Experimental and theoretical investigations of coherent anti-Stokes Raman scattering as a nonintrusive diagnostic for gas-phase systems

In this study, two coherent anti-Stokes Raman scattering (CARS) diagnostic techniques were investigated. The first study involved the experimental examination of high-resolution N₂ CARS measurements using a modeless dye laser as the Stokes beam source to reduce the presence of mode noise. A new spectra-fitting procedure was developed to avoid starting-point bias in the least-squares fitting results. Time-averaged and single-shot measurements of pressure were made in a static pressure vessel over the range from 0.1 to 4.0 atm to test the pressure sensitivity of the technique. In addition, the precision of these single-shot measurements is indicative of the baseline pressure-fluctuation detection limit. The precision uncertainty of the measurements was studied to investigate the possibility of making property fluctuation measurements in high-speed flows. Centerline measurements of pressure and temperature in an underexpanded jet (M_j = 1.85) were obtained to determine the performance of the technique in a compressible flowfield. Improvements in accuracy for time-averaged and single-shot mean measurements and increased precision were found for pressure levels above 1.0 atm. For subatmospheric pressure levels, the results indicated that the method is incapable of making fluctuation measurements due to limited precision. Nevertheless, the increased precision above 1.0 atm indicates that fluctuation measurements may be possible with further modifications.; The second portion of the work concerned the development of a theoretical model for electronic-resonance-enhanced CARS (ERE CARS) spectra of nitric oxide. This model incorporates the effects of the Raman resonance in conjunction with an electronic resonance to provide the first known theoretical predictions for NO. To determine the model's accuracy and effectiveness, predictions were compared to previously obtained experimental data. The comparisons displayed close agreement between spectral peak locations and relative intensities. Experimental linewidths were larger than predicted by the theory, which is attributed to saturation or Stark effects in the experiment. The model also allowed for the correct assignment of the molecular constants for the split ground states. This model can be used for investigations of NO formation in hypersonic flow and combustion and can be extended to other species for rapid detection of air pollutants and toxins.