An investigation into the role of metastable argon atoms in the afterglow plasma of a low pressure discharge

An investigation into the behaviour of metastable argon atoms in a low pressure (250 Pa) pulsed electrical discharge was undertaken in an effort to find the cause of the persisting emission from sputtered metal atoms in the afterglow of an atomic fluorimeter. Results obtained by time-resolved emission and absorption measurements of several argon and copper spectral lines indicate that low energy electrons in the afterglow are converted to high energy electrons via the recombination of electrons with argon ions and the subsequent collisions of pairs of metastable argon atoms. The high energy electrons excite the sputtered metal atoms to give rise to a slow decaying emission tail in the afterglow. A probable change in the electron energy distribution in the afterglow may also have an effect on the observed emission. This phenomenon may be reduced by the use of a suitable quenching gas.     

Some fundamental measurements of the atmospheric-pressure,microwave-induced discharge

Measurements of several fundamental parameters in a microwave-induced atmospheric-pressure flowing plasma are presented. Optical and electrical measurements were performed on argon and argon/nitrogen plasmas in the region 1–7 cm outside the cavity, as the applied microwave power, and plasma composition were varied.The stability of the plasma, atomic emission from the argon support gas, and emission from the analyte species, are proportional to the electron density. The observed electron density was varied when the power was changed,-when an electrophilic species was added, and as the observation zone was moved relative to the microwave field. In all cases, the change in the emission signal followed the change in electron density.The electron temperature, as measured by the double-probe method, is related to the kinetic energy of the fastest-moving electrons in the plasma. It is unchanged by variations in power, plasma gas composition, flow rate, and is independent of the location of the probes relative to the cavity. The spectroscopic and electrical data are consistent with excitation by ion—electron radiative recombination.     

Electronic perturbation investigations into excitation and ionization in the millisecond pulsed glow discharge plasma

This study employed a power perturbation method to examine the energy transfer processes at different locations within the afterpeak regime of a millisecond pulsed glow discharge plasma. Brief power perturbation pulses were applied during the afterpeak regime altering the environment of the collapsing plasma. Responses of several transitions to the power perturbations were measured via atomic emission and absorption spectroscopic methods at various distances from the surface of the cathode. The experimental data provide further insight into the energy transfer processes that occur at different spatial locations and in different temporal regimes of these pulsed glow discharge plasmas. Although the enhancement of the large population of metastable argon atoms is again confirmed, the mechanism responsible for this enhancement remains unclear. The most likely possibility involves some form of ion–electron recombination followed by radiative relaxation of the resulting species. The metastable argon atoms subsequently Penning ionize sputtered copper atoms which then appear to undergo a similar ion–electron recombination process yielding variable degrees of observable afterpeak emission for copper atom transitions. The kinetic information of these processes was approximated from the corresponding relaxation time. The electron thermalization time allowing for recombination with ions was found to be ∼25 μs after the discharge power termination.     

Non-thermal features of atmospheric-pressure argon and helium microwave-induced plasmas observed by laser-light Thomson scattering and Rayleigh scattering

Laser-light Thomson scattering and Rayleigh scattering have been measured from a microwave-induced plasma sustained at atmospheric pressure, using both argon and helium as a support gas. The measurements were performed at several spatial positions in each plasma, and at forward microwave power levels of 350 W for argon, and at 350 W and 100 W for helium. It was found from these measurements that both argon and helium plasmas deviate substantially from local thermodynamic equilibrium (LTE), Measured electron temperatures range from 13 000–21 500 K, whereas gas temperatures are generally lower by a factor of 2 to 10, depending on the support gas and the spatial position in the discharge. At the same forward microwave power, the electron temperature of the helium plasma is about 3500–7000 K higher than that of the argon plasma. Yet, the argon plasma has a higher electron number density than the helium plasma. Electron number densities in both argon and helium plasmas are roughly two to three orders of magnitude lower than what LTE would predict, based on the measured electron temperatures and the Saha Equation. Even more interestingly, signals in the far-wing portion of the Thomson-scattering spectrum were found to be significantly higher than are predicted by a fitted Maxwellian curve, indicating that there exists an over-population of high-energy electrons. It is concluded that, compared to the inductively coupled plasma, the microwave-induced plasma is highly non-thermal and remains in an ionizing mode in the analytical zone.     

