Metastable Argon Research Articles

Femtosecond Laser Excitation in Argon–Nitrogen Mixtures

Argon gas is demonstrated to enhance femtosecond laser electronic excitation tagging at atmospheric pressure and temperature for unseeded velocimetry applications, primarily through two to four orders of magnitude increase of excited species that may radiate through nitrogen’s second positive system at early timescales of interest. The first positive system continues to play an important role in maintaining this emission at longer delays. A detailed kinetic model is implemented to explain this observed behavior in nitrogen and argon mixtures. Dominant processes governing the creation of and include a slower decay of electron temperature through increased ionization processes, reduced nitrogen quenching of electrons, nitrogen atom creation and recombination, the formation and dissociation of , and a number of argon–nitrogen direct and indirect excitation pathways. The production of ions through charge-transfer reactions and affect excited C- and B-state nitrogen population delays at later times (). The pooling reactions play minor roles in the formation of and at timescales useful for measurements in the femtosecond laser electronic excitation tagging argon plasma chemistry. In mixtures where nitrogen is dominant, metastable argon species are less instrumental in direct nitrogen excitation transfer, , than in facilitating further reactions through maintaining a higher electron temperature, whereas this excitation transfer begins to play a larger role as the percentage of argon is further increased. The model yields results that agree with sub-100 torr argon–nitrogen discharge experiments and theoretical results derived from other studies. It is concluded that not one single process can be credited for the enhancement, but a combination of ionization and heating produces the increased emission observed in argon mixtures.

Emission spectral diagnosis of argon-helium plasma produced by radio frequency capacitive discharge

Optically pumped metastable rare-gas laser (OPRGL) have been proposed to overcome the shortcomings of diode-pumped alkali-vapor laser in the recent years. The OPRGL promises to realize high-scale output. But how to achieve enough particle density of metastable atoms is still an open problem. Usually, plasma produced by discharge serves as a gain medium of the OPRGL. Here in this paper, we are to reveal the effects of different discharge parameters on the plasma properties, such as particle density of metastable argon atoms. Gas discharge at a radio frequency of 13.56 MHz is adopted to excite argon atoms. Emission spectrum is employed to study argon and helium radio frequency discharge of optically pumped argon laser at high pressure, different powers of discharge and various content of argon. Gas temperature is obtained by analyzing rotational spectrum (A2∑+ → X2Π) of OH radical generated by residual water vapor and comparing simulated spectrum with the measured spectrum. The electronic excitation temperature relating to electron temperature is obtained by the method of Boltzmann's plot. Stark broadening of the spectrum is used to determine the electron density. The results show that gas temperature rises slightly with the increase of pressure and varies little with content and discharge power changing. The electronic excitation temperature increases with the decrease of pressure evidently and decreases slightly with the increase of content. The electron density is on the order of 1015 cm-3 under various conditions controlled by us. Long time discharge test reveals that residual water vapor can lead to the decrease of electron temperature, and thus reducing the yield of argon metastable state. In conclusion, considering that the higher gas temperature can improve the collision relaxation rate of helium and argon, and the higher electron temperature can improve the rate of production of argon metastable state. Thus a proposal is put forward that appropriately heating gas and reducing gas pressure can obtain higher particle density of metastable argon. Furthermore, It can be found from these results that heating and cleaning the gas during discharge may be candidate methods to obtain and sustain the higher particle density in the plasma.

Fluid modeling of plasma dynamics in pulsed RF capacitive glow discharges in low pressure argon

Based on the drift-diffusive approximation, one-dimensional fluid modeling is carried out for the pulsed RF capacitive glow discharges in low pressure argon. Investigated are the effects of various discharge parameters, such as the duty cycle ratio and frequency of the pulsed modulation, and the material properties of the electrode, on the plasma characteristics such as the electron recombination rate, during both the initial and the steady state phases of the discharge. The modeling results show that, after switching off the applied voltage during the pulsed modulation of the RF discharge, the electron density increases first and then decreases. This phenomenon is particularly pronounced before the discharge reaches steady state. Meanwhile, independent of whether the discharge has reached steady state or not, right after the applied voltage is switched on during each modulation period, the electron and ion densities and the metastable argon atom density, as well as their generation rate, experience a time delay (phase lag) with respect to the applied voltage. The results also show that, at the initial phase of the pulsed modulation, during the steady state discharge, the electron temperature in the center of the bulk plasma is almost not affected by the applied voltage, or by the material properties of the electrode such as the secondary electron emission rate. The electron density, however, does increase with these parameters, resulting in increased power density dissipation of the plasma. On the other hand, at fixed applied voltage, the central electron temperature of the bulk plasma is reduced by increasing several parameters, including the modulation duty ratio, the distance between two electrodes, and the modulation frequency, as well as the electron recombination rate due to different choices of the electrode material. This eventually leads to a reduction of the dissipated power density in the plasma. In particular, with the increase of the modulation duty ratio, the distance between electrodes, or the RF modulation frequency, the electron temperature decays slower after switching off the applied voltage within the modulation period. All the above studies are also accompanied by a systematic investigation of the temporal and spatial distributions of the electron energy conversion during the pulsed RF discharge.

