Parent Gas Research Articles

Ion drift in a gas in an external electric field

A model of ion collisions with gas atoms is constructed that takes into account resonant ion-atom charge exchange, polarization interaction, and short-range (gas-kinetic) collisions. The cross sections for resonant charge exchange of the ions of noble gases, as well as rubidium, cesium, and mercury ions, with atoms are calculated based on the experimental data and the results of numerical simulations of ion collisions with parent gas atoms in a uniform electric field. The effect of the electric field, the temperature of the buffer gas atoms, and the percentage composition of the gas mixture on the ion velocity distribution function is investigated.

Extended negative glow and “hollow anode” discharges for submillimeter-wave observation of CN−, C2H−, and C4H−

Three molecular anions, CN(-), C(2)H(-), and C(4)H(-), have been detected in an extended negative glow discharge and a "hollow anode" discharge in the submillimeter-wave region. These electrical discharges have been unexpectedly found to be reasonable anion sources. The measurements have been extended up to 830 GHz, and the molecular constants have been improved significantly compared with those determined by microwave and millimeter-wave spectroscopy. The parent gas mixture of C(2)N(2) or C(2)H(2) of 2 mTorr and Ar or Ne buffer of 15 mTorr was used for the production of CN(-) or C(2)H(-) and C(4)H(-). The number densities of the anions in the cell were estimated to be approximately 1x10(9), approximately 3x10(7), and approximately 3x10(6) cm(-3) for CN(-), C(2)H(-), and C(4)H(-), respectively.

Temperature Effects on Gaseous Fuel Cracking Studies Using a Dielectric Barrier Discharge

Fuel cracking using a plasma is one of the methods to enhance combustion by forming lighter hydrocarbons and hydrogen from heavier ones. Lighter hydrocarbons and hydrogen burn more cleanly and under much leaner burn conditions, thereby increasing combustion stability and producing less pollution (particularly oxides of nitrogen - NOx). The cracked fuel fragments are also expected to efficiently burn more, and thus, more energy can possibly be extracted from the fuel. Fuel cracking is accomplished at Los Alamos National Laboratory by using an AC annular dielectric barrier discharge (also called a silent electrical discharge). This forms a nonthermal plasma between the barrier (a ceramic tube) and the central electrode, which is typically charged to 5-10 kV. The nonthermal plasma is relatively cold, since electrons carry most of the energy and the ions and neutrals remain at approximately ambient temperature. Our experiments show that increased temperature inside the reactor elevates the stable concentration of CH4 (methane), C2H2 (acetylene), C2H4 (ethylene), and C3H8 (propane) from a C2H6 (ethane) parent gas. The AC waveform was used to heat the dielectric barrier, which in turn heated the nonthermal plasma in the reactor. Results of these experiments were compared to a KINEMA model of ethane plasma chemistry. The model predicts the same global trends as the experimental data. This modeling shows that certain radical-forming reactions, and direct pathways for H2, CH4, and C3H8, have enhanced reaction rates at higher temperatures compared to the ambient case.

Gas-Phase Chemistry of Pulsed n-Hexane Discharge

Many types of carbon films are being investigated for use in many different applications for their electrical, mechanical, and optical properties. Organic based plasmas are beginning to be used for the production of a wide variety of novel film stacks. The use of these stacks can range from sensor to dielectrics for flexible electronics to biocompatible surfaces. Knowledge of how these films actually grow is largely unknown. In light of this, we have performed Fourier transform infrared (FTIR) spectroscopy to the study of hexane plasmas. We use this diagnostic to identify different plasma-produced daughter species produced from n-hexane. For example, we observe the creation of methane, ethylene, and acetylene. From this we develop a likely dissociation model for the parent gas.

Self-broadening and shifting of the Li(2 s–2 p) and K(4 s–4 p) line cores

The aim of this work is to determine quantum mechanically the width Γ and the shift Δ of the lithium Li ( 2 p → 2 s ) and potassium K ( 4 p → 4 s ) resonance lines when these atoms are evolving in their parent gases. The interaction potentials along which the atoms Li ( 2 p ) + Li ( 2 s ) and K ( 4 p ) + K ( 4 s ) approach each other are constructed from reliable data. The radial wave equation is then solved numerically by using these potentials to compute the elastic phase shifts. By adopting the simplified Baranger model for the pressure broadening, which assumes the impact approximation, the cross sections effective in linewidth and lineshift are analyzed. The analysis leads in particular to the determination of the width and shift rates and the computations show that these rates have constant values and, mainly, do not depend on temperature. An approximate method is further applied to the calculations of the cross sections. The results reveal the influence of the long-range - C 3 / R 3 interactions and confirm the universality of the obtained formulas of Γ and Δ .

