Negative Bands Of N2 Research Articles

One dimensional temperature measurements by resonantly ionized photoemission thermometry of molecular nitrogen

This paper presents an extensive parameter study of a non-intrusive and non-seeded laser diagnostic method for measuring one dimensional (1D) rotational temperature of molecular nitrogen (N2) at 165 - 450 K. Compared to previous efforts using molecular oxygen, here resonantly ionized and photoelectron induced fluorescence of molecular nitrogen for thermometry (N2 RIPT) was demonstrated. The RIPT signal is generated by directly probing various rotational levels within the rovibrational absorption band of N2, corresponding to the 3-photon transition of N2 (X1Σ g +,v=0→b1Π u ,v′=6) near 285 nm, without involving collisional effects of molecular oxygen and nitrogen. The photoionized N2 produces strong first negative band of N2+ (B2Σ u +−X2Σ g +) near 390 nm, 420 nm, and 425 nm. Boltzmann analyses of various discrete fluorescence emission lines yield rotational temperatures of molecular nitrogen. By empirically choosing multiple rotational levels within the absorption band, non-scanning thermometry can be accurately achieved for molecular nitrogen. It is demonstrated that the N2 RIPT technique can measure 1D temperature profile up to ∼5 cm in length within a pure N2 environment. Multiple wavelengths are thoroughly analyzed and listed that are accurate for RIPT for various temperature ranges.

O2 based resonantly ionized photoemission thermometry analysis of supersonic flows.

Characterization of the thermal gradients within supersonic and hypersonic flows is essential for understanding transition, turbulence, and aerodynamic heating. Developments in novel, impactful non-intrusive techniques are key for enabling flow characterizations of sufficient detail that provide experimental validation datasets for computational simulations. In this work, Resonantly Ionized Photoemission Thermometry (RIPT) signals are directly imaged using an ICCD camera to realize the techniques 1D measurement capability for the first time. The direct imaging scheme presented for oxygen-based RIPT (O2 RIPT) uses the previously established calibration data to direct excite various resonant rotational peaks within the S-branch of the C3Π, (v = 2) ← X3Σ(v' = 0) absorption band of O2. The efficient ionization of O2 liberates electrons that induce electron avalanche ionization of local N2 molecules generating N2 +, which primarily deexcites via photoemissions of the first negative band of N2+(B 2 Σ u+-X 2 Σ g+). When sufficient lasing energy is used, the ionization region and subsequent photoemission signal is achieved along a 1D line thus, if directly imaged can allow for gas temperature assignments along said line; demonstrated here of up to five centimeters in length. The temperature gradients present within the ensuing shock train of a supersonic under expanded free jet serves as a basis of characterization for this new RIPT imaging scheme. The O2 RIPT results are extensively compared and validated against well-known and established techniques (i.e., CARS and CFD). The direct imaging capability fully realizes the technique's fundamental potential and is expected to be the standard of implementation going forward. The direct imaging capability can play instrumental roles in future scientific studies that rely upon acute characterization of thermal gradients within a medium that cannot be easily resolved by a point. Furthermore, the removal of the spectrometer greatly reduces the cost, complexity, and optical alignment associated with prior RIPT measurements.

One-dimensional air temperature measurements by air resonance enhanced multiphoton Ionization thermometry (ART).

In this work, a detailed calibration study is performed to establish non-intrusive one-dimensional (1D) rovibrational temperature measurements in unseeded air, based on air resonance enhanced multiphoton ionization thermometry (ART). ART is generated by REMPI (resonance enhanced multi-photon ionization) of molecular oxygen and subsequent avalanche ionization of molecular nitrogen in a single laser pulse. ART signal, the fluorescence from the first negative band of molecular nitrogen, is directly proportional to the 2-photon transition of molecular oxygen C3Π (v = 2) ← X3Σ (v'=0), which is used to determine temperature. Experimentally, hyperfine structures of the O2 rotational branches with high temperature sensitivity are selectively excited through a frequency-doubled dye laser. Electron-avalanche ionization of N2 results in the fluorescence emissions from the first negative bands of N2 + near 390, 425, and 430nm, which are captured as a 1D line by a gated intensified camera. Post processing of the N2 + fluorescence yields a 1D thermometry line that is representative of the air temperature. It is demonstrated that the technique provides ART fluorescence of ∼5cm in length in the unseeded air, presenting an attractive thermometry solution for high-speed wind tunnels and other ground test facilities.

