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

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. Previously unobserved types of reactive species formed on the core ionization of liquid water have been identified using a combination of liquid microjet photoemission spectroscopy and ab initio calculations. The charge-separated di-cationic species are formed within a few femtoseconds, through proton-transfer-mediated processes followed by autoionization.