Optical Absorption Measurements of Ground-State CN in Active Nitrogen Flames

Concentrations of CN in the $X^2\text{Σ, υ\hspace{0.17em}=\hspace{0.17em}0}$ level as low as 1 × 10⁹ molecules cm⁻³ have been detected in active nitrogen flames at pressures from 0.5 to 20 torr by optical absorption spectroscopy using a CN discharge light source and a refined ac detection method. A systematic study of $\mathrm{CN}{B}^2{\Sigma }^{+}{\mathrm{\hspace{0.17em}\leftrightarrow \hspace{0.17em}X}}^2{\Sigma }^{+}$ absorption and emission intensity as a function of pressure and fuel has provided additional information about the mechanisms of formation and removal of ground‐state CN in these systems. An estimate of 3×10⁻¹⁴ cm³ molecule⁻¹·sec⁻¹ for the rate constant for removal of $X^2\Sigma$ CN by H₂ has been obtained by determining the effect of added gases on steady‐state concentrations. High‐resolution measurements have revealed that the rotational population distribution in the ground vibrational level is thermal at pressures as low as 0.5 torr; this result is consistent with the estimated lifetime of ground‐state CN in active nitrogen flames.