Excited Argon Research Articles

Theoretical analysis of argon 2p states' density ratios for nanosecond plasma optical emission spectroscopy

A theoretical analysis of excited argon state densities responsible for optical emission spectra of atmospheric pressure argon plasma is presented for its use in plasma diagnostics. Nanosecond pulsed barrier discharges are simulated using spatially one- and two-dimensional fluid-Poisson models using the reaction kinetics model presented by Stankov et al. [1], which considers all ten argon 2p states (Paschen notation) separately. The very first (single) discharge and repetitive discharges with frequencies from 5 kHz to 100 kHz are considered. A semi-automated procedure is utilized to find appropriate 2p states for electric field determination using an intensity ratio method, which is based on a time-dependent collisional-radiative model. The fluid simulations in combination with the semi-automated procedure are used to quantify the sensitivity of selected 2p-state ratios to given preionization of the gas. A highly sensitive time-correlated single photon counting experiment shows clearly that the selected ratio is sensitive to the electric field variation in the streamer head, yet additional calibration is needed for absolute values determination. Different approaches for effective lifetime determination are tested and applied also to measured data. The influence of radial and axial 2p state density integration on the intensity ratio method is discussed. The above mentioned models and procedures result in a flexible theory-based methodology applicable for development of new diagnostic techniques.

Nanometer-thick TiO2 channel thin film transistors as radical monitors for plasma enhanced atomic layer deposition

Nanometer-thick-TiO2-channel thin-film transistors (TFTs) are examined as oxidizing-agent monitors for room-temperature atomic layer deposition (RTALD). RTALD is a plasma-based process using plasma-excited humidified argon where OH radicals oxidize the precursor-gas adsorbed surface. In the TFT, the anatase TiO2 channel, with a thickness of 16 nm, is attached to a 300 nm thick gate capacitor of SiO2, while the channel surface is exposed to the ALD ambient. A heavily doped n-type Si substrate attached to the gate SiO2 is used as the gate electrode. The gate width and length are 1000 and 60 μm, respectively. When the TFT is installed in the RTALD chamber, the drain-current waveform is recorded in the course of the metal organic gas adsorption, evacuation of the residual gas, oxidization, and evacuation. In the present study, three kinds of metal organic (MO) precursors, tetrakis(dimethyl)amino titanium, tris(dimethyl)amino silane, and trimethyl aluminum are examined. The drain current exhibits strong responses upon the exposure of the plasma excited humidified argon. This suggests that OH radicals in the plasma might oxidize the adsorbed MO precursors to produce the OH moieties, where the surface polarization of OH moieties might enhance channel conductivity. The experimental results suggest the applicability of the present TFT as a plasma monitor in RTALD.

Kinetic analysis of an optically pumped rare gas lasing medium with a nanosecond repetitively pulsed discharge in Ar-He mixture

A kinetic model of a lasing medium for an optically pumped rare gas laser (OPRGL) was developed that accounts for the production of excited argon atoms Ar* in a nanosecond repetitively pulsed discharge (NRPD). Analytical formulas were derived for the frequency of Ar* radiation losses, population of the energy levels involved in the laser cycle and specific powers for pump absorption and lasing in the limit of bleaching of pump and lasing transitions. The formulas use the kinetic constants of the model as parameters and Ar* number density as the independent variable. Periodic solutions for number density of plasma components, pump absorption and small signal gain, specific pump absorption and lasing were obtained numerically. Mean over the NRPD period values of Ar* number density, specific pump absorption and lasing were calculated as the functions of the preset peak values of the reduced electric field E/N. Triangular electric field pulses of 80 ns FWHM duration and 200 kHz frequency were considered. Optical pumping increases Ar* loss by more than a factor of 10, due to excess spontaneous emission from Ar* levels that populates the 1s4 state of argon, resulting in a reduced NRPD plasma ionization and Ar* production. As a result, without auxiliary optical pumping from the 1s4 state that compensates this loss, the averaged across the discharge period Ar* number density decreases by a factor of 30. Moreover, the average specific heat release in plasma becomes almost equal to the specific lasing power in the acceptable operating modes, making auxiliary optical pumping mandatory for scaling of an OPRGL. The sensitivity of the results to the values of kinetic constants in the model is analyzed.

