大气压氩直流微放电光谱研究

微空心阴极放电或微放电是一种能够实现高气压下放电的有效方法。利用不锈钢空心针作阴极,不锈钢网作阳极,进行了大气压氩直流微放电实验研究。测量了大气压氩微放电光谱,发现氩气的发 射谱线主要集中在690～860 nm范围,且全部为氩原子4p—4s的跃迁。实验研究了不同放电电流、气体压强、气体流量与谱线强度之间的关系,发现谱线强度随放电电流、气体流量增加均增加,而谱线强 度随压强变化呈现不同特征：谱线强度随压强的增加先增加后降低,在13.3 kPa时强度最大。此外,选用跃迁波长为763.51和772.42 nm的两条光谱线,利用发射谱线强度比值法测量了氩气微放电等离子 体的电子激发温度。结果显示,其电子激发温度处于2 000～2 800 K之间,且随放电电流的增加而增加,随气体压强和气体流量的增加而降低。     

Electron impact ionization and excitation rate coefficients in the negative glow region of a glow discharge

Some easy to use reasonable approximations for electron impact rate coefficients have been considered. The most important rate coefficients for electron collisions in noble gases are electron-neutral ionization and electron impact excitation. Electron-neutral ionization besides electron impact excitation of some states of the argon and helium atom in direct current (dc) glow discharge plasma has been calculated. The plasma parameters of electron are significant factors for computing the rate coefficients. We present first results of probe diagnostic that includes the double probe measurements of the plasma parameters, namely, electron temperature (T_e) and electron density (n_e). Electron properties obtained from the double probe characteristic curves including T_e and n_e as well as the calculated rate coefficients (ionization and excitation) were studied as a function of the axial distance from the cathode while the discharge operating parameters of voltage and pressure were varied. Two regions of the glow discharge were investigated: cathode fall region and negative glow. Particular emphasis was placed on the negative glow region.     

Excitation of Doubly‐Charged Ion Emission Lines from a Grimm‐Style Glow Discharge Plasma—Comparison Between Yttrium and Zirconium

Spectra of yttrium and zirconium emitted from a Grimm‐style glow discharge plasma were investigated to elucidate the excitation mechanism of doubly‐charged ionic lines when using argon–helium mixed gas as well as argon gas alone. The energy sum for exciting doubly‐charged ion species of yttrium is slightly smaller compared to the case of zirconium, which yields an interesting correlation in the excitation energy between their ionic species and excited species of helium or argon. The Y III emission lines which were assigned to the 4p⁶5p–4p⁶5s(4p⁶4d) transitions could be observed in the argon–helium mixed gas plasma, but those were hardly excited with argon gas only. The Zr III emission lines did not appear in the spectra emitted by the argon gas plasma nor by the mixed gas plasma. A possible explanation for these phenomena is that the excitation of these ionic species is caused principally by collisional energy transfer from helium species to the analyte atoms.     

Excitation of helium atoms in collisions with plasma electrons in an electric field

The rate constants are evaluated for excitation of helium atoms in metastable states by electron impact if ionized helium is located in an external electric field and is supported by it, such that a typical electron energy is small compared to the atomic excitation energy. Under these conditions, atomic excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atomic excitation, which is a self-consistent process because it leads to a sharp decrease in the energy distribution function of electrons, which in turn determines the excitation rate. The excitation rate constant is calculated for the regimes of low and high electron densities, and in the last case, it is small compared to the equilibrium rate constant where the Maxwell distribution function is realized including its tail. Quenching of metastable atomic states by electron impact results in excitation of higher excited states, rather than transition to the ground electron state for the electric field strengths under consideration. Therefore, at restricted electron number densities, the rate of emission of resonant photons of the wavelength 58 nm, which results from the transition from the 2¹ P state of the helium atom to the ground state, is close to the excitation rate of metastable atomic states. The efficiency of atomic excitation in ionized helium, i.e., the part of energy of an electric field injected in ionized helium that is spent on atomic excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field.     

