Ionization Dynamics Of Atoms And Molecules In Intense Laser Fields

For over a century, the ionization of atoms and molecules has been the focus of attention by physicist. It is not only plays a crucial role for the establishment and development of quantum theory, but also the important tool for revealing the details of laser-matter interaction. The development of ultrafast intense laser pulse technology provides an unprecedented tool for the study of light and matter interaction. The study of the interaction between light and matter is expanded to a new level. In the ionization of atoms and molecules by ultrafast intense laser pulse, a series of high order non-linear phenomena are found, such as above threshold ionization, nonsequential double ionization. The study of these strong field ionization phenomena makes it possible to observe the movement of electrons in attosecond time scale, which provides an unprecedented condition for the development and improvement of the quantum theory. Moreover, strong field nonsequential double ionization shows strong electron correlation behavior. It provides the simple and effective pathway for people to study the electron correlation, which is the most universal effect in the world. Therefore, above threshold ionization and nonsequential double ionization has been very active branchs of the strong field physics. Although the three-step recollision model provides a basic physical picture for above threshold ionization and nonsequential double ionization, it can not reflect the detailed dynamic processes. With the development of experimental techniques, a large number of new phenomena are emerging. These novel experimental phenomena repeatedly demonstrated that the Coulomb potential of the nucleus, molecular structure and orientation and the polarization properties of the laser will significantly affect microscopic dynamics in strong field ionization. In order to further understand the microscopic dynamics in above threshold ionization and nonsequential double ionization, several efforts have been done in this thesis. They include:(1) We theoretically investigate the focusing of the electron transverse momentum in above threshold ionization of atoms driven by linearly polarized laser pulses. The result of the soft-core potential reproduces the focusing of the electron transverse momentum. This indicates that the focusing of the electron transverse momentum do not originate from the Coulomb singularity. Moreover, the result of the short-range potential does not reproduce the focusing of the electron transverse momentum. This indicates that the long-range Coulomb interactions between the electron and the parent ion are responsible for the focusing of the electron transverse momentum. Our analysis shows that the attraction of the parent ion to the return electron results in the focusing of the transverse momentum. This is confirmed by the dependence of the focusing strength on the laser wavelength and intensity.(2) We investigate electron emissions from H2+by circularly polarized laser pulses. The result indicates that most of electrons are emitted not at the instant when the electric field is parallel to the molecular axis, but hundreds of attoseconds later. The time delay results in the shift of electron emission direction with respective to the prediction of the tunneling theory. Furthermore, we find that the angular shift decreases with increasing the laser wavelength.(3) We investigate electron emissions in strong field enhanced ionization of asymmetric diatomic molecules by quantum calculations. It is demonstrated that the widely-used intuitive physical picture, i.e., electron wave packet direct ionization from the up-field site (DIU), is incomplete. Besides DIU, we find another two new ionization channels, the field-induced excitation with subsequent ionization from the down-field site (ESID), and the up-field site (ESIU). The contributions from these channels depend on the molecular asymmetry and internuclear distance. Our work provides a more comprehensive physical picture for the long-standing issue about enhanced ionization of diatomic molecules.(4) We investigate the correlated electron emission in nonsequential double ionization (NSDI) of argon atoms by few-cycle laser pulses. By tracing these NSDI trajectories, we find that besides the process of recollision-induced excitation with subsequent ionization just before the next field maximum, the recollision ionization also significantly contributes to the cross-shaped structure. Back analysis indicates that the two mechanisms have the comparable contributions to the cross-shaped structure in the correlated two-electron momentum spectrum. Moreover, the electron ionized first after recollision often escapes with a near-zero final momentum in the polarization direction, whereas the other electron most likely drifts out with a non-zero final longitudinal momentum.(5) We have investigated the effect of molecular alignment on the electron-electron correlation in nonsequential double ionization. The result shows that for perpendicular molecules the two electrons involved in NSDI more likely exit the molecules into the opposite hemispheres. Compared with parallel molecules, the suppressed potential barrier for perpendicular molecules is higher, and the second electron is more difficult to ionize. Thus for perpendicular molecules the second ionization has longer time delay and has a larger probability to occur before the second field maximum, which results in more anticorrelated emissions.(6) We have demonstrated the accurate measurement of the transient internuclear distance with the diffraction patterns of single-photon ionization of H2+by ultrashort XUV pulses. For such high-energy electrons, the effect of the parent ion is negligible. Thus their diffraction pattern is clear and undistorted, which can be excellently described by the simple double-slit model. This avoids the complicated recovering process of the diffraction pattern in the imaging with rescattered electrons.