N2 Vibrational Temperature Research Articles

The three temperature model for the fast-axial-flow CO2 laser

It is pointed out that the vibrational temperatures of the symmetric stretch and bending modes of CO2 being almost equal, are close to the translational temperature (T) of the fast-axial-flow continuous wave CO2 laser plasma. This allows one to base the analysis of the processes in such plasmas on the three temperature approximation (translational temperature, vibrational temperature of the asymmetric stretch mode of CO2 (T3) and vibrational temperature of N2 (T4)) leading to a substantial reduction of computer time when compared with the conventional five temperature approximation. Approximations of the three temperature model are supported by the results of our computations and by the available experimental data. When applied to the industrial fast-axial-flow laser, our model predicts a gradual increase in T, T3 and T4 along the laser axis, with T reaching about 360 K, T3 and T4 reaching about 2500 K, which agrees with experimental observations.

Rate constants for the reaction of Ar+(2P3/2) with N2 as a function of N2 vibrational temperature and energy level

Rate constants have been measured for the reaction of Ar+(2P3/2) with N2 as a function of N2 vibrational temperature for a rotational and translational temperature of 300 K. The rate constants increase from a value of 1.2×10−11 cm3 s−1 at a N2 vibrational temperature of 300 K to 2.0×10−10 cm3 s−1 at a N2 vibrational temperature of 4700 K. The data are used to derive rate constants for specific vibrational levels. The rates constants for v=0–3 are 1.2×10−11, 3.0×10−10, 7.6×10−10, and 7.6×10−10 cm3 s−1, respectively. Additionally, the data suggest that the rate constant for v≳3 drops from the v=3 value, although this is less certain. The increase in the rate constants with increasing v is postulated to arise from the N2(v)→N2+(v+1) channel becoming less endothermic with increasing v and therefore closer to being resonant. Vibrational energy is found to be more effective at promoting this reaction than are translational or rotational energy.

Diagnostics of a CO2 radiofrequency waveguide laser active medium

The CO2 active medium of an 86 MHz radiofrequency flowing waveguide laser has been investigated. Optical diagnostics in the visible-infrared spectral region, mass spectrometry and the Langmuir probe technique have been employed to study He-N2-CO2 discharge at various pressures, compositions and discharge powers. The vibrational distributions of CO(X) and N2(B and C) electronic states have been measured. Evidence for a moderate vibrational disequilibrium has been found. The N2(X, v=1) vibrational and gas temperatures have been deduced. CO2 dissociation yields have also been measured. A very simple technique consisting of trapping CO2 from the discharge output stream in a liquid nitrogen cooled sample has been applied. Dissociation yield varies from 15% to 90% depending on the flow rate, mixture composition, pressure (35-80 Torr) and power. The electron energy distribution function measured by Langmuir probes at low pressure in a large parallel plate reactor shows that the addition of 6% CO2 to a He-20% N2 mixture enhances both the tail and the bulk of the EEDF.

Determination of N2(B3πg) and N2(C3πu) vibrational temperatures in e-beam pumped Ar-N2 and He-Ar-N2 mixtures

We report the results of a study to determine the vibrational temperatures of N2(B3πg) and N2(C3πu) in e-beam pumped N2 rare-gas mixtures. They are analyzed in terms of the kinetics of the e-beam pumped gas mixtures, and a number of qualitative conclusions are reached.

Radar measurements of high‐latitude ion composition between 140 and 300 km altitude

The Chatanika radar has been used to measure the ratio of atomic (O+) ions to molecular (O2+, NO+) ions in the high‐latitude ionosphere. The radar results agreed well with simultaneous in situ rocket data, giving confidence in the radar method of deducing ion composition. Measurements made over long periods of time show seasonal variations, diurnal variations, and variations due to auroral processes. The transition altitude, where the number densities of atomic and molecular ions are equal, is a convenient parameter for describing the composition variation with altitude or ‘composition altitude profile.’ The transition altitude occurs at ∼190 km at night and ∼170 km during the day, in agreement with midlatitude results. During the winter the daytime transition altitude is 15 km lower than in summer, a seasonal variation similar to that at midlatitudes. Energetic particle precipitation results in the lowering of the transition altitude, by 10 km in one case when energetic particles deposited ∼20 ergs/cm² s in the atmosphere. The largest variations in ion composition were found during periods of large joule heat input resulting from electric fields on the order of 50 mV/m. The transition altitude increased by 50 km in a case where the joule heat input rate was 30 ergs/cm² s. These observations were compared to calculations from a simple steady state model involving the principal consituents and reactions. The results indicate that the transition altitude during particle precipitation is most influenced by the increased ion production. There do not appear to be significant effects from possible increases of N2 vibrational temperature. A number of interrelated effects contribute to the increase in transition altitude during joule heating. The most important effect is the electric field contribution in raising the effective ion temperature. In addition, it appears that increased N2 density is also required to account for the observed change.

