Low temperature reaction kinetics inside an extended Laval nozzle: REMPI characterization and detection by broadband rotational spectroscopy.

This article focuses on the development and characterization of an extended Laval nozzle coupled with Chirped-Pulse Fourier-Transform millimeter wave (CP-FTmmW) spectroscopy for low-temperature reaction kinetics studies using the CRESU (Reaction Kinetics in Uniform Supersonic Flows) technique. The extended nozzle creates a uniform supersonic flow within the nozzle itself, followed by a shock-free secondary expansion to low temperature and pressure conditions suitable for CP-FTmmW detection. Resonance-enhanced multiphoton ionization (REMPI) detection of nitric oxide (NO) was implemented inside the nozzle to characterize flow conditions, confirming computational fluid dynamics (CFD) simulations. The system was applied to measure the bimolecular rate coefficient of the reaction between formyl radical (HCO) and oxygen (O₂) at 20 K, yielding a rate of 6.66 ± 0.47 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, significantly faster than room temperature rates. This approach offers a shock-free, multiplexed detection method for studying low-temperature reaction kinetics relevant to atmospheric and astrochemical environments.