Dissociation Fraction Research Articles

A Catalytic Sensor for Measurement of Radical Density in CO2 Plasmas

A catalytic sensor for the measurement of radical density in weakly ionized CO2 plasmas, created in a low-pressure electrodeless discharge, is presented. The CO2 plasma was created in a 4 cm wide borosilicate glass tube inside a copper coil connected to a radio frequency generator operating at 27.12 MHz with a nominal power of 250 W. The dissociation fraction of the CO2 molecules was measured in the early afterglow at pressures ranging from 10 Pa to 100 Pa, and at distances of up to 35 cm along the gas stream from the glowing plasma. The radical density peaked (2 × 1020 m−3) at 80 Pa. The density quickly decreased with increasing distance from the glowing plasma despite a rather large drift velocity. The dissociation fraction showed similar behavior, except that the maximum was obtained at somewhat lower pressure. The results were explained by rather intense surface recombination of radicals.

Ionization and dissociation fractions of CO2under 10–30-keV H−, C−, and O−negative-ion impact

The ionization and dissociation fractions of the CO${}_{2}$ molecule under the impact of 10--30-keV negative ions H${}^{\ensuremath{-}}$, C${}^{\ensuremath{-}}$, and O${}^{\ensuremath{-}}$ are studied. The four recoil ions originating from the target molecule CO${}_{2}$ (CO${}_{2}{}^{+}$, CO${}^{+}$, O${}^{+}$, and C${}^{+}$) are measured and identified in coincidence with projectiles in two final charge states ($q=0$ and $q=+1$) by using a time-of flight spectrometer. The ionization and dissociation fractions of CO${}_{2}$ are found to associate with the momentum of the impacting ions. We also analyze the fractions for ionization and dissociation from a physical point. No comparison is given since no other experimental and theoretical data exist in the investigated energy range.

Optical emission diagnostics with electric probe measurements of inductively coupled Ar/O2/Ar-O2 plasmas

Physical properties of low-pressure inductively coupled argon, oxygen, and Ar-O2 mixture plasmas are investigated using optical emission spectroscopy (OES) combined with an rf-compensated Langmuir probe measurement. In each gas discharge, the electron density and the electron temperature were obtained by using the probe. The electron temperature was also obtained by OES models and compared with that measured by the probe. The electron temperature was observed to decrease with increasing power and pressure and also observed to decrease with increasing Ar content. Argon metastable densities were calculated based on an optical transition model. In Ar-O2 discharges, the dissociation fraction of O2 molecules was estimated using optical emission actinometry. The dissociation fraction was observed to increase with increasing power and Ar content.

Enhancement of the electrical properties of carbon nanotubes with Ar–N2 plasma treatment

Optical emission spectroscopy and Langmuir probe are used to investigate the low pressure inductively coupled Ar–N2 plasmas as function of rf power, filling pressure and Ar content in N2 discharge. It is observed that the active species generation, dissociation fraction and electron temperature significantly depends on discharge parameters and may be used to optimize the plasma reactor. Mixture of SWCNTs and MWCNTs are treated for different treatment time (0–120 min) at optimum discharge conditions. Changes induced in the elemental composition, surface morphology, crystallographic structure, and structural disorder in the plasma irradiated CNTs are analyzed by EDX, FTIR, SEM, XRD and Raman spectroscopy, respectively. Ar–N2 mixture plasma treatment of CNTs lead to a significant increase in the electrical conductivity, modify the microstructure and induce structural disorder and cause a transition of crystalline phase from well crystalline to an amorphous structure.

