低于再碰撞阈值下强激光场原子非序列双电离

在最近的实验和理论研究中,我们探讨了氩原子和氖原子在红外强激光场中低于再碰撞阈值的非序列双电离问题。在研究中,我们发现在非序列双电离过程中,氖原子的电子关联表现为在激光偏振面内肩并肩出射,而对于氩原子的电子关联行为表现为在激光偏振面内的背对背出射,我们采用三维半经典模型（考虑电子隧道电离）很好地解释了实验结果。在阈值附近,我们发现电子在激光场中的多次散射以及电子再碰撞激发后电子隧道电离是氩原子反关联行为的主要原因,而电子在激光场作用下的单次碰撞是电子关联行为的主要原因。通过测量双电离过程中产生电子的横向电子动量分布,观察到了库伦聚焦效应,我们认为这是非经典的关联行为。最后,我们给出了氩原子和氖原子在激光场中阈值的解析模型,并给出了原子的关联和反关联激光强度区域.     

低于再碰撞阈值下强激光场原子非序列双电离(英文)

在最近的实验和理论研究中,我们探讨了氩原子和氖原子在红外强激光场中低于再碰撞阈值的非序列双电离问题。在研究中,我们发现在非序列双电离过程中,氖原子的电子关联表现为在激光偏振面内肩并肩出射,而对于氩原子的电子关联行为表现为在激光偏振面内的背对背出射,我们采用三维半经典模型(考虑电子隧道电离)很好地解释了实验结果。在阈值附近,我们发现电子在激光场中的多次散射以及电子再碰撞激发后电子隧道电离是氩原子反关联行为的主要原因,而电子在激光场作用下的单次碰撞是电子关联行为的主要原因。通过测量双电离过程中产生电子的横向电子动量分布,观察到了库伦聚焦效应,我们认为这是非经典的关联行为。最后,我们给出了氩原子和氖原子在激光场中阈值的解析模型,并给出了原子的关联和反关联激光强度区域。     

Multiple recollisions in nonsequential double ionization below the recollision-ionization threshold

By using the three-dimensional classical ensemble model, the recollision dynamics in nonsequential double ionization (NSDI) of Ar by 780-nm laser pulses at (6-1.2)×10¹⁴ W/cm² was extensively studied. We revealed the picture of multiple-recollision in the double ionization events at the laser intensity region below the recollision-ionization threshold. Via tracing the NSDI trajectories, it was found that the contribution of these multiple-recollision events increases as the laser intensity decreases. In this low intensity region, many multiple-recollision induced NSDI trajectories occur through the doubly excited states. The decay speed of the doubly excited state decreases with the decreasing laser intensity.     

Transition from strong-field sequential to nonsequential double ionization at near-infrared wavelengths and low intensities

Within a quantum-mechanical model, we investigate strong-field double ionization of a model helium atom by near-infrared, linearly polarized laser pulses at intensities far below the recollision threshold. The quantum simulations show a clear mechanism change from sequential to nonsequential double ionization (NSDI) as the laser intensity increases. For NSDI, the two-electron correlated momentum distribution exhibits a strong final-state Coulomb repulsion effect for high-energy photoelectrons, but absent for low-energy photoelectrons. This repulsion effect is ascribed to field double ionization from doubly-excited states populated by recollision of the first ionized electron when it returns to the parent ion. Such recollision-induced excited states are absent at ultraviolet wavelengths due to the very low returning kinetic energies, resulting to the absence of final-state repulsion effect in NSDI.     

Influence of molecular structure on double ionization of N2 and O2 by high intensity ultrashort laser pulses

The electron momentum correlation after nonsequential double ionization of N2 and O2 in ultrashort light pulses at light intensities near 1.5 x 10(14) W/cm(2) has been investigated. The experimental results reveal distinctive differences between the molecular species and between molecules and atoms of similar ionization threshold. We provide evidence that recollision double ionization is the essential mechanism and trace the origin of the differences back to the symmetry of the orbitals occupied by the valence electrons.     

Effect of elliptical polarizations on nonsequential double ionization in two-color elliptically polarized laser fields

Using the classical ensemble model, we investigate the nonsequential double ionization(NSDI) of Ar and Mg in the two-color elliptically polarized laser pulse for different elliptical polarizations. Numerical results show that for Ar atoms the NSDI yield increases as the ellipticity increases, which is different from the case of Mg atoms. Moreover, the correlated behavior in the correlated electron momentum along the x direction and ion momentum distributions of Ar atoms are influenced by the ellipticity. By statistical analysis of different times, we can conclude that the ellipticity may be responsible for the NSDI processes. The correlated momenta distributions along the x direction at the recollision time are demonstrated and the results show that the travelling time and ellipticity can affect the emitted directions of both electrons.     