Evaluation eines Elektronendiffusionsmodelles zur Berechnung von nicht Gleichgewichts-Elektronenkonzentrationen in Induktiv gekoppelten Argonplasma für die spektrochemische Analyse

The electron density in argon ICP discharges has been found experimentally by other investigators to be higher than that calculated from the temperature distribution and Saha equation assuming local thermodynamic equilibrium (LTE). The results of the present study suggest that this non equilibrium concentration has mainly two causes: first, the kinetic energy of electrons owing to the power input of the rf field is higher than the kinetic energy of the gas particles. Second, as an effect of the extremely high gradients in the electron density and the temperature distribution, ambipolar diffusion of electrons results in a non LTE situation. With the help of the ambipolar diffusion constant and with recombination being taken into account, the electron concentration and the electron temperature in an ICP have been calculated. The so calculated electron density distributions are compared with literature values, found experimentally by other investigators. Finally a new model is proposed which explains the high ion concentration found experimentally for important analytical species.     

Boltzmann statistical consideration on the excitation mechanism of iron atomic lines emitted from glow discharge plasmas

A Boltzmann plot for many iron atomic lines having excitation energies of 3.3–6.9 eV was investigated in glow discharge plasmas when argon or neon was employed as the plasma gas. The plot did not show a linear relationship over a wide range of the excitation energy, but showed that the emission lines having higher excitation energies largely deviated from a normal Boltzmann distribution whereas those having low excitation energies (3.3–4.3 eV) well followed it. This result would be derived from an overpopulation among the corresponding energy levels. A probable reason for this is that excitations for the high-lying excited levels would be caused predominantly through a Penning-type collision with the metastable atom of argon or neon, followed by recombination with an electron and then stepwise de-excitations which can populate the excited energy levels just below the ionization limit of iron atom. The non-thermal excitation occurred more actively in the argon plasma rather than the neon plasma, because of a difference in the number density between the argon and the neon metastables. The Boltzmann plots yields important information on the reason why lots of Fe I lines assigned to high-lying excited levels can be emitted from glow discharge plasmas.     

Spectroscopic characterization of a neon surface-wave sustained (2.45 GHz) discharge at atmospheric pressure

In this paper a spectroscopic study of a microwave (2.45 GHz) neon surface-wave sustained discharge (SWD) at atmospheric pressure in a quartz tube has been carried out in order to determine the plasma characteristic parameters (e.g. electron temperature and density, gas temperature, absorbed power per electron) and also to identify possible deviations from the thermodynamic equilibrium for this kind of microwave discharge. The results have been compared to experiments in the literature for other noble gas (helium and argon) SWDs generated under similar experimental conditions. Intermediate values between those of argon and helium plasmas were obtained for characteristic neon plasma parameters (temperatures and electron density). An important departure from the Saha equilibrium was exhibited by neon SWDs.     

Recombination-pumped triatomic hydrogen infrared lasers

Mid-infrared laser lines observed in hydrogen/rare gas discharges are assigned to three-body recombination processes involving an electron, a rare gas (He or Ne) atom, and the triatomic hydrogen ion (H(3)(+)). Calculations of radiative transitions between neutral H(3) Rydberg states support this interpretation, and link it to recent results for hydrogenic∕rare gas afterglow plasmas. A mechanism for the population inversion is proposed, and the potential generality and astrophysical implications of such molecular recombination laser systems are briefly discussed.     

Appreciation of a collisional-radiative model for the description of an inductively coupled argon plasma

The collisional-radiative model has been applied to the argon ICP discharge in order to elucidate the excitation mechanism in the plasma. The population density distributions of 25 argon energy levels were calculated under a steady-state approximation by using the literature values of electron number density, 5 × 10 ¹⁴cm^?3 and electron temperature, 9000 K. In the case of an optically thin plasma, in which the induced absorption can be neglected, the calculated population densities showed an overpopulation for low lying states, and were very close to LTE values for the upper levels. These results suggest the following excitation mechanisms in the argon ICP; corona model for lower levels and ladder-like excitation and ionization by electron impact for upper levels. According to the present calculation, the non-overpopulation of argon metastable can be interpreted by the interconversion between metastable and radiative states. It has been found that the induced absorption of resonance lines in an optically thick plasma and the motion of species in an inhomogeneous plasma have significant effects on the population densities. The non-linear processes by collision between heavy particles were not predominant compared to electron impact processes.     

Optical emission spectroscopy of electron-cyclotron-resonance-heated helium mirror plasmas

In this experiment emission spectroscopy in the 3000–5000 Å range has been utilized to determine the electron temperature (15–60 eV) and ion density (2–5 x 10¹¹ cm^–3) of helium plasmas produced by the Michigan mirror machine⁽¹⁾ (MIMI). The plasma is generated and heated by whistler-mode electron-cyclotron resonance (ECR) waves at 7.43 GHz with 400–900 W power in 80-ms-long pulses. Gas fueling is provided at the midplane region by a leak valve with a range in pressure of 3 x 10 to 2 x 10⁴ Torr. Emission line intensities are interpreted using a model of the important collisional and radiative processes occurring in the plasma. The model examines secondary processes such as radiation trapping, excitation transfer between levels of the carne principle quantum number, and excitation front metastable states for plasmas in the parameter range of MIMI (n _c = 1–6 x 10¹¹ cm^–3). Front the analysis of line intensity ratios for neutral helium, the electron temperature is measured and its dependence upon the gas pressure and microwave power is determined. These temperatures agree with those obtained by Langmuir probe measurements. Art analysis of the line intensity ratio between singly ionized helium and neutral helium yields a measurement of the ion density which is in good agreement with electron density measurements made by a microwave interferometer.     

