Investigation Of Ultrafast Dynamics Of Ionization Of Atoms Driven By Strong Laser Fields

Interaction of atoms or molecules with strong laser fields,many new nonlinear and non-perturbative physical phenomena have been observed,such as high harmonic generation(HHG),above threshold ionization(ATI),nonsequential double ionization(NSDI),frustrated double ionization(FDI),etc.The NSDI and FDI contain a lot of physical processes and this paper focuses on these two physical phenomena in atomic system.Recently,atomic FDI has been observed successfully using three-body coincidence detections.However,in theory,the physical conditions of recaptured electrons and the dependence on laser wavelength in the process of atomic FDI have not been solved.In addition,the orthogonal two-color(OTC)field is an important tool used to manipulate ionization processes.The control of the NSDI and FDI by adjusting the OTC field is also worth exploring.In view of the above problems,this paper uses the three-dimensional classical ensemble method to carry out the following research:(1)we theoretically investigated FDI of atoms with different laser wavelengths.Our results show that FDI can be more efficiently generated with shorter wavelengths and lower laser intensities.With proper laser parameters more FDI events can be generated than normal double ionization events.The physical condition under which FDI events happen is identified and explained,i.e.the initial electron velocity at the time of ionization must be equal(or very close)to the laser vector potential at the same time.The energy distribution of the FDI products-atomic ions in highly excited states-shows a sensitive wavelength dependency.(2)we theoretically investigated FDI of atoms driven by the OTC laser pulses.The numerical results show that the FDI yields could be controlled by changing the relative phase of the OTC fields.Moreover,the relative contribution of the two pathways of FDI,corresponding to the processes wherein the firstly or the secondly ionized electron is finally recaptured when the laser field is over,could also be manipulated by changing the relative phase of the OTC fields.The underlying dynamics for this relative-phase dependence of the FDI pathways is revealed by back-tracing the classical trajectories of the FDI events.Interestingly,the energy distributions of the highly excited states of the atom ions in the FDI events also display a sensitive relative phase dependence.(3)we theoretically investigated wavelength-dependent of NSDI driven by the OTC fields.The results show that the probability of NSDI can be controlled by changing the wavelength ratio of the OTC field.The dependence of the NSDI probability on the relative phase is mainly determined by the first electron return probability.The return probability is mainly affected by the travel time of the first electron and the recollision ionization time window.At the same time,the occurrence of NSDI is also related to the kinetic energy of the first electron.The return kinetic energy of the first electron shows a sensitive relative phase dependence and therefore also affects the NSDI probability.