周期量级激光脉冲驱动下非次序双电离的三维经典系综模拟

利用三维经典系综模型研究了周期量级激光脉冲驱动的氩原子非次序双电离,所得结果表明,Ar2+离子的纵向动量分布与载波包络相位有很强的依赖关系,随载波包络相位φ的增加,具有不对称双峰结构的离子纵向动量分布重心从负动量转移到正动量,并且φ每改变π时Ar2+离子的纵向动量呈现相反的分布.在重碰撞过程中核与电子之间的库仑势发生变化后,计算得到的Ar2+离子纵向动量分布随载波包络相位的变化与实验结果定量上一致.     

碰撞阈值下氩原子非次序双电离

利用三维经典系综模型研究了碰撞阈值下氩原子的非次序双电离.计算结果表明,关联电子末态纵向动量主要分布在二、四象限,且在原点附近几乎没有分布;Ar²⁺离子末态纵向动量谱在零动量附近呈单峰结构.上述结果与实验结果 定量一致.轨迹分析表明,在碰撞阈值下,氩原子非次序双电离的微观物理机理在不同激光强度下是不相同的.当激光强度I=0.7×10¹⁴ W/cm²时,一次碰撞主导重碰撞过程.而当I=0.4×10^{14关键词： 非次序双电离 库仑引力 碰撞阈值 电子关联}     

Recollision dynamics and phase diagram for nonsequential double ionization with circularly polarized laser fields

A semiclassical quasistatic model is used to investigate the recollision dynamics in circularly polarized laser fields. A velocity window for recollision to occur is found. Only when the return electron's orbits are irregular does significant double ionization take place. The model reproduces the experimental results for magnesium and explains the apparently conflicting experimental results in terms of an analytical formula that demarcates the phase diagram for the nonsequential double ionization in circularly polarized laser fields.     

Attosecond correlated dynamics of two electrons passing through a transition state

The strong-field induced decay of a doubly excited, transient Coulomb complex Ar**→Ar(2+)+2e(-) is explored by tracing correlated two-electron emission in nonsequential double ionization of Ar as a function of the carrier-envelope phase. Using <6 fs pulses, electron emission is essentially confined to one optical cycle. Classical model calculations support that the intermediate Coulomb complex has lost memory of its formation dynamics and allows for a consistent, though model-dependent definition of "emission time," empowering us to trace transition-state two-electron decay dynamics with sub-fs resolution. We find a most likely emission time difference of ～200±100 as.     

非序列双电离向饱和区过渡的电子最大关联度

本文采用三维半经典再散射模型系统地研究了 He 原子在 光强从域下到近序列双电离区强场作用下的非序列双电离问题, 统计了各种场强下最终电离的两个电子正负关联的几率之比, 发现发生最大正关联的场强处于向非序列 双电离饱和区过渡的区域, 并在关联纵向动量谱中展示了形成最大关联度的过程. 最后通过轨道"回溯"分析, 进一步研究了其背后的物理机理, 发现在碰撞后其中一个电子再散射和隧穿电子高速弱碰撞分别 是低场强和高场强下负关联比例增加的原因.     

Binary and recoil collisions in strong field double ionization of helium

We have investigated the correlated momentum distribution of both electrons from nonsequential double ionization of helium in a 800 nm, 4.5 x 10(14 W/cm2 laser field. Using very high resolution coincidence techniques, we find a so-far unobserved fingerlike structure in the correlated electron momentum distribution. The structure can be interpreted as a signature of the microscopic dynamics in the recollision process. We identify features corresponding to the binary and recoil lobe in field-free (e,2e) collisions. This interpretation is supported by analyzing ab initio solutions of a fully correlated three-dimensional helium model.     

Strong-field double ionization of Ar below the recollision threshold

Nonsequential double ionization of Ar by 45 fs laser pulses (800 nm) at (4-7)x10;{13} W/cm;{2} was explored in fully differential measurements. Well below the field-modified recollision threshold we enter the multiphoton regime. Strongly correlated back-to-back emission of the electrons along the polarization direction is observed to dominate in striking contrast to all previous data. No effect of Coulomb repulsion can be found, the predicted cutoff in the sum-energy spectra of two emitted electrons is confirmed, and the potential importance of multiple recollisions is discussed.     