发射光谱研究氩气/空气混合气体介质阻挡放电能量传递过程

使用水电极介质阻挡放电装置,对比氩气与氩气/少量空气的混合气体以及空气与空气/少量氩气的混合气体放电的发射光谱,研究了氩气与空气相混合时气体放电中的能量传递过程。实验发现, 当氩气中加入少量的空气时,氩原子谱线均变弱,说明空气中的氮分子对氩原子的各激发态具有猝灭作用。并且随着空气含量的增加,各谱线变弱的速率不同。越是与氮分子的激发电位接近的氩原子的激发态被猝灭的作用越明显。另一方面,当空气中加入少量氩气时,发现氮分子第二正带系和氮分子离子第一负带系的谱线均被增强。说明在空气/少量氩气放电中,氮分子的激发由于亚稳态氩原子的潘宁激发传能而增强。因此在氩气/空气混合气体放电中,气体成分及比例影响放电的发光特性和能量传输特性。     

Radiative Transitions and Temperature Time Dependences in Pulse Excited Microwave Discharges – (N2, Ar + N2)

Quantitative spectral and microwave measurements of vibrational temperatures and electron densities were performed for 2400 MHz non-isothermic pulse excited discharges in flowing nitrogen and argon at pressures (60–2700) Pa. A detailed analysis of the N₂ vibrational states population for the N₂ C³Π_u, X¹Σ_g⁺ electronic states has been carried out. The basic difficulties encountered when comparing the spectroscopically determined values of vibrational temperatures with corresponding quantities of the ground electronics state are mentioned and the time resolved dependences of the translational gas temperature in N₂ during the microwave pulse is evaluated. The steady state in the nitrogen pulse excited microwave plasma is reached within 3 · 10^?4 s, but generally, this time depends on the gas pressure in the discharge tube. In the Ar + N₂ mixtures the excitation conditions are complicated by the metastable argon atoms (3P_2,0) creating the nonequilibrium populations of electronic, vibrational and rotational N₂ states.     

Emission characteristics of a barrier discharge in an Ar-H2O mixture

We have experimentally studied the UV radiation of a low-temperature barrier discharge plasma in an Ar-H₂O mixture. The spectral interval 300–400 nm has been examined in detail. Addition of argon with a pressure of 24 kPa to a barrier discharge in water vapor at a pressure of ～0.1 kPa leads to a ninefold increase in the UV radiation power of excited hydroxyl molecules. An increase in the duration of the UV radiation pulse of the mixture in the longitudinal discharge decay has been achieved for the first time, which may be direct evidence of energy transfer from metastable argon atoms to water molecules. An estimate of the upper boundary of the dissociative excitation rate constant of hydroxyl molecules OH*(A ²Σ⁺) upon interaction of metastable argon atoms with water molecules has been obtained.     

Optical emission spectroscopy of Ar-N2 mixture plasma

Optical emission spectroscopy measurements are presented to characterize the different excitation and ionization processes of both atomic and molecular species in Ar-N₂ mixture plasma under different discharge conditions. Particularly, the emission intensities of nitrogen (0-0) band of second positive system at 337.1 nm and (0-0) band of first negative systems at 391.4 nm are used to determine the dependence of their radiative states on argon fraction in the mixture. It is observed that the addition of argon gas influences the radiative states differently due to their different populating mechanisms. The results demonstrate that the addition of argon to nitrogen plasma remarkably enhance the population of N₂(C³Π_u) radiative state through Penning excitation involving argon metastable states. The electron temperature is determined from Ar-I spectral line intensities, using Boltzmann's plot method and is found to depend on argon fraction in the mixture.     

Optical Emission of Molecular Hydrogen in a Recombining Hydrogen Plasma

We observed optical emission of molecular hydrogen in a recombining hydrogen plasma with an electron temperature of 0.1 eV and an electron density of 3 × 10¹²cm^–3. The optical emission intensities of molecular hydrogen in the recombining plasma were roughly 10%–45% of those in an ionizing plasma with an electron temperature of 4 eV. The ratio was greater for a transition line originated from an excited state with a larger vibrational quantum number. Because of the low electron temperature of 0.1 eV, the production processes of excited states are not considered electron impact excitation in the recombining plasma. Possible recombination processes are discussed which produce excited states of molecular hydrogen in the recombining plasma (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of the dependence of the radiative properties of high-frequency electrodeless lamps in an Hg-Ar mixture on the pressure of mercury vapor

