Nitrogen Atom Detection in Low-Pressure Flames by Two-Photon Laser-Excited Fluorescence

Nitrogen atoms have been detected in stoichiometric flat premixed H2/O2/N2 flames at 33 and 96 mbar doped with small amounts of NH3, HCN, and (CN)2 using two-photon laser excitation at 211 nm and fluorescence detection around 870nm. The shape of the fluorescence intensity profiles versus height above the burner surface is markedly different for the different additives. Using measured quenching rate coefficients and calibrating with the aid of known N-atom concentrations in a discharge flow reactor, peak N-atom concentrations in these flames are estimated to be on the order of 1012-5 × 1013 cm -3 ; the detection limit is about 1 × 1011 cm -3. PACS: 82.40Py, 33.50Dq In the combustion of nitrogen-containing fuels, a variety of conditions have been identified under which atomic nitrogen is of importance. For example, N atoms can influence the product distribution in the conversion of fuelbound nitrogen to N2 or NOx. Depending on the particular nitrogen source and the temperature and equivalence ratio of the combustion process, the reactions of N atoms with e.g. OH, NO, or CH3 can lead to the formation of NO, N2, or HCN. A detailed treatment of the gas-phase nitrogen chemistry in combustion can be found in the recent review by Miller and Bowman [1]. Nitrogen atom concentrations have been measured by ESR [2] and by atomic resonance absorption (ARAS) [3]. ESR spectroscopy has been used to detect N atoms in discharge flow reactors [2]. The ARAS technique requires vacuum UV radiation for the detection of N atoms; although it has proven most successful in shock tube experiments [3], flames usually are not transparent for these short wavelengths. Molecular beam sampling techniques coupled with mass spectrometry [4] were used in an investigation of the structure of a 45mbar NH3/O2/Ar flame. In this experiment, N concentrations were found to be below the detection limit (10 .5 ) of the apparatus throughout the flame. Their role in the kinetic mechanism was thus judged to be marginal [4]. The recent kinetic model of [1], however, predicts for this flame a mole fraction of N atoms which is about a factor of 30 higher than the upper limit stated in [4]. Upon addition of small amounts of HCN as N-containing fuel to a 33 mbar H2/O2/Ar flame, Miller etal. [5] find that the NO mole fraction is very sensitive to nitrogen atom reactions. Their conclusions on the importance of N atoms in the processes of NO formation and of conversion of NO to N2 under these conditions are in agreement with earlier work of Haynes [6] and Morley [7]. Although N atoms were not detected in their experiment, Miller et al. [5] predict N atom mole fractions of 5 x 10 .5 to 10 -4 in their flames. In the context of these different flame studies, the bxperimental det'ermination of N atom concentrations for specific combustion situations is expected to provide information which might be used to examine current chemical-kinetic models of the nitrogen chemistry in flames. Two-photon laser-induced fluorescence is a non-perturbing optical diagnostic technique that provides excellent spatial resolution and sufficient sensitivity for the in-situ measurement of light atom concentrations under combustion-relevant conditions. Two-photon excitation of N atoms at about 211 nm and fluorescence detection at about 870 nm has been used in discharge flow reactor studies [8-11] to investigate collisional energy transfer out of and among the N (3p 4D°) manifold. A similar two-photon laser-excited fluorescence scheme has led to the detection of atomic nitrogen in atmospheric-pressure flames [12]. Various multi-photon excitation techniques have been used to detect hydrogen and oxygen atoms in flames [13-18]. In our group, we have recently developed a calibration method which allows spatially resolved measurements of H and O atom concentrations in low-pressure flames [19, 20]. This method relies on Nitrogen Atom Detection in Low-Pressure Flames a discharge flow reactor as a calibration standard: The two-photon laser-excited fluorescence signal arising from a known atom concentration produced in the flow reactor under identical excitation and detection conditions is related to the fluorescence signal produced by the unknown atom concentration at a particular location in the flame. Considering the different contributions of, in particular, coltisional processes in both systems by solving the appropriate differential equations for the atomic levels involved, this calibration technique was capable of measuring H and 0 atom concentrations with a typical accuracy of 25% in low-pressure premixed H2/Oz/Ar flames. In this paper we report an extension of this method to the quantitative detection of N atoms in flames. This study aims at the spatially-resolved measurement of N atom concentrations in flat premixed low-pressure flames which might be suitable for a comparison with current modeling of the nitrogen chemistry. For this purpose, it seemed desirable to start from reasonably wellknown flame conditions as provided, for example, by the low-pressure H2/O2 flames studied in our group before [19-23]. Small amounts of HCN, (CN)2, or NH3 were added as N atom precursors to H2/O2 flames diluted with N2. As the detailed nitrogen chemistry might be dependent on the nature of the additive, the shape of the N atom profile and the position of the local N atom concentration maximum was expected to be different for each additive.