Article

The vibrational deactivation of the bending mode of CO2 by O2 and by N2

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Abstract

Shock tube measurements made between 1 500 and 350 K suggest that O2 is slightly more efficient at deactivating the bending mode of CO2 than is N2. The rate constant for its deactivation by air at 210 K has been estimated.

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... Taine et al. [1978] and Allen et al. [1980] measured the rate of collisions with molecular nitrogen at atmospheric temperatures but only for low-energy vibrational levels. There are also laboratory measurements of this coefficient reported by Simpson et al. [1977], Allen et al. [1977], Taine and Lepoutre [1979], Lunt et al. [1985] and Siddles et al. [1994]. All measurements gave similar values within their experimental errors. ...
... All measurements gave similar values within their experimental errors. Simpson et al. [1977], Taine et al. [1978] and Siddles et al. [1994] also provided the rate of CO 2 (u 2 ) relaxation in collisions with molecular oxygen (see Figure 7). The nominal values for both coefficients (k CO2-air ) used here are the parameterizations of Wintersteiner et al. [1992] obtained by fitting the aforementioned experimental data. ...
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The vast set of near-global and continuous atmospheric measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument since 2002, including daytime and nighttime kinetic temperature (Tk) from 20 to 105 km, is available to the scientific community. The temperature is retrieved from SABER measurements of the atmospheric 15 μm CO2 limb emission. This emission separates from local thermodynamic equilibrium (LTE) conditions in the rarefied mesosphere and thermosphere, making it necessary to consider the CO2 vibrational state non-LTE populations in the retrieval algorithm above 70 km. Those populations depend on kinetic parameters describing the rate at which energy exchange between atmospheric molecules take place, but some of these collisional rates are not well known. We consider current uncertainties in the rates of quenching of CO2(υ2) by N2, O2 and O, and the CO2(υ2) vibrational-vibrational exchange to estimate their impact on SABER Tk for different atmospheric conditions. The Tk is more sensitive to the uncertainty in the latter two, and their effects depend on altitude. The Tk combined systematic error due to non-LTE kinetic parameters does not exceed ±1.5 K below 95 km and ±4–5 K at 100 km for most latitudes and seasons (except for polar summer) if the Tk profile does not have pronounced vertical structure. The error is ±3 K at 80 km, ±6 K at 84 km and ±18 K at 100 km under the less favorable polar summer conditions. For strong temperature inversion layers, the errors reach ±3 K at 82 km and ±8 K at 90 km. This particularly affects tide amplitude estimates, with errors of up to ±3 K.
... Appendix A outlines the formulation necessary to incorporate this new 15-/zm cooling scheme in the two-dimensional model. The explicit ratio of Kco2-o and Kco2-co2 is now calculated, with the Kco2-co2 temperature dependence taken from Simpson et al. (1977). An expression for weak Kco2-o temperature dependence, similar to that of Sharma and Nadile (1981), is also given. ...
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Recent Pioneer Venus observations have prompted a return to comprehensive hydrodynamical modeling of the thermosphere of Venus. Our approach has been to reexamine the circulation and structure of the thermosphere using the framework of the R. E. Dickinson an E. C. Ridley (1977, Icarus30, 163–178), symmetric two-dimensional model. Sensitivity tests were conducted to see how large-scale winds, eddy diffusion and conduction, and strong 15-μm cooling affect day-night contrasts of densities and temperatures. The calculated densities and temperatures are compared to symmetric empirical model fields constructed from the Pioneer Venus data base. We find that the observed day-to-night variation of composition and temperatures can be derived largely by a wave-drag parameterization that gives a circulation system weaker than predicted prior to Pioneer Venus. The calculated mesospheric winds are consistent with Earth-based observations near 115 km. Our studies also suggest that eddy diffusion is only a minor contributor to the maintenance of observed day and nightside densities, and that eddy coefficients are smaller than values used by previous one-dimensional composition models. The mixing that occurs in the Venus thermosphere results from small-scale and large-scale motions. Strong CO2 15-μm cooling buffers solar perturbation such that the response by the general circulation to solar cycle variation is relatively weak.