Nitrogen Laser Emissions of Short and Long Durations Generated in Air

This article describes two sets of experiments. An eight-stage, pulse-forming network (PFN) Marx generator of 50- $\Omega $ internal impedance produces a “square”-shaped voltage waveform and induces multiphoton ionization, excitation of molecules and ions to generate laser pulses of long ( $ ) and short (~10 ns) durations in air, when the principal laser line of the N2 laser at 337.1 nm was observed. The visible blue laser lines coming from the first negative band of N2+ions due to the $\text{B}^{2}\sum ^{+}_{u}\to \text{X}^{2}\sum ^{+}_{g}$ transition at 391.4 and 427.8 nm were studied. The wideband (300–800 nm) Schott BG 39 interference filter was also employed to examine whether some other lines participate in the emission being generated. The scaling law was derived describing the attenuation of the emission versus the distance for long-duration pulses ( $ ) at 337.1 nm. The second set of experiments was introduced to support the data obtained. It offers information pertinent to the visible blue line emissions as well as to the photoionization processes, lighting phenomena, and processes occurring at high altitudes.

Influence of square wave duty ratio on pattern formation of atmospheric pressure glow discharge

Excited by a square wave voltage, atmospheric pressure glow discharge is generated in a needle to liquid configuration. With decreasing duty ratio of the square voltage, bright disk, bright disk with outer ring, triple ring, triple ring with concentric spot are observed on the water surface. Voltage and current waveforms indicate that the discharge occurs in the negative polarity. The discharge current consists of two parts, the pulse region and the constant region, which has a good periodicity. It is found that the inception voltage and the peak current increase with decreasing the duty ratio. Optical emission spectra are detected. It contains the second positive band of N2, the first positive band of N2, the first negative band of N2+ and the OH (309 nm). Using the Lifbase, emission spectra of the OH are used to estimate rotational temperature by fitting the experimental spectra to the simulated spectra. It is found that the gas temperature increases with increasing the duty ratio. By synchronously triggering discharge current and ICCD, temporally resolved images of the patterns are captured on the water surface. Results show that self-organized pattern gradually evolves in the pulse region, and it does not change any more in the constant region. Taking into account the space negative charge, above phenomena are analyzed and explained by using the basic theory of discharge.

A diffuse plasma generated by bipolar nanosecond pulsed dielectric barrier discharge in nitrogen

In this study, a bipolar high-voltage pulse with 20 ns rising time is employed to generate diffuse dielectric barrier discharge plasma using wire-plate electrode configuration in nitrogen at atmospheric pressure. The gas temperature of the plasma is determined by comparing the experimental and the best fitted optical emission spectra of the second positive bands of N2(C3Πu → B3 Πg, 0-2) and the first negative bands of N2 + (B2 Σu + → X2 Σg +, 0-0). The effects of the concentration of argon and oxygen on the emission intensities of N2 (C3Πu → B3Πg, 0-0, 337.1 nm), OH (A 2Σ → X2Π, 0-0) and N2 + (B2 Σu + → X2 Σg +, 0-0, 391.4 nm) are investigated. It is shown that the plasma gas temperature keeps almost constant with the pulse repetition rate and pulse peak voltage increasing. The emission intensities of N2 (C3Πu → B3Πg, 0-0, 337.1 nm), OH(A2Σ → X2Π, 0-0) and N2 + (B2 Σu + → X2 Σg +, 0-0, 391.4 nm) rise with increasing the concentration of argon, but decrease with increasing the concentration of oxygen, and the influences of oxygen concentration on the emission intensities of N2(C3Πu → B3Πg, 0-0, 337.1 nm) and OH (A2Σ → X2Π, 0-0) are more greater than that on the emission intensity of N2 + (B2 Σu + → X2 Σg +, 0-0, 391.4 nm).

