Studies On The Ultrafast Dynamics Of Atom And Molecules With Cold Target Recoil Ion Momentum Spectroscopy

Advances in ultrafast laser technology have opened the door to the new region of the intense laser-matter interaction. By measuring the momenta of the fragments induced by the interaction between the femtosecond laser pulses and the gas phase targets, the ultrafast dynamics in the ionization of atoms and molecules can be investigated. In this dissertation, we investigate the double ionization dynamics of molecular hydrogen in the few cycle laser pulses and the carrier envelope phase (CEP) effect in the ionization of the atoms with a newly home-made COLTRIMS combined with the phase-stabilized few-cycle laser pulses. The main contents are listed as follows:1. The COLTRIMS system which mainly includes three parts, vacuum chambers, cold gas jet and the device that resolves momentum of ions (and electrons), are illustrated from the building process and working principle of each part. By choosing accurate operation parameters, the ion and electron momentum can be measured in 4Ï€ solid angle under high resolution. This lays a solid foundation for studying the ultrafast dynamics in the ionization of atoms and molecules subjected to ultrashort intense laser field.2. We experimentally investigate the double ionization dynamics of molecular hydrogen in ultrashort intense laser fields. The total kinetic energy release of the two coincident H+ ions, which provides a diagnosis of different processes to double ionization of H2, is measured for two different pulse durations, i.e.,25 and 5 fs, and various laser intensities. It is found that, for the long pulse duration (i.e.,25 fs), the double ionization occurs mainly via two processes, i.e., the charge resonance enhanced ionization and recollision-induced double ionization. Moreover, the contributions from these two processes can be significantly modulated by changing the laser intensity. In contrast, for a few-cycle pulse of 5 fs, only the recollsion-induced double ionization survives, and in particular, this process could be solely induced by the first-return recollision at appropriate laser intensities, providing an efficient way to probe the sub-laser-cycle molecular dynamics.3. The CEP effects during the ionization of Argon driven by few cycle laser pulses are investigated experimentally and theoretically. We experimentally measure the differential momentum distributions of Ar+ and Ar2+ along the laser polarization direction by phase-stabilized few-cycle laser pulses. The left-right asymmetry of the measured momentum spectra exhibits a strong CEP dependence. Moreover, we find that there is a phase shift of the asymmetry curve as a function of CEP when the laser intensity increases. In order to understand the physical mechanism under these phenomena, we theoretically study the strong-field ionization of atom in few-cycle pulses with semiclassical method, the Coulomb-Volkov distorted-wave approximation (CVA) theory and strong-field approximation (SFA) theory, respectively. The measurements can be well reproduced by the CVA theroy. Our simulations show that the deviation of the asymmetry curves in CVA from the SFA simulations is attributed to the significant influence of the Coulomb potential on the forward rescattering electron which will get close to the core again after tunneling ionization. Furthermore, the laser-intensity dependence of the phase shift of the asymmetry curves in CVA is elucidated.