Interatomic processes play a crucial role in weakly bound complexes exposed to ionizing radiation; therefore, gaining a thorough understanding of their efficiency is of fundamental importance. Here, we directly measure the timescale of interatomic Coulombic decay (ICD) in resonantly excited helium nanodroplets using a high-resolution, tunable, extreme ultraviolet free-electron laser. Over an extensive range of droplet sizes and laser intensities, we discover the decay to be surprisingly fast, with decay times as short as 400 fs, nearly independent of the density of the excited states. Using a combination of time-dependent density functional theory and ab initio quantum chemistry calculations, we elucidate the mechanisms of this ultrafast decay process, where pairs of excited helium atoms in one droplet strongly attract each other and form merging void bubbles, which drastically accelerates ICD.

中文翻译：

---

量子流体动力学引起的超快共振原子间库伦衰变

原子间过程在暴露于电离辐射的弱结合复合物中起着至关重要的作用。因此，全面了解其效率至关重要。在这里，我们使用高分辨率，可调谐的，极紫外自由电子激光，直接测量共振激发的氦纳米液滴中原子间库伦衰变（ICD）的时间尺度。在大范围的液滴尺寸和激光强度范围内，我们发现衰减出奇地快，衰减时间短至400 fs，几乎与激发态的密度无关。结合时变密度泛函理论和从头算 通过量子化学计算，我们阐明了这种超快衰变过程的机制，其中一个液滴中的成对激发氦原子相互强烈吸引，并形成合并的气泡，从而极大地加速了ICD。