High-order harmonic generation by H in super-intense xuv ultrashort laser pulses

Abstract

Dynamics of the two-dimensional H under 30- and 40-cycle xuv (45 and 90 nm) laser pulses is investigated by numerical solution of the time-dependent Schrödinger equation beyond the dipole and Born–Oppenheimer approximations for two orientations (θ = 0° and θ = 90° referenced to the molecular axis) of the laser polarization. Dynamics of the electron and nuclei are treated in quantum and classical approaches, respectively. The ground, first and second excited states of H are considered as initial states for different laser field intensities in the range of (4 × 1016 – 4 × 1019 W cm−2). For parallel orientation (θ = 0°), a negligibly small difference is seen for high-order harmonic generation (HOHG) spectra obtained with the dipole and non-dipole Hamiltonians for all three electronic states. While, for the perpendicular orientation (θ = 90°), non-dipole forces attenuate the HOHG spectra for the initial ground and the second excited states. For the initial first excited state and orientation θ = 90°, the motion of nuclei considerably increases the HOHG yield within the dipole approximation, but decreases it partially beyond dipole approximation. It is shown that contribution of the motion of nuclei to the HOHG yield depends on the extent of the increase in the separation between nuclei.