Low-Temperature Product Detection and Branching Ratios

International audienceThe quantitative detection of reaction products at low temperature is crucial for the understanding of low-temperature chemistry, in part due to the increased importance of quantum effects as the temperature is lowered. Product information and branching ratios at very low temperatures are also vital input for astrochemical models. Early mass spectrometry experiments have allowed the detection of reaction products in a pulsed CRESU flow down to 90K, while spectroscopic techniques have enabled the measurement of H atom branching ratios down to 50K. Coupling CRESU flows to multiplex mass spectrometry at synchrotron facilities has already provided branching ratios between isomers formed by the same reactions below 100K. The newly designed CRESUSOL apparatus using photoion-photoelectron coincidence detection scheme has the ability of detecting reaction products down to 10K. Chirped-pulsed microwave spectroscopy has also appeared as a sensitive and universal technique for the detection of isomer-resolved products. The cold rotational distributions in the CRESU flow provide ideal conditions for detection of products and also give the advantage of being able to distinguish isomers of products through their rotational spectra. These universal, isomer-resolved detection techniques and others, like frequency comb spectroscopy, have only just started to be implemented to CRESU apparatuses and promise to provide a wealth of information about gas phase chemical and physical processes occurring close to the absolute zero. © 2022 by World Scientific Publishing Europe Ltd

Product branching fractions of the CH + propene reaction from synchrotron photoionization mass spectrometry

The CH(X2Π) + propene reaction is studied in the gas phase at 298 K and 4 Torr (533.3 Pa) using VUV synchrotron photoionization mass spectrometry. The dominant product channel is the formation of C4H6 (m/z 54) + H. By fitting experimental photoionization spectra to measured spectra of known C4H6 isomers, the following relative branching fractions are obtained: 1,3-butadiene (0.63 ± 0.13), 1,2-butadiene (0.25 ± 0.05), and 1-butyne (0.12 ± 0.03) with no detectable contribution from 2-butyne. The CD + propene reaction is also studied and two product channels are observed that correspond to C4H6 (m/z 54) + D and C4H5D (m/z 55) + H, formed at a ratio of 0.4 (m/z 54) to 1.0 (m/z 55). The D elimination channel forms almost exclusively 1,2-butadiene (0.97 ± 0.20) whereas the H elimination channel leads to the formation of deuterated 1,3-butadiene (0.89 ± 0.18) and 1-butyne (0.11 ± 0.02); photoionization spectra of undeuterated species are used in the fitting of the measured m/z 55 (C4H5D) spectrum. The results are generally consistent with a CH cycloaddition mechanism to the C═C bond of propene, forming 1-methylallyl followed by elimination of a H atom via several competing processes. The direct detection of 1,3-butadiene as a reaction product is an important validation of molecular weight growth schemes implicating the CH + propene reaction, for example, those reported recently for the formation of benzene in the interstellar medium (Jones, B. M. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 452−457)

Electron attachment on HI and DI in a uniform supersonic flow: Thermalization of the electrons

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The reaction of anthracene with OH radicals: An experimental study of the kinetics between 58 and 470K

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