Mass spectrometer-wall probe diagnostic of Ar discharges containing SF6 and/or O2: Reactive ions in etching plasmas

Positive and negative ions of Ar/SF₆ and Ar/SF₆/O₂ plasmas (etching plasmas) and of Ar/O₂ plasmas (cleaning plasmas) in Pyrex tubes have been investigated using a mass spectrometer-wall probe diagnostic technique. The measurement of negative ions proved to be a very sensitive method for the detection of wall material. In etching plasmas with small admixtures of SF₆, oxygen was found as the only representative of wall material. At larger amounts of SF₆, silicon could be detected. In cleaning plasmas with small admixtures of O₂ applied to a previously etched Pyrex surface, fluorine was found, indicating the reversal of fluoridation by oxygenation.     

Real-time monitoring traces of SF6 in near-source ambient air by ion mobility spectrometry

The leakage of sulphur hexafluoride (SF₆) gas threats the global climate changes and personnel safety. Monitoring the concentration of SF₆ in its application places is an industry regulation. In this study, ion mobility spectrometry (IMS) was developed for fast monitoring traces of SF₆ in near-source ambient air. Due to the water is an important part of the natural air and affects most atmospheric measurements, the operating parameters of IMS monitoring SF₆ were optimised for quantitative analysis of SF₆ at different relative humidity (RH). It is discovered two main product ions SF₆^? and SOF₄^? by IMS at different RH. The calibration curves of SF₆ were investigated by its relationship with the peak intensity of SOF₄^– for real application. The time resolution of the measurement was obtained less than 1 s and the limit of detection (LOD) achieved 0.16–0.68 ppm with a data averaging of 30 times. At last, the simulated application of monitoring SF₆ leakage was tested in the fume hood of our lab. The results showed a great potential application prospect of IMS in monitoring SF₆ in the ambient air of its application places.     

DC plasma etching of silicon by sulfur hexafluoride. Mass spectrometric study of the discharge products

Polycrystalline silicon wafers were etched in dc discharges of SF₆. SF_x species were extracted from the discharges and measured with a mass spectrometer. A systematic procedure was used to measure the SF _x ⁺ signals such that they are indicators of events in the discharge close to the sample undergoing etching. The picture that emerges is remarkably simple and shows the relative stability of several SF_x species including SF₆, SF₄, SF₂, and SF which are shown to be extracted from the discharge both in the presence and absence of the silicon sample. When silicon is being etched on the cathode of the discharge cell, the only significant additional products are SiF₄ and S₂F₂. A comparison of blank and sample data for opposite substrate polarities shows that there is only a small cation-assisted etching effect and suggests that ions do not play an important role in the etching of silicon by SF₆ discharges.     

Cluster-assisted decomposition reactions of the molecular anions of SF6 and C7F14

