Laser sensing strategies in shock-heated gases for near-IR potassium line shapes and mid-IR absorption spectra of hydrocarbons and oxygenates
Keywords: Mechanical Engineering.

Absorption spectroscopy is an important branch of spectroscopy that quantitatively measures the level of attenuation on electromagnetic radiation by a test sample. It offers the promise of in-situ, non-intrusive, fast, and sensitive diagnostics for application to transient harsh environments, such as exoplanets, flames, combustion systems, and hypersonic flows. In the endeavor to expand upon existing spectroscopic knowledge of infrared absorption and offer optical sensing solutions to the practical challenges in these complex environments, better experimental strategies of measurement and calibration for the associated high-temperature gas-phase atoms and molecules are warranted. This dissertation describes the development of two experimental approaches for the studies of potassium line shapes and broadband molecular absorption using state-of-the-art lasers at previously unexplored temperature conditions that are made possible by a shock tube. I first present a new approach to seed and produce alkali metal vapor in a shock tube and the resulting measurements of the high-temperature potassium vapor in a controlled laboratory environment. To overcome the experimental challenges associated with the extreme reactivity of potassium, the new method employs shock waves to break apart potassium chloride (KCl) salt precursors and produce atomic potassium in the shock-heated buffer gas. This potassium seeding approach was demonstrated to be effective between 1100 -- 1900 K and is readily deployable for other absorbing species of alkali metals. To overcome the hurdle of the relatively short test time of a shock tube, high-speed tunable diode laser absorption spectroscopy (TDLAS) was deployed. The lasers interrogated the potassium D1 and D2 transitions near 0.77 µm and yielded well-resolved absorption line shapes every 40 µs. The measured spectra were modeled as Voigt profiles. Line shape parameters are presented with temperature-dependent power-law relations for the potassium resonance doublets with argon, nitrogen, helium, and hydrogen as the collisional partners. Secondly, a novel methodology is presented of rapid-tuning broad-scan laser absorption spectroscopy that measures broadband mid-infrared absorption cross sections of gaseous molecules at elevated temperatures. The new method deploys rapid-tuning, broad-scan external-cavity quantum-cascade lasers (EC-QCLs) in a shock tube and can provide quantitative absorption information at a rate over 30,000 cm-1/s at spectral intervals between 0.35 -- 0.6 cm-1. Within the shock tube test time of a few milliseconds, the lasers can sweep across over 100 cm-1 to cover the entire branches, or even entire bands, of the absorption spectra for these species. In total, this method was used to measure the cross section profiles of ethylene, propene, 1-butene, i-butene, cis-2-butene, trans-2-butene, 1,3-butadiene, methanol, ethanol, formaldehyde, acetaldehyde, and acetone. The measurements focus on their strongest mid-infrared absorption bands between 5.4 -- 6.1 µm and 8.4 -- 11.7 µm for various temperatures and pressures up to 1600 K and 5 atm, respectively. The resulting spectra are distributed in a plain text format and archived as the Stanford ShockGas-IR database through a permanent URL https://purl.stanford.edu/cy149sv5686