Ultrafast Resonant Interatomic Coulombic Decay Induced by Quantum Fluid Dynamics

Abstract

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.

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Plain Language Summary

When a free atom or molecule is excited by an energetic photon, the only means to release its energy is through internal decay or radiation emission. In contrast, when atoms or molecules are weakly bound to one another, the excitation energy from one site can be transferred to a neighboring one. A particularly interesting energy-sharing process is interatomic Coulombic decay (ICD), in which the release of energy from one atom or molecule leads to ionization of a neighbor. This process plays an important role in the response of biological tissue to radiation. Here, we show that ICD is dramatically enhanced by the response of the medium surrounding the excitations.

In many condensed systems, such as fluids and small droplets, not only do the interacting atoms and molecules matter, but also the local environment can strongly influence the interatomic decay process. To study this influence, we use ultrashort, extreme-ultraviolet laser pulses to directly map ICD of laser-excited superfluid helium nanodroplets over time. State-of-the-art theoretical modeling of the process reveals that a localized bubble, or cavity, forms around each excited atom. Neighboring bubbles then merge into one, thereby pushing the excited atoms together. This causes the atoms to decay by ICD within a few hundred femtoseconds, which is orders of magnitude faster than previously expected.

Similar processes are likely to occur in other fluids such as water, where the formation of nanobubbles 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.