On the energy dependence of the yield of doubly charged negative ions during the capture of free electrons by C60(CF3)12 trifluoromethylfullerene molecules

Peaks of C₆₀(CF₃) _n ^2? doubly charged negative ions (n = 6–12) have been observed in the mass spectra of the resonance electron capture by trifluoromethylfullerene C₆₀(CF₃)₁₂ molecules. It has been established that these ions are formed owing to the attachment of two free isoenergetic electrons. The autodetachment of an extra electron has been detected for the doubly charged molecular ions (n = 12). It has been established from the observation of the delayed fragmentation of the most abundant ions with n = 8 and 10 that the doubly charged negative ions, like their singly charged analogs, are metastable with respect to the separation of the CF₃ fragment(s). The yield of doubly charged negative ions has been obtained as a function of the electron energy. By comparing them with the analogous dependences for the singly charged ions, the specific features have been revealed which were associated with the presence of the repulsive Coulomb barrier and the regular effect of the doubled energy of two additional electrons on the energy dependence of the dissociative decay of the doubly charged negative ions. The absolute cross section for the formation of the C₆₀(CF₃) ₁₀ ^2? ions has been measured. At the energy of their yield maximum near the 5 eV, it is ～1 × 10^?19 cm².     

Boltzmann Equation Analysis of the Electron Gas in CF4 Discharge Plasma

The electron energy distribution function in CF₄ gas discharge was numerically calculated for density reduced electric field E/N = 15 - 300 × 10^?17 Vcm² using the Boltzmann kinetic equation. The momentum transfer and vibrational cross-sections were derived from available experimental swarm parameters while the cross-sections for inelastic electron-molecule collisional processes included were taken from the literature. The two-term spherical harmonic expansion of the distribution function was used in the calculations.     

Relaxing Phenomena in Negative Corona Discharge: New Aspects

Two theories, explaining the time dependence of the negative corona discharge current in air, are confronted with new experimental results. The influence of the ozone concentration on the discharge current was experimentally confirmed in dry air and in mixtures of nitrogen with oxygen. Assuming that only the dissociative electron attachment to ozone molecule is a process being responsible for a reduction of the electron component in the total mean discharge current, the mean value of the electron attachment rate constant k = (3 - 5.5) × 10^?9 cm^?3 s^?1 was derived from the measured dependence of the discharge current on the ozone concentration. The calculated value of the rate constant k corresponds to the dissociative attachment of electron to ozone molecule via process e + O₃ → products (O^? or O^?₂ negative ions).     

Feasibility of a low-voltage discharge in pure molecular hydrogen

The feasibility of initiating a low-voltage discharge in pure (free of readily ionizable impurity) molecular hydrogen is studied theoretically. A discharge with cathode fall φ₁ = 10 V, interelectrode gap L = 2 cm, and total hydrogen concentration \(N_{H_2 }^{(0)} = 2 \times 10^{15}\) cm^?3 is considered by way of example. The plasma parameters, including concentration \(N_{H^ - }\) of negative hydrogen ions H^?, are calculated. The concentration of H^? ions is maximal in the near-anode plasma and may reach \(\left( {N_{H^ - } } \right)_{\max } = 0.5 \times 10^{12}\) cm^?3. Concentration \(N_{H^ - }\) can be increased severalfold by introducing a small amount of cesium into the discharge, \(N_{Cs}^{(0)} \leqslant 10^{13}\) cm^?3. Cesium ionizes completely and concentrates in narrow near-electrode layers, which are depleted with the plasma in the purely hydrogen discharge. The discharge modifications studied in this work may be of interest as low-voltage volume plasma sources of H^? ions under conditions when a high concentration of cesium in the source plasma is undesirable.     

Generation of H− ions in a low-pressure xenon-hydrogen discharge. I

A theory of a low-pressure discharge in a xenon-molecular hydrogen mixture is developed. It is shown that, in such a discharge, at an interelectrode distance of L = 1 cm and a total plasma pressure of p ₀ ～ 1 Torr, the density of negative hydrogen ions produced via the dissociative attachment of thermal electrons to vibrationally excited molecules H₂ can reach a value as high as N_H ^? ≥ 10¹² cm^?3. According to calculations, the electron temperature in discharge operating regimes under study attains T _e ≈ 1?2 eV, which corresponds to the maximum of the e-v exchange rate constant of H₂ molecules. This ensures a relatively high rate of vibrational pumping of H₂ molecules in the discharge.     