Preparing For Hyperseed MAC: An Observing Campaign To Monitor The Entry Of The Genesis Sample Return Capsule

The return of the Genesis Sample Return Capsule (SRC) from the Earth’s L1 point on September 8, 2004, represents the first opportunity since the Apollo era to study the atmospheric entry of a meter-sized body at or above the Earth’s escape speed. Until now, reentry heating models are based on only one successful reentry with an instrumented vehicle at higher than escape speed, the 22 May 1965 NASA “FIRE 2” experiment. In preparation of an instrumented airborne and ground-based observing campaign, we examined the expected bolide radiation for the reentry of the Genesis SRC. We find that the expected emission spectrum consists mostly of blackbody emission from the SRC surface (T∼2630 K@peak heating), slightly skewed in shape because of a range of surface temperatures. At high enough spectral resolution, shock emission from nitrogen and oxygen atoms, as well as the first positive and first negative bands of N2 +, will stand out above this continuum. Carbon atom lines and the 389-nm CN band emission may also be detected, as well as the mid-IR 4.6-μm CO band. The ablation rate can be studied from the signature of trace sodium in the heat shield material, calibrated by the total amount of matter lost from the recovered shield. A pristine collection of the heat shield would also permit the sampling of products of ablation.

230 空気中の強い衝撃波背後における発光の時間分解画像分光計測

This paper describes the imaging spectroscopic technique applied to the experimental study of radiation emanating from strong shock waves in air. Shock velocity is about 11km/s and initial pressure is 13.3Pa in air. The spectral diagnostic system consists of an imaging spectrograph, a streak camera, a gated image-intensified CCD camera and a personal computer for data acquisition/processing. This spectral diagnostic system is capable of simultaneous wavelength-, intensity and time-resolved spectroscopic measurements in the nanosecond order. Wavelength range is chosen primarily to investigate the first negative band of N2+ and the second positive band of N2. Spectra with enhanced spectral resolution are obtained.

Energetic oxygen precipitation as a source of vibrationally excited N2+ in emissions observed at low latitudes

Observations have been made at Mt. Haleakala, Hawaii (dip lat.∼22°N) and Cachoeira Paulista, Brasil (dip lat.∼12°S) of emissions excited by particle precipitation during periods of magnetic activity. The first negative bands of N2+ were found to have a high degree of vibrational excitation at both sites, and with the absence of emissions attributable to hydrogen and helium, this finding leads to the interpretation that the excitation was due to a flux of precipitating oxygen atoms or ions, more plausibly the former, produced by charge exchange of ring current O+ ions with exospheric neutral constituents. More laboratory work is needed to properly interpret the data, but crude estimates of the associated energy deposition and ionization production fall in the range 10−1 to 10+1mWm−2, and 100‐10² cm−3s−1 respectively.

Daytime ion chemistry of N2+

Rocket measurements of the emission of the (0, 0) first negative band of N2+ at 3914 A in the day airglow have been made between 120 and 300 km. The solar zenith angle at the time of the flight was 60 deg, and the zenith intensity above 120 km was 1.6 kR averaged over both legs of the flight. The data are compared with a model in which charge exchange of N2 with metastable O+(2D) ions is included as an additional source of N2 ionization. Above 240 km this mechanism is dominant over the photoionization of N2 by solar extreme ultraviolet radiation. The derived N2+ density is consistent with the results of rocket-borne mass spectrometric observations.

