Characterization of an argon-hydrogen microwave discharge used as an atomic hydrogen source. Effect of hydrogen dilution on the atomic hydrogen production

This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon on atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the Ha(656.3 nm) and Hβ(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratioP _{H 2}/P _Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar⁺ and H₂ which lead to ArH⁺ and H ₂ ⁺ ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the electron temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H⁺, H ₂ ⁺ , and H ₃ ⁺ ) or excited species H(n=2) with H₂ producing H atoms.     

CCl<Subscript>4</Subscript> Decomposition in RF Thermal Plasma in Inert and Oxidative Environments

The decomposition of carbon tetrachloride was investigated in an RF inductively coupled thermal plasma reactor in inert CCl₄–Ar and in oxidative CCl₄–O₂–Ar systems, respectively. The exhaust gases were analyzed by gas chromatography-mass spectrometry. The kinetics of CCl₄ decomposition at the experimental conditions was modeled in the temperature range of 300–7,000 K. The simulations predicted 67.0 and 97.9% net conversions of CCl₄ for CCl₄–Ar and for CCl₄–O₂–Ar, respectively. These values are close to the experimentally determined values of 60.6 and 92.5%. We concluded that in RF thermal plasma much less CCl₄ reconstructed in oxidative environment than in an oxygen-free mixture.     

Radical reactions of C84

Pulse radiolysis and steady-state radiolysis experiments describing the radical and electron transfer reactions of C₈₄ are reported here for the first time. C₈₄ reacts readily with radiolytically generated chloromethyl (^•CCl₃) and trichloromethylperoxyl (CCl₃OO^•) radicals in CCl₄. The formation of the radical adduct has been confirmed from its characteristic absorption in the UV (320 nm) and visible (480 nm). Radical-induced oxidation in 1,2-dichloroethane (1,2-DCE) resulted in a short lived transient absorbing at 920 nm. Reduction of C₈₄ in toluene/2-propanol/acetone could be conveniently followed by formation of an absorption band with an absorption maximum at 960 nm.     

Chemical mechanisms in C3F8-H2 radiofrequency discharges

Discharges fed with C₃F₈-H₂ mixtures have been studied by means of mass spectrometry in a tubular reactor operated at 0.5 torr and 50 W. Comparison of the results with those obtained with emission actinometry give additional evidence that emission actinometry and mass spectrometry are powerful diagnostic tools to monitor stable and unstable species in discharges utilized for dry etching and polymer depositions. Mechanisms for end product formation and polymer deposition are also discussed.     

Deposition characteristics of γ-ureidopropyltrimethoxysilane onto E-glass fibres using toluene and carbon tetrachloride as carrier solvents

Industrial grade γ-ureidopropyltrimethoxysilane was adsorbed onto industrial grade E-glass fibres from toluene and carbon tetrachloride at 23 °C. The treated surfaces were characterised using thermogravimetry, diffuse reflectance Fourier transform IR spectroscopy, electron spectroscopy for chemical analysis, scanning electron microscopy and the zeta potential. When ureidosilane was deposited from toluene and CCl₄, both of which have very low hydrogen-bonding ability, a ureido surface was obtained. The ureidosilane deposited from CCl₄ hydrolysed at the surface and condensed to form a solvent-swollen Si–O–Si network. The ureidosilane deposited from toluene also hydrolysed at the E-glass surface. Condensation of the silane deposited from toluene was not apparent. Aluminium dissolved from the E-glass surface was condensed into the network of the silane deposited from CCl₄. Aluminium was not detected at the surface of the samples with ureidosilane deposited from toluene. Received: 10 November 2000 Accepted: 23 March 2001     

Insertion of singlet chlorocarbenes across C-H bonds in alkanes: Evidence for two phase mechanism

Transition states for the insertion reactions of singlet mono and dichlorocarbenes (¹CHCl and ¹CCl₂) into C-H bonds of alkanes (methane, ethane, propane and n-butane) have been investigated at MP2 and DFT levels with 6–31g (d, p) basis set. The p _π of ¹CHCl and ¹CCl₂ may interact with alkane’s filled fragment orbital of either σ or π symmetry. So chlorocarbenes insertion reactions have been investigated for both (σ/π) approaches. The σ approach has been adjudicated to be the minimum energy path over the π approach both at the MP2 and DFT levels. Mulliken, NPA and ESP derived charge analyses have been carried out along the minimal energy reaction path using the IRC method for ¹CHCl and ¹CCl₂ insertions into the primary and secondary C-H bonds of propane. The occurrence of TSs either in the electrophilic or nucleophilic phase has been identified through NBO charge analyses in addition to the net charge flow from alkane to the carbene moiety. Dedicated to our senior colleague Dr V Sethuraman on the occasion of his retirement from collegiate service     

Effect of solvent media and condensed benzene rings on thermochemical behaviour of dibenzo-18-crown-6 in solution

The specific and non-specific interactions of twelve activated carbon cloth samples prepared from commercial cotton fabric, and that present different activation degrees are studied through the determination of immersion enthalpies in CCl₄ and H₂O, and in aqueous solutions of NaOH and HCl. The immersion enthalpies found for the solvents CCl₄ and H₂O are in a range of 5.49–45.84 and 1.77–7.76 J g⁻¹, respectively. The enthalpic values for the materials in aqueous solutions of NaOH and HCl, allow characterizing the chemical surface of these materials, which are in a range of 6.63 and 21.49 J g⁻¹, finding through them important relations in company with other characterizing techniques used in the study of these materials.     

