Optical emission study of nonthermal plasma confirms reaction mechanisms involving neutral rather than charged species.

Charge transfer reactions are commonly used to explain NOx conversion in nonthermal plasma. An analysis of optical emission spectra induced by pulsed corona discharge in NOx-containing argon suggests that, in fact, the contribution of charge transfer reactions to NOx conversion in nonthermal plasma is negligible. During electrical discharge in such gas mixtures, NO(B), an electronic excited state of NO formed due to the dissociative recombination reactions of NO2+ and N2O+ and the optical emission of NO(B) could be a proof that cations are responsible for NOx conversion. However, the optical emission of NO(B) is not observed, leading to the conclusion that cations are not involved to any measurable degree. Therefore, charge transfer reactions cannot play a significant role in nonthermal plasma largely because the cations are neutralized with electrons before any charge transfer reactions can occur and concentrations of radicals are far higher than those of cations, which inhibits charged particle reactions. Instead, neutral active species, such as atoms, molecular fragments, and excited molecules, are the major active species contributing to nonthermal plasma reactions. [ABSTRACT FROM AUTHOR]