Rate constant of the recombination of free electrons and holes in AgCl at 295 K

The transient kinetics of the loss of electrons generated by light pulses in powdered AgCl has been studied by the microwave photoconductivity method (36 GHz) at 295 K. At high light intensities,I ₀ > 10¹⁴ photon cm^–2 per pulse, the kinetics obeys the second-order law. The rate constant of the recombination of free electrons and holes is equal to 2·10^–12 cm³ s^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2234–2236, September, 1996.     

Drift Mobility of Electrons in Pyrazoline-Containing Copolymers

Drift mobility of electrons in copolymers containing pyrazolyl fragment in the main or side chain of the copolymer is studied experimentally using the time-of-flight method. The mobility is an exponential function of the applied electric field, and its values for the copolymers range from 5 × 10^–7 to 2 × 10^–5 cm² V^–1 s^–1 at the field strength ranging from 5 × 10⁴ to 1 × 10⁶ V cm^–1. Based on the measured mobilities and the molecule parameters, which were obtained by a PM3 quantum-chemical calculation, the role of copolymer fragments in the transport of charge carriers is discussed.     

Rate constant of free electron-hole recombination in thin cadmium sulfide films

The transient decay kinetics of electrons generated in thin cadmium sulfide films by short laser pulses was studied by the microwave photoconductivity method (9 and 36 GHz) at 295 K. The films were prepared by the pulverization method from thiocarbamide coordination compounds. At the high light intensity I ₀ > 10¹⁴ photon cm^–2 per pulse, the decay kinetics of photoelectrons corresponded to a reaction of the second order. Analysis of the kinetic data made it possible to determine the rate constant of recombination of free electrons and holes: k ₃ 2(±1)·10^–13 cm³ s^–1.     

Gas-phase combination reactions of SF4 and SF5 with F in plasmas of SF6

Reactions of both SF₄ and SF₅ with F have been studied at 295 K in a gas-flow reactor sampled by a mass spectrometer. The rate coefficient for the combination reaction of F with SF₄ to produce SF₅ was found to increase from (0.9 to 3.0)×10^–12 cm³ s^–1 when the helium bath gas number density was increased from (2 to 26)×10¹⁶ cm^–3. The values obtained here are three orders of magnitude higher than a recent estimate of the high-pressure value based on the modelling of photochemical studies. The experimental results have been compared with RRKM and master equation calculations in which a simplified Gorin model has been used to determine the structure of the transition state. These calculations show that reasonable agreement can be obtained between the experimental data and the calculation if a small (2 KJ/mol) activation energy is assumed. The rate coefficient for the reaction between SF₅ and F to produce SF₆ was found to be independent of helium bath gas number density within the range given above. The value obtained for the rate coefficient was 9×10^–12 cm³ s^–1 with an uncertainty of a factor of 2. This value is close to that of 1×10^–11 cm³ s^–1 computed from the simplified Gorin model and to the value of 1.7×10^–11 cm³ s^–1 deduced from modelling of photochemical experiments.     

Gas-phase reactions of SF5, SF2, and SOF with O(3 P): Their significance in plasma processing

Reactions of both SF₅ and SF₂ with O(³ P) and molecular oxygen have been studied at 295 K in a gas flow reactor sampled by a mass spectrometer. For reactions with O(³ P), rate coefficients of (2.0±0.5)×10^–11 cm³ s^–1 and (10.8±2.0)×10^–11 cm³ s^–1 were obtained for SF₅ and SF₂ respectively. The rate coefficients for reactions with O₂ are orders of magnitude lower, with an estimated upper limit of 5×10^–16 cm³ s^–1 for both SF₅ and SF₂. Reaction of SF₂ with O(³ P) leads to the production of SOF which then reacts with O(³ P) with a rate coefficient of (7.9±2.0)×10^–11 cm³ s^–1. Both SO and SO₂ are products in the reaction sequence initiated by reaction between SF₂ and O(³ P). Although considerable uncertainty exists for the heat of formation of SOF, it appears that SO arises only from reaction between SOF and O atoms which is also the source of SO₂. These results are discussed in terms of a reaction scheme proposed earlier to explain processes occurring during the plasma etching of Si in SF₆/O₂ plasmas. A comparison between the results obtained here and those reported earlier for reactions of both CF₃ and CF₂ with O and O₂ shows that there is a marked similarity in the free radical chemistry which occurs in SF₆/O₂ and CF₄/O₂ plasmas.     

