Flame bands: CO + O chemiluminescence as a measure of gas temperature

Carbon monoxide flame band emission (CO + O → CO₂ + hν) in CO₂ microwave plasma is quantified by obtaining absolute calibrated emission spectra at various locations in the plasma afterglow while simultaneously measuring gas temperatures using rotational Raman scattering. Comparison of our results to literature reveals a contribution of O₂ Schumann–Runge UV emission at T > 1500 K. This UV component likely results from the collisional exchange of energy between CO₂(¹B) and O₂. Limiting further analysis to T < 1500 K, we demonstrate the utility of CO flame band emission by analyzing afterglows at different plasma conditions. We show that the highest energy efficiency for CO production coincides with an operating condition where very little heat has been lost to the environment prior to ∼3 cm downstream, while simultaneously, T ends up below the level required to effectively freeze in CO. This observation demonstrates that, in CO₂ plasma conversion, optimizing for energy efficiency does not require a sophisticated downstream cooling method. (paper)