Emission characteristics of Grimm-style glow discharge plasmas with helium matrix plasma gas containing small amounts of nitrogen

Glow discharge plasmas with helium–(0–16%) nitrogen mixed gas were investigated as an excitation source in optical emission spectrometry. The addition increases the sputtering rate as well as the discharge current, because nitrogen molecular ions, which act as primary ions for the cathode sputtering, are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. The intensity of a silver atomic line, Ag I 338.29 nm, is monotonically elevated along with the nitrogen partial pressure added. However, the intensities of silver ionic lines, such as Ag II 243.78 nm and Ag II 224.36 nm, gave different dependence from the intensity of the atomic line: Their intensities had maximum values at a nitrogen pressure of 30 Pa when the helium pressure and the discharge voltage were kept at 2000 Pa and 1300 V. This effect is principally because the excitations of these ionic lines are caused by collisions of the second kind with helium excited species such as helium metastables and helium ion, which are quenched through collisions with nitrogen molecules added to the helium plasma. The sputtering rate could be controlled by adding small amounts of nitrogen to the helium plasma, whereas the cathode sputtering hardly occurs in the pure helium plasma.     

Emission characteristics of Grimm-style glow discharge plasmas with helium matrix plasma gas containing small amounts of nitrogen

Glow discharge plasmas with helium–(0–16%) nitrogen mixed gas were investigated as an excitation source in optical emission spectrometry. The addition increases the sputtering rate as well as the discharge current, because nitrogen molecular ions, which act as primary ions for the cathode sputtering, are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. The intensity of a silver atomic line, Ag I 338.29 nm, is monotonically elevated along with the nitrogen partial pressure added. However, the intensities of silver ionic lines, such as Ag II 243.78 nm and Ag II 224.36 nm, gave different dependence from the intensity of the atomic line: Their intensities had maximum values at a nitrogen pressure of 30 Pa when the helium pressure and the discharge voltage were kept at 2000 Pa and 1300 V. This effect is principally because the excitations of these ionic lines are caused by collisions of the second kind with helium excited species such as helium metastables and helium ion, which are quenched through collisions with nitrogen molecules added to the helium plasma. The sputtering rate could be controlled by adding small amounts of nitrogen to the helium plasma, whereas the cathode sputtering hardly occurs in the pure helium plasma.     

A microwave induced plasma system for the maintenance of moderate power plasmas of helium,argon, nitrogen and air

A microwave induced plasma system capable of maintaining stable plasmas of each of the gases helium, argon, nitrogen and air is presented. The system is capable of operation at powers of up to 500 W. The TM₀₁₀ cavity design is similar to that previously described in the literature with some modifications. A demountable torch facilitates centering of diffuse plasmas of helium, nitrogen and air by providing 6 flows directed tangentially within the quartz tube. This torch was not useful for argon plasmas. Toroidal argon plasmas were maintained with a threaded quartz tube arrangement. The heat generated by these plasmas was dissipated by an outer sheath of coolant air. Details of the design and preliminary characterization of each plasma system is presented.     

Selective excitation of singly-ionized silver emission lines by Grimm glow discharge plasmas using several different plasma gases

The relative intensities of silver emission lines from Grimm glow discharge plasmas were investigated in the wavelength range from 160 to 600 nm when using different plasma gases. It was characteristic of the plasma excitation that the spectral patterns were strongly dependent on the nature of the plasma gas employed. Intense emission lines of silver ion were observed when argonhelium mixed gases were employed as the plasma gas. Selective excitation of the ionic lines could be principally attributed to the charge transfer collisions between silver atoms and helium ions.     

