Dissociation Fraction Research Articles

Note: Iodine dissociation with a pulsed arc discharge.

A new method is presented to dissociate iodine molecules in gas discharge. A high repetition rate spark gap is designed and used to trigger the circuit. Absorption spectroscopy is applied to measure the dissociation fraction of iodine molecules. It is found that more than 35% of iodine molecules are dissociated. The effect of switching frequency on the dissociation coefficient is investigated by changing the frequency of the rotating spark gap. Pulsed and equivalent direct current circuits are compared, and it is found that in the constant current glow discharge the dissociation fraction is near to the zero and negligible.

Insights to scaling remote plasma sources sustained in NF3 mixtures

Remote plasma sources (RPSs) are being developed for low damage materials processing during semiconductor fabrication. Plasmas sustained in NF3 are often used as a source of F atoms. NF3 containing gas mixtures such as NF3/O2 and NF3/H2 provide additional opportunities to produce and control desirable reactive species such as F and NO. In this paper, results from computational investigations of RPS sustained in capacitively coupled plasmas are discussed using zero-dimensional global and two-dimensional reactor scale models. A comprehensive reaction mechanism for plasmas sustained in Ar/NF3/O2 was developed using electron impact cross sections for NF2 and NF calculated by ab initio molecular R-matrix methods. For validation of the reaction mechanism, results from the simulations were compared with optical emission spectroscopy measurements of radical densities. Dissociative attachment and dissociative excitation of NFx are the major sources of F radicals. The exothermicity from these Franck–Condon dissociative processes is the dominant gas heating mechanism, producing gas temperatures in excess of 1500 K. The large fractional dissociation of the feedstock gases enables a larger variety of end-products. Reactions between NFx and O atom containing species lead to the formation of NO and N2O through endothermic reactions facilitated by the gas heating, followed by the formation of NO2 and FNO from exothermic reactions. The downstream composition in the flowing afterglow is an ion–ion plasma maintained by, in oxygen containing mixtures, [F−] ≈ [NO+] since NO has the lowest ionization potential and F has the highest electron affinity among the major neutral species.

Relative dissociation fractions ofN2Ounder15–30−keVH−,C−, andO−negative-ion impact

The relative dissociation fractions of ${\mathrm{N}}_{2}\mathrm{O}$ are studied under 15--30-keV negative ions ${\mathrm{H}}^{\ensuremath{-}},{\mathrm{C}}^{\ensuremath{-}}$, and ${\mathrm{O}}^{\ensuremath{-}}$ impact. The recoil ions and ion pairs originating from the target molecule ${\mathrm{N}}_{2}\mathrm{O}$ are detected and identified in coincidence with scattered ions in single electron loss (SL) and double electron loss (DL) channels using a time-of-flight mass spectrometer. The dissociation fractions for the production of the fragment ions are obtained. We find that the relative dissociation fractions in SL are smaller than those in DL, and the degree of fragmentation will become greater with the impact energy increasing. We also analyze the coincident TOF spectra of two fragment ions which are produced from dissociation of ${\mathrm{N}}_{2}{\mathrm{O}}^{2+}$ and give the possible dissociation pathways of ${\mathrm{N}}_{2}{\mathrm{O}}^{2+}$ with $15--30\text{\ensuremath{-}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}{\mathrm{H}}^{\ensuremath{-}},{\mathrm{C}}^{\ensuremath{-}}$, and ${\mathrm{O}}^{\ensuremath{-}}$ impact. There are many studies on ${\mathrm{N}}_{2}\mathrm{O}$ with positive-ion, photon, and electron impact, and we compare our results under negative-ion impact with those works.

Structural fingerprints of yielding mechanisms in attractive colloidal gels.

Core-Modified Dissipative Particle Dynamics (CM-DPD) with a modified depletion potential and full hydrodynamics description is used to study non-equilibrium properties of colloidal gels with short range attraction potentials at an intermediate volume fraction (ϕ = 0.2) under start-up shear deformation. Full structural and rheological analysis using the stress fabric tensor complemented by bond number and bond distribution evolution under flow reveals that similarly to dilute colloidal gels, flow-induced anisotropy and strain-induced stretching of bonds are present during the first yielding transition. Unlike in low volume fraction depletion gels however, a small fraction of bond dissociation is required to facilitate bond rotation at intermediate volume fractions. The strain at which structural stretching and anisotropy in bond distribution emerge coincides with the first maximum in the shear stress (first yielding transition). At higher strains, depending on flow strength, a second peak in stress signal appears which is attributed to the compaction and melting of clusters. In this work, for the first time we provide evidence that multibody hydrodynamic interactions are essential to predict the correct dynamics of depletion gels under flow, namely two-step yielding process.