... The 4 dependence in the first term implies a temperature-independent cross-section at low temperatures, but our choice of this form is empirical. For collisions of C02 with N 2 and 02, the constants in these expressions were chosen to provide a fit to the published experimental data ( Lunt et al, 1985;Taine et al, 1978;Taine and Lepoutre, 1979;Simpson et al, 1977). Figures 2 and 3 show these data for the deactivation of C0 2 (01101), and our approximations to them. Figure 4 shows the scant data available for the V-T deactivation of C02(02201) by N 2 , and our parameterization. ...
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The acoustic absorption in mixtures of CO2 and O2 has been measured for 90%, 80%, and 70% concentrations of O2 at 300 °K. The absorption has been analyzed to extract the rate of energy transfer between the vibration of the bending mode of CO2 and the O2 vibration and the rate at which O2 de‐excites the bending mode of CO2 in a v‐t process. The rate constants for the v‐t de‐excitation of the bending mode of CO2 by O2 was found to be about the same as de‐excitation by CO2. The v‐v exchange between CO2 and O2 increases only slightly from 1.6×105 (sec atm)−1 at 300 °K to 2.5×105 at 600°K.
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Vibrational relaxation in CO2-N2 and CO2-O2 mixtures is studied via the optic-acoustic effect.The relaxation of the bending vibration of CO2 by N2 and O2 can be explained by a simple one-step relaxation. For CO2-N2 a relaxation time of 12.8 ± 1.5 μs atm is obtained, for CO2-O2 8.8 ± 0.3 μs atm.The relaxation of the asymmetric stretching vibration of CO2 by N2 can be described by a two-step mechanism. The result of 11.9 ± 0.25 μs atm for the relaxation time is in reasonable agreement with laser-fluorescence results.The frequency and pressure dependences of the relaxation observed in the CO2-O2 mixtures cannot be explained by a two-step mechanism.
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Vibrational relaxation times have been measured in CO2 and CO2 mixtures from 360 to 1500 K by a laser-schlieren method. The scatter in the results is small and there is excellent agreement with recent ultrasonic measurements. Within the experimental error the relaxation times for a given pressure are a function of the translational temperature alone and do not depend upon how close the system is to equilibrium. Simple theories of vibrational relaxation do not account for the results satisfactorily. This applies particularly to the temperature coefficients of the relaxation times of CO2 relaxed by H2 and D2. In general it seems that above 1000 K the relaxation time may be controlled largely by the reduced mass of the collision partners while at lower temperatures this may be true for the inert gases, but not for other molecules.
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Vibrational relaxation in mixtures of CO2 with 10% He-3, 10% o-D2 or 3% HD was measured in the temperature range 1000-360 K using a shock tube and a laser schlieren system, and results compared with data on other hydrogen isotopes and isomers. There is clear evidence of vibration-rotation exchange between CO2 and n-H2, HD, and D2. An important role of long-range interactions in the coupling reaction is not indicated, since the relaxation of CO2 by p-H2, in which a rotational transition near resonant with the v2 vibration of CO2 is not possible, was found to be temperature-dependent. The high efficiency of HD in relaxing CO2 suggests that the rate of rotational relaxation is a limiting process for H2 and D2 but not for HD. Calculations based on these measurements indicate that the first-order Sharma (1969) model underestimates the coupling rate by a factor of 2.
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The optic-acoustic method has been used to study, at 300 K, the vibrational collisional transfers in a CO2CO mixture, after CO2(00°1) or CO (1) has been excited. Parameters, not obtained through optical or thermodynamic techniques (such as flourescence, shockwaves, etc.), are thus determined.
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In order to study the thermal vibrational relaxation of CO2, the gas is given a perturbation such that the bending mode, b, and the symmetric-stretching mode, s, remain in equilibrium with each other while the asymmetric stretching mode, a, undergoes a different evolution. Two relaxation times, τVT and τVV, occur in the collisional exchanges of energy: respectively, that of vibration-translation (V-T) between the set bs and the degrees of translation-rotation; and that of vibration-vibration (V-V) between a and bs. A third parameter, G, characterizes the ratio of energies, gained by bs and lost by a, during the V-V exchange. τVT can be obtained from thermodynamic experiments and τVV by a fluorescence technique. The spectrophone method used here allows the simultaneous determination of τVT, τVV and G; one can therefore determine the quantum levels concerned in the exchanges and compute the corresponding transition probabilities.
