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| Titel |
Low Temperature Studies of the Removal Reactions of 1CH2 with Relevance to the Atmosphere of Titan |
| VerfasserIn |
Kevin Douglas, Eloise Slater, Wuhu Feng, Mark Blitz, John Plane, Dwayne Heard, Paul Seakins |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250152749
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| Publikation (Nr.) |
 EGU/EGU2017-17628.pdf |
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| Zusammenfassung |
| The photolysis of methane by UV photons is the primary source of hydrocarbon radicals in
the atmosphere of Titan and the giant planets. Although there is still significant uncertainty in
the branching ratios of products, the production of the first singlet excited state of methylene,
1CH2, is thought to be a significant channel. Reactions of 1CH2 with methane (R1a) and
hydrogen (R2a) are a significant source of methyl radicals, the recombination of which is the
primary route to ethane on Titan (R3). The reaction of 1CH2 with acetylene is also a
source of propargyl, C3H3, the recombination of which is the primary route to
benzene on Titan. However, in addition to these reactions of 1CH2 leading to chemical
products, there is also competition between inelastic electronic relaxation to form
ground triplet state methylene, 3CH2 (R1b and R2b). Triplet methylene is much less
reactive, and cannot undergo the complex insertion elimination reactions of singlet
methylene. The main reaction of 3CH2 occurs with other radical species such as H
(R4).
1CH2 + CH4 → CH3 + H2 (R1a)
1CH2 + CH4 → 3CH2 + CH4 (R1b)
1CH2 + H2 → CH3 + H (R2a)
1CH2 + H2 → 3CH2 + H2 (R2a)
CH3 + CH3 (+M) → C2H6 (R3)
3CH2 + H → CH + H2 (R4)
Using pulsed laser photolysis laser-induced fluorescence, we have studied the reaction
kinetics for the removal of 1CH2 with N2, H2, CH4, C2H6, C2H4, C2H6, and O2 as a
function of temperature. Low temperatures between 43 and 135 K were obtained using a
pulsed Laval nozzle apparatus, while data at 160 K was obtained using a low flow
reaction cell with cryogenic cooling. In addition to measuring total removal rates,
the fraction of 1CH2 removed via electronic relaxation versus chemical reaction
to products has also been investigated for H2 and CH4 at 160 and 73 K. Results
show that that removal of 1CH2 by electronic relaxation increases with decreasing
temperature.
These experimental results indicate that the majority of 1CH2 formed in Titan’s
atmosphere will be rapidly relaxed to its ground state via collisions with both reactive and
non-reactive species, and thus is likely to play a less significant role in the formation of larger
hydrocarbons than previously thought. However, for a full understanding of the implications
of these results, the new measurements have been included in a 1D model of Titan’s
atmosphere. The model results show a significant reduction in ethane concentrations (10 – 50
%), due to reduction in CH3 production via reactions R1a and R1b. In addition we also
observe an increase in ethylene concentrations, the result of increased amounts of 3CH2
reacting with H radicals to form CH (R4), which primarily react with methane to form
ethylene.
Additional work is also underway to determine branching ratios between reaction and
relaxation of 1CH2 with ethane, ethylene, and acetylene. Model results have shown that if a
similar trend to reactions with H2 and CH4 is observed, there would be significant reductions
in benzene production on Titan. |
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