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

Abstract

To understand the yield and patterns of damage in aqueous condensed matter, including biological systems, it is essential to identify the initial products subsequent to the interaction of high-energy radiation with liquid water. Until now, the observation of several fast reactions induced by energetic particles in water was not possible on their characteristic timescales. Therefore, some of the reaction intermediates involved, particularly those that require nuclear motion, were not considered when 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 occur on the same timescale as electron autoionization. Proton transfer leads to the formation of a Zundel-type intermediate [HO*···H···H2O]+, which further ionizes to form a so-far unnoticed type of dicationic charge-separated species with high internal energy. We call the process proton-transfer mediated charge separation.

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Figure 1: Schematic of the three autoionization mechanisms of core-ionized liquid water considered here.
Figure 2: Auger-electron spectra from normal (blue) and heavy (red) liquid water, together with the absolute intensity difference between the two spectra (black).
Figure 3: Potential energy curves of ionized water dimers.
Figure 4: The main findings of this work summarized in a graphical form.

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Acknowledgements

We thank N. Kryzhevoi and L. S. Cederbaum for stimulating discussions. We acknowledge the support of the Grant agency of the Czech Republic via grants no. P208/10/1724 and P208/11/0161 to P.S., the US National Science Foundation (CHE-0957869) to S.E.B., the Deutsche Forschungsgemeinschaft (DFG) via projects WI 1327/3-1, UH 3060/5-1, and the DFG Research Unit FOR 1789.

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S.T. and B.W. conceived, designed and performed the experiments, and analysed the data. P.S. and M.O. conducted the calculations and contributed to data interpretation. N.O. and R.S. contributed materials and/or analysis tools. B.W., P.S., S.T., U.H. and S.E.B. co-wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to Petr Slavíček or Bernd Winter.

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Thürmer, S., Ončák, M., Ottosson, N. et al. On the nature and origin of dicationic, charge-separated species formed in liquid water on X-ray irradiation. Nature Chem 5, 590–596 (2013). https://doi.org/10.1038/nchem.1680

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