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Vibrational relaxation times in gases are calculated with the method of Zener using an exponential repulsion in a one‐dimensional model. The constants of the interaction potential are determined by fitting it to the data of Hirschfelder, et al. The great effect which some impurities have is accounted for either by their low mass and resultant high velocity or by ``near resonance'' transfers in which the vibrational quantum of the substratum is used partly to excite the vibration of the impurity, only the difference being transferred to translation. However, there are other impurities, the action of which cannot be explained in this manner. The theoretical values for the relaxation times are 10 to 30 times shorter than the experimental ones, which difference may be accounted for by the use of the one‐dimensional model. Macroscopic equations governing the more complex relaxation processes in polyatomic gases and gas mixtures are developed.
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Vibrational relaxation has been studied in CO2 and CO2☒Ar mixtures over the temperature range 360°–3000°K using a laser–schlieren method. The purpose of this investigation was to compare the results obtained by this technique with those deduced using a Mach–Zehnder interferometer and to provide further evidence as to whether relaxation times depend upon how far the system is from equilibrium as well as on the translational temperature. Relaxation times measured by these two methods agree well. The laser–schlieren method is the more satisfactory for measuring the rate of relaxation and the Mach–Zehnder for finding the total density change during the process. These new results support the view that relaxation times do not depend upon how far the system is from equilibrium. The measurements using CO2 and CO2☒Ar mixtures show that CO2 is about five times as effective as Ar in causing relaxation at 360°K and two times as effective at 2000°K.
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Theoretical and experimental evidence is presented which leads to a vibrational relaxation time appropriate for the v2 vibration of CO2 at 15 μ under atmospheric conditions at 210°K and standard pressure of 6·0 × 10−6sec. The effect of this on the atmospheric cooling rate near 90 km due to emission by CO2 is discussed. It is shown that absorption of solar radiation by the v3 band (at 4·3 μ) and the combination bands of CO2 (at 2·7 μ) leads to a heating rate of about 2°C (12 hr)−1 near 80 km, this being one of the largest contributions to the radiative heating rate at this altitude. The processes by which relaxation from the v3 vibration of CO2 occurs involve vibrationally excited oxygen and the v2 vibration of H2O. The magnitude of heating-rate depends considerably, therefore, on the H2O concentration. For thermal radiative exchange by the v3 band, thermodynamic equilibrium begins to break down at 30 km; its contribution to the radiative budget of the mesosphere is consequently very small.
X to5 s-r ~trn-~, Cannemeyer and de Vries (0.89 +O.IO) X IO5 s-l aim-r, and Tame [7] ako using spectrophone (9.20 kO.65) X IO4 s-l atm-* . (This last value was obtained from the kbs quoted by the author
  • Merrillandammegiving
MerrillandAmmegiving(l.lOtO.t3)X to5 s-r ~trn-~, Cannemeyer and de Vries (0.89 +O.IO) X IO5 s-l aim-r, and Tame [7] ako using spectrophone (9.20 kO.65) X IO4 s-l atm-*. (This last value was obtained from the kbs quoted by the author.) An impact tube me~urement by Chevalier and Huetz-Aubert
Ia4 s-1 atm-* _ Although the difference in efficienciesof 02 and N2 is small it seems that 02 does relax CO&-,) faster than does N2_ One cannot expect crude theo-[Ill LT
  • C J S M Simpson
  • T R D Chandler
  • . D C Allen
  • T J Price
  • C J S M Simpson
gives a rate constant of (5.15 t 0.45) X Ia4 s-1 atm-* _ Although the difference in efficienciesof 02 and N2 is small it seems that 02 does relax CO&-,) faster than does N2_ One cannot expect crude theo-[Ill LT. Houghton, Quart. J. Roy. Met. Sot. 95 (1969) 1. C.J.S.M. Simpson and T.R.D. Chandler, Proc. Roy. Sot. A317 (1970) 265. D.C. Allen, T.J. Price and C.J.S.M. Simpson, Chem. Phys. Letters 45 (1977) 183. C.J.S.M. Simpson, T.R.D. Chandler and AC. Strawson, J. Chem. Phys. 51 (1969) 2214. P. Chevalier and M. Huetz-Hubert, Advan. Mol. Relaxa-tion Processes 2 (1970) 101. J. Taint, Chcm. Phys. Letters 41 (1976) 297. J. Taine, Thesis, l'l'niversitt Pierre et Marie Curie, Paris VI (1976).