Atoms and molecules attached to rare gas clusters are ionized by an interatomic autoionization process traditionally termed 'Penning ionization' when the host cluster is resonantly excited. Here we analyze this process in the light of the interatomic Coulombic decay (ICD) mechanism, which usually contains a contribution from charge 1 arXiv:1910.06230v1 [physics.atm-clus] 14 Oct 2019 exchange at short interatomic distance, and one from virtual photon transfer at large interatomic distance. For helium (He) nanodroplets doped with alkali metal atoms (Li, Rb), we show that long-range and short-range contributions to the interatomic autoionization can be clearly distinguished by detecting electrons and ions in coincidence.Surprisingly, ab initio calculations show that even for alkali metal atoms floating in dimples at large distance from the nanodroplet surface, autoionization is largely dominated by charge exchange ICD. Furthermore, the measured electron spectra manifest ultrafast internal relaxation of the droplet into mainly the 1s2s 1 S state and partially into the metastable 1s2s 3 S state.Interatomic decay processes have recently been found to play a crucial role in the interaction of biological matter with energetic radiation. Both free radicals and low-energy electrons produced by ICD processes can induce irreparable damage of the genome (double strand breaks in DNA) causing cancer or cell death. 1-3 Upon electronic excitation, weakly bound systems such as van der Waals or hydrogen bonded complexes and clusters can relax by interatomic autoionization if the excited state energy exceeds their adiabatic ionization energy. In the case of rare gas clusters doped with atomic or molecular impurities, this process has traditionally been termed Penning ionization, 4-10 in analogy to the collisional autoionization occurring in crossed atomic beams involving excited atoms, mostly rare gases prepared in metastable excited states. 11 This process is mainly driven by charge exchange between two interacting atoms or molecules which come so close to one another that their valence orbitals overlap. However, already in the early days of systematic Penning ionization studies, it was realized that the autoionization rate contains a second contribution describing energy transfer in the form of a virtual photon exchange. 12Since the seminal work by L. Cederbaum in 1997, such non-local autoionization processes involving two or more atomic or molecular centers have been formulated in the theoretical framework termed interatomic/intermolecular Coulombic decay (ICD). 13 This approach mainly refers to the autoionization of weakly bound systems that are inner-shell excited by

Penning spectroscopy of acetylene molecules dissolved in superfluid He nanodroplets reveals the loosely held molecular aggregate collapsing into a covalently bound oligomer ion upon indirect ionization effected by the photoexcited He* in the host.

Alkali metal dimers attached to the surface of helium nanodroplets are found to be efficiently doubly ionized by electron transfer-mediated decay (ETMD) when photoionizing the helium droplets. This process is evidenced by detecting in coincidence two energetic ions created by Coulomb explosion and one low-kinetic energy electron. The kinetic energy spectra of ions and electrons are reproduced by simple model calculations based on diatomic potential energy curves, and are in agreement with ab initio calculations for the HeNa 2 and HeKRb systems. This work demonstrates that ETMD is an important decay channel in heterogeneous nanosystems exposed to ionizing radiation.

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