The cluster ions formed by the attachment of dimethylsulfoxide (DMSO) and methanol to the molecular negative ions of C₇F₁₄ and SF₆ have been studied by a pulsed e-beam high pressure mass spectrometer (PHPMS) and by an atmospheric pressure ionization mass spectrometer (APIMS). The free energy change (ΔG°) for the clustering equilibria reaction, M⁻ + S M⁻S, at 35 °C are found to be −7.7 and −7.s kcal/mol for S = DMSO and M⁻ = C₇F⁻₁₄ and SF⁻₆, respectively, and −6.4 and −4.5 kcal/mol for S = methanol and M⁻ = C₇F⁻₁₄ and SF⁻₆, respectively. While the cluster ions formed by DMSO are found to be stable against side reactions, those formed by methanol undergo decomposition processes in which the central core ion is fragmented. At 35 °C, the rate law for the decomposition of the SF⁻₆ (CH₃OH)₁ ion is second-order, involving the M⁻ (CH₃OH)₁ cluster ion and another methanol molecule. While the C₇F⁻₁₄(CH₃OH)₁ ion also decomposes through this second-order process, a competing unimolecular mechanism is also operative at 35 °C. With increases in the PHPMS ion source temperature to 150 °C, the unimolecular decomposition process becomes progressively dominant for both of the M⁻(CH₃OH)₁ cluster ions of C₇F₁₄ and SF₆. Methanol cluster ions of the type M⁻S₂ are not observed under any of the conditions examined here. When methanol or water partial pressures of a few torr or higher are present in the buffer gas of the APIMS ion source, the decomposition reactions are very fast and only the fragment ions produced by these reactions are observed in the electron-capture (EC)-APIMS spectra of C₇F₁₄ and SF₆. Also, in the methanol-containing APIMS ion source, the course of the SF⁻₆ decomposition reaction is altered so that fragment ions of the type F⁻(S)_n dominate the EC-APIMS spectrum of SF₆ at all ion source temperatures. For C₇F₁₄, fragment ions of the type F⁻(S)_n become dominant at lower ion source temperatures. These previously unknown reactions are expected to be important in the analysis of perfluorinated compounds by mass spectrometric methods that utilize ionization by electron capture or negative chemical ionization. The nature of the fragment ions produced in these cluster-assisted reactions may also provide a new source of information concerning the structures of the molecular negative ions of SF₆ and C₇F₁₄.     

Anisotropy of fragment ions from SF6 by photoexcitation between 23 and 210 eV

The anisotropy of the ionic photofragments produced from SF₆ has been measured using synchrotron radiation in the range of 23–210 eV. Despite the highly symmetrical molecule a strong anisotropy is observed below ∼35 eV. The behavior of the asymmetry parameter involving all the fragment ions has been interpreted by simulation using partial oscillator strengths for the formation of individual species. Only SF₅⁺ ions produced via superexcited states of valence type are assumed to have an anisotropic angular distribution. The observed decrease in the asymmetry parameter with increasing photon energy can be ascribed to the dominance of direct photoionization and the decrease in the branching ratio for SF₅⁺ formation.     

Electron impact ionization of CCl4 and SF6 embedded in superfluid helium droplets

Electron impact ionization of helium nano-droplets containing several 10⁴ He atoms and doped with CCl₄ or SF₆ molecules is studied with high-mass resolution. The mass spectra show significant clustering of CCl₄ molecules, less so for SF₆ under our experimental conditions. Positive ion efficiency curves as a function of electron energy indicate complete immersion of the molecules inside the helium droplets in both cases. For CCl₄ we observe the molecular parent cation CCl₄⁺ that preferentially is formed via Penning ionization upon collisions with He*. In contrast, no parent cation SF₆⁺ is seen for He droplets doped with SF₆. The fragmentation patterns for both molecules embedded in He are compared with gas phase studies. Ionization via electron transfer to He⁺ forms highly excited ions that cannot be stabilized by the surrounding He droplet. Besides the atomic fragments F⁺ and Cl⁺ several molecular fragment cations are observed with He atoms attached.     

Mass-spectrometric investigation of the anions and cations in a hydrocarbon flame doped with sulphur/fluorine additives

A fuel-lean, premixed, CH₄O₂ flame at atmospheric pressure was doped with 0.2 mol% of SF₆ and 0.1 mol% of triflic anhydride (CF₃SO₂)₂O. Primarily the anions, but also the cations, occurring in the flame reaction zone and in the burnt-gas region downstream were observed by sampling the flame through a nozzle into a mass spectrometer. The main objective was to ascertain the ability of these sulphur/fluorine (S/F) additives and their combustion products to scavenge free electrons by forming negative ions in the flame gas. With either additive present, both total anions and cations increased in the reaction zone by a factor of between three and four. In the burnt gas, the total cations were essentially unchanged but the total anions showed a three-fold increase. With SF₆ additive only, major S/F cations were observed in the reaction zone (e.g.; SF⁺₃, SF₃O⁺, SF⁺₃, etc.) although H₃O⁺ with both additives was completely dominant downstream. Both additives produced major S/F anion signals in the flame reaction zone. Some of these were related to the individual additive structure (e.g. SF⁻₅ with SF₆; CF₃SO⁻₃ with (CF₃SO₂)₂O). Others were essentially unrelated but were observed with both additives (e.g. FSO⁻₃, HSO⁻₄, etc.) and persisted throughout the burnt gas. Some important features of the ion chemistry are discussed.     

