Molecular Nitrogen Emissions Research Articles

Measurement of non-linearity in the cathodoluminescence yield for non-doped scintillators

We propose a method for studying nonlinearity in the cathodoluminescence (CL) characteristics of wide-bandgap materials based on the measurements of the CL energy dependence on the total energy of a pulsed electron beam Eb using its bremsstrahlung. It is shown that the x-ray radiant energy produced by a high-power (∼10MW/cm2) electron beam with particle energies of 50–300 keV is proportional to Eb in the case of weak variations in the electron energy distribution of the beam. This direct proportionality between Eb and the x-ray radiant energy is experimentally confirmed in the current experiment by measuring the dependence of the molecular nitrogen emission radiant energy at 337 nm excited by a direct electron impact (the 0-0 vibrational transition of the second positive system of the emission bands of N2 molecule) on the total electron beam energy Eb. Using this result, the dependencies of the CL radiant energy on Eb are studied for undoped Bi4Ge3O12, PbWO4, CeF3, and BaF2 crystals with bright intrinsic luminescence. An interpretation of these dependencies is given using a simple theoretical model and photoluminescence nonlinearity data published in the literature. We estimate the average concentration of the electronic excitations (EE) provided by the electron beam (1018–1019 e.–h.p./cm3) and obtain the approximate dependencies of the CL yield on the EE density for the studied materials. For the CeF3 crystal, different CL yield dependencies on the EE density are found for the bands at 300 and 350 nm.

Picosecond laser electronic excitation tagging velocimetry using a picosecond burst-mode laser.

Detailed characterizations of picosecond laser electronic excitation tagging (PLEET) in pure nitrogen (N2) and air with a 24 ps burst-mode laser system have been conducted. The burst-mode laser system is seeded with a 200 fs broadband seeding laser to achieve short pulse duration. As a non-intrusive molecular tagging velocimetry (MTV) technique, PLEET achieves "writing" via photo-dissociating nitrogen molecules and "tracking" by imaging the molecular nitrogen emissions. Key characteristics and performance of utilization of a 24 ps pulse-burst laser for MTV were obtained, including lifetime of the nitrogen emissions, power dependence, pressure dependence, and local flow heating by the laser pulses. Based on the experimental results and physical mechanisms of PLEET, 24 ps PLEET can produce similar 100 kHz molecular nitrogen emissions by photodissociation, while generating less flow disturbance by reducing laser joule heating than 100 ps PLEET.

Electric field dynamics in an atmospheric pressure helium plasma jet impinging on a substrate

Time and spatially resolved electric field measurements by Stark polarization spectroscopy in a nanosecond pulsed atmospheric pressure helium jet operating in ambient air and impinging on a indium tin oxide coated glass slide are reported. An automatic fitting procedure of the Stark shifted spectra taking into consideration constraints regarding Stark components' positions and intensities as well as molecular nitrogen emission subtraction was implemented. This allowed electric field vector component measurements both in the gas phase and at the interface when the jet impinges on the substrate and during the development of a surface ionization wave. The obtained results show an increase in the axial electric field in the jet effluent in the gas phase with a peak magnitude from 12 to 18 kV cm−1 before the ionization wave impinges on the substrate. A steep electric field enhancement to a peak value of about 24 kV cm−1 was observed when the ionization wave impinges on the surface. A peak radial electric field of about 27 kV cm−1 was measured off-axis in the surface ionization wave. These results are consistent with previously reported modelling predictions. While Stark polarization spectroscopy is limited to electric field measurements from regions with emission, we illustrate that the capability to measure near surface electric fields in helium makes it a valuable complementary technique for the electric field-induced second harmonic (EFISH) technique.

Nanosecond-pulsed discharge in liquid nitrogen: optical characterization and production of an energetic non-molecular form of nitrogen-rich material

Nanosecond-pulsed discharge in liquid nitrogen ignited using a needle electrode and positive 60 kV high voltage pulses was characterized using fast and shadow imaging, as well as optical emission spectroscopy. Estimation of temperature was done using molecular nitrogen emission of second positive system ro-vibrational transition spectra, and the maximum temperature increase is estimated to be ~60 K. We also report on the first observations of plasma-generated unstable material from liquid nitrogen, presumably a form of polynitrogen compound.

Investigation of the transition from streamer to uniform ‘overvoltage’ mode of atmospheric air nanosecond-pulsed dielectric barrier discharge

A nanosecond-pulsed dielectric barrier discharge ignited in atmospheric air was studied by optical emission spectroscopy to investigate the time- and space-resolved development of the reduced electric field. The discharge operated in two distinct modes that correlate with the values of the applied electric field: non-uniform streamer regime and uniform breakdown regime. It is shown that the reduced electric field in the non-uniform mode corresponds to the propagation of cathode-directed streamers, while in uniform discharge both molecular nitrogen emission and the reduced electric field are uniformly distributed along the gap during the initial discharge development stage with corresponding lower reduced electric field values.

