Electron Dynamics Manipulation And Nonadiabatic Effect In Strong Field Atomic Ionization

The interaction between high intensity laser pulses and atoms or molecules can produce a series of nonlinear physical phenomena that cannot be explained by traditional perturbation theory,such as above threshold ionization,nonsequential double ionization,high-order harmonic generation and so on.The three-step model based on recollision process provides a basic physical image for understanding these physical phenomena,i.e.,the bound electrons pass through the Coulomb barrier suppressed by the laser field through the tunneling effect,then the moving direction of electrons is reversed under the action of laser field,and finally the electrons may return to the parent nucleus and recollide.The tunneling process is the first step in the recollision process,and the accurate description of the initial state of the tunneling electron wave packet is of great significance for the accurate description of various strong field physical phenomena.The recollision process is the immediate cause of many strong field physical phenomena.Studying the electron dynamics in this process is helpful to understand the physical images of strong field phenomena,and to control the results of strong field phenomena by controlling the electronic behaviors in the recollision process.In this paper,the above two processes in the recollision model are studied.By designing an experimental scheme with two elliptically polarized laser fields,the characteristic structure of the nonadiabatic effect of electron tunneling is observed for the first time.At the same time,the electron dynamics in recollision process of single ionization and non-sequence double ionization are controlled by using the two-color laser field.The main research contents of this paper are shown as follows,Firstly,the formation of photoelectron angular distributions from above-threshold ionization of xenon atomic under parallel two-color laser field is studied.By measuring the photoelectron momentum spectrum from above-threshold ionization of xenon atoms in the parallel-polarized two-color laser field,it is found that the electron yield has different asymmetrical distribution under different electron emission angles and relative phases of two-color field.Based on a semi-classical model,the relative contributions of non-scattering and rescattering trajectories on the photoelectron angular distributions of the above-threshold ionization are separated,and it is proved that the rescattering electrons make a nonnegligible contribution to the formation of the photoelectron angular distributions in the two-color laser field.Depending on the relative phase between the two colors,the rescattering trajectories can be emitted to the direction with a large angle relative to the laser polarization,and the electrons along the laser polarization direction are mainly dominated by the nonscattering trajectories.The relative contributions of the nonscattering and the rescattering trajectories differ significantly for different emission angles,leading to the angular-dependent asymmetry.By tuning the relative phase of the two-color pulse,the electron emission angle and the relative contributions of the rescattering trajectories can be precisely controlled.Secondly,the recollision time of the electrons in the nonsequential double ionization is controlled by using the two-color laser field.It is found that the momentum distribution of ions oscillates obviously with the change of relative phase of the two-color laser field.A semi-classical ensemble model is used to analyze the momentum distribution of the associated electrons and it is found that the energy sharing between the two doubly ionized electrons at the moment of recollision is extremely uneven.Further analysis shows that the shift of the ion momentum corresponds to the change of electric field vector potential at the time of electron recollision,so the recollision time can be mapped onto the ion momentum.By adjusting the relative phase of the two-color laser field,the recollision time of the electrons in the nonsequential double ionization can be controlled with the accuracy of attosecond time.Finally,the nonadiabatic effect of the strong laser field tunneling ionization is studied.An experimental scheme is designed to detect the nonadiabatic behavior of tunnel ionization,without counting on the laser intensity calibration or the modeling of the Coulomb effect.In this scheme,the degree of nonadiabaticity for tunneling scenarios in elliptically polarized laser fields can be steered continuously simply with the pulse ellipticity,while the critical instantaneous vector potentials remain identical.We observe the characteristic feature of the measured photoelectron momentum distributions,which matches the distinctive prediction of nonadiabatic theories.Further experiments show that the nonadiabatic initial transverse momentum at the tunnel exit is approximately proportional to the instantaneous effective Keldysh parameters in the tunneling regime,and the adiabatic approximation for tunnel ionization in a rotating field is inaccurate under the typical conditions in the laboratory.At the same time,based on the strong-field approximation,we obtain analytical expressions for the initial momentum for the most probable trajectory at the tunnel exit and instantaneous ionization rate of tunneling ionization in elliptically polarized laser fields with arbitrary ellipticity.We further found that the initial momentum of the tunneling electron has an important effect on instantaneous ionization rate.These study results are significant for the improvement of many semiclassical models in strong-field physics,and have important implications in the appealing strong-field phenomena which are triggered by tunnel ionization in rotating laser fields,e.g.,elliptically polarized high-harmonic generation,nonsequential double ionization,spin polarization,and photoelectron holography.