Advanced

Pressure dependent isotopic fractionation in the photolysis of formaldehyde-d(2)

Heimdal Nilsson, Elna LU ; Schmidt, J. A. and Johnson, M. S. (2014) In Atmospheric Chemistry and Physics 14(2). p.551-558
Abstract
The isotope effects in formaldehyde photolysis are the key link between the delta D of methane emissions and the delta D 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... (More)
The isotope effects in formaldehyde photolysis are the key link between the delta D of methane emissions and the delta D 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 H-2 or D-2. 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 S-0 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 S-1 to S-0, which are not considered in the present model. (Less)
Please use this url to cite or link to this publication:
author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Atmospheric Chemistry and Physics
volume
14
issue
2
pages
551 - 558
publisher
Copernicus Gesellschaft Mbh
external identifiers
  • wos:000330853800003
  • scopus:84892708277
ISSN
1680-7324
DOI
10.5194/acp-14-551-2014
language
English
LU publication?
yes
id
d5b15ae4-44d6-4c1d-9c04-458bb7d5bac6 (old id 4376535)
date added to LUP
2014-04-14 12:53:22
date last changed
2017-01-01 03:25:07
@article{d5b15ae4-44d6-4c1d-9c04-458bb7d5bac6,
  abstract     = {The isotope effects in formaldehyde photolysis are the key link between the delta D of methane emissions and the delta D 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 H-2 or D-2. 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 S-0 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 S-1 to S-0, which are not considered in the present model.},
  author       = {Heimdal Nilsson, Elna and Schmidt, J. A. and Johnson, M. S.},
  issn         = {1680-7324},
  language     = {eng},
  number       = {2},
  pages        = {551--558},
  publisher    = {Copernicus Gesellschaft Mbh},
  series       = {Atmospheric Chemistry and Physics},
  title        = {Pressure dependent isotopic fractionation in the photolysis of formaldehyde-d(2)},
  url          = {http://dx.doi.org/10.5194/acp-14-551-2014},
  volume       = {14},
  year         = {2014},
}