Vibrational relaxation of CO₂(ν₂) by atomic oxygen

In the Earth's upper atmosphere, collisions with ambient O atoms efficiently excite the CO 2 [00 0 0] vibrational ground‐state population to the first excited, [01 1 0] or ν 2 , vibrational bend state. Subsequent relaxation of the ν 2 population occurs through spontaneous emission of 15‐μm radiation. Much of this radiation escapes into space, thereby removing ambient kinetic energy from the atmosphere. This cooling mechanism is especially important at altitudes between the mesopause and the lower thermosphere, approximately 80–120 km, where the O‐atom density is relatively high and the kinetic temperature is rising. Laboratory measurements have been performed to better characterize the CO 2 (ν 2 )‐O vibrational relaxation rate coefficient k O (ν 2 ). A 266‐nm laser pulse photolyzed trace amounts of O 3 in a CO 2 ‐O 3 ‐rare gas mixture, simultaneously creating O atoms and raising the gas temperature to create a nonequilibrium CO 2 vibrational distribution. Transient diode laser absorption spectroscopy was used to monitor CO 2 vibrational level population reequilibration. A global nonlinear least squares fitting technique was used to interpret the kinetic data, yielding k O (ν 2 ) = (1.8 ± 0.3) × 10 −12 cm 3 s −1 . The result is in good agreement with previous laboratory measurements, with published k O (ν 2 ) values in the (1.2–1.5) × 10 −12 cm 3 s −1 range and at the low end of the (2–6) × 10 −12 cm 3 s −1 range estimated from the analysis of upper atmospheric data.