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The isotope effects in formaldehyde photolysis are the key link between theδD of methane emissions and the δD of atmospheric in situhydrogen production. A few recent studies have suggested that a pressuredependence in the isotopic fractionation can partly explain enrichment ofdeuterium with altitude in the atmosphere. The mechanism and the extent ofthis pressure dependency is, however, not adequately described. In thepresent work D2CO and H2CO were photolyzed in a static reaction chamberat bath gas pressures of 50, 200, 400, 600 and 1000 mbar; these experimentscompliment and extend our earlier work with HDCO vs. H2CO. The UV lampsused for photolysis emit light at wavelengths that primarily dissociateformaldehyde into molecular products, CO and H2 or D2. The isotopeeffect k(H2CO)/k(D2CO) = 3.16 ± 0.03 at 1000 mbar is ingood agreement with results from previous studies. Similarly to what waspreviously shown for k(H2CO)/k(HDCO), the isotope effect decreased aspressure decreased. In addition, a model was constructed using RRKM theory tocalculate the lifetime of excited formaldehyde on the S0 surface, toinvestigate its role in the observed pressure dependent photolyticfractionation of deuterium. The model shows that part of the fractionation isa result of competition between the isotopologue dependent rates ofunimolecular dissociation and collisional relaxation. We suggest that theremaining fractionation is due to isotope effects in the rate of thenon-radiative transition from S1 to S0, which are not considered in thepresent model

Pressure dependent isotopic fractionation in the photolysis of formaldehyde-d&lt;sub&gt;2&lt;/sub&gt;

Pressure dependent isotopic fractionation in the photolysis of formaldehyde-d<sub>2</sub>