Causes of the F region winter anomaly

Measurements made by the Atmosphere Explorer satellites are used to determine the causes of the F‐region winter anomaly, i.e., seasonal variations which result in peak electron densities which are larger in winter than in summer. It is found that for the data examined there are 3 causes of the winter anomaly. 1) Seasonal changes in neutral composition; 2) an increase in the vibrational temperature of N2 and hence in the rate coefficient for the reaction O+ (4S) + N2 → NO+ + N; 3) an increase in winter in the production of O+ (4S) due to enhanced electron quenching of O+ (²D). The presence of vibrational excitation is inferred from an increase in the rate coefficient for the reaction from winter to summer at heights above 260 km. We quantitatively demonstrate that the initial increase in electron density produced in winter by causes (1) and (2) results in an increase in the quenching of O+ (²D) thereby enhancing the source of O+ (4S) ions. We estimate that causes (1) to (3) account for ∼55%, ∼20% and ∼25% respectively of the seasonal change in O+ (4S) from summer to winter at solar minimum.

Experimental verification of the vibration–vibration exchange quasi-steady state in N2+ CO mixtures

Using a discharge–flow system at 3 Torr pressure, small quantities of CO were added to vibrationally excited N2. At a residence time of 13 ms, the vibrational temperatures of N2 and CO were measured using an electron beam probe and observation of infrared radiation, respectively. The CO partial pressure was varied from 1 to 100 mTorr; over the entire range a non equilibrium quasi-steady state relationship was observed, in good quantitative agreement with theoretical prediction.

The SAR arc event observed during the December 1971 magnetic storm

Observations from five separate experiments on the Isis 2 spacecraft are used to study the atmosphere/ionosphere during the magnetic storm of December 16-20, 1971. The data are most complete in the midlatitude region, permitting a study of the SAR arc (subauroral red arc) which developed during the night of December 17-18. Ion composition and temperature, electron temperature, electron-density height profiles from the spacecraft to the F region, and the intensity of the 6300-A oxygen emission are all presented for the region of interest. It is found that the H(+) concentration had sharp gradients near the SAR arc and that the plasma temperature was significantly enhanced over typical nighttime values, reaching nearly 7000 K at 1400 km on the field line which intersected the arc. A system of time-dependent equations for atmospheric/ionospheric composition and temperature is solved using boundary conditions which were selected so that the solutions are in agreement with the observations. From these solutions, an assessment is made of the influence of (1) the efflux of plasma from the ionosphere to the magnetosphere, (2) the decrease in O/N2 at the turbopause, (3) the increase in the loss coefficient as a result of an increase in the vibrational temperature of N2, and (4) the conduction of thermal energy into the ionosphere.

Effect of electric fields on the daytime high-latitudeEandFregions

We have obtained solutions of the coupled continuity, momentum, and energy equations for NO+, O2+, and O+ ions for conditions appropriate to the daytime high-latitude E and F regions. Owing to the rapid increase of the reaction O+ + N2 → NO+ + N with ion energy, high-latitude electric fields and consequent E⊥ × B drifts deplete O+ in favor of NO+. For electric field strengths less than about 10 mV m−1 the depletion of O+ is small, and the altitude profiles of ion density are similar to those found at mid-latitudes. However, for moderate electric field strengths (∼50 mV m−1), NO+ is substantially increased in relation to O+ and becomes an important ion throughout the F region. For these conditions the electron density has a tendency to become nearly constant with altitude in the range 160–360 km. For large electric fields (∼200 mV m−1), NO+ completely dominates the ion composition to at least 600 km, decreasing at high altitudes with a diffusive equilibrium scale height. Since the overall F region electron density decreases markedly with increasing electric field strength, it appears that high-latitude, daytime electron density troughs are directly related to the presence of ionospheric electric fields. In addition, since increases in the N2 density or the N2 vibrational temperature also affect ion composition and electron densities in a manner similar to that of electric fields, the observed daytime troughs may arise from both processes acting simultaneously.

