The Influence Of Atomic Orbital Angular Momenta On Photoelectron Vortices And Scattering Phases

The interaction between intense laser and matter produces many new physical phenomena,such as the generation of high order harmonics,Nonsequential double ionization and so on.The first step in all of these strong field phenomena is strong field ionization.Strong field ionization of atoms is the simplest ionization model,which plays an important role in the ionization dynamics research of complex systems,and is of great significance to the research of strong field physics.Although strong field ionization of atoms has received a lot of attention,the hot issues such as the defintion of the transition regime from tunneling to multiphoton ionization and the influence of the atomic orbital angular momentum on strong field ionization have not been solved.In view of the above problems,the following research work is carried out in this thesis.(1)The transition regime from tunneling to multiphoton ionization is quantitative defined.The average ionization delay time as a function of Keldysh parameter obtained by virtual detector,with two distinct increasing trends and a plateau-like structure,can serve as a more reliable indicator for characterizing the transition from tunneling to multiphoton ionization.The method of using Gaussian functions fitting the probability currents is proposed,in which the probability currents can be decoupled into weighted tunneling and multiphoton ionization portions.This confirms that the observed plateau-like structure in intermediate regime originates from the competition between tunneling and multiphoton ionization.A hybrid quantum and classical approach is developed to obtain the fraction of tunneling electrons under different Keldysh parameters,in which the boundary of the transition regime can be clearly defined.Moreover,the well separated tunneling and multiphoton electrons are further propagated forward in classical way for reproducing the signatures of photoelectron momentum distributions and energy spectrums.(2)A scheme is proposed to detect ring currents associated with atomic orbital angular momenta via photoelectron vortices.The three-photon ionization system is designed by a counter-rotating circularly polarized pulse pair.In the case of the first pulse opposite to the ground state,the two-photon resonance excitation can introduce an additional cross path,which break the rotational symmetry of the electron vortices.Hence,the defined sign of the ring current can be judged according to the character of electron vortices in the counter-rotating circularly polarized pulse pairs with a different sequence.Furthermore,the vortex-shaped photoelectron momentum distributions for the superposition states composed of p_±orbitals in different proportion are dependent on the dominant ring current.It is proved that this method can also be used to detect the net ring current of superposition states.In addition,by comparing with the results of non-resonant five-photon ionization,it is proved that this method is strongly dependent on resonant enhanced ionization.(3)The influence of atomic orbital angular momentum on photoelectron Wigner phase shift is investigated.A scheme to extract the Wigner phase shift is designed by a strong left-handed circularly polarized laser field and a weakly linearly polarized laser field.The Wigner phase shift can be used to characterize the different interactions between the rotating electrons and the Coulomb potential.It is found that the Wigner phase shifts between the_+p and p_-orbitals are negative in one-photon ionization,but positive in three-photon ionization.It is found that in both one-and three-photon ionization schemes the electrons liberated more easily by the circularly polarized laser field suffer less influence of the Coulomb potential and then accumulate less Winger phase.This result indicates that the strength of the interaction between the rotating electrons and the Coulomb potential can explain the helicity-dependent ionization for different ionization mechanisms universally.Moreover,the classical ensemble model is used to make supporting calculations by analyzing the electronic classical trajectories.In addition,it is found that the phase jump caused by resonance ionization can propagate through the above-threshold ionization peaks,which goes against the extraction of Wigner phase shift.