The I2 dissociation mechanisms in the chemical oxygen-iodine laser revisited

The recently suggested mechanism of I(2) dissociation in the chemical oxygen-iodine laser (COIL) [K. Waichman, B. D. Barmashenko, and S. Rosenwaks, J. Appl. Phys. 106, 063108 (2009); and J. Chem. Phys. 133, 084301 (2010)] was largely based on the suggestion of V. N. Azyazov, S. Yu. Pichugin, and M. C. Heaven [J. Chem. Phys. 130, 104306 (2009)] that the vibrational population of O(2)(a) produced in the chemical generator is high enough to play an essential role in the dissociation. The results of model calculations based on this mechanism agreed very well with measurements of the small signal gain g, I(2) dissociation fraction F, and temperature T in the COIL. This mechanism is here revisited, following the recent experiments of M. V. Zagidullin [Quantum Electron. 40, 794 (2010)] where the observed low population of O(2)(b, v = 1) led to the conclusion that the vibrational population of O(2)(a) at the outlet of the generator is close to thermal equilibrium value. This value corresponds to a very small probability, ∼0.05, of O(2)(a) energy pooling to the states O(2)(X,a,b, v > 0). We show that the dissociation mechanism can reproduce the experimentally observed values of g, F, and T in the COIL only if most of the energy released in the processes of O(2)(a) energy pooling and O(2)(b) quenching by H(2)O ends up as vibrational energy of the products, O(2)(X,a,b), where the vibrational states v = 2 and 3 are significantly populated. We discuss possible reasons for the differences in the suggested vibrational population and explain how these differences can be reconciled.

Two-dimensional visualization of excitation temperature and chemical species in divertor plasmas: Trace CD molecules by chemical sputtering of fusion plasma reactor wall

The divertor region is among the most important section of nuclear fusion reactors. In order to investigate the behavior of deuterium in divertor plasmas, emission profiles of the Balmer series lines of deuterium atoms, molecular bands of D2 and CD intensities have been two-dimensionally visualized with a CCD camera fitted with a bandpass filter, using a test system, the Nagoya Divertor Simulator II (NAGDIS-II). It has been clarified that profiles of emission from deuterium atoms and excitation temperature varied significantly with changes of the discharge current. From profiles of the ratio of the atomic emission intensity to the deuterium molecular emission, the increase in the dissociation fraction of deuterium molecule with the increase of the discharge current has been confirmed. Two dimensional visualization of the divertor plasma is a promising method for studies in nuclear fusion science.

Phase transformation in tungsten carbide–cobalt composite during high temperature treatment in microwave hydrogen plasma

To study phase formation up to 2300K, tungsten carbide–cobalt (WC–Co) samples were exposed to concentrated solar radiation in hydrogen plasma. The reducing atmosphere was a non-equilibrium hydrogen plasma created in a microwave cavity at the nominal power of 1000W. The dissociation fraction of hydrogen molecules in plasma was of the order of 10% assuring for almost optimal reduction of any oxide that could be formed on the surface of samples due to the residual atmosphere. Micro-structural characterization was performed by XRD and AES depth profiling. The results showed the appearance of Co6W6C phase at the temperature of 1050±50K. This phase almost vanished at the temperature of 1300±90K and was replaced by the Co3W3C phase. This phase vanished at 1690±150K where only WC and Co peaks were detected by XRD. The AES depth profiles showed enrichment of the surface film with Co at elevated temperatures. At extremely high temperature of 2300K, the Co vanished from the surface layer but remained in crystalline form in the bulk material. SEM imaging showed an evolution of the material crystallinity up to perfectly recognizable crystals of the dimension of approx. 1μm at the maximum temperature. The results are explained by mobility of Co atoms, surface segregation and sublimation at very high temperature.

Comparison of one- and three-dimensional computational fluid dynamics models of the supersonic chemical oxygen–iodine laser

A simple one-dimensional (1D) computational fluid dynamics (CFD) model of the chemical oxygen iodine laser (COIL) with supersonic mixing is compared with three-dimensional (3D) CFD models and with experimental measurements of the COIL parameters. Dependence of the gain, iodine dissociation fraction and temperature at the resonator optical axis and of the output lasing power on the iodine flow rate predicted by the 1D model is in good agreement with that found using 3D models and experimental results. Hence the 1D model can be used instead of much more complicated 3D models for estimates of the working parameters of supersonic COILs.

