Complex Dynamics Of Atomic And Molecular Ionization In Ultra-short Super-strong Laser Fields

During the past decades, the investigation of atoms and molecules interacting with ultra-short super-strong laser fields has lead to the discovery of a series of non-perturbative ion-ization phenomena, including multi-photon ionization, above-threshold ionization, tunneling ionization, over-the barrier ionization, and non-sequential double ionization (NSDI), to name only a few. Among all, NSDI is the most complex because it deals with the correlated dy-namics of two highly entangled electrons.The first experimental evidence of non-sequential double ionization can be dated back to the early 1980s. Up to now, it has received intensive studies for more than two decades. In spite of this, novel experimental data still constantly emerge and challenge our exist-ing knowledge. For instance, with the help of many-particle imaging techniques, it is now possible to record differential information on strong-field few-electron reactions. Plotting these data in the parallel momentum plane of (p1‖, p2‖), two experimental groups indepen-dently found a surprising V-shaped (or finger-like) structure in the first and third quadrants for helium NSDI that was not observed before, giving rise to an extensive discussion on its underlying physical mechanisms. Moreover, the pattern is found to sensitively depend on the laser intensity and vary with the atomic species. With decreasing the intensity slightly from 0.09 PW/cm2 to 0.07 PW/cm2, it was found that a transition from correlation (domi-nant population in the first and third quadrants) to anticorrelation (dominant population in the second and fourth quadrants) emerged for argon NSDI. This kind of transition, however, is not observed yet for neon even though the laser intensity was decreased down to the regime where no NSDI event was detected over weeks of data collection. These observations have triggered a new surge of investigation on NSDI.The explanation of these surprising observations constitute the main purpose of this thesis, which is divided into eight chapters, as following:In the 1 st chapter, after briefly introducing the ultra-short super-strong laser technology, we turn to review some remarkable breakthroughs in the field of laser-atom(-molecule) in-teraction with focusing on the investigating history, the newest experimental observations, and the existing theoretical approaches for the hot topic of non-sequential double ionization. We also point out the theoretical drawbacks and introduce some of our main achievements in this field.The next chapter details the semi-classical theories, the simple-man model and the semi-classical rescattering model set up by Jie Liu et al.. We also show, how to extend this ded-icated model to the regime below the recollision threshold, to the molecular case, to the over-the-barrier regime, and to the relativistic regime.In the 3rd chapter, we investigate the underlying mechanisms that give rise to the pe-culiar V-shape structure in the correlated momentum distribution. The distinctively different role of laser field, e-e Coulomb repulsion, and nuclear Coulomb focusing effect are identified with virtual'numerical experiments'. It was found that the field-assisted backscattering upon the parent core is decisive for the production of V-shape structure. This conclusion is con-firmed with tracing back the evolution history of individual trajectories. We also analyze the dependence of the correlated momentum spectra upon the relative perpendicular momentum between two electrons, and reveal the physical origin of the butterfly-like structure observed in earlier results based on direct solution of the 1D Schrodinger equation.In the 4th chapter, we investigate the strong-field double ionization at the transition to below the recollision threshold. We find that both multiple recollision and recollision-induced excitation-tunneling significantly contribute to the anti-correlated emission in this extremely low intensity regime. While the former is a purely classical mechanism, the lat-ter is inherently quantum-mechanical. The quantitative contribution of these two different mechanisms can be computed numerically, and more importantly, the different footprints left by them are identified. Moreover, we also perform some analytical deduction, yielding the new criterion of the threshold intensity that demarcate the correlated and anti-correlated regime, which show good agreement with the existing experimental data.In the 5th chapter, we present model calculations for single and double ionization of Ar and Ne in infrared laser fields in the DI threshold regime, at the lowest intensities ever inves-tigated. Whereas for Ar anticorrelation in the longitudinal two-electron momentum spectra is observed in agreement with previous measurements at slightly higher intensity, correlation dominates for the Ne target similarly close to the threshold. Further lowering the intensity led to the complete absence of any double ionization event over weeks of data taking time. Inspecting the transverse momentum distribution of electrons coincident with singly and doubly charged Ne and Ar ions, we shed light on the dynamics and provide clear evidence that our essentially classical calculations do not capture all of the dynamics:Whereas the cusplike behavior for single ionization is in good agreement with the experimental data, the experimental transverse electron momentum distribution for Ne2+ is significantly broader as predicted pointing to the existence of correlated quantum motion not taken into account by the classical model and underlining the quest for many-particle quantum approaches.In the 6th chapter, we investigate the nonsequential double ionization induced by few-cycle laser pulses with focusing on the carrier-envelope phase effect. We show that with adjusting the absolute phase we can steer the ionization time, return energy of the tunneled electron, and thus manipulate the recollision dynamics. Such manipulation is found to be most efficient at relatively lower intensity. Meanwhile, we also look insight into the sub-cycle dynamics of the correlated electrons, improving our understanding about the underly-ing mechanisms.The 7th chapter focuses on the atomic ionization processes in the relativistic regime. The current research interest in this regime is stimulated by the following two considera-tions:i. the traditional measurement of less powerful lasers becomes less viable because the mirrors involved in the measurement process may not sustain such intensities anymore. We discuss the possibility of determining the peak intensity of a super-strong laser pulse via the ionization fraction of atoms. ii. since the electrons are speeded up to near the speed of light in a fraction of an optical cycle and the Lorentz force starts to play important role, the return probability of an electron to the parent ion is significantly reduced. We thus briefly introduce several approaches to enhance the recollision processes.In the last chapter, with N2 as an example, we demonstrate the alignment effect of the molecular double ionization. With tracing back the dynamical evolution of lots of individual trajectories at collision and post-collision, we sketch several typical trajectories. We find that, in contrast to atomic NSDI, the time delay between recollision and double ionization is surprisingly long in the molecular case, up to several optical cycles. We propose to observe this time delay effect by comparing the correlated momentum spectra with longer and shorter laser pulses. Finally, we close the thesis by drawing a conclusion on the major achievements and giving a perspective on the promising directions in strong field double ionization.