Hyperspectral imaging of a microwave argon plasma jet expanding in ambient air.

Non-equilibrium plasmas at atmospheric pressure are often characterized by optical emission spectroscopy. Despite the simplicity of recording optical emission spectra in plasmas, the determination of spatially resolved plasma properties (e.g., electron temperature) in an efficient way is very challenging. In this study, spatially resolved optical images of a microwave argon plasma jet expanding into the ambient air are recorded over a wide range of wavelengths using a hyperspectral imaging system based on a tunable Bragg-grating imager coupled to a scientific complementary metal-oxide-semiconductor camera. The system's working principle is detailed, along with the necessary post-processing steps. Further analysis of the spatial-spectral data, including the Abel transform used to determine 2D radially resolved spatial mappings, is also presented. Overall, the proposed approach provides unprecedented cartographies of key plasma parameters, such as argon and oxygen line emission intensities, Ar metastable number densities, and argon excitation temperatures. Considering that all these plasma parameters are obtained from measurements performed in a reasonable time, Bragg-grating-based hyperspectral imaging constitutes an advantageous plasma diagnostic technique for detailed analysis of microwave plasma jets used in several applications.

State-resolved modeling of electronic excitation in weakly ionized oxygen mixtures.

Electronic excitation and ionization in oxygen-argon mixtures are analyzed using a three-temperature electronic state-resolved model and evaluated using recent experimental data from reflected shock experiments. A detailed description of the model formulation and parameter selection is provided. Excellent agreement is obtained between model predictions and experimental measurements of O_{2} number density during dissociation in mixtures of 2%-5% O_{2} dilute in argon. Next, electron number density measurements are leveraged to infer a rate constant for the heavy particle impact excitation of argon, facilitating improved modeling of net ionization and a clearer understanding of the electronic excitation kinetics of oxygen. The electronic state-resolved model is then assessed using measured data for three electronic states of atomic oxygen. The model successfully reproduces the multistage behavior observed in the measured time histories and yields new insights into the multistage behavior that revises previous interpretations. For several experiments, the modeling choices involved in the calculation of escape factors significantly influence the predicted time histories. A global sensitivity analysis considering nearly 300 parameters is then conducted to identify which model parameters most sensitively influence the predicted excited state populations. Excitation of the measured states from the metastable levels and collisional excitation between the three measured states are important across all conditions. The excited state populations demonstrate complex sensitivities involving a large number of collisional and radiative processes, highlighting the importance of adopting a detailed modeling approach when interpreting excited state measurements.

Modeling of the influence of field electron emission from a cathode with a thin insulating film on its sputtering in a gas discharge in a mixture of argon and mercury vapor

A model of the low-current gas discharge in a mixture of argon and mercury vapor in the presence of a thin insulating film on the cathode surface is proposed. The model takes into account that in such a mixture a substantial contribution to the ionization of the working gas can come from the ionization of mercury atoms during their collisions with metastable excited argon atoms. In the discharge, positive charges accumulate on the film surface, creating an electric field in the film sufficient to cause field emission of electrons from the cathode metal substrate into the insulator. Such electrons are accelerated in the film by the field and can escape from it into the discharge volume. As a result, the effective yield of ion-electron emission from the cathode increases. The temperature dependences of discharge characteristics are calculated and it is shown that, due to a rapid decrease in the concentration of mercury vapor in the mixture with decreasing temperature, the electric field strength in the discharge gap and the discharge voltage increase. The presence of a thin insulating film on the cathode can result in an improvement in its emission characteristics and a significant reduction in the discharge voltage. This causes a decrease in the energies of the ions and atoms bombarding the cathode surface, and, consequently, in the intensity of cathode sputtering in the discharge.

Experimental study on the full cycle evolution of high-intensity atmospheric dc arc discharge from breakdown to extinguishment.

