Mechanisms of one-photon two-site double ionization after resonant inner-valence excitation in Ne clusters

The role of interatomic and intermolecular energy and charge-transfer processes in weakly bound matter is currently lively debated due to emerging destructive low-energy electrons and radicals. Here, we discuss two mechanisms of single-photon two-site double ionization occurring competitively or subsequently to resonant interatomic Coulombic decay (rICD) in inner-valence (2s -> np) excited Ne dimers and clusters. The first mechanism is photoelectron-impact ionization which is, in general, not related to resonant excitation, but in the present case strongly enhanced and, thus, observable due to resonant excitation. Studying this mechanism at its energetic threshold enables addressing a subset of Ne dimers with selected bond lengths. The second mechanism is collisional ionization of energetic Rydberg atoms, which are produced by rICD in Ne clusters and may be ionized by collisions with neutrals on their way through the medium. Both mechanisms are identified by the coincident detection of charged products and, for the case of collisional ionization, confirmed by calculations. These mechanisms produce one more low-energy electron and ion than conventional rICD and, thus, should be considered in the discussion of the biochemical impact of photoinduced rICD processes.