Spatially resolved characterization of a dc magnetron plasma using optical emission spectroscopy

In this work, a reactive argon−nitrogen dc magnetron plasma for sputtering of a chromium (Cr) target is characterized with high spatial resolution by optical emission spectroscopy (OES) using molecular nitrogen emission bands at 0.5 Pa and 100 W. Beside the global gas temperature Tg, the electron temperature Te, electron density ne, and the steady-state Cr density nCr are also determined spatially resolved using a movable OES setup inside the chamber and Abel inversion. ne and Te are found to be consistent with the values measured by a Langmuir probe (LP) within the non-magnetized region along the magnetron axis in a pure Ar plasma for the same process parameters. Finally, a nitrogen content cN of 4% in the target surface is found for the reactive plasma by matching the mean steady-state chromium density measured by OES and calculated from TRIDYN simulations.

New observations of molecular nitrogen in the Martian upper atmosphere by IUVS on MAVEN

AbstractWe identify molecular nitrogen (N2) emissions in the Martian upper atmosphere using the Imaging Ultraviolet Spectrograph (IUVS) on NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We report the first observations of the N2 Lyman‐Birge‐Hopfield (LBH) bands at Mars and confirm the tentative identification of the N2 Vegard‐Kaplan (VK) bands. We retrieve N2 density profiles from the VK limb emissions and compare calculated limb radiances between 90 and 210 km against both observations and predictions from a Mars general circulation model (GCM). Contrary to earlier analyses using other satellite data, we find that N2 abundances exceed GCM results by about a factor of 2 at 130 km but are in agreement at 150 km. The analysis and interpretation are enabled by a linear regression method used to extract components of UV spectra from IUVS limb observations.

Modelling N21P contamination in auroral O+ emissions

Modelling of N21P (5,3) contamination at the O+ doublet emissions in the region of 732nm is presented. The method is derived from a known relationship between emission from the N21P (5,3) band and emissions from the N21P (5,2), (4,1) and (3,0) bands. A synthetic molecular spectrum is used to quantify a temperature-dependent emission ratio of these band systems as a function of filter characteristics and emission altitude. Five optical observations of high-energy auroral periods on 9 January 2008 are compared with results from the synthetic spectrum. Two cameras from a high sensitivity, high frame rate (20Hz) ground based imager in combination with a co-located high resolution spectrograph are used to identify events which are dominated by molecular nitrogen emissions. There is good agreement between the observed and modelled ratios. The temperatures associated with these ratios agree well with temperature profiles extracted from fitting the synthetic spectra to the spectrograph data. A synthetic spectrum is important for future work when the removal of N21P (5,3) contamination from O+ (2P) doublet emissions is required at high temporal resolution.

Optical emission spectroscopy study in the VUV–VIS regimes of a developing low-temperature plasma in nitrogen gas

The mechanisms leading to the development of an atmospheric low temperature plasma along a surface under pulsed conditions is of current interest. In the early plasma phase, high energy photons are a contributing factor to the process of generating electron avalanches resulting in surface flashover. Since only photons in the vacuum ultraviolet (VUV) regime are energetic enough to cause step-ionization or direct ionization of atmospheric gases, an experiment has been set up to enable observations of photons with wavelengths shorter than 200 nm while still allowing observation up to 800 nm. A spectrum simulation software package has been developed to allow for temperature analysis on the developing plasma in the VUV region. Observations below 200 nm revealed a Boltzmann distributed excited state population corresponding to a temperature of 3.1 eV. Time-resolved emission spectroscopy measurements of the entire electrode region during the streamer phase of breakdown demonstrate the presence of molecular nitrogen emission lines from the second positive system. Further photomultiplier tube measurements of the spark phase showed a rapid decrease in intensity of the second positive system compared to that of a representative atomic emission line in the VUV regime. This emission dominates the ultraviolet–visible (UV–VIS) spectrum during the initial phases of breakdown with little detection of other sources of emission during this phase.

Spectral intensity of the N2 emission in argon low-pressure arc discharges for lighting purposes

Nitrogen is discussed as an alternative to hazardous mercury in lamps for general lighting. Molecular nitrogen bands emit in both the near-UV (the second positive system C3Πu → B3Πg) and the visible spectral range (the first positive system B3Πg → A3Σ+u), which reduces conversion losses. To analyse the potential of nitrogen, low-pressure arc discharges in an argon background were characterized by means of optical emission spectroscopy. The spectral intensity of the molecular nitrogen emission rises with increasing nitrogen content in the discharge and shows a maximum around 4 mbar of absolute pressure. With regard to the application as a light source, radiation efficiencies were determined, which are around 5% at maximum. In order to identify the main population processes a collisional radiative model for the nitrogen–argon system was established which reveals the high relevance of heavy-particle collisions due to a pressure of a few mbar. The decisive excitation reactions for the state N2 C3Πu are the well-known processes of energy pooling between metastable nitrogen molecules and energy transfer from metastable argon atoms. For the state N2 B3Πg the main population channels are collision-induced crossings within the nitrogen states, where the collision partner can be either a nitrogen molecule or an argon atom, and the quenching collisions with argon.

