Observation of a transient intermediate in the ultrafast relaxation dynamics of the excess electron in strong-field-ionized liquid water

Abstract

A unified picture of the electronic relaxation dynamics of ionized liquid water has remained elusive despite decades of study. Here, we employ sub-two-cycle visible to short-wave infrared pump-probe spectroscopy and ab initio nonadiabatic molecular dynamics simulations to reveal that the excess electron injected into the conduction band (CB) of ionized liquid water undergoes sequential relaxation to the hydrated electron s ground state via an intermediate state, identified as the elusive p excited state. The measured CB and p-electron lifetimes are 0.26 ± 0.02 ps and 62 ± 10 fs, respectively. Ab initio quantum dynamics yield similar lifetimes and furthermore reveal vibrational modes that participate in the different stages of electronic relaxation, with initial relaxation within the dense CB manifold coupled to hindered translational motions whereas subsequent p-to-s relaxation facilitated by librational and even intramolecular bending modes of water. Finally, energetic considerations suggest that a hitherto unobserved trap state resides ~0.3-eV below the CB edge of liquid water. Our results provide a detailed atomistic picture of the electronic relaxation dynamics of ionized liquid water with unprecedented time resolution.

Ultrafast Vibrational Wave Packet Dynamics of the Aqueous Guanine Radical Anion Induced by Photodetachment

“… The broad absorption above 550 nm in the contour plot can be attributed to the absorption of the hydrated electron, which is known to absorb in the 300–1000 nm range. , The decrease in the Δ A signal of the hydrated electron at ∼700 nm may be due to plasma-induced blue shifting of the probe spectrum (Section S5). This plasma-induced blue shift prevents detailed analysis of the time-resolved Δ A spectrum to retrieve the hydrated electron formation dynamics, which is the subject of numerous previous investigations. − …”

“…Under an aqueous environment, the ionization of biomolecules leads to the formation of the hydrated electron and the associated radical species, which trigger a cascade of reactions that are of fundamental importance in radiation chemistry and radiation biology. − Advancements in the generation of ultrashort laser pulses and time-resolved spectroscopic techniques over the years offer a glimpse into the dynamics of these photoinduced processes. While the dynamics of the hydrated electron have been studied extensively, − the ultrafast structural rearrangement dynamics of the associated radical species remain vastly unexplored. The ultrafast structural rearrangement of radical species impacts key biological processes such as long-range electron transfer in proteins − and DNA. − Thus, further research is needed to extend our understanding of the ultrafast structural rearrangement involving aqueous biomolecules.…”

Tracking Cavity Formation in Electron Solvation: Insights from X-ray Spectroscopy and Theory

“…Thus, many critical aspects of the electron solvation process remain unclear. Open questions include the solvation time scale, for which studies have reported values ranging from a few hundred femtoseconds to 2 ps, ,,, the influence of the preceding ionization mechanism, ,, or the specific nature of e aq – ’s immediate precursor, ,− to name a few. At the origin of this general lack of consensus lies the inherent difficulty of disentangling the vibrational and electronic responses when using optical techniques .…”

Early and Transient Formation of Highly Acidic pH Spikes in Water Radiolysis under the Combined Effect of High Dose Rate and High Linear Energy Transfer

“…The H 3 O + and • OH produced in reaction (1) are located very close to the position at which ionization occurred. On the other hand, the secondary electron (also called "dry" electron) created during the ionization event is ejected with a kinetic energy of a few tens of electron-volts [25][26][27], allowing it to travel, on average, a distance of the order of ~10 nm [28] before becoming thermalized and hydrated (at about 1.3 ps [29,30]). At this point in time, the hydrated electron (e − aq ), on the one hand, and H 3 O + and the • OH radical, on the other hand, are quite far apart [31][32][33].…”