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| Titel |
Pressure dependent isotopic fractionation in the photolysis of formaldehyde-d2 |
| VerfasserIn |
E. J. K. Nilsson, J. A. Schmidt, M. S. Johnson |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 2 ; Nr. 14, no. 2 (2014-01-20), S.551-558 |
| Datensatznummer |
250118291
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| Publikation (Nr.) |
 copernicus.org/acp-14-551-2014.pdf |
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| Zusammenfassung |
| The isotope effects in formaldehyde photolysis are the key link between the
δD of methane emissions and the δD of atmospheric in situ
hydrogen production. A few recent studies have suggested that a pressure
dependence in the isotopic fractionation can partly explain enrichment of
deuterium with altitude in the atmosphere. The mechanism and the extent of
this pressure dependency is, however, not adequately described. In the
present work D2CO and H2CO were photolyzed in a static reaction chamber
at bath gas pressures of 50, 200, 400, 600 and 1000 mbar; these experiments
compliment and extend our earlier work with HDCO vs. H2CO. The UV lamps
used for photolysis emit light at wavelengths that primarily dissociate
formaldehyde into molecular products, CO and H2 or D2. The isotope
effect k(H2CO)/k(D2CO) = 3.16 ± 0.03 at 1000 mbar is in
good agreement with results from previous studies. Similarly to what was
previously shown for k(H2CO)/k(HDCO), the isotope effect decreased as
pressure decreased. In addition, a model was constructed using RRKM theory to
calculate the lifetime of excited formaldehyde on the S0 surface, to
investigate its role in the observed pressure dependent photolytic
fractionation of deuterium. The model shows that part of the fractionation is
a result of competition between the isotopologue dependent rates of
unimolecular dissociation and collisional relaxation. We suggest that the
remaining fractionation is due to isotope effects in the rate of the
non-radiative transition from S1 to S0, which are not considered in the
present model. |
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