Investigation Of Electron Vortices In Photoionization Of Atom In Bichromatic Circularly Polarized Laser Fields

The interaction between the intense laser field with atoms and molecules contains many high-order nonlinear physical phenomena.More and more researchers have studied and explored these novel physics phenomena.Ionization is one of the fundamental processes within laser-matter interaction.The photoelectron momentum distribution(PMD)of ionized electrons contains a lot of electronic dynamics information and material structure information.Therefore,people pay much attention to the research of the PMD.In this article,we systematically investigate the PMD of hydrogen atoms and helium ions under the action of combined circularly polarized lasers,the main research contents can be summarized as follows:(1)We theoretically study the electron vortices in photoionization of hydrogen atom by bichromatic circularly polarized attosecond pulses by numerically solving the two-dimensional(2D)time-dependent Schr(?)dinger equation(TDSE).Firstly,the effect of time delay on the electron vortex was discussed.When the time delay changes within one cycle,the angle of the vortex-shaped PMD will rotate.And when the time delay varies in multiple cycles,the spiral arms of the vortex pattern will become thinner and longer as the time delay increases.Secondly,the influence of the laser wavelength on the electron vortex was explored.One sees that there are two interference spiral arms with?₁=?₂=50 nm,whereas three spiral arms are induced with?₁=156 nm,?₂=78 nm.However,the PMDs exhibit four arms with?₁=?₂=156 nm.Finally,we discuss the effect of the helicity and carrier envelope phase on the electron vortices.Through the above research,we found that the vortex structure in the PMD of hydrogen atom is sensitive to time delay,wavelength,laser helicity and carrier envelope phase.And we use the PMD of ionized electron wave packet interference to explain the cause of electron vortex in photoionization.The angle of the PMD is accurately calculated by the attosecond perturbation ionization theory,which can realize the control of the PMD.(2)We theoretically investigate the cross channel contributions to the vortex patterns in photoionization of helium ion.First of all,the influence of the time delay on the cross channel is analyzed,and then the contribution of the cross channel to the vortex pattern is explored.When the time delay is short,the cross channel plays an important role in the ionization process,and the intensities of the vortex-shaped PMD are different at this time;when the time delay is long,the contribution of the cross channel is ignored,and the intensities of the spiral arms are the same.It concludes that the uneven distributions of the vortex patterns in the PMDs are mainly due to the ionization cross channel which is sensitive to the time delay.Subsequently,the interference between the electronic wave packets generated by different paths was analyzed to explore the influence of the cross channel on the electron vortex patterns by varying the different wavelengths.For the short time delay,the presence or absence of cross channels has little effect on the electron vortex.However,for the long time delay,the vortex pattern still exists in the case of no cross channels,and the vortex structure almost disappears in the case of having the cross channels.Through the above research,we found the contribution of the cross channel to the electron vortex.The attosecond perturbation ionization theory is used to calculate the transition amplitude generated by the cross channel,and the non-uniform distribution of spiral arms is explained by the interference effect between different channels.(3)The PMD of helium ions under the polarization-gated pulse is studied.When the polarization-gated pulse with the wavelengths of?₁=?₂=20 nm and?₁=?₂=40 nm are used,the PMD shows the two-lobed and four-lobed structure,respectively.But none of them exhibited a vortex structure.Subsequently,we study the PMD of helium ions by tricolor circular laser pules.When the time delay between both laser pules is2 o.c.with the wavelengths of 76 nm and 38 nm,the PMD presents six spiral arms;we add another laser pulse with the wavelength of 19 nm,and the time delay between it and the second laser pulse is 2 o.c..If the helicity of this laser pulse is the same as that of the first laser pulse,the electron vortex pattern shows three arms;if its helicity is the same as that of the second laser pulse,a spiral arm appears in the vortex pattern.By investigating the PMD under the action of the tricolor circular laser field,we find that the time delay between the third laser pulse and the first two pulses,as well as the helicity of the third laser pulse,will affect the electron vortices.(4)We theoretically study the effects of different electronic states as the initial state on the vortex patterns in PMD of helium ions with a pair of counter-rotating circularly polarized attosecond pulses.It concludes that the number of spiral arms in vortex patterns is the same as the number of the absorbed photons when the initial state is the s state and₊p state.However,when the initial state is the excited p_x state,the number of spiral arms in vortex patterns is always two more than the number of the absorbed photons.The initial electron density distributions are presented to interpret the difference of PMDs on the initial electronic state.We proposed to use the interference between wave packets to explain the effect on the number of vortex arms.The physical mechanism of the difference in intensity of the electron vortex under different conditions is deeply studied.We compare the PMDs of different initial electronic states with the same wavelengths and find that the physical mechanisms underlying the creation of the discrepancy in distribution are different.The attosecond perturbation ionization theory and the initial wave packet interference are used to illustrate these phenomena.Finally,we use p₊ state and p_- state as the initial electronic state to explore the effect of orbital spin on the electron vortex.In our research,we have discovered the dependence of the electron vortex on the initial electronic state and orbital spin.