Ozone Isotope Enrichment: Isotopomer-Specific Rate Coefficients

Abstract

Six rate coefficients of relative ozone formation contradict the role of molecular symmetry in the process that results in the enrichment of heavy ozone isotopomers. The results show that collisions between light atoms, such as 16O, and heavy molecules, such as 34O2 and 36O2, have a rate coefficient advantage of about 25 and 50 percent, respectively, over collisions involving heavy atoms and light molecules. These results suggest that the observed isotope effect for each isotopomer may be caused by the preponderance of a single reaction channel and not through molecular symmetry selection.

On the electron-induced isotope fractionation in low temperature32O2/36O2ices—ozone as a case study

“…[28][29][30] A weak n 2 band was also observed in the irradiated ices (Fig. 1b) 18 O species, only appeared when the ice was warmed to 45 K. We note that a previous study in our groups suggested that the irradiated oxygen ices trap oxygen atoms at 11 K 27 such that, as the temperature increases, these oxygen atoms become mobile and can react with an oxygen atom or molecule to form molecular oxygen and ozone respectively. During the heating process additional bands of 16 (Fig.…”

“…27 Although O( 1 D) can be Fig. 3 Temporal evolution of ozone isotopomers and isotopologues in the ice during the irradiation at 10 K together with kinetic fits corresponding to pseudo first order reaction (given in eqn (3)) for (a) 18 (4) respectively. Dotted lines show the fits using eqn (4) for a-d and using eqn (3) e-f. produced in the matrix during irradiation, after 3600 s in the isothermal condition all these atoms will have relaxed into their ground state leaving the irradiated ice rich in atomic oxygen in its ground state ( 3 P).…”

“…1 First measured in chondritic meteorites, such isotopic enrichments have been found in Martian rocks and are now routinely utilized to identify Martian meteoritic material that are deposited on Earth. 1,2 In 1965, Taylor suggested a classification of rocks in the solar system according to their 18 O/ 16 O ratio. 3 Recently Ozima et al 4 classified 400 bulk meteorite samples using their oxygen isotopic data and commented upon the similarity between solar and planetary oxygen.…”

“…The 18 O enrichment in ozone has received particular attention. In the Earth's atmosphere, ozone is formed by the Chapman mechanism (eqn (1) and (2)), where M is a third body (usually nitrogen in the terrestrial atmosphere) necessary to stabilize the ozone molecule, which would otherwise decompose back to the reactants due to the internal energy it has upon formation.…”

“…Initially, these molecules were thought to form via a cyclic ozone intermediate, whose ring opened to the acyclic form. However, this interpretation has been challenged, and fast isotopic exchange reactions involving 18 O enriched molecular oxygen ( 16 O 18 O) have to be considered as well. 21,22 …”