In this study, the spatiotemporal evolution of full cycle of high-intensity dc argon arc discharge at atmospheric pressure is investigated by using a transferred arc device, which is easy to be directly observed in the experiment. Combining the voltage and current waveforms with high-speed images, the full cycle evolution process of high-intensity atmospheric dc arc can be divided into five different stages: breakdown pulse stage, cathode heating stage, current climbing stage, stable arc discharge stage, and finally arc extinguishing stage. The characteristics of each different stage are analyzed in detail through the electrical properties, high-speed pictures, and spectroscopic measurements. The results show that the strong luminescence region develops from the vicinity of cathode and anode to the middle in the breakdown pulse stage, which is explained from the spatiotemporal evolution of distributions of excited argon atom and ions. The development velocity of emission intensity of argon ions is mainly determined by the dominant stepwise ionization process. Then the cathode heating stage appears with many bright and nonuniformly distributed light spots on the cathode surface, and the electron emission mechanism of cathode gradually changes to the thermionic emission as the surface temperature rises. With the increase of arc current, the discharge channel significantly expands, then becomes stable due to the increment of the Lorentz force. The characteristics of arc extinguishing stage are clarified in terms of the decay of charged particles density.

Influence of Voltage, Pulselength and Presence of a Reverse Polarized Pulse on an Argon–Gold Plasma during a High-Power Impulse Magnetron Sputtering Process

This work aims to provide information about the deposition of gold via bipolar high-power impulse magnetron sputtering (HIPIMS) in order to identify suitable process parameters. The influences of voltage, pulse length and the kick-pulse on an argon–gold plasma during a bipolar high-power impulse magnetron sputtering deposition process were analysed via optical emission spectroscopy (OES) and oscilloscope. The voltage was varied between 700 V and 1000 V, the pulse length was varied between 20 µs and 100 µs and the process was observed once with kick-pulse and once without. The influence of the voltage on the plasma was more pronounced than the influence of the pulse width. While the intensity of several Au I lines increased up to 13-fold with increasing voltages, only a less-than linear increase in Au I brightness with time could be identified for changes in pulse length. The intensity of excited argon is only minimally affected by changes in voltages, but follows the evolution of the discharge current, with increasing pulse lengths. Contrary to the excited argon, the intensity emitted by ionized argon grows nearly linearly with voltage and pulse length. The reverse polarised pulse mainly affects the excited argon atoms in the plasma, while the influence on the ionized argon is less pronounced, as can be seen in the the spectra. Unlike the excited argon atoms, the excited gold atoms appear to be completely unaffected by the kick-pulse. No ionization of gold was observed. During the pulse, a strong rarefaction of plasma takes place. Very short pulses of less than 50 µs and high voltages of about 1000 V are to be preferred for the deposition of gold layers. This paper offers a comprehensive overview of the gold spectrum during a HIPIMS process and makes use of optical emission spectroscopy as a simple measuring approach for evaluation of the reverse polarized pulse during a bipolar process. Future uses of the process may include the metallization of polymers.

Influence of the magnetic field on the extension of the ionization region in high power impulse magnetron sputtering discharges

The high power impulse magnetron sputtering (HiPIMS) discharge brings about increased ionization of the sputtered atoms due to an increased electron density and efficient electron energization during the active period of the pulse. The ionization is effective mainly within the electron trapping zone, an ionization region (IR), defined by the magnet configuration. Here, the average extension and the volume of the IR are determined based on measuring the optical emission from an excited level of the argon working gas atoms. For particular HiPIMS conditions, argon species ionization and excitation processes are assumed to be proportional. Hence, the light emission from certain excited atoms is assumed to reflect the IR extension. The light emission was recorded above a 100 mm diameter titanium target through a 763 nm bandpass filter using a gated camera. The recorded images directly indicate the effect of the magnet configuration on the average IR size. It is observed that the shape of the IR matches the shape of the magnetic field lines rather well. The IR is found to expand from 10 and 17 mm from the target surface when the parallel magnetic field strength 11 mm above the racetrack is lowered from 24 to 12 mT at a constant peak discharge current.

