Crossed Molecular Beam Investigation of 18O(3P)+NO2 and 18O(1D)+NO2: Isotope exchange and O2+NO formation
Nitrogen oxides (NO and NO2, collectively referred to as NOx) play central roles in the chemistry of both the troposphere and the stratosphere. In the troposphere, reactions of NOx can lead to photochemical production of ozone, whereas in the stratosphere NOx takes part in catalytic cycles of ozone destruction. In addition, NO and NO2 are expected to participate in oxygen atom isotope exchange reactions, which have recently become a subject of particular interest because they are an important mechanism by which the unusual isotopic signature of stratospheric ozone can be transferred to other molecules. Anomalous isotopic compositions resulting from isotope exchange reactions have been predicted for numerous oxygen-containing atmospheric species, including NO2. Furthermore, such anomalous isotopic compositions have the potential to serve as new tracers of atmosphere and biosphere dynamics and interactions.- In particular, Michalski et al., have used the anomalous oxygen isotopic signature found in atmospheric nitrates, which has its origin in the anomalous isotopic composition of ozone, to distinguish between atmospheric deposition and biological production as sources of nitrate in various ecosystems. Since nitrates in the atmosphere are formed predominantly by oxidation of NO2 (e.g., NO2+OH+M→HNO3+M), understanding the isotope exchange reactions that influence the isotopic composition of NO2 is important for tracer applications. Dynamical studies in particular can provide a means of evaluating whether the reactant oxygen atom is incorporated into the product molecule with statistical probability during isotope exchange or whether anomalous isotope effects may exist. In order to investigate the molecular level details of O+NO2 isotope exchange and probe the unresolved dynamics of O2+NO formation from O(3P)+NO2, we perform crossed molecular beam studies of 18O(3P)+NO2 and 18O(1D)+NO2.
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