Heat of Adsorption of Pure Sulfur Hexafluoride on Micro-Mesoporous Adsorbents

The isotherms and the isosteric heats of adsorption of pure SF₆ were measured on two microporous zeolites (NaX and Silicalite), one mesoporous alumina, and two activated carbons (BPL and PCB) at 305 K. The adsorption isotherms were Type I by Brunauer classification. The PCB carbon adsorbed SF₆ most strongly and the alumina adsorbed SF₆ most weakly. The adsorption of SF₆ on the other three materials were comparable in the low pressure region despite their drastic differences in the physicochemical properties. The heat of adsorption of SF₆ on the silicalite and the alumina remained practically constant over a large range of coverage. The heat of adsorption of SF₆ increased with increasing adsorbate loading on the NaX zeolite in the high coverage region. The heat of adsorption of SF₆ on the activated carbons decreased with increasing adsorbate loading before leveling off in the high coverage region.     

TEA CO2-Laser Induced SF6 + Ba Beam-Surface Ionization

A TEA CO₂-laser induced SF₆ + Ba beam-surface ionization process has been studied when vibrational excitation of SF₆ molecules was carried out at (and near) the polished surface of the electrically heated up to 675 K polycristalline Ba. Electron emission and negative molecular ion signal were detected. The dependence of the molecular ion signal on laser fluence and frequency (on SF₆ molecular absorption) as well as on the SF₆ gas pressure in the nozzle were studied. The results reveal a nonlinear, probably multiphoton character of the molecular ion formation and a clear vibrational selectivity i.e. vibrational enhancement, of the SF₆ + Ba beam-surface ionization process. Possible mechanisms of the negative molecular ion formation in the IR laser induced SF₆ + Ba beam-surface ionization process are discussed.     

Selective Dissociation of Sulfur Hexafluoride by Intense CO2 Laser Radiation in Pulsed Gas Dynamic Flow

Isotopically selective IR multiphoton dissociation (MPD) of SF₆ in a pulsed gas dynamic flow was studied. The dependence of the yield of the product SF₄ on the frequency of CO₂ laser radiation exciting SF₆ molecules was obtained. The ³⁴S enrichment coefficient in SF₄ was measured. The enrichment factor was found to agree well with the value predicted from comparison of spectral dependences for the SF₄ yield from ³²SF₆ and ³⁴SF₆. The obtained results are compared with the data on SF₆ dissociation in a low-temperature cell and in a molecular beam.     

Identification of corona discharge-induced SF6 oxidation mechanisms using SF6/18O2/H2 16O and SF6/16O2/H2 18O gas mixtures

The absolute yields of gaseous oxyfluorides SOF₂, SO₂F₂, and SOF₄ from negative, point-plane corona discharges in pressurized gas mixtures of SF₆ with O₂ and H₂O enriched with¹⁸O₂ and H₂ ¹⁸O have been measured using a gas chromatograph-mass spectrometer. The predominant SF₆ oxidation mechanisms have been revealed from a determination of the relative¹⁸O and¹⁶O isotope content of the observed oxyfluoride by-product. The results are consistent with previously proposed production mechanisms and indicate that SOF₂ and SO₂F₂ derive oxygen predominantly from H₂O and O₂, respectively, in slow, gas-phase reactions involving SF₄, SF₃, and SF₂ that occur outside of the discharge region. The species SOF₄ derives oxygen from both H₂O and O₂ through fast reactions in the active discharge region involving free radicals or ions such as OH and O, with SF₅ and SF₄.     

