Generation of Highly Damaging H₂O⁺ Radicals by Inner Valence Shell Ionization of Water

Abstract

[a] Water molecules surround all biological matter and have a crucial role as the matrix of life. Owing to the close proximity of water molecules to all biological systems, it is important to understand the response of water to ionizing sources, since the emitted electrons and open-shell fragments play a major role in various damaging mechanisms of biomolecules. Very recent experiments have investigated the inner valence ionization of water in the water dimer [1] and in larger water clusters, [2] and have found that the system decays emitting electrons of low energy and radical cations H 2 O + . Low-energy electrons can effectively interact and induce breakage of biomolecules. The water radical cation is also capable of great damage. [3,4] It is a strong acid and will immediately transfer a proton to any nearby group with proton-accepting capability. The proton transfer results in OHC, which is extremely reactive itself. In nature and experimentally the inner valence ionization can be accomplished by interaction with light. Other preparations are also conceivable, for example, an inner-valence ionized state can be the result of a cascade of processes, such as Auger decay, started by ionization or excitation of a core electron by an energetic particle or photon. The process by which an inner-valence ionized state decays by emission of an electron is called intermolecular Coulombic decay (ICD). ICD is an electronic deexcitation process that was first theoretically predicted for hydrogen-bonded systems [5,6] and van der Waals clusters of noble gas atoms. [7] Later, experimental evidence of ICD in van der Waals clusters was reported, [8,9] and more recently, evidence of ICD in water has been reported as well.[1, 2, 10] ICD can take place if the inner valence ionized state is energetically above the double-ionization threshold of the system. This is usually not the case for isolated atoms or small molecules. However, for aggregates, the double ionization threshold can fall below the inner valence ionization energy, opening the possibility for ICD to occur. The process can be described as a relaxation of the ionized atom or molecule, in which an outer valence electron fills the inner valence hole. The energy released is transferred to a neighboring species, from which a second outer valence electron is expelled. In the case of the water dimer, the double ionization threshold is about 3 to 4 eV below the inner valence ionization energy, and therefore ICD follows an inner valence ionization process. [6] In the experiments on the water dimer of ref.[1], the ICD process is triggered by ionization from an inner valence shell of one of the water molecules. The two emitted electrons (direct photo-electron and ICD electron) and the cationic fragments resulting from the Coulomb explosion of the dimer are then measured in coincidence, determining their directions, and the charges and masses of the fragments. Such experiments clearly show that ICD results in two H 2 O + fragments, which immediately separate in a Coulomb explo...

Efficient site-specific low-energy electron production via interatomic Coulombic decay following resonant Auger decay in argon dimers

“…The relevance of ICD and radiation damage was noted several times in the literature [9,[23][24][25][26][27] due to the following reason. Genotoxic damage by high-energy ionizing radiation, including the breaking of DNA strands in living cells, is not caused by direct ionization but is induced by the secondary electrons produced by the primary ionizing radiation.…”

“…The relevance of ICD processes with radiation damage is also noted [9,[23][24][25][26][27]. Noting that ICD produces low-energy electrons locally at the site where an excitation takes place, one may think one step further: one may consider the relevance of the production of low-energy electrons via ICD with radiation therapy that requires localized radiation damage.…”

On the nature and origin of dicationic, charge-separated species formed in liquid water on X-ray irradiation

“…It follows from our arguments that the core ionization of liquid water leads to formation 21,51 But neither the proton-transfer mediated ICD-type nor the Auger-type charge separated states were considered so far. We can only speculate on the reactions following the autoionization process, and we argue that even the relatively subtle differences in energy and structure for ICD vs. PTM-ICD will have an effect on the subsequent chemical reactions.…”

Site- and energy-selective slow-electron production through intermolecular Coulombic decay

“…We note that only for electrons with energies below 15 eV the microscopic mechanisms for strand breakage have been investigated [10,11,[37][38][39] (for brief discussion on higher energy electrons, see Supplementary Materials). In all the ICD processes during the cascade two or more neighboring ions are produced directly leading to damaging Coulomb explosion [3,40]. The damage to DNA through the ICD electrons and the Coulomb explosion following RA-ICD cascades will happen in the immediate vicinity of the site where the energy was initially deposited.…”