The Theoretical And Computational Development To The Dynamics Of Multi-electron Atomic And Molecular Systems In Intense Laser Fields

The study of the strong-field multi-photon process of atomic and molecular system is a subject of current significance both theoretically and experimentally. Recent progress of laser technology has reached a burst of attosecond science where the electron dynamics plays an important role in physics. In particular, the recent advancement of intense laser field has led to a set of strong-field phenomena, such as multiphoton ionization, multiphoton resonance, high-order harmonic generation, etc, are all beyond the perturbative regime. To advance the strong-field physics, this dissertation aims at developing the new theoretical and computational methods for ab initio studies of atomic and molecular process in intense laser fields. The main achievements are summarized below:1. The ionization dynamics of helium atom and lithium atom under intense laser pulses was explored by using classical ensemble method. 1) Probabilities of double-ionization of helium in intense laser fields in different conditions are calculated by the symplectic method. The wavelength dependence of NSDI in He is investigated. The non-sequential ionization (NSDI) is observed in classical simulations at the laser wavelength of 532 nm, 780 nm and 1024 nm, respectively, while the sequential double ionization (SDI) is the dominant process at the laser wavelength of 248 nm. The double to single ionization ratio of helium with different laser intensities are also investigated, and the results shows the double-ionization of helium shifts from non-sequential to sequential as the intensity increases at the wavelength of 532 nm, 780 nm and 1024 nm, which is in agreement with the corresponding experimental and quantum results qualitatively. The result demonstrated the non-sequential double ionization mechanism could be observed in terms of one-electron energy from a classical view. 2) The classical ensemble method is applied to study ionization processes of a 1D model lithium atom interacting with an intense laser pulse. The motion of electrons is described by the classical Hamiltonian canonical system of equations. The ratio of double-to-single ionization with the increasing laser intensity is calculated and explained in terms of the energy distribution of electron. The triple ionization of lithium is also investigated and the primary triple ionization path is found. Our results show a clear transition from non-sequential to sequential double ionization as the intensity increases, which is in agreement with quantum calculation.2. We extended the coupled coherent-states (CCS) approach to simulate the strong field ionization of atoms and molecules in long wavelength. This approach uses a basis of trajectories guided by Frozen Gaussian coherent states, sampled from a Monte-Carlo distribution, as the initial states of the quantum time-dependent Schrodinger equations. The CCS trajectories move over averaged potentials, which can remove the Columbic singularities exactly. 1) The behavior of helium in intense laser field is investigated by CCS method. The novel low-energy structure (LES) is predicted by our CCS calculation and the ''rescattering'' event is clearly identified in the higher energy regime. Besides, the non-sequential double ionization is also explored and the"rescattering"event can be identified as the major mechanism. In addition, we also studied the electron angular distribution of helium and found that the maximum angle between the electron and electric field directions becomes smaller with the increasing of laser intensity and wavelength. 2) The strong-field ionization, low energy structure region and the orientation dependence of CO2 and N2 in intense laser fields are investigated by the improved coupled coherent-states (CCS) approach. The effects of laser intensity on the orientation dependence of total ionization probability of N2 and CO2 are calculated and the results are in agreement with experimental research. The normalized electron yield and angular distribution with different parameters are also investigated.3. I have learned the time-dependent generalized pseudospectral (TDGPS) method and time dependent density functional theory (TDDFT) in Prof. Shih-I Chu's group for atomic and molecular systems in intense laser fields, and applied them to Ar cases. The generalized pseudospectral technique is extended to perform an optimal spatial grid discretization, leading to significant improvement of the quality of the wavefunction over that obtained by the equal-spacing spatial discretization techniques. The accuracy of the method is demonstrated by several benchmark calculations including the excellent agreement of the HHG power spectra in length and acceleration form.