Proton transfer and hydronium formation in ionized water

Abstract

Aqueous radiation chemistry emerges through ultrafast proton transfer and ion-radical formation with unexplored energy-redistribution dynamics steering the subsequent reactions. We performed time-resolved disruptive probing on pure water dimer, (H$_2$O)$_2$, to disentangle the post-ionization reactions. Through kinetic-energy-resolved ion imaging, we unraveled the dynamics in the (H$_2$O)$_2^+$ ground state: at low-energy ($\sim$0.05 eV) ultrafast proton transfer ($\sim$19 fs) is followed by H$_3$O$^+$+OH fragmentation ($\sim$360~fs). At higher energies, proton transfer becomes hindered ($\sim$60 fs) while the subsequent fragmentation becomes faster ($\sim$210 fs), evolving into coupled dynamics ($>0.15$ eV, $\sim$100 fs). Moreover, we observed H$_2$O)$_2^+$ stabilization proceeding through a Zundel-like structure. This reveals how ion-radical formation in ionized hydrogen-bonded networks shapes reactivity in aqueous dynamics.

Figures (4)

• Figure 1: Schematic picture of the experiment. Water dimers were strong-field ionized to launch the ultrafast dynamics. The dynamics were perturbed by a delayed, weak probe pulse that redistributed population and modified the fragmentation pattern. Monitoring all ion species over the pump–probe delay provided time-resolved signatures of PT, ion–radical stabilization, or fragmentation. Velocity-map imaging further yielded delay-dependent momentum distributions that reveal the interplay between PT and fragmentation.
• Figure 2: Recorded transient signals related to disruptive probing of the $\text{H}_3\text{O}^+$ channel and their analysis.A and B display transient signals of the $\text{H}_3\text{O}^+$ and $\text{H}_2\text{O}^+$ channels (blue dots and black error bars), respectively. Fits (red lines) were obtained using the parametric model described in Materials and Methods. The data (gray points) in the range $(-500,-50)$ fs were excluded from the fitting procedure due to probe-induced pre-excitation effects. The inset in A shows a velocity map of $\text{H}_3\text{O}^+$ with only the central part (yellow circle) originating from $(\text{H}_2\text{O}\xspace)_2^+$ dynamics. C displays the result of simulations of $(\text{H}_2\text{O}\xspace)_2^+$ dynamics in its ground electronic state ($\text{D}_0$), corresponding data for $\text{D}_1$…$\text{D}_3$ are provided in Supplementary Text. D represents the background corrected delay- and energy-resolved $\text{H}_3\text{O}^+$ signal. E shows the comparison of the bleached (blue), pump-only (yellow), and simulated ($\text{D}_0$, red) kinetic energy distributions Vinklarek:water2-moadk:inprep. F displays the timescales of the $\text{H}_3\text{O}^+$ formation obtained from the kinetic energy-resolved analysis using the parametric models described in Materials and Methods; see also Supplementary Text for the individual fits.
• Figure 3: Recorded transient signals related to disruptive probing of the $(\text{H}_2\text{O}\xspace)_2^+$ channel.A and B display transient signals of $(\text{H}_2\text{O}\xspace)_2^+$ and $\text{H}^+$, respectively. C shows the delay- and energy-resolved $\text{H}^+$ signals after subtraction of the pump-only contributions. D and E show transient $\text{H}^+$ signals for selected kinetic energy ranges. All fits (red lines) were obtained using the parametric model described in Materials and Methods. The data (gray points) in the range $(-500,-50)$ fs were excluded from the fitting procedure due to probe-induced pre-excitation effects.
• Figure 4: The observed $(\text{H}_2\text{O}\xspace)_2^+$ dynamics. Energy-resolved PT and fragmentation pathways are identified from the $\text{H}_3\text{O}^+$ signal and low-energy stabilization via inter-to-intra-molecular vibrational relaxation (IVR) is observed as the $(\text{H}_2\text{O}\xspace)_2^+$ signal.