Electron Interferences And Correlations In Strong-Field Ionization Of Atoms And Molecules

The advent of ultrafast intense laser pulse has extended the research on the interaction of light with matter into a new level. The interaction of such ultrafast intense laser pulses with atoms and molecules leads to a number of high-order nonlinear phenomena such as above-threshold ionization (ATI), nonsequential double ionization (NSDI), high-order harmonic generation (HHG) and Coulomb explosion. Among these high-order nonlinear phenomena, as a powerful tool for probing the strucure of atoms and molecules and ultrafast electron dynamics on atomic level, above-threshold ionization has attracted utmost attentions. Moreover, in strong field nonsequential double ionization the emission of two electrons exhibits strong correlated behaviors. Thus, it provides a simple and effective pathway to study the electron correlation which is the most universal effect in the world. Therefore, above threshold ionization and nonsequential double ionization has become a hot topic in the field of the strong field physics. In this dissertation, we perform researches on the above threshold ionization and nonsequential double ionization as follows:(1) The electron interferences in above-threshold ionization is investigated systematically by numerically solving time-dependent Schrodinger equation (TDSE) and a quantum orbit model within SFA. These interferences include the interference between direct electrons, the interference between the direct and scattered electrons. The quantum orbits responsible for these interferences are identified and the corresponding interference structures are obtained in the photoelectron momentum distribution. This provides insight into experimental results on above threshold ionization.(2) The molecular photoelectron holography by an attosecond XUV pulse for different molecular orbitals is investigated. By combining a infrared laser pulse and an attosecond XUV pulse, the interference between direct electron wave packets is suppressed and a clear holographic interference structure is obtained in the photoelectron momentum distribution. This holographic interference structure strongly depends on the molecular structure. For the molecular orbitals with σ symmetry where the electron cloud is concentrated along the molecular axis, the numbers of the interference peaks in the holographic structure increase as the number of the nodal axis increases. Moreover, for this molecular orbital the numbers and locations of the interference peaks in the photoelectron momentum distribution along the laser polarization direction are sensitively dependent on the internuclear distance. For the molecular orbitals with π symmetry where the electron cloud is localized off the nuclei, there is a nodal line along the laser polarization direction in the photoelectron momentum distribution. These results indicate that the photoelectron holography by an attosecond XUV pulse can be used as an efficient tool for molecular imaging.(3) The attosecond photoionization of diatomic molecules at different alignment angles in a strong infrared field is investigated. The results show that the holographic interference structure in the photoelectron momentum distribution strongly depends on the alignment. When the laser polarization does not coincide with symmetry axes of the molecule, there is a nodal line in the holographic pattern and it changes gradually as the alignment angle increases. A careful examination of the electron wave function shows that the alignment dependence of the holographic interference structure originates from asymmetric amplitude distribution of the continuum electron wave function at ionization.(4) The correlated electron dynamics in nonsequential double ionization (NSDI) of atoms by the orthogonally polarized two-color pulses is investigated. The results show that the correlated electron momentum distribution is strongly dependent on the relative phase of the two pulses. By tracing the history of double ionization trajectories, we find that the revisit time of the recolliding electron wave packet is controlled with attosecond accuracy. After recollision, one electron is ionized immediately while the other electron is either released immediately or excited with subsequent field ionization. The release time of the excited electron is also steered with attosecond resolution by changing the relative phase of the orthogonal two-color pulses. The attosecond-resolved control of the revisit time of the returning electron wave packet and the release time of the excited electron is responsible for the phase dependence of the correlated behaviors of the two electrons.