Chemical and physical properties of gas jets in comets: I. Monte Carlo model of an inner cometary coma

We describe a 3-dimensional, time-dependent Monte Carlo model developed to analyze the chemical and physical nature of a cometary gas coma. Our model includes the necessary physics and chemistry to recreate the conditions applicable to Comet Hale–Bopp when the comet was near 1 AU from the Sun. Two base models were designed and are described here. The first is an isotropic model that emits particles (parents of the observed gases) from the entire nucleus; the second is a jet model that ejects parent particles solely from discrete active areas on the surface of the comet nucleus, resulting in coma jets. The two models are combined to produce the final model, which is compared with observations. The physical processes incorporated in both base models include: (1) isotropic ejection of daughter molecules (the observed gases) in the parent's frame of reference, (2) solar radiation pressure, (3) solar insolation effects, (4) collisions of daughter products with other molecules in the coma, and (5) acceleration of the gas in the coma. The observed daughter molecules are produced when a parent decays, which is represented by either an exponential decay distribution (photodissociation of the parent gas) or a triangular distribution (production from a grain extended source). Application of this model to the analysis the OH, C 2 and CN gas jets observed in the coma of Comet Hale–Bopp is the focus of the accompanying paper [Lederer, S.M., Campins, H., Osip, D.J., 2008. Icarus, in press (this issue)].

Nitrogen Atmospheric Pressure Post Discharges for Surface Biological Decontamination inside Small Diameter Tubes

AbstractA nitrogen afterglow at atmospheric pressure has recently been described as able to transport active species over long distances in small diameter tubes, with a biocidal effect. For a discharge gas composed of nitrogen, either of high purity or with some controlled ppm of oxygen, survival curves are presented. The afterglow, flowing at 40 slm in a cylindrical quartz tube with 8 mm internal diameter is studied using emission spectroscopy. Fundamental or excited states of atomic or molecular species of parent gases are detected and evaluated. Their absolute concentration is measured along the tube axis. Correlated to transport equations, results give information on the creation and destruction reactions of these species, especially of the O(1S) metastable state of O, the species that has been shown to boost the biocidal effect.magnified image

The Sena effect

The Sena effect relates to the transport of atomic ions in a parent gas, when this transport results from the resonant charge exchange process involving ions and atoms. The physics of this process and its role in a gas discharge plasma are analyzed briefly.

Kinetics and mechanisms of Cl2 or HCl plasma etching of copper

An investigation by experiment and computer simulation is reported into the kinetics and mechanisms of the interaction between copper and impinging reactive species as the former is etched with a chlorine or a hydrogen chloride plasma. It is established that the manner in which etching proceeds is substantially the same for the two parent gases. There are, however, considerable differences in etch rate that are related to those in the types and stationary concentrations of reactive plasma species and in their fluxes to the surface being processed. Two regimes of etching are identified over the temperature range 373–653 K that differ in the character of the etch-rate-temperature relationship and in the shape of kinetics graphs. In a higher temperature regime, the plasma-etching reaction with copper involves complete removal of etch products from the surface; it proceeds under steady-state conditions and is of first order in terms of the concentration of reactive species in the bulk plasma. This occurs at temperatures above 500–520 K for Cl2 plasmas and at above 580–600 K for HCl plasmas.

Calculation of breakdown voltage in plasma display panels

Plasma display panels play an important role in flat display panel market. The description of physical phenomena present in the alternative current plasma display panel discharges needs to know the breakdown voltage. This leads to minimize the applied voltage. The Paschen law gives the breakdown voltage for only pure gases. For this reason an analytical calculation has been done for pure gases and a xenon-neon mixture in order to estimate the breakdown voltage. This study makes it possible to see the importance of the parent gas. The results were obtained in an analytical approach to the numerical and experimental data.

