Laboratory Measurements of the O2(a1Δ g, v = 0) and O2(b1Σ ^+g, v = 0) Yields Following Collisional Removal of O2(A3Σ ^+u, v = 6--10)

Three-body oxygen atom recombination presents a major source of the nightglows of Earth and Venus. In this process the O₂ molecule is formed in the seven electronic states that lie below the O(³P) + O(³P) dissociation limit. Information on the yields of the lowest O₂ electronically excited states, a¹Δ_g and b¹Σ ^+_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 O₂(a¹Δ _g--X³Σ ^-_g) and O₂(b¹Σ ^+_g--X³Σ ^-_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 (c¹Σ ^-_u, A'³Δ _u, A³Σ ^+_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 a¹Δ _g and b¹Σ ^+_g states, following collisional deactivation of the v = 6-10 levels of the A³Σ ^+_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, O₂, N₂, and CO₂, are used. We employ a state-selective two-laser method, in which pulsed output of the first laser excites O₂ to O₂(A, v = 6--10). A time-delayed second laser pulse detects O₂(a, v = 0) and O₂(b, v = 0) by resonance-enhanced multiphoton ionization via the d¹Π _g Rydberg state. Temporal evolution of the O₂(a, v = 0) and O₂(b, v = 0) populations is determined by varying the time delay between the two laser pulses. We find that the O₂(a, v = 0) yields from O₂(A, v = 7--9) are nearly equal, while the yields from O₂(A, v = 6 and 10) are lower by 30-40%. The O₂(b, v = 0) yields are nearly equal for O₂(A, v = 8--10), and lower by 40-50% for O₂(A, v = 6 and 7). Temporal evolution of the O₂(a, v = 0) signal following excitation to O₂(A, v = 8) shows that production of O₂(a, v = 0) occurs through a rapid several-step collision-driven process. Experiments with air show O₂(a, v = 0) yield about 60% lower than the yield from pure O₂. Production of O₂(a, v = 0) in air is about 3.5 times slower than in pure O₂. The O₂(b, v = 0) yield from air is about the same as the yield from pure O₂. The results indicate that N₂ collider is not more efficient than O₂ in promptly producing O₂(a, v = 0) and O₂(b, v = 0). Preliminary experiments with CO₂ show that CO₂ collider is likely less efficient than O₂ in producing O₂(a, v = 0). We will outline our current efforts to use comparison with products from a O₃/O₂ mixture to determine the absolute O₂(a, v = 0) and O₂(b, v = 0) yields as well as relative yields of these two states. Preliminary results show that about 2.5 times as much O₂(a, v = 0) as O₂(b, v = 0) is promptly formed upon O₂(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.