Gas-phase reactions in plasmas of SF6 with O2 in He

Processes which occur in microwave discharges of dilute mixtures of SF₆ and O₂ in He have been examined using a flow reactor sampled by a mass spectrometer. Two classes of experiments were performed. In the first set of experiments, mixtures containing 6×10¹¹ cm^–3 SF₆, 6×10¹⁶ cm^–3 He, and O₂ in the range (0–3.6)×10¹³ cm^–3 were passed through a 20-W 2450-MHz microwave discharge. The gas mixtures arriving at a sample point downstream from the discharge were examined for SF₆, SF₄, SOF₂, SOF₄, SO₂F₂, SO₂, F, and O. In the second class of experiments, rate coefficients were measured for the reactions of SF₄ with O and O₂ and for the reaction of SF with O. The rate coefficient for the reaction of SF with O was found to be (4.2±1.5)×10^–11 cm^–3 s^–1. SF₄ was found to react so slowly with both oxygen atoms and oxygen molecules that only upper limits could be placed on the rate coefficients for these reactions. These values were 2×10^–14 cm³ s^–1 and 5×10^–15 cm³ s^–1 for reactions with O and O₂ respectively. The observed distribution of products from the discharged mixtures is discussed in terms of the measured rate coefficients.     

Study of a hollow cathode arc discharge in the presence of chromium oxide

A hollow cathode arc discharge in hydrogen has been used for the purpose of chromium oxide reduction, the solid oxide being placed inside the anode. Mass transport from the oxide to the gas phase and excitation conditions in the plasma have been investigated. The results show that a substantial amount of oxide is transferred to the gas phase with subsequent reduction and deposition inside the cathode cavity, in the form of a pure metal. The residual part condenses on the discharge chamber wall as an amorphous substance, containing 50–60% of Cr metal, and on the anode surface under the form of a mixture of chromium oxide and metal crystals (10%). From spectroscopic investigations it follows that, inside the anode zone, total Cr concentration in the gas phase is of the order of 10¹⁴ cm^–3, the excitation temperature of the atoms and ions being 4500 and 5500 K, respectively, and the ionization temperature being about 6000 K.Notation I absolute spectral line intensity (W cm^–2 sr^–1) - emission coefficient (W cm^–3 sr^–1) - A relative absorption - absorption coefficient (cm^–1) - L plasma diameter (mm) - f _tk oscillator strength - _D full Doppler width (cm^–1) - S( ₀ L) Ladenburg-Levy function - wave number (cm^–1) - k _pl mass transport rate (mol cm^–2 s^–1) - k _th thermal reduction rate (mol cm^–2 s^–1) - u ion mobility (mm V^–1 s^–1 ) - E electric field strength (V mm^–1) - drift velocity (cm s^–1)     

Gas-phase reactions of CF3 and CF2 with atomic and molecular fluorine: Their significance in plasma etching

