High-order Harmonic Generation From Complex Molecules Irradiated By The Intense Laser

With the development of ultra-strong and ultra-short laser pulse technology,the study of the interaction of lasers with atoms and molecules has attracted widespread attention from strong field physicists.In experiments,strong field physical phenomena such as multi-photon ionization,tunneling ionization,high-order harmonic generation and molecular dissociation have been observed.Among them,high-order harmonic generation are potentially tunable table-top light sources for coherent extreme ultraviolet and soft x-ray radiation,and are widely used in coherent diffraction imaging,ultrafast holography,and ultra-high time-resolved measurement.Since molecules are more complex than atom and have additional degrees of freedom and symmetries,the high-order harmonic generation from molecules exhibit richer characteristics.It provides new opportunities to further optimize the efficiency of the harmonics,extend the cut-off energy of the harmonic spectrum and control the polarization characteristics of the harmonics.Thus,we theoretically studied the high-order harmonic generation from complex molecules in the intense laser electric field.The main work include the following contents:Firstly,the time-dependent density-functional theory method is used to numerically simulate the high-order harmonic generation radiated by the long-chain molecule 1-nonene(C₉H₁₈)irradiated by the linearly polarized laser field.The harmonic emission efficiency of C₉H₁₈ molecule is significantly higher than that of nitrogen molecule(N₂)under the same laser pulse,and the cut-off energy of the harmonic spectrum is also significantly greater than the harmonic generated from the N₂ molecule.By analyzing the time-frequency behavior of harmonics and semi-classical simulations,it is found that the source of the harmonics in the energy-increasing part of the harmonic spectrum of the C₉H₁₈ molecule is generated by the recombine with other atom in the molecule of the ionized electrons.The harmonics generated by this mechanism is used to synthesis high-intensity isolated attosecond pulses.In addition,we have also studied the high-order harmonic generation from chain molecules of different spatial scales irradiated by the circularly polarized laser pulses.It is found that chain-shaped molecules emit intense harmonic emission under the action of a circularly polarized laser field.The larger the spatial scale of the molecule,the greater the harmonic cutoff energy.Secondly,we have studied the high-order harmonic generation of ring molecules under a circularly polarized laser electric field.High-order harmonic generation from the C₆H₆ molecules with a few-cycle circularly polarized laser pulse is investigated.Compared with the harmonics from atoms and diatomic molecules with similar ionization energy irradiated by the same laser pulse,the harmonics from C₆H₆molecules possess higher efficiency and cut-off energy.Further research reveals that the harmonic efficiency and cut-off energy of the C₆H₆ molecule increase gradually as the wavelength of the driving laser decreases.Through the analysis of the time-dependent evolution of electronic wave packets,it is found that the harmonic generation can be attributed to the recombination of the ionized electrons and their parent ions.By optimizing the driving laser pulse,we demonstrated that high-order harmonic generation in the benzene molecule can be used to produce isolated circularly polarized attosecond pulse.On this basis,we further studied the high-order harmonic generated from the ring-shaped molecular cyclo[18]carbon(C₁₈(ring))with a larger molecular size irradiated by the circularly polarized laser.Compared with C₆H₆ molecule,for the same incident laser intensity and wavelength,the harmonic efficiency of C₁₈(ring)molecule is higher than that of C₆H₆ molecule,and the harmonic cut-off energy is also larger than that of C₆H₆ molecule.Through the analysis of the wave packet motion behavior,it is found that the increase in harmonic efficiency can be attributed to the fact that C₁₈(ring)molecule has more chance of recollision than the ionization wave packet of C₆H₆ molecule irradiated by the circularly polarized laser field.Thirdly,we studied the influence of multi-orbital effects on harmonic emission.Compared with atoms,the ionization energies of different electrionic orbitals of molecules are closer.Irradiated by the strong laser pulse,multi-orbital will contribute to the molecular high-order harmonic generation.We investigated high-order harmonic generation in molecules N₂ and C₆H₆ irradiated by bichromatic counterrotating circularly polarized laser pulses.It is found that 3N(N is an integer)order harmonics in C₆H₆ disappear.Whereas,N order harmonics are obtained in N₂.Analysis of the high harmonic generated from individual electronic orbital of the molecule,shows that,the characteristics of harmonic spectra are determined by the spatial distribution of electronic orbits.For the C₆H₆ molecules,the spatial distribution of the 12 molecular orbitals that mainly contribute to the harmonic emission presents a nearly spherical distribution as a whole.Moreover,effects of molecular orientation on harmonic emission are also presented.However,it was found that the harmonic generated from non-aligned molecule were consistent with the experimental measurement.In addition,the harmonic spectrum of acetylene molecule(C₂H₂)irradiated by the counter-rotating bi-circular laser pulses was studied,and it was found that there is a 3N order suppression phenomenon in the low-energy region of the harmonic spectrum.With the increase of the harmonic energy,the sub-peak structure appeared in the the harmonic spectrum.With the increase of harmonic energy,the sub-peak structure in the harmonic spectrum disappears,and all integer order harmonics can be observed.By analyzing the evolution of the ionization probability of each molecular orbital and the contribution of different molecular orbital in the harmonic spectrum,it is found that this special harmonic spectrum is generated from the contribution of different numbers of molecular orbitals to the harmonic spectrum and the interference of each orbital.By changing the intensity of the driving laser,its harmonic emission can be controlled effectively.