Electron-ion recombination in dense gaseous and liquid argon: effects due to argon cation clusters allow to explain the experimental data

The role of cation clusters in the bulk electron-ion recombination in dense gaseous and liquid argon is investigated. The size and structure of cation clusters in those systems are determined by a Monte Carlo simulation. Then, the rate constants of electron-ion recombination are calculated by another simulation method that takes into account the presence of cation clusters in the considered systems. A good agreement with experiment for both dense gaseous and liquid argon is obtained.     

Excimer formation mechanism in gaseous krypton and Kr/N2 mixtures

A new approach for the understanding of the energy relaxation dynamics of excited atoms involving a long-lived molecular precursor is presented here for krypton. Excitation of the gas close to the 5s[3/2]₂ metastable atomic level (E _at. ?E _exc.<kT) is achieved with an intense VUV laser source (I ≈ 10¹² photon/pulse) realized by resonantly enhanced 4-wave mixing (2ω₁ + ω₂) in room temperature mercury vapor (N _Hg ≈ 10¹³ at./cm³). The decay of the II. continuum luminescence (145 nm) is studied. In the pressure range 200–500 mbar, decay rates depend linearly on pressure but have a negative zero-pressure intersect. We show here that this result can be understood as an effect of the exchange of energy between two different “reservoirs” of atomic (5s[3/2]₂) and molecular (1_g) nature, and can be an inherent peculiarity of the recombination kinetics of excited atoms with several product channels. The efficiency of the model is checked for the Kr/N₂ system. Rate constants for relaxation processes are determined in pure krypton and in Kr/N₂ mixtures.     

Etching of GaAs (100) with gaseous H/CH3 mixtures

GaAs (100) wafers were etched in mixtures of hydrogen atoms and methyl radicals. The atoms were formed in a remote hydrogen plasma, and a fraction of these were converted into methyl radicals by introducing methane into the flow system upstream from the semiconductor surface. The flux of hydrogen atoms into the reaction chamber was determined by isothermal calorimetry. The methyl radical flux passing over the substrate was then calculated using previously determined rate parameters for the reaction between atomic hydrogen and methane, and a simple modeling program. The GaAs etch rates were about an order of magnitude faster when methyl radicals were present in the hydrogen atom stream, and were found to follow a first-order dependence on the partial pressure of methyl radicals. Absolute rate constants were determined and an Arrhenius activation energy of 1.2 kcal mol^?1 was calculated. The values of k and E_a are consistent with a diffusion-controlled process. SEM photographs of the surface revealed small crystallographic features that made the surface appear very rough. XPS analysis indicated that these surfaces were arsenic deficient. A mechanism is proposed for the etching of GaAs by a combination of methyl radicals and hydrogen atoms. © 1994 John Wiley & Sons, Inc.     

Xenon sensitized photolysis of CCl4/CH4 mixtures

The kinetics and mechanism of energy transfer from Xe/³P₁/ atoms to CH₄ molecule was investigated by XeCl/B-X/ transition fluorescence intensity measurements in xenon sensitized photolysis of CCl₄/CH₄ mixtures. The kinetic analysis of the experimental I_B-X=f/CH₄/ relation at constant CCl₄ and xenon concentrations showed that the “third order” energy transfer reaction should be included into the reaction mechanism: $$Xe/^3 P_1 / + Xe + CH_4 \to products$$ with the rate constant /2.5±0.5/×10^?28 cm⁶s^?1.     

Chemical reaction studies in CH4/Ar and CH4/N2 gas mixtures of a dielectric barrier discharge

Chemical reactions in a dielectric barrier discharge at medium pressure of 250-300 mbar have been studied in CH(4)/Ar and CH(4)/N(2) gas mixtures by means of mass spectrometry. The main reaction scheme is production of H(2) by fragmentation of CH(4), but also production of higher order hydrocarbon molecules such as C(n)H(m) with n up to 9 including formation of different functional CN groups is observed. Formation of C(2)H(2), C(2)H(4), and C(2)H(6) molecules has been investigated in some detail. Significant differences are noted in comparison to a theoretical estimate.     

Rapid diffusion of CH4/H2 mixtures in single-walled carbon nanotubes

Equilibrium molecular dynamics (EMD) are used to examine the self-diffusion and macroscopic diffusion of CH4/H2 mixtures adsorbed inside a (10,10) single-walled carbon nanotube. EMD can be used to determine the macroscopic diffusion coefficients of adsorbed mixtures by evaluating the matrix of Onsager transport coefficients. Earlier studies have indicated the diffusion of light gases adsorbed as single components in carbon nanotubes is extremely rapid compared to that in other known nanoporous materials. The results presented here indicate that extremely rapid diffusion can also occur for mixtures of adsorbed molecules. The rapid diffusion of adsorbed molecules and the strong coupling between the fluxes of the adsorbed species in a mixture have interesting implications for uses of carbon nanotubes in membrane-based applications.     

