Theoretical Exploration Of High-order Harmonic Generation From Hydrogen Molecular Ion In Intense Laser Field

High-order harmonic generation (HHG) is an interesting physics phenomenon arising from the dynamics of atoms or molecules exposed to intense laser field. In recent decades, as a powerful tool to obtain soft X radiation source and isolated attosecond pulses, HHG is the subject of many theoretical and experimental investigations. In recent years, harmonic generation from molecules has drawn increasing attention from researchers, since it contains abundant physical information than atom. As the simplest molecule, H2+ molecular ion is the researching focus in this field. In this article, by solving the time-dependent Schrodinger equation, we not only investigate the quantum path control on harmonic emission from H2+, but also explore the interference minimum in harmonic generation and the maximum in below-threshold harmonic generation.The main contents in this paper are:(1) The quantum path control on the harmonic emission from H2+ in a few-cycle laser pulse combined with a half-cycle pulse (HCP). The results show that the HHG cutoff is dramatically extended and the quantum paths in harmonic emission are effectively controlled with the introduction of the HCP. Additionally, by means of the time-frequency distributions and the electronic wave packet density distributions, we particularly analyze the physical mechanism of harmonic emission processes. Finally, an isolated 61 as pulse is generated by superposing proper harmonic orders. (2) Theoretical exploration of interference minimum in HHG from H2+. By adjusting the wavelength of the laser and initial condition of nucleus, we calculate the different harmonic spectra. The results show that the minimum in the low-order region is largely dependent on both the nuclear mass and the initial vibrational level, which derives from molecular structure. Furthermore, by performing the time-frequency distributions and electronic wave packet density, we demonstrate that the minimum in the high-order region is due to the electronic dynamics behavior induced by the laser field. (3) Theoretical investigation of maximum in below-threshold HHG from H2+.Based on Born-Oppenheimer approximation, the harmonic spectra under different conditions are simulated by adjusting the parameters of laser and the internuclear distance. The results interpret that the maximum in the below-threshold harmonic region is independent of the laser wavelength, while it is determined by the laser intensity and the internuclear distance. By performing both the time-frequency analysis and double-well model, we explain that the maximum is attributed to the intramolecular electronic transfer induced by laser field.