Rare Gas Mixtures Research Articles

VUV lasing in diffuse discharges formed by runaway electrons

The parameters of stimulated emission in diffuse discharges formed in a sharply inhomogeneous electric field by runaway electrons in mixtures of rare gases with the addition of H2 and F2 at pressures up to 10 atm are studied. Efficient VUV lasing was obtained at wavelengths from 148 to 193 nm on the transitions of H2, F2 and exciplex ArF* molecules. It was shown that the addition of He buffer gas increases the pulse duration, while Ne addition improves the output energy of the VUV laser on the H2 Lyman band. A laser pulse duration over 10 ns and an output of 0.12 mJ were obtained. The diffuse discharge in mixtures of He with F2 was found to form as a result of successive ionization waves. It was shown that the laser pulse at 157 nm has three peaks, which correspond to the maxima of the diffuse discharge current. Therewith, the first or second peak of the laser radiation has the maximum intensity, depending on the amplitude of the conduction current in the primary ionization wave. A maximal F2* laser electrical efficiency of η 0 = 0.18% and an output of Q 157 = 3.8 mJ were obtained in a He–F2 gas mixture at pressure of 10 atm, which exceeds the efficiency of lasers of this type pumped by transverse volume discharges with UV preionization. Long-pulse operation of the ArF* laser was achieved in a He–Ne–Ar–F2 gas mixture. Lasing at 193 nm continued during two periods of the diffuse discharge current. The total duration of the laser pulse was as long as 40 ns, and the radiation energy at 193 nm was as high as 2 mJ from an active volume of 20 cm3.

Electron-Impact Excitation and Dissociation of Heavy Rare Gas Heteronuclear Ions via Transitions to Charge Transfer States

Heteronuclear diatomic rare gas molecular cations feature excited electronic terms with charge transfer character located several eV above the ground term. The role of such terms in collisions involving heteronuclear ions is studied theoretically under conditions typical of the plasma-based sources of UV and IR radiation. Calculations were carried out for processes of dissociative excitation, dissociative recombination and electron impact bound–bound excitation in Ar/Xe and Kr/Xe plasmas using the recently developed semiclassical approach combined with the ab initio data for potential energy curves and oscillator strengths of electronic transitions. The approach consistently describes the contributions from the entire rovibrational manifold to the processes studied. The cross sections of the processes mentioned are calculated for wide ranges of gas temperatures and electron energies. We show that the processes considered are quite effective when they are accompanied by transitions to charge transfer terms. For the range of electron energies typical of active media of UV and IR radiation sources the cross sections exceed those reported for the processes usually considered to involve transitions between the ground and first excited electronic state. The excitation of charge transfer electronic terms can play an important role in the kinetics of rare gas mixture plasmas.

Numerical Analysis of Corona Pre-Ionization for High-Power Discharge-Pumped XeCl* Excimer Lasers

The working gas of discharge-pumped excimer lasers is generally a mixture of rare gases and monohalide. Typically, the working gas is preconized to generate reproducible uniform glow discharge. In high-power (more than 100 W) excimer lasers, this pre-ionization is achieved through spark discharge in the discharge chamber. In this study, a high-power discharge-pumped XeCl* excimer laser with corona preionisation was designed to investigate the feasibility of applying corona preionisation in high-power excimer lasers as well as analyse the correlation of corona discharges characteristics with laser discharge properties. The electron avalanche triggered by ultraviolet preionisation in the laser was numerically studied by creating 2D models. The simulation results revealed that the additional ionisation induced by the secondary electron emission of the cathode together with the dielectric constant of the corona bars significantly affect the discharge current of the principal electrodes. Under the condition of the design values, the peak preionisation electron density between electrodes was approximately 108 cm−3, and the laser discharge was quite uniform, which theoretically proved the feasibility of using corona preionisation instead of spark discharge preionisation in high-power practical devices.

