Research On The Characteristics Of High-order Harmonic Generation Influenced By Molecular Nuclear Motion

High-order harmonic generation（HHG）through the interaction of femtosecond laser with atoms and molecules is one of the research hotspots in strong field physics.Compared to atoms,Molecules have richer internal structure and additional freedom of nuclear motion.Therefore,in the femtosecond lasers,the molecular HHG presents many unique phenomena,such as the two-center interference minimum,the dynamic minimum caused by multi-orbit interference.In addition to the complex molecular orbital structure,the dynamics of molecules can also influence the HHG spectral characteristics,such as resonance enhancement,harmonic intensity and frequency modulations caused by molecular nuclear vibration.This paper focuses on the influence of molecular dynamics,such as molecular nuclear vibration and rotation,on HHG.Moreover,we study the methods for detecting molecular ultra-fast vibration and rotational processes.The main researches in this paper are as follows:（1）Wavelength scaling of HHG yield of H₂⁺ modulated by molecular nuclear motionWe have investigated the HHG yield of H₂⁺ by numerical simulations base on the time-dependent Schr?dinger equation（TDSE）in the non-Born-Oppenheimer approximate.The results show that the decrease of wavelength-dependent HHG yield is considerably slowed down,which is due to the charge-resonance-enhanced ionization effect induced by molecular vibration.In the longer driving laser wavelengths,the nuclear separation is stretched,which then considerably increases the ionization rate.In addition,we find an oscillation structure in the wavelength scaling of HHG.When decreasing the laser intensity or increasing the nuclear mass,the oscillation structure will shift towards a longer wavelength of the laser pulse.These results show that an efficient harmonic spectrum in the midinfrared regime can be generated by manipulating the nuclear dynamics of molecules.（2）The dependence of carrier-envelope phase of multi-cycle laser pulse onH₂⁺HHG frequency shift by molecular vibrationWe have investigated the HHG frequency shift ofH₂⁺by numerical simulations of the time-dependent Schr?dinger equation（TDSE）in the non-Born-Oppenheimer approximate.The results show that HHG in the plateau exhibit counterintuitive frequency modulation（FM）as the CEP of the multi-cycle laser varies.Based on the classical electron trajectories and time-frequency analysis,this multi-cycle CEP-dependent FM is demonstrated to result from the interference of half-cycle HHG radiations with long time interval,which is modulated by laser-driven nuclear motion.The mechanism of the CEP-dependent FM is further confirmed by simulations based on a simple algorithm in the time domain.Moreover,we demonstrate to retrieve the nuclear dynamics from the CEP-dependent FM with attosecond resolution.（3）Efficient HHG in water window region based on resonance-enhanced ionization induced by molecular motion.We have proposed a two-pulse scheme for efficient HHG in water window region based onH₂⁺molecular vibration.H₂⁺is prepared in the dissociation state by the pump pulse.In the probe pulse,the harmonic efficiency is enhanced by 2-3 orders of magnitude due to relatively large nuclear separation.Furthermore,efficient harmonics in the water window region is generated by using a probe pulse with longer wavelength.（4）Investigation of HHG influenced by molecular rotation.We investigated the influence of molecular rotation on HHG frequency shift.We find that molecular rotation can induce the nonadiabatic frequency shift of HHG.Frequency shifts will be different for different harmonic orders and quantum orbitals.Based on the quantitative rescattering theory,including the rotational nonadiabatic effect,we well reproduce the experimental observation results.On the other hand,we demonstrated an all-optical measurement of high-order fractional molecular echoes.By using pump-probe experiment,the signatures of full and high order fractional（1/2and 1/3）alignment echoes are observed in the CO₂HHG spectrum.By increasing the time delay of the pump pulses,much higher order fractional（1/4）alignment echo is also observed in N₂O molecules,which is the highest order fractional molecular arrangement echo ever observed.