Line mixing studies of diatomic rovibrational fundamental bands in the infrared
Keywords: Engineering.
Mechanical Engineering.

Laser absorption spectroscopy enables quantitative, non-intrusive, and short-time-scale measurements of gaseous systems, especially at extreme temperature and pressure conditions where traditional sensors may fail. As we develop more powerful energy systems and optical metrology facilities, the need for accurate sensing in these extreme conditions has grown to include studies of combustion systems, propulsion facilities, and more recently, even exoplanetary bodies in space. Unfortunately, at higher pressure conditions, collisional "line mixing" effects cause the traditional absorption models that inform most absorption-based sensing techniques to fail. The characterization and modeling of these line mixing effects are therefore important to maintain accurate sensing across environments of interest. This dissertation will focus on the experimental measurement and modeling of broadband mid-infrared light absorption across elevated pressure and temperature conditions, especially where collisional line mixing effects are significant. In this work, I present quantitative, broadband absorbance measurements of the fundamental rovibrational band of carbon monoxide (CO) between 1965 and 2230 cm-1 in bath gases of nitrogen (N2), helium (He), and hydrogen (H2), and nitric oxide (NO) between 1700 to 2000 cm-1 in N2. These measurements were taken using a static cell and a narrow-linewidth, broad-scan external-cavity quantum-cascade laser at pressures of 15-35 atm and temperatures of 293, 453 (CO/H2) and 802 K (CO/N2, CO/He, NO/N2). The measurements are then compared to multiple constructed models capable of reproducing the effects of line mixing present in the measured results. The first line mixing approach, based on the modified exponential gap (MEG) law with fitted inter-branch factors, shows improved agreement with the measured spectra across different pressures and broadening partners relative to purely Lorentzian models. The NO model required two fitted inter-branch factors to achieve satisfactory agreement, necessary due to its additional Q-branch structure relative to that of CO. At the elevated temperatures, similar agreement is observed; however, for CO, a mismatch is present between extrapolated HITRAN broadening parameters and those observed in the measured spectra. This is likely due to the known deficiency of the single power law over large temperature ranges, and hence a minor scaling of the line-by-line temperature-dependence exponents is incorporated that is supported by previous studies in the literature. The second, more empirical line mixing approach involves extracting MEG line mixing parameters through a direct fit to the measured spectra. The Direct Fit method bypasses the need for known line shape parameters and produces even stronger agreement with measured CO data, with a CO/H2 root-mean-square error of 0.4% at the highest-number-density condition of 293K and 35atm. This approach is slightly less effective for modeling NO due to its additional spin-split lines, but satisfactory agreement is still demonstrated with constraints placed on the spin-split MEG parameters. Finally, the last line mixing approach utilizes the Energy Corrected Sudden (ECS) scaling law, which eliminates the need for any additional fitted factors, and produces reasonable agreement across the measured NO spectra, excluding the Q-branch. In the NO Q-branch peak, the ECS model overpredicts the measured data by about 7%, possibly due to the presence of additional collisional coupling between the NO spin-split lines. The line mixing models presented will be useful across a number of applications, from accurate NO thermometry in hypersonic propulsion test facilities on the ground to the interpretation of future infrared observations of CO in exoplanetary gas giant atmospheres deep in space