Investigations On Atomic And Molecular Ionization Dynamics In Ultra-short Intense Mid-infrared Laser Fields

Ultrashort intense laser-matter interactions, which have broken up the framework of the traditional nonlinear optics, have reached the realm of extreme nonlinear optics and become an important frontier research field in modern physics. In this realm, the interactions of ultrafast intense laser with atoms and molecules lead in a series of interesting and novel highly nonlinear phenomena, such as above threshold ionization (ATI), nonsequential double ionization(NSDI), high-order harmonic generation (HHG) and Coulomb explosion. Research on the strong field ionization of atoms and molecules, which is a fundamental process, not only provides ways for understanding the dynamics of electrons in atomic time and spatial scales, but also greatly prompts the development of ultrafast imaging of the molecular structure and dynamics, opening a new avenue for monitoring and controlling the chemical reaction of molecules.However, limited by the ultrafast laser technology, most studies on strong field ionization of atoms and molecules have been practically confined to Ti:Sapphire laser wavelength (i.e., around800nm). Until recently, with the rapid advancement of the optical parametric amplification (OPA) technique, intense wavelength-tunable mid-infrared femtosecond laser pulses becomes available, opening new avenues for research in the strong field atomic and molecular physics. Intense laser with long wavelengths offers a convenient experimental knob to push the ionization regime into the unprecedented deep tunneling limits and facilitate a comprehensive understanding of the strong field ionization.In this dissertation, by virtue of the intense wavelength-tunable mid-infrared femtosecond laser, we perform a systematic experimental investigation on the ionization process of various noble gas atoms and homonuclear diatomic molecules under different laser parameters, i.e., wavelength and intensity, and have discovered several novel strong field atomic phenomena and effects. The underlying physical mechanisms behind these findings are uncovered by comparing the experimental data with appropriate theoretical simulations. Our works reveal an important influence of the atomic and molecular structure and the molecular orbital symmetry on the strong field ionization. The major achievements and innovations are listed as follows: 1. We have performed a systematic investigation on the ATI process of the noble gas atoms (Ar, Kr and Xe) and the diatomic molecules (N2and O2) exposed to the intense mid-infrared laser fields. The low-energy part of the measured ATI photoelectron spectra exhibits two unexpected peak-like structures with different energies, i.e., the very low-energy structure (VLES) and the high low-energy structure (HLES). The two structures were observed in all atomic and molecular species investigated and thus seemed to be universal. Moreover, it is found that VLES and HLES exhibit different dependences of laser parameters. These experimental features have been well reproduced by a semiclassical model simulation. By analyzing in detail the calculated photoelectron energy spectra, two-dimensional electron momentum distributions and the initial tunnel ionization phase distributions, we are able to ascribe the production of the low-energy structure to the long-range Coulomb interaction between the parent ionic Coulomb potential and the outgoing electron.2. Resorting to a semiclassical model including a Coulomb potential (CP) softened by different softening parameters, we further reveal that though the very low-energy structure (VLES) and the high low-energy structure (HLES) are both due to the interaction between the ionic CP and the electron, the two structures have different physical mechanisms:the VLES can be attributed to the electron-parent ion Coulomb interaction at a rather small distance and the HLES is more likely to be ascribed to the electron-parent ion Coulomb interaction at large distance.3. We have performed a comparison study of the ionization process of molecules (N2and O2) and their companion atoms (Ar and Xe) with mid-infrared laser wavelengths. Experimental data recorded at a mid-infrared laser wavelength and its comparison with that at a near-infrared wavelength revealed a peculiar wavelength and intensity dependence of the suppressed ionization of O2with respect to its companion atom of Xe, while N2behaves like a structureless atom. It is found that the S-matrix theory calculation can reproduce well the experimental observations and unambiguously identifies that the ionization suppression of O2is due to the effect of destructive interference between ionizing wave packets emitted from the two atomic centers of O2with an antibonding molecular ground state. Our results clarify the underlying physical mechanism of the peculiar ionization suppression behavior of molecule O2, which has been a hot topic of debate for decades.