Optical Emission Spectroscopic Investigation of CN-Related Emission Behavior and C-N-O Coupling in CO2/N2 Plasmas

Optical emission spectroscopy was used to investigate CN-related emission in atmospheric-pressure microsecond-pulsed spark discharges of pure N2, pure CO2, and CO2/N2 mixtures. This study examines how gas composition and pulse repetition frequency affect the relative evolution of C-, N-, O-, and CN-containing emitting species in plasma-assisted CO2/N2 carbon–nitrogen coupling. Pure N2 discharge was dominated by N2 second positive bands and N atomic lines, whereas pure CO2 discharge mainly showed C- and O-related emissions. In mixed gases, the CN violet band was observed together with C, N, and O emissions, indicating the coexistence of carbon- and nitrogen-containing reactive species. CN band fitting and atomic-line analysis indicated a non-equilibrium plasma state, with Tvib and Trot of approximately 7850 and 2150 K, an excitation temperature of 0.8–1.0 eV, and an electron density on the order of 1015 cm−3. An emission-based CN characterization index was introduced to compare the relative variation in CN-related emission. Changes in gas composition and pulse frequency modified the relative CN-, N-, and O-related emission responses, consistent with competition between C–N coupling and oxygen-related pathways. The index is a semi-quantitative spectral descriptor, not a direct measure of CN concentration or formation efficiency.