Influence of argon fraction on plasma parameters in H2-N2mixture discharge with cathodic cage

Low-pressure H 2 -N 2 mixture pulsed DC plasmas with a cathodic cage (active screen) are widely used for plasma nitriding applications. In this study, the low-pressure H 2 -N 2 mixture plasma with a cathodic cage generated by 50 Hz pulsed DC source is investigated with triple Langmuir probe and optical emission spectroscopy. The electron temperature ( T e LP ) and electron number density ( n e ) are measured using a triple Langmuir probe (TLP). The excitation temperature ( T exc OES ) is calculated spectroscopically using Boltzmann plot method whereas nitrogen dissociation fraction is estimated using actinometry as well as the intensity ratio method ( I N (746.83 nm)/ I N2 (337.1 nm)). The results show that the electron and excitation temperatures, electron density and nitrogen atomic species density [N] all increase with the argon admixture, however, the important molecular ionized species density [N 2 + ] significantly decreases beyond 30% addition. This study provides useful information about the influence of the argon addition on plasma parameters and active species generation. As a result it helps to optimize the plasma nitriding system as a function of argon admixture to avoid random trials in the processing.

Spectroscopic and Langmuir probe measurements of Ar-N2 mixture plasma in magnetic pole enhanced ICP source

Over the last decade, the plasma sources based on electromagnetic power absorption mechanisms have emerged for large scale semiconductor manufacturing. For this purpose inductively coupled plasmas (ICPs), helicons, electron cyclotron resonance (ECR) etc have been widely explored. Recently, new designs of ICPs have been proposed for the improvement of plasma processing reactors including ICPs enhanced by a ferromagnetic core and ICPs with multiple antennas for a uniform spread of plasma over a large processing area. Present paper deals with variation in plasma processing parameters of a low pressure Ar-N2 inductively coupled plasma source with magnetic pole enhancement (MaPE-ICP). Optical emission spectroscopy (OES) and an RF compensated Langmuir probe are employed to examine the trends of parameters including the electron density, electron temperature and electron energy probability functions (EEPFs). Moreover, Ar metastable fractions are also calculated from line ratio method using optical emission intensities of the Ar spectral lines. The variation in emission intensities of the selected lines and in metastable fractions can be related with electron densities in various parts of the EEPFs. Furthermore, at low RF powers the evolution in EEPFs from ‘bi-Maxwellian’ to ‘Maxwellian’ distribution with increasing Ar concentration in the mixture is also observed. The dissociation fraction of N2 determined by ‘advanced actinometry’ technique, based on Trace rare gas-actinometry (TRG-actinometry), is also reported. It is found that dissociation fraction increases with Ar concentration in the discharge as well as RF power and filling gas pressure.

An ionization region model of the reactive Ar/O2 high power impulse magnetron sputtering discharge

A new reactive ionization region model (R-IRM) is developed to describe the reactive Ar/O2 high power impulse magnetron sputtering (HiPIMS) discharge with a titanium target. It is then applied to study the temporal behavior of the discharge plasma parameters such as electron density, the neutral and ion composition, the ionization fraction of the sputtered vapor, the oxygen dissociation fraction, and the composition of the discharge current. We study and compare the discharge properties when the discharge is operated in the two well established operating modes, the metal mode and the poisoned mode. Experimentally, it is found that in the metal mode the discharge current waveform displays a typical non-reactive evolution, while in the poisoned mode the discharge current waveform becomes distinctly triangular and the current increases significantly. Using the R-IRM we explore the current increase and find that when the discharge is operated in the metal mode Ar+ and Ti+ -ions contribute most significantly (roughly equal amounts) to the discharge current while in the poisoned mode the Ar+ -ions contribute most significantly to the discharge current and the contribution of O+ -ions, Ti+ -ions, and secondary electron emission is much smaller. Furthermore, we find that recycling of atoms coming from the target, that are subsequently ionized, is required for the current generation in both modes of operation. From the R-IRM results it is found that in the metal mode self-sputter recycling dominates and in the poisoned mode working gas recycling dominates. We also show that working gas recycling can lead to very high discharge currents but never to a runaway. It is concluded that the dominating type of recycling determines the discharge current waveform.

Hyperthermal molecular beam source using a non-diaphragm-type small shock tube.

