Theoretical Study Of Single-and Multi-electron Dynamics Of Atoms And Molecules In Intense Laser Pulses

The interaction of atoms and molecules with ultraintense and ultrafast laser pulses is one of the leading and central subjects of scientific disciplines. Strong-field processes allow to probe and manipulate the structure of matters within Angstrom spatial resolution and attosecond temporal resolution, which is of huge scientific significance and application values. Subjected to intense laser fields, atoms and molecules will undergo many highly nonlinear processes, such as tunnel ionization（TI）, above-threshold ionization,nonsequential double ionization, and high-order harmonic generation（HHG）, which require nonperturbative theoretical treatment in return. With increasing number of electrons,great challenge is encountered for theoretical investigations and developments. As a result, new theories and methods as well as more dedicated studies are desired in strong-field physics. In this paper, we develop a series of strong-field model and ab-initio methods and programs. The electronic dynamics in various atomic and molecular systems, from singleelectron to multielectron, from single-center to two-center, are investigated systematically with these methods.Firstly, for the first time, we demonstrate that the dynamic polarization of core electrons have huge impacts on the behavior of electrons in the highest occupied molecular orbitals（HOMO）, and we successfully explain the experimental finding on the strongfield ionization（SFI） of CO molecules. By incorporating the polarization potential into the single active electron（SAE） method, we identify that the dynamic core polarization（DCP） plays an important role in the strong-field TI of CO molecules, and we find the physical origin that leads to the disagreement between molecular tunneling ionization theory and experiment. We identify that both the DCP and dynamic orbital deformation of HOMO are important in the orientation-dependent HHG of CO molecules subjected to intense few cycle laser fields. These polarization dynamics allow for the observation of strong orientation effects and dynamic minimum in the harmonic spectra. The generated attosecond pulses can be greatly affected by these multielectron effects. This work sheds light on future development of dynamic orbital imaging on attosecond time scale. The further studies on the transient absorption of CO molecules show that multielectron effects have two important impacts: the intensity of absorption spectra and position of absorption peaks of HOMO electrons can be modified by DCP; the core electrons will contribute directly to the total absorption spectra, which may overrun the results of HOMO. These two effects depend critically on molecular orientations.Secondly, we develop the three-dimensional time-dependent Hartree-Fock（TDHF）and multiconfiguration time-dependent Hartree-Fock（MCTDHF） methods and programs for diatomic molecules, which benefit the development of multielectron ab-initio methods. We adopt the prolate spheroidal coordinates（which can treat the two-center Coulomb potential accurately） and highly efficient two-electron integration algorithms, and the programs are optimized for parallel computations. With the TDHF method, we calculate the ionization results of H₂ and CO molecules subjected to multicycle laser pulses, which yield very good agreement with experiments. For the first time, we investigate the alignmentdependent ionization results of H₂ molecules in intense XUV pulses with MCTDHF theory. Our results show that, electron correlation has little effects in single ionization process, but it plays an important role in double ionization process, leading to the decrease in the ionization probability. This method underlies the basis for future studies of electron correlation effects in strong laser fields.Thirdly, we develop a strong-field model calculation method based on quantum chemistry programs, which advances the strong-field model calculations and greatly benefits the fast check and comparisons of experimental results. Our method provides an efficient way for the calculation of SFI and HHG directly from output files of the general purpose quantum chemistry programs: GAMESS, Firefly and Gaussian, and it can be applied to quite large basis sets and complex molecules with many atoms. For ionization, the molecular Ammosov-Delone-Krainov（MO-ADK） theory, and both the length gauge（LG） and velocity gauge（VG） Keldysh-Faisal-Reiss（KFR） theories are implemented, while the Lewenstein model is used for harmonic spectra. Furthermore, it’s also efficient for the evaluation of orbital coordinates wavefunction, momentum wavefunction, orbital dipole moment and calculation of orbital integrations.Fourthly,we develop a method which solves accurately the 3D single-electron timedependent Schr?dinger equation（TDSE） of two-center systems, and we investigate the alignment-dependent SFI and HHG for the ground state of H+2. The study shows that, the ratio of ionization rates between perpendicular and parallel alignments displays a steplike structure against the Keldysh parameter γ, which is recognized as the competition between the multiphoton ionization（MPI） and TI. MO-ADK and LG-KFR models agree with the TI results, which VG-SFA model is found to be better for MPI. Due to the twocenter interference, highly elliptical HHG is yielded from the TDSE, while the standard strong-field approximation（SFA） model fails to predict this. By including the coulomb potential and the Stark shift corrections, we yield qualitative agreement results with the TDSE.Finally, we investigate the generation of Rydberg states of hydrogen atoms with intense laser pulses by solving the 3D TDSE and by means of classical-trajectory Monte Carlo simulations. Comparisons between these methods show that both the Coulomb force and initial lateral momentum, which have effects on the n distribution and l distribution of the population of excited states, are important in the generation of Rydberg states. The results may provide insights into the generation of Rydberg states in multielectron systems.