The radiative characteristics of high-frequency electrodeless lamps in a mixture of mercury and argon have been studied theoretically and experimentally as functions of the cold spot temperature (the pressure of mercury vapor). The intensity of the mercury lines at 404.7, 435.8, and 546.1 nm, corresponding to the triplet transition (7³ S ₁-6³ P _0,1,2) in the visible spectral region, as well as the intensity of the UV resonance line (6³ P ₁-6¹ S ₀) at 253.7 nm, has been measured. A model describing the physical processes in the discharge plasma and including the kinetics of excited states of mercury and argon atoms has been suggested. The parameters of the discharge plasma and the electromagnetic field have been calculated self-consistently through the numerical solution of the system of equations of electron density and energy balance and population balance of excited levels of argon and mercury atoms, as well as the Maxwell equations. The model developed has allowed us to calculate the intensities of the mercury emission lines at 253.7, 404.7, 435.8, and 546.1 nm and to compare the results with experimental data. The relative intensities of the mercury spectral lines corresponding to the triplet transition 7³ S ₁-6³ P _0,1,2 have been calculated for the first time on the basis of a self-consistent model of the discharge.     

Fe II level populations in a hollow cathode discharge

Relative populations of 232 excited levels of the Fe⁺ ion have been measured in a hollow cathode discharge in neon and in argon. The population distribution shows that thermal charge exchange between noble gas ions and ground level Fe atoms is responsible for most of the excited Fe⁺ population. the cross section for thermal charge exchange is enhanced when the excited Fe⁺ final level has the same 3d⁶(⁵D) core configuration as the initial Fe atom ground level.     

Characteristics of an argon rf plasma: Active discharge and laminar sonic flow region

The electron density, the excitation and the gas temperatures have been obtained from optical spectrometric, microwave interferometric and thermocouple measurements in an argon rf plasma. The investigation was carried out in an active discharge and a channel region characterized by sonic laminar flow. Optical measurements of the excitation temperature were made in the 4000–5000 Å range for discharge pressures of ∼ 63–644 torr and input rf power of 42–60 kW. Excitation temperatures in the discharge ranged between ∼ 7000–10,000 K. Electron densities measured by optical and microwave techniques showed good agreement. Thermocouple measurements in the channel and N₂ rotational spectra from traces of N₂ injected in the argon, as well as gas dynamic considerations, indicated that the gas temperature in the discharge and the channel regions were 2900–4400K and 1900–3300K, respectively. These values were substantially lower than the excitation temperatures corresponding to these regions, indicating that the plasma was in a two-temperature state in both regions. Standard tests for local thermal equilibrium (LTE) showed that the first excited level of argon constituted the bottleneck level.     

Level populations in plasmas RF discharges and sonic channel

An absolute intensity calibration has been made to previously reported spectrographic measurements in an extensively studied argon rf discharge downstream of the exciting coil, and sonic afterglow. This calibration allows calculation of neutral argon atom density and of individual level population. Neutral atom densities were calculated from the electron density, the electron temperature, and Saha's equation, assuming that the electron temperatures were equal to the excitation temperatures. The densities calculated by these methods were far higher than those obtained from the perfect gas law, the gas temperature, and the pressure. The observed under- population of the ground level is shown to correspond to the “terminal non-equilibrium” defined by PARK.⁽³⁾ The present data are compared with two existing experiments to show that agreement exists among seemingly inconsistent and confusing reports, provided the non-equilibrium phenomena are properly accounted for.     

Electron terms and resonant charge exchange involving oxygen atoms and ions

The electron terms are constructed for oxygen dimer ions at large ion-atom distances taking into account a certain scheme of summation of electron momenta on the basis of a hierarchy of various ion-atom interactions. Because the number of interaction types exceeds that in the Hund scheme, a realistic hierarchy of interactions and corresponding quantum numbers of the diatomic ion are outside the Hund coupling scheme. Electron terms are evaluated for the oxygen dimer ion in the case where the ground and first excited states of an atom and an ion belong to the respective valence electron shells p⁴ and p³ and correspond to the range of separations that determine the cross sections of resonant charge exchange in plasma. These electron terms allow us to calculate the partial and average cross sections for resonant charge exchange involving an oxygen ion and atom in the ground and first excited states in the range of collision energies of interest for oxygen plasmas. The specific features of electron terms of the oxygen ion dimer and the cross section of electron transfer are analyzed.     

