Theoretical Investigations On Ultrafast Molecular Dynamic Imaging By High-order Harmonic Generation And Above-threshold-ionization Spectra

When atoms and molecules are irradiated by an intense laser field(above 1013 W/cm2),a series of non-perturbative effects occur,for example,high-order harmonic generation,above-threshold ionization and non-sequential double ionization,etc.These new phenomena further promote the development and application of attosecond physics.The high-order harmonic spectrum has been used to synthesize the ultra-short isolated attosecond pulse.The above-threshold ionization spectrum can be applied to probe the ionization and dissociation dynamics.The non-sequential double ionization can be used to investigate the electron correlation effect.The nuclear dynamic has to be considered generally in the interaction between molecules and a strong laser pulse,due to its significant influence in many processes like ionization,recombination and re-scattering in non-Born-Oppenheimer approximations.Although the time scale of nuclear motion is much larger than the electron motion,with the electron ionizing and the nuclear inner restrain weakening,the charge-resonance enhanced ionization effect will acutely increase the ionization rate at some critical inter-nuclear separations.In high-order harmonic generation,the dipole will be influenced by the nuclear wavefunction evolution between the electron ionization and recombination moments.Besides,the nuclear dissociation pathways,including above-threshold dissociation,vibrational trapping and bond softening,will also influence the spectra.We try to build the connection between nuclear dynamics and the high-order harmonic generation and the correlated electron-nuclear energy spectrum,and we extract nuclear information from the spectra and propose a new method to probe nuclear motion.The main work is listed as follows:1.Based on the frequency modulation of molecular high-order harmonic generation,a probing method for the alignment angle and the vibrational state as well as the dissociation velocity is proposed theoretically.In non-Born-Oppenheimer approximations,when H2+ is irradiated by an intense laser field,the redshifts in the high-order harmonic generation and its dependence on the alignment angle are theoretically investigated.Compared with atoms,the redshifts are unique in molecular high-order harmonics both in the tunneling and multiphoton ionization regimes,which has been verified by experiments.The nuclear motion influences the ionization rate,leading to the asymmetry between harmonic yields generated in the rising and falling parts of the laser pulse,and redshifts occur.We quantify the asymmetry parameter and clarify the above model.Besides,the influence of the initial vibrational state oin redshifts and the dependence of the width of harmonic peaks on the alignment angles are also investigated.Thus,we propose a scheme of probing nuclear alignment angle and vibrational state by the redshifts.2.An ultrafast molecular clock for dynamic imaging by correlated electron-nuclear spectra is proposed which can be applied to probe the subcycle time-resolved Coulomb explosion channel.By numerically calculating the ionization of H2+ driven by an intense linearly polarized laser pulse,we find that photoelectron yields in the opposite directions along the polarization axis are different,and the asymmetry function between the t.wo yields shows peaks and valleys alternately as a function of the nuclear energy.This phenomenon is explained by the Coulomb Explosion channel and is used to probe the nuclear motion with subcycle resolution(?1 femtosecond).This probing method is applicable in various laser conditions even when the spatial distribution of the laser beam is considered.Different from the traditional pump-probe method,the new method is more concise,and the nuclear motion can be obtained by a single-shot experiment.3.A theoretical method for controlling polarization of the elliptically polarized harmonics by changing molecular alignment angle is proposed.The molecular high-order harmonic generation driven by a counter-rotating bicircular laser field is investigated.The definitions of ellipticity and rotation angle of elliptically polarized harmonics are optimized.When a ?/3? counter-rotating bicircular laser field with 4-fold symmetry is applied on the H2+,the major axis of the elliptically polarized harmonic is almost perpendicular to the molecular alignment axis with a small tilt angle.The harmonic ellipticity changes dramatically with different molecular alignment.Even at some molecular alignment angle around 45°,the ellipticities of most harmonic orders are the same.Based on this feature,the molecular alignment information can be extracted from the harmonic polarization,and a new method for controlling the h armonic polarization is possible conversely.