Composition of a non-self-sustained discharge plasma in an N2: O2: H2O mixture at atmospheric pressure

The plasma composition of a discharge sustained by a pulsed ionization source of μs duration is computed. It is shown that, within a time interval of ∼10⁻⁶ s after the ionization pulse, the dependences of the ion densities on the electric field and ionization source power show features that should be taken into account when developing laser systems for controlling electric discharges in long air gaps. The effect of the plasma composition on the efficiency of electron photodetachment from negative O ₂ ⁻ ions is investigated by the example of a discharge initiation system consisting of two lasers with different pulse durations and wavelengths. Plasmochemical processes under conditions of enhanced electron photodetachment from negative O ₂ ⁻ ions are simulated. It is shown that photodetachment can increase the electron density for a time of <10⁻⁵ s.     

Verbreiterung von Hauptserienlinien des Kaliums durch Wechselwirkung mit Kalium- und Cäsiumatomen bei hohen Teilchendichten

For certain conditions the afterglow of low pressure discharges can be dominated by three body recombination. These conditions — electron densities of about 10¹² cm^?3 and electron temperatures of about 500 °K— are realized 20 μsec after switch off of the discharge for gas pressures (hydrogen) between 2 and 6 mTorr. In this pressure range the energy transfer from the electrons via the ions to the neutrals and subsequently to the wall is high enough to permit a decay of the electron temperature to low values before the electron density has decreased appreciably. A theoretical model demonstrates that the electron density decaydn _e/dt～n _e ³ f(T _e) is in good approximation equivalent todn _e/dt=const·n _e ² for a certain range of time and of parameters. This proportionality is observed. The measured values of the constant differ from the calculated ones by about 15%.     

Negative ion effects in CO2 convection laser discharges

Negative ions are computed to be formed on a time scale and in quantities such that they may be a cause of plasma instability observed in low pressure electrical discharge convection CO₂ lasers. In a typical CO₂?N₂?He?H₂O laser mixture the principal ions are CO ₃ ^? , CO ₄ ^? and H^? with the total negative ion densityn _? given by 0.1n _e <n _?<n _e , wheren _e is the electron density: but if the gases are re-cycled or if there is an air leak NO ₂ ^? and NO ₃ ^? are formed in significant amounts andn _? can become greater thann _e in a time considerably less than the gas dwell time in the electrical excitation discharge. CO is effective in reducingn _? in a system without re-cycling, but is ineffective in a re-cycled system with the oxides of nitrogen present.     

Plasma-volume generation of H<Superscript>−</Superscript> ions in a low-voltage xenon-hydrogen discharge. II

A low-voltage xenon-hydrogen discharge is considered theoretically at an interelectrode distance of L = 1 cm and cathode emission current densities of j _s = 2–20 A/cm². Basic parameters of the discharge plasma, in particular, the total hydrogen and xenon densities, are optimized to attain the maximum possible density of negative hydrogen ions \(N_{H^ - } (L)\) at the plasma-anode boundary. The distributions of the plasma parameters over the discharge gap are calculated for optimized regimes. According to calculations, at intermediate cathode emission current densities (j _s ≈ 5–10 A/cm²) in optimized discharge regimes, the density of negative hydrogen ions in the anode region of the plasma is \(N_{H^ - } (L)\) ≈ (1.5–2.5) × 10¹² cm^?3 and the total plasma pressure is p ₀ = 0.5–0.6 Torr.     

Electron kinetics in dc CF4 + O2 mixture discharge

The electron distribution function, transport coefficients and rate coefficients for the dissociation of CF₄ and the excitation of a¹ _g and b¹ _g ⁺ states of oxygen have been calculated for a CF₄ + O₂ mixture discharge. The two-term approximation was used in solving the electron Boltzmann equation. The influence of the dissociation was ignored, so that the results are parametric in the density reduced electric fieldE/N and the ratio of the mixture components. Only slight dependence of the calculated quantities on the composition of the gas mixture has been observed. The critical electric field for the onset of the discharge was found to beE/N=1.1–1.25 × 10^–15 Vcm² and it is weakly decreasing with growth of the oxygen content. With a small amount of oxygen added to CF₄ the electron distribution function and the mean kinetic energy remain practically unchanged.     

Spektroskopische Untersuchungen über das thermodynamische Gleichgewicht der Anregung in den Plasmen kurzdauernder Impulsentladungen

The light emission of gases conducting electric discharges is of considerable complexity. The observed phenomenas are controlled to a great part by the electric parameters of the circuit, as well as by the time constants of the specific plasma properties as the relation times for reaching Maxwellian distribution and thermal equilibrium of electrons, ions, and atoms. The influence of the nonequilibrium between the excitation and the electron temperature on the population of states was studied in short time discharges by means of singulet and triplet lines of elements of the second group of the periodic system. In these discharges a nonequilibrium was found between the excitation and the electron temperature. While the time constant measured for the cooling of the electron gas was found to 7·10^?7 s in a discharge switched off 9·10^?7 s past the ignition the minimal time constant of relaxation of excitation is equal to the life time of the 2p ₂ state — this value of cadmium being 2.4·10^?6 s -.The use of Boltzmann and Saha equation must be considered very carefully in this type of short time discharges.     

