Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsa11a1107h&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #SA11A-1107
Other
5405 Atmospheres: Composition And Chemistry, 0300 Atmospheric Composition And Structure, 0355 Thermosphere: Composition And Chemistry
Scientific paper
In the atmospheres of Earth, Venus, and Mars photodissociation of O2 and CO2 produces oxygen atoms that eventually undergo three-body recombination: O + O + M -> O2* + M. The competition between photodissociation, recombination, and diffusive vertical transport controls the atomic and molecular composition of the mesosphere and lower thermosphere. Knowledge of the rate coefficient for recombination of atomic oxygen is essential for modeling atmospheric composition. The most recent measurement of O-atom recombination rate coefficient is over thirty years old [1]. The published values of this rate coefficient have large divergence for both M = O2 and M = N2. For N2 as the third body, the room temperature coefficient varies between about 3 × 10-33 cm6s-1, which is the value recommended in the combustion science community, and 5 × 10-33 cm6s-1, a value used in the atmospheric modeling community. Previous laboratory investigations [2] of the process O + O + N2 -> O2* + N2 shared the same basic approach, which was to use N2 discharge flow system with NO added downstream to generate O-atoms in the absence of O2 through the reaction N + NO -> O + N2. This approach is vulnerable to heterogeneous recombination and other processes that may obscure the reaction of interest, mostly due to the low O-atom densities and, consequently, long reaction times. We employ an F2 laser with up to 50 mJ of 157 nm pulsed output to achieve nearly complete photodissociation of molecular oxygen. In a high-pressure (760 Torr) background of N2 the oxygen atoms recombine in a time scale of several milliseconds. Oxygen atom population is monitored by detecting 845-nm fluorescence, which is induced by the 226 nm output of the second laser via a two-photon process O(2p4 3P) + 2hν -> O(2p33p ^3P). Our measurements give a preliminary value for the O + O + N_2 recombination rate coefficient of approximately 3 \times 10^{-33} cm^6s^{-1}, which favors the value recommended in the combustion community. Implications of this result for atmospheric modeling will be discussed. This work is supported by the NASA Geospace Sciences Program under grant NAG5-12992. The F_2$ laser was purchased under grant ATM-0216583 from the NSF Major Research Instrumentation Program. [1] I. M. Campbell and C. N. Gray, Chem. Phys. Lett. 18, 607 (1973). [2] D. L. Baulch, D. D. Drysdale, J. Duxbury, and S. J. Grant, Evaluated Kinetic Data for High Temperature Reactions Vol. 3 (Butterworths, London, 1976).
Copeland Richard A.
Huestis David L.
Kalogerakis Konstantinos S.
Pejaković Dušan A.
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