High-temperature absorption cross-sections and interference-immune sensing method for formaldehyde near 3.6-µm

Abstract

Absorption cross-sections of formaldehyde (CH2O) were measured at elevated temperatures near 3.6 µm, a region of strong intensity due to the overlapping QQ5 branch of the v1 band and the PQ7 branch of the v2+v4 combination band. 1,3,5-trioxane was shock-heated in an argon bath gas to produce controlled mixtures of formaldehyde over a range of temperatures (900–1500 K) and pressures (0.4–4.8 atm). Spectrally-resolved absorption measurements of formaldehyde were obtained using a rapidly-tunable distributed-feedback interband cascade laser scanned at 40 kHz over 2778.1–2778.9 cm−1 behind reflected shock waves. The resulting absorption cross-sections were fit with functions reflecting the temperature and pressure dependence of the targeted features, enabling spectral reconstruction at any temperature and pressure within the range of the study. By exploiting the strong temperature and pressure sensitivity of the cross-sections, it is demonstrated that CH2O concentration, temperature, and pressure can be inferred from a measured absorbance spectrum. Cross-sections of common combustion species were also measured over the selected wavelength range to validate that the region is largely free of narrow-band interference. Further, the multi-parameter spectral fitting method is shown to correct for baseline interference and demonstrated in experiments involving transient gas properties.