Analysis of the second virial coefficient, and application to rare gas mixtures

Abstract The second virial coefficients characterize the real-gas non-ideality caused by the interaction between molecular pairs and ensure a link between macroscopic thermodynamic properties and microscopic molecular interactions because they depend on intermolecular interaction energy and temperature. Therefore, the second virial coefficients that are suitable for calculating the thermodynamic properties of gases used in the many fields in this work are preferred. In this study, a semi-analytic representation for the second virial (SV) coefficient over exponent–spline-Morse-spline-van der Waals potential (ESMSV), investigating the thermodynamic properties of rare gases, is presented. In the study the series formulae of the hypergeometric function, exponential function, gamma function, Meijer function, and binomial expansion have used in the suggested method. The numerical approach has been used mostly to evaluate the SV coefficient with ESMSV potential in literature. This unified formula can be applied and tested for rare gases. The obtained results for the SV coefficient over ESMSV potential of 4He–4He, 4He–Ne, 4He–Ar, 4He–Kr, 4He–Xe, Ne–Ne, O2–O2, and Ar–O2 rare gases have been compared with alternative experimental data and numerical calculations and shown that semi-analytical expression can be successfully applied to evaluate simple fluids.

Guided plasma jets directed onto wet surfaces: angular dependence and control

The optimal use of atmospheric pressure plasma jets (APPJs) for treatment of surfaces—inorganic, organic and liquid—depends on being able to control the flow of plasma-generated reactive species onto the surface. The typical APPJ is a rare gas mixture (RGM) flowed through a tube to which voltage is applied, producing an RGM plasma plume that extends into the ambient air. The RGM plasma plume is guided by a surrounding shroud of air due to the higher electric field required for an ionization wave (IW) to propagate into the air. The mixing of the ambient air with the RGM plasma plume then determines the production of reactive oxygen and nitrogen species (RONS). The APPJ is usually oriented perpendicular to the surface being treated. However, the angle of the APPJ with respect to the surface may be a method to control the production of reactive species to the surface due to the change in APPJ propagation properties and the resulting gas dynamics. In this paper, we discuss results from computational and experimental investigations addressing two points—propagation of IWs in APPJs with and without a guiding gas shroud as a function of angle of the APPJ with respect to the surface; and the use of this angle to control plasma activation of thin water layers. We found that APPJs propagating out of the plasma tube into a same-gas environment lack any of the directional properties of shroud-guided jets, and largely follow electric field lines as the angle of the plasma tube is changed. Guided APPJs propagate coaxially with the tube as the angle is changed, and turn perpendicularly towards the surface only a few mm above the surface. The angle of the APPJ produces different gas dynamic distributions, which enable some degree of control over the content of RONS transferred to thin water layers.

Ethanol monomer revisited: Thermal isomerisation between anti and gauche conformers in Ar and N2 matrix

The anti-gauche conformational distribution of ethanol has been investigated using the O-H and symmetric C-C-O stretching infrared spectra in argon and nitrogen matrix. The dipole moment of the gauche conformer is higher compared to the anti conformer. The relative population of the anti and gauche conformer of ethanol depends on the rare gas mixture in the gas phase and also in the matrix after thermal cyclisation. In the gas phase, the anti conformer is more populated in nitrogen mixture whereas the gauche conformer is more populated in the argon mixture. After thermal cyclisation in the matrix, the anti conformer isomerise to the gauche conformer in nitrogen matrix and reverse happens in the argon matrix. Thermal cyclisation at 30 K in nitrogen matrix leads to the splitting of the gauche± states that is probed by monitoring symmetric C-C-O stretching frequency mode.

Laser-induced multiphoton and collisional ionizations in sodium-argon mixture

It is attractive to investigate the interaction mechanism between metal and rare gas atoms within ionization process, especially for the mixture of metal vapor and rare gas. In this paper, laser-induced breakdown spectroscopy is used to study the ionization and recombination kinetic processes in sodium-argon mixture. Sodium-argon excimer is formed by the atomic collision, and it plays an important role in the ionization process, sodium atoms on 3P state are produced after the dissociation of excimer, then multiphoton ionization happens, the electrons in plasma are accelerated to collide with argon atoms, electron-impact ionization of argon will take place, identified with time-resolved atomic emission spectrum. Population accumulate on 3D,4P, and 5S states after the recombination process of Na+ with electron, then most population will transit to 3P state mainly by collisional relaxation.

