We report improved measurements of the temperature-dependent cross sections for the quenching of fluorescence from the AΣ+2(v′=0) state of NO. Cross sections were measured for gas temperatures ranging from 294to1300K for quenching by NO(XΠ2), H2O, CO2, O2, CO, N2, and C2H2. The AΣ+2(v′=0) state was populated via two-photon excitation with a picosecond laser at 454nm, and the decay rate of the fluorescence originating from AΣ+2(v′=0) was measured directly. Thermally averaged quenching cross sections were determined from the dependence of the fluorescence decay rate on the quencher gas pressure. Our measurements are compared to previous measurements and models of the quenching cross sections, and new empirical fits to the data are presented. Our new cross-section data enable predictions in excellent agreement with prior measurements of the fluorescence lifetime in an atmospheric-pressure methane-air diffusion flame. The agreement resolves discrepancies between the lifetime measurements and predictions based on the previous quenching models, primarily through improved models for the quenching by H2O, CO2, and O2 at temperatures less than 1300K.

We describe the use of linear Raman scattering for the investigation of fuel-rich sooting flames. In comparison, the frequency-tripled and -quadrupled fundamental wavelengths of a Nd:YAG laser have been used as an excitation source for study of the applicability of these laser wavelengths for analysis of sooting flames. The results obtained show that, for the investigation of strongly sooting flames, 266-nm excitation is better than 355-nm excitation. Although the entire fluorescence intensity of polycyclic aromatic hydrocarbons (PAHs) decreases with rising excitation wavelength, there is increased interference with the Raman signals by displacement of the spectral region of the Raman signals toward the fluorescence maximum of the laser-induced fluorescence emissions. Besides the broadband signals of PAHs, narrowband emissions of laser-produced C₂ occur in the spectra of sooting flames and affect the Raman signals. These C₂ emission bands are completely depolarized and can be separated by polarization-resolved detection. A comparison of the laser-induced fluorescence emissions of an ethylene flame with those of a methane flame shows the same spectral features, but the intensity of the emissions is larger by a factor of 5 for the ethylene fuel. Using 266-nm radiation for Raman signal excitation makes possible measurements in the ethylene flame also.

Time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy using picosecond-duration laser pulses is investigated for gas thermometry. The use of picosecond laser pulses significantly reduces background caused by scattering of the probe beam, and delayed probing of the Raman coherence enables elimination of interference from nonresonant four-wave mixing processes. Temperatures inferred from rotational spectra are sensitive to the probe delay because of the rotational-level dependence of collisional dephasing of Raman coherences. The sensitivity decreases, however, with increasing temperature, and accurate temperature measurements in a flame are demonstrated using a standard frequency-domain analysis of the spectra.