Experimental Determination of the Dissociative Recombination Rate Coefficient for Rotationally Cold CH+ and Its Implications for Diffuse Cloud Chemistry

Observations of CH + are used to trace the physical properties of diffuse clouds, but this requires an accurate understanding of the underlying CH + chemistry. Until this work, the most uncertain reaction in that chemistry was dissociative recombination (DR) of CH + . Using an electron–ion merged-be...

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Published in:The Astrophysical journal 2022-11, Vol.939 (2), p.122
Main Authors: Paul, Daniel, Grieser, Manfred, Grussie, Florian, von Hahn, Robert, Isberner, Leonard W., Kálosi, Ábel, Krantz, Claude, Kreckel, Holger, Müll, Damian, Neufeld, David A., Savin, Daniel W., Schippers, Stefan, Wilhelm, Patrick, Wolf, Andreas, Wolfire, Mark G., Novotný, Oldřich
Format: Article
Language:English
Subjects:
Astrophysics
Chemistry
Cloud chemistry
Clouds
Cryogenic storage
Diffuse molecular clouds
Electron beams
Electron recombination reactions
Ion beams
Laboratory astrophysics
Physical properties
Reaction rates
Recombination coefficient
Rotational states
Storage rings (particle accelerators)
Temperature rise
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Summary:Observations of CH + are used to trace the physical properties of diffuse clouds, but this requires an accurate understanding of the underlying CH + chemistry. Until this work, the most uncertain reaction in that chemistry was dissociative recombination (DR) of CH + . Using an electron–ion merged-beams experiment at the Cryogenic Storage Ring, we have determined the DR rate coefficient of the CH + electronic, vibrational, and rotational ground state applicable for different diffuse cloud conditions. Our results reduce the previously unrecognized order-of-magnitude uncertainty in the CH + DR rate coefficient to ∼20% and are applicable at all temperatures relevant to diffuse clouds, ranging from quiescent gas to gas locally heated by processes such as shocks and turbulence. Based on a simple chemical network, we find that DR can be an important destruction mechanism at temperatures relevant to quiescent gas. As the temperature increases locally, DR can continue to be important up to temperatures of ∼600 K, if there is also a corresponding increase in the electron fraction of the gas. Our new CH + DR rate-coefficient data will increase the reliability of future studies of diffuse cloud physical properties via CH + abundance observations.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ac8e02