Intermolecular Coulombic Decay in water dimers -
observation of an ultrafast decay mechanism
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We observed a novel decay mechanism called "Intermolecular Coulombic Decay" (ICD) in water dimers. The process is triggered by removing an inner valence electron from one of the water molecules of the dimer (a). As the ionized molecule deexcites, the deexcitation energy is transferred to the neighboring molecule (b). That neighbor to the initially ionized molecule emits a low energy electron, the so called ICD electron. Our investigations reveal that the decay process is ultrafast, it happens even before the water molecules are able to rearrange: usually a protonated water molecule, i.e. H3O+ is formed after ionization of compounds of water molecules. As the proton rearrangement happens typically on a time scale of 50 fs, ICD seems to be a highly efficient mechanism to "convert" incoming radiation into a low energy electron. This fact might be of interest to other disciplines: during the last years low energy electrons have been observed to efficiently cause single and double strand breaks in DNA. As bio matter is often surrounded by water, ICD in water may be a so far overseen source for radiation damage of living tissue. 
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