Christof Janssen

Relative formation rates of 50O3 and 52O3 in 16O–18O mixtures

Tunable diode laser (TDL) and mass spectrometry have been combined to measure relative formation rate coefficients of each of the four channels contributing to ozone of mass 50 u and 52 u produced in 16O–18O mixtures. Only one channel has a large rate coefficient advantage causing almost exclusively the observed isotope enrichment. Collisions to form ozone are end-on reactions. Molecular symmetry plays no apparent role in the ozone isotope enrichment process, regardless, whether or not ozone is produced in collisions with homo- or heteronuclear molecular oxygen. The oxygen isotope exchange process may hold a key in explaining the rate coefficient results.

Isotope dependence of the O+O2 exchange reaction: Experiment and theory

The isotope dependence of the O+O2 exchange reaction is investigated by means of kinetic experiments and classical trajectory calculations on an accurate potential energy surface. The measurements confirm the previously reported negative temperature dependence and yield the rate coefficients for both the exothermic 18O+16O2→18O16O+16O and the endothermic 16O+18O2→16O18O+18O reaction between 233 and 353 K: k8=(3.4±0.6)×10−12 (300 K/T)1.1±0.5 cm3 s−1 and k6=(2.7±0.4)×10−12 (300 K/T)0.9±0.5 cm3 s−1. In addition, the ratio of these two rates, R, has been measured with comparatively higher precision. It is 1.27±0.04 at 300 K and also shows a distinct negative temperature dependence. Four types of classical trajectory calculations are performed in order to interpret the experimental result. They differ by the way in which the quantum mechanical zero-point energy of the reactants and the differences of zero-point energies between reactants and products, ΔEZPE≈±22 cm−1, are phenomenologically incorporated. Only c...

Temperature dependence of ozone rate coefficients and isotopologue fractionation in 16O-18O oxygen mixtures

Abstract The temperature dependence of five ozone isotope-specific rate coefficient ratios and of isotopologue fractionation has been determined. Large formation rate coefficient ratios of 1.5 like 16 O + 18 O 18 O vs. 16 O + 16 O 16 O show no temperature dependence while small ratios such as 18 O + 16 O 16 O vs. 16 O + 16 O 16 O with a value of 0.92 decrease with decreasing temperatures. Temperature-related changes of isotopologue fractionation values for 50 O 3 and 52 O 3 are explained in terms of changes in rate coefficient ratios and contributions from isotope exchange reactions. The latter reactions exclusively control the large isotope fractionation of 54 O 3 while the rate coefficient ratio 18 O + 18 O 18 O vs. 16 O + 16 O 16 O remains constant at 1.02.

Assessment of the Ozone Isotope Effect

Publisher Summary This chapter discusses the ozone isotope effect from a molecular perspective and presents results from atmospheric investigations. The chapter introduces a number of aspects of the oxygen-ozone system because O and O 2 contribute in specific ways to the final enrichments observed in the ozone isotopologue formation. The chapter discusses the oxygen isotope exchange reaction, which plays an intrinsic role in the ozone formation. Ozone isotopes produced in natural oxygen and in specific oxygen mixtures to unfold the complexity of the effect are also explained. Further, the expression “mass-independent” is employed to characterize the ozone isotope effect in 49 O 3 and 50 O 3 and in results observed for other trace gases that may be affected by reactions with ozone. Mass independence is a descriptive label that carries little scientific information except that the process in question is not standard mass-dependent, while mass dependence in chemical and physical processes is well-defined: Molecules that have 18 O substituted are about twice as much fractionated as those with 17 O. The multitude of ozone isotopologues and rate coefficient ratios require a clear and precise description related to physical parameters.

Oxygen Isotope Processes and Transfer Reactions

A unique kinetic isotope effect has been found in the formation process of ozone molecules. Isotope enrichments of about 10% above statistically expected values were first discovered in atmospheric isotopomers 49O3 and 50O3 and later in many other molecular combinations. Most recently the source of this effect was identified through measurement of isotope-specific ozone formation rate coefficients which show a large variability of over 50%. Ozone molecule formation is a complex process since different reaction channels contribute to a specific isotopomer. In addition, fast oxygen isotope exchange reactions determine the abundance of atomic oxygen participating in ozone formation. The isotope enrichments observed are both pressure and temperature-dependent and they decrease at pressures above 100 mbar and toward lower temperatures. Ozone possesses not only one of the most unusual isotope anomalies, it also serves as a mediator by transferring heavy oxygen from the O2 reservoir to other species. Stratospheric isotope composition of CO2 has been recently measured with high accuracy and a pronounced isotopic signature was found which shows that “17Ois preferentially transferred from 03 into CO2.

Isotope evidence for ozone formation on surfaces.

Ozone formation in the gas phase is associated with a large and unusual isotope effect of widespread use in geochemistry and climate research. Little is known whether similar nonstandard mass dependent fractionations also occur in other recombination reactions. Here we report on the pressure and temperature dependence of the isotopic composition of ozone formed by electric discharge in molecular oxygen. Isotope signatures at low pressures show a standard mass dependent depletion, their magnitudes strongly depending on temperature. Our analysis confirms the formation of ozone at Pyrex reactor walls with an atom recombination coefficient gamma = (0.4 ± 0.1)% at room temperature and slightly higher values at lower temperatures. Thus, although neglected so far, wall assisted ozone formation is an essential part of oxygen plasma chemistry and it could also provide a mechanism explaining the presence of ozone on icy satellites. Recombination reactions on the surface are not likely to show the isotope anomalies associated with ozone formation in the gas phase.

Unambiguous identification of 17O containing ozone isotopomers for symmetry selective detection

Symmetry selective detection of the 17O containing ozone isotopomers 16O16O17O and 16O17O16O requires the unambiguous identification of absorption lines. We report high resolution tuneable diode laser spectrometer measurements of 17O containing ozone isotopomers in the R-branch of the ν3 band and present a purely experimental technique that discriminates between 16O16O17O and 16O17O16O. Around 1040 cm−1, differences in line positions of 16O17O16O upto 4 × 10−3 cm−1 between our measurements and present spectroscopic database records (HITRAN 2004) are found.