Non-empirical calculations of rotovibrational band wings: Carbon dioxide–rare gas mixtures

We consider spectral characteristics of rotational perturbation for CO2-Rg collisions (Rg = Ar, Xe) using molecular torques time correlation function and three-dimensional classical trajectories without several conventionally used simplifications. The results are applied to the band wing calculation. The calculated wing profiles depend on the potential energy surface and agree well with the experimental data. The relative contributions of collision-induced absorption and the role of dimer absorption are discussed.

Rydberg states population via three-body and dissociative recombination in low-temperature plasmas of rare gas mixtures

We study various physical mechanisms of electron-ion recombination accompanied by the formation of Rydberg atoms in plasmas containing atomic and molecular ions. Analytical approach to the description of resonant mechanism of ternary electron capture associated with the non-adiabatic energy transfer from free electron to the electronic shell of a quasimolecular ion is developed. Similar technique is used for the evaluation of the integral contribution of all rovibrational states of a bound molecular ion to the Boltzmann-averaged cross section and rate constant of dissociative recombination (DR). Simple expressions for cross sections and rate constants of non-resonant electron capture to Rydberg states in ternary electron-ion-atom collisions are derived in the impulse approximation. We perform a comparative analysis of the efficiencies of resonant and non-resonant three-body recombination of electrons with atomic ions and DR of molecular ions in plasmas of rare gas mixtures, Rg/Xe, with small relative concentration of xenon ([Xe] ≪ [Rg], Rg = He, Ne, Ar, Kr). It is shown that in a wide range of plasma parameters the resonant recombination mechanisms are predominant. The behavior and magnitudes of the recombination rate constants turn out to be quite different for heteronuclear ions with small (HeXe+ and NeXe+, D0 = 13.1 and 33 meV) and moderate (ArXe+ and KrXe+, D0 = 171 and 400 meV) values of the dissociation energy.

Анализ эффективностей процессов захвата электронов ионами в ридберговские состояния и неупругих переходов n n-=SUP=--=/SUP=- в плазме смесей инертных газов

We have carried out the comparative analysis of the efficiencies of resonant and non-resonant mechanisms of electron capture by ions into Rydberg states of atoms, Xe(n), and the inelastic n → n′ transitions between the highly-excited states in the plasmas of rare gas mixtures, Rg/Xe, containing atomic, Xe$^{+}$, and molecular, RgXe$^{+}$ and Xe$_{2}^{+}$, ions (Rg = Ne, Ar and Kr, [Xe]≪[Rg]). The rate constants of resonant electron capture by Xe$^{+}$ ions in ternary collisions with rare gas atoms, Rg($^{1}S_{0}$), and dissociative recombination of heteronuclear, RgXe$^{+}$, and homonuclear, Xe$_{2}^{+}$, ions were evaluated on the basis of the original approach within the framework of the theory of non-adiabatic transitions between electronic terms of RgXe$^{+}+e$ system. For the alternative mechanism of the three-body electron capture by Xe$^{+}$ ions in collisions with Ne, Ar and Kr atoms the calculations of the rate constants were carried out in the impulse approximation by taking into account both short-range and polarization electron-atom interactions. The rate constants of the three-body electron capture by ions and n → n′ transitions in collisions with electrons were calculated on the basis of the well-known theoretical models. We have determined the ranges of plasma ionization degree and electronic and gas temperatures as well as the principal quantum number of Xe atom which correspond to situations when the resonant free-bound and bound-bound transitions of electron play a key role.

Collision-induced light scattering (CILS) and collision-induced absorption (CIA) spectra for monatomic rare gas mixtures

The isotropic and anisotropic collision-induced light scattering (CILS) for gaseous binary mixtures of neon with argon and krypton with xenon are computed using pair polarizabilities trace and anisotropy as input. Also the line shapes of collision-induced absorption (CIA) at different temperatures are determined using theoretical induced dipole moment and interatomic potential. Comparison with measured scattering and absorption spectra and literature shows a good agreement, despite the uncertainty about spectral moments. Good agreement between computed and experimental line shapes of scattering and absorption is also obtained when potential models which are constructed from the thermo-physical and transport properties are used.

Using a passively stable attosecond beamline for relative photoemission time delays at high XUV photon energies.