Nonsequential double ionization of Xe by mid-infrared laser pulses

With the three-dimensional semiclassical ensemble model, we studied the correlated electron dynamics in strong-field nonsequential double ionization of Xe by mid-infrared laser pulses over a wide range of laser intensities. Numerical results show that the correlated electron momentum distribution exhibits the strong back-to-back pattern at the lower laser intensity. It evolves into the side-by-side behavior as the laser intensity increases and presents the experimentally observed crosslike shape at high laser intensity. Different from the case of near-infrared region where recollision mainly occurs at the first returning, at the mid-infrared region recollision dominantly occurs at the later returnings. The windows of initial transverse velocity for the various multiple-returning recollision trajectories are different and are unambiguously determined by this semiclassical ensemble model.     

Microscopic Dynamics of Nonsequential Double Ionization by Elliptically Polarized Few-Cycle Laser Pulses

With the classical ensemble model, we investigate nonsequential double ionization (NSDI) of xenon atoms using 780 nm, 0.25 PW/cm2elliptically polarized few-cycle laser pulses. The momentum distribution of correlated electron along the long axis of the laser polarization plane shows an obvious V-like structure locating at the third quadrant, and the momentum along the short axis of the laser polarization plane are mainly distributed in the second and fourth quadrants. Moreover, we demonstrate that the Coulomb repulsion interaction plays a decisive role to the above results.By back analyzing the classical trajectories of NSDI, we find that there are two kinds of recollision trajectories mainly contribute to NSDI, and the different microscopic dynamics for the two kinds of trajectories are clearly explored.     

氦原子非次序双电离对正交双色场强度比的依赖关系

利用经典系综模型研究了正交双色场中氦原子非次序双电离对双色场强度比的依赖关系.研究表明, 该依赖关系与双色场相对相位φ 有关. φ =0.25π 时,沿长波长激光偏振方向的相关动量谱随强度比的增大从相关模式转变为反相关模式. φ =0.35π, 0.45π 时,相关动量主要分布在第一和第三象限,相关模式几乎不随强度比的变化而变化.对双电离轨迹碰撞时间、碰撞角、碰撞动量的向后分析可以解释上述结果,并显示了正交双色场对非次序双电离中碰撞时间、碰撞角的控制作用.     

Dynamics of strong-field double ionization of H_2 in co-rotating two-color circularly polarized laser fields

Double ionization of H_2 in a co-rotating two-color circularly polarized(TCCP) laser field is theoretically investigated. By changing the ratio of electric field peak amplitudes of the TCCP laser pulses, the double ionization probability as a function of the laser intensity shows a clear knee structure, which is suppressed significantly in the case of the atom. Due to the large spatial range of the electronic initial distribution, with the analysis of classical trajectories of ionized electrons, it is found that the ionization of the electron in the farther distance increases the probability of recollision. Furthermore, the yield of nonsequential double ionization created by the recollision can be enhanced by controlling the amplitude ratio of the TCCP laser field.     

Electron correlations in nonsequential double ionization of argon atoms by elliptically polarized laser pulses

Using a classical ensemble model, we investigate the correlation behaviour of electrons originating from nonsequential double ionization (NSDI) of argon atoms by the elliptically polarized laser pulses. Because of the ellipticity, not only the first electron to return but also the later return of tunneled electrons contribute significantly to NSDI. We mainly discuss two kinds of events of NSDI originating from the first and the second return separately. For the NSDI resulting from the recollision of the first return, the correlated electron momentum spectrum along the long axis of the laser polarization plane reveals an obvious V-like shape, located at the first and third quadrant. However, for the NSDI resulting from the recollision of the second return, the momenta of two electrons are distributed in the four quadrants uniformly. By analysing the trajectories of these two kinds, we find that the recollision energy and the laser phase at recollision are different for the first and second returning trajectories, which are responsible for the difference in the correlated behavior of the final electron momentum.     

Nonsequential two-photon double ionization of atoms: identifying the mechanism

We develop an approximate model for the process of direct (nonsequential) two-photon double ionization of atoms. Employing the model, we calculate (generalized) total cross sections as well as energy-resolved differential cross sections of helium for photon energies ranging from 39 to 54 eV. A comparison with results of ab initio calculations reveals that the agreement is at a quantitative level. We thus demonstrate that this complex ionization process can be described by the simple model, providing insight into the underlying physical mechanism. Finally, we use the model to calculate generalized cross sections for the two-photon double ionization of neon in the nonsequential regime.     

Controlling electron collision by counterrotating circular two-color laser fields

Electron collision as well as its controlling lies in the core of study on nonsequential double ionization(NSDI).A single collision occurred in a convergent time is important to disclose the essential features of the electron correlation.However,it is difficult to form such a collision.By using counterrotating circular two-color(CRTC)laser fields,we show that a single electron collision can be achieved in a convergent time and a net electron correlation is set up within the sub-femtosecond time scale in the NSDI process of Ar atoms.The proposed method is also valid for other atoms,provided that one chooses the frequency and intensity of the CRTC field according to a scaling law.