DBD‐based plasma polymerization from monomer‐argon mixtures: Analytical model of monomer reactions with excited argon species

Experimental studies of DBD‐based atmospheric‐pressure plasma polymerization from mixtures of hexamethyldisiloxane (HMDSO) with argon are presented and interpreted using an analytical model of the plasma‐chemical kinetics. Gradient films are obtained using a special DBD‐PACVD reactor. Growth‐rate profiles and their monomer fraction dependence show that long‐living excited Ar species play a major role in the deposition mechanism. Reactions of HMDSO with metastable and resonance argon atoms as well as argon excimers are considered within the presented model which may be simplified by introducing a lumped excited argon species Ar*. Model predictions are compared with experimental results. Recombination reactions must be included in order to reproduce characteristic features of the measured growth‐rate curves.

Metastable argon atoms under significant neutral depletion in helicon plasmas by laser-induced fluorescence

We study the spatial distribution of the metastable-state argon atoms in high density helicon plasmas by means of laser-induced fluorescence. It is observed that the neutral argon in metastable-state has an anomalous radial distribution in density; it has a caldera-like shape radially, which is rare in typical low-temperature plasmas such as inductively coupled plasmas wherein the density increases toward the discharge center, as previously reported. The formation of the distribution can be explained as it forms by the combined effects of significant neutral depletion in high plasma density, off-axis electron-density distribution, and increasing diffusive loss toward the wall. To establish the assertion with the underlying physics, we calculate a simple global model and obtain the neutral density distribution in metastable-state under various conditions. The calculated results qualitatively agree with the experimental results.

Spatially resolved modeling and measurements of metastable argon atoms in argon-helium microplasmas

Microwave-driven plasmas operating near atmospheric pressure have been shown to be a promising technique for producing the high density of argon metastable atoms required for optically pumped rare gas laser systems. Stable microwave-driven plasmas can be generated at high pressures using microstrip-based resonator circuits. We present results from computational modeling and laser absorption measurements of argon metastable densities in such plasmas operating in argon-helium gas mixtures at pressures up to 300 Torr. The model and measurements resolve the plasma characteristics both perpendicular to the substrate surface and along the resonator length. The measurements qualitatively and in many aspects quantitatively confirm the accuracy of the model. The plasmas exhibit distinct behaviors depending on whether the operating gas is mostly argon or mostly helium. In high-argon plasmas, the metastable density has a large peak value but is confined very closely to the electrode surfaces as well as being reduced near the discharge gap itself. In contrast, metastable densities in high helium-fraction mixtures extend through most of the plasma. In all systems, increasing the power extends the region of metastable along the resonator length, while the extent away from the substrate surface remains approximately constant.

Plasma-catalytic hydrogenation of CO2 for the cogeneration of CO and CH4 in a dielectric barrier discharge reactor: effect of argon addition

