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.
Radiation chemistry in aqueous environments is largely governed by water radiolysis [1][2][3]. Upon irradiation, water undergoes a cascade of reactions initiated by the formation of highly reactive ion-radical species [3,4] that give rise to fundamental phenomena such as biological radiation damage, catalytic transformations, material corrosion, and radical-driven oxidation. A molecular-level characterization of transient reactive species formation and the associated energy redistribution is therefore essential for advancing technologies including radiation therapy [4,5], wastewater treatment [6,7], nuclear reactor coolant systems [4,8], and spaceflight applications [9,10].
Surprisingly, the initial elementary steps driving the subsequent post-ionization dynamics in water were only recently explored in a time-resolved fashion [2,3,11]. These studies provide a coarse picture lacking state-and energy-resolved details. The dominant initial step following valence ionization of liquid water is the ultrafast proton transfer (PT) reaction occurring on a sub-50fs timescales [3,12,13], producing a hydronium cation (H 3 O + ) and a hydroxyl radical (OH):
These products drive secondary PTs, oxidative reactions, or undergo recombination. Early pump-probe studies lacked the temporal resolution to capture the initial PT dynamics [12]. Time-resolved x-ray absorption spectroscopy (XAS), enabled by x-ray free-electron lasers, revealed an ultrafast ∼46 fs PT event [3], while ultrafast electron diffraction captured H 3 O + • • • OH formation in ∼140 fs followed by the ion-radical separation occurring on a ∼250 fs timescale [14]. These seemingly different timescales reflect the complementary sensitivities of the probes: XAS probes local electronic structure and diffraction probes atomic motion. Recently, experiments on water-dimer clusters ionized by 24 eV photons reported a PT time of ∼55 fs, consistent with the earlier XAS results [2].
Molecular clusters permit direct detection of ionic fragments, providing detailed insights into energy redistribution along ultrafast reaction pathways [15], whereas secondary processes characteristic of the bulk liquid [16] remain inaccessible. We used water dimer, (H 2 O) 2 , a controllable and well-defined model [17][18][19], to investigate post-ionization chemistry in aqueous systems. Previously, we found a surprising diversity of ion products after single ionization of (H 2 O) 2 . Our approach using the electrostatic deflector [17,20] allows the assignment of the originally ionized system without the need for simultaneous detection of multiple ions, as in coincidence-detection methods [2]. Similar to earlier studies [3,11], we populated (H 2 O) + 2 via strong-field ionization, which raises the question of the initial electronic states. Our recent study showed that ionization predominantly accesses the four energetically lowest states, D 0 . . . D 3 [21]. According to our previous ab initio simulations [2] these undergo PT, forming the ion-radical complex (H 3 O • • • OH) + , which either stabilizes, predominantly for ionization into D 0 , or fragments to H 3 O + + OH.
To explore the elementary dynamics of ionized water dimer, we employed the mass-spectrometric disruptiveprobing (DP) method. DP was recently presented as a panoramic approach [22] that synchronously monitors delay-dependent signal changes arising from perturbations of evolving electronic and nuclear wavepackets in 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.
multiple independent photochemical pathways [23]. A schematic overview of the approach is shown in Figure 1: A strong-field (I pump = 2.3 • 10 14 W/cm 2 , 800 nm) pump pulse initiates the dynamics, while a weaker, delayed NIR-probe pulse perturbs the ongoing evolution. When temporally separated from the pump, the probe is too weak to induce ionization. Regardless of the precise disruption mechanism [22,23], the probe modifies the yields of individual ion species as the delay is scanned, thereby encoding the reaction dynamics. So far, DP experiments only took ion yields into account [22]. Our combination of DP with velocity-map imaging (VMI) adds kinetic-energy resolution to the transient ion signals. Complementing these experiments with ab initio simulations of the dynamics to 1 ps allowed us to assign fragmentation pathways beyond the initial PT event.
We disentangled the ultrafast dynamics of (H 2 O) + 2 in its electronic ground state, determined energyresolved timescales for PT (20 . . . 100 fs), fragmentation (360 . . . 100 fs), and their coalescence at higher energies as well as (H 2 O) + 2 stabilization (∼1 ps
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