Interatomic Coulombic decay
Interatomic Coulombic decay (ICD) is a general, fundamental property of atoms and molecules which have neighbors. Interatomic (intermolecular) Coulombic decay is a very efficient interatomic (intermolecular) relaxation process of an electronically excited atom or molecule embedded in an environment. Without the environment the process cannot take place. Until now it has been mainly demonstrated for atomic and molecular clusters, independently of whether they are of van-der-Waals or hydrogen bonded type. The nature of the process can be depicted as follows: Consider a cluster with two subunits, "A" and "B". Suppose an inner-valence electron  is removed from subunit "A". If the resulting (ionized) state is higher in energy than the double ionization threshold of subunit "A" then an intraatomic (intramolecular) process (autoionization, in the case of core ionization Auger decay) sets in. Even though the excitation is energetically not higher than the double ionization threshold of subunit "A" itself, it may be higher than the double ionization threshold of the cluster which is lowered due to charge separation. If this is the case, an interatomic (intermolecular) process sets in which is called ICD. During the ICD the excess energy of subunit "A" is used  to remove (due to electronic correlation) an outer-valence electron from subunit "B". As a result, a doubly ionized cluster is formed with a single positive charge on "A" and "B". Thus, charge separation in the final state is a fingerprint of ICD. As a consequence of the charge separation the cluster typically breaks apart via Coulomb explosion. ICD is characterized by its decay rate or the lifetime of the excited state. The decay rate depends on the interatomic (intermolecular) distance of "A" and "B" and its dependence allows to draw conclusions on the mechanism of ICD. Particularly important is  the determination of the kinetic energy spectrum of the electron emitted from subunit "B" which is denoted as ICD electron. ICD electrons are often measured in ICD experiments. Typically, ICD takes place on the femto second time scale, many orders of magnitude faster than those of the competing photon emission and other relaxation processes. ICD in water. Very recently, ICD has been identified to be an additional source of low energy electrons in water. There, ICD is faster than the competing proton transfer that is usually the prominent pathway in the case of electronic excitation of water clusters. The response  of condensed water to electronic excitations is of utmost importance for biological systems. For instance, it was shown in experiments that low energy electrons do affect constituents of DNA effectively. Furthermore, ICD was reported after core-electron excitations of hydroxide in dissolved water. Related processes. Interatomic (Intermolecular) processes do not only occur after ionization as described above. Independent of what kind of electronic excitation is at hand, an interatomic (intermolecular) process can set in if an atom or molecule is in a state energetically higher than the ionization threshold of other atoms or molecules in the neighborhood. The following ICD  related processes, which were for convenience considered below for clusters, are known: