The Experimental Study Of High-order Harmonic Generation From Aligned Molecules

The non-sequential double ionization?high-order harmonic generation?HHG??coulomb explosion and so on was the nonlinear production of an intense optical field by light-matter interactions.High-order harmonic generation in strong laser fields with short wavelength,high coherence and ultrashort pulse width has high frequency spectrum characteristics,it has become one of the methods to produce a soft X ray source or attosecond pulse light source.Molecule has the more complex structure and spatial degrees of freedom,the process of high-order harmonic generation contains more abundant physical information.High-order harmonic generation is directly related to the optical field parameters and the molecular structure,and the distribution of the harmonic may reflect the contribution of molecular orbitals.This has promoted the study of high-order harmonic generation from molecular and expected to be more widely used,such as the acquisition of molecular structure information,probing the ultrafast dynamics in molecules and molecular orbital tomography.This thesis is devoted to the theortical study of femtosecond laser pulse alignment molecules and the experimental study of HHG from moleculars.In this thesis,we first introduce the theortical model of molecular alignment generated by femtosecond laser pulses.By solving the time-dependent Schrodinger equation,we analyze the process of rotational excitation and non adiabatic moleculars alignment with femtosecond laser.And by simulating molecular alignment,we study the effect under different field intensity and different initial rotational temperature,the results provide a theoretical basis for the experimental study of the high-order harmonic generation from aligned molecules.We introduce the experimental setup and its principle of high-order harmonic generation,including the alignment detection system,the femtosecond laser system,and the flat field grating spectrometer and its calibration are described in detail.Study on the characteristics of high-order harmonic generation spectrum from aligned molecules by using the high-order harmonic spectrometer system,the results indicate that the variation of the harmonic yield with the delay time is in good agreement with the theoretical simulation.We also study the modulation inversion phenomenon of HHG from aligned CO₂ molecules,the two-center interference effect can be controlled by changing the intensity of the driving laser and the distribution of the alignment angle.In addition,we study the influence of the highest occupied orbital on the HHG from aligned molecules.Since the highest occupied orbital?HOMO?of N₂ is ags orbital,and the highest occupied orbital?HOMO?of the O₂ is agp orbital.Thus the alignment dependence of the HHG yield again is a direct consequence of the symmetry property of the molecular orbitals.The experimental results indicate that when the N₂ axis is parallel to the polarization direction laser,the intensity of high-order harmonic generation is maximal,and the intensity of the high-order harmonic generation begins to weaken as the molecular axis deviates from the laser polarization.For the O₂ molecule,when the molecular axis is parallel or perpendicular to the laser polarization direction,the intensity of the high-order harmonic generation is suppressed.The HHG yield peaks when the molecules are aligned at about 40 o from the polarization axis.