DOI: 10.7274/25527739
title: Experimental and Theoretical Studies of the Solvated Electron at Plasma-Liquid Interfaces

description: Plasma-liquid interactions is an extremely wide field, encompassing a wide variety of applications: the reduction of metal cations in solution to form colloidal nanoparticles, the generation of reducing and oxidizing species to destroy contaminants in wastewater, the application of plasma to tissue in order to promote wound healing or kill malignant cells, and as an analytical tool to determine the constituent species in a solution. There are also a number of ways to apply plasmas to a liquid, including forming a discharge inside a gas bubble in solution, via advection in a plasma jet, and with a direct electrical connection as in plasma electrolysis, which is the focus of this work. 
Many of the reactive species formed at the plasma-water interface are well known, most notably the solvated electron and hydroxyl radical, and their reactions have been well characterized in the field of radiation chemistry. However, there are many open questions about the physics of their formation, and their subsequent dynamics and lifetime in a plasma-liquid system. It is only recently that solvated electrons were directly observed at the plasma-liquid interface, using total internal reflection absorption spectroscopy (TIRAS). This work builds on these results, and uses the TIRAS diagnostic to assess theoretical models of the behavior of primary products at the plasma-liquid interface. First, a study is conducted of the dynamics of the TIRAS discharge, observing its changing size as a function of current to determine the corresponding effect on the plasma-laser overlap. The size is observed to change throughout the duration of a TIRAS cycle at 20 kHz, but slowly enough for the discharge to reach a steady state at its maximum and minimum current. Quantities extracted from TIRAS, interfacial concentration and penetration depth, are also observed to be extremely sensitive to even small errors in measurement of current density. This motivates modifications to the TIRAS system, setting the low current state to 0 A, which simplifies measurement of current density and removes a large source of potential error. Experimental measurements are then used to confirm an outstanding theoretical model of interfacial solvated electron concentration and penetration depth as a function of current density in plasma electrolysis with a liquid anode, by observing the scaling behavior of current density. These results are used to develop an improved theoretical model, which accounts for both current density and modulation frequency, and predict upper bounds of penetration depth and concentration in plasma electrolysis, 120 nm and 0.6 mM respectively. Next, the formation of solvated electrons in a discharge with a liquid cathode is studied. TIRAS is used to confirm the presence of solvated electrons in this configuration. Previous measurements of yield from the dissociative attachment of chloroacetate are used to measure their yield, 1.04±0.59 per incident ion. Based on this high yield, a mechanism is proposed for the ionization of water in a liquid-cathode discharge, wherein excess energy from ion neutralization drives a proton-coupled electron transfer to an existing solvation trap. To test this, measurements of secondary emission coefficient are were carried out in both krypton and xenon, which have a lower ionization energy and should drive fewer ionization events. However, secondary emission was instead found to increase in these gases, as a function of ion mass, indicating secondary emission is a separate process from solvated electron formation driven by physical emission of water molecules. Solvated electron yield is then measured by comparing the faradaic efficiency of chloride production from chloroacetate in krypton and argon discharges, which is found to be the same within error. Lastly, these results, as well as TIRAS’ current limitations, serve as motivation for a proposal of future experiments using this measurement, and of a proposed future system, which might enable TIRAS to perform more consistent, quantitative measurements.