Electron Collisions Research Articles

Spectral-width variation of N2+ lasing

The complicated collisional processes inside a femtosecond laser-induced plasma column can show a great impact on the light emissions from it. A known example is that the lifetime of noncoherent fluorescence emitting from gas plasma can be greatly decreased due to the fierce ultrafast plasma collisions. However, for coherent stimulated emissions in plasma surrounding, to what extent it can be influenced by the collisional process has not been fully understood. Herein, we report an experimental spectral width analysis on coherent ${\mathrm{N}}_{2}^{+}$ lasing that is generated in weakly ionized gas plasma with a frequently adopted pump-probe scheme. It is found unexpectedly that the spectral width of ${\mathrm{N}}_{2}^{+}$ lasing evidently shortens with the increasing delay between the pump and the probe pulses at a pressure of 40 mbars, whereas an invariable spectral width appears when the gas pressure is decreased below 10 mbars. Moreover, it was also observed that the spectral width narrows with the increment of polarization angle between the pump and the probe pulses. These observations update the understandings of molecular spectral manipulation on the basis of the previous that the spectral modulations mainly occur in the vicinity of molecular alignment. Our results indicate that by combining the spectral width and intensity analysis, a more comprehensive understanding regarding the impact of ultrafast electronic plasma-ion collisions on the ${\mathrm{N}}_{2}^{+}$ lasing generation can be obtained. These findings also shed light on the spectral manipulation for coherent irradiations produced during strong laser-matter interaction.

Using Collisional Electron Spectroscopy to Detect Gas Impurities in an Open Environment: CH4-Containing Mixtures.

The collisional electron spectroscopy method for analyzing and determining gaseous impurities was further developed to realize the operation in an open environment. In addition, the method not only facilitates the registration of the impurity components, but also the reactive radicals generated from the discharge reaction. The sandwich-like discharge structure was used to generate a stable, non-local, negative glow equipotential plasma in an open environment, and the I–V characteristic curve of the plasma was collected using an additional sensor electrode. The collisional electron spectroscopy was obtained from the first derivative of the probe current I with respect to the probe potential V by adding a diffusion function to correct it. In addition, our experiment verifies the reliability of the sink theory.

Elastic Electron Scattering by Diborane(6) and Diborane(4) Molecules

We present integral, differential, and momentum transfer cross sections for elastic electron collisions with the two most stable isomers of the B2H4 molecule, hereafter referred to as isomer I (C2v symmetry) and isomer II (D2d symmetry), and with the B2H6 molecule (D2h symmetry). The isomer I of B2H4 and the B2H6 molecule are known for their electrodeficiency in the "three-center two-electron" (3c-2e) bond, a region in which a hydrogen atom occupies a bridged position between two non-hydrogen atoms. The cross sections were computed by using the Schwinger multichannel method implemented with norm-conserving pseudopotentials, and the scattering calculations were carried out in the static-exchange and static-exchange plus polarization levels of approximation for energies from 0.1 to 50 eV. We discuss the presence of shape resonances in the scattering cross sections. We also made a direct comparison among the cross sections of these three targets.

Electron-neutral collision cross sections for H2O: I. Complete and consistent set

This work proposes a complete and consistent set of cross sections (CSs) for electron collisions with water molecules to be published in the IST-Lisbon database on LXCat. The set is validated from the comparison between experimental and computed electron swarm parameters. The former are collected from literature while the latter are calculated using a space-homogeneous two-term Boltzmann solver, assuming isotropic scattering in inelastic collisions. Rotational CSs, based on the Born approximation, are optimised by means of the electron swarm analysis technique. Superelastic rotational and vibrational collisions are accounted for in the calculations and found to be particularly important for low-energy electrons interacting with water molecules. The set can be used with codes assuming space-homogeneous conditions, in particular common two-term Boltzmann solvers, ensuring a good agreement with experiments. Therefore, it constitutes an important tool for fast calculations and modelling of complex plasma chemistries.

