Anisotropy parameters describing the angular dependence of the photoionization delay are defined. The formalism is applied to results obtained with the relativistic random phase approximation with exchange for photoionization delay from the outermost $s$-orbitals in selected rare-gas atoms. Any angular dependence in the Wigner delay is induced here by relativistic effects, while the measurable atomic delay exhibits such a dependence also in the nonrelativistic limit. The contributions to the anisotropy from the different sources are disentangled and discussed. For the heavier rare gases, it is shown that measurements of the delay for electrons ejected in specific angles, relative to, e.g., those ejected along the laser polarization, are directly related here to the Wigner delay. For a considerable range of angles, the contributions from the second photon largely get canceled when the results in different angles are compared, and this angle-relative atomic delay is then close to the corresponding Wigner delay.

中文翻译：

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光电离延迟的角各向异性参数

定义了描述光电离延迟的角度依赖性的各向异性参数。将形式化应用于通过相对论随机相位近似获得的结果，并从最外层交换光电离延迟$秒$-选定的稀有气体原子中的轨道。Wigner 延迟中的任何角度依赖性都是由相对论效应引起的，而可测量的原子延迟在非相对论极限中也表现出这种依赖性。对不同来源对各向异性的贡献进行了梳理和讨论。对于较重的稀有气体，显示以特定角度喷射的电子延迟的测量值，例如相对于沿激光偏振喷射的电子，这里与 Wigner 延迟直接相关。对于相当大的角度范围，当比较不同角度的结果时，第二个光子的贡献在很大程度上被抵消了，这个角度相关的原子延迟接近相应的维格纳延迟。