Kinetics of Metastable Argon Optical Excitation and Gain in Ar/He Microplasmas.

The optically pumped rare-gas metastable laser is capable of high-intensity lasing on a broad range of near-infrared transitions for excited-state rare gas atoms (Ar*, Kr*, Ne*, Xe*) diluted in flowing He. The lasing action is generated by photoexcitation of the metastable atom to an upper state, followed by collisional energy transfer with He to a neighboring state and lasing back to the metastable state. The metastables are generated in a high-efficiency electric discharge at pressures of ∼0.4 to 1 atm. The diode-pumped rare-gas laser (DPRGL) is a chemically inert analogue to diode-pumped alkali laser (DPAL) systems, with similar optical and power scaling characteristics for high-energy laser applications. We used a continuous-wave linear microplasma array in Ar/He mixtures to produce Ar(1s5) (Paschen notation) metastables at number densities exceeding 1013 cm-3. The gain medium was optically pumped by both a narrow-line 1 W titanium-sapphire laser and a 30 W diode laser. Tunable diode laser absorption and gain spectroscopy determined Ar(1s5) number densities and small-signal gains up to ∼2.5 cm-1. Continuous-wave lasing was observed using the diode pump laser. The results were analyzed with a steady-state kinetics model relating the gain and the Ar(1s5) number density.

Investigation of the afterpeaks in pulsed microwave argon plasma at atmospheric pressure

We studied the energy transport process in pulsed microwave argon plasmas at atmospheric pressure, focusing on the optical emission burst during the pulse-off time called the afterpeak. Guided by experimental observations using nanosecond time resolution imaging and spectroscopic diagnostics, we developed a global simulation model considering time-varying reaction rate coefficients and non-thermal electron energy distribution. Experimental and simulation results show that the afterpeak can be maximized by choosing an appropriate pulse period. Our analysis of the generation and consumption of excited argon species reveals that the rapid drop in electron temperature during the inter-pulse time reduces the diffusive loss of ions and enhances the recombination reactions, which produce the afterpeak. We also reveal that the radiation trapping and high energy level argon must be considered to simulate the afterpeak in atmospheric conditions. The improved understanding of the afterpeak dynamics can be utilized to optimize the power coupling and/or generation of reactive species.

DISCHARGING OF DUST PARTICLES OF DIFFERENT SIZES IN AN ARGON AFTERGLOW PLASMA

The dust charge distribution function (DCDF) in an argon plasma afterglow is obtained by solving numerically the master equation describing dust discharging as a one-step stochastic process. The calculated DCDFs are compared with Gaussian distributions, and it is found that the dust charge distribution functions can be approximated quite well by the latter ones for different external conditions. It is found how the DCDF, mean dust charge, variance and charging time depend on dust size. For late afterglow times, it is also analyzed how the emission of electrons in the collisions of excited argon atoms with dust particles affects the DCDF. It is shown that the emission effect is more essential for larger nanoparticles than for smaller ones.

Mechanism and Reactive Species in a Fountain-Strip DBD Plasma for Degrading Perfluorooctanoic Acid (PFOA)

Perfluorooctanoic acid (PFOA) is an artificially synthesized perfluorinated chemical widely used in industries. It is often released into the environment without treatment, which causes pollution in groundwater. Recently, we have reported a rapid and efficient removal of PFOA in aqueous solution by using a fountain-strip dielectric barrier discharge reactor (SF-DBD). This design allows for the gaseous–liquid interaction to happen in a large space at atmospheric pressure, so it is a promising method to efficiently remove PFOA from water. Recently, we reported the effects of the process parameters, including power mode, pulse time, sinusoidal wave discharge, the discharge gas, initial concentration, pH, conductivity, and positive and negative discharges, on the efficiency of this method for PFOA degradation. Understanding the reaction mechanism is key to further improve the efficiency of the system. In this work, we reported the decomposition mechanism of the SF-DBD for PFOA degradation. The mass spectrum (MS) showed that PFOA was degraded to perfluoroheptanoic acid, perfluorohexanoic acid, perfluoropentanoic acid, perfluorobutanoic acid, perfluoropropionic acid, and trifluoroacetic acid after the plasma treatment. The optical emission spectroscope (OES) and the radical scavenger experiments indicated that the excited argon atoms and hydroxyl radicals played a major role in PFOA degradation, while the contributions from the solvated electrons (e−aq), superoxide anion radical (·O2−), and singlet oxygen (1O2) were negligible in initiating the cleavage reaction.