Analytical Formulas for Approximating Cross Sections of Electron Collisions with Hydrogen, Noble Gases, Alkali and Other Atoms

This paper presents an analysis of data on the cross sections of elastic and inelastic collisions of electrons with noble gases, alkali and other atoms. For the selected sets of experimental and theoretical data, optimal analytical formulas are found, and approximation coefficients are calculated. The obtained semi-empirical formulas reproduce the values of the transport (diffusion), excitation and ionization cross sections for noble gases. Much attention is paid to the ionization cross sections of metal atoms, which are often present as an impurity in gas-discharge plasma. The approximation formulas reproduce the values of the ionization cross sections for hydrogen, metal and other elements in a wide range of energies with accurate orders of errors of the available theoretical and experimental data. For some elements with a two-hump plot of the dependence of the ionization cross section on the collision energy, it is proposed to use a two-term formula that takes into account ionization from both external and internal shells.

Dynamics of electron collision with potential biofuel: N-butanol

A detailed theoretical investigation of electron impact collision on n-butanol, a potential biofuel, is carried out over a wide energy impact range from 0.1 to 5000 eV. The low energy scattering calculations from 0.1 to 20 eV are performed using UK molecular R-matrix method via Quantemol-N software. This scattering investigation aims to predict various resonances by providing dissociative electron attachment via C–O bond cleavage. Furthermore, the study focuses on estimating several cross-sections namely elastic, differential, momentum transfer, excitation, ionization and total cross-section. In addition, we also computed higher energy cross-section from the ionization threshold to 5000 eV using the spherical complex optical potential (SCOP) method. A good agreement is observed between the computed cross-section at the overlapping energies of employing (SCOP and R-matrix) formalisms. Apart from the scattering study, we also obtained various resonances having significance at low energy. Theoretically predicted resonance peaks agree well with previously reported data and a few new resonance states are also identified. The cross-sectional data presented here will be beneficial as an input parameter modelling electron transport processes during the spark ignition.

Flux Concentrator Optimization for Future Positron Sources

Future lepton colliders, such as CLIC, FCC-ee or ILC require high quality positron sources. Positrons are created after the collision of electrons on a tungsten target and then focused to match the aperture of the capture structures with a flux concentrator (FC) often called an Adiabatic Matching Device (AMD), a strong pulsed magnet. The FC is a tapered solenoid powered with fast pulses (microsecond) of high current (kA). The current pulse produces a strong magnetic field (3-8 T) at the magnet entrance that rapidly decays (over few cm) to zero. This paper describes the finite element model of the FC and the transient electromagnetic simulation capable of describing the experimental data in terms of current, field and voltage. The computed field map is transferred into particle tracking codes to compute the figure of merit, the positron yield. The coil configuration is optimized to minimize the voltage and Lorentz forces and to maximize the yield. This paper presents the numerical model and the result of the optimization process.

Contribution of Liénard–Wiechert potential to the broadening of spectral lines by electron collisions in plasmas

In this work, we have calculated the relativistic collision operator representing the line broadening, by collision with free electrons, of isolated lines emitted by hydrogen-like ions (Ly-α line 2p 2P3/2o→1s 2S1/2) of hydrogenic ions FeXXVI, CrXXIV, and CoXXVII in the temperature range of 108 to 5×109 K and electron densities of 1021 to 1026cm−3. To accomplish this task, we have considered the fine structure of these ions and taking into account the relativistic effects related to the free electrons. Specifically, two relativistic effects are considered: the electric field of Liénard–Wiechert, created by the free electron at the emitter ion, and the modification of the hyperbolic trajectory due to the dependent mass on the free electron velocity. The average over the velocities of the free electrons is accomplished by using the Maxwell–Juttner distribution, which is more adequate for the fast (relativistic) electrons. The results are compared to the classical case (when the electric field is the Coulomb's field) to Doppler broadening and to some experimental results (K. Koyama and M. G. Haines) available in the literature. It turns out that, at high temperatures and high densities, the Stark broadening by the relativistic electrons overcomes the Doppler broadening.