A molecular orbital study of bond energies in compounds of sulfur and fluorine

CNDO/2 molecular orbital theory is employed in a study of the binding energies of the molecules SF, SF₂, SF₄, SF₆, their cations and anions, and of the molecules SSF₂, FSSF and S₂F₁₀. Computed energies, when rescaled according to energy partitioning concepts, compare favorably with available experimental data. Ionization energies and electron affinities are calculated for SF, SF₂, SF₄ and SF₆.

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Zusammenfassung Mit Hilfe der CNDO/2 Theorie werden die Bindungsenergien der SF, SF₂, SF₄ und SF₆ Moleküle, von deren positiven und negativen Ionen und von SSF₂, FSSF und S₂F₁₀ berechnet. Die berechneten Energien stimmen gut mit experimentellen Daten überein, wenn sie nach Energieaufteilungsprinzipien wiederberechnet werden. Ferner werden die Ionisierungsenergien und Elektronenaffinitäten für SF, SF₂, SF₄ und SF₆ angegeben.

     

A full dimensional direct ab initio dynamics study of the electron capture by SF6

The dynamics of electron capture by sulfur hexafluoride (SF₆) have been investigated by means of full dimensional direct ab initio dynamics calculations at the HF/6-311G(d)+sp (a diffuse sp-function is further added to sulfur atom) level. A rigid SF₆ structure and narrow Franck–Condon region for electron capture was assumed in choosing initial conditions for the trajectories. The direct ab initio dynamics calculations for the electron capture processes by SF₆ indicated that the SF⁻₆ ion formed by the vertical electron attachment to SF₆ decomposes into SF⁻₅ and F via short-lived complex formation (SF⁻₆). The lifetime of SF⁻₆ was distributed from 0.1–0.2 ps, which is quite short as lifetime of the intermediate complex. The dynamics calculations showed that 12% of the total available energy is partitioned into the relative translational mode between SF₅⁻ and F. The effect of solid phase on the dynamics has been examined by introducing a model bath-relaxation time for energy dispersion. There was a possibility that the SF₆⁻ anion exists in solid phase. The mechanism of the electron capture by SF₆ in gas phase and in solid phase was discussed on the basis of the results.     

Electron transfer from laser excited rydberg atoms to molecules. Absolute rate constants at low and intermediate principal quantum numbers

Using mass spectrometric detection of positive and negative ions, we have investigated ionizing reactions of Ne(ns,nd) Rydberg atoms, efficiently excited by resonant two-photon excitation of metastable Ne(3s ³ P ₂) atoms, with electron attaching moleculesBC (BC=SF₆, CCl₄, CS₂, O₂) at thermal collision energies. Absolute rate constants have been determined in the range of low and intermediate principal quantum numbersn(5≦n?30) by utilizing the photoionization signal caused by room temperature black-body radiation and the loss of Ne(3s ³ P ₂) atoms, associated with the laser excitation. Substantially differentn-dependences of the electron transfer cross section have been found for the larger molecules (BC = SF₆, CCl₄) and the smaller molecules (BC = CS₂, O₂). Simple model calculations have been performed to gain new insight into the dynamics of the electron transfer process; forBC = SF₆, our results at lown(5 ≦n ≦ 10) suggest that internal energy conversion in the Coulombic complex Ne⁺ — SF ₆ ^? is important for the formation of the detected ions.     

Hot-atom initiated chemical laser study of the H* + SF6 reaction

Chemical laser methods have been used to study the “hot-atom” reaction H* + SF₆ → products at KE_rel < 102 kcal mole^?1. Collision-induced dissociation of SF₆ by H* is the dominant reaction channel. Internal excitation of SF₆ may be required to promote the abstraction reaction: H + SF₆ → HF + SF₅.     

