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

Abstract

Identifying the initial products of the interaction of high-energy radiation with liquidwater is essential for understanding the yield and patterns of damage in aqueous condensed matter, including biological systems. Up until now several fast reactions induced by energetic particles in water could not be observed on their characteristic timescales, and hence some of the reaction intermediates involved, particularly those requiring nuclear motion, have not been considered in describing radiation chemistry.Here, through a combined experimental and theoretical study, we elucidate the ultrafast proton dynamics in the first few femtoseconds after X-ray core-level ionization of liquid water. We show through isotope analysis of the Auger-spectra that proton-transfer dynamics occurs on the same timescale as electron autoionization. Proton transfer leads to formation of a Zundel-type intermediate [HO*··H··OH 2 ] + , which further ionizes, forming a so-far unnoticed type of di-cationic, charge-separated species with high internal energy. We call the process proton-transfer mediated charge separation.The primary processes in water initiated by X-radiation are poorly understood despite their paramount importance in different fields. Understanding the energy and charge redistribution in water upon X-ray photon absorption is vital for a design of more efficient radio-oncology schemes, 1-2 for disentangling the physical basis of genotoxic effects on living tissues, [3][4][5] for minimizing the damage of biological samples during X-ray diffraction 2 experiments, 6 as well as for controlling the performance of nuclear reactors under operating conditions. 7 Current understanding of electron-initiated processes in aqueous systems, following energy deposition, and the subsequent radical chemistry have been recently reviewed. 8 An explicit consideration of radicals and molecular species formed via multiple ionization processes of water, involving for instance atomic oxygen and hydrogen peroxide, can be found in the radiolysis literature, e.g. in refs. 7,9 However, the knowledge of the ultrafast processes and mechanisms in water radiolysis remains to large extent unexplored.In the present work we focus on the processes following O1s core-level ionization of water. The highly excited species formed by the core ionization relaxes primarily via Augerelectron decay. As shown in Figure 1b, Auger decay of a water molecule involves refilling the water core-hole by one of the valence electrons, and the simultaneous emission of another valence electron, the Auger electron, from the same water molecule. The resulting highly reactive doubly ionized H 2 O 2+ (aq) molecule, with both vacancies (holes) located at the same site (denoted here as 2h state), then undergoes ultrafast Coulomb explosion, forming dominantly O + 2H + . [10][11] In recent years a set of novel non-local autoionization processes has been identified to play an important role in weakly bonded atomic and molecular systems. [12][13][14] One such relaxation process is Intermolecu...

X-ray emission spectroscopy of bulk liquid water in “no-man’s land”

“…However, there is a small and almost negligible conversion of some 1b 1 ′ to 1b 1 ′′ intensity with increasing fluence that will be discussed below. Although the currently tested radical species are the most probable 62 we cannot definitely rule out that there could exist other species not tested here that could give rise to spectral intensity at lower energies. We note that non-linear effects in terms of reabsorption can affect the spectra but only at significantly higher fluence than the peak fluence in the present work.…”

“…A 4th hypothesis is that there could be radical species building up in the liquid, not from ultrafast dissociation, but from the Auger decay cascade. 62 These could include various fragmented water species, such as the OH radical, that could contribute to the 1b 1 ′ peak. This is contradicted by theoretical calculations of the 1b 1 to O1s emission energy for OH, OH + , O, and O + as fragments, which all lead to a contribution at the 1b 1 ′′ position of intact water or at higher energy (Fig.…”

Isotope effects in liquid water probed by transmission mode x-ray absorption spectroscopy at the oxygen K-edge

“…Over the last years, x-ray spectroscopy and x-ray scattering have been used increasingly to study liquid water [1,6,[25][26][27][28][29][30][31][32][33][34][35][36][37].…”

Conformational and nuclear dynamics effects in molecular Auger spectra: fluorine core-hole decay in CF4

“…This strategy will break down however, when repulsive states come into play, and when the decay amplitudes depend explicitly on the molecular conformation. Both factors might gain in importance when Auger decay of molecules in a chemical environment is considered, as shown in a recent study of liquid water [5].…”