Electron impact cross-sections of tetraethyl silicate

Understanding the interactions of electrons with molecules in plasma is of vital importance from both academic and technological points of view. Reliable electron collision data is required to model the electron and ion components of low-temperature and nonequilibrium plasmas. Various electron impact cross sections such as the differential, integral, momentum transfer, partial and total ionizations are reported for tetraethyl silicate (TEOS), a plasma-relevant molecule in the energy range between the ionization threshold and 5 keV. The elastic (differential, integral and momentum transfer) cross sections are obtained by invoking the molecular approach and local potential approximation within the single center expansion formalism. The dissociative ionization cross sections are reported within the binary encounter Bethe (BEB) model formalism. The elastic and ionization cross sections are summed incoherently to estimate total cross sections. A good agreement is observed between the present results and others that are available. This work validates the efficacy of the modified BEB model in computing the partial ionization cross sections.

The 2021 release of the Quantemol database (QDB) of plasma chemistries and reactions

The Quantemol database (QDB) provides cross sections and rates of processes important for plasma models; heavy particle collisions (chemical reactions) and electron collision processes are considered. The current version of QDB has data on 28 917 processes between 2485 distinct species plus data for surface processes. These data are available via a web interface or can be delivered directly to plasma models using an application program interface; data are available in formats suitable for direct input into a variety of popular plasma modeling codes including HPEM, COMSOL, ChemKIN, CFD-ACE+, and VisGlow. QDB provides ready assembled plasma chemistries plus the ability to build bespoke chemistries. The database also provides a Boltzmann solver for electron dynamics and a zero-dimensional model. Thesedevelopments, use cases involving O2, Ar/NF3, Ar/NF3/O2, and He/H2O/O2 chemistries, and plans for the future are presented.

Research on Inductively Coupled Low-Temperature Plasma Spectroscopy Diagnosis

The electron density distribution of a closed-cavity inductively coupled plasma (ICP), along the incident direction of electromagnetic waves, is a key factor affecting its electromagnetic wave attenuation effect. Therefore, an investigation of the various parameters of the ICP, including the electron density inside a closed cavity, is of great significance in the advancement of plasma technology. In this study, an ICP generator was used to generate argon plasma, and the radiation intensity and full-width at half-maximum (FWHM) of the spectral line were measured using emission spectroscopy. The electron collision section model was then used to calculate the electron temperature of the plasma, which was found to be above 0.9 eV. In addition, the electron temperature was found to decrease as the pressure increases and increase significantly as the power increases. The Stark broadening method was used to calculate the electron density, where the Doppler broadening of spectral lines was considered, and the Voigt convolution function was introduced. An accurate value for the Stark broadening was obtained through the convolution calculation, which improves the accuracy of the plasma electron density. Subsequently, the FWHM of the 794.82-nm spectral line was used to obtain the electron density of the plasma. The influence of different power levels and pressures on the electron density was analyzed under pure argon conditions. Finally, the distributions of the plasma electron temperature and electron density in the axial and radial directions of the cavity were investigated. In order to verify the accuracy of spectral diagnosis in the current experimental environment, the diagnostic results of Langmuir single probe were used to verify it. The electron density of argon plasma measured by the probe method and spectroscopic method is of the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{17}\,\,\text {m}^{-3}$ </tex-math></inline-formula> , and the axial distribution of the electron density and the variation trend with the pressure tend to be consistent. The reliability of spectral diagnosis under the current experimental conditions is verified.

The effect of water and electron collisions in the rotational excitation of HF in comets

ABSTRACT We present the first set of rate coefficients for the rotational excitation of the 7 lowest levels of hydrogen fluoride (HF) induced by collision with water molecules, the dominant collider in cometary comas, in the 5–150 K temperature range. The calculations are performed with a quantum statistical approach from an accurate rigid rotor ab initio interaction potential. Rate coefficients for excitation of HF by electron-impact are also computed, within the Born approximation, in the 10–10 000 K temperature range. These rate coefficients are then used in a simple non-local thermodynamic equilibrium (non-LTE) model of a cometary coma that also includes solar radiative pumping and radiative decay. We investigate the range of H2O densities that lead to non-LTE populations of the rotational levels of HF. We show that to describe the excitation of HF in comets, considering collisions with both water molecules and electrons is needed as a result of the large dipole of HF.

