Open AccessUltra-sensitive spectroscopy of OH radical in high-temperature transient reactions
Author(s) -
Shengkai Wang,
Ronald K. Hanson
Publication year - 2018
Publication title -
optics letters/optics index
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.524
H-Index - 272
eISSN - 1071-2763
pISSN - 0146-9592
DOI - 10.1364/ol.43.003518
Subject(s) - shock tube , microsecond , combustion , hydroxyl radical , absorbance , materials science , spectroscopy , optics , analytical chemistry (journal) , microwave , radical , chemistry , computer science , shock wave , physics , telecommunications , quantum mechanics , thermodynamics , chromatography , organic chemistry
The hydroxyl (OH) radical is arguably the most important transient radical in high-temperature gas-phase combustion reactions, yet it is very difficult to measure because of its high reactivity and, thus, short lifetime and low concentration. This work reports the development of a novel method for ultra-sensitive, quantitative, and microsecond-resolved detection of OH based on UV frequency-modulation spectroscopy (FMS). To the best of the authors' knowledge, this is the first FMS demonstration in the near-UV spectral region for detection of short-lived radical species. Shot-noise-limited detection was achieved at an optical power of 25 mW. A proof-of-concept experiment in a tabletop H 2 O/He microwave discharge cell has reached a 1σ minimum detectable absorbance (MDA) of less than 2×10 -4 over 1 MHz measurement bandwidth. High-temperature OH measurement was demonstrated in a 15 cm diameter shock tube, where a typical MDA of 3.0×10 -4 was achieved at 1330 K, 0.38 atm, and 1 MHz. These preliminary results have outperformed the previous best MDA by more than a factor of 3; further improvement by another order of magnitude is anticipated, following the strategies outlined at the end of this Letter. The current method paves the path to parts per billion (ppb) -level OH detection capability and offers prospects to significantly advance fundamental combustion research by enabling direct observation of OH formation and scavenging kinetics during key stages of fuel oxidation that were inaccessible with previous methods.