Development of soft plasma ionization (SPI) source for analysis of organic compounds

We present a newly designed soft plasma ionization (SPI) source developed for mass spectrometric study of organic compounds in this study. The SPI cell having a relatively small size consists of a hollow anode and a hollow mesh cathode. The voltage–current characteristic depending on the pressure was investigated, indicating that it has similar characteristics to conventional hollow cathode glow discharges. To investigate the emission characteristics of the SPI source, some molecular band emission spectra (N₂, N₂⁺ and OH⁺) were measured by using argon and helium discharge gases. The SPI source was installed to a commercially used quadrupole mass analyzer for analyzing organic compounds. To demonstrate the SPI source, the mass spectra of some organic compounds (methylene chloride, toluene, benzene, cyclohexane and chloroform) were measured. The organic compounds were ionized with good stability in the plasma, and the fragmentation depended on the applied current. When helium and argon gases were used as the discharge gas, the helium plasma was more suitable for SPI-MS rather than argon because the argon plasma not only suffers from spectral interference but also has lower sensitivity.     

The development of microplasmas for spectrochemical analysis

Miniaturized microwave, high-frequency, and dc-powered microplasmas are discussed, with emphasis on the state-of-the-art and development trends. Specific atomic emission sources discussed include the microstrip microwave plasma operated in argon and helium at ca 10-30 W and below 1 L min(-1) gas at atmospheric pressure, the capacitively coupled microplasma, operated at 13.56 MHz, 5-25 W, and 17-150 mL min(-1) helium, the miniaturized inductively coupled plasma operated at several watts and reduced pressure, and dc glow-discharge plasmas on a chip, including a barrier-layer discharge as atom reservoir for atomic absorption spectrometry. Diagnostics for these sources are discussed and some of their figures of merit are compared with those of conventional sources. Current possibilities for introduction of gaseous samples are reported and scope for further development and outlook are both discussed.     

Fundamental studies of mixed-gas inductively coupled plasmas

The effects of adding foreign gases to the central-gas flow or the intermediate-gas flow of an argon inductively coupled plasma are presented. In particular, the influence of up to 16.7% added helium, nitrogen or hydrogen on radially-resolved electron number density, electron temperature, gas-kinetic temperature and calcium ion emission profiles is examined. It is shown that these gases affect not only the fundamental parameters and bulk properties of the plasma, but also how energy is coupled and transported through the discharge and how that energy interacts with the sample. For example, added helium causes an increase in the gas-kinetic temperature, most likely due to the higher thermal conductivity of helium compared to argon but, in general, does not appear to affect significantly either the electron temperature or electron concentration. The shift in the calcium ion emission profile towards lower regions in the discharge with added helium may be attributable to higher droplet desolvation and particle vaporization rates. In contrast, the addition of nitrogen or hydrogen to an Inductively Coupled Argon Plasma (Ar ICP) results in dramatic changes in all three fundamental plasma parameters: electron number density, electron temperature, and gas-kinetic temperature. The net effect of these molecular gases (N₂ or H₂) on calcium ion emission and on the fundamental plasma parameters is shown to be dependent on the amount of gas added to the plasma and whether the gas is introduced as part of the central- or intermediate-gas flow. In general, nitrogen added to the central-gas flow causes a significant reduction in the number of electrons throughout most of the discharge (over an order of magnitude in certain regions), mainly in the central and upper zones of the ICP. A drop of 3000–5000 K in the central channel electron temperature and a smaller drop in the gas-kinetic temperature are also observed when N₂ is added to the central-gas flow. In contrast, the introduction of nitrogen in the intermediate flow causes about a 1 × 10¹⁵ electrons cm⁻³ increase in the electron concentration in the low, toroidal regions of the plasma and an increase in the gas-kinetic temperature of around 1000 K throughout most of the discharge. As seen with the addition of nitrogen to the central-gas flow, the electron temperature is found to increase in the toroidal zones of the plasma when N₂ is added to the intermediate flow. These combined effects cause a 20-fold depression in the calcium ion emission intensity only a 1.7-fold depression when N₂ is added to the central- or intermediate-gas flows, respectively. On the other hand, hydrogen causes a depression in the electron concentration in the upper areas of the plasma when this gas is added to the central flow but increases the number of electrons in the same region when added to the intermediate flow. Hydrogen also causes a dramatic effect on the electron and gas-kinetic temperatures, significantly increasing both of these parameters throughout the discharge. An increase in the calcium ion emission intensity, accompanied by a downward shift, elongation and broadening of the calcium ion emission profile is also observed with H₂ addition.     