Reaction rate coefficients have been measured at 295 K for both CF₃ and CF₂ with atomic and molecular fluorine. The reaction between CF₃ and F was studied over a gas number density range of (2.4–23)×10¹⁶ cm^–3 with helium as the bath gas. The measured rate coefficient increased from (1.1–1.7)×10^–11 cm³ s^–1 as the gas number density increased over this range. In contrast to this relatively small change in rate coefficient with gas number density, the rate coefficient for CF₂+F increased from (0.4–2.3)×10^–12 cm³ s^–1 as the helium gas number density increased from (3.4–28.4)×10¹⁶ cm^–3. Even for the highest bath gas number density employed, the rate coefficient was still more than an order of magnitude lower than earlier measurements of this coefficient performed at comparable gas number densities.Both these association reactions are examined from the standpoint of the Gorin model for association of radicals and use is made of unimolecular dissociation theory to examine the expected dependence on gas number density. The calculations reveal that CF₃+F can be explained satisfactorily in these terms but CF₂+F is not well described by the simple Gorin model for association.CF₃ was found to react with molecular fluorine with a rate coefficient of (7±2)×10^–14 cm³ s^–1 whereas only an upper limit of 2×10^–15 cm³ s^–1 could be placed on the rate coefficient for the reaction between CF₂ and F₂. The values obtained for this set of reactions mean that the reaction between CF₃ and F will play an important role in plasmas containing CF₄. The high rate coefficient will mean that, under certain conditions, this particular reaction will control the amount of CF₄ consumed. On the other hand, the much lower rate coefficient for reactions between CF₂ and F means that CF₂ will attain much higher concentrations than CF₃ in plasmas where these combination reactions are dominant.     

Cyclic voltammetric study of the catalytic behavior of nickel(I) salen electrogenerated at a glassy carbon electrode in an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, BMIMBF4)

Cyclic voltammetry has been employed to examine the electrochemistry of nickel(II) salen at a glassy carbon electrode in an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, BMIM⁺BF₄⁻). Residual water in the ionic liquid can be eliminated by introduction of activated molecular sieves into the electrochemical cell. Nickel(II) salen exhibits a one-electron, quasi-reversible reduction to nickel(I) salen, and the latter species serves as a catalyst for the cleavage of carbon–halogen bonds in iodoethane and 1,1,2-trichlorotrifluoroethane (Freon^® 113). In BMIM⁺BF₄⁻ the diffusion coefficient for nickel(II) salen at room temperature has been determined to be 1.8×10⁻⁸ cm² s⁻¹, which is more than 500 times smaller than that (1.0×10⁻⁵ cm² s⁻¹) in a typical organic solvent–electrolyte system such as dimethylformamide (DMF) containing 0.10 M tetramethylammonium tetrafluoroborate.     

Far infrared spectrum,conformational stability,barriers to internal rotation,vibrational assignment,and ab initio calculations of 2-chloro-3-fluoropropene

The far infrared spectrum (375 to 30 cm^–1) of gaseous 2-chloro-3-fluoropropene, CH₂=C(CH₂F)CI, has been recorded at a resolution of 0.10 cm^–1. The fundamental asymmetric torsional mode is observed at 117.5 cm^–1 with ten excited states falling to low frequency for thes-cis (fluorine atom eclipsing the double bond) conformer. For the higher energy gauche conformer, the asymmetric torsion is estimated to be at 94 cm^–1. From these data the asymmetric torsional potential function has been calculated. The potential function coefficients are calculated to be in cm^–1):V ₁=803±21,V ₂=–94±21,V ₃= 1025±10,V ₄=95±10, andV ₆=2±1, with an enthalpy difference between the more stables-cis and gauche conformera of 550±100 cm^–1 (1.57±0.29 kcal/mol). This function gives values of 1227±50cm^–1(3.51±0.14kcal/mol), 1266±200 cm^–1 (3.62±0.57 kcal/mol), and 665±100 cm^–1 (1.90±0.29 kcal/mol), for thes-cis to gauche, gauche to gauche, and gauche tos-cis barriers, respectively. From the relative intensities of the Raman lines of the gas at 652 cm^–1 (gauche) and 731 cm^–1 (s-cis) as a function temperature, the enthalpy difference is found to be 565±96 cm^–1 (1.62±0.27 kcal/mol). However, the more polar gauche conformer remains in the crystalline solid. The Raman spectrum of the gas has been recorded from 3500 to 70 cm^–1 and, utilizing these data and the previously reported infrared data, a complete vibrational analysis is proposed for both conformers. The conformational stability, barriers to internal rotation, fundamental vibrational frequencies, and structural parameters that have been determined experimentally are compared to those obtained from ab initio Hartree-Fock gradient calculations employing both the 3–21 G^* and 6–31G^* basis sets and to the corresponding quantities for some similar molecules.     

