A New Isotope Effect In Molecular Physics Discovered In The Formation Process Of Ozone Molecules

January 14, 1999

Scientists at the Max Planck Institute for Nuclear Physics, Division of Atmospheric Physics, in Heidelberg/Germany have investigated during the past four years the formation process and physical parameters of the ozone molecule. This research was inspired by the unusually large enrichments in heavy ozone - molecules of masses other than the standard molecule 16O16O16O. Since oxygen has three stable isotopes (16O, 17O, and 18O), a wide variety of ozone molecules can be formed. Laboratory experiments have recently been completed that isolate ozone formation channels for specific oxygen atoms and molecules. Results are expressed as ozone formation rate coefficients which describe how fast a certain reaction will proceed.

The nine rate coefficients measured suggest a new rule in the ozone formation process: collisions between light atoms (e.g. 16O) and heavy molecules (e.g. 18O18O) have a much higher formation rate over collisions involving heavy atoms (18O) and light molecules (16O16O). This rule was confirmed in experiments with 17O. The results will considerably advance our understanding of the large isotope effect in heavy ozone that has puzzled scientists for almost two decades.

Rate coefficients and their interpretation are published in Science Vol. 283, Jan 15, 1999 by K. Mauersberger, B. Erbacher, D. Krankowsky, J. Gunther and R. Nickel, Max Planck Institute for Nuclear Physics, Heidelberg/Germany. Ozone is a unique molecule containing three oxygen atoms that form an isosceles open triangle. It is produced in a three-body reaction O+O2+M R O3+M with M being a molecule (e.g. N2 or O2) that stabilizes the collision product O-O2. 117O and 18O are minor species in atmospheric oxygen, and thus only 48O3, 49O3, and 50O3 are present in measurable quantities with 17O or 18O substituted for 16O, respectively. Initially observed in the atmosphere (Mauersberger, Geophys. Res. Lett. 8, 935, 1981) and later studied in great details in laboratory experiments, the occurrence of the two heavy ozone molecules cannot be described by standard molecular formation processes which predict a slight depletion in 49O3 and 50O3 when compared to 48O3. Contrary, enrichments of over 10% were found that were about equal for both. Furthermore, ozone produced in isotopically enriched oxygen mixtures has shown similar large enrichments in many of the isotopes investigated (Krankowsky and Mauersberger, Science Vol. 274, 1324, 1996).

Theories to explain the enrichments have so far centered around molecular symmetry: asymmetric molecules such as 16O16O18O were supposed to be preferentially formed over symmetric species (16O18O16O or 17O17O17O). The new rate coefficients contradict the role of molecular symmetry in the ozone formation process. Single, specific reaction channels are responsible for the enrichments of certain heavy ozone molecules. Future research must investigate how the new rule for ozone molecule formation will apply to other processes. Ozone specific research should extend the rate coefficient studies to cover as many additional formation channels as possible. Finally, the tools are now on hand to begin a theoretical investigation of the ozone formation process including its isotopic variants based on the rate coefficient study reported in the Science paper.
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Max-Planck-Gesellschaft

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