Radiation-induced synthesis of branched liquid alkanes

The irradiation of gaseous alkane mixtures under circulation conditions was used for the synthesis of liquid branched hydrocarbons. It was found that the synthesized liquid product was a mixture of alkanes with the average molecular weight higher than the molecular weight of the parent gas by a factor of 3–4. The resulting liquids were characterized by boiling range from 35 to 200°C in atmospheric distillation. The average degree of molecular branching in the synthesized liquids was evaluated on the basis of their knock resistance. The octane ratings of liquid mixtures were above 95 (motor octane number) or 103 (research octane number). The fractional composition and detonation properties of the synthesized liquids suggested the prevalence of C5–C11 isomers with highly branched structures in these liquids. Depending on irradiation conditions, 2,3-dimethylbutane, 2-methylpentane, or 3-methylpentane was predominant among hexanes. As a rule, 2,2,3-trimethylbutane and 2,3-dimethylpentane prevailed among heptanes.

Control of reactive plasmas for low-k/Cu integration

We proposed models for controlling surface reactions during etching of SiOCH and organic material. The etch rate of each material can be determined by the balance between the total atom fluxes of O, C, F, N, and H that were supplied from both the plasma and the etched material to the reactive layer. Low-k films (SiOCH, porous SiOCH, and organic material) have narrow process windows for obtaining good etching properties, such as selectivity, because the polymer and reactive layers on these films can be changed by only slight changes in the plasma parameters. Therefore, the partial pressure and dissociation of parent gas molecules in fluorocarbon or N–H plasma as well as plasma–wall interaction must be controlled. To create highly reliable interconnects, the interfaces between the metal and low-k must be optimized during the etching of stopper material and ashing. The surface of Cu is very reactive, the remaining F induces degradation of Cu. SiOCH can easily be oxidized during ashing processes, and the adsorption of H2O on damaged SiOCH causes interconnect failure during electrical tests. To suppress problems in the etching and ashing processes, the balance of the total atom fluxes should be quantitatively and instantaneously controlled to the optimum point for each material.

Effect of ion collisions on the parameters of dust-plasma structures

A study is made of how collisions of ions with gas atoms affect the parameters of an ion flow and the interaction between dust grains, as well as their interaction with the flow. The ion velocity distribution in a gas discharge is analyzed with allowance for both resonant charge exchange of the ions with parent gas atoms and polarizing collisions. The interaction forces between a dust grain and an ion flow and among the grains due to the charge exchange of ions with gas atoms near the grain are examined.

Ion chemistries in hexamethyldisiloxane

Dissociative ionization of hexamethyldisiloxane (HMDSO) by electron impact is investigated using Fourier transform mass spectrometry. Absolute total and partial ionization cross sections are measured and the results compared with previous studies. The collision-induced dissociation technique is used to probe the fragmentation pathways of ions, providing insight into the production of neutral products from the electron impact ionization. Ion-molecule reactions of Ar+ and HMDSO ions with the parent gas molecule are studied, and isotope studies are used to evaluate the possible reaction mechanisms.

Ionization of octafluorocyclopentene, c-C5F8

The ion chemistry in octafluorocyclopentene (c-C5F8) is examined by Fourier-transform mass spectrometry under single-collision conditions in the 10−7 Torr pressure range. Absolute total and partial cross-sections of electron impact ionization on c-C5F8 are measured as a function of the electron energy in the range 10–200 eV. The molecular ion and 11 fragment ions are observed with a total cross-section of 1.0 × 10−15 cm2 at ∼70 eV. The ion population at low energies (< 40 eV) is dominated by , , , and . Ar+ charge transfer reaction with c-C5F8 produces the same important ions except the parent ion . All ions from c-C5F8 are non-reactive with the parent gas molecule, except CF+ and , which react to form .

A Safety Net for Fast Migrators: Interactions between Gap‐opening and Sub–Gap‐opening Bodies in a Protoplanetary Disk

Young planets interact with their parent gas disks through tidal torques. An imbalance between inner and outer torques causes bodies of mass $\ga 0.1$ Earth masses to lose angular momentum and migrate inward rapidly relative to the disk; this is known as ``Type I'' migration. However, protoplanets that grow to gas giant mass, O($10^2) M_\oplus$, open a gap in the disk and are subsequently constrained to migrate more slowly, locked into the disk's viscous evolution in what is called migration. In a young planetary system, both Type I and Type II bodies likely coexist; if so, differential migration ought to result in close encounters when the former originate on orbits exterior to the latter. We investigate the resulting dynamics, using two different numerical approaches: an N-body code with dissipative forces added to simulate the effect of the gas disk, and a hybrid code which combines an N-body component with a 1-dimensional viscous disk model, treating planet-disk interactions in a more self-consistent manner. In both cases, we find that sub-gap-opening bodies have a high likelihood of being resonantly captured when they encounter a gap-opening body. A giant planet thus tends to act as a barrier in a protoplanetary disk, collecting smaller protoplanets outside of its orbit. Such behavior has two important implications for giant planet formation: First, for captured protoplanets it mitigates the problem of the migration timescale becoming shorter than the growth timescale. Secondly, it suggests one path to forming systems with multiple giant planets: Once the first has formed, it traps/accretes the future solid core of the second in an exterior mean-motion resonance, and so on. The most critical step in giant planet formation may thus be the formation of the very first one.

