmonics. The present TDDFT is thus capable of providi ng a unified and self-consistent dynamical picture of the HHG processe s. [S1050-2947(98)04601-0].