Tolerance effect of a shock-free atmospheric plasma on human skin.

In this work, a shock-free argon-fed plasma plume was generated by a variable-frequency power supply and the discharge characteristics were investigated from the voltage and current waveforms between 72 and 92 kHz frequencies. The higher electron temperature dominates the plasma chemical process and the average plasma temperature is about 30 ℃ under these conditions. The influence of non-thermal atmospheric plasma plume length and plume temperature on Ar gas flow is optimized at 7 sL/min. The average charge accumulation on the plume tip area and the dependence of flow rate on the plasma irradiation area were also explored. This atmospheric pressure plasma jet (APPJ) has been proposed for human-skin irradiation on different areas (even on the tongue) owing to its less painful, tingling and burning effect. Optical emission spectroscopy (OES) confirmed the presence of excited argon with reactive nitrogen (RNS) and oxygen species (ROS). This study contributes to a better understanding of non-thermal plasma effects on the human body which may find prospects for disinfection and prevention of different diseases during the current pandemic time.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00339-022-06022-w.

The Effect of Power on Inductively Coupled Plasma Parameters

In this work, we studied the effect of power variation ​​on inductively coupled plasma parameters using numerical simulation. Different values ​​were used for input power (750 W-1500 W), gas temperature 300K, gas pressure (0.02torr), 5 tourns of the copper coil and the plasma was produced at radio frequency (RF) 13.56 MHZ on the coil above the quartz chamber. For the previous purpose, a computer simulation in two dimensions axisymmetric, based on finite element method, was implemented for argon plasma. Based on the results we were able to obtain plasma with a higher density, which was represented by obtaining the plasma parameters (electron density, electric potential, total power, number density of argon ions, electron temperature, number density of excited argon atoms) where the high density in the generated plasma provides a greater degree in material processing, which increases the efficiency of the system. These results may aid in future research towards the development of more efficient optimization of plasma parameters which are (electron density, electric potential, total power, number density of argon ions, electron temperature, and number density of excited argon atoms).

Modeling results on the dust charge distribution in a plasma afterglow

Discharging of dust particles in an argon plasma afterglow is investigated using different approaches. First, the dust charge distribution function (DCDF) is obtained by solving numerically the master equation describing dust discharging as a one-step stochastic process. Second, the DCDF is calculated as a Gaussian distribution with mean dust charge and variance, which are functions of time. Additionally, the time-dependencies for the mean dust charge are obtained assuming that the charge changes continuously in the afterglow plasma. Calculation results are compared with available experimental data and are found to be in good qualitative agreement if the dust discharging model accounts for the emission of electrons in the collisions of excited argon atoms with dust particles. This study is carried out taking into account the transition from ambipolar to free diffusion as well as multistep ionization, excitation, and deexcitation of argon atoms in the plasma afterglow.

Role of excimer formation and induced photoemission on the Ar metastable kinetics in atmospheric pressure Ar–NH3 dielectric barrier discharges

