Electron Impact Ionization Of N2 Research Articles

Fluid simulation of species concentrations in capacitively coupled N2/Ar plasmas: Effect of gas proportion

A two-dimensional self-consistent fluid model and the experimental diagnostic are employed to investigate the dependencies of species concentrations on the gas proportion in the capacitive N2/Ar discharges operated at 60 MHz, 50 Pa, and 140 W. The results indicate that the N2/Ar proportion has a considerable impact on the species densities. As the N2 fraction increases, the electron density, as well as the Ar+ and Arm densities, decreases remarkably. On the contrary, the N2+ density is demonstrated to increase monotonically with the N2 fraction. Moreover, the N density is observed to increase significantly with the N2 fraction at the N2 fractions below 40%, beyond which it decreases slightly. The electrons are primarily generated via the electron impact ionization of the feed gases. The electron impact ionization of Ar essentially determines the Ar+ density. For the N2+ production, the charge transition process between the Ar+ ions and the feed gas N2 dominates at low N2 fraction, while the electron impact ionization of N2 plays the more important role at high N2 fraction. At any gas mixtures, more than 60% Arm atoms are generated through the radiative decay process from Ar(4p). The dissociation of the feed gas N2 by the excited Ar atoms and by the electrons is responsible for the N formation at low N2 fraction and high N2 fraction, respectively. To validate the simulation results, the floating double probe and the optical emission spectroscopy are employed to measure the total positive ion density and the emission intensity originating from Ar(4p) transitions, respectively. The results from the simulation show a qualitative agreement with that from the experiment, which indicates the reliable model.

Characterization of the large area plane-symmetric low-pressure DC glow discharge

Electron density and temperature as well as nitrogen dissociation degree in the low-pressure (10–50mTorr) large area plane–symmetric DC glow discharge in Ar-N2 mixtures are studied by probes and spectral methods. Electron density measured by a hairpin probe is in good agreement with that derived from the intensity ratio of the N2 2nd positive system bands IC,1−3/IC,0−2 and from the intensity ratio of argon ions and atom lines IArII/IArI, while Langmuir probe data provides slightly higher values of electron density. Electron density in the low-pressure DC glow discharge varies with the discharge conditions in the limits of ~108–1010cm−3. The concept of electron temperature can be used in low-pressure glow discharges with reservations. The intensity ratio of (0–0) vibrational bands of N2 1st negative and 2nd positive systems I391.4/I337.1 exhibits the electron temperature of 1.5–2.5eV when argon fraction in the mixture is higher than nitrogen fraction and this ratio quickly increases with nitrogen fraction up to 10eV in pure nitrogen. The electron temperature calculated from Langmuir probe I-V characteristics assuming a Maxwellian EEDF, gives Te~0.3–0.4eV. In-depth analysis of the EEDF using the second derivative of Langmuir probe I-V characteristics shows that in a low-pressure glow discharge the EEDF is non-Maxwellian. The EEDF has two populations of electrons: the main background non-Maxwellian population of “cold” electrons with the mean electron energy of ~0.3–0.4eV and the small Maxwellian population of “hot” electrons with the mean electron energy of ~1.0–2.5eV. Estimations show that with electron temperature lower than 1eV the rate of the direct electron impact ionization of N2 is low and the main mechanism of N2 ionization becomes most likely Penning and associative ionization. In this case, assumptions of the intensity ratio IN2+,391/IN2,337 method are violated.In the glow discharge, N2 dissociation degree reaches about 7% with the argon fraction in the Ar-N2 mixture<10% and decreases afterwards approaching to ~1–2% when the argon percentage becomes 90% and higher. The atomic nitrogen species is produced by electron-impact processes such as, collisions between electrons and nitrogen molecules or between electrons and N2+ ions. At small Ar fraction in Ar-N2 mixtures, the atomic nitrogen species is most likely produced by the collisions between electrons and N2+ ions.

Ionization balance in Titan’s nightside ionosphere

Based on a multi-instrumental Cassini dataset we make model versus observation comparisons of plasma number densities, nP=(nenI)1/2 (ne and nI being the electron number density and total positive ion number density, respectively) and short-lived ion number densities (N+, CH2+, CH3+, CH4+) in the southern hemisphere of Titan’s nightside ionosphere over altitudes ranging from 1100 and 1200km and from 1100 to 1350km, respectively. The nP model assumes photochemical equilibrium, ion–electron pair production driven by magnetospheric electron precipitation and dissociative recombination as the principal plasma neutralization process. The model to derive short-lived-ion number densities assumes photochemical equilibrium for the short-lived ions, primary ion production by electron-impact ionization of N2 and CH4 and removal of the short-lived ions through reactions with CH4. It is shown that the models reasonably reproduce the observations, both with regards to nP and the number densities of the short-lived ions. This is contrasted by the difficulties in accurately reproducing ion and electron number densities in Titan’s sunlit ionosphere.