Chemical and Biological Sensors Based on Electrochemical Detection Using Conducting Electroactive Polymers

The electrochemical behavior of composites of conducting electroactive polyaniline (PAn) and polypyrrole (PPy) formulated within cross-linked hydrogel networks was investigated by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Composite PAn gels displayed similar anodic charge density compared to the pristine conducting polymer (80mC/cm² and 84mC/cm², respectively), suggesting a similar degree of electroactivity between the two systems. Composite gels of PAn displayed fast cation transport with K⁺ diffusivity (D_appt= 5.31×10^–7cm²s^–1) that were three orders of magnitude larger than that of pristine PAn (D_appt=3.12×10^–10cm²s^–1), while PPy composite gels showed similar ferrocene anion diffusivity (D_appt=7.05×10^–5cm²s^–1) compared to electropolymerized PPy (D_appt=6.54×10^–5cm²s^–1). The electrochemical interactions between CYP2D6, a cytochrome P450 isoenzyme, and fluoxetine mediated by electroactive polyaniline films on glassy carbon electrodes (GCEs) were investigated. Cyclic voltammograms indicate that PAn is an effective mediator of CYP2D6 activity under anaerobic conditions. An analytical interrogation methodology based on small-amplitude, pulsed DC was developed and incorporated into the Electroconductive Polymer Sensor Interrogation System (EPSIS). Polypyrrole membranes were rendered biospecific by either copolymerization of pyrrole (Py) with 4-(1-pyrrolyl) butyric acid (4PyBA), followed by direct conjugation with 5-(biotinamido)pentyl amine (5BPA), or by reacting 4PyBA with 5BPA to form pyrrolyl-biotin conjugates. The biotinylated PPy was made responsive to glucose or urea by exploiting strong biotin-streptavidin binding to either streptavidin-glucose oxidase or biotin-urease conjugates. These bioactive conducting polymer membranes were demonstrated as conductimetric glucose and urea biosensing layers using the EPSIS. The rate of conductivity of the bioactive PPy membranes was observed to double upon increasing glucose concentration from 100µM (4×10^–6Scm^–1s^–1) to 600µM (9×10^–6Scm^–1s^–1).     

The use of a gold-containing membrane for ion-sensitive electrodes and their application in analysing systems. I

Summary Pellets of a material, resulting from the precipitation of silver salts (AgCl, AgBr, AgI and Ag₂S) onto finely divided gold (particle size less than 3 m) showed a selective ion-sensitive behaviour for halogens, silver and sulphide, resp. Silver iodide on gold turned out to be sensitive for cyanide as well.Electrodes with this type of membrane combined a low redox sensitivity with good mechanical strength and chemical resistance and with a low membrane impedance (1–500 kOhm mm^–1 as a maximum). Settling times vary in the range of 0.1–10 sec. A linear Nernstian response was shown in the ranges 10^–1×10^–5 M (chloride), 10^–1 –10^–6 M (bromide, iodide, cyanide) and 10^–1–10^–7 M (silver, sulphide). In order to obtain good cleaning facilities for heavy duty applications (e. g. measurements in slurries) the membrane pellet was mounted in the electrode body in a way that its cylindrical side contacted the solution to be measured or monitored.