Dissociation of molecular iodine in RF discharge for oxygen-iodine lasers

The dissociation of molecular iodine in 40 MHz-RF discharge was studied experimentally. This generation of atomic iodine is aimed at use in oxygen-iodine lasers. The discharge was ignited in a mixture of I2 + buffer gas fast-flowing through the cylindrical chamber and the discharge products were injected into a supersonic flow of nitrogen. The atomic iodine number density was measured in a low-pressure cavity after mixing with nitrogen and the dissociation fraction was calculated related to the input I2 flow rate. The dissociation fraction of 46.2% was achieved at 0.22 mmol/s of I2 and 7 mmol/s of Ar and RF power of 500 W. Argon and helium were used as a buffer gas; discharge stability and dissociation efficiency were better with argon. At the I2 flow rate corresponding to the operation of a 1 kW chemical oxygen-iodine laser, the dissociation fraction was about 20%. The dissociation efficiency (the fraction of absorbed energy used for the dissociation) significantly decreased with increasing in the specific energy. At a reasonable I2 flow rate (0.32 mmol/s), the maximum achieved efficiency was 8.5% and the corresponding energy cost was 8.9 eV per dissociating of one I2 molecule. The input energy of more than 3 kJ per 1 mmol of I2 is needed for dissociating at least 50% of I2. The obtained dependencies on the gas flow rates infer a good chance for scaling-up of the tested RF discharge generator for the intended application.

3温度モデルを用いた水素MPDスラスタの電磁流体解析

Plasma flowfields and performances of an MPD thruster (MY-II thruster), which uses hydrogen as a propellant, are studied by solving axi-symmetrical two-dimensional magnetoplasmadynamic equations. The three-temperature model including the translation Ttr, vibration Tvib, and electron Te temperatures is incorporated into the flow solver. When the mass flow rate (m) is 1.37g/s, there is reasonable agreement on the thrust with measured data, especially at a discharge current (Jdis) of 7, 8kA. Under the conditions of m = 0.4g/s and Jdis = 5kA, the calculated thrust, specific impulse and thrust efficiency are 7.50N, 1,911s and 21.3% (without considering sheath voltage drop), respectively. The calculated flow shows a temperature nonequilibrium state (Te = 2.53eV, Tvib = 1.32eV, Ttr = 0.60eV) at the edge of the straight part, and it is found that the relaxation time between the vibration and the electron energy is equal to the characteristic time of the flow at (z, r) = (30mm, 20mm). Along the symmetrical axis, the dissociation and ionization fractions reach 1.0 (fully dissociation) and 0.2, respectively. Because the discharge current path by the three-temperature model can explain the important feature of hydrogen MPD (current expansion to the downstream), it is concluded that the three-temperature model can reproduce the experimental results.

J053012 衝撃波加熱分子線の飛行時間分布計測

We have developed the molecular beam source with non-diaphragm type small shock tube and evaluate the beam characteristics by the time-of-flight (TOF) method. We measured TOF distributions of the molecular beam of nitrogen. The shock tube with optimized geometry can generate the molecular beam with the translational energy of more than 1 eV at the repetition rate of 0.5 Hz. The translational energy of the beams is controlled by the initial pressure ratio. The beam source can also generate dissociated atomic oxygen beam with the fraction of dissociation controlled by the initial pressure ratio. These results demonstrate that our beam source is a promising tool for scattering experiments and surface reaction studies.

Optical Emission From Oxygen Plasma in E and H Modes

Highly reactive oxygen plasma with a very high dissociation fraction of oxygen molecules is often created in electrodeless discharges. The discharge runs either in the E or H mode. The E mode is obtained at low power while the H mode dominates at high power. The transition between two modes is easily observed by the naked eye or, even better, by optical emission spectroscopy. In this paper, we show major differences between the two modes obtained in an experimental plasma reactor with a 1200-W RF generator operating at the frequency of 13.56 MHz.

