Multi-electron Dynamics Of Molecular System In Intense Laser Field

Strong field physics is a science to study the interaction between the intense laser fields and matters.The super intense and ultra-short pulse laser technologies not only provide people with an important means to explore the microscopic material structure and its law of movement,but also allow people to real-time observe and manipulate the dynamics of electrons in atoms and molecules.In the past few decades,interaction between intense laser fields and atoms caused a series of novel high-order nonlinear physical phenomena,for example multi-photon ionization(MPI),above threshold ionization(ATI),high-order above threshold ionization(HATI),non-sequential double ionization(NSDI)and high harmonic generation(HHG)and so on.These phenomena help people more deeply understand the process of interaction between light and matter.The unique phenomenon of high harmonic generation promotes the emergence of a series of attosecond technologies.HHG can be used not only to reconstruct the molecular orbitals but also to synthesize the attosecond pulses which can be used for direct imaging of the electric field of a laser pulse and to probe the ultrafast electronic dynamics in atoms and molecules.In addition,the attoclock technology can be used to experimentally measure tunneling time.Quantum tunneling is a fundamental physical phenomenon in quantum mechanics.In recent years,the tunneling time has become a hot topic.Recent attoclock experimental measurements of tunneling time make people pay attention to this problem in strong field physics.However,there are still no consensus about the tunneling time both in theories and experiments.Recently,the study about HHG has been gradually expanded from atomic systems to complex molecular systems and even solids.A series of more novel and interesting physical phenomena appear when the intense laser fields interact with molecular systems,such as two-center interference,multi-channel interference,dynamic exchange in molecular orientation ionization and multi-electron polarization effect,etc.With the increase of the number of electrons in the research system,the theoretical descriptions of the multi-electron dynamical process under intense laser fields face many difficulties and challenges.In this thesis,based on the time-dependent Hartree-Fock theory,we have studied the multi-electron dynamical process when CO2 molecule exposed to intense laser fields.In addition,we theoretically study the tunneling time problem.Firstly,we investigate the high harmonic spectroscopy of CO2 molecule exposed to a one-color laser field.When the polarization of laser field is parallel to the molecular axis,we identify a minimum structure in the HHG spectra in different laser intensities,which is different from the two-center interference and dynamical interference minima.By propagating different orbitals in the numerical calculation,we find that this minimum is mainly resulted from the HOMO orbital.In order to understand the mechanism of this minimum occurring,we calculate the relation between ionization time and harmonic order by the classical trajectory Monte Carlo(CTMC)method.By comparing the HOMO orbital ionization rate with the result of CTMC,it can be found that the instants of ionization suppression of the HOMO orbital correspond to the ionization moments of the dip structure In order to further understand the ionization suppression of the HOMO orbital,we introduce three different approximate models to investigate the roles of multi-electron interactions in the HHG of CO2 molecule and find that the new dip structure can be attributed to the dynamic Coulomb effect between the HOMO orbital electrons.Secondly,we investigate the coherent modulation of the high harmonic generation of CO2 molecule in two-color laser field by numerically solving the time-dependent Hartree-Fock equation.Our theoretical calculation results are qualitatively consistent with the experimental observations.We can see that the intensities of even order harmonics are modulated by the relative phase of the two-color laser field with different patterns.By theoretical analysis,we find that the multi-orbital participation and dynamical interference between HOMO and HOMO-2 orbitals change the modulation of two-color phase difference on the intensities of even order harmonics.Finally,we investigate the problem of tunneling time in strong field physics.We introduce a new definition of quantum travel time ?t in one-dimensional rectangular barrier tunneling process.Based on the new introduced definition of quantum travel time,we bridge the connections between different tunneling times including the Bohmian time,the Büttiker-Landauer time,the Lamor time and its associated times.The quantum travel time ?t is obtained without any approximation in one-dimensional rectangular tunneling process.Thus the quantum travel time ?t can be retrieved to the Büttiker-Landauer time under opaque barrier approximation.Moreover,this quantum travel time can also give a reasonable description in the very thin barrier where the Büttiker-Landauer time is not well defined.We calculate the tunneling delay time proposed by Keitel et al.by numerically solving the one-dimension time-dependent Schrodinger equation.According to our definition of quantum travel time,the tunneling delay time can be interpreted as a quantum travel time spent by the electron to tunnel from a point xF under the barrier to the tunnel exit.The peak of current density at position xF coincides with the peak of the laser field.In addition,a peculiar oscillation structure with constant period in the wavelength dependence of tunneling delay time are observed.Further analysis shows that this oscillation structure can be attributed to the interference between the ground state tunneling channel and the excited states tunneling channels.