Experimental investigation of the triple differential cross section for electron impact ionization of N2 and CO2 molecules at intermediate impact energy and large ion recoil momentum

The (e,2e) triple differential cross sections (TDCS) are measured for the ionization of nitrogen and carbon dioxide molecules in a coplanar asymmetric geometry for a wide range of ejected electron energies and at an incident energy about 500–700 eV. This kinematics corresponds to a large momentum imparted to the ion, and is meant to enhance the recoil scattering. The experimental binary and recoil angular distributions of the TDCS are characterized both by a shift towards larger angles with respect to the momentum transfer direction and by a large intensity in the recoil region, in particular for the ionization of the ‘inner’ N2(2σg) molecular orbital. The data are compared with the results of calculations using the first Born approximation–two centre continuum (FBA–TCC) theoretical model for treating differential electron impact ionization. The experimentally observed shifts and recoil intensity enhancement are not predicted by the model calculations, which rather yield a TDCS symmetrically distributed around the momentum transfer direction, and completely fail in describing the recoil distribution. It is hoped that these new results will stimulate the development of more refined theories for correctly modelling single ionization of molecules.

Direct experimental measurement of electron impact ionization‐excitation branching ratios: 1. Results for N2 at 100 eV

A new laboratory experiment has been developed to measure electron impact ionization‐excitation branching ratios for formation of positive ions of atmospheric species in the ground and excited electronic states of the ion. In the experiments described here the distribution of N2+ ions produced in the X ²Σg+, A ²Πu, and B ²Σu+ electronic states by 100‐eV electron impact ionization of N2 has been measured directly for the first time. The experiment is a modification of general purpose (e,2e) electron coincidence ionization experiments which have been developed in recent years to study ionization processes. We have added a second electron energy analyzer to an existing electron energy loss spectrometer so that both the scattered primary and ejected secondary electrons can be detected in coincidence. The final ion state is determined from the energy of the ejected electron and the energy lost by the incident electron. Spectra of coincidence rate versus primary electron energy loss have been obtained at fixed ejected secondary electron energies of 2.5, 5.0, 10.0, and 15.0 eV. Peaks in these spectra are observed corresponding to the three final states of the N2+ ion. The relative peak areas are proportional to the partial branching ratios at each secondary electron energy. Branching ratios at 100 eV integrated over angle and energy variables have been computed from the data. They are 0.50, 0.40, and 0.10 for the X ²Σg+, A ²Πu, and B ²Σu+ electronic states respectively. These values are in excellent agreement with branching ratios obtained by other techniques.

Nascent rotational distributions of N+2(X 2Σ+g) produced by electron-impact ionization of N2 in a supersonic beam

Laser-induced fluorescence from nascent N+2(X 2Σ+g) ions produced by electron impact on a N2 supersonic beam was observed. An analysis of the B 2Σ+u−X 2Σ+g (0,0) band shows that the rotational state distributions cannot be represented by a single Boltzmann function, higher N″ levels being overpopulated. Experimental and analytical efforts were made to minimize the influence of cascading and relaxation on the rotational distributions. The rotational energy of N+2(X) thus estimated increases with decreasing electron energy from 2.26±0.16 meV at 300 eV to 4.24±0.27 meV at 25 eV. This trend is explained qualitatively in terms of angular momentum transfer through multipole electron–molecule interactions.

Polarized Laser Induced Fluorescence of N2+(X2Σg+) Produced in the Electron Impact Ionization of N2

Abstract When nitrogen molecule was ionized in collision with electrons of 80–300 eV, the angular distribution of the rotation vector of N2+(X2Σg+) was found to be slightly anisotropic with respect to the direction of the electron beam in the vicinity of 80 eV. This anisotropy indicates that the ejection of a πu electron is the dominant channel in the ionization process in this energy range.

