Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. The relaxation occurs primarily via Auger emission, so excited-state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained elusive in XFELs owing to inherent timing and phase jitter, which can be orders of magnitude larger than the timescale of Auger decay. Here we develop an approach termed ‘self-referenced attosecond streaking’ that provides subfemtosecond resolution in spite of jitter, enabling time-domain measurement of the delay between photoemission and Auger emission in atomic neon excited by intense, femtosecond pulses from an XFEL. Using a fully quantum-mechanical description that treats the ionization, core-hole formation and Auger emission as a single process, the observed delay yields an Auger decay lifetime of \(2.2_{ - 0.3}^{ + 0.2}\) fs for the KLL decay channel.

强 X 射线自由电子激光器 (XFEL) 可以快速激发物质，使其处于固有的不稳定状态，并在飞秒时间尺度上衰减。弛豫主要通过俄歇发射发生，因此激发态观测受到俄歇衰变的限制。因此，该过程的原位测量至关重要，但迄今为止，由于固有的时序和相位抖动，它在 XFEL 中仍然难以捉摸，这可能比俄歇衰减的时间尺度大几个数量级。在这里，我们开发了一种称为“自参考阿秒条纹”的方法，尽管存在抖动，但仍可提供亚飞秒分辨率，从而能够时域测量由 XFEL 的强烈飞秒脉冲激发的原子氖中的光电发射和俄歇发射之间的延迟。使用处理电离的完全量子力学描述，\(2.2_{ - 0.3}^{ + 0.2}\) fs 用于 KLL 衰减通道。