Plasma-catalytic CO2 hydrogenation over a Ni/Al2O3 catalyst for the cogeneration of CO and CH4 has been carried out in a dielectric barrier discharge (DBD) reactor at 150 °C. The presence of the Ni catalyst in the DBD reactor has clearly demonstrated a plasma-catalytic synergistic effect at low temperatures, as the reaction performance of the plasma-catalytic CO2 hydrogenation is significantly higher than that of the sum of the individual processes (plasma process and thermal catalytic process) at the same temperature. The addition of argon (up to 60%) in the reaction enhances the conversion of CO2, the yield of CO and CH4 and the energy efficiency of the plasma process. The formation of metastable argon (Ar*) in the plasma could create new reaction routes which make a significant contribution to the enhanced CO2 conversion and production of CO and CH4. The introduction of Ar decreases the breakdown voltage of the feed gas and promotes charge transfer through the reactor. In addition, we find that the selectivity of CO is almost independent of the Ar content in the feed gas, while increasing the Ar content from 0 to 60% enhances the CH4 selectivity by 85%. This phenomenon suggests that the presence of Ar* might promote the methanation of CO and CO2 with hydrogen at low temperatures. Moreover, the molar ratio of CO/CH4 in the plasma-catalytic hydrogenation of CO2 can also be controlled by changing the Ar content in the feed gas.

Low-pressure low-frequency inductive discharge with ferrite cores for large-scale plasma processing

Electrophysical characteristics of a low–frequency (100 kHz) low–pressure (10–100 Pa) argon inductive discharge with ferrite cores have been investigated for discharge currents of 1–50 A, discharge chamber diameter of 230 mm. The dependencies of electric field strength on the argon pressure and discharge current were measured. Radial profiles of electron density and temperature were determined with double electric probes. Experimental results were compared with numerical results obtained by a self-consistent kinetic model of the low-frequency ICP, based on the simultaneous solution of a non-local Boltzmann equation for electron energy distribution function and balance equations for the metastable argon atoms density and gas temperature. Comparison of numerical and experimental results is presented.

Modeling of a dual-wavelength pumped metastable argon laser

Optically pumped metastable argon laser is an attractive research topic of innovative gas lasers, but the slow collisional relaxation rates of 1s4–1s5 (in Paschen notation) may form a bottleneck on the cycling of active atoms and decrease the laser output at room temperature. Here, by employing a method of a dual-wavelength pump, we demonstrate the removal of accumulation on the 1s4 level and the improvement of output power in the simulation. The simulated results show that a large increase in laser output is possible with a relatively weak assistant pump intensity. This method offers a feasible way to scale the laser to higher gain.

3-D Numerical Characterization of a Microwave Argon PECVD Plasma Reactor at Low Pressure

A 3-D model of a microwave plasma (mwp)-enhanced chemical vapor deposition (PECVD) reactor at 2.45 GHz in argon at low pressure describing self-consistently, the coupling of the microwave energy into the plasma is presented. The characteristics of the discharge are simulated using a fluid plasma model which solves the electron and ion continuity equations, electron energy balance equation, and the Poisson’s equation by finite element method, using COMSOL Multiphysics software. The physical behavior of the microwave PECVD discharge, such as plasma density, electron temperature, electric field and plasma potential, are simulated and analyzed. The chemical reactions considered in this paper are: elastic, superelastic, excitation, ionization, penning ionization and metastable quenching processes, involving electrons, ions (Ar+), neutral atoms (Ar), and excited metastable argon atoms (Ar*). The plasma characterization results are studied for a gas temperature of 300 K, a gas pressure of 100 mtorr and a microwave power of 600 W. The effect of varying gas pressure from 50 to 200 mTorr has been studied. The obtained results turn out to be in agreement with previous measurements and show that this kind of model can lead to a better understanding of the physical processes occurring in this kind of microwave reactor and thus allow optimization of this device.

Metastable argon atom density in complex argon/acetylene plasmas determined by means of optical absorption and emission spectroscopy

Optical emission and absorption spectroscopy has been utilized to investigate the instability of acetylene-containing dusty plasmas induced by growing nano-particles. The density of Ar(1s5) metastable atoms was derived by two methods: tunable diode laser absorption spectroscopy and with the help of the branching ratio method of emitted spectral lines. Results of the two techniques agree well with each other. The density of Ar(1s3) metastable atoms was also measured by means of optical emission spectroscopy. The observed growth instability leads to pronounced temporal variations of the metastable and other excited state densities. An analysis of optical line ratios provides evidence for a depletion of free electrons during the growth cycle but no indication for electron temperature variations.

