Abstract
Recently, the study of near- and below- threshold regime harmonics as a potential source of intense coherent vacuum-ultraviolet radiation has received considerable attention. However, the dynamical origin of these lower harmonics, particularly for the molecular systems, is less understood and largely unexplored. Here we perform the first fully ab initio and high precision 3D quantum study of the below- and near-threshold harmonic generation of molecules in an intense 800-nm near-infrared (NIR) laser field. Combining with a synchrosqueezing transform of the quantum time-frequency spectrum and an extended semiclassical analysis, we explore in-depth the roles of various quantum trajectories, including short- and long trajectories, multiphoton trajectories, resonance-enhanced trajectories, and multiple rescattering trajectories of the below- and near- threshold harmonic generation processes. Our results shed new light on the dynamical origin of the below- and near-threshold harmonic generation and various quantum trajectories for diatomic molecules for the first time.
Highlights
Mechanism for the below- and near-threshold regimes for atoms
We find the multiple returns have strong contributions to near-threshold harmonic generation, and the multi-rescattering process depends on the intensity of the laser field as observed in the previous studies of atomic systems[23,24,31]
We find that multiphoton-dominated short trajectories, long trajectories, multi-rescattering trajectories and resonant trajectories in the below- and near-threshold High-order-harmonic generation (HHG) involve only the electron scattered off the combined molecule-field potential wall followed by the absorption of photons
Summary
Mechanism for the below- and near-threshold regimes for atoms. Power et al.[14] and Yost et al.[15] suggested that the generation of the below-threshold harmonics is dominated by long trajectories, owing to the influence of the atomic potential. Theoretical investigations by Li et al uncover the subtle electron dynamics for near- and below-threshold harmonic generation in H23 and Cs24 atoms in strong laser fields. By comparing the SST time-frequency spectra and the extended semiclassical calculations, we unravel for the first time the contributions of the short trajectories, long trajectories, multi-scattering trajectories, “resonant” or bounded trajectories, and “continuum” trajectories in the below-, near- and above-threshold harmonic generation, and demonstrate the features of the spectral dynamical phase for the H+2 molecule. We find that the multiphoton-dominated processes for the long trajectories only occur when the electrons are scattered off the high part of the combined molecule-field barrier potential (HBP) followed by the absorption of many photons in the below- and near-threshold harmonic generation. These near-threshold multi-rescattering trajectories are the only type that have locked dynamical phases, where the long-trajectories are very sensitive to the laser profile
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