Excitation of singly-ionized argon species in helium-matrix Grimm glow discharge plasmas II – Comparison between argon and neon

A considerable intensity enhancement of several Ar II lines assigned to the 3p(4)4p-3p(4)4s transition in a helium-argon Grimm glow discharge plasma has been previously reported and attributed to argon ions excited by metastable helium atoms. In this paper the behavior of Ne II lines assigned to the 2p(4)3p-2p(4)3s transition in a helium-neon plasma was investigated to obtain detailed information on the excitation of plasma gases in the helium-matrix plasmas. No Ne II lines with enhanced emission intensities have been found; on the contrary, the intensities of the doublet Ne II lines decreased in the helium-matrix plasma.     

Characterization of High Voltage Cold Atmospheric Plasma Generation in Sealed Packages as a Function of Container Material and Fill Gas

While numerous experiments have demonstrated the efficacy of high voltage cold atmospheric pressure plasmas (HVCAPs) in sealed packages for microbial inactivation, the influence of the package on the emitted species measured during HVCAP discharge is poorly understood. This study elucidates the impact of the package on plasma generation in sealed packages for four separate gases (ambient air, commercial grade compressed air, a helium/air mixture, and nitrogen) placed in commercially available transparent plastic containers and bags representative of the materials used in the food industry. The container and bag individually reduced emission signal intensity by an average of 63 and 45%, respectively, across the measured wavelengths of 200–1100 nm, demonstrating that they acted as broadband absorbers. Neither the container nor bag caused additional emission lines to appear, indicating no significant effect on the types of species generated. Considering the minimum applied voltage necessary to induce a discharge, the power dissipated by the nitrogen and ambient air plasma generated at 72 ± 3.7 kV RMS were comparable to the compressed dry air discharge generated at 80 ± 3.7 kV RMS. The helium discharge at 37 ± 3.7 kV RMS absorbed approximately 92% more power than these gases. Rotational temperatures ranged from 285 K for helium to 479 K for compressed air. These results indicate that the package impacts the intensity distribution but not the presence of the most dominant peaks, although further studies are required to elucidate the impact on less intense peaks.     

Applicability of microwave induced plasma optical emission spectrometry (MIP-OES) for continuous monitoring of mercury in flue gases

The applicability of microwave-induced plasma optical emission spectrometry (MIP-OES) for continuous monitoring of the environmentally hazardous element mercury in flue gases has been studied. Microwave induced plasmas have been sustained using both a TM₀₁₀ cavity (Beenakker resonator) and a so-called Surfatron. The analytical figures of merit for mercury in argon and helium discharges with both types of low-power micro-wave discharges have been examined. To determine mercury in artificial stack gases non-mixed argon/nitrogen discharges have been tested using a tangential flow torch design which allows to introduce a metal-loaded nitrogen gas flow as external gas and argon as internal gas. The addition of main flue gas components such as water vapour (concentration <6 g/m³), oxygen (<4% v/v) and carbon dioxide (<15% v/v) decrease the mercury line intensities to a considerable extent. Trace gases (CO, HCl, SO₂, NO) in concentrations typical to waste incineration processes have been found to have no effect on the mercury and the argon line intensities. The detection limit of mercury in nitrogen is 8 g/m³ using the TM₀₁₀ MIP and 10 g/m³ using the Surfatron. As such low detection limits are below the emission limit values of present-day environmental legislation MIP-OES is useful for on-line monitoring of mercury.Dedicated to Professor Dr. Dieter Klockow on the occasion of his 60th birthday     

