A quasiclassical procedure for the examination of the collision dynamics of atom—diatomic-molecule reactions with activation energy is introduced. By means of Monte Carlo averages over a large number of appropriately chosen three-dimensional classical trajectories, the total reaction cross section (Sr) and other reaction attributes can be determined as a function of the initial relative velocity (Vr) and the initial molecular rotation-vibration state (J, ν). The method is applied to the exchange reaction resulting from a hydrogen atom and a hydrogen molecule moving on a simple barrier potential of the London—Eyring—Polanyi—Sato type. It is found that Sr is a monotonically increasing function of relative velocity that rises smoothly from a threshold at ∼0.9×106 cm/sec to its asymptotic value of ∼4.5a02 at ∼1.8×106 cm/sec. The zero-point vibrational energy of the molecule contributes to the energy required for reaction, but the rotational energy does not. The reaction probability, which depends on VR, ν, and J, is found to be a smoothly decreasing function of the impact parameter (b) with a maximum of ∼0.6 for b=0 at high velocities. By integration of Sr over the distributions of VR, ν, and J corresponding to temperatures between 300° and 1000°K, a rate constant that can be fitted to an expression of the form K(T)=ATα exp[—Ea°/RT] is obtained, where A, Ea°, and α(α=1.1762) are constants. This temperature dependence is analyzed and contrasted with the results obtained from an absolute rate theory treatment based on the same surface. A detailed examination of the collision trajectories shows no evidence for a long-lived ``collision complex''; instead the results are best represented by a direct interaction model with an interaction time on the order of that required for the atom to pass unimpeded by the molecule. For reactive collisions, the transition configuration is nonlinear with average angles of 170° for VR=0.93×106 cm/sec and 150° for VR=1.30×106 cm/sec; for nonreactive collisions, the average angle is between 95° and 125°, but has no simple relation to VR.

[1] The dramatic solar storm events of April 2002 deposited a large amount of energy into the Earth's upper atmosphere, substantially altering the thermal structure, the chemical composition, the dynamics, and the radiative environment. We examine the flow of energy within the thermosphere during this storm period from the perspective of infrared radiation transport and heat conduction. Observations from the SABER instrument on the TIMED satellite are coupled with computations based on the ASPEN thermospheric general circulation model to assess the energy flow. The dominant radiative response is associated with dramatically enhanced infrared emission from nitric oxide at 5.3 mm from which a total of $7.7 Â 10 23 ergs of energy are radiated during the storm. Energy loss rates due to NO emission exceed 2200 Kelvin per day. In contrast, energy loss from carbon dioxide emission at 15 mm is only $2.3% that of nitric oxide. Atomic oxygen emission at 63 mm is essentially constant during the storm. Energy loss from molecular heat conduction may be as large as 3.8% of the NO emission. These results confirm the ''natural thermostat'' effect of nitric oxide emission as the primary mechanism by which storm energy is lost from the thermosphere below 210 km.

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A new value of the rate coefficient for the deactivation of the bending mode of carbon dioxide by atomic oxygen at low temperatures is derived from the observation of 15 μm emission from the atmosphere of the Earth. This new value gives a cooling rate for the lower thermosphere that is two to three times the rate previously calculated, and it may resolve a long‐standing problem in the Mars‐Venus aeronomy.

On the basis of experimental and theoretical studies, this paper proposes a new mechanism that contributes to nocturnal 4.3 µm CO2 emissions. It suggests that collisions of ground state O atoms with highly vibrationally excited OH(v), produced by the reaction of H with O3, remove a substantial fraction of the OH(v) vibrational energy by a fast, spin‐allowed, multiquantum vibration‐to‐electronic energy transfer (ET) process that generates O(1D): OH(v ≥ 5) + O(3P) → OH(0 ≤ v′ ≤ v − 5) + O(1D). The electronically excited O(1D) atom is subsequently deactivated by collisions with N2 in a fast spin‐forbidden ET process that leaves the N2 molecule with an average of 2.2 vibrational quanta. Finally, the vibrational excitation of N2 is transferred by a fast, near‐resonant vibration‐to‐vibration ET process to the asymmetric stretch (v3) mode of CO2, which promptly radiates near 4.3 µm.

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[1] The temperature dependence of the rate coefficient of the N( 2 D)+O 2 !NO+O reaction has been determined using ab initio potential energy surfaces (PES) and classical dynamics. The calculation agrees with the recommended rate coefficient at 300 K ($110 km altitude). The rate coefficient is given by the expression k(T) = 6.2 Â 10 À12 (T/ 300) cm 3 /s/molec. In contrast to the nearly temperatureindependent value of this rate coefficient previously recommended, the value given here increases by almost a factor of about four as the altitude increases from 110 to 200 km. It is also shown that even though N( 2 D) atoms in the thermosphere are produced with large translational energies, using the value of the rate coefficient at the local temperature introduces negligible error in the amount of NO produced. The new value of this rate coefficient will significantly increase the amount of NO computed in the aeronomic models causing a re-evaluation of the heat budget and temperature and density structure of the thermosphere. In particular, implications of the larger rate coefficient for the recent observations of dramatically enhanced 5.3 mm emission from NO in the thermosphere due to solar storms are discussed.

The cross section for the near-resonant transfer of vibrational energy from CO2(001) to N2(0), CO2(001) + N2(0)→CO2(000) + N2(1) + ΔE, is calculated for the isotopes N214 (ΔE = 18 cm−1) and N215 (ΔE = 97 cm−1). The impact parameter (semi-classical) approximation is used, and it is assumed that the vibrational-energy transfer is caused by the interaction of the instantaneous CO2 dipole moment with the N2 quadrupole moment. When proper account is taken of the rotational motions of the molecules it is found that in collisions of CO2 with 14N2 only the low rotational levels of the CO2 and N2 molecules contribute to Reaction (1). In collisions of CO2 with 15N2, only those rotational levels contribute which undergo transitions cancelling most of the relatively large (97 cm−1) vibrational-resonance defect. For 14N2 below about 1000°K, where the cross section displays a negative temperature dependence, the results are in excellent qualitative and quantitative agreement with available experimental data, with no adjustable parameters in the theory. Above about 1000°K the experimental cross-section data display a positive temperature dependence indicating that some other mechanism becomes important there. For 15N2 the calculations are in good agreement with an experimental measurement at 300°K. Data at other temperatures are not presently available. The theoretical results indicate that the cross section for the vibrational-energy transfer should have maximum around 200°K. Below this temperature the higher rotational levels which dominate the energy-transfer process are unfavorably weighted in the Boltzmann distribution.