We have developed a hyperthermal molecular beam source employing a non-diaphragm-type small shock tube for gas-surface interaction studies. Unlike conventional shock-heated beam sources, the capability of repetitive beam generation without the need for replacing a diaphragm makes our beam source suitable for scattering experiments, which require signal accumulation for a large number of beam pulses. The short duration of shock heating alleviates the usual temperature limit due to the nozzle material, enabling the generation of a molecular beam with higher translational energy or that containing dissociated species. The shock-heated beam is substantially free from surface-contaminating impurities that are pronounced in arc-heated beams. We characterize the properties of nitrogen and oxygen molecular beams using the time-of-flight method. When both the timing of beam extraction and the supply quantity of nitrogen gas are appropriately regulated, our beam source can generate a nitrogen molecular beam with translational energy of approximately 1 eV, which corresponds to the typical activation energy of surface reactions. Furthermore, our beam source can generate an oxygen molecular beam containing dissociated oxygen atoms, which can be a useful probe for surface oxidation. The dissociation fraction along with the translational energy can be adjusted through the supply quantity of oxygen gas.

Vertical Distribution of Ammonium Ion Concentration to the Air-Cathode of a Single-Chamber Microbial Fuel Cell

A microbial fuel cell (MFC) is a promising technology for wastewater treatment that can achieve removal of organic matter from wastewater and electricity generation simultaneously. In addition, it is known that not only organic matter but also ammonia in wastewater can be removed by single chamber microbial fuel cell (SCMFC). However, little is known of this ammonia removal mechanism. Some researchers reported that reduction of ammonium ion concentration occurred through biological nitrification / denitrification process by biofilm on the surface of an air-cathode in the SCMFC. Meanwhile, there is another hypothesis that ammonia volatilization occurs through the air-cathode as a result of decrease in the degree of ammonia dissociation and promotion of conversion to ammonia molecule, induced by local pH increase near the air-cathode. However, no direct evidence about the ammonia volatilization has not been reported. Therefore, in this study, SCMFC with a new (biofilm-less) air-cathode was operated and vertical distribution of ammonium ion concentration to the air-cathode was measured. If ammonia volatilization occurs, a sharp decrease in the concentration must be observed near the air-cathode. In our previous study, the vertical pH distribution to the air-cathode was measured with an ion selective microelectrode. Therefore, we applied this method to a micro-ammonium ion electrode. The concentration of ammonium ion at 10 mm from the air-cathode was 2 mM and there was a trend to decrease gradually as it approached to the air-cathode. Especially, the concentration of ammonium ion decreased sharply at approximately 0.6 mm from the air-cathode, and it became less than one-tenth within the distance of 0.5 mm. In our previous study under almost the same operational condition, the maximum pH was observed on the surface of the air-cathode, and it was about 9.3. Since the fraction of ammonium ion dissociation under this pH is about 50 % at most, the sharp decrease in the ammonium ion concentration cannot be explained only by this reason. Hence, something that caused the decrease in ammonium ion concentration must have happened near the air-cathode. The effect of nitrification can be denied because the biofilms were not formed on the surface of the air-cathode. From these above reasons, it is natural to think that ammonia volatilization through the air-cathode occurred in this study. Acknowledgments This research was financially supported by Grant-in-Aid for Japan Society for the Promotion of Science Fellows (Project No. 15J0490902). Figure 1

Reduction of a thin chromium oxide film on Inconel surface upon treatment with hydrogen plasma

Inconel samples with a surface oxide film composed of solely chromium oxide with a thickness of approximately 700nm were exposed to low-pressure hydrogen plasma at elevated temperatures to determine the suitable parameters for reduction of the oxide film. The hydrogen pressure during treatment was set to 60Pa. Plasma was created by a surfaguide microwave discharge in a quartz glass tube to allow for a high dissociation fraction of hydrogen molecules. Auger electron depth profiling (AES) was used to determine the decay of the oxygen in the surface film and X-ray diffraction (XRD) to measure structural modifications. During hydrogen plasma treatment, the oxidized Inconel samples were heated to elevated temperatures. The reduction of the oxide film started at temperatures of approximately 1300K (considering the emissivity of 0.85) and the oxide was reduced in about 10s of treatment as revealed by AES. The XRD showed sharper substrate peaks after the reduction. Samples treated in hydrogen atmosphere under the same conditions have not been reduced up to approximately 1500K indicating usefulness of plasma treatment.

Rayleigh Scattering Measurements of Heating and Gas Perturbations Accompanying Femtosecond Laser Tagging

Planar Rayleigh scattering and Rayleigh scattering polarimetry are applied toward measurements of molecular dissociation, thermometry, and the visualization of fluid dynamics in the afterglow of a femtosecond laser plasma under conditions relevant to the femtosecond laser tagging diagnostic. The generation of shock waves and a high-temperature region are observed, with a corresponding temperature increase of (, ) and (, ) produced with a 175 mm focusing lens. The spatial profile of the temperature rise along the beam is found to scale linearly with the optical emission intensity. The transverse width of the emission region is also observed to correlate with increased temperature. Rayleigh polarimetry measurements show a high dissociation fraction of up to (, ). Modeling of three-body recombination into the state using the measured dissociation fraction is able to reproduce the emission intensity time history between 1 and . Local heating is found to play a key role in the observed trends in the spatial profile, width, intensity, and temporal evolution of femtosecond laser tagging at laser intensities near or above .