Electrical Features of Radio-frequency,Atmospheric-pressure,Bare-metallic-electrode Glow Discharges

Radio-frequency (RF), atmospheric-pressure glow discharge (APGD) plasmas with bare metallic electrodes have promising prospects in the fields of plasma-aided etching, deposition, disinfection and sterilization, etc. In this paper, an induced gas discharge approach is proposed for obtaining the RF, atmospheric-pressure, γ-mode, glow discharges with pure nitrogen or air as the primary plasma-working gas using bare metallic electrodes. The discharge characteristics, including the discharge mode, the breakdown voltage and discharge voltage for sustaining α mode and/or γ mode discharges, of the RF APGD plasmas of helium, argon, nitrogen, air or their mixtures using a planar-type plasma generator are presented in this study. The uniformity (no filaments) of the discharges is confirmed by the images taken by an iCCD with a short exposure time (10 ns). The effects of different gap spacings and electrode materials on the discharge characteristics, the variations of the sheath thickness and the electron number density are also studied in this paper.     

Asymmetric charge transfer with hydrogen ions — An important factor in the “hydrogen effect” in glow discharge optical emission spectroscopy

It is now well known that traces of hydrogen or nitrogen in the argon plasma gas in glow discharge optical emission spectrometry (GD-OES) may affect the sputtering rate. More seriously, such traces can also selectively affect the absolute and relative intensities of individual lines, and thereby have a major effect on the accuracy of analytical results. This problem is becoming more severe as the potential of GD-OES as an analytical tool is steadily increasing, and the technique is now used for the analysis of more complex samples.The results presented form part of an extensive study of the effects of hydrogen and nitrogen on the spectra of a number of elements — Fe, Ti, V, Ni, Zn etc. Two systems have been used to record the spectra for the present results — high resolution Fourier transform spectroscopy at Imperial College, London, and the LECO GDS500A, with CCD spectrometer at LECO Instrumente, Plzeň. The two approaches have yielded consistent results.During the analysis of the data for Fe II and Ti II spectra, it has become clear that asymmetric charge transfer involving hydrogen ions (H-ACT) is a very important selective excitation mechanism for spectral lines with a total excitation energy close to 13.6 eV, the ionisation potential of hydrogen. Detailed evidence for this mechanism is presented. The magnitude of the effect varies for different elements and spectral lines but great care must be taken before choosing ionic lines with a total excitation energy of between 12.5–14 eV for analytical use.     

Correlation Between the Electrical and Optical Properties of an Atmospheric Pressure Plasma During Siloxane Coating Deposition

The effect of varying process parameters on atmospheric plasma characteristics and properties of nanometre thick siloxane coatings is investigated in a reel-to-reel deposition process. Varying plasma operation modes were observed with increasing applied power for helium and helium/oxygen plasmas. The electrical and optical behaviour of the dielectric barrier discharge were determined from current/voltage, emission spectroscopy and time resolved light emission measurements. As applied power increased, multiple discharge events occurred, producing a uniform multi-peak pseudoglow discharge, resulting in an increase in the discharge gas temperature. The effects of different operating modes on coating oxidation and growth rates were examined by injecting hexamethyldisiloxane liquid precursor into the chamber under varying operating conditions. A quenching effect on the plasma was observed, causing a decrease in plasma input power and emission intensity. Siloxane coatings deposited in helium plasmas had a higher organic component and higher growth rates than those deposited in helium/oxygen plasmas.     

Basic investigations for laser microanalysis: III. Application of different buffer gases for laser-produced sample plumes

We have measured the diameters and depths of craters in a copper sample and the amount of material ablated by the 1.06-m radiation of a pulsed Nd: YAG laser in the buffer gases argon, neon, helium, air and nitrogen as well as the emission intensities of analyte atoms in dependence on laser power and buffer gas pressure. The results are correlated with corresponding data of the plasma temperatures and the relative electron densities in the plasma. Criteria for the choice of the buffer gas, the buffer gas pressure and the laser power for optical emission spectrometry of microplasmas are given.     

