椭圆偏振激光脉冲驱动的氩原子非次序双电离

利用经典系综模型研究了椭圆偏振激光脉冲驱动的氩原子非次序双电离.计算结果表明,非次序双电离产率随着椭偏率的增大而减小;双电离得到的电子对在激光偏振平面长轴方向的末态关联动量谱呈现正关联,在激光偏振平面短轴方向的末态关联动量谱呈现反关联;Ar²⁺在激光偏振平面短轴方向的末态动量谱呈现单峰结构,并且随着椭偏率增大而变宽.轨迹分析显示,椭圆偏振激光脉冲驱动下,非次序双电离仍然是通过再碰撞而发生;随着椭偏率的增大,有效碰撞和单电离之间的时间延迟增加,这是因为椭偏率较大时第一个电子需要经过多次往返才能与母核离子发生有效碰撞.     

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。     

椭圆偏振激光脉冲驱动的氩原子非次序双电离对激光强度的依赖

利用经典系综模型研究了椭圆偏振激光脉冲驱动的氩原子非次序双电离对激光强度的依赖.计算结果显示,沿激光偏振平面长轴方向,电子对的关联行为依赖于激光强度,在较高的激光强度下,关联电子对的动量谱呈正关联,并且在一、三象限呈现明显的V-型结构.在较低的激光强度下,关联电子对的动量谱呈强烈的反关联行为.在激光偏振平面短轴方向,关联电子对的动量谱在不同的激光强度下均呈现强烈的反关联行为.通过分析非次序双电离的经典运动轨迹,证明末态电子之间的排斥作用对关联电子动量谱在激光偏振平面长轴方向的V-型结构,以及短轴方向的反关联行为起决定性作用.     

不同分子取向下氢分子非次序双电离对核间距的依赖关系

利用三维经典系综模型研究了氢分子非次序双电离对核间距的依赖性.在不同分子取向下,氢分子双电离率随核间距的增大而先增大后减小,在核间距为4a.u.时取最大值.氢分子非次序双电离对核间距的依赖性随分子轴与激光偏振方向间夹角φ的增大而减弱.φ=0时,氢分子非次序双电离包含丰富的相关模式,对核间距有强烈的依赖性φ=π/2时,不同核间距下的相关模式相似,重碰撞在双电离过程中发挥更重要的作用.这些结果表明分子结构对双原子分子非次序双电离有重要影响. 关键词： 非次序双电离 核间距 关联动量分布     

空间非均匀激光场驱动的原子非次序双电离

利用三维经典系综模型,研究空间非均匀激光场驱动的氙原子非次序双电离,并对比了空间均匀激光场的情况.结果显示,波长较短时,空间非均匀激光场与空间均匀激光场的非次序双电离的产率较为相近.随着波长的增大,较高激光强度时空间非均匀激光场下非次序双电离受到越来越明显的抑制.相比于空间均匀激光场,空间非均匀激光场下非次序双电离两电子的末态发射角表现出更强烈的关联特性,特别在较大的激光波长下,两电子的末态发射角几乎全部集中在0°附近,这意味着两电子往往是平行发射到相同方向.此外,波长由近红外增大到中红外时,空间非均匀激光场下非次序双电离的有效再碰撞均由第1个电子的第1次返回主导,而空间均匀激光场下则呈现由第1次返回主导到第2次返回主导的转变.进一步,通过反演分析非次序双电离的经典轨迹,揭示了空间非均匀激光场下关联电子超快动力学过程的更多细节.     

飞秒激光激发空气电离的阈值研究

报道在脉宽50fs—22ps,波长800nm脉冲激光作用下的空气电离阈值的研究结果.利用探测等离子体发光信号的方法,实验测量了激发空气电离所需的阈值激光强度.结果表明,当激光脉冲宽度从50fs增加到22ps时,阈值光强I_th从8.7×10¹⁴W/cm²下降到2.7×10¹³W/cm²;I_th经历了由迅速降低逐渐发展为缓慢降低的过程.在50fs—1p 关键词： 飞秒激光 阈值 多光子电离 碰撞电离     

原子非序列双电离的多次返回碰撞电离机理分析

本文采用三维半经典再散射模型研究了He原子在高光强(1.5×10¹⁵ W/cm²)、少周波激光脉冲作用下的非序列双电离问题,重点分析了沿激光电场极化方向的动量关联谱.发现两个电子沿相反方向发射的比例明显比中等光强区(7×10¹⁴ W/cm²)和低光强区(2.5×10¹⁴ W/cm²),以及同等光强的长脉冲情形都偏高, 同时V形结构也更加明显.通过轨道"回溯"分析, 进一步深入 关键词： 强场 非序列双电离 再散射     

强场非次序双电离中再碰撞动力学的强度依赖

利用三维经典系综模型系统地研究了不同强度线偏振激光脉冲驱动下He原子的非次序双电离.结果表明在非次序双电离中回碰电子的返回次数、两电子的碰撞距离和电子对的关联特性都强烈地依赖于激光强度.对于750 nm,随着激光强度的增加,单次返回诱导的非次序双电离事件逐渐减少,而多次返回事件的比例显著增加.对于1500 nm,随着激光强度的增加,前三次返回诱导的非次序双电离事件都会减少,返回次数大于3的轨道对非次序双电离的贡献逐渐增加.这是因为在高强度下每次返回过程中母核的库仑吸引对返回电子横向偏离的补偿较弱,所以需要更多次的返回来补偿电子的横向偏离以实现再碰撞.轨道分析表明非次序双电离中两电子的碰撞距离随激光波长和强度的增加而逐渐减小.最后讨论了非次序双电离中电子对的关联特性对返回次数的依赖.     

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.     

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.     

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.     

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.     

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.     

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.     

Evidence of potential change in nonsequential double ionization

Recently,the quantitative rescattering model(QRS)for nonsequential double ionization(NSDI)is modified by taking into account the potential change(PC)due to the presence of electric field at the time of recollision.Using the improved QRS model,we simulate the longitudinal momentum distributions of doubly charged ions He2+by projecting the correlated two-electron momentum distributions for NSDI of He onto the main diagonal.The obtained results are compared directly with the experimental data at different intensities.It is found that when the PC is considered,the width of momentum distributions reduces and the agreement between theory and experiment is improved.     

Experimental study and computer simulations of the implantation of laser plasma ions in pulsed electric fields

Results are presented from experimental studies of pulsed plasma flows generated by nanosecond laser pulses with an intensity of 7 × 10⁸ W/cm² from a solid-state target in a strong electric field. The current pulses through the laser target and the depth distributions of the iron ions implanted in a silicon substrate to which a negative high-voltage pulse was applied are measured. The physical processes occurring in laser plasma with an initial iron ion density of 6 × 10¹⁰ cm⁻³ are simulated numerically by the particle-in-cell method for different delay times and different shapes of the accelerating high-voltage pulse. The model developed allows one to calculate the ion flows onto the processed substrate, the electron flows onto the target, and the energy spectra of the implanted ions. The results from computer simulations are found to be in good agreement the experimental data.