Apparatus and Techniques for Electron Beam Fluorescence Probe Measurements

A laboratory apparatus and experimental techniques have been developed to investigate the properties of a 5–30 keV electron beam fluorescence probe system in air, nitrogen, and helium test gases over a pressure range from 10−3 to 10 Torr at 302 K. The design and performance characteristics of the electron beam generating system and optical detector system are described. The electron beam system used in this work was especially designed for wind tunnel remote measurements of local gas density in low density high velocity flow fields. A 1 mm diam collimated beam of 5–30 keV electrons was used to irradiate each of the test gases and excite fluorescence radiation, specifically the 391.44 nm (0,0) First negative band of N2+ in air and nitrogen, and the 501.57 nm (21S–31P) He line in helium. A precision optical detector system has been designed and used to observe the spatial distribution and pressure dependence of this fluorescence radiation excited by the electron beam. Calibration and test procedures have been implemented to reduce significantly experimental errors and provide relative fluorescence intensity measurements with a precision better than ±2%. Typical results are presented and discussed which illustrate data obtained using the developed apparatus and test procedures.

Mid-Latitude Atmospheric Emission of N2+ First Negative Bands

THE first negative bands of N2+ are excited in the atmosphere by energetic particle precipitation. The cross section data of Srivastava and Mirza1 imply an associated N2+ ion production rate of 14.1 ion pairs per photon in the 0–0 band or 41.5 ion pairs per photon in the 0–1 band, The total ion production rate for all species may be a few times greater and depends on the relative concentration of N2, a function of the height at which the ionizing radiation is stopped.

Electrostatic Probe Studies of the Nitrogen Pink Afterglow

Studies of the short-duration nitrogen pink afterglow (PA) are reported for different electrostatic probe types (spherical and cylindrical) and materials (Pt and Au). Various collision-dominated probe theories were applied to determine the electron density Ne and temperature Te. Experimental radial distribution curves are presented for the parameters kTe, Ie / Ii (electron to ion current ratio), and Ne at various points in the PA. An apparent increase in the ion current caused by emission of electrons from the probe surface was found to correlate with the light emission intensity of the first negative bands of N2+. kTe was found to be about 1 eV upstream of the PA in the dark space, reaching a maximum of nearly 4.5 eV before decreasing again to 1 eV downstream. The maxima in the axial profiles through the PA of Te, the light emission intensity and the ion current coincide, while Ne (in the range 1–5 × 109 cm−3) shows its maximum a few milliseconds later. The effect of impurities on the PA was examined. The role played by electrons in the process of PA formation is discussed.

Calculations of auroral intensities from electron impact

The energy deposited in the excited states of N2, O2, and O from the slowdown of electrons in the 0.1–30-kev range is calculated from the extensive sets of electron impact cross sections of Green et al. These results are then used to calculate the radiation from many of the important auroral transitions. Comparison with experiment is then obtained by normalizing to the first negative bands of N2+. Also, the average ev/ion pair in the slowdown process was calculated with consideration given to the details of the low-energy secondaries and tertiaries.

Rocket Measurements of the Visible Dayglow

Recent rocket measurement of the spectrum and altitude distribution of the visible dayglow are reviewed. The current theory of the daygolw is discussed in the light of these data and the results of new laboratory studies on atomic processes of aeronomic interest. The important role played by solar radiation in the excitation of the visible dayglow is illustrated in a detailed discussion of the emission of the (0, 0) negative band of N2+ and the red line (6300A) of atomic oxygen in the dayglow.

Elektronendichtebestimmungen mit einer Mikrowellenbrücke in schwach ionisierten Plasmen

