Contribution of the pre-ionized H2 and the ionized H2+ subsystems to the HHG Spectra of H2 in intense laser fields
Abstract
Contributions of the pre-ionized H2 (PI-H2) and ionized H2+ subsystems of the two-electron H2 system to its high-order harmonic generation in 8-cycle 2-like ultrafast intense laser pulses are calculated and analyzed based on the solution of the time-dependent Schr\"odinger equation (TDSE) for the one-dimensional two-electronic H2 system with fixed nuclei. The laser pulses have λ\ =\ 390\ \&\ 532 nm wavelengths and I=1× 1014, 5× 1014, 1× 1015 \&\ 5× 1015 Wcm-2 intensities. It is found that at the two lower intensities, the PI-H2 subsystem dominantly produces the HHG spectra. While, at the two higher intensities, both PI-H2 and ionized H2+ subsystems contribute comparably to the HHG spectra. In the H2+ subsystem, the symmetry of the populations of H2+(I) and H2+(II) regions (left and right regions of H2+ subsystem) is broken by increasing the laser intensity. Complex patterns and even harmonics also appear at these two higher intensities. For instance, at 1× 1015 Wcm-2 intensity and λ = 532 nm, the even harmonics are appeared near cut-off region. Interestingly, at 5× 1015 Wcm-2 intensity and λ = 390 nm, the even harmonics replaced by the odd harmonics with red shift. At λ = 390 \&\ 532 nm and I=1× 1015 intensity, the two-electron cutoffs corresponding to nonsequential double-recombination (NSDR) with maximum return kinetic energy of 4.70Up are detected. The HHG spectra of the whole H2 system obtained with and without nuclear dynamics treated classically are approximately similar. However, at 1× 1015 Wcm-2 intensity and λ = 532 nm, if we take into account nuclear dynamics, the even harmonics which are appeared near cutoff region, replaced by the odd harmonics with blue shift.