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

Abstract

Concentrations of CN in the X 2Σ, υ = 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 CN B 2Σ+ ↔ X 2Σ+ 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 2Σ 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.