Impact of two-electron dynamics and correlations on high-order harmonic generation in He

The interaction of a helium atom with intense short 800 nm laser pulse is studied theoretically beyond the single-active-electron approximation. For this purpose, the time-dependent Schr\"odinger equation for the two-electron wave packet driven by a linearly-polarized infrared pulse is solved by the time-dependent restricted-active-space configuration-interaction method (TD-RASCI) in the dipole velocity gauge. By systematically extending the space of active configurations, we investigate the role of the collective two-electron dynamics in the strong field ionization and high-order harmonic generation (HHG) processes. Our numerical results demonstrate that allowing both electrons in He to be dynamically active results in a considerable extension of the computed HHG spectrum.