Dissociation and Ionization Due to Direct Electron Impact onto Liquid Surfaces

When using negative discharges onto liquids at atmospheric pressure, an anode-like sheath forms at the liquid surface onto which electrons can be accelerated with energies of a few to tens of eV. Electrons striking the liquid surface can result in dissociation and ionization, which can be also induced by heavy particle and photons. These processes can be dominant mechanism in plasma-based water treatment intended to degrade surfactant-like molecules such as PFAS. When using positive discharges, analogous processes can occur with the formation of a cathode-like sheath accelerating positive ions into the liquid. The contributions of these direct electron and ion impact processes to chemical activation of the liquid are poorly known.

In this paper, reactions between gas phase electrons and surface-resident molecules in liquids will be computationally investigated using nonPDPSIM, a 2-dimensional model in which multiphase plasmas can be addressed. The investigation aims to evaluate the contribution of electron impact dissociation and ionization onto the water surface compared to that of heavy particle impact, photon stimulated processes, and solvation. The test systems are pulsed negative discharges onto the water to quantify the sources of OH and H₂O⁺ production in the surface layer. The dissociation of surface-resident fluorocarbon molecules will be discussed as an example of electron impact reactions onto water with a surfactant.