Experimental Investigations On Atomic Ultrafast Ionization Dynamics In Strong Laser Fields

With the development of ultrafast laser technology, light-matter interaction enters a new era. When subjected to an ultrashort strong laser pulse, an atom may exhibit a variety of novel ultrafast phenomena, such as above-threshold ionization (ATI), non-sequential double ionization (NSDI), high-order harmonic generation (HHG), and so on. These findings provide new methods to investigate ultrafast imaging of atomic and molecular dynamics and electron-electron correlation, and practical method to generate attosecond pulses. Presently, electron dynamics in atoms and molecules can be investigated and manipulated in attosecond time-scale, resulting in the flourish of a new science: attosecond physics.Ionization is the primary response of an atom to an intense laser field, and serves as the basis for understanding of the more complex processes in the context of intense laser-atom interaction. For linear polarization, the study of strong field ionization has gained fruitful achievements, with discovery of new experimental features and uncovery of new physical mechanisms. While for elliptical polarization, with a non-vanishing electric component perpendicular to the major polarization axis, ionization dynamics shows even more aspects, which receives extensive attention nowadays. In this thesis, we investigate atomic above-threshold ionization and non-sequential dou-ble ionization in strong laser fields under various parameters, e.g., polarization, pulse duration and pulse chirp. The major achievements and innovations are listed as fol-lows:1. High-order above-threshold ionization of Ar, Kr and Xe in elliptically polar-ized laser fields. With increasing of ellipticity, different part of the plateau in photo-electron spectra exhibits different decline in yields. Based on quantum orbit theory, the experimental features are ascribed to the ellipticity-dependence of electron recol-lision orbits. Our results show that with increasing ellipticity, contribution of multiple return collisions becomes more and more important.2. Non-sequential double ionization of argon in many-cycle and few-cycle ellip-tically polarized laser pulses. With increasing ellipticity the ratio between double and single ionization yields decreases faster for few-cycle pulses than that for many-cycle pulses. This feature can be attributed to the suppression of multiple return collisions in few-cycle pulses. Based on the recollision model, the dependence of electron recolli-sion orbits on ellipticity is investigated, which is closely related to the initial transverse momentum distribution of tunneling electron.3. Tunneling ionization in elliptically polarized laser fields. In experiments with Keldysh parameter γ≈1, single ionization yields decrease rapidly with increasing ellipticity, and the decrease becomes more dramatic for lower laser intensity. The experimental features can be qualitatively reproduced by ADK theory, which is based on the quasi-static approximation, and S-matrix theory, in which the non-adiabatic effect is included. Quantitatively, the S-matrix calculations are in a better agreement with the experimental data than the ADK simulations. Our investigation implies that the non-adiabatic effect may play an important role in tunneling ionization with γ≈1.4. Above-threshold ionization of xenon in femtosecond chirped pulses. Chirp dependence of Freeman resonance is found for the first time in experiments. For the up-chirped pulses the energy of the resonant peak is higher, while it is lower for the down-chirped pulses. Our experimental observation provides a support that the Free-man resonance fine structure is due to a multi-photon excitation with subsequent ion-ization in a two-step process, rather than an excitation and instantaneous ionization in an one-step process.