| High-order harmonic generation（HHG）is one significant consequence of interaction of strong field laser with atoms or molecules.It is a highly nonlinear effect with wide range of applications.The HHG spectrum can reflect the molecular structure and wave function information because of the process of HHG is closely related to the transition of electrons.The spatial structure and wave function of molecules are more complex than atoms’and the total harmonic spectrum is determined by HHG from different molecular orbital.In this paper,using the non-perturbative quantum electrodynamics（QED）theory,we studied the HHG from different molecules.The main contents of this thesis are as follows:1.Based on the non-perturbative QED theory,the zero potential correction and coulomb potential correction are added and verified the feasibility of the correction.The molecular orbital function is obtained by using the molecular orbital theory of atomic orbital linear combination and Gaussian 09.The wave function is more accurate than the wave function obtained by the analytical method,and the wave function of the complex molecules can be obtained.2.Using the modified transition matrix,the HHG from different molecules is obtained.By changing the position of the molecule in space,it is found that wave function of different molecular orbital affect the intensity of HHG.Analysing the HHG from N2 and H2O,we found that intensity of HHG is related to the wave function distributed in the polarization direction of the laser.3.Using the zero potential correction we can obtain the below-threshold harmonics（BTH）in O2.The variation of the intensity of BTH with the laser wavelength is studied.It is found that the minimum of harmonics generation from oxygen molecules by high light intensity laser field is the result of interference of multiple molecular orbitals.As the wavelength of laser changes,the phase of harmonics of single molecular orbital appear to beπ-phase mutations,resulting in interference minimum of the total harmonic.When the light intensity is low,the total harmonic is dominated by the harmonics generated by the Highest Occupied Molecular Orbital（HOMO）orbital.The total harmonic minimum is the minimum of the HOMO orbital harmonic.The phase of harmonics generated by a single return channel will also have a mutation ofπ-phase with the change of wavelength of laser.The harmonics generated by different return channels are coherently summed up to cause a interference minimum of a single molecular orbital harmonics.4.Based on the non-perturbative QED theory,we developed an iterative algorithm for imaging molecular wave functions.Firstly,we theoretically explain the principle of iterative algorithm.We extract the final state wave function from the coherent superposition of different return channels.We design the iteration algorithm with the scaling law of HHG.After the calculation of iterative algorithm,we found great difference between the calculated HHG spectrum intensity and the exact HHG spectrum intensity.Because the initial wave function obtained by the interpolation method changes with the number of iterations,which leads the instability of the HHG spectrum.Further improvements of the transition matrix are needed to solve this problem. |
