Attosecond photoionization time delays reveal information about the potential energy landscape that an outgoing electron wavepacket probes upon ionization. In this study, we experimentally quantify the dependence of the time delay on the angular momentum of the liberated photoelectrons. For this purpose, we resolved electron quantum-path interference spectra in energy and angle using a two-color attosecond pump–probe photoionization experiment in helium. A fitting procedure of the angle-dependent interference pattern allows us to disentangle the relative phase of all four quantum pathways that are known to contribute to the final photoelectron signal. In particular, we resolve the dependence on angular momentum of the delay of one-photon transitions between continuum states, which is an essential and universal contribution to the total photoionization delay observed in attosecond pump–probe measurements. For such continuum–continuum transitions, we measure a delay between outgoing $ s $ and $ d $ electrons as large as 12 attoseconds, close to the ionization threshold in helium. Both single-active-electron and first-principles ab initio simulations confirm this observation for helium and hydrogen, demonstrating the universality of the observed delays.

中文翻译：

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连续体中从单光子跃迁的时间延迟

阿秒光电离时间延迟揭示了有关电离电子波包在电离时探测到的势能分布的信息。在这项研究中，我们通过实验来量化时间延迟对所释放的光电子的角动量的依赖性。为此，我们在氦气中使用了两色阿秒泵浦-探针光电离实验，解析了能量和角度的电子量子路径干涉光谱。角度相关干涉图样的拟合过程使我们能够解开已知对最终光电子信号有贡献的所有四个量子路径的相对相位。特别是，我们解决了连续态之间单光子跃迁延迟对角动量的依赖，这是对在阿秒泵浦-探针测量中观察到的总光电离延迟的重要而普遍的贡献。对于这样的连续体-连续体过渡，我们测量了传出之间的延迟$ s $和$ d $电子大至12阿秒，接近氦中的电离阈值。单活性电子和第一性原理从头算都证实了氦和氢的这一观察结果，证明了观察到的延迟的普遍性。