HR: 0800h
AN: SA11A-1107    [Abstracts]
TI: New Measurement of the Rate Coefficient for Three-Body Recombination of Oxygen Atoms in Presence of N$_2$
AU: * Huestis, D L
EM: david.huestis@sri.com
AF: Molecular Physics Laboratory, SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 United States
AU: Pejakovi\'c, D A
EM: dusan.pejakovic@sri.com
AF: Molecular Physics Laboratory, SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 United States
AU: Copeland, R A
EM: richard.copeland@sri.com
AF: Molecular Physics Laboratory, SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 United States
AU: Kalogerakis, K S
EM: ksk@sri.com
AF: Molecular Physics Laboratory, SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 United States
AB: In the atmospheres of Earth, Venus, and Mars photodissociation of O$_2$ and CO$_2$ produces oxygen atoms that eventually undergo three-body recombination: O + O + M $\rightarrow$ O$_2^*$ + 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 = O$_2$ and M = N$_2$. For N$_2$ as the third body, the room temperature coefficient varies between about 3 $\times$ 10$^{-33}$ cm$^6$s$^{-1}$, which is the value recommended in the combustion science community, and 5 $\times$ 10$^{-33}$ cm$^6$s$^{-1}$, a value used in the atmospheric modeling community. Previous laboratory investigations [2] of the process O + O + N$_2$ $\rightarrow$ O$_2^*$ + N$_2$ shared the same basic approach, which was to use N$_2$ discharge flow system with NO added downstream to generate O-atoms in the absence of O$_2$ through the reaction N + NO $\rightarrow$ O + N$_2$. 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 F$_2$ 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 N$_2$ 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($2p^4$ $^3P$) + $2h\nu$ $\rightarrow$ O($2p^33p$ ^3P$). Our measurements give a preliminary value for the O + O + N$_2$ recombination rate coefficient of approximately 3 $\times$ 10$^{-33}$ cm$^6$s$^{-1}$, 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).
DE: 5405 Atmospheres--composition and chemistry
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0355 Thermosphere--composition and chemistry
SC: SPA-Aeronomy [SA]
MN: 2004 AGU Fall Meeting