Crossed Molecular Beam Investigation of 18O(3P)+O2 isotope exchange
The abundance of the stable isotopes of oxygen (16O, 17O, 18O) in ozone and other oxygen-containing species is of particular interest because of its potential to provide new constraints on the chemistry of odd-oxygen species in the troposphere and stratosphere, and has been the subject of much study. Nonstandard oxygen isotope effects occurring in Earth's atmosphere first gained considerable attention in 1981 when Mauersberger used a mass spectrometer to measure the stratospheric abundance of heavy ozone (18O16O16O, molecular mass 50) relative to ozone of mass 48 and found that the enrichment in 18O was significantly greater than the statistically predicted abundance.1 Since then, unusual isotope effects have been observed in the O3 formation reaction in laboratory experiments as well,2,3 although a complete quantitative understanding of their origins remains elusive. In order to gain insight into the molecular-level details of the O+O2 system, we have investigated the dynamics of the 18O(3P)+32O2 &rarr 50O3* &rarr 16O(3P)+34O2 isotope exchange reaction (which proceeds on the same potential energy surface as ozone formation) using the crossed molecular beam technique in collaboration with Professors Jim Jr-Min Lin and Yuan T. Lee at the Institute of Atomic and Molecular Sciences in Taipei, Taiwan. The results of our experiments, in combination with theoretical predictions from quantum statistical calculations (performed by Shiying Lin and Hua Guo) and quasi-classical trajectory calculations (performed by George Lendvay), provide important experimental constraints on the structure of the potential energy surface for ozone formation as well as the first experimental evidence that the dissociation of excited ozone exhibits non-statistical behavior.4 Our results facilitate a deeper understanding of the unusual isotope effects in the formation of ozone, a species whose chemistry is of utmost importance in both the stratosphere and troposphere.
1. K. Mauersberger, Measurement of Heavy Ozone in the Stratosphere, Geophys. Res. Lett. 8, 935 (1981).
2. K. Mauersberger, B. Erbacher, D. Krankowsky, J. Günther and R. Nickel, Ozone isotope enrichment: Isotopomer-specific rate coefficients, Science 283, 370 (1999).
3. J. Guenther, D. Krankowsky and K. Mauersberger, Third-body dependence of rate coefficients for ozone formation in 16O-18O mixtures, Chem. Phys. Lett. 324, 31 (2000).
4. A. Van Wyngarden, K. A. Mar, K. Boering, J. J. Lin, Y. T. Lee, S. Y. Lin, H. Guo, and G. Lendvay, Non-statistical behavior of the reactive scattering in the 18O+32O2 isotope exchange reaction, J. Am. Chem. Soc., 129(10), 2866-2870, 2007.