Catalytic Degradation of Sulfur Hexafluoride by Rhodium Complexes

The development of a safe and efficient method for the degradation of SF₆ is of current environmental interest, because SF₆ is one of the most potent greenhouse gases. SF₆ is thermally and chemically extremely inert, and therefore, it has been used in various industrial applications. However, this inertness results in a major challenge for its depletion. We report on a process for a catalytic degradation of SF₆ in the homogeneous phase by using rhodium complexes as precatalysts. The SF₆ activation reactions feature mild reaction conditions, low catalyst loadings, and a high selectivity. The employment of phosphines and hydrosilanes for scavenging the sulfur and fluorine atoms of the SF₆ molecule allows the selective transformation of SF₆ into nongaseous and nontoxic compounds.     

Nucleophilic Activation of Sulfur Hexafluoride: Metal‐Free,Selective Degradation by Phosphines

The development of new methods for the chemical activation of the extremely inert greenhouse gas sulfur hexafluoride (SF₆) not only is of current environmental interest, but also offers new opportunities for applications of SF₆ as a reagent in organic synthesis. We herein report the first nucleophilic activation of SF₆ by Lewis bases, namely by phosphines, which results either in its complete degradation to phosphine sulfides and difluorophosphoranes or in the selective conversion of SF₆ into a bench‐stable, crystalline salt containing the SF₅^? anion. Quantum chemical calculations reveal a nucleophilic substitution mechanism (S_N2) for the initial fluorine abstraction from SF₆ by the phosphine. Furthermore, a scalable one‐pot procedure for the complete decomposition of SF₆ into solid, nonvolatile products is presented based on cheap and commercially available starting materials.     

Electron scattering and dissociative attachment by SF6 and its electrical-discharge by-products

Discrete electron-molecule processes relevant to SF₆ etching plasmas are examined. Absolute, total scattering cross sections for 0.2–12-eV electrons on SF₆, SO₂, SOF₂, SO₂F₂, SOF₄, and SF₄, as well as cross sections for negative-ion formation by attachment of electrons, have been measured. These are used to calculate dissociative-attachment rate coefficients as a function ofE/N for SF₆ by-products in SF₆.     

Experimental investigation and simulation of hollow fiber membrane process for SF6 recovery from GIS

In this present study, polyethersulfone hollow fiber membrane was used to recover sulfur hexafluoride (SF₆) from gas‐insulated switchgear (GIS). SF₆, N₂ pure gas and mixed gas (12.5 vol.% of SF₆) experiment was initiated to observe permeation behavior according to temperature and pressure difference and retentate flow rate. Scanning electron microscopy was used to investigate the morphological characteristics and the structure of asymmetric hollow fibers. The permeation rates of SF₆ and N₂ were measured by the variable pressure method. As a result, permeance of N₂ was 9.5–16.3 GPU, and selectivity of N₂/SF₆ was 10.5–13.3. Moreover, the concentration of SF₆ in the retentate stream reached to 99.2% by the control of the operating condition. Based on the experimental results, tree‐stage membrane process was designed using simulation program. As a result, demanded membrane area reduced about 74% according to operating condition difference. Copyright © 2013 John Wiley & Sons, Ltd.     

Nucleophilic Activation of Sulfur Hexafluoride: Metal‐Free,Selective Degradation by Phosphines

The development of new methods for the chemical activation of the extremely inert greenhouse gas sulfur hexafluoride (SF₆) not only is of current environmental interest, but also offers new opportunities for applications of SF₆ as a reagent in organic synthesis. We herein report the first nucleophilic activation of SF₆ by Lewis bases, namely by phosphines, which results either in its complete degradation to phosphine sulfides and difluorophosphoranes or in the selective conversion of SF₆ into a bench‐stable, crystalline salt containing the SF₅⁻ anion. Quantum chemical calculations reveal a nucleophilic substitution mechanism (S_N2) for the initial fluorine abstraction from SF₆ by the phosphine. Furthermore, a scalable one‐pot procedure for the complete decomposition of SF₆ into solid, nonvolatile products is presented based on cheap and commercially available starting materials.