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Verwendung einer goldhaltigen Membran für ionensensitive Elektroden und ihre Anwendung in analytischen Systemen. IHerstellung und Charakteristik ionensensitiver Elektroden

Zusammenfassung Tabletten eines Materials, das durch Fällung von Silbersalzen (AgCl, AgBr, AgJ und Ag₂S) auf fein-pulverisiertem Gold (Teilchengröße weniger als 3 m) entstand, wurden zur Fertigung selektiver ionen-sensitiver Elektroden für Halogene, Silber und Sulfid eingesetzt. Silberjodid auf Gold war auch für Cyanid empfindlich. Diese Elektroden vereinigen eine niedrige Redox-Empfindlichkeit mit einer guten mechanischen und chemischen Stabilität sowie einer niedrigen Elektrodenimpedanz (Höchstwert: 1–500 kOhm mm^–1). Der Signalwert bildet sich in 0,1–10 sec. Das Elektrodensignal ist proportional der Ionenaktivität gemäß dem Nernstschen Gesetz im Bereich von 10^–1–3·10^–5 M (Chlorid), 10^–1–10^–6 M (Bromid, Jodid, Cyanid) und 10^–1–10^–7 M (Silber, Sulfid). Die Tablette ist im Elektrodengehäuse so montiert, daß nur die cylindrische Außenseite von der zu messenden Flüssigkeit berührt wird. Die Reinigung der Elektrode im anspruchsvollen Dauerbetrieb (z. B. kontinuierliche Messungen in Festkörper enthaltenden Abwässern) wird durch diese Anordnung erheblich vereinfacht.

     

Experimental Evidence for Acceleration of Reaction between Iodine Monoxide and Chlorine Monoxide at the Reactor Surface

The reaction of iodine monoxide with chlorine monoxide resulting in atom escape to the gas phase is studied at T = (303 ± 5) K and P = 2.5 Torr using a flow setup for measuring the resonance fluorescence signals of atomic iodine and chlorine. The heterogeneous reaction between chlorine monoxide and iodine monoxide occurring at the reactor surface covered with an F32-L Teflon-like compound and treated by the reaction products is characterized by the rate constant k = (4.9 ± 0.2) × 10^–11 cm³ molecule^–1 s^–1. This value is substantially higher than the rate constant for the homogeneous reaction IO^· + ClO^· (k ₁ 1 × 10^–12 cm³ molecule^–1 s^–1).     

Experimental determination of the dependence of the free electron—hole recombination rate constant on the band gap in semiconductors of the AIIBVI and AIBVII types

A correlation between the recombination rate constant of free electrons and holes (k _r) and the band gap (E _g) of semiconductors (AgCl, AgBr, Cd_xZn_1−x S, CdSe, CdTe, and their solid solutions) at 295 K was found. The experimental data were obtained by the UHF photoconductivity (36 GHz) using current carrier generation by laser pulses (λ = 337 nm, pulse duration 8 ns). A decrease in E _g in a range of 1.5–3 eV increases k _r by 1.5 orders of magnitude according to the law close to exponential. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 856–860, May, 2007.     

Application of open-shell coupled cluster theory to the ground state of GaAs

Summary Theoretical calculations at the coupled cluster level of theory including all single, double and perturbative triple excitations, CCSD(T), are carried out for the^3– ground state of GaAs. Employing a (7s5p3d1f) basis set, the theoretical predictions forr _e (2.560 Å), _e (217 cm^–1),D _e (1.84 eV), and IP (7.80 eV), are in good agreement with recent experimental results. The importance of includingf-type polarization functions in the basis set and the effect of correlating 3d electrons are discussed in detail.     

