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This Letter reports a femtosecond ultraviolet laser absorption spectroscopy (fs-UV-LAS) for simultaneous in situ measurements of temperature and species. This fs-UV-LAS technique was demonstrated based on X 2 Π-A 2 Σ + transitions of OH radicals near 308 nm generated in low temperature plasmas and flames. The fs-UV-LAS technique has revealed three major diagnostic benefits. First, a series of absorption features within a spectral bandwidth of ∼3.2 nm near 308 nm were simultaneously measured and then enabled simultaneous multi-parameter measurements with enhanced accuracy. The results show that the temperature and OH concentration could be measured with accuracy enhanced by 29–88% and 58–91%, respectively, compared to those obtained with past two-narrow-line absorption methods. Second, an ultrafast time resolution of ∼120 picoseconds was accomplished for the measurements. Third, due to the large OH X 2 Π-A 2 Σ + transitions in the UV range, a simple single-pass absorption with a 3-cm path length was allowed for measurements in plasmas with low OH number density down to ∼2 × 10 13 cm −3 . Also due to the large OH UV transitions, single-shot fs absorption measurements were accomplished in flames, which was expected to offer more insights into chemically reactive flow dynamics.
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This content will become publicly available on April 18, 2026
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−1sufficient 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.
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- Award ID(s):
- 2329124
- PAR ID:
- 10583423
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Applied Optics
- Volume:
- 64
- Issue:
- 13
- ISSN:
- 1559-128X; APOPAI
- Format(s):
- Medium: X Size: Article No. 3412
- Size(s):
- Article No. 3412
- Sponsoring Org:
- National Science Foundation
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