Ionization Of Atoms In Intense Laser Fields

Due to the development of ultrashort intense laser pulse and applications of new measurement technique, such as high resolution time-of flight electron spectroscopy and cold target recoil ion momentum spectroscopy (COLTRIMS), ionization of atoms in intense laser field has become one of the research hotspots in atomic physics field. The perturbative approach fails to describe the ionization process in intense laser field. Resently, high resolution experiments have revealed the low energy structure (LES) in above-threshold ionization (ATI) process and the electron correlation in nonsequential double ionization (NSDI) process, the mechanisms behind which are still not clear and deserve to be further investigated.In this thesis we have studied the dynamics of low energy electrons in above-threshold ionization (ATI) process by solving the time-dependent Schrodinger equation, and studied the dynamics of electrons in nonsequential double ionizatoin (NSDI) process using semiclassical model. We found that the classical effects, such as long range Coulomb potential, play an important role in ionization process, while the quantum effects, such as resonance and longitudinal quantum diffusion of electron wavepacket, also contribute to ionization process. The main results are as follows:We study the dynamics of low energy electrons in ATI process by solving3D time dependent Schrodinger equation numerically. We find that the high-energy low-energy structure (HLES) becomes more pronouced with increasing wavelength and intensity, which is in agreement with experimental data (Nat. Phys.,2009,5,335; Phys. Rev. Lett,2009,103,093001). It is noted that a sharp peak structure with electron energy well below1eV (very-low-energy structure, VLES) in energy spectrum, corresponding to the double-hump structure in the longitudinal momentum distribution, is also identified as in the experimental observation (Phys. Rev. Lett.,2009,103,093001). It is interesting to note that VLES and HLES depend on the detail forms of atomic potentials. If the long range potential is replaced by a short range one, both of the two structures disappear. So we believe that the long-range Coulomb potential plays an important role in the production of these structures. In addition, we find that the resonance channels still contribute considerably to the ionization process in tunnelling region, and it will smear the classical effect, resulting in a less prominent double-hump structure as reported in the experiment data.A semiclassical model is developed to investigate the atomic NSDI process in elliptically polarized intense laser field. First, the ellipticity dependence of the ion yield of Ne2+is calculated and a good agreement with the experiment observation is achieved. Second, the wavelength dependence of the ratio of Ne2+/Ne+is investigated for fields with different ellipticities. In short-wavelength regime, the ratio increases rapidly with increasing wavelength and is not dependent on the ellipticity. However, the ratio reaches maximum which decreases with increasing ellipticity and decreases with wavelength in the long-wavelength regime.From the quantum mechanical point of view, the initial wave packet generated in the tunnelling ionization process should possess a finite width of momentum due to the uncertainty principle, which will lead to diffusion in its subsequential propagation process. The transverse quantum diffusion effect of the electron has been taken into account in semiclassical model in [Phys. Rev. A,1996,54, R2551]. To take into account the longitudinal quantum diffusion effect of the electron after its tunnelling ionization, a non-zero initial longitudinal velocity has been introduced into the model. We find that the non-zero initial longitudinal velocity has a suppressing effect on single-return collision (SRC) events in NSDI process, more specifically, it results in an obvious reduction in the center part of the correlation momentum distributions in the direction of z axis (axis of polarization) and makes the distribution of SRC in the direction of x axis (the minor axis of polarization in elliptically polarized laser field) become narrow. Moreover, a better agreement with the experimental observation is achieved when the nonzero initial longitudinal velocity is taken into account in linear polarization laser field.Correlated electron momentum distributions for NSDI of Ne in linearly polarized laser field with different intensities are studied using the semiclassical model. Our analysis shows that relative contributions of the two kinds of collision trajectories (single return collision (SRC) and multi return collision (MRC)) change with intensities:SRC dominates at low intensity and MRC dominats at high intensity. By comparing with the calculations of quantum S-matrix theory, we find that due to neglection of ion potential in the present S-matrix theory, which induces omission of the Coulomb focusing effect of the ion on the ionized electron wave packet, the contribution from the MRC is underestimated in the S-matrix theory.