Plasma-induced reversible surface modification and its impact on oxygen heterogeneous recombination

Abstract

A novel model is developed for atomic oxygen surface kinetics in silica-like walls, introducing a plasma-induced surface modification, which may impact intermediate pressure plasma reactors. The model is the first to reproduce experimental measurements in an oxygen glow discharge operating in the pressure range between 0.27 mbar (0.2 Torr) and 4 mbar (3 Torr), showing a decrease with pressure of the O recombination probability on Pyrex between 0.27 mbar and 1 mbar. The numerical simulations suggest that a modification is induced by the production and destruction of metastable chemisorption sites at the surface. As such, the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) recombination mechanisms take place involving not only physisorption (1) and stable chemisorption (2) sites, but also metastable chemisorption sites (3), produced by the impact of fast O2 ions and neutrals. The presence of metastable sites can be reversed by increasing the plasma pressure.

Surface recombination in Pyrex in oxygen DC glow discharges: mesoscopic modelling and comparison with experiments

“…As explained in the works by Guerra andMarinov 2016, Marinov et al 2017), KMC allows to obtain more accurate results than the deterministic method that takes approximations concerning the diffusion of physisorbed species. In this work, KMC is not employed, since the studied conditions are similar to those in the works by Marinov et al (2017) and by Afonso et al (2024), where it was confirmed that the KMC and deterministic results are close, within a 20% difference.…”

“…This effect was attributed by Booth et al (2019) to incident ions and fast neutrals with sufficient kinetic energy to clean or chemically modify the surface, generating new adsorption sites. This idea was further developed by Afonso et al (2024). At pressures above 0.75 Torr, Booth et al (2019) found an approximate Arrhenius behaviour of γ O with the incident atom temperature.…”

“…In these conditions, ions are assumed to have low energies and thus to not modify the surface. The surface kinetics of atomic oxygen recombination in Pyrex for pressures below 0.75 Torr, conditions in which the ion kinetic energy can have an effect on surfaces, was recently addressed by Booth et al (2019) and by Afonso et al (2024). The cylindrical tube outer surface is kept at a constant temperature variable between −20 • C and 50 • C. This is guaranteed by a water/ethanol mixture flowing through an outer envelope and connected to a thermostatic bath.…”

“…In this work the deterministic method is numerically employed for different reaction schemes, as explained by Marinov et al (2017) and recently recalled by Afonso et al (2024). Through the deterministic method, a system of rate balance equations is solved for the surface densities n i of every species i.…”

“…In the works by Guerra and Loureiro (2004), Guerra and Marinov (2016), Marinov et al (2017)and Afonso et al (2024), another method is used to solve the system of surface reactions, the Kinetic Monte Carlo (KMC) method. Instead of dealing with average surface coverages the KMC method follows every particle and every site on the surface.…”