Spectroscopic anatomy of a meteor with the very large telescope (ESO)

A meteor spectrum was recorded serendipitously with the ESO Very Large Telescope during a long exposure in long-slit spectroscopic mode with the instrument FORS1. The −8 magnitude fireball crossed the narrow (1 arcsec × 7 arcmin) slit during the observation of a high z supernova in normal service mode operation on May 12, 2002. The spectrum covers the range of 637–1050 nm, where the meteor’s air plasma emissions from N 2, N and O dominate. The meteor trail appears moreover resolved along the slit but we conclude that this is because the meteor at 100 km altitude was out of focus for the VLT. The plasma excitation temperature varies only from about 4300 to 4365 K across the trail, based on the ratio of atomic and molecular nitrogen emissions. This is in agreement with the fact that the trail is not actually spatially resolved. Finally, carbon atom emission is not detected in the relatively unexplored range above 900 nm.

Gas kinetic studies using a table-top set-up with electron beam excitation: quenching of molecular nitrogen emission by water vapour

A table-top set-up for studying gas kinetic processes in dense gases via time resolved optical spectroscopy is presented. The set-up uses low energy electron beams for gas excitation. A thin silicon nitride membrane separating the electron source and the gas cell allows to use electron energies as low as 10–15 keV. The electron source is operated in a fast pulsing mode with a pulse width of 5 ns and repetition rates up to 30 kHz. Light emission from the target gas sample is studied and time resolved photon counting is used to measure decay times and collisional rate constants for specific excited states. The experimental concept is applied to measure the rate constants for quenching of molecular nitrogen in the C 3Πu state (vibrational levels 0 and 1) by water vapour. Quenching rate constants of (7.1 ±0.7) × 10-10 s-1 cm3 and (6.7 ±0.7) × 10-10 s-1 cm3 were obtained for the C 3Πu (v = 0) and C 3Πu (v = 1) vibrational levels, respectively.

Spectroscopic anatomy of a meteor trail cross section with the European Southern Observatory Very Large Telescope

Abstract— A meteor spectrum was recorded serendipitously at the European Southern Obrervatory (ESO) Very Large Telescope (VLT) during a long exposure in long‐slit spectroscopic mode with FORS1. The −8 magnitude fireball crossed the narrow 1Î × 7î slit during the observation of a high z supernova in normal service mode operation on May 12, 2002. The spectrum covered the range of 637–1050 nm, where the meteor's air plasma emissions from N2, N, and O dominate. Carbon atom emission was not detected in the relatively unexplored wavelength range above 900 nm, but the observed upper limit was only 3 sigma less than expected from the dissociation of atmospheric CO2. The meteor trail was resolved along the slit, and the emission had a Gaussian distribution with a dimension of FWHM = 7.0 (±0.4) * sin(α) * H (km)/90 m, where α is the unknown angle between the orientation of the meteor path and slit and H the assumed altitude of the meteor in km. To our knowledge, this is the first observation of a spatially resolved spectrum across a meteor trail. Unlike model predictions, the plasma excitation temperature varied only from about 4,300 to 4,365 K across the trail, based on the ratio of atomic and molecular nitrogen emissions. Unfortunately, we conclude that this was because the meteor at 100 km altitude was out of focus.

Nitrogen atom energy distributions in a hollow-cathode planar sputtering magnetron

Energy distributions of nitrogen atoms (N) in a hollow-cathode planar sputtering magnetron were obtained by use of optical emission spectroscopy. A characteristic line, N I 8216.3 A, well separated from molecular nitrogen emission bands, was identified. Jansson's nonlinear spectral deconvolution method, refined by minimization of chi(2)(w), was used to obtain the optimal deconvolved spectra. These showed nitrogen atom energies from 1 eV to beyond 500 eV. Based on comparisons with VFTRIM computer code results, it is proposed that the energetic N's are generated from N+2 ions after these ions are accelerated through the sheath and dissociatively reflect from the cathode.

