Molecular Orbital Imaging Based On High-order Harmonic Scheme

With the deepening of the exploration of high-order harmonic,the development of high-order harmonic has made achievements in some fields.One of the most important applications of high-order harmonic is to detect complex molecular structures and dynamic processes.This is because the generation process of high-order harmonic itself comes from the ultrafast motion of electrons,so it can be used as a tool to detect the internal structure of molecules and atoms and capture the internal motion of electrons.The use of high-order harmonic in molecular orbital tomography also provides an unprecedented way to study the internal structure of molecules.In this paper,the more accurate N₂ molecular orbital wave function is obtained by using Molpro quantization software,and the momentum space dipole moment is calculated according to the orbital wave function.Next,the high-field approximation theory is used to calculate the high-order harmonic spectrum of N₂ molecule under high-intensity laser pulse.It is found that the image of harmonic intensity with electron backtouch momentum is strikingly similar to the dipole moment obtained from momentum representation in node position,symmetry and other aspects,then using the theory of tomography completed the N₂ molecules of the highest occupied orbital reconstruction.Further research shows that the driving laser parameters affect the molecular orbital reconstruction results.When the intensity of laser pulse is weak,the harmonic spectrum is not wide enough and the high energy part is missing.Therefore,the characteristics of N₂ orbit cannot be well reproduced in the reconstruction process,and the effect is better under the action of long pulse.When the duration of laser pulse is constant,the cutoff position of harmonic spectrum expands with the increase of laser pulse intensity,and a larger spectrum range of harmonic spectrum can be obtained.In this case,because the harmonic spectrum corresponds to a larger range of momentum space,the resolution of coordinate space is finer,and the reconstruction effect of orbital wave function is better.Since the conventional linear polarization driven laser irradiates molecules and then performs molecular orbital imaging,it is necessary to rotate the polarization direction of the driving field many times.Therefore,in order to simplify the experimental steps,this paper attempts to reconstruct molecular orbital by using a high order harmonic beam interacting with molecules whose polarization direction rotates with time.To this end,we first study the characteristics of molecular high harmonics under rotating linearly polarized laser,and find that the HOMO intensity of N₂ and CO molecules is greater than that of the HOMO-1.And with the increase of laser pulse speed,the harmonic efficiency decreases.The reason for this phenomenon is that the probability of ionized electrons backcollision decreases,which leads to the reduction of harmonic intensity.In addition,it is found that the peak splitting phenomenon occurs after the rotation of high-order harmonic in the polarization direction of the laser field,and the distance between the two adjacent sub-peaks of the splitting phenomenon is about two times of the rotation angular frequency of the driving field.After understanding its harmonic characteristics,we try to reconstruct the HOMO orbital of N₂ molecule by rotating linearly polarized laser pulse,and get the result similar to the molecular orbital space wave function.This provides a new idea and useful reference for the study of molecular orbital imaging.