On the reaction of CH with CO studied in high temperature CH4/Ar + CO mixtures

In this study a ring dye laser spectrometer was employed for in-situ measurements of CH concentrations in the reaction zone behind shock waves. The time dependent absorption in the Q-branch of the A²Δ — X²Π band of CH at 431.1311 nm caused by the formation and consumption of CH radicals during the shock induced pyrolysis of a few ppm methane in argon was recorded. The CH concentration could directly be calculated from the measured absorption by applying the Lambert-Beer law. By adding a few percent CO to the initial mixtures, the CH concentration profiles were significantly perturbed. Both the perturbed and unperturbed CH concentration profiles have been compared with those calculated from reaction kinetic simulations. A reaction mechanism describing the CH concentration history in the CH₄/Ar and CH₄/CO/Ar systems between 2900 K and 3500 K was developed. By a fitting procedure, a value of k₁ = 1.85 × 10¹¹ cm³ mol^?1 s^?1 was obtained for the most important perturbation reaction CH + CO → C₂O + H.     

Role of spin-rotation relaxation in solid CH4-Kr mixtures

The nuclear spin-rotation interaction, the “quenched” modification proposed by Blicharski, is found to be relevant for interpretation of spin-lattice relaxation rates. Our results demonstrate that spin-rotation can be important in solids and give support to the quenched spin-rotation model, which, to our knowledge, has previously been applied only to liquids.     

Separation of CO2/CH4 and CH4/N2 mixtures using MOF-5 and Cu3(BTC)2

In this paper we used MOF-5 and Cu₃(BTC)₂ to separate CO₂/CH₄ and CH₄/N₂ mixtures under dynamic conditions. Both materials were synthesized and pelletized, thus allowing for a meaningful characterization in view of process scale-up. The materials were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). By performing breakthrough experiments, we found that Cu₃(BTC)₂ separated CO₂/CH₄ slightly better than MOF-5. Because the crystal structure of Cu₃(BTC)₂ includes unsaturated accessible metal sites formed via dehydration, it predominantly interacted with CO₂ molecules and more easily captured them. Conversely, MOF-5 with a suitable pore size separated CH₄/N₂ more efficiently in our breakthrough test.     

A spectroscopic determination of the methyl radical recombination rate constant in shock waves

The reactions CH₃ +

and CD₃ +

have been studied in shoch waves at 1200–1500 K and densities of 2 × 10^?6 ?2 × 10^?4 mol cm^?3 using UV absorption near 216 nm. The rate constants at the highest densities: k_H = (1.7 ± 0.6) × 10^?11 cm³ s^?1 and k_D = (2.2 ± 0.9) × 10^?11 cm³ s^?1 are close to the second order limit. At the lowest densities the rates are lower by a factor of 5. The experimental results agree well with theoretical predictions based on the statistical adiabatic channel model but differ from those of conventional RRKM calculations. A direct observation of the equilibrium C₂H₆ ? 2CH₃ favours the “high” value for ΔH⁰₀ (87.76 kcal/mol).     

Determination of the rate constant for sulfur recombination by quasiclassical trajectory calculations

The sulfur recombination reaction has been thought of as one of the most important chemical reactions in the volcanic activities of the planet. It is also important in determining the propagation of elemental sulfur in the atmosphere. There have been two experimental attempts to determine the reaction rate of the S+S-->S(2) recombination, however their results differ by four orders of magnitude. In this work, we determine the rate constant of S+S-->S(2) from quasiclassical trajectory calculations. The third order rate constant at 298.15 K predicted by the present calculations is 4.19 x 10(-33) cm(6) molecules(-2) s(-1), which is in excellent agreement with the determination of Fair and Thrush [Trans. Faraday Soc. 65, 1208 (1969)]. The temperature dependent rate constant is determined to be 3.94 x 10(-33) exp[205.56(1T-1298.15)], which was determined from the temperature range of 100-500 K.     

Reaction rate constant of SO3 + CH3OH in the gas phase

The reaction rates of SO₃ with CH₃OH in He were measured at total pressures of 0.7–1.6 torr in flow tubes. The concentration of SO₃ was monitored by the SO₂* fluorescence from excitation of SO₃ at 147 nm. The reaction rate constant of SO₃ + CH₃OH in the gas phase is determined to be (1.17 ± 0.16) × 10^?13 cm³ molec^?1 s^?1 at room temperature.     

