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1.
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.  相似文献   

2.
Electron transmission spectra, dissociative attachment studies and threshold electron-impact excitation spectra are presented for CFCl3, CF2Cl2, CF3Cl and CF4. Sharp vibrational progressions, which are ascribed to a new type of resonance, have been discovered in transmission spectra below 2 eV. A survey is given of the dissociative attachment processes in the compounds studied. Some of these processes have not been described previously. New assignments are suggested for the optical spectrum of CF3Cl. Approximate triplet energies of the chlorofluoromethanes and the chlorornethanes are given.  相似文献   

3.
A two-dimensional model of the non-equilibrium unipolar discharge occurring in the plasma–sheath boundary region of a transferred-arc was developed. This model was used to study the current transfer to the nozzle (1 mm diameter) of a 30 A arc cutting torch operated with oxygen. The energy balance and chemistry processes in the discharge were described by using a kinetic block of 45 elementary reactions and processes with the participation of 13 species including electronically excited particles. The nonlocal transport of electrons was accounted for into the fluid model. The dependence of the ion mobility with the electric field was also considered. Basic discharge properties were described. It has been found that a large part (~ 80%) of the total electric power (1700 mW) delivered in the bulk of the sheath region is spent in heating the positive ions and further dissipated through collisions with the neutral particles. The results also showed that the electron energy loss in inelastic collisions represents only ~ 25% of the electron power and that about 63% of the power spent on gas heating is produced by the ion–molecule reaction, the electron–ion and ion–ion recombination reactions, and by the electron attachment. The rest of the power converted into heat is contributed by dissociation by electron-impact, dissociative ionization and quenching of O(1D). Some fast gas heating channels which are expected to play a key role in the double-arcing phenomena in oxygen gas were also identified.  相似文献   

4.
We present cross sections for the elastic scattering of low-energy electrons by phosphine (PH3) and arsine (AsH3), and for electron-impact excitation of the (1t 2 3sa 1)3 T 2 and1 T 2 states in methane (CH4). These results were calculated using the Schwinger multichannel method as implemented on distributed-memory parallel computers. The PH3 and AsH3 cross sections show a pronounced low-energy shape resonance which may provide a pathway to dissociative attachment. The1,3 T 2 cross sections for CH4 correlate fairly well with recent measurements of CH2 production via electron-impact dissociation of methane.Contribution No. 8587  相似文献   

5.
A global plasma model is used to investigate the chemistry and energy deposition in 13.56 MHz radio-frequency capacitively coupled oxygen–argon discharges under conditions usually used for the deposition of tin oxide films. These models are based on the solution of a stationary electron Boltzmann equation coupled to species balance equations including the vibrational kinetics equations of O2. The results obtained showed that vibrational non equilibrium of O2-molecule is not significant. The dissociation degree of O2 was found to be around a few percents and the discharge was often moderately electronegative even for small O2 contents in the feed gas. The ionization and energy dissipation mechanisms are mainly governed by the collisional processes involving O2 for an oxygen feed gas composition greater than 20%. Results also showed that the predicted densities of the charged species and the electronegative character of the discharge are strongly linked to the assumption made on the homogeneity of the power deposition. On the contrary, the predicted density of O-atom is not sensitive to this assumption.  相似文献   

6.
The information concerning dissociative adsorption of H2S on Li surface is inadequate and the mechanistic insight for its complete dissociation is yet to be explored. The present investigation aims to scrutinize the dissociative adsorption of H2S on Li(110) surface using density functional theory calculations. The climbing image nudged elastic band calculation was employed to unveil the relative energy profiles for S−H dissociation. To elucidate the components of interaction energy responsible for stabilizing the adsorbed moieties on the surface, periodic energy decomposition analysis was performed. A Car-Parrinello molecular dynamics (CPMD) simulation was performed to understand the dynamic behaviour of H2S on Li(110). Results vividly demonstrates: (i) partially dissociated product with perpendicular S−H is comparatively stable than the parallel SH, (ii) completely dissociated moieties H/H/S are the most stable among all, (iii) dissociation of first S−H is barrierless and the second S−H dissociation is a low energy barrier reaction, (iv) complete dissociation of H2S occurs in a stepwise manner, (v) orbital and electrostatic contributions of the interaction energy plays a vital role in stabilizing the dissociated moieties, and (vi) stepwise dissociation of H2S was further reinforced by CPMD.  相似文献   

7.
We have investigated the dissociation of a resonant state that can be formed in low energy electron scattering from H2O+. We have chosen the second triplet resonance above the state of H2O+ whose autoionization mainly produces H2O+ ( ). We have considered both dissociation of the resonant state itself, dissociative recombination (DR), or the dissociation of the H2O+ cation after autodetachment, dissociative excitation (DE). The time-evolution of a wave packet on the potential energy surfaces of the resonance and cationic states shows, for the initial conditions studied, that the probability for DR is about 38 % while the probability for DE is negligible.  相似文献   

8.
The boundary trajectories separating non-reactive, reactive, and dissociative bands of trajectories in atom—diatom collision-induced dissociation are defined. Trajectory calculations are reported for the collinear H + H2 reaction which demonstrate how the boundary trajectories can be used to obtained information on the non-reactive, reactive, and dissociative regions of the reagents phase space.  相似文献   

9.
The nonequilibrium vibrational excitation of reacting molecules can result in a substantial (up to 100%) decrease in the temperature threshold of self-ignition in the H2 + air system. In this case, dissociation reactions are responsible for the formation of active species that initiate chain combustion. An increase in temperature during the induction period caused by the recombination of oxygen and hydrogen atoms and OH radicals formed plays an important role in the intensification of chain reactions and self-ignition of the mixture at low temperatures (∼300-400 K)  相似文献   

