HR: 0800h
AN: SA11A-1106    [Abstracts]
TI: Collisional Removal of O$_2$($b^1\Sigma^+_g$, $v$ = 1) by Atomic Oxygen
AU: * Kalogerakis, K S
EM: ksk@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: Slanger, T G
EM: tom.slanger@sri.com
AF: Molecular Physics Laboratory, SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 United States
AB: In the thermosphere, energy transfer between excited O atoms and ground-state molecular oxygen produces O$_2$ in the first two vibrational levels of the $b^1\Sigma^+_g$ state: O($^1D$) + O$_2$ $\rightarrow$ O($^3P$) + O$_2$($b^1\Sigma^+_g$, $v$ = 0, 1). Subsequent radiative decay of O$_2$($b^1\Sigma^+_g$, $v$ = 0, 1) to the ground state O$_2$($X^3\Sigma^-_g$) results in the Atmospheric Band emissions. Atmospheric observations suggest that above $\sim$120 km O($^3P$) plays an important role in removing O$_2$($b^1\Sigma^+_g$, $v$ = 1). Therefore, knowledge of the rate coefficient for collisional removal of O$_2$($b^1\Sigma^+_g$, $v$ = 1) by O($^3P$) is important for detailed understanding of the Atmospheric Band emissions. Measurements are reported of the room-temperature rate coefficient for removal of O$_2$($b^1\Sigma^+_g$, $v$ = 1) by O($^3P$). A commercial F$_2$ laser with pulsed energy output of up to 50 mJ at 157 nm is used to photodissociate a large fraction of molecular oxygen in a O$_2$/N$_2$ mixture. Photodissociation of an O$_2$ molecule produces a ground-state oxygen atom O($^3P$) and an excited oxygen atom O($^1D$), and O($^1D$) rapidly transfers energy to the remaining O$_2$ to produce O$_2$($b^1\Sigma^+_g$, $v$ = 0, 1). The O$_2$($b^1\Sigma^+_g$, $v$ = 1) population is monitored by observing emission in the O$_2$ ($b-X$) 1--1 band at 771 nm. To extract the O$_2$($b^1\Sigma^+_g$, $v$ = 1) + O($^3P$) rate coefficient, knowledge of the O($^3P$) partial pressure or, equivalently, the fraction of dissociated O$_2$, is necessary. Based on the F$_2$ laser fluence, the signal dependence on the fraction of dissociation, and computer modeling, the signals measured in our experiments correspond to about 50% dissociation. Our measurements yield a preliminary value of the rate coefficient for O$_2$($b^1\Sigma^+_g$, $v$ = 1) removal by O($^3P$) of 6 $\times$ 10$^{-12}$ cm$^3$s$^{-1}$. The results will be compared to the rate coefficients for corresponding processes in the ground and $a^1\Delta_g$ states of O$_2$, and implications of the results for modeling of the upper atmosphere will be discussed. This work is supported by the NSF Aeronomy Program under grant ATM-0209229. The F$_2$ laser was purchased under grant ATM-0216583 from the NSF Major Research Instrumentation Program.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0310 Airglow and aurora
DE: 0340 Middle atmosphere--composition and chemistry
DE: 0355 Thermosphere--composition and chemistry
DE: 0399 General or miscellaneous
SC: SPA-Aeronomy [SA]
MN: 2004 AGU Fall Meeting