Vibrational temperature and molecular density of thermospheric nitrogen measured by rocket-borne electron beam induced luminescence

The vibrational temperature and molecular density of thermospheric nitrogen were probed in situ above Fort Churchill, Manitoba, Canada, on March 13, 1970. The rocket-borne system utilized electron beam induced luminescence of the atmosphere as a diagnostic technique in effecting the measurements. The vibrational populations of N2+ ions in the B²Σu+ state were inferred from the intensities of selected transitions of the N2+ first negative (1N) band system (B²Σu+−X²Σg+) induced by a 2.5-keV electron impact. Four narrow band photometers were used to measure the radiant intensity of the (0–1), (0–2), (1–2), and (2–4) N2+ 1N transitions. In the technique employed the vibrational population of N2(X¹Σg+) is derived from the N2+(B²Σu+, υ′ = 0 and 1) populations if it is assumed that the relative cross sections for production of the excited N2+ ions by simultaneous ionization and excitation of the neutral molecule are proportional to the appropriate Franck-Condon factors. The inferred population of N2+(B²Σu+, υ′ = 2) is not consistent with this simple model, appearing anomalously large at low vibrational temperatures. Within the limits of uncertainty imposed by the nature of the experimental technique and by the precision realized in the measurements the inferred vibrational population ratio, [N2(υ = 1)]/[N2(υ = 0)], was not recognizably greater than that based on model atmospheric kinetic temperatures appropriate for the time of the flight (950°K exospheric temperature). An upper limit for the N2 vibrational temperature is given in the altitude range 80–175 km (1500°K at 175 km, 1200°K at 155 km, 1000°K at 135 km, and 800°K for altitudes less than 115 km). Measured N2 molecular densities are in substantial agreement (<±10% disparity) with the Jacchia (1971) model atmosphere in the 145- to 175-km altitude range. At lower altitudes the measured concentrations are significantly less than the model values. For example, the measured molecular density at 120 km [2.3 × 1011 cm−3] is approximately 61% of the model value. Further departure of the results from the model at altitudes less than 120 km is attributed in part to the effects of supersonic flow about the vehicle.

Infrared processes in the auroral zone

Recently auroral observations have indicated the strong enhancement of certain infrared bands during periods of auroral activity. A theoretical model has been created in order to study the infrared processes associated with the aurora. The quantitative results of this study depend upon a thorough knowledge of the rates of all processes involved. Some of the rate constants are well known, whereas for others we have made only the crudest approximation. Qualitatively, however, our results indicate that certain infrared bands of CO2, N2O, NO, and NO+ are strongly enhanced during periods of auroral activity. This enhancement is primarily the result of two basic energy exchange mechanisms. The energy deposited by the primary auroral electron flux goes into increasing the internal energy of N2, O2, and O. That is, the vibrational temperature of N2 is significantly increased in an aurorally disturbed atmosphere. This energy is then transferred to various infrared active minor constituents such as CO2, N2O, NO, etc. through near-resonant vibrational-vibrational exchange processes, increasing the higher-level Vibrational populations of these species and thereby enhancing their infrared radiative output. In addition to energetic auroral particles as an energy source we have also considered electric fields. Qualitatively the results are similar except that the energy tends to be deposited at altitudes somewhat higher than those in direct precipitation processes.

Temperature dependence of the quenching of vibrationally excited nitrogen by atomic oxygen

A photo-ionization detector for vibrationally excited nitrogen (N2*) has been used to obtain the rate of quenching of N2* by atomic oxygen in the temperature range 300°–723°K. The quenching rate coefficient was found to be 3.2 × 10−15 cm³ s−1 at 300°K, 1.3 × 10−14 cm³ s−1 at 463°K, 2.7 × 10−14 cm³ s−1 at 633°K, and 4.0 × 10−14 cm³ s−1 at 723°K. These rate coefficients are extremely large compared with the expected values for a conventional vibrational-translational (VT) energy transfer process in this temperature range. They are also large compared with the predictions of the vibrational relaxation theory developed by Fisher and Bauer for the N2-O system based upon crossing of covalent potential energy curves. The measured coefficients combined with higher temperature data obtained elsewhere with different experimental methods indicate that the VT process in the N2-O system is anomalously efficient over the range 300°–4500°K and has only a moderate temperature dependence. This result has important consequences in the upper atmosphere, where the effect of an efficient VT process involving N2 and O is to hold the atmospheric N2 vibrational temperature at or very near the ambient kinetic temperature, in agreement with the results of recent rocket probe experiments.