Characterization of an atomic hydrogen source for charge exchange experiments

We characterized the dissociation fraction of a thermal dissociation atomic hydrogen source by injecting the mixed atomic and molecular output of the source into an electron beam ion trap containing highly charged ions and recording the x-ray spectrum generated by charge exchange using a high-resolution x-ray calorimeter spectrometer. We exploit the fact that the charge exchange state-selective capture cross sections are very different for atomic and molecular hydrogen incident on the same ions, enabling a clear spectroscopic diagnostic of the neutral species.

Characterization of RF He-N2/Ar mixture plasma via Langmuir probe and optical emission spectroscopy techniques

A Magnetic Pole Enhanced inductively coupled RF He- N2/ Ar plasma is characterized using a Langmuir probe and optical emission spectroscopy (OES) techniques. The effect of helium mixing on electron density (ne) and temperature (Te), electron energy probability functions (EEPFs), [N] atomic density, and N2 dissociation is investigated. A Langmuir probe and a zero slope method based on trace rare gas-optical emission spectroscopy (TRG-OES) are employed to measure the electron temperature. It is noted that the electron temperature shows an increasing trend for both methods. However, the temperature measured by a zero slope method Te(Z·S) approaches the temperature measured by a Langmuir probe; Te(L·P) at 56% and above helium concentration in the discharge. “Advance actinometry” is employed to monitor the variation in [N] atomic density with helium concentration and gas pressure. It is noted that [N] atomic density increases at 56% and above helium in the discharge, which is consistent with the trend of electron temperature and EEPFs. A drastic enhancement in N2 dissociation fraction D1 determined by “advance actinometry” is noted at 56% and above helium concentration in the mixture due to modifications in different population and depopulation mechanisms. However, it is also noted that the dissociation fraction D2 determined by intensity ratio method increases linearly with helium addition.

Relative dissociation fractions of CF4 under 15–30 keV H−, C− and O− negative ion impact

The relative dissociation fractions to produce the fragments of CF4 molecule are studied under the impact of 15 keV to 30 keV H−, C− and O− negative ions. By using a time-of-flight mass spectrometer, the recoil ions and ion pairs originating from the target molecule CF4 are detected and identified in coincidence with scattered ions in q = 0 and q = +1 charge states. The fractions for the production of the fragment ions are obtained relative to the yield, while that of the ion pairs relative to the (C+, F+) coincidence yield.

Influence of pulsed power supply parameters on active screen plasma nitriding

It is well known that active screen plasma nitriding (ASPN) is driven using a pulsed D.C. power supply, due to several advantages such as improved control over temperature, nitrided layer phase composition and active species generation. In addition, the pulsed power supply helps to avoid the transition to the arc regime. The ASPN configuration in particular is preferred over conventional plasma nitriding due to complete elimination of edge effect, as plasma is not in direct contact with the samples. In this work, the influence of pulsed power supply parameters such as duty cycle, peak voltage, peak current and time averaged current is studied in an ASPN setup, on nitrided layer phase composition and surface hardness of austenitic stainless steel (ASS). In addition, optical emission spectroscopy (OES) is used to measure the excitation, vibrational and rotational temperatures (Texc, Tvib, Trot), nitrogen dissociation fraction and concentration of active species N2(C3Πu) and N2+(B2Σu+) (in terms of emission intensities) to clarify the results obtained from surface characterization. The results show that keeping time-averaged current constant, the surface hardness decreases with the duty cycle of the pulsed D.C. power supply. The results from OES and surface analysis are correlated, and as a result plasma parameters (Texc, Tvib, Trot) and active species concentration (N, N2, N2+) are found to be important precursors in surface hardening of ASS.

Optical emission spectroscopy of 50 Hz pulsed dc nitrogen–hydrogen plasma in the presence of active screen cage

ABSTRACTThe N2-H2 plasma gas mixture, generated in a 50 Hz pulsed dc discharge system with active screen cage, is characterized by optical emission spectroscopy (OES), as a function of gas pressure, the fractions of hydrogen and current density. The N2 dissociation degree and N atomic density was measured with actinometery where argon gas is used as actinometer. It was shown that the increase in hydrogen fraction enhances the dissociation of N2, until the maximum of 40%. The excitation temperature is determined from Ar-I emission line intensities by using the simple Boltzmann plot method. The dissociation fraction and excitation temperature is found to increase with hydrogen mixing in nitrogen plasma.

