The article deals with a process that has previously been proposed theoretically and only recently have M. Mudrich and colleagues succeeded in observing it in experiments performed at the local ASTRID2 synchrotron facility.

These results hopefully will help to better understand how ionizing radiation causes damage in matter, which is the basis for devising better radiation protection and radiotherapies schemes.

Helium atoms in which the two electrons are simultaneously excited by one photon have been extensively studied for many years as they represent a paradigm system for so-called Fano resonances. This is resonances in the absorption cross section of an atom, molecule or nanoparticle high up above the threshold for its single ionization. The well-known asymmetric line shape of a Fano resonance results from the quantum interference of two paths leading from the common initial state (ground state of the helium atom) to the same final state (a helium ion and a free electron); One path is the double excitation followed by autoionization of the helium atom (the spontaneous decay of one electron to ground state and emission of the other electron), whereas the other path is the direct photoionization.

Autoionization of the lowest of the doubly excited states of helium is an ultrafast process which lasts only 17 femtoseconds; It is therefore all the more astonishing that when the doubly excited helium atom is embedded in a helium nanodroplet, another process starts to compete with autoionization.

This process – Interatomic Coulombic Decay (ICD) -- was first theoretically proposed by Cederbaum in 1997 and has since been observed in many different weakly bound atomic and molecular systems, including water and solvated biomolecules. In this particular variant of ICD, one of the two excited electrons in the helium atom decays back to the ground state and the released energy is transferred to a neighboring helium atom which in turn is ionized.

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