Multi-orbit Effect Of N₂ Molecular High-order Harmonic Generation Process

The development of laser technology will directly affect the development of the interaction between laser and atomic molecules.The interaction of laser and atomic molecules has many phenomenabeen observed.For example,high harmonic generation?HHG?is a highly nonlinear process.Highly linearly polarized lasers interact with atomic molecules to produce ultra-wideband coherent radiation from infrared to extreme ultraviolet and even soft X-ray segments.Now,with the appearance of polarization and other shaping technology,some new characteristics of the high harmonics driven by these complex laser pulse are found,such as the atoms in the bicircular field produce the circularly polarized high harmonics.At present,people have made a series of researches on the research of the higher harmonic generation of atoms.Because atoms have a simpler structure than molecules,so,at beginning they have been widely used and explored.Compared with atoms,molecules have more complex symmetries.Therefore,the study of molecules become more complicated.Moreover,the harmonic emission of a molecule has a great dependence on the vibrational and dynamic states of the molecule.Therefore,the research on the high harmonics generation of molecule or more complex molecules will face great difficulties and challenges.So,we start with relatively simple molecules of N₂.In this paper,we simulate the high-order harmonics generated by the aligented N₂ molecules under the action of the infrared polarized laser pulse and the bicircular field by numerically simulated the strong field approximation scheme.The variation of the higher harmonics generation by the N₂ single molecular orbits HOMO or HOMO-1,the HHG was studied under the condition that the angle between the N₂molecular axis and the laser polarization direction was continuously changed,and the HOMO and HOMO-1 orbits were jointly generated.The similarities and differences between the total harmonics and the independent orbital harmonics,the influence of the multi-orbital effect on the"minimum"structural characteristics of the N₂ molecule high harmonic spectrum,and the harmonic generation of the HOMO and HOMO-1independent molecular orbitals under the bicircular field,The spectrum of the forbidden,harmonic polarization,multi-track effects on the bicircular field harmonics.The theoretical study shows that under the action of the line polarization laser pulse,the intensity of the harmonics generation by the aligented N₂ molecular HOMO orbits decreases with the increase of the aligented,and the harmonic intensity generation by the HOMO-1 orbital is aligented increases and increases.At a small angle,the harmonics generated by the HOMO orbit are stronger than the harmonics generated by the HOMO-1 orbit.At a large angle,the HOMO-1 orbital harmonics stronger than the HOMO orbital harmonics.Under certain angles of aligented,the high harmonics generation by the HOMO or HOMO-1 usually quite different in intensity.In the process of total harmonic generation,one orbit always dominates the contribution,resulting in the generation of high harmonics between the two orbits.Coherent has little effect on the harmonics that are commonly generated by the total multi-orbit.With the increase of the aligented of the N₂ molecule,the dominance of the total harmonics shifts from the HOMO orbit to the HOMO-1 orbit.Therefore,the total harmonic intensity shows a decrease with the angle and increasing,the position of the"minima"point appears at the intersection of the two independent orbital harmonic intensity curves with the angle.The"minima"structure of the total N₂molecular harmonic spectrum is shifted from the"minima"position of the harmonics of the HOMO orbit,which is usually considered to be the main contribution,to the higher frequency direction in frequency and to the smaller angle direction in the angle.Under the action of the bicircular field laser pulse,the electron undergoes an ionization-bump collision emission harmonic process in each one third of the low frequency period driven by the“three-lobed”form of the electric field.The direction of the three“petal”electric fields in the adjacent low frequency period is different,so that the electrons are ionized from the bound state wave packet to collision again with the parent ion wave packet.The bound wave packets experienced are different.As a result,the harmonics generated by the single orbit of the aligented N₂ molecule are caused,and the 3q harmonic suppression rule is broken.For the harmonic electric field,because the single molecular orbital wave function does not have good rotational symmetry,it causes the harmonics generation by the single orbit of the aligented N₂ molecule.Although the law of the rotational direction of the atomic harmonics is preserved,it does not Then the circular harmonics of the atomic harmonics become elliptically polarized.Moreover,the direction of the major axis of the ellipse is determined by the symmetry of the molecular orbital and rotates synchronously with the rotation of the molecular orbital.For the effect of the two orbits on the harmonics,we find that there is a significant difference between coherent superposed harmonics and incoherent superposed harmonics.This shows that the harmonics generation by the N₂ molecules driven by the bicircular field are not only multi-orbit effects,but also important.Moreover,it is also very important for the interference effect between the orbital harmonic.This interference effect will bring about the enhancement or suppression of specific order harmonics.On the total coherent superimposed harmonic spectrum contributed by multiple orbits,the reappearance of the 3q forbidden law indicates that the coherence effect between the N₂ molecule HOMO and the HOMO-1 orbital harmonics plays an intuitive role in the overall harmonic generation process.The important role,its role makes the structural characteristics of the harmonic spectrum have undergone essential changes.This research of the molecule harmonic system driven by the bicircular fielddeepens the understanding of the new characteristics and laws of the molecule high harmonicsdriven bythe complex light field.It has accumulated experience for people studying the higher harmonics of simple molecules in complex light fields.