Vibrational temperature of N2in theEandFregions

The vibrational energy content of atmospheric N2 has been determined through numerical integration of the time-dependent species continuity equation and the species equation of motion. Generalized diffusion through the nonuniform thermosphere was included by using the method of Shimazaki. Sources of vibrational excitation included the reaction of N with NO as well as O(1D) quenching by N2 and thermal electron collisions. Vibration-translation energy transfer from the vibrationally excited N2 to O atoms, relaxation of N2 by electrons, and vibration-vibration exchange to CO2 were included as loss mechanisms. The results, which predict a vibrational temperature of about 300°K for noon at 100 km and less than 1300°K for noon at 300 km, suggest that the actual N2 vibrational temperature is inadequate to explain E and F region positive ion data for a quiet atmosphere but is in excellent agreement with the recent rocket probe measurements of O'Neil.

Measurement of vibrational temperature of CO and N2 using the He(23S) Penning ionization technique

The vibrational temperatures of N2 and CO in binary gas mixture and in the VV quasiequilibrium regime of high vibrational temperature and low kinetic temperature were measured in a flow tube apparatus. The Penning ionization with He (23S) technique was used to determine the vibrational temperatures of both gases. The Treanor-Teare expression relating vibrational temperatures for such conditions was verified. The formulation of the Penning ionization technique for CO allowed the CO rotational energy and the kinetic energy of relative motion between CO and He to play a role in the normally nonenergetically allowed CO(X, 0)→CO+(B, 1) excitation reaction. The unexpected result of a collisional quenching rate that is smaller for CO+(B, 1) than for CO+(B, 0) was also observed.

Infrared Emission at 2.7 μm from Flowing CO2—N2* Mixtures

A flow tube has been constructed wherein ground-state CO2 and vibrationally excited N2 molecular streams interact and the combined flow is spectrally investigated at selected downstream stations. The close coupling between the lower vibrational levels of N2(X 1Σg+) and the CO2 asymmetric stretch mode (00° v3) allows rapid energy exchange to these levels, followed by internal redistribution among the vibrational states of the various CO2 modes. When emission at the 2.7−μm bands is monitored, it has been observed that such redistribution produces substantial population of the mixed-mode levels associated with this radiation. Comparison of the observed band profiles with analytically derived counterparts has indicated that for an N2 vibrational temperature of 1900°K, CO2 difference bands at 2.7 μm contribute approximately 2.5 times as much radiant energy as is emitted in the 10°1–00°0 and 02°1–00°0 summation bands.

Sodium-Atom Excitation in Nitrogen Afterglows

The addition of Na metal vapor to nitrogen discharge afterglows in a fast flow system produced strong Na atomic excitation and ionization. Spectroscopic techniques were used to determine the relative populations of several Na atomic levels for Na excitation in the nitrogen pink and Lewis–Rayleigh afterglows. Three sets of Na f values were used. For nitrogen afterglow pressures of 2–10 torr, the populations of the Na 42P and several higher atomic levels appear related by a Maxwell–Boltzmann distribution. Measured Na excitation temperatures were 4500 ± 300°K in the N2 pink and 4000 ± 200°K in the Lewis–Rayleigh afterglows. A double-probe measurement of the electron temperature in the pink afterglow yielded a value of 5000°K. Comparison with earlier work indicates that the Na excitation temperature corresponds to the N2(X1Σ) vibrational temperature. These results support the hypothesis that the Na atomic excitation and ionization are produced by energy transfer between the N2(X1Σ) vibrational states and the Na-atom electronic states.

Afterglow Studies of the Reactions He+, He(23S), and O+ with Vibrationally Excited N2

The following reactions have been measured as a function of N2 vibrational temperature from 300°K to 6000°K: He++N2→N++N+He →N2+ + He, He(23S) + N2→N2 + + He + e, O+ + N2→NO+ + N. Reactions (1) and (3) are the first ion–molecule reactions to be measured as a function of vibrational temperature of the neutral. The rate constants for Reactions (1) and (2) are found to be insensitive to the N2 vibrational temperature. Reaction (3) increases its rate constant by about a factor of 40 with increased N2 vibrational temperature. The branching ratio (1a) to (1b) increases measurably with N2 vibrational temperature.

