Microwave spectroscopic detection of flame-sampled combustion intermediates

Abstract

Microwave spectroscopy probes the rotational transitions of polar molecules in the gas phase and is a proven technique for the detection and identification of short-lived molecules produced from a variety of molecular sources. In this explorative study, we demonstrate that two prerequisites can be met for microwave spectroscopy to become a quantitative tool for the analysis of high-temperature gas mixtures as found in combustion environments. First, we show that the rotational temperature of the targeted species can be sufficiently cooled to allow for a sensitive detection of low-lying rotational states after sampling from hot (∼2200 K) flames. Second, we show that signal intensity profiles can be assembled which, after correcting for the different flame temperatures at various sampling positions, agree well with mole fraction profiles obtained via flame-sampling molecular-beam mass spectrometry. Based on the described results, it is conceivable that rotational spectroscopy can contribute towards the unraveling of complex, high-temperature reaction networks.