Reinvestigation of the spectroscopy of the (A)over-tilde3Πu-(X)over-tilde3Σg- transition of the NCN radical at high temperature: Application to quantitative NCN measurement in flames

The initiation reaction of the prompt-NO mechanism in flames (CH + N-2 = NCN + H) involves the NCN radical. This radical is a very minor species reaching only a few hundreds of ppb in mole fraction in hydrocarbon/air flames, which makes the experimental determination of its absolute concentration profile challenging. The aim of this study is (1) to show that unambiguous assignment of absorption spectral lines of the NCN radical can be obtained in selected flames and (2) to provide the temperature-dependent NCN absorption cross sections needed to determine the absolute NCN concentration. In this work, NCN spectroscopy of the A-X band is fully revisited using theoretical spectroscopic calculations. Assessment of the spectroscopic constants is based on comparison of the theoretical spectrum, simulated with the PGOPHER program, with an extended LIP spectrum obtained in a low-pressure acetylene/oxygen/nitrogen flame. From the calculated electronic transition moments and the partition function, the temperature-dependent absorption cross section of NCN sigma(T) has been calculated at the most intense bandhead wavelength (329.13 nm). Cross-section temperature dependence is in very good agreement with that experimentally obtained by Dammeier and Friedrichs [J. Phys. Chem. A 114 (2010) 12963] using NCN3 pyrolysis in a shock tube followed by single-pass NCN absorption measurement, though their values differ by a factor of 2.6. The first NCN absorption spectrum between 329.0 and 329.3 nm obtained by CRDS at high temperature in a flame is provided here, from which the bandhead is clearly identified. Occurrence of possible interferences has been examined. Finally, peak mole fractions obtained by CRDS are given in three low-pressure premixed flames by reconsidering previous published data and adding a new datum. The experimental absorption cross section required for this determination is derived from the theoretical one, taking into account the laser bandwidth. The accuracy of the NCN measurements is discussed and highlights the benefit of coupling CRDS and LIF. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.