Monte Carlo simulation of the motion of argon ions in the vicinity of the cylindrical Langmuir probe

The motion of argon ions in its parent gas in the electric field was studied using standard Monte Carlo collision method in order to fit the collision cross sections for momentum transfer and for resonant charge transfer on the experimental data. The classical Langevin formula for polarization scattering was used for the latter. As a fitted parameter the momentum transfer cross section was used. The experimental dependency of the drift velocity on the ratio of the electric field intensity and the neutral gas number density was used as a fitted data. The fitted collision cross sections were used for the Monte Carlo study of the influence of ion-neutral collisions on the saturated ion probe current. The calculated ion current characteristics were compared with the results of Langmuir probe measurements under different plasma conditions.

Ion mobilities in Xe/Ne and other rare-gas mixtures.

The ion mobility or drift velocity data important for modeling glow discharges in rare gas mixtures are not generally available, nor are the ion-neutral scattering cross sections needed to calculate these data. In this paper we propose a set of cross sections for Xe+ and Ne+ collisions with Xe and Ne atoms. Ion mobilities at 300 K calculated using this cross section set in a Monte Carlo simulation are reported for reduced field strengths, E/N, up to 1500 x 10(-21) V m(2), in pure gases and in Xe/Ne mixtures containing 5% and 20% Xe/Ne, which are mixtures of interest for plasma display panels (PDPs). The calculated Xe+ mobilities depend strongly on the mixture composition, but the Ne+ mobility varies only slightly with increasing Xe in the mixture over the range studied here. The mobilities in pure gases compare well with available experimental values, and mobilities in gas mixtures at low E/N compare well with our recent measurements which will be published separately. Results from these calculations of ion mobilities are used to evaluate the predictions of Blanc's law and of the mixture rule proposed by Mason and Hahn [Phys. Rev. A 5, 438 (1972)] for determining the ion mobilities in mixtures from a knowledge of the mobilities in each of the pure gases. The mixture rule of Mason and Hahn is accurate to better than 10% at high field strengths over a wide range of conditions of interest for modeling PDPs. We conclude that a good estimate of ion mobilities at high E/N in Xe/Ne and other binary rare gas mixtures can be obtained using this mixture rule combined with known values of mobilities in parent gases and with the Langevin form for mobility of rare gas ions ion in other gases. This conclusion is supported by results in Ar/Ne mixtures which are also presented here.

Warming Potential Reduction of C4F8 Using Inductively Coupled Plasma

The global warming potential (GWP) reduction of the perfluorinated compound (PFC) semiconductor manufacturing gas, C4F8, has been studied in an inductively coupled high-frequency (13.56 MHz) plasma system operating under multimode antenna. Mass spectral analysis of the decomposed species showed that major ionic species in the present study were (CFx+ (x=1,3) and C2F4+). Weak signals of CF4+ ions were detected for the first time in a C4F8 plasma. The behavior of the CF4+, C2F4+ and CF3+ ion peaks at various gas pressures (1–50 mTorr) and the effect of additive oxygen on the decomposition of the parent gas and CF4 have been presented. The mechanism of electron energy control was proposed for the effective decomposition of the C4F8 gas. The overall results showed that, at a higher azimuthal mode of antenna (m=1) the C4F8 decomposition and its GWP reduction was significant and it was considered to be due to the electron energy control at that mode.

Momentum transfer theory of ion transport under the influence of resonant charge transfer collisions: the case of argon and neon ions in parent gases

Transport properties of ion swarms in presence of Resonant Charge Transfer (RCT) collisions are studied using Momentum Transfer Theory (MTT). It was shown that, not surprisingly, RCT collisions may be represented as a special case of elastic scattering. Using the developed MTT we tested a previously available anisotropic set of cross-sections for Ar+Ar + collisions by making the comparisons with the available data for the transverse diffusion coefficient. We also developed an anisotropic set of Ne+Ne + integral cross-sections based on the available data for mobility, longitudinal and transverse diffusion. Anisotropic sets of cross-sections are needed for Monte Carlo simulations of ion transport and plasma models.