Effect of Sodium Thiosulfate on the Kinetics of Electron–Ion Processes in Powdered Silver Chloride

The results of a microwave photoconductivity study on the effect of pretreatment of a silver chloride powder with sodium thiosulfate (STS) aqueous solutions on the kinetics of electron–ion processes in silver chloride are reported. At an UV light fluence of <10¹⁴ photon/cm^–2 per pulse, photoresponse decay after the end of the pulse consisted of two exponential components over the entire concentration range 10^–8–10^–2 mol STS/mol AgCl examined. The amplitude of the components and the ratio between the slow ^s and fast ^f components depended on the sodium thiosulfate concentration, which was due to the formation of an additional number of new electron traps in AgCl. An analysis of the kinetics in terms of models taking into account various types of decay of an excess electron made it possible to obtain data on the depth of both old traps (0.45 eV) and new STS-created traps (0.45–0.63 eV) in AgCl.     

Potential-Dependent Adsorption of Transferring Ions Having Asymmetric Charge Distribution at the 1,2-Dichloroethane|Water Interface and Its Ion-Transfer Kinetics Studied by AC-modulated Voltfluorometry

The kinetics of the transfer of Erythrosine B dianions (EB^2–) and Eosin Y dianions (EY^2–) across the 1,2-dichloroethane|water interface, studied using ac-modulated voltfluorometry, is compared with that of Rose Bengal dianions (RB^2–). All three exhibit anomalous phase angles, showing the transient adsorption of these ions at the interface. The apparent standard rate constants are 9.0 × 10^–3 and 1.3 × 10^–2 cm s^–1 for EY^2– and EB^2– at 25° (cp. 1.4 × 10^–2 cm s^–1 for RB^2–). The slight decrease in the rate in the order RB^2–, EB^2–, and EY^2– may be associated with the increasing polarity of the ions, which endows the ions with a stronger interaction with water, resulting in the slower rotational dynamics of the ions on crossing the interface.     

Low Energy Electron Attachment by Some Chlorosilanes

In this paper, the rate coefficients (k) and activation energies (E_a) for SiCl₄, SiHCl₃, and Si(CH₃)₂(CH₂Cl)Cl molecules in the gas phase were measured using the pulsed Townsend technique. The experiment was performed in the temperature range of 298–378 K, and carbon dioxide was used as a buffer gas. The obtained k depended on temperature in accordance with the Arrhenius equation. From the fit to the experimental data points with function described by the Arrhenius equation, the activation energies (E_a) were determined. The obtained k values at 298 K are equal to (5.18 ± 0.22) × 10⁻¹⁰ cm³·s⁻¹, (3.98 ± 1.8) × 10⁻⁹ cm³·s⁻¹ and (8.46 ± 0.23) × 10⁻¹¹ cm³·s⁻¹ and E_a values were equal to 0.25 ± 0.01 eV, 0.20 ± 0.01 eV, and 0.27 ± 0.01 eV for SiHCl₃, SiCl₄, and Si(CH₃)₂(CH₂Cl)Cl, respectively. The linear relation between rate coefficients and activation energies for chlorosilanes was demonstrated. The DFT/B3LYP level coupled with the 6-31G(d) basis sets method was used for calculations of the geometry change associated with negative ion formation for simple chlorosilanes. The relationship between these changes and the polarizability of the attaching center (α_centre) was found. Additionally, the calculated adiabatic electron affinities (AEA) are related to the α_centre.     

Pulse radiolysis of ethyl propionate in aqueous solution

Hydrated electrons (e _aq ^– ) formed in water radiolysis react with ethyl propionate with a rate parameter of 7.5×10⁷ mol^–1 dm³ s^–1. The electron adduct in acidic solutions immediately (<100 ns) dissociates, yielding CH₃CH₂C=0 radical. This process in alkaline solutions is slower, k=1.4×10⁵ s^–1. The hydroxyl radicals abstract H atoms in about 50% from the -position of propionate.     

Ionic Lithium-Conducting Solid Li2S–Sb2S x Electrolytes

The electroconductivity of lithium-containing solid electrolyte Li₂S–Sb₂S₃is studied. At room temperature it equals (1.5–2.5) × 10^–4 S cm^–1. The activation energy for conduction, determined from the temperature dependence of the electroconductivity, is 0.35–0.43 eV.