We report a picosecond laser study of the transient absorption of hydrated electrons generated by the 3-5 eV multiphoton ionization of liquid water. The geminate kinetics indicate that e aqis produced by at least three different mechanisms over this energy range. Power dependence of the signal amplitude shows a two-photon threshold for 4.0 eV excitation and a three-photon threshold absorption at 3.47 eV, consistent with two-or three-photon excitation of the A ˜(1 B 1 ) lowest excited state. For (three-photon) excitation in the range 3.02-3.47 eV very little (e15%) geminate recombination is observed while for the (two-photon) excitation at shorter wavelengths significant recombination (g55%) is observed. In the region of 3.85-4.54 eV, photonenergy-independent kinetics indicate that e aqis produced via two-photon excitation of the A ˜state followed by an ionization process in which the electrons do not obtain any excess kinetic energy. For photon energies in the range of 4.75-5.05 eV, the escape fraction increases slightly, consistent with two-photon excitation of higher energy states. Simulation with a diffusion model shows that the electron is ejected at least 25 Å farther into the bulk for the 3.02-3.47 eV photon energies relative to two-photon ionization in the 3.67-5.0 eV range. We conclude that the larger distances result from a (3 + 1)-photon resonance-enhanced multiphoton ionization (REMPI) process, made possible by visible/near-UV absorption of the water excited states. Possible mechanisms of the water ionization are discussed. A new mechanism is proposed to explain the production of solvated electrons from excitation of the A ˜(1 B 1 ) state of liquid water, well below the Born-Oppenheimer ionization threshold. On the basis of the gas phase properties of this state, we assume a direct dissociation to give OH radical and H atoms, with the excess energy almost entirely transferred to kinetic energy of the H atoms: H 2 O* f OH + H(hot). It is proposed that the hot H atoms immediately react with an adjacent water molecule to form hydronium ion and a solvated electron, in a process analogous to the thermal reaction of H atoms with water at elevated temperatures: H(hot) + H 2 O f H 3 O + + e aq -.

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Solvated electrons are typically generated by radiolysis or photoionization of solutes. While plasmas containing free electrons have been brought into contact with liquids in studies dating back centuries, there has been little evidence that electrons are solvated by this approach. Here we report direct measurements of solvated electrons generated by an atmospheric-pressure plasma in contact with the surface of an aqueous solution. The electrons are measured by their optical absorbance using a total internal reflection geometry. The measured absorption spectrum is unexpectedly blue shifted, which is potentially due to the intense electric field in the interfacial Debye layer. We estimate an average penetration depth of 2.5±1.0 nm, indicating that the electrons fully solvate before reacting through second-order recombination. Reactions with various electron scavengers including H+, NO2−, NO3− and H2O2 show that the kinetics are similar, but not identical, to those for solvated electrons formed in bulk water by radiolysis.

Spectra of the hydrated electron in pressurized light and heavy water at temperatures up to and beyond the critical temperature are reported, for wavelengths between 0.4 and 1.7 microm. In agreement with previous work, spectra can be approximately represented by a Gaussian function on the low-energy side, and a Lorentzian function on the high-energy side in subcritical water, but deviations from this form are very clear above 200 degrees C. The spectrum shifts strongly to the red as temperature rises. At supercritical temperatures, the spectrum shifts slightly to the red as density decreases, and the Gaussian-Lorentzian form is a very poor description. Application of spectral moment theory allows one to make an estimate of the average size of the electron wave function and of its kinetic energy. It appears that for water densities below about 0.6 g/cc, and down to below 0.1 g/cc, the average radius of gyration for the electron remains constant at around 3.4 angstroms, and its absorption maximum is near 0.9 eV. For higher densities, the electron is squeezed into a smaller cavity and the spectrum is shifted to the blue. The enthalpy and free energy of electron hydration are derived as a function of temperature on the basis of existing equilibrium data and absolute proton hydration energies derived from the cluster-based common point method. In a discussion, we compare the effective "size" of the hydrated electron derived from both methods.

Since its discovery over 50 years ago, the “structure” and properties of the hydrated electron has 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 (UMJ: Uhlig, Marsalek, and Jungwirth, Journal of Physical Chemistry Letters 2012, 3, 3071-5), 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 time we also successfully calculate the EPR g-factor and (low temperature ice) hyperfine couplings. The simple tetrahedral anion cluster model conforms very well to experiment, suggesting it does in fact represent the dominant structural motif of the hydrated electron.