NCN quantitative measurement in a laminar low pressure flame

摘要

The present work has been performed to quantitatively determine the NCN radical concentration profile in a laminar low-pressure CH_4/O_2/N_2 (Φ = 1.25) flame. Laser induced fluorescence (LIF) technique has been implemented in order to record a NCN spectrum in the 326.7-330.1 nm range in flame conditions, and secondly to measure the spatially resolved NCN profile in the flame by exciting the main congested branch of NCN around 329.13 nm. The LIF profile has been calibrated into absorbance using cavity ring-down spectroscopy (CRDS). The calibration into absolute concentration has been done by using the PGOPHER spectral simulation program. The experimental and the simulated NCN spectra were found to be in very good agreement. It has allowed reconsidering the exact assignment of the transitions involved in the strong bandhead at flame temperature, which actually originate from the contribution of both the origin (000) band and the (010∑~+) component of the (010) band contrary to ambient temperature where only the (000) band contribution is significant around 329.13 nm. Thanks to the simulation and using electronic transition dipole moments extracted from available NCN fluorescence lifetimes, we provide a direct determination of the absorption cross-section: σ(λ = 329.13 nm) = (9.4 ± 1.8) × 10~(-17) cm~2/molecule at 1830 K. This cross-section was found constant within ±15% in the 1500-1900 K temperature range. Its determination is very important for research groups concerned with NCN chemistry at high temperature and prompt-NO formation. From that cross-section the peak mole fraction of the NCN radical has been determined to be equal to 170 ± 90 ppb in the low-pressure flame. The experimental uncertainty is mainly linked to the CRDS measurement conditions close to the detection limit and should be reduced in flame conditions yielding higher NCN concentration.