Electrical and optical properties of Ar/NH3 atmospheric pressure plasma jet

Inspired by the Penning effect, we obtain a glow-like plasma jet by mixing ammonia (NH3) into argon (Ar) gas under atmospheric pressure. The basic electrical and optical properties of an atmospheric pressure plasma jet (APPJ) are investigated. It can be seen that the discharge mode transforms from filamentary to glow-like when a little ammonia is added into the pure argon. The electrical and optical analyses contribute to the explanation of this phenomenon. The discharge mode, power, and current density are analyzed to understand the electrical behavior of the APPJ. Meanwhile, the discharge images, APPJ's length, and the components of plasma are also obtained to express its optical characteristics. Finally, we diagnose several parameters, such as gas temperature, electron temperature, and density, as well as the density number of metastable argon atoms of Ar/NH3 APPJ to help judge the usability in its applications.

Stable 811.53 nm diode laser pump source for optically pumped metastable Ar laser

A stable external cavity diode laser coupled with volume Bragg grating for metastable argon atoms pumping is presented. The measured maximum output power of the continuous wave is 6.5W when the spectral width (FWHM) is less than 21pm around 811.53nm and the power efficiency is 68%. The tuning range of the emission wavelength is bigger than 270pm. The calculated deviation in relative absorption efficiency caused by the fluctuations of wavelength and power is less than 4%.

A simple velocity-tunable pulsed atomic source of slow metastable argon

A pulsed beam of metastable argon atoms having a low tunable velocity (10 to 150 m s−1) is produced with a very substantial brightness (9 × 108Ar* s−1 sr−1). The present original experimental configuration leads to a variable velocity dispersion that can be smaller than the standard Brownian one. This behaviour, analysed using Monte Carlo simulations, exhibits momentum stretching (heating) or narrowing (cooling) entirely due to a subtle combination of Doppler and Zeeman effects.

Photoassociation Spectroscopy in Penning Ionization Reactions at Sub-Kelvin Temperatures.

Penning ionization reactions in merged beams with precisely controlled collision energies have been shown to accurately probe quantum mechanical effects in reactive collisions. A complete microscopic understanding of the reaction is, however, faced with two major challenges-the highly excited character of the reaction's entrance channel and the limited precision of even the best state-of-the-art ab initio potential energy surfaces. Here, we suggest photoassociation spectroscopy as a tool to identify the character of orbiting resonances in the entrance channel and probe the ionization width as a function of interparticle separation. We introduce the basic concept, using the example of metastable helium and argon, and discuss the general conditions under which this type of spectroscopy will be successful.

Nonequilibrium Atmospheric Pressure Ar/O2 Plasma Jet: Properties and Application to Surface Cleaning**supported by National Natural Science Foundation of China (No. 11305017)

In this study an atmospheric pressure Ar/O2 plasma jet is generated to study the effects of applied voltage and gas flux rate to the behavior of discharge and the metal surface cleaning. The increase in applied voltage leads to increases of the root mean square (rms) current, the input power and the gas temperature. Furthermore, the optical emission spectra show that the emission intensities of metastable argon and atomic oxygen increase with increasing applied voltage. However, the increase in gas flux rate leads to a reduction of the rms current, the input power and the gas temperature. Furthermore, the emission intensities of metastable argon and atomic oxygen decrease when gas flux rate increases. Contact angles are measured to estimate the cleaning performance, and the results show that the increase of applied voltage can improve the cleaning performance. Nevertheless, the increase of gas flux rate cannot improve the cleaning performance. Contact angles are compared for different input powers and gas flux rates to search for a better understanding of the major mechanism for surface cleaning by plasma jets.

On the plasma chemistry of a cold atmospheric argon plasma jet with shielding gas device

A novel approach combining experimental and numerical methods for the study of reaction mechanisms in a cold atmospheric plasma jet is introduced. The jet is operated with a shielding gas device that produces a gas curtain of defined composition around the plasma plume. The shielding gas composition is varied from pure to pure .The density of metastable argon in the plasma plume was quantified using laser atom absorption spectroscopy. The density of long-living reactive oxygen and nitrogen species (RONS), namely , , , , and , was quantified in the downstream region of the jet in a multipass cell using Fourier-transform infrared spectroscopy (FTIR).The jet produces a turbulent flow field and features guided streamers propagating at several that follow the chaotic argon flow pattern, yielding a plasma plume with steep spatial gradients and a time dependence on the scale while the downstream chemistry unfolds within several seconds. The fast and highly localized electron impact reactions in the guided streamer head and the slower gas phase reactions of neutrals occurring in the plasma plume and experimental apparatus are therefore represented in two separate kinetic models. The first electron impact reaction kinetics model is correlated to the LAAS measurements and shows that in the guided streamer head primary reactive oxygen and nitrogen species are dominantly generated from . The second neutral species plug-flow model hence uses an source term as sole energy input and yields good agreement with the RONS measured by FTIR spectroscopy.

