Non-thermal effects and temperature disparities in uniform supersonic flows

Abstract Over the last 40 years, low-temperature reaction rate coefficients in astrochemical databases have largely come from CRESU ( Cinétique de Réaction en Ecoulement Supersonique Uniforme ) experiments, where the flow is described by a single temperature under an assumption of local thermal equilibrium. Here we directly test this assumption using infrared frequency comb spectroscopy, laser-induced fluorescence, Pitot measurements, and computational fluid dynamics in seeded supersonic flows. We observe persistent vibrational excitation in carbonyl sulfide (OCS) and nitrous oxide (N 2 O) under all conditions examined, despite efficient rotational cooling, demonstrating a breakdown of local thermal equilibrium within the flow. In parallel, we find that conventional Pitot-derived temperatures do not consistently reflect the energetic state of the reacting ensemble. These results show that the reacting ensemble in CRESU flows cannot always be described by a single thermodynamic temperature, particularly for species with low-lying vibrational modes, and that internal-state distributions should be carefully considered when interpreting low-temperature reaction rate coefficients in astrochemical kinetic databases.