Numerical study on characteristics of nitrogen discharge at high pressure with induced plasma

Based on the fluid theory of plasma, a model is built to study the characteristics of nitrogen discharge at high pressure with induced argon plasma. In the model, the spices such as electron, N₂⁺, N₄⁺, Ar⁺, and two metastable states (N₂ (A³∑_u⁺), N₂ (a¹∑_u^-)) are taken into account. The model includes particle's continuity equations, electron's energy balance equation, and Poisson equation. The model is solved with a finite difference method. The numerical results are obtained and used to investigate the effect of time taken to add nitrogen gas and initially-induced argon plasma pressure. It is found that lower speeds of adding the nitrogen gas and varying the gas pressure can induce higher plasma density, and inversely lower electron temperature. At high-pressure discharge, the electron density increases when the proportion of nitrogen component is below 40%, while the electron density will keep constant as the nitrogen component further increases. It is also shown that with the increase of initially-induced argon plasma pressure, the density of charged particles increases, and the electron temperature as well as the electric field decrease.     

Acoustic perturbations in a pulsedRF-plasma

The response of a stationary weakly ionized plasma to a density perturbation in the neutral gas component was studied in a neon plasma with the following typical properties: electron density ¯N _e≈8×10¹² cm^?3, electron temperature on the axis of the vesselT _e0≈3.0 eV; neutral gas densityN _n≈1×10¹⁷cm^?3 and neutral gas temperatureT _n0≈600 °K. A neutral density perturbation, generated 50 cm apart from the plasma, produces a fluctuation in the ion density and a sharp spike in the differential voltage of a floating double probe. The experimental observations demonstrate the propagation of an ion sheath and of an electric field perturbation together with the neutral density perturbation. An interpretation of the plasma response to acoustic wave pulses has been proposed by Ingard and Schulz in a theory on acoustic wave modes in a weakly ionized gas. The experimental results are in good agreement with the theoretical expectations.     

The fragmentation of negative ions of fullerene C60 trifluoromethyl derivatives

The formation and decay of gas-phase negative ions of trifluoromethylated fullerenes C₆₀(CF₃) _n (n = 2–10) were studied. The resonance electron capture mass spectra were measured to find that the main fragmentation channel of negative ions was the detachment of trifluoromethyl groups. The degree of fragmentation directly depended on the energy of electrons and reached the complete splitting off of all the CF₃ addends with the formation of C₆₀⁻ ions. The observed metastable ion signals were analyzed to determine the scheme of sequential fragmentation of negative ions.     

Untersuchungen über die Entwicklung des Kanaldurchschlags in Gasen

The temporal development of the current of a discharge leading to breakdown was investigated under static and homogeneous field conditions with wide band oscilloscopic techniques. The discharges were started by 10³ to 10⁵ electrons released within some 10^?9 sec along traces of single α-particles parallel to the electric field. Measurements have shown that streamer mechanism occurs in gases as CH₄ and CO₂ at static breakdown, if a sufficient density of space charge is produced by a great number of overlapping avalanches. The gas amplification of thesingle avalanche only has a value of about 10⁵. Also in O₂ and dry air at highpd-values breakdown develops with streamer mechanism. — In electronegative gases as freon 12 and O₂ a prolongation of the time necessary for streamer formation is caused by the reduction of the effective total space charge by the negative ions. The results of a detailed study of the time necessary for streamer development and the time constant of the increase of the current leading to breakdown confirm the model of streamer mechanism.     

Determination of the Electron Density in an Argon Laser Plasma by Spectroscopy of the Hydrogen Hα and Hβ Lines

From measurements of the H_α and H_β spectral line profiles in a plasma, a method is developed which allows to separate the contributions of Doppler and Stark broadening. This method is superior to the deconvolution of Voigt profiles, in particular, when the lines are of low intensity. The electron density in the plasma can be calculated from the Stark broadening. An example is the low pressure (p ≈ 1 hPa) arc discharge of argon ion lasers which is characteristised by electron densities of approximately 10¹⁴ cm^?3 at heavy particle temperatures of about 10⁴ K. These plasma parameters lead to a broadening of the Balmer H_α and H_β spectral lines of hydrogen, which has a low concentration within the discharge area. The spectral lines are broadened due to the electron density dependent Stark effect and the temperature responsive Doppler effect. The results are consistent with predictions of the argon ion laser modelling.     