A Large Electrodeless Plasma Reactor as a Source of Radicals

Low-pressure plasma created in gases of two- or multiatom molecules is often used for dissociation of molecules to parent atoms. The dissociation fraction depends on the type of discharge, the discharge power and coupling, the properties of the discharge reactor, and the pumping speed. Large quantities of atoms are produced in large reactors providing poor loss due to heterogeneous surface recombination. The best results are obtained in electrodeless radio-frequency discharges created in glass discharge systems. An example of such a system is presented in this paper.

Pressure dependence of dissociation fraction and optical emission characteristics in low-pressure inductively coupled N2-Ar plasmas

A diagnostics study of low-pressure inductively coupled N2-Ar plasmas was performed by using optical emission spectroscopy (OES) and an rf-compensated Langmuir probe under the conditions of pressures of 1 - 30 mTorr and powers of 300 - 600 W. In the OES experiments, the argon was used as an actinometer and as an adding gas. The effect of the argon content in the gas mixture was examined in the range of 5 - 30%. The investigation of the effects of pressure on the dissociation fraction of nitrogen molecules and on the optical emission characteristics were carried out. The correction factors for estimating the dissociation fraction by OES actinometry accounting for argon effect were formulated and calculated. It was found that the dissociation fraction increased with increasing power and Ar content, while it decreased with increasing pressure. In addition, the electron energy probability function (EEPF), the electron density, and the electron temperature were obtained by using a Langmuir probe to investigate the effects of the plasma parameters on the optical emission characteristics and the dissociation fraction.

Measurement of the O2 Dissociation Fraction in RF Low Pressure O2/Ar Plasma Using Optical Emission Spectrometry

Measurement of the oxygen dissociation fraction in RF low pressure oxygen/argon plasma using optical emission spectrometry is presented. The oxygen dissociation fraction and its evolutions as functions of operational parameters were determined using argon as the actinometer. At a pressure of 30 Pa, the oxygen dissociation fraction decreased from 13.4% to 9.5% as the input power increased from 10 W to 70 W. At an input power of 50 W, the oxygen dissociation fraction decreased from 12.3% to 7.7% when the gas pressure increased from 10 Pa to 40 Pa. The influences of operational parameters on the generation of atomic oxygen were also discussed.

Surface Phase Behavior and Domain Topography of Ascorbyl Palmitate Monolayers

Ascorbyl palmitate (ASC(16)) is a molecule of potential pharmacological interest due to its antioxidant properties and amphiphilic nature. The surface behavior of ASC(16) was studied using Langmuir monolayers and Brewster angle microscopy. This molecule formed stable monolayers at room temperature that showed phase transition from a liquid-expanded to liquid-condensed or crystalline phase, depending on the subphase conditions. Using a theoretical approach, we were able to explain the behavior of the ASC(16) film at different bulk pH values and salt conditions based on the surface pH and the dissociation fraction of the film. Both condensed phases corresponded to highly packed conditions with the crystalline phase occurring at a low charge density, showing molecular tilting and preferential growth at characteristic angles, while the liquid-condensed phase formed in highly charged surfaces revealed small flowerlike domains probably as a consequence of internal dipole repulsion. A smaller perpendicular dipole moment was observed for the crystalline than the liquid-condensed phase which may explain the domain features. In conclusion, ASC(16) showed a complex surface behavior that was highly sensitive to subphase conditions.

Dissociation of diatomic molecules and the exact-exchange Kohn-Sham potential: The case of LiF

We examine the role of the exact-exchange (EXX) Kohn-Sham potential in curing the problem of fractional molecular dissociation. This is achieved by performing EXX calculations for the illustrative case of the LiF molecule. We show that by choosing the lowest-energy electronic configuration for each interatomic distance, a qualitatively correct binding energy curve, reflecting integer dissociation, is obtained. Surprisingly, for LiF this comes at the cost of violating the Aufbau principle, a phenomenon we discuss at length. Furthermore, we numerically confirm that in the EXX potential of the diatomic molecule, one of the atomic potentials is shifted by a constant while the other one is not, depending on where the highest occupied molecular orbital is localized. This changes the relative positions of the energies of each atom and enforces the integer configuration by preventing spurious charge transfer. The size of the constant shift becomes increasingly unstable numerically the larger the interatomic separation is, reflecting the increasing absence of coupling between the atoms.

