Theoretical Studies On Atomic Ionization In Strong Laser Fields

When atoms and molecules are exposed to an intense laser field, photoionization is one of the most basic physical processes. The above-threshold ionization (ATI), as a very important phenomenon of strong-field ionization, has always been a hot topic in the strong field atomic physics. Especially, in recent years, a nontrivial structure has been found in the low energy part of the ATI photonelectron energy spectrum and advances the understanding of the strong-field ionization of atoms. Moreover, the nonsequential double ionization (NSDI) process has also drawn intensive attention since an anomalously high degree of electron correlation was involved in this process, thus providing an ideal prototype to investigate the electron correlation effect in laser driven atomic phenomena. However, when a strong laser field interacts with atoms, the perturbation theory is no longer applicable. As a consequence, the non-perturbation theories are developed to deal with various phenomena in the strong field, such as high-order harmonic generation (HHG), ATI and NSDI. In this Ph.D. dissertation, a semiclassical theoretical method which based on three-step model is used. With this method, the low energy structures (LES) in ATI photonelectron spectrum and NSDI process in elliptically polarized laser fields are studied theoretically. The main works and achievements in this Ph.D. dissertation are as follows:1. Using semiclassical method, a theoretical study focusing on the nontrivial structure in the low energy part of the ATI ionization photonelectron energy spectrum is carried out. The results show that the Coulomb interaction between ionic core and the ionizied electrons is responsible for the LES. More than that, by theoretically_studying the energy resolved angular distribution (AD) of the ATI electrons and the evolution of the photoelectron kinetic energy with time, it is find that the Coulomb focusing effect can affect the electrons which possess high return energy significantly as well. Moreover, a theoretical study on the very low energy structures (VLES) in photonelectron spectrum and the corresponding double-hump structure in longitudinal momentum distributions from different noble gas atoms masured by the Wu et.al. in Peking University is carried out. The results unveil that long-range Coulomb potential plays important role in these special structures.2. Influence of the nonzero initial longitudinal momentum of the electron on the LES in photonelectron energy spectrum is studied. The results show that the effect of Coulomb potential from ionic core on the LES is closely related to initial longitudinal momentum direction of the tunneled electron.3. The NSDI behavior in strong laser field with different ellipticities is studied. The results show that the correlated electron momentum distributions change significantly with the increasing ellipticity. Theoretical analysis shows that the evolution of the momentum distribution can be attributed to combined effect of the transverse electric field and the ionic Coulomb potential on the movement of the first tunneling ionized electron.