Effect of microwave pulse parameters on energy coupling and enhancement of microwave assisted ignition

Microwave Assisted Ignition (MAI) is a promising technology to optimize the lean combustion characteristics of internal combustion engines. This research investigated the effects of microwave pulse waveform and delay time on energy coupling and ignition enhancement by using a power diagnostic with high time-resolution. The pulse width ranged from 80 to 200 μs and the corresponding peak power was adjusted between 1000 and 400 W to keep the incident energy of microwave pulse constant. Results showed that a high power and short width pulse waveform was conducive to the microwave energy absorption. The coupled energy generally decreased with delay time. There was a significant ignition enhancement with only 6∼7 mJ energy coupled into the flame kernel, which was approximately 1/5 of total spark energy. Moreover, the ignition enhancement was well correlated with the coupled energy, suggesting that ignition enhancement was determined by coupled energy via electron collision reactions. Finally, the main influencing factor of coupled energy was the energy coupling pattern which changed from none-coupling to saturated coupling. A high pulse power and short delay time facilitated this change. This study revealed the saturated coupling pattern independent of waveform and delay time, which is caused by a stable electron number density due to the cut-off effect.

The Kinetic Monte Carlo Model of the Auroral Electron Precipitation into N2-O2 Planetary Atmospheres

Auroral events are the prominent manifestation of solar/stellar forcing on planetary atmospheres. They are closely related to the energy deposition by and evolution of planetary atmospheres, and their observations are widely used to analyze the composition, structure, and chemistry of the atmosphere under study, as well as energy fluxes of the precipitating particles that affect the atmosphere. A numerical kinetic Monte Carlo model had been developed, allowing us to study the processes of precipitation of high-energy auroral electrons into the N2-O2 atmospheres of the rocky planets in the Solar and exosolar planetary systems. This model describes on a molecular level the collisions of auroral electrons and atmospheric gas, taking into account the stochastic nature of collisional scattering at high kinetic energies. The current status of the kinetic model is illustrated in the applications to the auroral events on the Earth such as the production of suprathermal nitrogen atoms due to the electron impact dissociation of N2. It was found that electron impact dissociation of N2 can potentially be an important source of suprathermal N atoms in the auroral regions of the N2-O2 atmosphere of terrestrial-type planets. Such research will allow us to study the odd nitrogen chemistry as an atmospheric marker of the N2-O2 atmosphere of rocky exoplanets.

Plasma activation of methane for hydrogen in microwave liquid discharge by auxiliary gases: a way to realize efficient utilization of resources

Given the excited state species generated by the discharge gas can activate CH4 in gas-phase discharge and water in liquid-phase discharge. In this work, to make full use of the activation of auxiliary gases and maximize the energy utilization in liquid-phase discharge, the microwave liquid-phase plasma technology was used to study the activation of the CH4-H2O system by auxiliary gases for the first time, the photoelectric characteristics of discharge and discharge plasma parameters were analyzed. N2 and Ar were selected as auxiliary gases. The results showed that auxiliary gas could also activate CH4 and H2O in addition to the effective dissociation of reactants by electron collision, and the discharge gas-phase products were mainly H2. Compared with the discharge in the pure CH4-H2O system, under N2 and Ar atmosphere, the hydrogen production energy efficiency increased by 43.0% and 72.8%, respectively. This difference is mainly related to the selectivity of the active species produced by the two auxiliary gases to the original reactants and radicals. This study provides a simple and efficient method for plasma-assisted CH4 activation for hydrogen production.

Resonances in nitrobenzene probed by the electron attachment to neutral and by the photodetachment from anion.

We probe resonances (transient anions) in nitrobenzene with the focus on the electron emission from these. Experimentally, we populate resonances in two ways: either by the impact of free electrons on the neutral molecule or by the photoexcitation of the bound molecular anion. These two excitation means lead to transient anions in different initial geometries. In both cases, the anions decay by electron emission and we record the electron spectra. Several types of emission are recognized, differing by the way in which the resulting molecule is vibrationally excited. In the excitation of specific vibrational modes, distinctly different modes are visible in electron collision and photodetachment experiments. The unspecific vibrational excitation, which leads to the emission of thermal electrons following the internal vibrational redistribution, shows similar features in both experiments. A model for the thermal emission based on a detailed balance principle agrees with the experimental findings very well. Finally, a similar behavior in the two experiments is also observed for a third type of electron emission, the vibrational autodetachment, which yields electrons with constant final energies over a broad range of excitation energies. The entrance channels for the vibrational autodetachment are examined in detail, and they point to a new mechanism involving a reverse valence to non-valence internal conversion.

