Role of metastable excited molecules in non-equilibrium plasma of dielectric barrier discharges in air at different humidity

Low-temperature air plasmas in dielectric barrier discharges (DBD) are of great interest for science and practice, e.g., in materials processing [1,2]. But despite the advances in DBD plasma physics, the non-equilibrium kinetics of DBD plasma chemistry in humid air is not very clear, especially regarding metastable electronically excited states (MES) of reactive oxygen and nitrogen species (RONS). We tried to study this issue in more detail. Experiments were carried out in oxygen-nitrogen mixtures at different humidity using electrical and optical spectroscopic diagnostics. Measurements indicated that DBD plasma processes in humid air are accompanied by the formation of long-lived RONS. Calculations with using 0D-kinetic and 1D-fluid models, considering ionization, excitation, dissociation-recombination, relaxation, collisional quenching, and radiative decay, revealed the most probable mechanisms of plasma-chemical reactions. Effects of MES involving singlet oxygen and triplet nitrogen on the production of ozone and hydroxyls in various timescales were examined. The study showed the relative role of different MES/RONS in humid air DBD plasma and pointed the way to more efficient DBD plasma processing. [1] A. Fridman, L.A. Kennedy, Plasma Physics and Engineering, Taylor & Frencis, 2021. [2] I. Adamovich et al., J. Phys. D: Appl. Phys. 55, 373001 (2022).