According to the asymmetric molecular orbital reconstruction algorithm, which divides orbital into gerade and ungerade components and which does not depend on the unidirectional recollisional condition, we obtain the two-dimensional highest occupied molecular orbital (HOMO) of CO based on the directly calculated transition dipole moment and the harmonic spectra calculated by the Lewenstein model, respectively, which is the three-dimensional (3D) HOMO projected onto the plane perpendicular to the laser propagation direction. In order to retrieve the full orbital function, a 3D molecular orbital tomography (MOT) method is developed and is successfully applied to the reconstructions of the HOMO of CO, which simplifies the 3D imaging process of orbitals of linear molecules, and is expected to be extended to reconstruct the 3D orbitals of nonlinear molecules. In addition, the time-dependent density functional theory is employed to acquire the harmonic spectra of CO in a 800 nm and 1500 nm wavelength laser, respectively. The comparison of these two reconstruction results helps identify the multi-electron effects for asymmetric MOT, which requires further study. This work advances the development of MOT and is expected to reveal multi-electron effects in orbital imaging of complex polyatomic molecules.
Compared with nonpolar molecules, due to the inherent asymmetry, polar molecules exhibit richer and more complex electronic dynamics under the interaction with strong laser fields. In this work, high-order harmonic generation (HHG) of polar molecules CO is investigated using the three-dimensional time-dependent Hartree-Fock (3D-TDHF) theory with all electrons active. Through the high harmonic spectra and time-frequency analyses, it is found that when the laser field polarizes along the molecular axis, the ionized electrons from the two sides (C side and O side) contribute differently to the harmonic radiation. On one hand, the harmonic intensity from C side is greater than that from O side, which is caused by the ionization rate. On the other hand, for the lower-order (7th - 17th) harmonics of plateau region, only the electrons from C side participate in the HHG. However, for its higher part (18th - 36th), the electrons from both C and O sides contribute to high harmonics simultaneously. Moreover, the contribution difference from two sides is related with the alignment angle <i>θ</i> between the laser polarization and the molecular axis. It reaches maximum around <i>θ</i> and gets its minimum around <i>θ</i>=90°. There are two strong resonances around harmonic order H12.6 (19.5 eV) and H18 (27.9 eV) in the harmonic spectra when <i>θ</i>=0°. The first resonance around H12.6 reveals that part of electrons ionized from C side recombine to the vicinity of the further O nucleus. For the second resonance around H18, it originates from shape resonance. Nevertheless, the shape resonances from C and O sides are disparate. Based on the strong-field approximation theory, the ratio of photoionization cross sections from C and O sides around the shape resonance is calculated. The ratio is about 5.5 from 3D-TDHF, which is greater than the result of 3 simulated by ePloyScat, where only HOMO is considered. This discrepancy reveals that multi-electron effects enhance the asymmetry of polar molecules. This work provides deep insights into the asymmetry in HHG of polar molecules, which benefits the isolated attosecond pulse generation. It also promotes the application of higher harmonic spectra in tracking the ultrafast dynamics of electrons.
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