Alcohol addition improves the liquid-phase plasma process for “Green” reduction of graphene oxide

Graphene oxide (GO) and reduced graphene oxide (r-GO) continue to attract considerable research due to their applicability in many academic and industrial fields.Here, we report a liquid-phase plasma approach with fast reduction and high reactivity for directly converting GO to r-GO at mild temperatures. The liquid-phase plasma process is at the forefront, and it is a strong candidate for facilitating large-scale applications of GO and r-GO. In this work, using methanol as an additive rather than other hazardous reducing agents, the results demonstrated that methanol could provide more hydrogen radicals (·H) to improve the reduction process of the liquid-phase plasma. Meanwhile, the ·CH2, ·CH, ·C, ·C2, ·OH, ·H, ·O were formed during discharge, suggesting that the collision of energetic electrons in plasma with methanol and water, which subsequently removes oxygen-containing functional groups and restores the π-conjugated structure in GO through a radical process. The analytical results revealed that r-GO has been successfully reduced by liquid-phase plasma. In conclusion, our findings can provide underlying insights into liquid-phase plasma treatment for GO reduction.

Platform for probing radiation transport properties of hydrogen at conditions found in the deep interiors of red dwarfs

We describe an experimental concept at the National Ignition Facility for specifically tailored spherical implosions to compress hydrogen to extreme densities (up to ∼800× solid density, electron number density ne∼4×1025 cm−3) at moderate temperatures (T∼200 eV), i.e., to conditions, which are relevant to the interiors of red dwarf stars. The dense plasma will be probed by laser-generated x-ray radiation of different photon energy to determine the plasma opacity due to collisional (free–free) absorption and Thomson scattering. The obtained results will benchmark radiation transport models, which in the case for free–free absorption show strong deviations at conditions relevant to red dwarfs. This very first experimental test of free–free opacity models at these extreme states will help to constrain where inside those celestial objects energy transport is dominated by radiation or convection. Moreover, our study will inform models for other important processes in dense plasmas, which are based on electron–ion collisions, e.g., stopping of swift ions or electron–ion temperature relaxation.

Self-consistent formation and steady-state characterization of trapped high-energy electron clouds in the presence of a neutral gas background

This study considers the self-consistent formation and dynamics of electron clouds interacting with a background neutral gas through elastic and inelastic (ionization) collisions in coaxial geometries similar to gyrotron electron guns. These clouds remain axially trapped as the result of crossed magnetic field lines and electric equipotential lines creating potential wells similar to those used in Penning traps. Contrary to standard Penning traps, in this study, we consider a strong externally applied radial electric field which is of the same order as that of the space-charge field. In particular, the combination of coaxial geometry, strong radial electric fields, and electron collisions with the residual neutral gas (RNG) present in the chamber induce non-negligible radial particle transport and ionization. In this paper, the dynamics of the cloud density and currents resulting from electron–neutral collisions are studied using a 2D3V particle-in-cell code. Simulation results and parametric scans are hereby presented. Finally, a fluid model is derived to explain and predict the cloud peak density and peak radial current depending on the externally applied electric and magnetic fields, and on the RNG pressure.

Development of particle detectors for electron–ion collision spectroscopy with highly charged ions at the storage ring CSRe

The electron cooler at the experimental Cooler Storage Ring (CSRe) has been upgraded with an embedded electron energy fast detuning system for the merged-beams electron–ion collision experiments. A plastic scintillation detector (PSD) and a multi-wire proportional chamber (MWPC) detector have been developed and installed downstream of the electron cooler to detect the recombined and ionized ions in the electron–ion collision experiments at the CSRe. Both detectors have been tested successfully in a recent dielectronic recombination (DR) experiment of Na-like Kr25+ ions at the CSRe. In addition, the measured DR rate coefficients for Kr25+ ions from both detectors were compared with the flexible atomic code (FAC) calculations, and a very good agreement is achieved. The present experimental results demonstrate that the new experimental setups at the CSRe including the electron energy fast detuning system and the particle detectors have high stability and efficiency and meet the needs of the forthcoming electron–ion collision precision spectroscopy at the CSRe.