Laboratory Investigations of the Collisional Removal of O2(X3Σ g-, υ = 1, 2, and 3)

One of the tasks of the SABER instrument on the TIMED satellite is to measure atmospheric water vapor by making observations of H₂O emission. Two important processes in the production of this emission are the collisional removal of O₂(X^{3Σ _g-}, υ = 1) by H₂O and O(³P). Mlynczak et al.¹ have recently highlighted the need for an improved laboratory measurement of the collisional removal rate coefficient of O₂(X³Σ _g^-, \upsilon} = 1) by oxygen atoms as essential to a reliable interpretation of the SABER data. We have initiated an experimental program aiming to resolve the uncertainty in laboratory measurements involving O₂(X³Σ _g^-, υ = 1) and oxygen atoms. In our experiments, laser light at 266 nm photolyzes ozone in a mixture of molecular oxygen and ozone. This step produces vibrationally excited O₂(a¹Δ g) which rapidly populates O₂(X³Σ g^-, υ = 1 - 3) in a resonant process. In addition, a large amount of O atoms is generated. A second laser pulse near 193 nm excites O₂(X³Σ g^-, υ = 1 - 3) via the (7, 1), (10, 2) and (14, 3) B³Σ _u^--X³Σ _g^- bands, respectively, and the fluorescence is detected through a 360 nm interference filter by a photomultiplier tube. The time evolution of the population of O₂(X³Σ _g^-, \upsilon} = 1, 2, and 3) is monitored by varying the delay between the two laser pulses. The concentration of ozone in the cell is determined by absorption measurements at 253.7 nm. Our results indicate that the collisional removal rate coefficients for O₂(X³Σ _g^-, \upsilon} = 2, 3) by O₂ at room temperature have values of (1.3 +/- 0.4) x 10^-13 and (1.9 +/- 0.3) x 10^-13 cm³s^-1, respectively. These values represent the first laboratory measurement of these rate coefficients and are in good agreement with recent theoretical calculations.² Because the removal of O₂(X³Σ _g^-, \upsilon} = 1) by O₂ at room temperature is extremely slow ( ~3 x 10^-18 cm³s^-1), collisions with the photolytically generated O atoms control the lifetime of υ = 1. We have made preliminary measurements of this process and are working to make the measurements quantitative. This work is supported by the NASA Ionospheric, Thermospheric, and Mesospheric Physics, Supporting Research & Technology Program. ¹ M. G. Mlynczak, D. K. Zhou, M. Lopez-Puertas, G. Zaragoza, and J. M. Russell III, Geophys. Res. Lett. 26, 63 (1999). ² C. Coletti, G. Billing, personal communication.