HR: 1330h
AN: SA12A-1075    [PDF]
TI: Laboratory Measurements of the O$_2$($a^1\Delta_g$, $v$ = 0) and O$_2$($b^1\Sigma^+_g$, $v$ = 0) Yields Following Collisional Removal of O$_2$($A^3\Sigma^+_u$, $v$ = 6--10)
AU: * Pejakovi\'c, D A
EM: dusan.pejakovic@sri.com
AF: Molecular Physics Laboratory, SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025 United States
AU: Cosby, P C
EM: philip.cosby@sri.com
AF: Molecular Physics Laboratory, SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025 United States
AU: Copeland, R A
EM: richard.copeland@sri.com
AF: Molecular Physics Laboratory, SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025 United States
AU: Slanger, T G
EM: tom.slanger@sri.com
AF: Molecular Physics Laboratory, SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025 United States
AB: Three-body oxygen atom recombination presents a major source of the nightglows of Earth and Venus. In this process the O$_2$ molecule is formed in the seven electronic states that lie below the O($^3$P) + O($^3$P) dissociation limit. Information on the yields of the lowest O$_2$ electronically excited states, $a^1\Delta_g$ and $b^1\Sigma^+_g$, from O-atom recombination can be used to extract O-atom densities in the emitting atmospheric region from the atmospheric emission intensities in the O$_2$($a^1\Delta_g$--$X^3\Sigma^-_g$) and O$_2$($b^1\Sigma^+_g$--$X^3\Sigma^-_g$) bands. Previous SRI experiments showed rapid collisional transfer between the excited states in the energy region close to the dissociation limit [1]. Based on this, we argue that direct optical excitation to these states ($c^1\Sigma^-_u$, $A'^3\Delta_u$, $A^3\Sigma^+_u$) and the resulting energy flow is a reasonable proxy for the three-body O-atom recombination. This work presents studies of vibration level- and collider-dependent yields into the $v$ = 0 levels of the $a^1\Delta_g$ and $b^1\Sigma^+_g$ states, following collisional deactivation of the $v$ = 6-10 levels of the $A^3\Sigma^+_u$ state. Experiments are done at 240 K, a temperature slightly higher than temperatures relevant for the Earth$'$s mesopause (altitude 80-100 km). Colliders pertinent to the atmospheres of Earth and Venus, O$_2$, N$_2$, and CO$_2$, are used. We employ a state-selective two-laser method, in which pulsed output of the first laser excites O$_2$ to O$_2$($A$, $v$ = 6--10). A time-delayed second laser pulse detects O$_2$($a$, $v$ = 0) and O$_2$($b$, $v$ = 0) by resonance-enhanced multiphoton ionization via the $d^1\Pi_g$ Rydberg state. Temporal evolution of the O$_2$($a$, $v$ = 0) and O$_2$($b$, $v$ = 0) populations is determined by varying the time delay between the two laser pulses. We find that the O$_2$($a$, $v$ = 0) yields from O$_2$($A$, $v$ = 7--9) are nearly equal, while the yields from O$_2$($A$, $v$ = 6 and 10) are lower by 30-40%. The O$_2$($b$, $v$ = 0) yields are nearly equal for O$_2$($A$, $v$ = 8--10), and lower by 40-50% for O$_2$($A$, $v$ = 6 and 7). Temporal evolution of the O$_2$($a$, $v$ = 0) signal following excitation to O$_2$($A$, $v$ = 8) shows that production of O$_2$($a$, $v$ = 0) occurs through a rapid several-step collision-driven process. Experiments with air show O$_2$($a$, $v$ = 0) yield about 60% lower than the yield from pure O$_2$. Production of O$_2$($a$, $v$ = 0) in air is about 3.5 times slower than in pure O$_2$. The O$_2$($b$, $v$ = 0) yield from air is about the same as the yield from pure O$_2$. The results indicate that N$_2$ collider is not more efficient than O$_2$ in promptly producing O$_2$($a$, $v$ = 0) and O$_2$($b$, $v$ = 0). Preliminary experiments with CO$_2$ show that CO$_2$ collider is likely less efficient than O$_2$ in producing O$_2$($a$, $v$ = 0). We will outline our current efforts to use comparison with products from a O$_3$/O$_2$ mixture to determine the $absolute$ O$_2$($a$, $v$ = 0) and O$_2$($b$, $v$ = 0) yields as well as relative yields of these two states. Preliminary results show that about 2.5 times as much O$_2$($a$, $v$ = 0) as O$_2$($b$, $v$ = 0) is promptly formed upon O$_2$($A$) excitation. This work is funded by the NASA Ionosphere, Thermosphere, and Mesosphere Supporting Research \& Technology Program and the NASA Planetary Atmospheres Program. [1] T.G. Slanger and R. A. Copeland, ``Energetic Oxygen in the Upper Atmosphere and the Laboratory'', Chem. Rev., in press.
DE: 0310 Airglow and aurora
DE: 0343 Planetary atmospheres (5405, 5407, 5409, 5704, 5705, 5707)
SC: SPA - Aeronomy [SA]
MN: 2003 Fall Meeting