Chemistry in mixtures of Kr with Ar and He illuminated with Kr resonance radiation

The formation and destruction of excited rare-gas dimers have been studied using resonance optical excitation of high-pressure gas mixtures containing Kr. From an analysis of the resulting emission, 19 reaction rate coefficients have been inferred. It is concluded that atom–atom interchange processes play a major role in rare-gas eximer formation, and that stepwise relaxation down the eximer vibrational ladder is less important than multistep relaxation due to atom–atom exchange in the excited eximer.     

Microdetermination of nitrous oxide in gaseous mixtures

Zusammenfassung Eine kritische Untersuchung der Entwicklung der Mikrogasanalyse, beginnend mit den Arbeiten vonTimiriazeff undKrogh, führte zur Konstruktion einer Gasbürette, die die Vorteile früherer Modelle beibehält und gleichzeitig deren Nachteile zu vermeiden sucht. Außerdem mußten die spezifischen Schwierigkeiten bei der Analyse sehr kleiner Proben von Stickstoffoxydulmischungen wie sie in der Anästhesie vorkommen, berücksichtigt werden.Die gasvolumetrische Messung in engen Kapillaren hat den Nachteil, daß aus den Absorptionspipetten mitgeführte Verunreinigungen einerseits den verfügbaren freien Raum in der Kapillare verkleinern und anderseits die Oberflächenspannung an den Menisken derart beeinflussen können, daß die genaue Einstellung des Druckes in der gemessenen Gassäule in Frage gestellt wird. Aus diesem Grunde wird in der neuen Bürette das Gas für die Volumsbestimmung in eine verhältnismäßig weite Kapillare gebracht, die nur dort verengt wird, wo sich der Meniskus zur Zeit der Messung befindet. Die Messung erfolgt mit Hilfe eines Stempels, dessen Verschiebung mikrometrisch gemessen werden kann. Die Mikrometerstellung wird abgelesen, wenn die beiden Menisken der Gasblase eine Ringmarke im verengten Teil der Kapillare passieren. Aus der Differenz der Ablesungen und dem Durchmesser des Stempels ergibt sich das Volumen der Gasblase. Zur genauen automatischen Korrektur von während der Verdrängung der Gasblase durch den Stempel auftretenden Druck- und Temperaturschwankungen wird das Thermobarometer-Prinzip vonZuntz undHempel benutzt, wobei dafür gesorgt ist, daß die Menisken der abgesperrten Luftprobe und der Gasblase nur durch eine kurze und verhältnismäßig massive Quecksilbersäule getrennt sind. Bei kleinen Gasblasen kann man eine absolute Genauigkeit der Volumsbestimmung von ± 0,05 erreichen. Die Präzision der Bestimmung des Volumens großer Gasblasen (etwa 200) läßt sich jedoch nicht über einen Teil in zweitausend Teilen hinaus steigern.Die Spitze der Bürette endet in einer kleinen Quecksilberwanne, in der sich auch die Reagenzgläser befinden, die als Absorptionspipetten und Gassammelgefäße dienen. Die Arbeitsweise vonSeevers undStormont, die vonTreadwell an leicht zugänglicher Stelle beschrieben wurde, wird beim Überführen der Gasblasen von der Bürette in die Pipette und umgekehrt beibehalten.Die Konstruktion der Bürette und die Arbeitsweise mit derselben sind genau beschrieben. Beleganalysen sind in Tabellen angeführt.

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Résumé Une étude critique du développement de la microgazométrie depuis les travaux deTimiriazeff etKrogh, a conduit à la construction d'une burette à gaz, présentant les mêmes avantages que les modèles précédents, tout en réduisant les inconvénients.De plus, on devait envisager les difficultés spécifiques à l'analyse des échantillons très réduits d'oxyde azoteux en mélanges, comme ceux qui servent en anesthésie.La mesure gazométrique en capillaire étroit, présente l'inconvénient que les impuretés introduites dans les pipettes capillaires d'absorption réduisent le volume utilisable, et que d'autre part, la tension superficielle au niveau du ménisque peut en être influencée au point que l'établissement de la pression exacte dans la colonne gazeuse à mesurer soit sujet à caution.Dans la nouvelle burette, le gaz à mesurer, est introduit, pour cette raison dans un capillaire relativement large, retréci seulement là où l'on amène le ménisque au moment de la mesure.La mesure se fait à l'aide d'un plongeur, dont les déplacements sont appréciés micrométriquement.La lecture est faite, lorsque les deux ménisques de la bulle gazeuse passent dans la partie retrécie du capillaire en y dessinant chacun un cercle. De la différence des lectures et du diamètre du plongeur se déduit le volume de la bulle gazeuse.Les corrections précises de température et de pression sont rendues automatiques par l'emploi du thermobaromètre deZuntz etHempel, qui assure la séparation des ménisques des bulles d'air et de gaz au moyen d'une colonne de mercure relativement importante. La précision absolue dans la détermination de petits volumes gazeux atteint ± 0,05; celle dans la détermination des volumes plus grands (environ 200) ne descend pas au-dessous d'une partie pour 2000.La pointe de la burette aboutit dans un petit réservoir à mercure, dans lequel se trouve aussi un tube à réaction servant de pipette d'absorption et de réservoir à gaz.Le mode opératoire deSeevers etStormont, décrit parTreadwell, est maintenu par déplacement de la bulle gazeuse de la burette à la pipette et inversement.La construction de la burette et le mode opératoire sont décrits avec précision. Les analyses de références sont données en tableaux.