Differences in near UV (∼3400–4300 Å) optical emissions from midday cusp and nighttime auroras

Our ground‐based spectrophotometric observations of nighttime auroras over Fairbanks (Λ = 64.6°N), Alaska, and from dayside cusp auroras over Longyearbyen (Λ = 75°N), Svalbard, show that while molecular bands contribute most of optical radiation from nighttime auroras, atomic line emissions, particularly from metastable atmospheric species, dominate midday cusp auroral spectra. In the near UV (∼3400–4300 Å) nighttime auroral optical emissions (excited by electrons with average energy of a few keV) consist mostly of N2+ first negative group (1 NG), N2 2P and N2 Vegard‐Kaplan (VK) bands; the only prominent atomic feature is [N I] 3466A line and its intensity is about 1/300 of N2+ 1 NG (0, 0) band; the [O II] 3726‐3729 Å lines are very weak (&lt;2R). In midday cusp regions, where magnetosheath electrons with average energy &lt;100 eV precipitate, the N2 2P and the N2 VK bands are extremely weak (&lt;2R), on the other hand [N I] 3466 Å and [O II] 3726–3727 Å lines are relatively intense. N2+ 1 NG bands are observed in midday cusp auroras, but unlike night auroras, where these bands are excited by electron impact ionization of N2, in cusp auroras most of the N2+ 1 NG band emissions come from resonant scattering of sunlight by N2+ ions; these ions are formed mostly through charge exchange reaction: N2(X¹Σu+) + O+[²D]→ N2+ (A²π) + O (³P). The vibrational rotational distributions of these bands vary with shadow height but on the average they are approximately similar to bands excited by electron impact ionization of N2 thermalized at ∼2500°K. In midday cusp auroras, the ratio of [N I] 3466‐Å line intensity to particle‐excited component of N2+ 1 NG (0, 0) band intensity is ∼1/10. Relatively more energetic (average energy ∼1 keV) electrons precipitate equatorward of the cusp, exciting both atomic and molecular emissions; the optical spectra of these auroras, which occur temporally coincident with but spatially separated from the cusp auroras, are similar to those observed on the nightside. Even in the cusp region, the average energy of precipitating particles can be higher during pre and post local magnetic noon periods; cusp auroral spectra observed during these periods contain molecular bands. Hence, major differences in auroral optical emissions are most noticeable when nightside spectroscopic observations are compared with similar measurements from the cusp region made around (±1 hour) local magnetic noon.

EUV emission from Titan's upper atmosphere: Voyager 1 encounter

Analysis of Titan's EUV emission spectra obtained at the Voyager 1 encounter demonstrates that electron impact on N2 above 3600 km accounts for the bulk of the observed emission short of Lyman α. In conjunction with the UVS solar occultation data it is concluded that N2 is the major component of Titan's upper atmosphere, with upper limit mixing ratios at 3900 km on NeI, ArI, CO, H2, and HI of 0.01, 0.06, 0.05, 0.06, and 0.1, respectively. Magnetospheric electrons interact with Titan's sunlit hemisphere to produce a power dissipation rate of ≃2 × 109 W in the exosphere and ≃3 × 109 W below the exobase, with optical signatures from numerous N2 bands, NI, and NII multiplets. The N2 C4′ (0‐0) Rydberg band at 958 Å acts as an optical probe of Titan's exosphere because of transmission losses caused by fluorescence and predissociation. Magnetospheric electron precipitation produces an average dayside electron density of ≃3 × 10³ cm−3 between 3600 and 4000 km, the region of bright limb emission. When Titan is within Saturn's magnetosphere, magnetospheric electron impact dissociation of N2 generates an N atom escape rate of ≃3 × 1026 s−1 from Titan's exosphere. A nonthermal H atom escape rate of ≃2 × 1026 s−1 is estimated from magnetospheric electron impact ionization of N2 followed by reactions with CH4 and H2 and recombination to produce hot H atoms.

Sequential mass spectrometry: nitrogen and the determination of appearance potentials

Electron impact ionization of N2 below 70 eV has been studied under conditions favourable to the trapping of ions in the electron beam. The ionization of N2 is observed to produce N2+, N2+(*), N22+, N+ + N(4So), N+ + N(2Po), (N+ + N?), N2+ + N and (N2+ + N?), where product pairs in parentheses have excess energy distributed in an undetermined manner Electron collisions with N2+ give N+, N22+ and N2+ + N, and collisions with N22+ give N2+ + N. The problem of determining appearance potentials under these conditions is discussed, and a new method is presented and applied to the above processes. Comparisons with known or expected appearance potentials show that the error in determining thresholds is 0 37 ± 0 23 eV.