Theoretical Study On Ionization Of Hydrogen And Molecular Hydrogen Ion In An Elliptically Polarized Laser Field

With the development of laser pulse technology,ultrafast dynamics of the ionization of atoms and molecules driven by strong laser fields have been studied extensively,and the ultrafast dynamics of atomic and molecular ionization is an important basis to understand various natural sciences such as physics,chemistry and biology.In an elliptically polarized laser field,electronic ionization time can be mapped to the photoelectron momentum distribution of the deflection angle due to the rotational property of the laser field.Based on the relation of angle-time mapping,new ultrafast detection techniques of the attosecond clock have been developed.By accurately measuring the deflection angle in the photoelectron momentum distribution,the ionization time of the electron wave packet is obtained,and attosecond temporal resolution is achieved.Therefore,photoionization driven by elliptically polarized laser fields is a hot issue.This thesis focuses on the ultrafast dynamic process of the ionization of atoms and molecules driven by elliptical polarized laser field.The contents are shown as follows,Firstly,we study the photoelectron momentum vortices of the hydrogen atom and hydrogen molecular ion in the elliptically polarized laser fields.Previous studies mostly focused on the amplitude distribution of ionized electrons,and the phase distribution of the wave packet was ignored.The phase distribution of the electron wave packet is key to understanding the wavelike property of the electron and thus it is the basis for matter wave interferometry.The photoelectron momentum vortices of the hydrogen atom ionized by time-delayed counter-rotating laser pulse consisting of a circularly polarized pulse and an elliptically polarized pulse,which is similar to time domain double-slit interference.The phase distribution of the electron wave packet in the elliptically polarized laser field is extracted by the variation of the angular fringe spacing of adjacent interference fringes with the photoelectron emission angle.The maximum value of the phase distribution of the electron wave packet obviously deviates from the result without considering the Coulomb potential,and its value is consistent with the attoclock shift.It is found that the amplitude of the phase distribution is very sensitive to the ellipticity of the laser pulse,providing a new method to precisely calibrate the laser ellipticity.Next,we study the Coulomb asymmetry of the photoelectron momentum distribution for the hydrogen molecular ion driven by elliptically polarized laser field.Due to the existence of multiple nuclei in a molecule,the polyatomic Coulomb potential are formed under the interaction of laser field and Coulomb potential.Electrons can be released by tunneling through different potential barriers.Thus the ionization process is more complicated than that of atoms.In this paper,we study the Coulomb asymmetry of molecules with different internuclear distance ionized by elliptically polarized laser field.With the increase of internuclear distance of molecule,The Coulomb asymmetry is changed with respect to the major and minor axes of the laser ellipse.When the internuclear distance of the molecule corresponding to the enhanced ionization region,the Coulomb asymmetry of the photoelectron momentum distribution is opposite to that of the companion atom,which is mainly due to the effect of the internal scattered electrons.By tracing the electron density and using a classical-trajectory model,it is found that the internal scattered electrons are mainly released at an earlier ionization moment with respect to the peak of the laser electric field within each half cycle.The Coulomb asymmetry of the photoelectron momentum distribution from molecular ionization in elliptically polarized laser pulses records abundant ultrafast dynamic information,which can not only be used to identify the tunneling site of the molecule,but also to record the electron emission time within half a cycle of the laser field.Finally,we study the Autler-Townes effect for the hydrogen molecular ion driven by elliptically polarized laser fields.When the photon energy of the laser is equal to the energy level difference between the ground and first excited states of the molecule,the molecules absorb a single photon and resonatively coupled the two bound states,resulting in the Autler-Townes splitting of the bound state level and the generation of new dreesed state.Via the photoionization,the splitting of the energy level of the bound state can be mapped onto the continuum,which manifests itself as two peak structure in the photoelectron energy spectrum,i.e.,Autler-Townes doublet.We found that the maximal values of the angular distribution of the Autler-Townes doublet reveal different angular shifts with respect to the molecular axis.Using an improved strong-field approximation method,we reproduce the difference of the angular shifts for the Autler-Townes,which originates from the interference of the electron wave packets released from the ground and excited states of the molecule.By tracing the time evolution of the electron density distribution along the molecular axis,the results show that electron delocalizations on the two nuclei appear within several cycles of the laser pulse,i.e.,the spatial distribution of the electrons along the molecular axis is symmetric,and there is a phase jump of π for the phase difference between the ground and first excited states,leading to a distorted interference pattern for the electron electron wave packets released from the ground and excited states.Our study provides unprecedented information about the subcycle dynamics for the Autler-Townes splitting of a molecule.