Impact of NH Emission on Determining the Rotational Temperature From the Second Positive System of N2

Abstract

The gas temperature is a key parameter in low-temperature plasmas, strongly influencing plasma chemistry and energy balance. It is commonly determined from the second positive system (SPS) of N2, particularly the (0,0) band at 337 nm, assuming rotational–translational equilibrium. However, trace impurities can induce NH (A–X) emission that overlaps with this band and distorts temperature evaluation. We analyze optical emission from an atmospheric-pressure N2 dielectric barrier discharge and a low-pressure radiofrequency plasma. Small amounts of water vapor lead to NH formation, despite the absence of other hydrogen-related markers in the DBD. Neglecting NH contributions causes significant errors in rotational temperatures. Simultaneous fitting of N2 SPS (Δ𝜈 =0) and NH emission mitigates this effect and improves consistency.