Vibrational relaxation of CO2(ν2) by atomic oxygen

Abstract

In the Earth's upper atmosphere, collisions with ambient O atoms efficiently excite the CO2 [0000] vibrational ground‐state population to the first excited, [0110] or ν2, vibrational bend state. Subsequent relaxation of the ν2 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 CO2(ν2)‐O vibrational relaxation rate coefficient kO(ν2). A 266‐nm laser pulse photolyzed trace amounts of O3 in a CO2‐O3‐rare gas mixture, simultaneously creating O atoms and raising the gas temperature to create a nonequilibrium CO2 vibrational distribution. Transient diode laser absorption spectroscopy was used to monitor CO2 vibrational level population reequilibration. A global nonlinear least squares fitting technique was used to interpret the kinetic data, yielding kO(ν2) = (1.8 ± 0.3) × 10−12 cm3s−1. The result is in good agreement with previous laboratory measurements, with published kO(ν2) values in the (1.2–1.5) × 10−12 cm3s−1 range and at the low end of the (2–6) × 10−12 cm3s−1 range estimated from the analysis of upper atmospheric data.