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

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.