Multi-Plateau Molecular High Harmonic Generation From Multi-Functional Group Recombination

Permanent dipole moments induced High-order Harmonic Generation (HHG) signals offer a potential approach to producing elliptically or even circularly polarized X-ray attosecond sources. Previous studies on this topic have mainly focused on diatomic molecules such as CO and HeH. Based on this scheme, significant HHG signals in the direction perpendicular to the molecular axis can be observed in both the high-energy and low-energy regions. However, we found that the high-order harmonics induced by the permanent dipole moments of polyatomic complex molecules involve more intricate physical processes. Using time-dependent density functional theory (TDDFT), we simulated the dynamics of HHG from NH₂COOH and NH₂COSH interacting with linearly polarized lasers. We found that the harmonic signals in the direction perpendicular to the N-C bond significantly enhanced in the high-energy photon region. Our analysis indicates that this is due to the complex molecular configuration of NH₂COOH and NH₂COSH: the NH₂ group has C_2v symmetry, whereas the COOH and COSH groups lack this symmetry. This structural characteristic results in the permanent dipole moments being felt only when the electron returns to the COSH or COOH groups, and not to the NH₂ group. Additionally, our results reveal a multi-plateau structure in the HHG signal along the laser polarization direction, a phenomenon that arises from multi-electron and multi-orbital effects in the interaction between the complex molecule and the strong laser field.