Tunable diode laser absorption spectroscopy was used to record the space-and time-resolved number density of argon metastable atoms, Ar(1s3) (Paschen notation), in plane-to-plane dielectric barrier discharges (DBDs) operated in a Penning Ar–NH3 mixture at atmospheric pressure. In both low-frequency (LF 650 V, 50 kHz) discharges and dual LF–radiofrequency (RF 190 V, 5 MHz) discharges operated in α–γ mode, the density of Ar(1s3) revealed a single peak per half-period of the LF voltage, with rise and decay times in the sub-microsecond time scale. These results were compared to the predictions of a 1D fluid model based on continuity and momentum equations for electrons, argon ions (Ar+ and Ar2 +) and excited argon 1s atoms as well electron energy balance equation. Using the scheme commonly reported for Ar-based DBDs in the homogeneous regime, the Ar metastable kinetics exhibited much slower rise and decay times than the ones seen in the experiments. The model was improved by considering the fast creation of Ar2 * excimers through three-body reactions involving Ar(1s) atoms and the rapid loss of Ar2 * by vacuum ultraviolet light emission. In optically thin media for such photons, they can readily reach the dielectric barriers of the DBD electrodes and induce secondary electron emission. It is shown that Ar2 * and photoemission play a significant role not only on the Ar metastable kinetics, but also on the dominant ionization pathways and possible α–γ transition in dual frequency RF–LF discharges.

Optimization of Atmospheric Pressure Plasma Jet with Single-Pin Electrode Configuration and Its Application in Polyaniline Thin Film Growth.

This study systematically investigated an atmospheric pressure plasma reactor with a centered single pin electrode inside a dielectric tube for depositing the polyaniline (PANI) thin film based on the experimental case studies relative to variations in pin electrode configurations (cases I, II, and III), bluff-body heights, and argon (Ar) gas flow rates. In these cases, the intensified charge-coupled device and optical emission spectroscopy were analyzed to investigate the factors affecting intensive glow-like plasma generation for deposition with a large area. Compared to case I, the intense glow-like plasma of the cases II and III generated abundant reactive nitrogen species (RNSs) and excited argon radical species for fragmentation and recombination of PANI. In case III, the film thickness and deposition rate of the PANI thin film were about 450 nm and 7.5 nm/min, respectively. This increase may imply that the increase in the excited radical species contributes to the fragmentation and recombination due to the increase in RNSs and excited argon radicals during the atmospheric pressure (AP) plasma polymerization to obtain the PANI thin film. This intense glow-like plasma generated broadly by the AP plasma reactor can uniformly deposit the PANI thin film, which is confirmed by field emission-scanning electron microscopy and Fourier transform infrared spectroscopy.

On the population density of the argon excited levels in a high power impulse magnetron sputtering discharge

Population densities of excited states of argon atoms in a high power impulse magnetron sputtering (HiPIMS) discharge are examined using a global discharge model and a collisional-radiative model. Here, the ionization region model (IRM) and the Orsay Boltzmann equation for electrons coupled with ionization and excited states kinetics (OBELIX) model are combined to obtain the population densities of the excited levels of the argon atom in a HiPIMS discharge. The IRM is a global plasma chemistry model based on particle and energy conservation of HiPIMS discharges. OBELIX is a collisional-radiative model where the electron energy distribution is calculated self-consistently from an isotropic Boltzmann equation. The collisional model constitutes 65 individual and effective excited levels of the argon atom. We demonstrate that the reduced population density of high-lying excited argon states scales with (p*)−6, where p* is the effective quantum number, indicating the presence of a multistep ladder-like excitation scheme, also called an excitation saturation. The reason for this is the dominance of electron impact processes in the population and de-population of high-lying argon states in combination with a negligible electron–ion recombination.

Влияние процентного содержания аргона в смеси газов Ar-N2 на относительное количество частиц Ar+, N2+, N и N+ в плазме несамостоятельного тлеющего разряда низкого давления с полым катодом

The paper presents the results of studies of argon-nitrogen plasma of a non-self-sustaining low-pressure glow discharge with a hollow cathode by optical emission spectrometry. The plasma was generated in a mixture of Ar-N2 gases with an argon percentage from 0 to 100% at a total pressure of 1 Pa. The discharge current and voltage were maintained constant and amounted to 18 A and 165 V, respectively. A significant increase in the amount of Ar+ (up to 30%) at a nitrogen content of 10-25% was shown, which is associated with a shift in the reaction of nitrogen recharging with argon towards the generation of argon ions. It was found that at a low partial pressure of nitrogen (≈ 10%), a sharp increase in the content of atomic nitrogen is observed, which is probably due to the dissociation of nitrogen molecules upon collisions with an excited argon atom.