Time-resolved molecular non-equilibrium spectra in nanosecond laser induced air plasma

We performed a quantitative analysis of time-resolved laser-induced breakdown air plasma spectra to obtain the evolution of temperatures and species relative fractions. The air plasma was generated by focusing a 100 mJ Nd:YAG laser pulse, and the time-resolved spectra were recorded by an intensified charge-coupled device camera with incremental delay. The attention was mainly focused on the emission spectra of the first negative system of nitrogen (${\mathrm{N}}_2^{+}$, ${\mathrm{B}}^2{\Sigma }_{\mathrm{u}}^{-}$–${\mathrm{X}}^2{\Sigma }_{\mathrm{g}}^{+}$) and the violet system of carbon nitride (CN, B²Σ⁺–X²Σ⁺) located at 383–396 nm. A custom-built model was developed to perform the simulation and fitting of the ${\mathrm{N}}_2^{+}$ and the CN spectra from the air plasma. The model was verified by comparing to a published model with a 0.9860 Spearman correlation coefficient. With this model, the time-resolved non-equilibrium temperatures and relative fractions of ${\mathrm{N}}_2^{+}$ and CN were obtained with a fitting correlation coefficient higher than 0.9108.