The Photochemical Birth of the Hydrated Electron in Liquid Water

TL;DR

This work demonstrates how UV excitation in liquid water birth the hydrated electron through two distinct excited-state pathways, HAT and PCET, and shows that ultrafast solvent motions and HB-network defects critically direct the outcome. By combining excited-state molecular dynamics with ROKS and rigorous spin-density analyses, the study links localization dynamics to transient species such as $\mathrm{HO^{\bullet}}$, $\mathrm{H_3O^+}$, and the hydrated electron, while predicting emission behavior tied to electron localization. The findings reconcile several time-resolved spectroscopic observations and establish a robust framework for studying photoinduced solvated electrons in bulk water, interfaces, and salted environments, with potential extensions to nonadiabatic effects and machine-learning enhancements. Overall, the paper provides a unified, defect-aware picture of how light initiates and steers the early-stage photochemistry of water, strengthening connections between theory and a broad spectrum of experimental data.

Abstract

The photophysics and photochemistry associated with irradiating UV light in liquid water is central to numerous physical, chemical and biological processes. One of the key events involved in this process is the generation of the hydrated electron. Despite long study from both experimental and theoretical fronts, a unified understanding of the underlying mechanisms associated with the generation of the solvated electron have remained elusive. Here, using excited-state molecular dynamics simulations of condensed phase photoexcited liquid water, we unravel the key sequence of chemical events leading to the creation of the hydrated electron on the excited state. The process begins through the excitation localized mostly on specific topological defects in the hydrogen-bond network of water which is subsequently followed by two main reaction pathways. The first, leads to the creation of a hydrogen atom culminating in non-radiative decay back to the ground-state within 100 femtoseconds. The second involves a proton coupled electron transfer, giving rise to the formation of the hydronium ion, hydroxyl radical and the hydrated excess electron on the excited-state. This process is facilitated by ultrafast coupled rotational and translational motions of water molecules leading to the formation of water mediated ion-radical pairs in the network. These species can survive on the picosecond timescale and ultimately modulate the emission of visible photons. All in all, our findings provide fresh perspectives into the interpretation of several independent time-dependent spectroscopies measured over the last decades, paving the way for new directions on both theoretical and experimental fronts.

Figures (18)

• Figure 1: Photo-initial absorption of neat liquid water. Panel A: Experimental absorption spectra in solid red line (reproduced from the ref yamamoto2020ultrafast) and the calculated $S_0\to S_1$ excitation using ROKS for 100 different conformations obtained from the electronic ground state. Panel B: Probability Counts of water molecules involved in the excitation using the Inverse Participation Ratio of the spin densities for all the conformations. The insets show two examples involving 1 and 5 water molecules. Panel C: Probability Counts of the different defects in the Hydrogen Bond (HB) Network of all the water molecules involved in the initial excitation. The insets show the molecular geometries of the normal and each type of defect in the HB network.
• Figure 2: Photo-physical decay mechanisms in neat liquid water upon excitation to the $S_1$ electronic state. The upper panels show the H Atom Transfer mechanism and the lower panels the Proton Coupled Electron Transfer mechanism. Panel A and D: evolution of the potential energies of the $S_1$ and $S_0$ electronic states along a representative trajectory for both mechanisms. Panel B: shows the evolution of the coordination number of the hydrogen that is dissociating in the excited state dynamic. Panel C: evolution of the distance between the electron center and the closest H. Panel E: shows the evolution of the gyration radius of the electron. Panel F: shows the evolution of the distance between the electron center and the closest H.
• Figure 3: H Atom Transfer Mechanism. Two-dimensional PDF of the coordination number of the closest H to the center of the electron and the coordination number of the closest O to this H. Red circles show the values for the conformations at the crossing $S_1\to S_0$ transition. Panel A: shows the water molecule at the initial steps. Panel B: shows the H atom in a empty cavity. Panel C: shows the formation of the radical $\mathrm{H_3O}^{\bullet}$. Panel D: shows the water molecules solvating the H atom.
• Figure 4: Proton Coupled Electron Transfer Mechanism. Panel A: Probability Density Function of the gyration radius and the energy gap for all the trajectories. Red circles represent the conformations at the crossing point. Insets show the electronic spin density for representative conformations of the system at two different times during the excited state dynamics and at different iso-values. Panel B: Probability Density Function of the gyration radius and the angle formed between the vector pointing from the Oxygen to the electron center and the vector of the O-H of the waters in the first solvation shell. Red circles represent the conformations at the crossing point. Insets show schematic representations of the water at the two gyration radius along with the vectors considered for the analysis.
• Figure 5: Collective motions in the excited states dynamics. Panel A: Average cosine angle between the dipole vector of each water with respect to the same vector at the end of the simulation. The waters that end up solvating the electron are plotted in purple and the rest of waters in green. Shadows represent the errors across all the trajectories and the waters. Panel B: Average diffusion distance of each water molecule. The waters that end up solvating the electron are plotted in purple and the rest of waters in green. Shadows represent the errors across all the trajectories and the waters. The upper axes represents the percentage of trajectories that remain in $S_1$ between all the trajectories undergoing the PCET mechanism. Panel C: 2D density plot between the gyration radius and the distance between the radical $\mathrm{HO}^{\bullet}$ and the ion $\mathrm{H_3O}^+$. Panel D: 2D density plot between the Energy gap $S_1$ and $S_0$ and the distance between the radical $\mathrm{HO}^{\bullet}$ and the ion $\mathrm{H_3O}^+$.