Quantitative Radar REMPI measurements of methyl radicals in flames at atmospheric pressure
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Quantitative Radar REMPI measurements of methyl radicals in flames at atmospheric pressure Yue Wu • Zhili Zhang • Timothy M. Ombrello Viswanath R. Katta
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Received: 20 August 2012 / Accepted: 16 January 2013 / Published online: 31 January 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Spatially resolved quantitative measurements of methyl radicals (CH3) in CH4/air flames at atmospheric pressure have been achieved using coherent microwave Rayleigh scattering from Resonance enhanced multi-photon ionization, Radar REMPI. Relative direct measurements of the methyl radicals were conducted by Radar REMPI via the two-photon resonance of the 3p2 A002 000 state and subsequent one-photon ionization. Due to the proximity of the argon resonance state of 2s22p54f [7/2, J = 4](4?1 REMPI by 332.5 nm) with the CH3 state of 3p2 A002 000 (2?1 REMPI by 333.6 nm), in situ calibration with argon was performed to quantify the absolute concentration of CH3. The REMPI cross sections of CH3 and argon were calculated based on time-dependent quantum perturbation theory. The measured CH3 concentration in CH4/air flames was in good agreement with numerical simulations performed using detailed chemical kinetics. The Radar REMPI method has shown great flexibility for spatial scanning, large signal-to-noise ratio for measurements at atmospheric pressures, and significant potential to be straightforwardly generalized for the quantitative measurements of other radicals and intermediate species in practical and relevant combustion environments.
Y. Wu Z. Zhang (&) Mechanical, Aerospace and Biomedical Engineering, University of Tennessee, Knoxville, TN 37996, USA e-mail: [email protected] T. M. Ombrello Air Force Research Laboratory, Aerospace Systems Directorate, Wright-Patterson AFB, Dayton, OH 45433, USA V. R. Katta Innovative Scientific Solutions Incorporated, Dayton, OH 45440, USA
1 Introduction The methyl radical (CH3) is one of the most important species in combustion [1], atmospheric chemistry [2], chemical vapor deposition [3], and ultrafast kinetics [4, 5]. In combustion, ignition and flame propagation reactions [via hydrogen abstraction by H atoms and hydroxyl radicals (OH)] is controlled by the CH3 radical [1]. The formation of polycyclic aromatic hydrocarbons (PAH) and soot is also highly dependent on this radical [6]. However, quantitative measurements of CH3 are very challenging, especially at elevated pressures. The methyl radical is characterized by a strong predissociation of its electronically excited states, preventing fluorescence detection. For this reason, direct measurements of CH3 have mostly been conducted using absorption-based methods [3, 7–9], conventional REMPI [10–12] and degenerate four wave mixing (DFWM) [13, 14]. Absorption-based methods, either direct absorption of CH3 or cavity ring-down spectroscopy, have the limitation of path integration, leading to limited spatial resolution. Conventional REMPI has a low signalto-noise ratio for measurement of the CH3 distribution in flames at atmospheric pressure [1
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