The nature of transient electronic states created by photoabsorption critically determines the dynamics of the subsequently evolving system. Here, we investigate $K$-shell photoionized atomic neon by absorbing a second photon within the Auger-decay lifetime of 2.4 fs using the European XFEL, a unique high-repetition-rate, wavelength-tunable x-ray free-electron laser. By high-resolution electron spectroscopy, we map out the transient Rydberg resonances unraveling the details of the subsequent decay of the hollow atom. So far, ultra-short-lived electronic transients, which are often inaccessible by experiments, were mainly inferred from theory but are now addressed by nonlinear x-ray absorption. The successful characterization of these resonances with femtosecond lifetimes provides the basis for a novel class of site-specific, nonlinear, and time-resolved studies with strong impact for a wide range of topics in physics and chemistry.

中文翻译：

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瞬变核电离原子中的共振结构映射

由光吸收产生的瞬态电子态的性质决定了随后发展中的系统的动力学。在这里，我们调查$ķ$通过使用欧洲XFEL（一种独特的高重复频率，波长可调的X射线自由电子激光器）吸收2.4 fs的俄歇衰变寿命内的第二个光子，使壳层电离化了原子氖。通过高分辨率电子光谱，我们绘制了瞬态Rydberg共振，从而揭示了中空原子随后衰减的细节。到目前为止，主要是从理论上推断出超短寿命的电子瞬态，而这些瞬态通常是无法通过实验获得的，但现在已经通过非线性X射线吸收解决了。这些共振的飞秒寿命的成功表征为新型一类针对特定地点，非线性和时间分辨的研究提供了基础，这些研究对物理和化学领域的广泛话题产生了重大影响。