Kinetics Driving H2(a) Continuum Emission in Low-Frequency Ar-NH3 Dielectric Barrier Discharges at Atmospheric Pressure

Abstract

Time-resolved optical emission and absorption spectroscopy was used to analyze a 50 kHz Ar-NH₃ dielectric barrier discharge operated in a homogeneous glow discharge regime at atmospheric pressure. In addition to the typical NH(A-X), N₂(C-B), and Ar(2p-1s) transitions, a continuum emission linked to de-excitation of \({{\text{H}}}_{2}\left({{\text{a}}}^{3}{\Sigma }_{{\text{g}}}^{+}\right)\) states was detected between 180 and 250 nm and lasted for a long time after discharge extinction. Over the range of experimental conditions investigated, the emitting \({{\text{H}}}_{2}\left({{\text{a}}}^{3}{\Sigma }_{{\text{g}}}^{+}\right)\) states are proposed to be populated by collisions of \({{\text{H}}}_{2}\left({{\text{X}}}^{1}{\Sigma }_{{\text{g}}}^{+}\right)\) with Ar(1s) states during discharge, and by dissociative recombination of the vibrationally-excited ammonia ion (NH₃⁺(v)) after the discharge. NH₃⁺(v) is produced by charge transfer from Ar₂⁺ to NH₃, and it breaks into \({{\text{H}}}_{2}\left({{\text{a}}}^{3}{\Sigma }_{{\text{g}}}^{+}\right)\) (or \({{\text{H}}}_{2}\left({{\text{c}}}^{3}{\Pi }_{u}\right)\) or \({{\text{H}}}_{2}\left({{\text{d}}}^{3}{\Pi }_{u}\right)\)) and NH upon gas phase recombination with a low-energy electron. Based on this proposed mechanism, a 1D fluid model was refined to include these reactions and used to simulate the emission intensity from \({{\text{H}}}_{2}\left({{\text{a}}}^{3}{\Sigma }_{{\text{g}}}^{+}\right)\) and revealed good agreement with experimental data.