E-H mode transition in low-pressure inductively coupled nitrogen-argon and oxygen-argon plasmas

This work investigates the characteristics of the E-H mode transition in low-pressure inductively coupled N2-Ar and O2-Ar discharges using rf-compensated Langmuir probe measurements and optical emission spectroscopy (OES). As the ICP power increases, the emission intensities from plasma species, the electron density, the electron temperature, and the plasma potential exhibit sudden changes. The Ar content in the gas mixture and total gas pressure have been varied in an attempt to fully characterize the plasma parameters. With these control parameters varying, the changes of the transition threshold power and the electron energy distribution function (EEDF) are explored. In N2-Ar and O2-Ar discharges at low-pressures of several millitorr, the transition thresholds are observed to decrease with Ar content and pressure. It is observed that in N2-Ar plasmas during the transition, the shape of the EEDF changes from an unusual distribution with a flat hole near the electron energy of 3 eV in the E mode to a Maxwellian distribution in the H mode. However, in O2 -Ar plasmas, the EEDFs in the E mode at low Ar contents show roughly bi-Maxwellian distributions, while the EEDFs in the H mode are observed to be nearly Maxwellian. In the E and H modes of O2-Ar discharges, the dissociation fraction of O2 molecules is estimated using optical emission actinometry. During the E-H mode transition, the dissociation fraction of molecules is also enhanced.

Extremely non-equilibrium oxygen plasma for direct synthesis of metal oxide nanowires on metallic substrates

A promising method for the synthesis of metal oxide nanowires is based on the application of the extremely non-equilibrium gaseous environment found in oxygen plasma created by some types of discharges. The kinetic temperature of neutral gas is kept close to the room temperature, the electron temperature is a few eV, the ionization fraction below 10−6 and the dissociation fraction close to 100%. Plasma with such characteristics is obtained using electrodeless high frequency discharges driven by radiofrequency or microwave generators. Plasma parameters such as the electron density and energy distribution function, the Debye length, the dissociation and ionization fractions, the density of negatively charged molecules, the ratio between the positively charged molecules and atoms and the distribution of atoms and molecules over excited states depend on discharge parameters. The most important discharge parameters are the generator power, frequency and coupling, the purity and pressure of working gas and the gas flow, the dimensions of the discharge chamber, the materials facing plasma, the residual atmosphere, and, usually very importantly though often neglected, the properties of the samples mounted into a discharge chamber. Proper construction of the experimental system for the synthesis of metal oxide nanowires allows for almost 100% dissociation fraction and thus extremely rapid growing of nanowires. The particularities of oxygen plasma as well as real-time monitoring of the dissociation fraction are elaborated in this contribution. The lack of reliable experimental results on characterization of extremely non-equilibrium oxygen plasma is stressed.

Dissociation fraction in low-pressure inductively coupled N 2–Ar and O 2–Ar plasmas

Experiments with a Langmuir probe and optical emission spectroscopy (OES) combined with actinometry were performed in inductively coupled rf 13.56 MHz N 2–Ar and O 2–Ar discharges under the pressure range of 1–30 mTorr and the power range of 400–600 W. The Ar contents in the gas mixture were varied from 5% to 80%. The effect of the Ar content on the variation of the emission line intensities from various species was investigated. The dissociation fraction of nitrogen and oxygen molecules was obtained from the measured intensity peaks of light emission by using the OES actinometry. The dissociation fraction of N 2 molecules has a peak value at a specific pressure (11 mTorr). The electron density and the electron temperature were obtained by using a Langmuir probe to investigate the effects of the plasma parameters on the dissociation fraction. The electron density was observed to increase with the power while the electron temperature to decrease with the pressure and power.