Spectrum-Line Reversal Measurements of Free-Electron and Coupled N2 Vibrational Temperatures in Expansion Flows

Spectrum-line reversal measurements of the excitation temperature of Na atoms in expansion flows of shock-heated Ar and 1% N2+Ar mixtures are described. The measurements were made in a conical nozzle attached to the end of a conventional shock tube. For expansion flows of pure Ar from reservoir temperatures of 3200° to 4200°K and a reservoir pressure of about 37 atm, the measured Na temperatures (≈2200°K) were considerably in excess of the local-translational temperature (≈400°K). These high excitation temperatures are interpreted in terms of the excitation of Na by free electrons produced from ionization of the Na in the reservoir. On this basis, free-electron temperatures are deduced which indicate freezing of the electron thermal energy at values corresponding to those expected at the nozzle throat. This result suggests a slow rate of transfer of electron thermal energy to the Ar translational mode. The addition of 1% N2 to the Ar expansions produced large reductions in the measured Na temperatures. The reduced temperatures are shown to correspond to the frozen N2 vibrational temperatures expected on the basis of previous results. These reductions are explained in terms of an efficient transfer of the excess-electron thermal energy to the N2 vibrational mode, the free-electron and N2 vibrational temperatures being thereby coupled and equilibrated during the expansion. The present results also substantiate the faster vibrational relaxation rates inferred previously from similar expansion-flow studies of pure N2; they further suggest that the simple Landau—Teller rate equation, when used with relaxation times measured in shock-wave flows, is not adequate to describe the vibrational relaxation process under the extreme nonequilibrium conditions associated with nozzle-expansion flows.

Temperature and Wavelength Dependence of the Thermal Emission and O–NO Recombination Spectra of NO2

The thermal emission spectrum of shock-heated NO2 has been measured from 4000 to 9500 Å at 2000°K. The bulk of the emission lies in the near infrared rather than in the visible. NO2 is at least 15 times as efficient as Ar in quenching NO2* due to the step NO2*+NO2→NO+NO3, k≈(9×108) T½ liter mole−1·sec−1. N2 as diluent behaves like a monatomic gas (Ar) at a translational temperature corresponding to rotational but not vibrational relaxation behind the front; the excitation is uncoupled to the rotational and vibrational temperature of N2. The radiative lifetime of NO2 is approximately equal to 10−7 sec. In systems containing O, NO, and NO2 the total emission can be resolved into contributions from thermal and recombination emissions; combining literature values the ``rate'' for O+NO→NO2+hv can be written (1.1×1011) T−1.59 exp (670/RT) cc mole−1·sec−1. Thermal and recombination contributions to the equilibrium emissivity of hot air are estimated.

Line-Reversal Studies of the Sodium Excitation Process Behind Shock Waves in N2

Spectrum-line reversal measurements of the time variation of Na excitation temperatures behind shock waves in N2 over the range 1720°—4340°K are described. From a comparison of the results with recent interferometric and infrared measurements of N2 vibrational relaxation, made also behind shock waves, it is shown that the observed Na excitation temperature is identified with the vibrational temperature of the relaxing nitrogen molecules. The present measurements thus confirm that the thermal excitation of Na by energy transfer from the N2 vibrational mode occurs with a much higher probability than by transfer from the rotational or translational modes. This result removes recent doubts concerning the interpretation of Na-line reversal temperatures in terms of N2 vibrational temperatures in earlier shock-tube studies. The probability of Na excitation by vibrationally excited N2 at the temperatures concerned is shown to be close to unity. The Na–N2 interaction process in various environments is briefly discussed in the light of this and other quenching measurements at low temperatures. The measurements were made with a double-beam spectrum-line reversal system in which the sodium emission and absorption intensities are both recorded in the same optical path. The high-temperature measurements were made using a calibrated xenon arc as the background source for reversal. For the low-temperature measurements a tungsten strip lamp was employed. The system is able to measure Na-line reversal temperatures from 1500° to 5500°K to within an accuracy of ±2% and with a time resolution of 1 μsec. The optical arrangement and xenon source characteristics are described in detail.