Plasma-chemical reactions in weakly decomposed CCl4

For the case of weak feed gas decomposition, where the concentration of CCl₄ exceeds those of decomposition and built-up products, the emission of CCl* is shown to originate from dissociative excitation of CCl₄. With electron concentration measured independently, the kinetics of CCl₄ decomposition has been extracted from the time dependence of the CCl* intensity. Supported by EPR determinations of radical concentrations in rapidly flowing CCl₄ and CCl₄/O₂ afterglows, the primary decomposition reaction is shown to be the electron impact dissociation into CCl₃ and Cl. Its rate constant (k ₁=4×10^–8 cm³s^–1) indicates strongly that dissociative electron attachment is the main reaction channel at least at r.f. power densities just above the threshold of a self-maintaining discharge. At extremely low mean electron energies the emission of a continuum is observed, which is tentatively ascribed to the radiative CCl₃-Cl recombination.     

Solvent extraction behaviour of iodine and bromine in aqueous-organic systems,analogy with radioactive decay

The extraction of iodine and bromine under various conditions from their saturated aqueous solutions by CCl₄, C₆H₆ and o-xylene has been studied. The data obtained from the experiments carried out at various temperatures, for H₂O(I₂)−CCl₄ and H₂O(I₂)−C₆H₆ systems, exhibit an Arrhenius behaviour. The overall activation energy calculated for the extraction in the H₂O(I₂)−CCl₄ system, 650±50 cal·mol⁻¹ is lower than that of H₂O(I₂)−C₆H₆, 3600±300 cal·mol⁻¹. The use of the solubility parameter for the interpretation of the data in the extraction of iodine is investigated. The data obtained in multiple extractions are treated by using the analogy between extraction and radioactive decay. The half number of extraction for each system is determined. The complex curves obtained in the H₂O(I₂)−CCl₄ and H₂O(I₂) −Br₂)−CCl₄ systems are resolved into two components.     

Shape resonances and partial photoemission cross sections of solid SF6 and CCl4

Photoelectron energy distribution curves from solid films of SF₆ and CCl₄ have been measured in the photon energy range 10 ? hr ? 40 eV using synchrotron radiation. The binding energies, peak widths and relative partial cross sections have been determined. In the photoelectron spectra a 1:1 correspondence to the gas phase is observed for the occupied molecular orbitals, and a straightforward assignment of the occupied valence bands emerges. Furthermore, the cross sections of the individual orbitals show for both samples great similarities to the gas phase. For SF₆ detailed structures are visible in the cross sections which are only partly interpreted as shape resonances. A new assignment for the 6t_1u shape resonance is proposed and the resonance energies are related to X-ray absorption and electron scattering data. Furthermore a comparison of the total photoemission cross section to the optical reflection spectrum of solid SF₆ is presented. For CCl₄ less structures are ob served in the partial cross sections. They are all interpreted as shape resonances. An energetic scheme of the virtual orbitals is proposed for CCl₄.     

Kinetics of tetrachloromethane fluorination by hydrogen fluoride in the presence of antimony pentachloride

Contradictory information concerning the kinetics of tetrachloromethane fluorination by anhydrous hydrogen fluoride in the presence of antimony pentachloride has been explained. The present studies were performed using a stainless steel autoclave. The exchange of the first chlorine atom was carried out in the temperature range of 35-95 °C, with the molar ratio of HF/CCl₄ varied within the range of 1.2-1.7 and at a constant molar ratio of SbCl₅/Cl₂ equal to 1.9. The degree of conversion of CCl₄ to CCl₃F and the reaction rate constant as a function of temperature fit to an Arrhenius straight line. The Arrhenius constants (pre-exponential factor) and the activation energy were determined. Studies concerning the exchange of the second chlorine atom, i.e. the conversion of CCl₃F to CCl₂F₂ have been performed. The results of kinetic investigations were created in the same way. Based on these results it was found that the Arrhenius equation is fulfilled also in the second stage of the fluorination. The same slope of the straight lines confirms that the fluorination stages proceed according to the same mechanism with the same activation energy but with different pre-exponential factors.Several kinetic problems associated with the performance of the reactor can be solved on the basis of the equations derived. However, knowledge of CCl₄ conversion as a function of time, temperature, catalyst concentration and the type of the reaction is required for this purpose. Such calculations enable the selection of the optimal process parameters.     

Calorimetric determination of activated carbons in aqueous solutions

The interactions among five samples of activated carbons, obtained from different lignocellulosic materials with different degrees of activation of approximately 20% and aqueous solutions of phenol and 4-nitro phenol are studied by means of the determination of immersion enthalpies. It is established that the obtained activated carbons are of a basic character and show values for the pH at the point of zero charge, pH_PZC, that range from 7.4 to 9.7 and, in all cases, higher total basicity contents than the values obtained for total acidity. The immersion heat of the activated carbons in CCl₄ and water is determined obtaining values which are higher for CCl₄ immersion and vary from 31.4 to 48.6 J g⁻¹. The hydrophobic factor, hf, it is calculated from the relation between of the immersion heat of the activated carbons in CCl₄ and the immersion heat in water, the obtained values were 2.98 and 6.75, which are greater than 1 due to the greater values obtained in CCl₄ when compared to the values obtained in water. Immersion enthalpies in phenol solution range from 7.6 to 13.9 J g⁻¹ and for the case of 4-nitro phenol such enthalpies range from 12.7 to 20.5 J g⁻¹; all the 5 samples studied showed a higher value for the heat of immersion in aqueous solutions of 4-nitro phenol.     

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.     

35Cl NQR study of CCl3 reorientations in the carbonyl-containing derivatives of 3,3,3-trichloropropene

Temperature dependences of³⁵Cl NQR frequencies and spin-lattice relaxation times were measured for the CCl₃ groups of compounds CCl₃CH=CHR and CCl₃CH=CR₂ with R=COCH₃ and COOCH₃. The activation energies of CCl₃ reorientations reflect the significant contribution of intermolecular steric interactions to the value of the potential barrier hindering these reorientations. In the CCl₃CH=CHCOCH₃ crystal, the reorientations of the CCl₃ group are hindered more than those of the molecule as a whole. Perm State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 2, pp. 54–60, March–April, 1994. Translated by L. Smolina     

Kinetics of the gas-phase chlorination of activated carbon with carbon tetrachloride

The chlorination of activated charcoals KAU and SCN with CCl₄ vapor in the temperature range 300–600 °C and also the thermal stability of the added chlorine has been studied. It is shown that chlorination with CCl₄ vapor led to the addition of up to 22 mass % (6.2 mmol/g) of chlorine to the surface layer of the carbon. The added chlorine is desorbed from the surface with formation of HCl, while at temperatures above 500 °C desorption of chlorine goes practically to completion. The effective rate constants and the energies of activation of the processes of chlorination and desorption have been found.     

Rate constants for reactions of Cl abstraction from CCl4 by CCl3CH2·CHR radicals and Br abstraction from CCl3CH2CHBrR (R=Bun, AcO, OCNC4H8, CN) by ·Re(CO)5 radicals

The rate constants for reactions of Cl abstraction from CCl₄ by CCl₃CH₂·CHR radicals and Br abstraction from CCl₃CH₂CHBrR (R=Buⁿ, AcO, OCNC₄H₈, CN) by·Re(CO)₅ radicals were determined by ESR spectroscopy using spin trapping technique. Replacement of H atoms at the C(β) atom by O or N atoms reduces the reactivity of the radicals in the reactions of Cl abstraction from CCl₄ by approximately an order of magnitude. The presence of two polar groups at the C(β) atom results in appreciable decrease in the strength of the C−Br bond in CCl₃CH₂CHBrR adducts. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 45–48, January, 2000.     

Theoretical study of the CCl3 radical

Employing the STO-3G and 4–31 G basis sets within the unrestricted Hartree-Fock method the equilibrium structure of the CCl₃ radical was calculated. The calculations predict the radical to be non-planar with both basis sets used. Using the UHF optimized geometries the SWX_α calculations have been performed to calculate energy levels, ionization potentials and electron affinities of CCl₃.     

Effect of the physical state on the scavenging of quasi-free electrons by a solute

The reactivity of carbon tetrachloride toward quasi-free electrons generated during the decay of ⁵⁷Co atoms in its alcohol solutions frozen at 80 K has been studied by Mössbauer emission spectroscopy. It has been found that an increase in CCl₄ concentration to 10 mol/L slightly affects the yield of ⁵⁷Fe²⁺ and ⁵⁷Fe³⁺ ions as ⁵⁷Co decay products. The inertness of CCl₄ has been interpreted as a consequence of a change of the energy state of quasi-free track electrons results from freezing the alcohol solution. This effect inhibits the dissociative attachment reaction of scavenging of track electrons and increases the ground-state energy of quasi-free electrons (on passing to the frozen matrix), thereby fundamentally altering the energy balance of the electron scavenging reaction in comparison with the liquid medium and, hence, decreasing the rate constant of electron scavenging by CCl₄.     

On the use of actinometric emission spectroscopy in SF6-O2 radiofrequency discharges: Theoretical and experimental analysis

A comparison of the results obtained by solving the Boltzmann equation with the experimental results from optical emissions obtained in SF₆-O₂ radiofrequency discharges, when N₂, Ar, and He are also admitted as actinometers, has allowed us to explore the potentialities and limits of actinometry. The use of different actinometers also allowed us to monitor the evolution of the electron distribution functions as a function of the plasma parameters.