  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.
