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International audience; The interplay between fundamental laboratory investigations, theoretical advances, and chemical modeling has led to tremendous progress in our understanding of the reactivity of polyatomic systems. Measured and/or calculated reaction rates are incorporated into models to identify the key pathways that control reaction outcomes. However, experimental studies often report the rate of reactant disappearance, leaving product identity and branching largely unknown. This limitation arises from considerable experimental challenges inherent to the quantitative detection of the full range of products of a given reaction, in particular for large molecules. To address these issues, we have developed a new approach incorporating chirped-pulse microwave spectroscopy in low-temperature uniform supersonic flows (Chirped-Pulse in Uniform Flow, CPUF). This technique provides clear quantifiable spectroscopic signatures for polyatomic products in bimolecular or unimolecular reactions for virtually any species with a modest dipole moment. In this talk, I will first illustrate the principle of this new experimental strategy and discuss a number of challenges related to the collisional environment of the flow. Results on the CN + C3H4 reaction at nearly 20 K and on the photodissociation of isoxazole C3H3NO at 193 nm, for which line intensities in the [60-90] GHz range have been measured, will then be presented. Overall, this work opens the door for new applications in kinetics/reaction dynamics with an unprecedented overlap with radioastronomy.

Chirped pulse (CP) Fourier transform rotational spectroscopy is uniquely suited for near universal quantitative detection and structural characterization of mixtures that contain multiple molecular and radical species. In this work we employ CP spectroscopy to measure product branching and extract information about the reaction mechanism guided by kinetic modeling. Pyrolysis of ethyl nitrite CH3CH2ONO is studied in a Chen type flash pyrolysis reactor at temperatures of 1000–1800 K. The branching between HNO CH2O and CH3CHO products is measured and compared to the kinetic models generated by the Reaction Mechanism Generator software. We find that roaming CH3CH2ONO ? CH3CHO + HNO plays an important role in the thermal decomposition of ethyl nitrite with its rate at 1000 K comparable to that of the radical elimination channel CH3CH2ONO ? CH3CH2O + NO. HNO is a signature of roaming in this system.

International audience; Nearly all kinetics studies report the observed rate of reactant disappearance, with product identity and branching largely unknown. This limitation arises from considerable experimental challenges inherent in the quantitative detection of the full range of products of a given reaction, particularly for large polyatomic systems. Recent advances have relied upon tunable synchrotron photoionization or low-energy electron impact ionization to achieve selective product detection in dynamics, kinetics, and flame studies. Challenges remain, however, as these studies require fitting of composite and often incompletely resolved spectra to infer branching, and clear product signatures are often lacking. To address these issues, we have developed an alternative approach, which incorporates chirped-pulse Fourier transform microwave spectroscopy in low-temperature uniform supersonic flows (“Chirped-Pulse in Uniform Flow”, C-PUF). This technique provides clear quantifiable spectroscopic signatures of polyatomic products in bimolecular or unimolecular reactions for virtually any species with a modest electric dipole moment.

International audience

International audience

A Chirped-Pulse millimeter-Wave (CPmmW) spectrometer is applied to the study of chemical reaction products that result from pyrolysis in a Chen nozzle heated to 1000 – 1800 K. Millimeter-wave rotational spectroscopy unambiguously determines, for each polar reaction product, the species, the conformers, relative concentrations, conversion percentage from precursor to each product, and, in some cases, vibrational state population distributions. A chirped-pulse spectrometer can, within the frequency range of a single chirp, sample spectral regions of up to ~10 GHz and simultaneously detect many reaction products. Here we introduce a modification to the CPmmW technique in which multiple chirps of different spectral content are applied to a molecular beam pulse that contains the pyrolysis reaction products. This technique allows for controlled allocation of its sensitivity to specific molecular transitions and effectively doubles the bandwidth of the spectrometer. As an example, the pyrolysis reaction of ethyl nitrite, CH[subscript 3]CH[subscript 2]ONO, is studied at different temperatures of the Chen reactor, and CH[subscript 3]CHO, H[subscript 2]CO, and HNO products are simultaneously observed, exploiting the multi-chirp CPmmW technique. Rotational and vibrational temperatures of some product molecules are determined. Subsequent to supersonic expansion from the heated nozzle, acetaldehyde molecules display a rotational temperature of 4 ± 1 K. Vibrational temperatures are found to be controlled by the collisional cooling in the expansion, and to be both species- and vibrational mode-dependent. Rotational transitions of vibrationally excited formaldehyde in levels ν[subscript 4], 2ν[subscript 4], 3ν[subscript 4], ν[subscipt 2], ν[subscript 3], and ν[subscript 6] are observed and effective vibrational temperatures for modes 2, 3, 4, and 6 are determined and discussed. Keywords: millimeter wave spectroscopy, microwave spectroscopy, broadband rotational spectroscopy, chirped pulse, free induction decay, fast passage, flash pyrolysis, branching, concentration of reaction products, kinetics, dynamics, thermal decomposition, ethyl nitrite, CH[subscript 3]CH[subscript 2]ONO, C[subscript 2]H[subscript 5]ONO, methyl nitrite, CH[subscript 3]ONO, formaldehyde, acetaldehyde, nitroxyl, H[subscript 2]CO, CH[subscript 2]O, CH[subscript 3]CHO, HNO, multi-chirp, Chen nozzle, pulsed valve, tubular reactor, vibrational relaxation, collisional cooling, vibrational temperature, vibrational population distribution, rotational temperature, Coriolis interaction, ortho, para, nuclear spin statistics., United States. Army Research Office (Award 58245-CH-11)