Theoretical Simulation For Asymmetrical Photoelectron Momentum Distributions Of Helium Ion In Dichromatic Laser Fields

The rapid development of laser technology provides an effective tool for physicists to explore the electronic dynamics in the microscopic world.When an atom or a molecule interacts with the strong laser field and,a variety of high order nonlinear physical phenomena are produced,such as above threshold ionization,nonsequential double ionization,tunneling ionization,multiphoton ionization,high-order harmonics generation and so on,and attract a lot of attention.Multiphoton ionization,one of the basic ionization processes,is our main research.Based on the solution of the time-dependent Schr?dinger equation,we study the one-dimensional and two-dimensional photoelectron momentum distributions of helium ion.The concrete contents of this paper as follows:(1)We theoretically investigate the property of ionization for the interaction between one dimensional helium ion and dichromatic linearly polarized laser pulses.In the monochromatic laser field,there is only one ionization pathway,and the photoelectron momentum distribution obtained is symmetrical.The distributions of photoelectron momentum are asymmetrical both in dichromatic laser fields with zero and nonzero time delays.In order to explore the physical mechanism of the asymmetry,we theoretically analysis the two ionization pathways in dichromatic laser pulses,i.e.,indirect ionization process and direct ionization process.First,by controlling the intensity of the two laser pulses,we respectively simulate the wave functions of the two ionization processes.Next,the wave functions of the two ionization processes are superposed based on the superposition principle.Finally,the asymmetrical photoelectron momentum distribution is obtained.Through above analysis,we attribute the asymmetrical photoelectron momentum distribution to the interference of the two ionization pathways.(2)For comparison with the photoelectron momentum distribution in experiment,we study the momentum distribution of helium ion in dichromatic circularly polarized laser fields.In this work,we solve the two-dimensional time-dependent Schr?dinger equation by using the soft-core potential in right-circularly polarized dichromatic laser fields.The results show that the asymmetrical round photoelectron momentum distribution in the zero time delay and the asymmetrical spiral-loop photoelectron momentum distribution in the nonzero time delay.In the dichromatic circularly polarized laser fields,the electron is ionized in two pathways,which is same as that in the one dimensional dichromatic laser fields.However,the first excited state is the degenerate state with 2_p+state and 2_p-state in the two-dimensional case.By examining the populations of the two degenerate states,we find that the electron can jump into the 2_p-state from the ground state firstly,and then is ionized.In order to explore the physical mechanisms of asymmetrical round photoelectron momentum distribution and the asymmetrical spiral-loop photoelectron momentum distribution,we subtract the 2_p-state from the propagating wave function to suppress the indirect ionization process and obtain the symmetrical round distribution.Thus we infer that the above photoelectron momentum distributions are caused by the interferences of the two ionization pathways.To verify our reasoning further,we split the two ionized process firstly and then superpose them,we obtain the asymmetrical round photoelectron momentum distribution in the zero time delay and the asymmetrical spiral-loop photoelectron momentum distribution in the nonzero time delay are obtained by using the method as same as in the one dimensional case.