| High-order harmonics are high-energy electromagnetic radiation generated during the interaction between strong laser fields and atoms,molecules or solids.Its waveband covers extreme ultraviolet to soft X-ray bands.Since the discovery of higher harmonic generation(HHG),the physical mechanism of HHG has attracted a lot of attention and has been extensively explored.The mechanism of atomic HHG in strong field has been clearly understood theoretically through a large number of studies.Experimentally,it has been found that extremely short XUV and X-rays and even attosecond pulses can be obtained using HHG,which paves the way for ultrafast science.Currently,some new research topics in the study of HHG are emerging.First,the mechanism of molecular HHG generation in strong field is studied to explain the deviation between theoretical predictions and experimental measurements of some properties of elliptically polarized HHG,and further reveal the physical mechanism of molecular HHG.Secondly,molecular HHG is used to study the properties of molecules in strong fields or the internal properties of molecules,such as molecular rotational wave packets and transition dipole,etc.,for the study of electron ultrafast dynamics and the development of molecular orbital ultrafast imaging technical method.Based on the above analysis,this thesis has carried out with research on the molecular inner orbital effect and retrieval of alignment distribution,ionization probability and transition dipole based on HHG.The main results are:(1)The contribution of the inner orbital to HHG is revealed by exploiting the ratio between perpendicular and parallel of HHG.Molecular HHG comes from the strong field ionization of electrons,which indicates that with the increase of the probe intensity,besides the electrons in the highest occupied molecular orbital(HOMO),the electrons in the inner orbital may also be ionized.In order to consider the contribution of inner electrons to HHG,we added the contribution of multiple inner orbitals(HOMO1,HOMO-2)to HHG based on the quantitative rescattering(QRS)theory of a single HOMO,and also considered the propagation of HHG in the macroscopic medium.Taking the N2 molecule as an example,numerical simulations show that the ratio between perpendicular and parallel of HHG by the QRS theory with two inner orbital(HOMO-1,HOMO-2)is more consistent with the experimental results.Moreover,the simulation results using a 1600nm probe field show that the prediction of the intensity ratio by the QRS theory including the inner orbital is different from the previous results when only the HOMO is considered.Our work not only proves the important contribution of inner orbital to molecular HHG,but also provides a new idea and method for studying molecular inner orbital information.(2)We propose a method to retrieve the molecular alignment distribution using polarized HHG.Based on the QRS theory,we then propose a retrieval method for obtaining the expansion coefficients of the alignment distribution with Legendre polynomials by linear fitting.Compared with the previous method that only uses the extreme point of the HHG to retrieve the several alignment parameters such as gas temperature and pump intensity,our method can directly retrieve the entire strong field alignment distribution of the molecule.Taking N2 molecule as an example,we not only accurately retrieved the molecular alignment distribution under the smaller and larger alignment degrees,but also quantitatively investigate the uncertainties of the probe intensity and the effect of the inner electron orbital on the retrieval.Our proposed retrieval method opens up a new avenue for studying the orientational alignment of molecules in strong fields.(3)We propose a method to retrieve the angle-dependent strong-field ionization of molecular HOMO using HHG.We uses Legendre polynomial to expand the ionization probability and uses the QRS theory to obtain the relationship between the HHG and the expansion coefficient.and finally obtains the ionization rate through the singular value decomposition method to obtain the expansion coefficient.Finally,the ionization probability is obtained with the expansion coefficients calculated by singular value decomposition(SVD).To mimic real experimental conditions,we use macro HHG in retrieval.Taking N2 molecule as an example,we not only give the ionization probability consistent with MO-ADK theory at higher and lower molecular alignment degrees,but also investigate the uncertainties in the alignment degree and the effect of the inner orbital on retrieval.Our proposed retrieval method provides a new way to study the fundamental problem of strong field ionization of molecules.(4)We propose a method to retrieve the angle-dependent transition dipole between the HOMO and continuum states of linear molecules using HHG.Different from realvalued physical quantities such as alignment distribution and ionization probability,the transition dipole is a complex number,so we use complex expansion coefficients in the Legendre polynomial expansion of the transition dipole.Finally,the magnitude and phase of the transition dipole are obtained with the expansion coefficients calculated by SVD.Compared with the previous method of using single-molecule HHG to retrieve the transition dipole,our method can use macro HHG for retrieval to mimic real experimental conditions.Taking N2 molecule as an example,we not only accurately retrieved the transition dipole at higher and lower molecular alignment degrees and probe intensity,but also study the uncertainties of probe intensity and alignment degree and the effect of inner orbital on retrieval.Our proposed retrieval method provides a new method for studying molecular orbitals and molecular internal structures.Our research not only can help people getting a deeper understanding of the high order harmonic generation process of molecules in strong fields,but also shows the new use of HHG in studying strong field molecular orientation,ionization and exploring molecular internal information. |
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