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.
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BibTeX
@article{wu2026nonthermal,
title = {{Non-thermal effects and temperature disparities in uniform supersonic flows}},
author = {Lok Yiu Wu and Luke Driver and Daniel I. Lucas and Madeleine E. Robertson and Nikil Kapur and Dwayne E. Heard and Gregory N. de Boer and Julia H. Lehman},
journal = {Communications Chemistry},
year = {2026},
month = oct,
doi = {10.1038/s42004-026-02244-8},
url = {https://www.nature.com/articles/s42004-026-02244-8},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Non-thermal effects and temperature disparities in uniform supersonic flows
AU - Lok Yiu Wu
AU - Luke Driver
AU - Daniel I. Lucas
AU - Madeleine E. Robertson
AU - Nikil Kapur
AU - Dwayne E. Heard
AU - Gregory N. de Boer
AU - Julia H. Lehman
PY - 2026
DA - 2026/10/13
JO - Communications Chemistry
DO - 10.1038/s42004-026-02244-8
UR - https://www.nature.com/articles/s42004-026-02244-8
AB - 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.
SN - 2399-3669
ER -