Ultrafast Resonant Interatomic Coulombic Decay Induced by Quantum Fluid Dynamics*

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 femtoseconds, nearly independent of the density of the excited states. Using a combination of time-dependent density functional theory andab 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. Similar processes are likely to occur in other fluids such as water, where the formation of nano-bubbles plays a role in the solvation of electrons and the unfolding and aggregation of proteins. Our results demonstrate the importance of bubble dynamics in interatomic decay processes and open up a new approach for understanding the basic processes causing radiation damage in biological systems.