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 subsystems of the two-electron H2 system to its high-order harmonic generation in eight-cycle sin2-like ultrafast intense laser pulses are calculated and analyzed based on the solution of the time-dependent Schrödinger equation for the one-dimensional two-electronic H2 system with fixed nuclei. The laser pulses have λ = 390 and 532 nm wavelengths and I = 1 ×1014, 5 ×1014, 1 ×1015 and 5 ×1015 W cm-2 intensities. It is found that at the two lower intensities, the PI-H2 subsystem dominantly produces the HHG spectra. However, at the two higher intensities, both PI-H2 and ionized subsystems contribute comparably to the HHG spectra. In the subsystem, the symmetry of the populations of (I) and (II) regions (left and right regions of 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 W cm-2 intensity and λ = 532 nm wavelength, the even harmonics are appeared near cutoff region. Interestingly, at 5 ×1015 W cm-2 intensity and λ = 390 nm wavelength, the even harmonics replaced by the odd harmonics with red shift. At λ = 390 and 532 nm wavelengths and I = 1 ×1015 intensity, the two-electron cutoffs corresponding to nonsequential double-recombination 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 W cm-2 intensity and λ = 532 nm wavelength, if we take into account nuclear dynamics, the even harmonics which are appeared near cutoff region, replaced by the odd harmonics with blue shift. © 2018 IOP Publishing Ltd.