介质阻挡均匀大气压氮气放电特性研究

基于一维流体力学模型，对介质阻挡均匀大气压氮气放电特性进行了数值计算研究.模型中考虑了氮气中主要的电离、激发过程，所包含的粒子种类为e，N₂，N⁺₂，N⁺₄，N₂(a¹∑^-_u)，N₂(A³∑⁺_u).模拟结果显示，氮中的放电具有低气压下汤生放电的特性.放电电流幅度较小，放电过程中气体电压变化缓慢，电子密度远低于离子密度，而且最大值出现在阳极，电子不能在放电间隙中被俘获，不存在中性等离子体区，气体中的电场趋于线性变化.亚稳态N₂(A³∑⁺_u)和N₂(a¹∑⁺_u)在整个放电空间都具有非常高的密度，比电子密度高三个量级以上，亚稳态密度的最大值出现在阳极，这样的分布决定了放电的空间结构.放电所需的种子电子主要由亚稳态之间潘宁电离提供，这种机理使放电的电离水平较低，导致氮气中的放电只能是汤生放电.随着放电参数的变化，多电流峰放电也可在氮气中获得. 关键词： 大气压均匀放电 介质阻挡放电 数值模拟 氮气     

Ionic composition of a humid air plasma under ionizing radiation

A kinetic model is proposed for ion–molecular processes involving charged particles of a humid air plasma produced by a fast electron beam. The model includes more than 600 processes involving electrons and 41 positive and 14 negative ions, including hydrated ions H₃O⁺ (H₂O) _n and O ₂ ^? (H₂O) _n with n = 1, 2, …, 12. The energy costs of production of electron–ion pairs and electronic and vibrational (for water molecules, also rotational) excitation of molecules are calculated in nitrogen, oxygen, water vapor, air, and humid air. A method is proposed for calculating the energy costs in mixtures by the calculation data in pure gases. The evolution of the plasma composition is studied by the numerical solution of a system of 56 time-dependent balance equations for the number of charged particles of plasma by the fourth-order Runge–Kutta method. The steady-state composition of plasma is determined by solving nonlinear steady-state balance equations for the ionization rates of humid air from 10 to 10¹⁶ cm^–3/s and the fraction of water molecules from 10^–3% to 1.5%. It is established that, for water vapor content (the ratio of the number density of water molecules to the total number density of air molecules) of 0.015–1.5% in air at atmospheric pressure and room temperature, the main ion species are two types of positive ions H₃O⁺ (H₂O) _n with the number of water molecules n = 5, 6 and three species of negative ions O ₂ ^? (H₂O) _n with n = 5, 8, 9.     

Nonlinear plasma spectroscopy of the hydrogen balmer-α line

The plasma line broadening of H_α fine-structure lines is investigated with Doppler-free saturation and polarization spectroscopy in He-H gas and are discharges at plasma densities of 10⁸ cm^?3 <N?1.4×10¹⁴ cm^?3. With a single-mode laser, the shift and broadening of four resolved H_α fs lines are measured in a low pressure discharge forN<10¹¹ cm^?3. With an intense, broadband multi-mode laser the plasma effects of H_α are investigated up toN=1.4×10¹⁴ cm^?3 in a hollow cathode are. Calculations in the classical phase shift and impact approximations can explain the experimental data and peculiarities of the low-density plasma effects and show that the ions are the dominant perturbers. Ion dynamical effects, perturber mass and temperature dependence, are observed and interpreted. Applications of the nonlinear techniques to other H and D lines, other atoms, and for H and D plasma diagnostics are discussed.     

A study of molybdenum radiation from alcator A

Radiation emitted by molybdenum impurity ions in the Alcator A tokamak has been investigated. A grazing incidence monochromator photometrically calibrated against synchrotron radiation was used to determine absolute brightnesses of emission lines and the role of molybdenum radiative losses in the plasma discharge power balance. At line average electron densities of 3 × 10¹⁴ cm^?3, molybdenum radiation from the central (0–3 cm) region was equal to ～17% of the local ohmic heating power, implying radiation from molybdenum ions does not dominate the power loss from the central part of the plasma. The temporal behavior of molybdenum ions injected by the laser blow off technique at $\text{n}{}_{\mathrm{e}}\text{= 3 × 10}{}^{14}\text{cm}{}^{\mathrm{?3}}$ indicated that the molybdenum ion confinement time was finite in conflict with neoclassical theory. At lower densities (1–2 × 10¹⁴ cm^?3) the radiation from intrinsic molybdenum increased faster than the electron density during the discharge, indicating a buildup of molybdenum ions in the plasma.