Absorption spectroscopy measurements of resonant and metastable atom densities in atmospheric-pressure discharges using a low-pressure lamp as a spectral-line source and comparison with a collisional-radiative model

The well-known absorption method for measuring metastable and resonant atom densities in reduced-pressure discharges using a low-pressure lamp as a spectral-line source is extended to plasmas sustained at atmospheric pressure where the line profiles are of the Voigt type rather than Gaussian. This technique is further applied to determine the axial distribution of metastable and resonant atom densities in a tubular argon surface-wave (microwave) discharge. At atmospheric pressure, the discharge is radially contracted and determination of the absorption length through lateral probing of the tube becomes a critical issue. Good agreement of the metastable and resonant atom densities calculated from a collisional-radiative model with the measured values supports the validity of the method developed. The model underlines the influence of the molecular ion kinetics on the Ar I 4s level population and reveals an efficient 3-step ionization process involving the 4s and 4p levels for atomic ion formation.     

Consideration on excitation mechanisms in a high-power two-jet plasma

The study of excitation mechanisms in the region before the jet confluence of a high-power two-jet plasma used for analysis of different powders has been undertaken. Distribution of excited levels of Fe atoms and ions according to the Boltzmann population was found. Measuring Fe atomic and ionic excitation temperatures showed their considerable difference (≈ 2000–2500 K). The effect of argon on line intensities of a wide range of elements was investigated by the experiment with argon covering. A negligible effect of argon covering on line intensities of atoms with ionization energy of < 8 eV allows one to assume their predominant excitation by electron impact. The argon participation in excitation of atoms having ionization energy of > 8 eV was revealed. This is likely to be due to Penning ionization by metastable argon followed by ion recombination with an electron and stepwise de-excitations. A more pronounced effect of argon covering was observed for ionic lines of investigated elements with total excitation energy ranging from 11 to 21 eV. Penning ionization followed by electron impact is believed to be a probable mechanism for ion excitation. The contribution of metastable argon to excitation processes results in departure from local thermodynamic equilibrium and different atomic and ionic excitation temperatures.     

Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production

Time resolved atomic emission, atomic absorbance, and laser-induced atomic fluorescence measurements of a millisecond pulsed glow discharge, made perpendicular to the insertion probe, provide temporal profiles of 1s₅ (³P₂) and 1s₃ (³P₀) metastable argon atom populations. Acquisition of these profiles at different spatial positions in the plasma provides data from which two-dimensional spatial plots of relative populations are constructed. Each map, the result of 368 individual pulse profiles, provides insight into the production of metastable argon atoms as a function of time and position within the plasma. During power application, intensities plateau after 3 ms as the plasma reaches a steady state condition. Metastable argon atoms are most abundant 1–2 mm above the cathode surface during this time. Excitation mechanisms such as electron excitation and fast atom/ion impact appear to dominate in this temporal regime. In contrast, argon ion–electron recombination dominates metastable formation after pulse termination. The relative population maximum for metastable argon atoms in the afterpeak shifts to 5–9 mm above the cathode surface. This shift should impact signals for analyte species generated by Penning processes in the plasma. Absorption and fluorescence measurements of the ³P₂ (11.55 eV) and the ³P₀ (11.72 eV) metastable argon atom states indicate possible differences in the populations of these two states between the plateau and afterpeak time regimes.     

Effects of Vibrational Nonequilibrium on the Chemistry of Two-Temperature Nitrogen Plasmas

A new two-temperature chemical kinetics model for nitrogen plasmas is presented. The model is used together with the vibrationally-specific collisional-radiative model to study the effects of vibrational nonequilibrium distributions on the chemical composition of two-temperature atmospheric pressure nitrogen plasmas. It is found that over a wide range of conditions the vibrational levels follow Boltzmann distributions and that the vibrational temperature T_v is well approximated by gas temperature T_g at low electron number densities and by electron temperature T_e at high electron number densities. This result suggests that simple kinetic models with two-temperature rate coefficients can be used to reliably model nonthermal plasmas. The calculation also yields a surprising result that, for a given constant gas temperature, the steady-state electron number density exhibits an S-shaped dependence on the electron temperature. This S-shaped behavior is caused by competing ionization, charge transfer reactions, two-body dissociative recombination, and three-body electron recombination reactions, and therefore is characteristic of molecular plasmas.     

