The Fano absorption line shape of an autoionizing state encodes information on its internal atomic structure and dynamics. When driven near-resonantly with intense extreme ultraviolet (XUV) electric fields, the absorption profile can be deliberately modified, including observable changes of both the line-shape asymmetry and strength of the resonance, revealing information on the underlying dynamics of the system in response to such external driving. We report on the influence of the XUV pulse parameters at high intensity that can be achieved with a free-electron laser (FEL) with statistically broadened spectra based on self-amplified spontaneous emission (SASE). More specifically, the impact of the FEL pulse duration is studied for the example of the doubly excited 2s2p resonance in helium, where line-shape modifications have been measured with XUV transient absorption spectroscopy in Fraunhofer-type transmission geometry. A computational few-level-model provides insight into the impact of different average pulse durations of the stochastic FEL pulses. These findings are supported by measurements performed at the Free-Electron Laser in Hamburg (FLASH) and provide further insight into XUV strong-coupling dynamics of resonant transitions driven by intense high-frequency FEL sources.

自电离态的Fano吸收线形状编码有关其内部原子结构和动力学的信息。当在强紫外线（XUV）电场的作用下接近共振时，可以有意地修改吸收曲线，包括可观察到的线形不对称性和共振强度的变化，从而揭示有关系统潜在动态的信息这样的外部驱动。我们报告了高强度XUV脉冲参数的影响，这可以通过自由电子激光器（FEL）来实现，自由电子激光器（FEL）具有基于自放大自发发射（SASE）的统计拓宽光谱。更具体地说，以氦气中双激发2s2p共振为例研究了FEL脉冲持续时间的影响，其中使用XUV瞬态吸收光谱法在Fraunhofer型透射几何中测量了线形变化。计算级模型可以洞悉随机FEL脉冲的不同平均脉冲持续时间的影响。这些发现得到了汉堡自由电子激光（FLASH）的测量的支持，并提供了对由强烈的高频FEL源驱动的共振跃迁的XUV强耦合动力学的进一步了解。