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Collisional excitation of HNC by He found to be stronger than for structural isomer HCN in experiments at the low temperatures of interstellar space

Abstract

HCN and its unstable isomer HNC are widely observed throughout the interstellar medium, with the HNC/HCN abundance ratio correlating strongly with temperature. In very cold environments HNC can even appear more abundant than HCN. Here we use a chirped pulse Fourier transform spectrometer to measure the pressure broadening of HCN and HNC, simultaneously formed in situ by laser photolysis and cooled to low temperatures in uniform supersonic flows of helium. Despite the apparent similarity of these systems, we find the HNC–He cross section to be more than twice as big as the HCN–He cross section at 10 K, confirming earlier quantum calculations. Our experimental results are supported by high-level scattering calculations and are also expected to apply with para-H2, demonstrating that HCN and HNC have different collisional excitation properties that strongly influence the derived interstellar abundances.

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Fig. 1: Schematic view of the experiment with 2D density map and T2 decay times.
Fig. 2: Examples of fits to FIDs for HCN and HNC in helium at 16 K.
Fig. 3: Theoretical and experimental pressure-broadening cross sections for HCN and HNC with helium.

Data availability

The source data for all the figures in the main article and in the Supplementary Information, and the source data for the experimental and theoretical cross sections, are publicly available in the Zenodo data repository55 at https://doi.org/10.5281/zenodo.6256226

Code availability

The codes used to analyse the experimental data and generate the theoretical data presented in this study are publicly available in the the Zenodo data repository55 at https://doi.org/10.5281/zenodo.6256226, or via the cited references.

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Acknowledgements

We thank J. Courbe, J. Thiévin, D. Biet, E. Gallou and A. Dapp for technical support, and A. Canosa for the design of some of the Laval nozzles used in the project. We acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under the European Research Council (ERC) grant agreement 695724-CRESUCHIRP (I.R.S.) and under the Marie Skłodowska-Curie grant agreement 845165-MIRAGE (I.R.C. and I.R.S.). We are also grateful for support from the European Regional Development Fund, the Region of Brittany and Rennes Métropole. This work was supported by the French National Programme ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES.

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B.M.H. and I.R.S. conceived the project. B.M.H., D.G., T.G., O.A.K. and I.R.C. performed the experiments and analysed the data. F.T. and F.L. performed the theoretical calculations. B.M.H., I.R.S., F.T. and F.L. wrote the paper. All authors contributed to discussions and gave comments on the manuscript.

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Correspondence to Ian R. Sims.

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Nature Chemistry thanks Bernadette M. Broderick, Kirill Prozument and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Experimental methods and results, including ‘Data analysis and fitting’, ‘Experimental conditions and results’, Supplementary Table 1, theoretical calculations including potential energy surfaces, scattering calculations and Supplementary Figs. 1 and 2.

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Hays, B.M., Gupta, D., Guillaume, T. et al. Collisional excitation of HNC by He found to be stronger than for structural isomer HCN in experiments at the low temperatures of interstellar space. Nat. Chem. 14, 811–815 (2022). https://doi.org/10.1038/s41557-022-00936-x

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