We present and demonstrate an experimental scheme that enables overlap-free reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) measurements at high extreme-ultraviolet (XUV) photon energies. A compact passively-stabilized attosecond beamline employing a multilayer (ML) mirror allows us to obtain XUV pulses consisting of only two odd high-harmonic orders from an attosecond pulse train (APT). We compare our new technique to existing schemes that are used to perform RABBITT measurements and discuss how our scheme resolves the limitations imposed by spectral complexity of the harmonic comb at high photon energies. We further demonstrate first applications of our scheme for rare gases and gas mixtures, and show that this scheme can be extended to gas-molecule mixtures.

Emission of fast hydrogen atoms at a plasma–solid interface in a low density plasma containing noble gases

The source of the broad radiation of fast hydrogen atoms in plasmas containing noble gases remains one of the most discussed problems relating to plasma–solid interface. In this paper, we present a detailed study of Balmer lines emission generated by fast hydrogen and deuterium atoms in an energy range between 40 and 300 eV in a linear magnetised plasma. The experiments were performed in gas mixtures containing hydrogen or deuterium and one of the noble gases (He, Ne, Ar, Kr or Xe). In the low-pressure regime (0.01–0.1 Pa) of plasma operation emission is detected by using high spectral and spatial resolution spectrometers at different lines-of-sight for different target materials (C, Fe, Rh, Pd, Ag and W). We observed the spatial evolution for Hα, Hβ and Hγ lines with a resolution of 50 μm in front of the targets, proving that emission is induced by reflected atoms only. The strongest radiation of fast atoms was observed in the case of Ar–D or Ar–H discharges. It is a factor of five less in Kr–D plasma and an order of magnitude less in other rare gas mixture plasmas. First, the present work shows that the maximum of emission is achieved for the kinetic energy of 70–120 eV/amu of fast atoms. Second, the emission profile depends on the target material as well as surface characteristics such as the particle reflection, e.g. angular and energy distribution, and the photon reflectivity. Finally, the source of emission of fast atoms is narrowed down to two processes: excitation caused by collisions with noble gas atoms in the ground state, and excitation transfer between the metastable levels of argon and the excited levels of hydrogen or deuterium.

On the importance of electron impact processes in excimer-pumped alkali laser-induced plasmas.

The excimer-pumped alkali laser (XPAL) system has recently been demonstrated in several different mixtures of alkali vapor and rare gas. Without special preventive measures, plasma formation during operation of XPAL is unavoidable. Recent advancements in the availability of reliable data for electron impact collisions with atoms and molecules have enabled development of a complete reaction mechanism to investigate XPAL-induced plasmas. We report on pathways leading to plasma formation in an Ar/C2H6/Cs XPAL sustained at different cell temperatures. We find that depending on the operating conditions, the contribution of electron impact processes can be as little as bringing the excitation of Cs(P2) states to higher level Cs** states, and can be as high as bringing Cs(P2) excited states to a full ionization. Increasing the input pumping power or cell temperature, or decreasing the C2H6 mole fraction leads to electron impact processes dominating in plasma formation over the energy pooling mechanisms previously reported in literature.

Effect of stable and metastable dimers on collision-induced rototranslational spectra: Carbon dioxide – rare gas mixtures

The role of stable and metastable dimers as well as of free collisions in the collision-induced rototranslational absorption by the compressed CO2–Ar and CO2–Xe gas mixtures is elucidated using the classical three-dimensional trajectories method. The contribution from the stable dimers is obtained via Fourier transform of the dipole correlation function. The spectral bandshape due to the unbound trajectories (metastable dimers and free collisions) is calculated as an averaged Fourier spectrum of the collision-induced dipole moment. The mean lifetimes of metastable dimers have been estimated as 3.8 ps for CO2–Ar and 5.9 ps for CO2–Xe pairs. Trajectory computations are complemented by calculations of zero spectral moments using pair distribution functions. The stable and metastable dimer contribution to the zero spectral moment is shown to be comparable with that from free collisions.

