[1] Vibrational excitation of ground-state NO through collisions with oxygen atoms produces NO(v = 1) in the lower thermosphere, representing a significant source of atmospheric cooling through the subsequent 5.3-mm radiative emission. A laser pumpprobe experiment has been used to measure the temperature dependence of the NO(v = 1)-O vibrational relaxation rate coefficient k O (v = 1) in the 295-825 K range, along with updated measurements of k O (v = 1,2) at room temperature. The experiment employed a continuous wave microwave source to form O atoms, combined with photolysis of a trace amount of added NO 2 to produce vibrationally excited NO. Oxygen atoms were detected through two-photon laser-induced fluorescence, cross-calibrated against a normalized O atom signal resulting from photolysis of a known concentration of NO 2 . No temperature dependence was observed for k O (v = 1) to within the uncertainty in the measurements. The measured room temperature value of k O (v = 1) = (4.2 ± 0.7) Â 10 À11 cm 2 s À1 is 75% larger than the value obtained previously in this laboratory, a significant difference at the 1s level. The present value is preferred owing to an improved experimental technique. The atmospherically relevant NO(v = 0)-O vibrational excitation rate coefficient can be derived from measured values of k O (v = 1) through detailed balance. The variable temperature measurements provide key information for aeronomic models of the lower thermospheric energy budget, infrared emission intensities, and neutral constituent densities.
Detailed spectroscopic analysis of hydroxyl fundamental vibration‐rotation and pure rotation emission lines has yielded OH(υ,N) absolute column densities for nighttime earthlimb spectra in the 20 to 110‐km tangent height region. High‐resolution spectra were obtained in the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) experiment. Rotationally thermalized populations in υ = 1–9 have been derived from the fundamental bands between 2000 and 4000 cm−1. Highly rotationally excited populations with N ≤ 33 ( ≤ 2.3 eV rotational energy) have been inferred from the pure rotation spectra between 400 and 1000 cm−1. These emissions originate in the airglow region near 85–90 km altitude. Spectral fits of the pure rotation lines imply equal populations in the spinrotation states F1 and F2 but a ratio Π(A′):Π(A″) = 1.8±0.3 for the Λ‐doublet populations. A forward predicting, first‐principles kinetic model has been developed for the resultant OH(υ,N) limb column densities. The kinetic model incorporates a necessary and sufficient number of processes known to generate and quench OH(υ,N) in the mesopause region and includes recently calculated vibration‐rotation Einstein coefficients for the high‐N levels. The model reproduces both the thermal and the highly rotationally excited OH(υ,N) column densities. The tangent height dependence of the rotationally excited OH(υ,N) column densities is consistent with two possible formation mechanisms: (1) transfer of vibrational to rotational energy induced by collisions with O atoms or (2) direct chemical production via H + O3 → OH(υ,N) + O2.
The RRKM and Marcus formalisms have been successfully applied in the past to the interpretation of gas-phase ion-molecule reaction rates. The analysis is complicated by the necessity of incorporating ion-molecule association energies, which are often unknown. Also, precise accounting of the well depths can lead to incorrect conclusions regarding ratelequilibrium relationships. We suggest defining the reactants and products to be noninteracting, and introduce a modified Marcus equation that includes the well depth factor. With this definition, calculated activation energies are often insensitive to the well depth. IntroductionThe interpretation of reaction rates plays a critical role in chemistry. There are two aspects of interpretation which require attention from the chemist. The first is the process by which the experimentally determined rate constant is related to values of the activation energy. The second is the process of relating the measured activation energy to the overall exothermicity in order to predict the behavior of other compounds with similar structure.Gas-phase ion chemistry has reached the point at which both of these aspects of reaction rate theory are important. Because
Time-resolved OH(X 2Πi,v=1–9) populations have been measured and analyzed to determine parameters relating to formation mechanisms and vibrational relaxation. OH(v) was formed in electron-irradiated Ar/H2/O3 mixtures containing added O2 or CO2 as relaxer species. OH(v→v−1,v−2) emission was observed using time-resolved Fourier spectroscopy. Spectra were then fit to determine time-dependent populations. Population data were analyzed using a single-quantum relaxation model, but the possible effects of multiquantum relaxation were also considered. The model includes provision for OH(v) production via H+O3→OH(v)+O2 after e-beam termination, which has been found to have a significant effect on the results. The following relaxation rate constants are obtained: kv=1–6(O2)=1.3±0.4, 2.7±0.8, 5.2±1.5, 8.8±3.0, 17±7, 30±15 (10−13 cm3s−1) and kv=1–4(CO2)=1.8±0.5, 4.8±1.5, 14±5, 28±10 (10−13 cm3s−1), respectively. Two different exponential decay rates are necessary to characterize the time dependence of the inferred H atom concentration. The role of O(1D)+H2→OH+H is also discussed.
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