Detecting combustion intermediates via broadband chirped-pulse microwave spectroscopy

Fritz SM, Mishra P, Wullenkord J, et al. Detecting combustion intermediates via broadband chirped-pulse microwave spectroscopy. Proceedings of the Combustion Institute. 2021;38(1):1761-1769.Developing new experimental techniques that allow for species identification and quantification in the high-temperature environment of reacting flows is a continuing challenge in combustion research. Here, we combine broadband chirped-pulse microwave (rotational) spectroscopy with an atmospheric-pressure jet-stirred reactor as a novel method to identify key reactive intermediates in low-temperature and ozone assisted oxidation processes. In these experiments, the gas sample, after being withdrawn from reactive dimethyl ether/O-2 /Ar, dimethoxy methane/O-2 /Ar, and ethylene/O-2 /O-3/Ar mixtures, expands via a supersonic expansion into the high vacuum of a microwave spectrometer, where the rotationally cold ensemble of polar molecules is excited with short MW radiation frequency ramps (chirps). The response of the molecular ensemble is detected in the time domain and after a Fourier transformation, the spectral composition of the transient emission is obtained in the frequency domain. The observed rotational frequencies are uniquely correlated to molecular structures and allow for an unambiguous identification of the sampled species. Detection and identification of intermediates such as formaldehyde, methyl formate, formic acid, formic acid anhydride, and the primary ethylene ozonide via literature-known rotational frequencies are evidence for the superb identification capabilities of broadband chirped-pulse microwave spectroscopy. Strong-field coherence breaking is employed to identify and assign transitions due to a specific component. The observation of van der Waals complexes provides an opportunity to detect combustion intermediates and products that are impossible to detect by rotational spectroscopy as isolated molecules. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved