The effects of surface terminal bonds and microstructure of SiO2 aerogel films on dry etching

The effects of microstructure and surface terminal bonds of SiO₂ aerogel films on dry etching were investigated using Ar, SF₆, and C₂F₆ plasma gases. With Ar plasma etching, physical effect of ion bombardment on porous film was found. In residue-free SF₆ plasma etching, reactive etchant transport and high-mass ion bombardment were observed. With C₂F₆ plasma etching, fluorocarbon residue layer was revealed to maintain surface morphology as acting a barrier to radical transport and ion bombardment. An etching of 450°C-annealed SiO₂ aerogel showed that a dense surface induced the decrease in reaction area, inhibition of etchant transport, and then uniform etching.     

Infrared diode laser spectroscopy of FCO: The ν1 and ν2 bands

The ν₁ (CO stretching) and ν₂ (CF stretching) bands of the FCO radical were observed with Doppler-limited resolution by an infrared diode laser spectrometer with Zeeman and source modulation. The FCO radical was generated by a 60-Hz discharge in one of the following three gas mixtures: O₂ + C₂F₄, CO + SF₆, and CO + C₂F₄, all diluted with He. The observed spectra were analyzed to determine the rotational constants, the centrifugal distortion constants, and the spin-rotation interaction constants. The band origins, 1861.6372(1) and 1026.1283(1) cm^?1 [with standard errors in parentheses], which were obtained, were found to agree well with matrix data, 1857 and 1023 cm^?1, respectively. The assignment of the observed spectra to the FCO radical was further supported by observing the ν₁ band of F¹³CO, which was obtained from ¹³CO and SF₆. The molecular structure and the force field of FCO are briefly discussed by using molecular constants obtained from the observed spectra.     

High resolution study of anion formation in low-energy electron attachment to SF<Subscript>6</Subscript> molecules in a seeded supersonic beam

Using two variants of the Laser Photoelectron Attachment (LPA) method involving a differentially-pumped, seeded supersonic beam (0.05% and 12.5% of SF₆ molecules in helium carrier gas, nozzle temperatures T₀= 300–600 K, stagnation pressures p₀= 1–5 bar) and mass spectrometric ion detection, we have investigated the energy dependence of anion formation in low-energy electron collisions with SF₆ molecules at high energy resolution. Using the standard LPA method, the yield for SF₆^- as well as SF₅^- and F^- anions was studied with an energy width around 1 meV over the electron energy range 0–200 meV. In addition, a variant of the LPA method with extended energy range (denoted as EXLPA) was developed and applied to measure the yield for SF₆^- and SF₅^- formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF₆^- decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF₅^- formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for T₀= 300 K and shifting to lower energies with rising T₀) and a monotonical decrease towards higher energies. SF₅^- attachment spectra taken at elevated temperatures exhibit changes with rising stagnation pressure which directly reflect rovibrational cooling of the SF₆ molecules with rising pressure. The SF₅^-/SF₆^- intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF₅^- spectra are used as thermometers for the internal temperature of the SF₆ molecules in the seeded supersonic beam which (at p₀= 1 bar) are found to be 50–100 K lower than the nozzle temperature. The energy dependence of the yield for F^- formation is similar to that for SF₆^-, but the F^- signals are three to four orders of magnitude lower than those for SF₆^-; in view of the rather high endothermicity of F^- formation the origin of the F^- signals is discussed in some detail.     

Measurements of thermal electron attachment rate coefficients to molecules using an electron swarm technique

An existing electron swarm apparatus has been redesigned and upgraded. In particular, the new design incorporates a novel planar radioactive foil to form an integral part of the drift tube, allowing us to overcome inherent problems present in our earlier system which used a cylindrical radioactive source. In addition to this, substantial upgrades have been made to improve the gating and amplification electronics and the data acquisition system. This has resulted in a much greater signal to noise ratio and improved accuracy. This paper describes the upgraded apparatus and its use in obtaining thermal (300 K) attachment rate coefficients to a number of molecules. The quality of the measurements and data are illustrated through the measurement of the thermal attachment rate coefficient for SF₆ (k_th(SF₆) = (2.38 ±0.15 ) ×10 ^-7 cm³ s^-1). Thermal electron attachment rate coefficients for four other molecules are presented, namely for two derivatives of SF₆, SF₅CF₃ and SF₅Cl, and two perfluorocarbons, c-C₄F₈ and 2-C₄F₈.     

Total electron scattering cross section of C3F6O at the intermediate-energy region for developing an alternative insulation gas to SF6

Perfluoro methyl vinyl ether (C₃F₆O) is a promising alternative gas to SF₆ for switching applications in the power industry. We measured the total electron scattering cross section (TCS) of C₃F₆O in the intermediate energy region from 15 to 90 eV using a magnetized electron beam apparatus. The overall TCS values were less than 20.8 × 10⁻¹⁶ cm² within the measuring energy region. The measured and observed TCS exhibited some features, a steep increase before 30 eV, the highest value near 30 eV, and a gradual decrease after 30 eV. The total uncertainty was evaluated as 3.5–8.5%, depending on the incident electron energies. These first measurement data of C₃F₆O are useful for understanding both the discharging characteristics and identifying the insulation properties and arc-extinguishing.     

Phase transition in RbNbWO6: The pyrochlore structure revisited

A transition from tetragonal to cubic symmetry has been observed in the pyrochlore RbNbWO₆ at ～ 120° C and is shown to be related to the Rb⁺ motion in the structure. A simple potential energy model is developed to study the motion of the A⁺ cation inside the framework (M₂X₆)^- of the pyrochlore structure : the site energy difference between the 16d and 8b positions is comparable to the experimental activation energy.     

Photolysis of SF6 adsorbed on Si(111)-7 × 7 by monochromatic soft X-ray

Continuous-time photoelectron spectroscopy (PES) and photon-exposure-dependent photon-stimulated desorption (PSD) were employed to investigate the monochromatic soft X-ray-induced dissociation of SF₆ molecules adsorbed on Si(111)-7 × 7 at 30 K (SF₆ dose = 3.4 × 10¹³ molecules/cm², ～ 0.5 monolayer). The photon-induced evolution of adsorbed SF₆ was monitored at photon energies of 98 and 120 eV [near the Si(2p) edge], and sequential valence-level PES spectra made it possible to deduce the photolysis cross section as a function of energy. It was found that the photolysis cross sections for 98 and 120 eV photons are ～ 2.7 × 10^? 17 and ～ 3.7 × 10^?17 cm², respectively. The changes in the F^? and F⁺ PSD ion yields were also measured during irradiation of 120 eV photons. The photon-exposure dependencies of the F^? and F⁺ ion yields show the characteristics: (a) the dissociation of adsorbed SF₆ molecules is ascribable to the substrate-mediated dissociations [dissociative attachment (DA) and dipolar dissociation (DD) induced by the photoelectrons emitting from the silicon substrate]; (b) at early stages of photolysis, the F^? yield is mainly due to DA and DD of the adsorbed SF₆ molecules, while at high photon exposure the F^? formation by electron capture of the F⁺ ion is likely to be the dominant mechanism; (c) the F⁺ ion desorption is associated with the bond breaking of the surface SiF species; (d) the surface SiF is formed by reaction of the surface Si atom with the fluorine atom or F^? ion produced by scission of S–F bond of SF_n (n = 1–6) species.     

Radical cations of n-alkanes in irradiated solutions as studied by time-resolved magnetic field effects

The time-resolved magnetic field effect in the recombination fluorescence of spin-correlated radical ion pairs has been measured to study n-alkane radical cations in irradiated solutions at room temperature. The magnetic field effect was recorded as a ratio of fluorescence decay curves in the 0.1 T and zero magnetic fields for solutions of C₈, C₉, C₁₀, C₁₂, and C₁₆ n-alkanes in n-hexane with addition of 3 × 10^?5 M p-terphenyl-d ₁₄. A distinct maximum at 10–30 ns followed by a slowly decaying plateau was observed for all the solutions. Simulation shows that the maximum corresponds to an unresolved ESR spectrum with the peak-to-peak line-width ranging from about 1.6mT to 0.5mT for C₈ to C₁₆ radical cations. The unresolved structure is believed to result from the hyperfine couplings with many protons of the radical cation, the increase in the number of interacting protons compared with low temperature matrices being caused by the methyl group rotation and conformational motion of the carbon chain. With increase in concentration of dissolved n-alkane, the maximum in the curves first moves to longer times and finally disappears; this was attributed to the narrowing of ESR spectrum contour due to degenerate electron exchange.     

PEO-based composite lithium polymer electrolyte, PEO-BaTiO3-Li(C2F5SO2)2N

Polyethylene oxide (PEO) based polymer electrolytes with BaTiO₃ as filler and Li(C₂F₅SO₂)₂N as salt have been examined in lithium polymer batteries. The aluminum disolution potential in PEO-Li(C₂F₅SO₂)₂N was estimated to be 4.1 V vs. Li/Li⁺ at 80 °C, which was compared to that of 3.8 V vs. Li/Li⁺ in PEO-Li(CF₃SO₂)₂N. The electrical conductivity of the system was measured as a function of O/Li ratio. The highest conductivity was observed in O/Li=8. The conductivity was 1.65×10⁻³ S/cm at 80 °C and 1.5×10⁻⁵ S/cm at 25 °C. The interfacial resistance of Li/polymer electrolyte/Li annealed at 80 °C for 15 days was lower than 100 Ωcm². Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.     

X-ray structure refinement and ionic conductivity of Na6.69Ca3.355(SO4)6Cl0.77F0.63

The structure of Na_6.69Ca_3.355(SO₄)₆Cl_0.77F_0.63, isostructural with fluorapatite, was determined by X-ray powder diffraction methods. The results of Rietveld refinement revealed a space group P6₃/m with lattice parameters of a?=?9.477 (2) Å, c?=?6.865 (5) Å. Final refinement led to R _F?=?1.83 % and R _B?=?7.64 %. The location of Na⁺ ions in the M (2) sites surrounding the channels was related particularly to the high polarizability of the Ca²⁺. The ionic conductivity over a wide range of temperature was investigated according to the complex impedance method. The highest overall conductivity values were found at σ _500 °C?=?1.03?×?10^?5?S?cm^?1 and E_a?=?0.70 eV.     

Mechanisms of inelastic scattering of low-energy protons by C6H6, C60, C6F12, and C60F48 molecules

The mechanisms of inelastic scattering of low-energy protons with a kinetic energy of 2–7 eV by C₆H₆, C₆F₁₂, C₆₀, and C₆₀F₄₈ molecules are studied using the methods of quantum chemistry and nonempirical molecular dynamics. It is shown that, for the C₆H₆ + proton and C₆₀ + proton systems, starting from a distance of 6 Å from the carbon skeleton, the electronic charge transfer from the aromatic molecule to H⁺ occurs with a probability close to unity and transforms the H⁺ ion into a hydrogen atom and the neutral C₆H₆ and C₆₀ molecules into cation radicals. The mechanism of interaction of low-energy protons with C₆F₁₂ and C₆₀F₄₈ molecules has a substantially different character and can be considered qualitatively as the interaction between a neutral molecule and a point charge. The Coulomb perturbation of the system arising from the interaction of the noncompensated proton charge with the Mulliken charges of fluorine atoms results in an inversion of the energies of the electronic states localized, on the one hand, on the positively charged hydrogen ion and, on the other hand, on the C₆F₁₂ and C₆₀F₄₈ molecules. As a result, the neutral molecule + proton state becomes the ground state. In turn, this inversion makes the electronic charge transfer energetically unfavorable. Quantum-chemical and molecular-dynamics calculations on different levels of theory showed that, for fluorine derivatives of some aromatic structures (C₆F₁₂, C₆₀F₄₈), the barriers to proton penetration through carbon hexagons are two to four times lower than for the corresponding parent systems (C₆H₆, C₆₀). This effect is explained by the absence of active π-electrons in the case of fluorinated molecules.     

Polyethylene Sandwich Technique for Far Infrared Powder Samples

Recent research on the far infrared absorption of the free radical “tanane” (nitroxyde tetramethyl-2, 2, 6, 6,-piperidine oxyle, C₉H₁₈NO) in connection with its ferroelectric-ferroelastic transition at 14° C^{1, 2, 3} has shown that conventional sample preparation techniques are unsuitable. A new method which offers certain advantages has been developed and is described here since it has potential applicability to other powder samples and can be readily adapted for measurements at higher frequencies.     

From quantum chemistry to dissociation kinetics: what we need to know †

The relationship between rate constants for dissociation and the reverse association reactions and their potential energy surfaces is illustrated. The reaction systems e^? + SF₆ ? SF₆ ^? →SF₅ ^? + F, H + CH₃ ?CH₄, 2 CF₂ ? C₂F₄, H + O₂ →HO₂, HO + O ?HO₂ ? H + O₂, and C + HO →CHO are chosen as representative examples. The necessity to know precise thermochemical data is emphasised. The interplay between attractive and anisotropic components of the potentials influences the rate constants. Spin–orbit and electronic–rotational coupling in reactions between electronic open-shell radicals so far generally has been neglected, but is shown to have a marked influence on low temperature rate constants.     

Die Anwendung des SF6 für δ34S-Bestimmungen

Eine neue Methode für die chemische Probenaufbereitung der Mineralsulfide zu SF₆ zur δ³¹S-Bestimmung wird beschrieben. Durch Fluorierung mit elementarem Fluor bei 150°C und 5 atm werden die Proben in SF₆ umgewandelt. Die chemischen und physikalischen Eigenschaften des SF₆ ermöglichen eine relativ einfache Reinigung des Produktes. Wegen des höheren Massenbereiches, niedrigeren “memory effect” und monoisotopen Fluors sind massenspektrometrische Messungen mit SF₆ einfacher als mit SO₂ Als Beispiel werden δ³¹S-Werte einiger Proben, gemessen mit SF₆ und mit SO₂ angegeben.     

Multiexponential decay of negative molecular ions as a result of target molecule distribution over vibrational states

The lifetimes of long-lived negative molecular ions SF₆⁻-, C₆H₅NO₂⁻, and C₆F₆⁻ are measured with a static mass spectrometer. A great spread in published data for the lifetimes of these ions is explained using a concept of multiexponential decay of molecular ions. The influence of the Boltzmann distribution of neutral target molecules over vibrational states on the lifetime of negative molecular ions is studied in terms of the Illenberger-Smirnov-Kompaneits simple statistical model. It is shown that this distribution has a profound effect on the multiexponential decay of molecular anions and, as a consequence, on the lifetime of negative molecular ions measured on different mass spectrometers.     

Electron spin resonance and structure of the CO- 2 radical ion

Analysis of the electron spin resonance spectrum shows that CO^? ₂ radical ions are trapped in sodium formate at room temperature after irradiation of the crystal with high energy γ-rays. This radical is bent with an angle of about 134° and has a ² A ₁ ground state. From the experimental values of the ¹³C coupling coefficients and the g values, coefficients are derived for the mixing of atomic orbitals in the half-filled 4a ₁ molecular orbital which appears to be 14 per cent carbon 2s, 66 per cent carbon 2p _z and 11 per cent of each oxygen 2p _z . There is also coupling to the sodium nucleus in an adjacent cation.     

Molecularly imprinted polymer based potentiometric sensor for the determination of 1-hexyl-3-methylimidazolium cation in aqueous solution

The molecular imprinting technique is powerful to prepare functional materials with molecular recognition properties. In this work, a potentiometric sensor was fabricated by dispersing molecularly imprinted polymers (MIPs) into plasticized PVC matrix and used for the determination of 1-hexyl-3-methylimidazolium cation ([C₆mim]⁺) in aqueous solution. The MIPs were synthesized by precipitation polymerization using 1-hexyl-3-methylimidazolium chloride ([C₆mim]Cl) as the template molecule, methacrylic acid (MAA) and ethylene glycol dimethacrylat (EGDMA) as the functional monomers, and EGDMA also as the cross-linking agent. The as-prepared electrode exhibited a Nernstian response (58.87 ± 0.3 mV per decade) to [C₆mim]⁺ in a concentration range from 1.0 × 10⁻⁶ to 0.1 mol kg⁻¹ with a low detection limit of 2.8 × 10⁻⁷ mol kg⁻¹, high selectivity, and little pH influence. The as-prepared electrode was used for the detection of the [C₆mim]⁺ in distilled water, tap water, and river water with a good recovery. It was also successfully applied in the determination of mean activity coefficients of [C₆mim]Br in fructose + water systems based on the potentiometric method at 298.15 K.

     

Synthesis and ionic conduction of phases in the AgF-BiF3 system

Two types of phases have been prepared and characterized in the AgF-BiF₃ system at 400°C : AgBiF₄ which is isostructural with NaNdF₄ and a solid solution Ag_1-xBi_xF_1+2x (0.65 ?×?0.73) of the fluorite derived type. Ag_0.35Bi_0.65F_2.30 has the highest ionic conductivity (σ_100°C=3.5 10^-4ω^-1·cm^-1). The influenc of structural data on electrical properties is discussed.     

Raman scattering in stage 1 europium intercalated graphite

The Raman spectrum of stage 1 europium intercalated graphite, EuC₆, has been recorded at room temperature. EuC₆ has a p(√3 × √3)R30° in-plane superlattice structure, the same as that of LiC₆. The c-axis stacking sequences of EuC₆ and LiC₆ are different. The Raman spectrum of EuC₆ exhibits a broad asymmetric Fano resonance peak at 1500 ± 5 cm^-1, similar to that observed in MC₈ (M = K, RbandCs). However, there is no evidence of any spectral feature around 580 cm^-1 as what has been observed in MC₈. Considering the Raman features of various graphite intercalation compounds in light of their superlattice structures, we suggest that: (1) The feature around 580 cm^-1 is a characteristic associated only with the MC₈p(2 × 2)R0° in-plane superlattice structure. It should originate from the M point phonons of pristine graphite, which become Raman active as predicted by phonon band structure calculations. (2) The continuous background in the spectra of EuC₆ and MC₈, which results in the Fano resonance peak at 1500 cm^-1, is associated with the staggered stacking sequence. Stacking faults are easy to occur in crystals with staggered stacking sequences, but not in crystals with complete elliptical sequences. Therefore, a disorder induced continuum is observed in the Raman spectra of EuC₆ and MC₈, but not in that of LiC₆.