Optical and Electron Spin Resonance Studies of Destruction of Porous Structures Formed by Nitrogen–Rare Gas Nanoclusters in Bulk Superfluid Helium

We studied optical and electron spin resonance spectra during destruction of porous structures formed by nitrogen–rare gas (RG) nanoclusters in bulk superfluid helium containing high concentrations of stabilized nitrogen atoms. Samples were created by injecting products of a radio frequency discharge of nitrogen–rare gas–helium gas mixtures into bulk superfluid helium. These samples have a high energy density allowing the study of energy release in chemical processes inside of nanocluster aggregates. The rare gases used in the studies were neon, argon, and krypton. We also studied the effects of changing the relative concentrations between nitrogen and rare gas on thermoluminescence spectra during destruction of the samples. At the beginning of the destructions, \(\alpha \)-group of nitrogen atoms, Vegard–Kaplan bands of \(\hbox {N}_2\) molecules, and \(\beta \)-group of O atoms were observed. The final destruction of the samples were characterized by a series bright flashes. Spectra obtained during these flashes contain M- and \(\beta \)-bands of NO molecules, the intensities of which depend on the concentration of molecular nitrogen in the gas mixture as well as the type of rare gas present in the gas mixture.

Optimization Based on Fuzzy Logic Control of Discharge Lamp-Electronic Ballast System for Water Purification

The low-pressure electric discharge established in mercury rare gas mixtures is the basis of many applications both in the field of lighting as for industrial applications. This present work presents the design of a high-frequency current supply based on a pulse width modulation inverter powering a low-pressure ultraviolet lamp dedicated for a germicidal effect in water purification. The basic open-loop system delivers voltage and current at a frequency of 50 kHz modulated in an envelop oscillating at 1 kHz, thus decreasing the operating efficiency of the system for an optimal germicide effect. A proportional-integral controller introduced in the ballast suppresses the modulation of the RMS intensity envelop but cannot assume robustness to external disturbances such as variation of the temperature of the water to disinfect. Thus, to increase the reliability of the whole system, the classic controller is originally replaced by a fuzzy controller, which significantly improves the quality of the waveforms of the lamp arc current and arc voltage. This new control strategy is more robust since it supports very large perturbations of the temperature of the water basin.

The role of the heating of the vacuum chamber on the water content in plasma and gas

The behavior of the H2O concentration in stainless steel chamber is compared for the cases of gas with and without plasma discharge at different temperatures of chamber wall. The experiments were performed with the use of mixtures of rare gases and water vapor. In absence of plasma discharge the dynamics of the density of the water molecules was determined by their interaction with the surface. In the presence of a gas discharge the concentration of water molecules in the volume of the chamber was determined by optical actinometry method. The role of the chamber wall temperature is discussed.

Lasing in mixtures of rare gases with fluorine pumped by volume diffuse discharges

Laser parameters in He–Ar(Kr, Xe)–F2 and He–F2 gas mixtures under pumping by runawayelectron preionized diffuse discharge (REP DD) are studied. It is shown that a REP DD is an efficient XeF* and KrF* laser emission source. Lasing on transitions of molecular fluorine in the VUV region (at 157 nm) was obtained for the first time. It is shown that high homogeneity of REP DD allows an increase in the pulse length in rare gas fluorine lasers. Lasing parameters under the REP DD pumping are comparable with those obtained under pumping by commonly used transverse discharges with preionization.

Dissociation degree of nitrogen molecule in low-pressure microwave-discharge nitrogen plasma with various rare-gas admixtures

The dissociation degree of nitrogen molecules is examined in a microwave discharge nitrogen–rare gas mixture plasma with a total discharge pressure of 1 Torr, by actinometry measurement. Although the spectral line from the excited nitrogen atoms is overlapped by the band spectrum of the N2 first positive system (1PS), the subtraction of the 1PS spectrum fitted theoretically can successfully extract the atomic nitrogen line, which enables actinometry measurement. The nitrogen dissociation degree decreases with increasing mixture ratio of Ar to Kr, whereas it increases with He, which is attributed to the variations in the electron temperature and density. When we dilute the nitrogen with neon, however, we find an anomalous increase in the nitrogen dissociation degree by several orders of magnitude even at a downstream region in the discharge tube. The reason for the dissociation enhancement upon adding neon is discussed in terms of atomic and molecular processes in the plasma.