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 , particularly the band at 337 nm, assuming rotational–translational equilibrium. However, trace impurities can induce (A–X) emission that overlaps with this band and distorts temperature evaluation. We analyze optical emission from an atmospheric‐pressure dielectric barrier discharge and a low‐pressure radiofrequency plasma. Small amounts of water vapor lead to formation, despite the absence of other hydrogen‐related markers in the DBD. Neglecting contributions causes significant errors in rotational temperatures. Simultaneous fitting of SPS () and emission mitigates this effect and improves consistency.
Houben et al. (2026) studied this question.