Spectroscopic study of 50Hz pulsed Ar–O2 mixture plasma

Abstract Plasma sterilization is widely used to disinfect the heat sensitive materials. In this study, we have investigated 50 Hz pulsed capacitively coupled plasma (CCP) system operated in Ar–O2 mixture. Optical emission spectroscopy (OES) is used to identify the optimum conditions suitable for plasma based sterilization. Explicitly, excitation temperature of Ar-I lines, atomic oxygen density, dissociation fraction and UV radiation intensity are measured as a function of discharge parameters. The excitation temperature is calculated using Boltzmann plot method whereas OES has also been employed to evaluate the ground state atomic oxygen density and dissociation fraction. Similarly the normalized UV radiation intensity is determined from UV spectrum in 200–400 nm range. The results show that the total UV emission has optimum value at 20% oxygen, 2 mbar pressure and 2.68 mA/cm2 current density in the mixture. While excitation temperature, atomic oxygen density and dissociation fraction increase with the current density. A comparison with the literature suggests 50 Hz CCP system operated in Ar–O2 mixture has the potential of a cheap and reliable system for plasma sterilization.

Hydrogen atom density in narrow-gap microwave hydrogen plasma determined by calorimetry

The density of hydrogen (H) atoms in the narrow-gap microwave hydrogen plasma generated under high-pressure conditions is expected to be very high because of the high input power density of the order of 104 W/cm3. For measuring the H atom density in such a high-pressure and high-density plasma, power-balance calorimetry is suited since a sufficient signal to noise ratio is expected. In this study, H atom density in the narrow-gap microwave hydrogen plasma has been determined by the power-balance calorimetry. The effective input power to the plasma is balanced with the sum of the powers related to the out-going energy per unit time from the plasma region via heat conduction, outflow of high-energy particles, and radiation. These powers can be estimated by simple temperature measurements using thermocouples and optical emission spectroscopy. From the power-balance data, the dissociation fraction of H2 molecules is determined, and the obtained maximum H atom density is (1.3 ± 0.2) × 1018 cm−3. It is found that the H atom density increases monotonically with increasing the energy invested per one H2 molecule within a constant plasma volume.

Carbon dioxide elimination and regeneration of resources in a microwave plasma torch

Carbon dioxide gas as a working gas produces a stable plasma-torch by making use of 2.45 GHz microwaves. The temperature of the torch flame is measured by making use of optical spectroscopy and a thermocouple device. Two distinctive regions are exhibited, a bright, whitish region of a high-temperature zone and a bluish, dimmer region of a relatively low-temperature zone. The bright, whitish region is a typical torch based on plasma species where an analytical investigation indicates dissociation of a substantial fraction of carbon dioxide molecules, forming carbon monoxides and oxygen atoms. The emission profiles of the oxygen atoms and the carbon monoxide molecules confirm the theoretical predictions of carbon dioxide disintegration in the torch. Various hydrocarbon materials may be introduced into the carbon dioxide torch, regenerating new resources and reducing carbon dioxide concentration in the torch. As an example, coal powders in the carbon dioxide torch are converted into carbon monoxide according to the reaction of CO2 + C → 2CO, reducing a substantial amount of carbon dioxide concentration in the torch. In this regards, the microwave plasma torch may be one of the best ways of converting the carbon dioxides into useful new materials.

Weakly Bound Free Radicals in Combustion: "Prompt" Dissociation of Formyl Radicals and Its Effect on Laminar Flame Speeds.

Weakly bound free radicals have low-dissociation thresholds such that at high temperatures, time scales for dissociation and collisional relaxation become comparable, leading to significant dissociation during the vibrational-rotational relaxation process. Here we characterize this "prompt" dissociation of formyl (HCO), an important combustion radical, using direct dynamics calculations for OH + CH2O and H + CH2O (key HCO-forming reactions). For all other HCO-forming reactions, presumption of a thermal incipient HCO distribution was used to derive prompt dissociation fractions. Inclusion of these theoretically derived HCO prompt dissociation fractions into combustion kinetics models provides an additional source for H-atoms that feeds chain-branching reactions. Simulations using these updated combustion models are therefore shown to enhance flame propagation in 1,3,5-trioxane and acetylene. The present results suggest that HCO prompt dissociation should be included when simulating flames of hydrocarbons and oxygenated molecules and that prompt dissociations of other weakly bound radicals may also impact combustion simulations.