Spectra of type‐B red lower borders

A low‐light‐level television camera and a television spectrograph were used to study the variation of color with elevation angle in auroras having type‐B red lower borders. It was found that much of what is commonly described as type‐B aurora results from a simple separation between the prompt red and blue molecular nitrogen emissions on the one hand and the persistent green oxygen line on the other hand. The red and blue emissions combine to produce magenta which appears on the leading edge of rapidly moving auroral features. True type‐B aurora, not dependent on motion, was observed to result from a suppression of the green line at low altitudes, presumably due to quenching. While the quenching of the green is the predominant effect, there are, in addition, changes in the shade of magenta resulting from an enhanced red/blue ratio associated with the phenomenon of auroral hems (bands of enhanced luminosity along the lower borders of auroral curtains). In one case there was also evidence for a redistribution of the intensities of the nitrogen first positive bands, possibly by intercollisional transfer of energy from the W states to the B states as proposed by Benesch [1981]. In sum, evidence was found to support each of the prominent theories for type‐B auroras and that the various mechanisms can coexist.

Pyroelectrically induced optical emission from potassium titanyl phosphate crystals

We have observed optical scintillations and corresponding electric current pulses when uniformly heating potassium titanyl phosphate (KTP) crystals at 0.1–0.4 K/s over the temperature range 8–300 K. The scintillations correspond to molecular nitrogen emission occurring during the electrical breakdown of air near the crystal surface, and imply the existence of pyroelectrically induced peak electric fields at the crystal surface exceeding 30 kV/cm, which is ten times larger than dc electric fields reported to induce electrochromic (EC) damage in this material. Recent optical damage studies on KTP under high repetition rate, high average-power laser irradiation reveal an EC-like damage, implying the existence of an internal electric field arising during laser irradiation. Our observation of a sizable total pyroelectric response suggests one possible mechanism for the origin of these internal fields in KTP and other nonlinear optical materials.

Molecular nitrogen emissions from denitrification during biomass burning

THE burning of biomass (forest vegetation, savannah grass, firewood and agricultural wastes) due to human activities in the tropics is an important source of nitrogen compounds in the atmosphere1–4. A recent experimental study5 identified a gap of 35–60% in the nitrogen balance between its content in the fuel and that recovered in the ash and in gaseous emissions of NOx, NH3, HCN, CH3CN and other nitriles, N2O, higher-molecular-weight organic compounds and in the smoke. It was suggested that the missing compound had to be molecular nitrogen. We have now carried out appropriate experiments and find that molecular nitrogen is indeed the most important nitrogen species emitted from biomass burning, with the largest contribution coming from flaming combustion. The loss of nutrient nitrogen by biomass burning, which is ∼10–50 Tg N yr−1 or 5–50% of global nitrogen fixation, may be particularly important for tropical ecosystems.

The ultraviolet spectrum of a dayside aurora: 530–1500 Å

Rocket-borne spectrometers have observed the high-latitude dayside aurora above Cape Parry, N.W.T. Ultraviolet spectra of optical emissions produced by ambient precipitating particles have been obtained in the wavelength region between 530 and 1500 Å, over a range of spectrometer line-of-sight orientations, from 100 km to the rocket apogee of 452 km. The spectrum below 1500 Å is dominated by transitions from neutral and singly ionized atomic oxygen. NI, NII, and molecular nitrogen emissions, which are prominent in day airglow and nighttime auroral spectra measured by the same instrumentation, are very weak, indicating energy from the dayside auroral particles is transferred to the atmosphere above most of the N2. Relative line strengths of OI and OII transitions in the high-latitude dayside aurora differ in comparison with either airglow or nighttime auroral observations. The differences arise from differing excitation mechanisms and radiative-transfer characteristics and indicate the far-UV region of the spectrum can serve as a useful probe for remotely sensing various auroral phenomena. Weak emission near 1170 Å is identified as the OI 3s′3D–2p42D intercombination transition at 1172 Å. Observed OI 1172 Å intensities are consistent with the branching-ratio measurements of Morrison

Some molecular nitrogen emission from Titan‐solar EUV interaction

Voyager 1 explored the satellite Titan at a solar zenith angle of 73°. Photoelectron fluxes in the atmosphere of Titan for this same angle (73°) have been calculated for an atmospheric model with molecular nitrogen. The Photoelectron impact excitation rates of a′πg and b′πu states of N2 are also calculated.

Far ultraviolet atomic and molecular nitrogen emissions in the dayglow

A scanning spectrophotometer was used to observe the far ultraviolet day airglow between 1130 and 1520 A at 4.4-A spectral resolution. Fourteen bands of the N2 Lyman-Birge-Hopfield (LBH) system are clearly resolved and suggest a total LBH system zenith column emission rate of 3810 plus or minus 520 R extrapolated to the subsolar point. A photoelectron flux model (based on recent photoelectron flux measurements and the observed LBH altitude profile) is used to derive the direct and dissociative excitation contributions to the atomic nitrogen emissions. The calculated atomic nitrogen density agrees with other measurements, although it is an order of magnitude greater than previous photochemical model results.