A soft ionization method for semivolatile compounds

This paper presents an improved conditioned glow discharge ionization method, based on Penning ionization by metastable argon atoms at fore-vacuum gas pressures. The afterglow flow passes through a capillary to remove charged species (so called “conditioning”) and then the analyte molecules are mixed with metastable atoms within the throat of a supersonic jet. The method provides exclusively M+. molecular ions accompanied by limited fragmentation, even for alkanes, phthalates, and nitro compounds. When in-source fragmentation is induced, spectra appear similar to NIST ones. The signal is a linear function of concentration within a dynamic range of at least four orders of magnitude and is notably independent on the presence of a chemical matrix. Sensitivity is uniform within a wide range of analyte compounds, including alkanes and fatty acids. A combination of soft ionization, NIST identification of fragment spectra, and quantitative analysis presents a novel opportunity in analytical chemistry. The method may be particularly suitable for the analysis of crude oils and for quantitative metabolomic measurements.

Development of a Laser Induced Fluorescence (LIF) system on thePlasma Material Interaction System (PLAMIS-II) device

A laser induced fluorescence (LIF) system has been developed for the plasma material interaction system (PLAMIS-II) device, which is equipped with a unique plasma gun composed of a LaB6 cathode and two anodes with electromagnets to generate a focused dense plasma. PLAMIS-II simulates the interactions of plasma with different materials and is to be used for the test of plasma facing components of fusion devices. The LIF system is composed of a seed laser with Littmann/Metcalf cavity and a master oscillator power amplifier to pump 3d4F7/2 metastable argon ion to 4p4D5/2 level at the wavelength of 668.61 nm, which has the following input parameters: laser power = 20 mW, line width < 100 kHz, and a mode-hop free tuning range > 70 GHz. For in-situ measurement of laser wavelength, the wavelength spectrum of an iodine cell was measured by a photo-transistor during LIF measurement. To measure argon ion temperature (Ti) and drift velocity (vd) in PLAMIS-II, the fluorescence light with the wavelength of 442.72 nm, emitted from 4p4D5/2 level to 4s4P3/2 level and passing through 1 nm band-width filter, was collected by the photomultiplier tube combined with a lock-in amplifier and a chopper with frequency of 3 kHz. Initial data of Ti and vd were analysed in terms of gas flow rate and applied power.

Argon metastables in HiPIMS: validation of the ionization region model by direct comparison to time resolved tunable diode-laser diagnostics

The volume plasma interactions of high power impulse magnetron sputtering (HiPIMS) discharges operated with a Ti target is analyzed in detail by combining time-resolved diagnostics with modeling of plasma kinetics. The model employed is the ionization region model (IRM) with an improved and detailed treatment of the kinetics of the argon metastable (Arm) state, called m-IRM. The diagnostics used is tunable diode-laser absorption spectroscopy (TD-LAS) of the Arm state, which gives the line-of-sight density integrated along the laser path parallel to the target surface. The TD-LAS recordings exhibit quite complex temporal evolutions Arm(t), with distinct features that are shown to reflect the time evolution of the plasma (the electron density and temperature), and of the argon gas (gas rarefaction and refill). The Arm(t) function is thus a tracer for the most important aspects of internal discharge physics, and therefore suitable for model testing and validation. The IRM model is constructed to be locked to obey specific experimental macroscopic discharge parameters, specifically the discharge current ID(t) and the voltage UD(t). It has to this purpose been run with the appropriate process gas pressures (from 0.67 to 2.67 Pa), with the experimentally applied voltage pulse profiles UD(t), and with the resulting current pulse profiles ID(t) (with maxima from 0.5 to 70 A). It is shown that the model reproduces the features in the TD-LAS measurements: both the Arm(t) evolution in single pulses, and how the pulse shapes change with gas pressure and with pulse amplitude. The good agreement between the measurements and model output is in this work taken to validate the basic assumptions of the m-IRM. In addition, the m-IRM results have been used to unravel the connections between volume plasma kinetics and various features recorded in the TD-LAS measurement, and to generalize the foremost characteristics of the studied discharges.