A microwave bridge, similar to the optical JAMIN interferometer, has been developed in order to determine the electron density in weakly ionized decaying plasmas. The discharge vessel was located in the open space between two horn antennas. A rather homogeneous excitation was accomplished by using an electrodeless high frequency (f = 13,6 MHz) discharge. Contrary to the common practice of using phase displacements several times of π, only very small displacements of the signal phase, traversing the plasma, have been used. The dimensions of the discharge vessel and the frequency of the microwave (f = 23 GHz; λ = 1,3 cm) have been properly chosen in order to have proportionality between the mean electron density and the oscilloscope trace. The range of electron-density investigated was about 108 to 1010 cm-3. The operation of the bridge has been investigated during the afterglow of a decaying nitrogen plasma. Simultaneous optical observations have shown that the intensity of the first negative bands of N2 + (B2Σu +-Χ2Σg +) is nearly proportional to the electron density. Using this correlation, the electron density profile across the discharge vessel has been evaluated. It could be shown that the time dependance of the electron density in the afterglow changes strongly within the discharge vessel. A comparison between the measurements of the electron density and the optical observations has shown that the microwave bridge is able to give some details of the electron density profile across the discharge vessel, inspite of the large wavelength used, compared with the dimensions of the discharge vessel.

An observation of the (0, 0) negative band of N2+in the dayglow

On May 7, 1963, at 1611 EST an Aerobee Hi rocket was launched from Wallops Island, Virginia, carrying a filtered photometer that measured the intensity of the (O, O) negative band of N2+ in the dayglow from 100 to 223 km. The total zenith intensity at 3914 A was observed to decrease from 6.7 kR at 100 km to 5.4 kR at the peak altitude, indicating that the majority of excited N2+ ions was located above 223 km in the F2 region. Resonance scattering of sunlight by pre-existing N2+ ions appears to be the principal excitation mechanism for the first negative bands in the dayglow. Simultaneous ionization-excitation of nitrogen by solar EUV radiation apparently plays a minor role, and an upper limit is placed on its contribution to the intensity of the (0, 0) negative band. The experimental results are analyzed by assuming that photoionization is the principal source mechanism for N2+ in the daytime ionosphere and that dissociative recombination, charge transfer, and ion-atom interchange with the atmospheric gases are the chief loss processes. It is shown that dissociative recombination is a minor loss mechanism below 200 km and that N2+ ions are destroyed principally in reactions with O and O2 at these altitudes.

Fluorescence and Pre-Ionization in Nitrogen Excited by Vacuum Ultraviolet Radiation

Fluorescence in nitrogen following absorption of vacuum ultraviolet radiation has been found to begin at a wavelength of 661.3 Å and to extend to at least 580 Å, the shortest wavelength available in this experiment. The energy of the longest wavelength at which the fluorescence appears is almost exactly equal to the energy required to excite the N2 molecule from the ground state, X1Σg+, to the B2Σu+ state of the N2+ ion. It is thought that the first negative bands of N2+, B2Σu+→X2Σ g+, are responsible for the observed fluorescence. The fluorescence is excited by dispersed radiation from a stabilized helium gas discharge which produces the Hopfield continuum in the region 580–1100 Å. Fluorescence is also detected in carbon monoxide and in oxygen, but it is too weak for accurate measurements of the appearance wavelength. Experiments with a quartz cell window, in the presence of the electric field from the end window of the photomultiplier, enabled the pre-ionized bands of nitrogen to be observed.

Experimental Determination of the Oscillator Strength of the First Negative Bands of N2+

The radiative lifetime of the upper state of the 3914 A transition of N2+ was measured to be (6.58±0.35) 10—8 sec, corresponding to an oscillator strength f of (3.48±0.20) 10—2. The apparent radiative lifetime of the upper state of the 3371 A transition of the second positive system of N2 was found to depend upon the conditions of excitation. Extrapolation to excitation threshold yields a lifetime of (4.45±0.6) 10—8 sec.

Transition Probabilities in Band-Systems of Diatomic Molecules: A Modified `Distortion' Process for the Wave Functions

A method previously used for `distorting' the wave functions of a simple harmonic oscillator to fit the potential curve for a particular electronic state has been modified, giving wave functions for the vibrational states which are close approximations to those of Morse, with comparatively little labour The transition probabilities obtained from these functions are compared with experimental values for the first negative bands of N2+, the Swan bands of C2, the CN violet bands, and the α-system of BO.