A hitherto unrecognized source of low-energy electrons in water

Abstract

Because ICD is expected to take place universally in weakly bound aggregates containing light atoms between carbon and neon in the periodic table 2,3 , these results could have implications for our understanding of ionization damage in living tissues. NPHYS-2009-06-00979a 2 Electronic vacancy states can be produced in matter by ionizing radiation, such as X-ray photons or fast charged particles. When a state with a high electronic excitation energy has been produced by impact of such particles, electron correlation can cause the ejection of electrons. Auger decay is the best known representative of this class of secondary processes that is more generally termed autoionization. In other words, the mechanism is a concerted transition in which a single hole in an inner shell is replaced by two vacancies in the outer valence shells of two adjacent molecules, and a free electron. This decay channel was termed Intermolecular (Interatomic, in the case of atomic clusters) Coulombic Decay and was subsequently observed in rare gas clusters 4-7 .The process is shown schematically in Fig. 1. A resonant variant of ICD, which may take place after photoexcitation into an unoccupied orbital, has also been discussed [7][8][9] . In the present paper, we consider ICD of inner valence vacancy states, for which case the ejected electrons have a low kinetic energy.On the basis of energetic considerations, ICD can take place whenever the binding energy of the ionized state lies above the double ionization threshold of the corresponding cluster or liquid. This prerequisite for ICD is fulfilled in hydrogen-bonded systems 2,10 , but so far the process has not been seen. Calculations of the energy spectrum of electrons ejected by ICD of small water clusters give a hint as to why it has escaped observation: A broad, rather unstructured distribution of energies is expected, which peaks at zero eV 10 . Ifwe consider an experiment with a conventional electron energy analyser on a bulk or liquid NPHYS-2009-06-00979a 3 sample, an electron spectrum with this shape can hardly be distinguished from the "universal curve" 1 for secondary electrons (Fig. 2). In this respect our work differs from earlier experiments, which were either restricted to dimers 5-7 , or dealt with simpler cases where an ICD feature appears from simple electron kinetic energy spectra 4,8,9 . Producing primary electrons of a well-defined energy by photoionization and detecting them in coincidence with the ICD electron has allowed us to overcome the aforementioned problem. Here, we demonstrate that ICD follows the photoionization of medium-sized water clusters and show that -above the corresponding photoionization threshold -ICD electrons make an important contribution to the low kinetic energy spectrum.In our experiment, a jet of water clusters with a mean size 〈N〉 of 40 or 200 was used.Such clusters are believed to form amorphous structures, which resemble the hydrogenbonded network of liquid water rather than that of crystalline ice 11 . Inner valence vacancies were p...

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

“…[12][13][14] One such relaxation process is Intermolecular Coulombic Decay (ICD), initially observed upon innervalence ionization of van der Waals-bonded clusters, 12,[15][16][17] and later also (hydrogen bonded) water clusters. 18 In the ICD process, the energy provided by an outer valence electron upon filling the vacancy is used to expel a second electron from an atom or molecule neighboring the initially ionized site. Throughout the process the two units (two water molecules in our case) can be considered as being electronically distinct; see scheme Figure 1c.…”

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

“…Recent experiments even suggest that ICD electrons contribute up to about 50% of the single-strand breaks (SSB) in DNA [13]. Moreover, these electrons possess energies at which much harder to repair double-strand breaks (DSB) occur [10,14] and the energetic cations produced in Coulomb explosions may additionally damage the DNA.…”

Importance of Electronic Relaxation for Inter-Coulombic Decay in Aqueous Systems

“…Atomic and molecular phases comprise mainly localized electronic orbitals, and the decay rate is consequently quite small for processes involving electron emission from any atomic site other than that originally excited. In such systems, this type of ''off-site'' emission has been labeled as ICD, inter-Coulombic (atomic or molecular) decay [3].Although ICD has been studied primarily in the context of valence excitations [3][4][5], recent experiments have shown that core-excited systems may also decay in this fashion, as shown schematically in Fig. 1 [6].…”

“…Although ICD has been studied primarily in the context of valence excitations [3][4][5], recent experiments have shown that core-excited systems may also decay in this fashion, as shown schematically in Fig. 1 [6].…”