We calculate the correlated two-electron momentum distributions for nonsequential double ionization of helium in a 400-nm laser pulse with a peak intensity of $6.5\times {10}^{14}$ $\mathrm{W}/{\mathrm{cm}}^2$, which is below the recollision threshold, by using the quantitative rescattering model in which the lowering of the threshold due to the presence of the electric field at the instant of recollision is taken into account. While distinct correlated back-to-back emission of the electrons along the polarization direction is predicted in accord with other existing theoretical simulations, we suggest an alternative mechanism that is responsible for the anticorrelation. At intensities below the recollision threshold, recollision excitation can only take place when the barrier of the Coulomb potential is sufficiently suppressed by the electric field. The excited electron begins to ionize at a “delayed” recollision time after a crossing of the field and hence the probability of being tunnel-ionized after the subsequent peak of the field is increased. It is demonstrated that the “delayed” recollision time predominantly determines the parallel momentum distribution of the tunneling electron and plays a decisive role in forming anticorrelation.

中文翻译：

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氦的非顺序双电离中的反相关

我们计算了400 nm激光脉冲中氦的非顺序双电离氦的相关双电子动量分布 $6.5\times {10}^{14}$ $\mathrm{w\; ^}/{\mathrm{厘米}}^{\mathrm{2个}}$通过使用定量重新散射模型，该阈值低于重新碰撞阈值，其中考虑了由于重新碰撞瞬间电场的存在而导致的阈值降低。虽然根据其他现有的理论模拟预测了沿极化方向的不同背对背电子发射，但我们建议了一种负责反相关的机制。在低于碰撞阈值的强度下，仅当通过电场充分抑制库仑电势的势垒时，才能发生碰撞激励。被激发的电子在场交叉之后的“延迟”重新碰撞时间开始电离，因此在随后的场峰之后被隧道电离的可能性增加。