Pure rotational far-infrared quantum cascade laser absorption tomography of hydrides in reacting flows

The high intrinsic polarity of many hydrides creates strong pure rotational absorption spectra in the THz domain. At high gas temperatures associated with reacting flows, pure rotational hydride spectra become active in the far-infrared and accessible with emerging semiconductor light sources. In this work, a pulsed far-IR quantum-cascade laser was utilized to probe rotational absorption lines of the hydroxyl radical (OH) and hydrogen fluoride (HF) in the reacting boundary layer of a solid fuel combustion experiment. Measurements targeted strong and isolated OH and HF transitions near 532cm^-1 (18.8µm), with a laser scanning range of ∼1.0cm^-1 sufficient to resolve both transitions within a single period. A mid-IR carbon monoxide line pair at 2008.5cm^-1 (4.98µm) provided complementary temperature measurements through two-line thermometry. Radially resolved temperature and species concentration were extracted through Tikhonov-regularized inversions of laser measurements across the exit plane of cylindrical fuel grains. This work demonstrates quantitative, spatially resolved measurements of key hydrides (OH and HF) in a high-temperature reacting boundary layer via far-infrared rotational laser absorption tomography.