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With 7 figures.     

Rovibrational energy transfer in mixtures of CH4 and CD4 with He and Ne

Using non-resonant photoacoustics, the rovibrational relaxation of the lowest bending modes of CH₄ and CD₄ by He and Ne has been studied in the temperature range 140–376 K. The results are compared with semiclassical calculations taking simultaneously into account processes of rotational and vibrational energy transfer.     

The rate constant for the recombination of trifluoromethyl radicals at T = 296 K

The high-pressure rate constant of the CF₃ + CF₃ → C₂F₆ reaction at T = 296 K was measured in the pulse photolysis (λ = 694.3 nm, ruby laser) of CF₃NO in the presence of NO by means of the time-resolved detection of CF₃NO by the intracavity absorption of He(SINGLE BOND)Ne laser radiation (λ = 632.8 nm). The obtained value is k_2∞ = (3.9 ± 1.3) × 10⁻¹² cm³/s. © 1996 John Wiley & Sons, Inc.     

Rotational and vibrational relaxation of methane excited to 2nu3 in CH4/H2 and CH4/He mixtures at 296 and 193 K from double-resonance measurements

A series of time-resolved IR-IR double-resonance experiments have been conducted where methane molecules are excited into a selected rovibrational level of the 2nu3(F2) vibrational substate of the tetradecad and where the time evolution of the population of the various energy levels is probed by a tunable continuous wave laser. The rotational relaxation and vibrational energy transfer processes occurring in methane upon inelastic CH4-H2 and CH4-He collisions have been investigated by this technique at room temperature and at 193 K. By probing transitions in which either the lower or the upper level is the laser-excited level, rotational depopulation rates in the 2nu3(F2) substate were measured. The rate constants for CH4-H2 collisions were found to be 17.7 +/- 2.0 and 18.9 +/- 2.0 micros(-1) Torr(-1) at 296 and 193 K, respectively, and for CH(4)-He collisions they are 12.1 +/- 1.5 and 16.0 +/- 2.0 micros(-1) Torr(-1) at the same temperatures. The vibrational relaxation was investigated by probing other stretching transitions such as 2nu3(F2) - nu3, nu3 + 2nu4 - 2nu4, and nu3 + nu4 - nu4. A kinetic model, taking into account the main collisional processes connecting energy levels up to 6000 cm(-1), that has been developed to describe the various relaxation pathways allowed us to calculate the temporal evolution of populations in these levels and to simulate double-resonance signals. The different rate coefficients of the vibrational relaxation processes involved in these mixtures were determined by fitting simulated signals to the observed signals corresponding to assigned transitions. For vibration to translation energy transfer processes, hydrogen is a much more efficient collision partner than helium, nitrogen, or methane itself at 193 K as well as at room temperature.     

Positive ion chemistry of SiH4/GeF4 gaseous mixtures studied by ion trap mass spectrometry and ab initio calculations

The positive ion chemistry occurring in SiH(4)/GeF(4) gaseous mixtures was investigated by ion trap mass spectrometry and ab initio theoretical calculations. The GeF(3)(+) cation, the only fragment obtained from ionized GeF(4), was unreactive towards SiH(4). All the primary ions SiH(n)(+) (n = 0-3) react instead with GeF(4) so to form SiF(+) or SiH(2)F(+). The latter species reacts in turn with SiH(4) and GeF(4) so to form SiH(3)(+) and SiHF(2)(+), respectively. The potential energy profiles conceivably involved in these reactions were investigated by ab initio calculations performed at the MP2 and coupled cluster (CCSD(T)) level of theory.