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Cinétique et dépendance en température des réactions neutre-neutre dans le gaz interstellaire froid

Abstract : The physical-chemistry of the cold interstellar gas is constrained by the extreme rarity of ternary associations, and by the low collision energy available. In this context , reactions between neutral species are generally ineffective, due to significant activation energy barriers. Since 1992, however, the CRESU experiment has shown that many neutral-neutral reactions involving radicals such as CN or OH are very fast at low temperatures, and surprisingly slow down at higher temperatures. Besides the consequences on interstellar chemistry models, these results question the use of long-range capture theories, which are unable to account for the strong decrease of the rate constants with increasing temperature. The purpose of this work is to investigate the influence of short and medium-range effects on the kinetics of the representative system CN+NH3 , and to bring out a generic mechanism able to reproduce the measured temperature dependence. We show, based on ab initio quantum chemical calculations, that the unusual reactivity of this system is not due to a peculiarity of the radical-molecule interaction at medium-range, but more likely to the presence of short-range dynamical effects. We underline, from quasi-classical trajectories, the influence of rotational excitation on the capture process, and we present an angular selection model which quantitatively reproduces the CRESU data. Finally, based on a simple triatomic study, we illustrate the role of short-range forces , and confirm the importance of rotational effects. Thus, our results emphasize the necessity to treat in detail the coupling between the capture process and the subsequent evolution of the reaction complex.
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Submitted on : Tuesday, August 21, 2012 - 1:11:13 PM
Last modification on : Friday, March 25, 2022 - 11:09:18 AM
Long-term archiving on: : Friday, December 16, 2016 - 7:11:47 AM

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  • HAL Id : tel-00724502, version 1

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Citation

Alexandre Faure. Cinétique et dépendance en température des réactions neutre-neutre dans le gaz interstellaire froid. Astrophysique stellaire et solaire [astro-ph.SR]. Université Joseph-Fourier - Grenoble I, 1999. Français. ⟨tel-00724502⟩

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