Theoretical Research On Extracting Structural Information Of Atoms And Molecules Based On High Order Harmonics

Because of its good monochromaticity and coherence,laser always has been an important tool to study the structure of matter.With the development of laser technology,especially the maturity of femtosecond laser technology in recent years,more and more experimental phenomena based on the interaction of femtosecond laser and matter have been discovered.High order harmonic generation is one of the important phenomena.On the one hand,high order harmonic generation provides an effective method to obtain shorter laser pulses,and on the other hand,it also reveals finer structural information inside atoms and molecules.High order harmonics are usually considered to generate from the ionization,acceleration and recollision process of electron under a strong laser field.In this process,the electrons are first ionized by the strong laser field,and then accelerated to a continuous state with high energy.When the accelerated electrons collide with the parent core,a series of high-energy photons will be emitted,which are high order harmonics.Since high order harmonics are generated by collisions between ionized electrons and the parent core,the high order harmonic spectrum will also contain information about the structure of the parent core.In addition,this kind of ionization,acceleration and collision process occurs within half optical cycles,which provides people with an effective method to detect the structure and dynamics of atoms and molecules on the attosecond time scale.Carrying out atomic and molecular structure detection based on high order harmonic generation not only helps people to more intuitively and deeply study the physical processes of the microscopic world of matter,but also has guiding value for chemical production,biopharmaceutical and other fields.In recent years,rapid development has been achieved in the detection of atomic and molecular structures based on high order harmonic generation,including the study of Cooper Minimum in high order harmonic spectra,and molecular orbital tomography based on molecular high order harmonic generation.However,many current researches are still limited to atoms in the ground state and simple symmetric molecules.Theoretical and experimental studies on the higher excited states of atoms and more complex asymmetric molecules are still lacking.On these foundations,this article has done the following:(1)A Monte Carlo model of quantum trajectory based on high order harmonic generation is proposed.Our model can provide a clear and intuitive physical picture to understand and analyze high order harmonic generation.Compared with the strong field approximation model,our model can calculate the higher harmonic spectrum more accurately on the Rydberg state of the atom because of the consideration of the Coulomb potential.In addition,because we use the accurate scattering cross-section as the transition dipole moment of the electron,our model can also provide a more accurate calculation result for the molecular high order harmonic spectrum.(2)Proposed a molecular transition dipole moment reconstruction algorithm.The high order harmonic spectrum measured in the experiment usually only contains intensity information,and the phase information of the high order harmonic radiation is lost.We propose a molecular transition dipole moment reconstruction algorithm based on particle swarm optimization.The algorithm can extract the amplitude and phase of the high order harmonic radiation from the intensity information of the high order harmonic spectrum measured in the experiment.Since high order harmonic radiation is generated by the transition of electrons between the continuous state and the ground state,we can also obtain the amplitude and phase of the molecular transition dipole moment from the amplitude and phase of the high order harmonic radiation.We applied this method to nitrogen and carbon dioxide molecules,and successfully reconstructed the amplitude and phase of the transition dipole moments of nitrogen and carbon dioxide molecules.(3)A molecular orbital tomography method for asymmetric molecules is proposed.In the past research on molecular orbital tomography,people often use one-dimensional laser field to perform tomographic imaging of molecular orbital.This limits the application of molecular orbital tomography theory to asymmetric molecules.We use a two-dimensional two-color orthogonally polarized laser field to control the two-dimensional motion trajectory of electrons,thereby realizing the molecular orbital tomography of asymmetric molecules carbon monoxide and nitric oxide.(4)Based on the Quantum Trajectory Monte Carlo model,the anomalous ellipticity dependence of near-threshold high order harmonics was calculated and explained.We propose that the anomalous ellipticity dependence of harmonics in the near-threshold region results from the circular trajectory around the core under the influence of the Coulomb potential.In this way,as the ellipticity of the driving laser increases,the circular trajectory has an ellipticity dependence that is different from the normal re-collision trajectory.Our research reveals the possibility of detecting the structural features of the Rydberg states of atoms and molecules based on the generation of near-threshold high order harmonics.