Strong-field ionization of atoms can be investigated on the attosecond time scale by using the attoclock method, i.e. by observing the peak of the photoelectron momentum distribution (PMD) after applying a laser pulse with a two-dimensional polarization form. Examples for such laser fields are close-to-circular or bicircular fields. Here, we report numerical solutions of the time-dependent Schrdinger equation for bicircular fields and a comparison with a compact classical model to demonstrate that the tunnel-exit position, i.e. the position where the electron emerges after tunnel ionization, is encoded in the PMD. We find that the tunnel-exit position depends on the transverse velocity of the tunneling electron. This gives rise to a momentum-dependent attoclock shift, meaning that the momentum shift due to the Coulomb force on the outgoing electron depends on which slice of the momentum distribution is analysed. Our finding is supported by a momentum-space-based implementation of the classical backpropagation method.

原子的强场电离可以使用阿托钟方法在阿秒时间尺度上进行研究，即通过在施加具有二维偏振形式的激光脉冲后观察光电子动量分布 (PMD) 的峰值。这种激光场的例子是接近圆形或双圆形的场。在这里，我们报告了双圆场的瞬态薛定谔方程的数值解，并与紧凑的经典模型进行了比较，以证明隧道出口位置，即隧道电离后电子出现的位置，是在 PMD 中编码的。我们发现隧道出口位置取决于隧道电子的横向速度。这导致了依赖于动量的时钟偏移，这意味着由于库仑力作用在出射电子上的动量偏移取决于所分析的动量分布的哪一部分。我们的发现得到了经典反向传播方法的基于动量空间的实现的支持。