Existence of a hydrated electron as a byproduct of water radiolysis was established more than 50 years ago, yet this species continues to attract significant attention due to its role in radiation chemistry, including DNA damage, and because questions persist regarding its detailed structure. This work provides an overview of what is known in regards to the structure and spectroscopy of the hydrated electron, both in liquid water and in clusters [Formula: see text], the latter of which provide model systems for how water networks accommodate an excess electron. In clusters, the existence of both surface-bound and internally bound states of the excess electron has elicited much debate, whereas in bulk water there are questions regarding how best to understand the structure of the excess electron's spin density. The energetics of the equilibrium species e-(aq) and its excited states, in bulk water and at the air/water interface, are also addressed.

The Hydrated Electron

Understanding redox and photochemical reactions in aqueous environments requires a precise knowledge of the ionization potential and electron affinity of liquid water. The former has been measured, but not the latter. We predict the electron affinity of liquid water and of its surface from first principles, coupling path-integral molecular dynamics with ab initio potentials, and many-body perturbation theory. Our results for the surface (0.8 eV) agree well with recent pump-probe spectroscopy measurements on amorphous ice. Those for the bulk (0.1-0.3 eV) differ from several estimates adopted in the literature, which we critically revisit. We show that the ionization potential of the bulk and surface are almost identical; instead their electron affinities differ substantially, with the conduction band edge of the surface much deeper in energy than that of the bulk. We also discuss the significant impact of nuclear quantum effects on the fundamental gap and band edges of the liquid.

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Electron affinity of liquid water

The wavelength dependence of the threshold for femtosecond optical breakdown in water provides information on the interplay of multiphoton, tunneling, and avalanche ionization and is of interest for parameter selection in laser surgery. We measured the bubble threshold from ultraviolet to near-infrared wavelengths and found a continuous decrease of the irradiance threshold with increasing wavelength $\ensuremath{\lambda}$. Results are compared to the predictions of a numerical model that assumes a band gap of 9.5 eV and considers the existence of a separate initiation channel via excitation of valence band electrons into a solvated state followed by rapid upconversion into the conduction band. Fits to experimental data yield an electron collision time of $\ensuremath{\approx}1\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$ and an estimate for the capacity of the initiation channel. Using that collision time, the breakdown dynamics were explored up to $\ensuremath{\lambda}=2\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$. The irradiance threshold first continues to decrease but levels out for wavelengths longer than 1.3 $\ensuremath{\mu}\mathrm{m}$. This opens promising perspectives for laser surgery at wavelengths around 1.3 and 1.7 $\ensuremath{\mu}\mathrm{m}$, which are attractive because of their large penetration depth into scattering tissues.

https://link.aps.org/accepted/10.1103/PhysRevB.94.024113

Wavelength dependence of femtosecond laser-induced breakdown in water and implications for laser surgery

Quantitative structure-activity relationship models for predicting reaction rate constants of organic contaminants with hydrated electrons and their mechanistic pathways.

The reductive conversion of CO2 into industrial products (e.g., oxalic acid, formic acid, methanol) can occur via aqueous CO2− as a transient intermediate. While the formation, structure, and reaction pathways of this radical anion have been modelled for decades using various spectroscopic and theoretical approaches, we present here, for the first time, a vibrational spectroscopic investigation in liquid water, using pulse radiolysis time-resolved resonance Raman spectroscopy for its preparation and observation. Excitation of the radical in resonance with its 235 nm absorption displays a transient Raman band at 1298 cm−1, attributed to the symmetric CO stretch, which is at ∼45 cm−1 higher frequency than in inert matrices. Isotopic substitution at C (13CO2−) shifts the frequency downwards by 22 cm−1, which confirms its origin and the assignment. A Raman band of moderate intensity compared to the stronger 1298 cm−1 band also appears at 742 cm−1 and is assignable to the OCO bending mode. A reasonable resonance enhancement of this mode is possible only in a bent CO2−(C2v/Cs) geometry. These resonance Raman features suggest a strong solute-solvent interaction, the water molecules acting as constituents of the radical structure, rather than exerting a minor solvent perturbation. However, there is no evidence of the non-equivalence (Cs) of the two CO bonds. A surprising resonance Raman feature is the lack of overtones of the symmetric CO stretch, which we interpret due to the detachment of the electron from the CO2− moiety towards the solvation shell. Electron detachment occurs at the energies of 0.28 ± 0.03 eV or higher with respect to the zero point energy of the ground electronic state. The issue of acid-base equilibrium of the radical, which has been in contention for decades, as reflected in a wide variation in the reported pKa (−0.2 to 3.9), has been resolved. A value of 3.4 ± 0.2 measured in this work is consistent with the vibrational properties, bond structure, and charge distribution in aqueous CO2−.

