Synthetic molecular spectra modeling for determining rotational, vibrational, and excitation temperatures of low-pressure nitrogen plasma

Nitrogen plasma is highly sought after in industries due to its unique properties, including high chemical reactivity and internal energy, which enhance process efficiency. However, a detailed understanding of thermal characteristics of nitrogen plasma is required to control nitrogen plasma properties and produce desirable reactants. Therefore, in this work, we proposed an advanced numerical simulation by clustering synthetic spectra of three molecular band systems (i.e., N₂ B-A, N₂ C-B, and N₂⁺ B-X). Through this method, rotational, vibrational, and excitation temperatures of excited species in inductively coupled nitrogen plasmas (5 mTorr, 50 W - 900 W) were determined. Results revealed that rotational and vibrational temperatures increased at various input power with an inflection point at around 250 W. However, the excitation temperature decreased with input power below 250 W but increased with input power above 250 W. We also found that energy states were not in thermal equilibrium and that vibrational temperature and excitation temperature were reciprocally related. Finally, correlations between dissociation, ionization, and excitation temperature were discussed by examining N I and N₂⁺ intensities as a function of input power.