Measurements of Ortho-to-para Nuclear Spin Conversion of H2 on Low-temperature Carbonaceous Grain Analogs: Diamond-like Carbon and Graphite

Hydrogen molecules have two nuclear spin isomers: ortho-H2 and para-H2. The ortho-to-para ratio (OPR) is known to affect chemical evolution as well as gas dynamics in space. Therefore, understanding the mechanism of OPR variation in astrophysical environments is important. In this work, the nuclear...

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Bibliographic Details
Published in:The Astrophysical journal 2021-12, Vol.923 (1)
Main Authors: Tsuge, Masashi, Kouchi, Akira, Watanabe, Naoki
Format: Article
Language:English
Subjects:
Analogs
Astrochemistry
Astrophysics
Carbon
Chemical evolution
Conversion
Dense interstellar clouds
Diamond-like carbon
Gas dynamics
Graphite
Hydrogen
Interstellar dust
Interstellar molecules
Ionization
Isomers
Laboratory astrophysics
Low temperature
Molecular clouds
Nuclear spin
Temperature
Temperature dependence
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Summary:Hydrogen molecules have two nuclear spin isomers: ortho-H2 and para-H2. The ortho-to-para ratio (OPR) is known to affect chemical evolution as well as gas dynamics in space. Therefore, understanding the mechanism of OPR variation in astrophysical environments is important. In this work, the nuclear spin conversion (NSC) processes of H2 molecules on diamond-like carbon and graphite surfaces are investigated experimentally by employing temperature-programmed desorption and resonance-enhanced multiphoton ionization methods. For the diamond-like carbon surface, the NSC time constants were determined at temperatures of 10–18 K and from 3900 ± 800 s at 10 K to 750 ± 40 s at 18 K. Similar NSC time constants and temperature dependence were observed for a graphite surface, indicating that bonding motifs (sp3 or sp2 hybridization) have little effect on the NSC rates.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ac2a33