Impact of the Vibrationally Excited States on the Macroscopic Behaviour of the Band-like Electron Beam Discharge Plasma in H/H2-Mixture

The investigation of the impact of the vibrationally excited molecules in the electronic ground state was performed by simultaneously solving a balance equation system for the main charge carriers, the H atoms, the metastable H atoms, the H₂ molecules in the different vibrational states and for the power transfer of the electrons in the beam discharge mixture plasma. The balance equations for the vibrational states include in particular one-quantum step excitation and deexcitation, electronic excitation, dissociation and ionization from each vibrational level in electron collisions as well as the finite life time of these states because of the gas transfer through the band-like plasma. A main finding is that due to the additional impact of vibrationally excited molecules there is a marked enhancement of the resulting dissociation and ionization degree in the beam discharge plasma at medium power input from the turbulent electric field. For discharge parameters of practical interest the ionization and dissociation budget, the population of the vibrational states, the different energy dissipation processes and the energy pumping into the ladder of the vibrational states were calculated and discussed in detail.     

Autoionization cross section of potassium atom excited by electron impact

The autoionization cross section of potassium atoms excited by electron impact is measured in the energy range from the first autoionization threshold at 18.72 eV to 202 eV. The data are obtained by deter-mining the total intensity of electron spectra resulting from the decay of the 3p ⁵ n ₁ l ₁ n ₂ l ₂ autoionizing states. The cross section has two maxima, 1.8 × 10^?16 and 2.2 × 10^?16 cm², at 21 and 32 eV, respectively. The excitation dynamics of autoionization states suggests that the first maximum is associated with the resonance character of the near-threshold excitation. The second maximum, as well as the behavior of the cross section at energies above 50 eV, reflects the dynamics of electron excitation of quartet and doublet autoionizing states. The measured autoionization cross section is compared with known data for the total single ionization cross section of potassium atom by electron impact. The relative contribution of the autoionization cross section to the total single ionization cross section is found to reach 30% at 32 eV.     

Role of atomic inelastic collisions in a Cs-He steady state discharge

The population of various excited states and the electron temperature in a mixture of cesium vapor and helium have been measured spectroscopically in a stationary electrical discharge where helium atoms remain in their ground state. An equilibrium between the populations of highly excited cesium levels was observed to be characterized by a Boltzmann temperature equal to the gas temperature. It is concluded that the populations of these high levels are more influenced by Cs¹+He inelastic collisions than by Cs¹+e collisions when [He]/[e]?10⁶. The influence of the helium atoms has also been observed on the relation between electron temperature and electron density.     

The quenching of Pb(6p 2 1 D 2) atoms in collisions with molecules

The collisions of metastable Pb(6p ^{2 1} D ₂) atoms with various molecules were studied by the diagnostics of radiation from a hollow cathode lamp and a laser on lead vapor. Experiments were performed for a gas flow of lead atoms with argon. The Pb(6p ^{2 1} D ₂) states were excited in a gas discharge in the presence of reagent gas molecules. The absolute rate constants for the quenching and chemical reactions of lead atoms in the ground and excited states were determined. The quantum efficiency of chemical reactions was close to one for the N₂O, CH₂Cl₂, SF₆, and CuBr molecules. Long-lived chemical compounds were formed in these reactions.     

Measurements of the spatially resolved electron temperature and plasma potential in ferrite-core side type Ar/He inductively-coupled plasmas

Spatial distributions of the effective electron temperature (T_eff) and plasma potential were studied from the measurement of an electron energy probability function in a side type ferrite-core inductively coupled plasma with an argon–helium mixture. As the helium gas was diluted at the fixed total gas pressure of 5 mTorr in an argon discharge, the distribution of the plasma density and plasma potential changed from a concave to a flat profile, and finally became a convex profile, while all spatial profiles of T_eff were hollow shapes with helium dilution in the argon discharge. This evolution of the plasma potential with helium gas could be explained by the increased energy relaxation length (λ_ε), indicating the transition of electron kinetics from local to non-local kinetics.     

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.