Electron production and loss processes in a spectrochemical inductively coupled argon plasma

The behavior of inductively coupled plasmas for spectroscopic purposes has been studied extensively in the past. However, many questions about production and loss of electrons, which have a major effect on this behavior, are unanswered. Power interruption is a powerful diagnostic method to study such processes. This paper presents time resolved Thomson scattering measurements of the electron density n_e and temperature T_e in an inductively coupled argon plasma during and after power interruption. In the center of the plasma the measured temporal development of n_e and T_e can be attributed to ambipolar diffusion, three-particle recombination and ionization. However, at the edge of the plasma an additional electron loss process must be involved. In addition, the high electron temperature during power interruption indicates the presence of an electron heating mechanism. The energy gain by recombination processes is shown to be insufficient to explain this electron heating. These discrepancies may be explained by the formation and destruction of molecular argon ions, which can be present in significant quantities.     

Spatially resolved spectroscopic diagnostics of an argon MIP produced by surface wave propagation (Surfatron)

Spatially resolved spectroscopic diagnostics of an argon MIP have been obtained after Abel inversion. The plasma is sustained by surface wave propagation and is working at atmospheric pressure. The inner diameter of the discharge tube has been selected to obtain one stable discharge filament. Electronic excitation temperatures with argon and rotational temperatures of some molecular species (OH and N₂⁺), electron number and metastable densities and absolute values of the continuum emission coefficient have been determined. In contrast to ICPs and DCPs, the origin of the continuum in the visible part of the spectrum cannot be attributed only to radiative recombination.     

Electron emission from naphthacene crystal due to the impact of argon and krypton metastable atoms

Energy distributions of electrons emitted from polycrystalline naphthacene due to the impact of metastable argon or krypton atoms were measured. The energy distribution peaks, except for large peaks appearing near zero eV, correspond to the kinetic energies estimated from photoelectron spectra on the assumption that the excitation energies of the metastable atoms are transferred to the electrons in the valence bands. The results are interpreted as the occurrence of Penning ionization (Auger de-excitation) on the naphthacene surface.     

Statistical models in the polarization radiation theory

The survey is devoted to general results for polarization free–free and free–bound radiative transitions in collisions of charged particles with heavy atoms and ions following from statistical atomic models. The atomic plasma model results for dynamic atomic polarizabilities are presented. Polarization Bremsstrahlung in collisions of fast and moderate energy electrons with Thomas–Fermi atoms is analyzed in details. A new polarization electron-heavy ions recombination channel is considered and compared with known radiative recombination channel. It is shown that recombination channels can be compared or even dominates over standards static radiation channels. Polarization radiation of fast multicharged ions in condensed media is also under consideration.     

Radiation effects, energy storage and its release in solid rare gases

An irradiation of solid argon sample by electrons ionizes the Ar atoms, and part of the beam energy is stored in the solid mainly in the form of self-trapped Ar(2)(+) holes. The pre-irradiated samples are investigated by methods of the so called "activation spectroscopy". During their controlled warm-up three thermally stimulated effects are observed and, in our experiments, simultaneously monitored: a VUV emission resulting from neutralization of the Ar(2)(+) holes by electrons, an anomalous desorption of surface atoms, and an exoelectron emission. A comparison of experiments with linear and step-wise sample heating shows clearly that all three processes are intimately connected. The heating detraps electrons, which neutralize the Ar(2)(+) holes resulting in a bound-free emission of argon dimers, centered around 9.7 eV. The excess energy set free during this process may dislodge surface atoms leading to an anomalous, low temperature, pressure rise. Some of the electrons can also be directly extracted from the sample and detected as an exoelectron current. The experiments provide information about the depth of electron traps, and indicate that there is a nearly continuous distribution of trapping energies.     

On the electron temperatures and densities in plasmas produced by the “torche à injection axiale”

The electron temperature and the electron density of plasmas created by the “Torche à Injection Axiale” (TIA) are determined using Thomson scattering. In the plasma with helium as the main gas, temperatures of around 25 000 K and densities of between 0.64 and 5.1 × 10²⁰m⁻³ are found. In an argon plasma the electron temperature is lower and the electron density is higher: 17 000 K and around 10²¹ m⁻³ respectively. From these results it can be established that the ionisation rates of both plasmas are much larger than the recombination rates, which means that the plasmas are far from Saha equilibrium. However, deviations from a Maxwell electron energy distribution function, as reported for the “Microwave Plasma Torch” (MPT), are not found in the TIA. The excitation and ionisation power of the TIA appears to be stronger than that of the MPT.