Multi-orbital Effects In High Harmonic Generation Of Molecules

High-order harmonic generation?HHG?is an important technique for obtaining table-top coherent extreme ultraviolet light source and synthesizing attosecond pulses.The investigation on HHG is contributing to understand and modulate the interaction process and realize the self-probe of the internal information of the target.Especially,compared to atoms,molecules have complex and abundant quantum state information.The orbital structure effects and dynamics in the process of HHG are one of the important frontier issues in current research.The multi-orbital,multi-electron and multi-center effects of molecules have important effects on the efficiency and distribution of HHG.Accurate modulation of HHG is beneficial to the high efficient output of short-wavelength light sources and the realization of attosecond pulse shaping.At the same time,the contribution of multi-orbital and multi-electron effects,as well as the dynamic Coulomb effects is more important for molecular orbital tomography based on HHG.In this dissertation,we study the HHG from simple diatomic molecules to triatomic molecules and extended to polyatomic molecules.We measured the angular dependence of HHG from aligned molecules and the dependence of harmonic yield on laser parameters?such as polarization,laser intensity,etc.?.The characteristics of HHG by molecular orbitals with different structures are revealed,and the multi-electron and multi-orbital effects in the process of HHG from molecules in intense laser field are understood.We obtained the following main results:Multi-orbital effects of HHG from aligned N₂ molecules.It is found that the angular distribution of HHG in the plateau and the cut-off regions show distinctive behaviors under the linearly polarized laser.The analysis shows that the highest occupied molecular orbital?HOMO?,as well as the lower one?HOMO-1?,contribute to the HHG of N2 molecules.By measuring the ellipticity dependence of harmonic yield as a function of harmonic orders under different alignment conditions,it is found that the ellipticity dependence of the harmonic yield at the perpendicular alignment is weaker than that of the parallel one,and the difference is more obvious with the increase of the harmonic order.It is also observed that the ellipticity dependence of 31 st harmonic yield of N2 molecules was suppressed at the parallel alignment.The results show that the influence of multi-orbital and the destructive interference of N2 molecules on HHG.Quantum interference of multi-orbital effects in HHG from aligned triatomic molecules CO₂ and N₂ O.By measuring the position of the minimum of harmonic spectrum of aligned CO2 and N2 O molecules with different intensities and ellipticity of the driving laser,it is found that the position of the minimum shifts to lower-order harmonics when decreasing the intensity or increasing the ellipticity of the driving laser.The analysis shows that the minimum originates from the dynamic interference of different channels.We measured the angular distribution of different harmonic orders at different ellipticity,the results show that the contribution of different molecular orbitals to HHG is identified.We extended the harmonic spectrum to the near-threshold region and found that the electron-electron Coulomb interaction has an important effect on the harmonic spectrum.Multi-orbital effects of HHG from aligned C₂H₂ molecules.A comparative study of the ellipticity dependence of HHG was made under different aligned conditions,and the influence of HOMO orbital on HHG is analyzed.By measuring the angular distribution of HHG from aligned C2H2 molecules at different ellipticities,we found that the harmonic yield increases monotonically as the angle between the molecular axis and the laser field is increasing.For a larger laser ellipticity,less difference is observed between the harmonic yield at the molecular axis aligned perpendicular and parallel to the laser field.The analysis shows that the contribution of HOMO and HOMO-1 orbitals on HHG of C2H2 is the main factor,which leads to the angular dependence of harmonic yield.