The nature of the CO2− radical anion in water

Since its discovery over 50 years ago, the “structure” and properties of the hydrated electron have been a subject for wonderment and also fierce debate. In the present work we seriously explore a minimal model for the aqueous electron, consisting of a small water anion cluster embedded in a polarized continuum, using several levels of ab initio calculation and basis set. The minimum energy “zero Kelvin” structure found for any 4-water (or larger) anion cluster, at any post-Hartree–Fock theory level, is very similar to a recently reported embedded-DFT-in-classical-water-MD simulation (Uhlig, Marsalek, and Jungwirth, J. Phys. Chem. Lett. 2012, 3, 3071−3075), with four OH bonds oriented toward the maximum charge density in a small central “void”. The minimum calculation with just four water molecules does a remarkably good job of reproducing the resonance Raman properties, the radius of gyration derived from the optical spectrum, the vertical detachment energy, and the hydration free energy. For the first t...

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A Simple ab Initio Model for the Hydrated Electron That Matches Experiment

The solvation structure of a solvated electron in methanol is investigated with ab initio calculations of small anion methanol clusters in a polarized dielectric continuum. We find that the lowest-energy structure in best agreement with experiment, calculated with CCSD, MP2, and B3LYP methods with aug-cc-pvdz basis set, is a tetrahedral arrangement of four methanol molecules with OH bonds oriented toward the center. The optimum distance from the tetrahedron center to the hydroxyl protons is ∼1.8 A, significantly smaller than previous estimates. We are able to reproduce experimental radius of gyration Rg (deduced from optical absorption), vertical detachment energy, and resonance Raman frequencies. The electron paramagnetic resonance g-factor shift is qualitatively reproduced using density functional theory.

A Simple ab Initio Model for the Solvated Electron in Methanol.

Relative diffusion coefficients were determined in water for the D, H, and Mu isotopes of atomic hydrogen by measuring their diffusion-limited spin-exchange rate constants with Ni2+ as a function of temperature. H and D atoms were generated by pulse radiolysis of water and measured by time-resolved pulsed EPR. Mu atoms are detected by muonium spin resonance. To isolate the atomic mass effect from solvent isotope effect, we measured all three spin-exchange rates in 90% D2O. The diffusion depends on the atomic mass, demonstrating breakdown of Stokes–Einstein behavior. The diffusion can be understood using a combination of water “cavity diffusion” and “hopping” mechanisms, as has been proposed in the literature. The H/D isotope effect agrees with previous modeling using ring polymer molecular dynamics. The “quantum swelling” effect on muonium due to its larger de Broglie wavelength does not seem to slow its “hopping” diffusion as much as predicted in previous work. Quantum effects of both the atom mass and t...

Isotope dependence and quantum effects on atomic hydrogen diffusion in liquid water

The self-recombination reaction of •CH2OH radicals in neutral aqueous solution has been studied at temperatures up to 300 °C at a pressure of 220 bar using pulse radiolysis and transient absorption. •CH2OH species decay by second-order kinetics independent of the applied dose, with a rate constant at 22 °C of 2k = 1.4 ± 0.1 × 109 M–1 s–1. The recombination follows Arrhenius behavior with the activation energy (Ea) 12.7 ± 0.9 kJ/mol and pre-exponential factor of 1.9 ± 0.4 × 1011 M–1 s–1. The overall recombination is significantly slower than the diffusion limit at elevated temperature, meaning that both disproportionation and dimerization channels have significant activation barriers. Ab initio calculations support the inference that the dimerization channel has no energy barrier, but has a large negative activation entropy barrier. The disproportionation channel (giving aqueous formaldehyde) almost certainly involves one or more specific water molecules to lower its activation energy relative to the gas p...

Hydroxymethyl radical self-recombination in high-temperature water.

Abstract The operational characteristics of Hall effect thrusters are altered by conductive surfaces in vacuum facilities. Conductive surfaces alter charge distribution in the plume by providing pathways for electron-ion recombination that do not exist in the spaceflight environment. Charge recombination pathways impact thruster performance and plume behavior through mechanisms that are not entirely understood. The incomplete understanding of the relationship between charge recombination pathways and thruster behavior limits the ability to characterize thruster performance through ground testing. This paper quantifies the effect of polarity and magnitude of body-to-cathode voltage on coupling between the thruster body and the local plasma environment. The effort operates the T-140 Hall thruster at a single, fixed operating condition of 300 V, 3.5 kW, with anode and cathode xenon flow rates of 11.6 ± 0.03 mg/s and 1.61 ± 0.12 mg/s, respectively. During data collection, the chamber was maintained at a pressure of 8.7 × 10 –6 Torr-Xe. The thruster body-to-ground voltage is manipulated by varying body-to-ground resistance. Results show the thruster pole face and body circumference couple to the local plasma environment through distinct sheaths. The polarity of the body-to-cathode voltage determines the characteristics of these sheaths. Therefore, the body-to-cathode voltage controls the interaction between the thruster body recombination pathway and the local plasma environment. 

/pdf/electrical-characteristics-of-a-hall-effect-thruster-body-in-25zge7al.pdf

Jonathan Walker

Papers

A Simple ab Initio Model for the Hydrated Electron That Matches Experiment

A Simple ab Initio Model for the Solvated Electron in Methanol.

Isotope dependence and quantum effects on atomic hydrogen diffusion in liquid water

Hydroxymethyl radical self-recombination in high-temperature water.

Electrical characteristics of a Hall effect thruster body in a vacuum facility testing environment