A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy ͑CARS͒ technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide ͑NO͒ to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the ͑1,0͒ band of the A 2 ⌺ ϩ-X 2 ⌸ electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the ͑0,0͒ band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.
A diode-laser-based sensor has been developed for ultraviolet absorption measurements of the nitric oxide (NO) molecule. The sensor is based on the sum-frequency mixing (SFM) of the output of a tunable, 395-nm external-cavity diode laser and a 532-nm diode-pumped, frequency-doubled Nd:YAG laser in a beta-barium borate crystal. The SFM process generates 325 +/- 75 nW of ultraviolet radiation at 226.8 nm, corresponding to the (v' = 0, v" = 0) band of the A2Sigma+-chi2II electronic transition of NO. Results from initial laboratory experiments in a gas cell are briefly discussed, followed by results from field demonstrations of the sensor for measurements in the exhaust streams of a gas turbine engine and a well-stirred reactor. It is demonstrated that the sensor is capable of fully resolving the absorption spectrum and accurately measuring the NO concentration in actual combustion environments. Absorption is clearly visible in the gas turbine exhaust even for the lowest concentrations of 9 parts per million (ppm) for idle conditions and for a path length of 0.51 m. The sensitivity of the current system is estimated at 0.23%, which corresponds to a detection limit of 0.8 ppm in 1 m for 1000 K gas. The estimated uncertainty in the absolute concentrations that we obtained using the sensor is 10%.
Understanding the saturation behavior of polarization-spectroscopy signals is a vital task for the development of this method as a versatile tool for quantitative detection of trace species. Recent progress in the theoretical treatment of the polarization-spectroscopy process offers the opportunity of studying its saturation behavior thoroughly. This theoretical treatment, referred to as direct numerical integration ͑DNI͒ calculations, is based on numerically demanding calculations; that is why we present a simple model that describes the curve shape of polarization spectroscopy power-dependence scans in both the saturated and the unsaturated regime. Polarization-spectroscopy-saturation curves in the copropagating beam geometry from the excitation of OH A 2 ⌺ ϩ-X 2 ⌸(0,0) at the Q 2 (8) line in a low-pressure flame were compared to both results from the DNI calculations and to our proposed analytical equation. Our simple model provides excellent fits to polarizationspectroscopy-saturation curves for absorption lines dominated by homogeneous broadening and for narrowbandwidth excitation sources. The model does not give a good agreement with experiment for lines dominated by inhomogeneous broadening. For this case an empirical equation is proposed and investigated. Our proposed model offers a starting point for a simplification of the underlying polarization-spectroscopy theory, the complexity of which has been a major obstacle to the further development of this theory.
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