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Accurate knowledge of the rate as well as the mechanism of excitation of thebending mode of CO2 is necessary for reliable modeling of themesosphere–lower thermosphere (MLT) region of the atmosphere. Assuming theexcitation mechanism to be thermal collisions with atomic oxygen, the ratecoefficient derived from the observed 15 μm emission by space-basedexperiments (kATM = 6.0 × 10−12 cm3s−1) differsfrom the laboratory measurements (kLAB =(1.5-2.5) × 10−12 cm3s−1) by a factor of 2–4. The general circulationmodels (GCMs) of Earth, Venus, and Mars have chosen to use a median value ofkGCM = 3.0 × 10−12 cm3s−1 for this ratecoefficient. As a first step to resolve the discrepancies between the threerate coefficients, we attempt to find the source of disagreement between thefirst two. It is pointed out that a large magnitude of the difference betweenthese two rate coefficients (kx ≡ kATM -kLAB) requires that the unknown mechanism involve one or bothmajor species: N2, O. Because of the rapidly decreasing volume mixingratio (VMR) of CO2 with altitude, the exciting partner must be longlived and transfer energy efficiently. It is shown that thermal collisionswith N2, mediated by a near-resonant rotation-to-vibration (RV) energytransfer process, while giving a reasonable rate coefficient kVRfor de-excitation of the bending mode of CO2, lead tovibration-to-translation kVT rate coefficients in the terrestrialatmosphere that are 1–2 orders of magnitude larger than those observed inthe laboratory. It is pointed out that the efficient near-resonantrotation-to-vibration (RV) energy transfer process has a chance of being theunknown mechanism if very high rotational levels of N2, produced by thereaction of N and NO and other collisional processes, have a super-thermalpopulation and are long lived. Since atomic oxygen plays a critical role inthe mechanisms discussed here, it suggested that its density be determinedexperimentally by ground- and space-based Raman lidars proposed earlier

Technical Note: On the possibly missing mechanism of 15 μm emission in the mesosphere–lower thermosphere (MLT)