An Optical Emission Study on Expanding Low-Temperature Cascade Arc Plasmas

The optical emission spectra from expanding low-temperature cascade arc plasmas were studied. The objective of this study was to examine the distinctive features of low-temperature cascade arc plasmas in comparison with a radio frequency (RF) plasma source. The principal results obtained in this study were: (1) in an expanding cascade arc plasma jet, active heavy particles (mainly excited argon or helium neutral species under our operating conditions), rather than electrons, are responsible for the excitation of reactive species when a reactive gas is injected into the plasma jet, (2) the excitation of reactive species was found to be controlled by the electronic energy levels of these excited argon or helium neutrals, (3) changing the operating parameters affected only the emission intensities of excited species, and no effect on the emission nature of plasmas was observed.     

Two-dimensional observation of emission spectra excited from laser induced plasmas and the application to emission spectrometric analysis.

The spatial distribution analysis of emission signals from a laser-induced plasma can provide information on the excitation mechanism as well as on the optimization of the analytical conditions when it is employed as a sampling and excitation source in optical emission spectrometry. A two-dimensionally imaging spectrometer system was employed to measure spatial variations in the emission intensities of a copper sample and plasma gases when krypton, argon, or helium was employed under various pressure conditions. The emission image of the Cu I 324.75-nm line consists of a breakdown spot and a plasma plume, where the breakdown zone expands toward the surrounding gas. The shape and the intensities of the plasma plume are strongly dependent on the kind and pressure of the plasma gas, while those of the breakdown zone are less influenced by these experimental parameters. This effect can be explained by the difference in the cross-section of collisions between krypton, argon, and helium. The signal-to-background ratio of the Cu I 324.75-nm line was estimated over two-dimensional images to determine the optimum position for analytical applications.     

Relationships of the synthesis of silicon dioxide nanoparticles in low-temperature plasma created in helium and argon at atmospheric pressure

Low-temperature plasma-chemical synthesis of silicon dioxide nanoparticles from tetraethoxysilane, performed at atmospheric pressure in a reactor with flat perforated electrodes coated with an insulating layer, was studied. The use of such electrodes allows reaching stable sustaining of RF discharge (α-mode) with helium and argon used as plasma-forming gases. The relationships of the synthesis of silicon dioxide nanoparticles from tetraethoxysilane in helium and argon plasma differ insignificantly. Most probably, nanoparticles with the size in the interval 100–150 nm are formed in the zone of the discharge sustaining, but, as the particles are transported with the gas flow beyond the plasma zone, they undergo agglomeration. An increase in the nanoparticle size to 100–150 nm, observed when using perforated electrodes with an insulating coating, is most probably due to an increase in the residence time of the reaction gas medium in the zone of the discharge sustaining.     

Excitation of singly ionized argon species in helium-matrix glow discharge plasma—role of the energy transfer from helium metastables

When a small amount of argon is added to the helium plasma in a Grimm-type glow discharge radiation source, the interaction between helium and argon species is investigated from analyzing the intensities of emission lines of of argon ion (ArII). The excitation energy as well as the term multiplicity concerning the optical transitions to which the ArII emission lines are identified are significant factors for determining their emission intensities in the helium-matrix plasma. In the case where the excitation energy of ArII lines is higher than the internal energy of the helium metastable states, the emission intensity in the helium-matrix plasma is observed to be much weaker than that obtained only with argon gas. On the other hand, the intensity is enhanced when the excitation energy is slightly lower. In the excited levels of argon ion having quartet multiplicity, closer interactions with the triplet rather than the singlet metastable level of helium atom are recognized, with the singlet helium metastable in the argon excited levels having doublet multiplicity.     

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.