As opposed to purely molecular systems where electron dynamics proceed only through intramolecular processes, weakly-bound complexes like helium droplets offer an environment where local excitations can interact with neighboring embedded molecules leading to new intermolecular relaxation mechanisms. Here, we report on a new decay mechanism leading to the double ionization of alkali dimers attached to helium droplets by intermolecular energy transfer. From the electron spectra, the process is similar to the well-known shakeoff mechanism observed in double Auger decay and single photon double ionization [1,2], however, in this case, the process is dominant, occurring with efficiencies equal to, or greater than, single ionization by energy transfer. Although an alkali dimer attached to a helium droplet is a model case, the decay mechanism is relevant for any system where the excitation energy of one constituent exceeds the double ionization potential of another neighboring molecule. The process is, in particular, relevant for biological systems, where radicals and slow electrons are known to cause radiation damage [3].

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

Acene molecules (anthracene, tetracene, pentacene) and fullerene (C) are embedded in He nanodroplets (He) and probed by EUV synchrotron radiation. When resonantly exciting the He nanodroplets, the embedded molecules M are efficiently ionized by the Penning reaction He + M → He + M + e. However, the Penning electron spectra are all broad and structureless, largely differing from those measured by binary Penning collisions, as well as from those measured for dopants bound to the He droplet surface. Simulations based on elastic binary electron-He collisions qualitatively reproduce the measured spectra only when assuming unexpectedly large He droplets, indicating that electron spectra of molecules embedded in helium nanodroplets are severely affected by collective electron-helium interactions.

Interatomic Coulombic decay (ICD) is induced in helium (He) nanodroplets by photoexciting the n = 2 excited state of He + using XUV synchrotron radiation. By recording multiple coincidence electron and ion images we find that ICD occurs in various locations at the droplet surface, inside the surface region, or in the droplet interior. ICD at the surface gives rise to energetic He + ions as previously observed for free He dimers. ICD deeper inside leads to the ejection of slow He + ions due to Coulomb explosion delayed by elastic collisions with neighboring He atoms, and to the formation of He + k complexes.Isolated atoms or molecules excited by energetic radiation typically decay through intramolecular processes such as the emission of an electron or photon. In contrast, in weakly bound complexes, locally generated electrons can additionally interact with neighboring atoms or molecules, leading to new interatomic or intermolecular interactions. Interatomic Coulombic decay (ICD) is a particularly interesting decay process which occurs when local electronic decay is energetically forbidden [1]. Thus, ICD offers a new, ultrafast decay path where energy is exchanged with a neighboring atom leading to its ionization. Since its discovery, ICD has been observed in a wide variety of weakly-bound systems from He dimers [2,3] and rare-gas clusters to biologically relevant systems such as water clusters; for reviews see [4,5]. Today, the focus is on condensed-phase systems where ICD is involved in complex relaxation mechanisms [6][7][8], which can generate genotoxic low-energy electrons and radical cations [9]. Recently, it was suggested to utilize this property of ICD for cancer treatment [10,11].Here we present the first study of ICD in helium (He) nanodroplets. He nanodroplets are generally considered as an ultracold, inert spectroscopic matrix for embedded, isolated molecules and clusters [12,13]. Upon ionization by intense or energetic radiation, however, He droplets turn into a highly reactive medium, inducing reactions and secondary ionization processes of the embedded species [14]. Their homogeneous quantum liquid density profile, and the simple structure of atomic constituents, make He droplets particularly beneficial as benchmark systems for elucidating correlated decay processes. Recent examples include the collective autoionization of multiply excited pure He droplets [15,16] and the creation of doubly charged species by one-photon ionization of doped He droplets [17]. In this work we fully characterize the product states generated by ICD and secondary processes in He nanodroplets using coincidence imaging techniques.The experiments were performed using a He droplet machine attached to a velocity map imaging photoelectron-photoion coincidence (VMI-PEPICO) detector at the GasPhase beamline of Elettra-Sincrotrone Trieste, Italy. The apparatus has been described in detail elsewhere [18,19]. Briefly, a beam of He nanodroplets is produced by continuously expanding pressurized He (50 bar) of high purity He out of a c...

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.