10.
A 3-parameter local hot spot model of gas-surface reactivity is employed to analyze and predict dissociative sticking coefficients for CH(4) incident on Ir(111) under varied nonequilibrium and equilibrium conditions. One Ir surface oscillator and the molecular vibrations, rotations, and translational energy directed along the surface normal are treated as active degrees of freedom in the 14 dimensional microcanonical kinetics. The threshold energy for CH(4) dissociative chemisorption on Ir(111) derived from modeling molecular beam experiments is E(0) = 39 kJ/mol. Over more than 4 orders of magnitude of variation in sticking, the average relative discrepancy between the beam and theoretically derived sticking coefficients is 88%. The experimentally observed enhancement in dissociative sticking as beam translational energies decrease below approximately 10 kJ/mol is consistent with a parallel dynamical trapping/energy transfer channel that likely fails to completely thermalize the molecules to the surface temperature. This trapping-mediated sticking, indicative of specific energy transfer pathways from the surface under nonequilibrium conditions, should be a minor contributor to the overall dissociative sticking at thermal equilibrium. Surprisingly, the CH(4) dissociative sticking coefficient predicted for Ir(111) surfaces at thermal equilibrium, based on the molecular beam experiments, is roughly 4 orders of magnitude higher than recent measurements on supported nanoscale Ir catalysts at 1 bar pressure, which suggests that substantial improvements in catalyst turnover rates may be possible.  相似文献   

11.
The mechanism of hexane decomposition under gliding arc gas discharge conditions is studied from both qualitative and quantitative analyses of its products for various hexane initial concentrations and different background atmospheres : nitrogen, argon, air (O2 21% N2 79% vol.) and N2–O2 mixtures. The decomposition rate, which decreases with increasing hexane initial concentration, can reach 94% when the carrier gas is air. Due to the electron energy consumed by the dissociation of nitrogen, the decomposition rate of hexane in nitrogen is lower than in argon. The radical channel plays a predominant role in the hexane decomposition process. With increasing oxygen concentration in the carrier gas, the hexane decomposition rate increases and promotes the conversion of CO– CO2, but it also leads to the formation of NO2.  相似文献   

12.
A numerical code, recently developed for describing the kinetics of H2 microwave discharges obtained in diamond deposition plasma reactors, was used to estimate the importance of dissociative attachment from H2 Rydberg states in enhancing the production of H in this kind of discharge. It was also used to investigate H production in multicusp low-pressure magnetic plasmas. Results show that the dissociative attachment from Rydberg states can be as important as the mechanism involving vibrationally excited molecules in both types of plasmas.  相似文献   

13.
Adsorption of hydrogen sulfide (H2S) on the external and internal surface of Zn12O12 nanocluster was studied by using density functional calculations. The results indicate that the H2S molecule is physically adsorbed or chemically dissociated by the nanocluster. It was found that the H2S molecule can dissociate into –H and–SH fragments, suggesting that the nanocluster might be a potential catalyst for dissociation of the H2S molecule. Also, dissociation of H2S to S species in internal surface of the Zn12O12 nanocluster is energetically impossible. The HOMO–LUMO energy gap of H2S dissociation configuration is changed about 27.68 %, indicating that the electronic properties of the nanocluster by dissociation process have strongly changed.  相似文献   

14.
Pulsed discharge plasma is typical oxidation technology for disposing organic compounds in aqueous solutions. When this electrical discharge plasma was applied in water, it may produce hydrogen peroxide (H2O2) without any catalyst or chemical agent. In order to increase H2O2 production by electrical discharge plasma in water, fine bubbles were introduced into the electrical discharge plasma in this experiment. Bipolar pulsed voltages were applied to cylindrical electrodes in the water while Ar or O2 bubbles were introduced, generating a pulsed discharge plasma. The introduction of the bubbles seemed to enhance the dissociation of water molecules and increased H2O2 formation, especially with O2 bubbling. Dissolved oxygen in the water contributed to H2O2 formation by pulsed discharge plasma with the bubbles, while dissociation of water molecules was the cause of H2O2 formation by pulsed discharge plasma without bubbles. More H2O2 was formed by pulsed discharge plasma with O2 bubbles, because the amount of dissolved oxygen in the water increased upon bubbling with O2.  相似文献   

15.
Tirapazamine (TPZ) has been tested in clinical trials on radio‐chemotherapy due to its potential highly selective toxicity towards hypoxic tumor cells. It was suggested that either the hydroxyl radical or benzotriazinyl radical may form as bioactive radical after the initial reduction of TPZ in solution. In the present work, we studied low‐energy electron attachment to TPZ in the gas phase and investigated the decomposition of the formed TPZ? anion by mass spectrometry. We observed the formation of the (TPZ–OH)? anion accompanied by the dissociation of the hydroxyl radical as by far the most abundant reaction pathway upon attachment of a low‐energy electron. Quantum chemical calculations suggest that NH2 pyramidalization is the key reaction coordinate for the reaction dynamics upon electron attachment. We propose an OH roaming mechanism for other reaction channels observed, in competition with the OH dissociation.  相似文献   

16.
17.
The available information on low-energy electron impact cross sections for Cl2, F2, and HCAl is reviewed and critically evaluated. Of interest are the cross sections for momentum transfer; rotational, vibrational, and electronic excitation (with possible dissociation); and attachment and ionization. The bulk of the data available for these molecules are on dissociative attachment, as that is the property, that has been of greatest interest in applications and is relatively easy to measure. Much of the process of critical evaluation is performed by computing electron transport, or swarm, coefficients using the published cross sections and comparing with published measurements. What little is known about electronic excitation or dissociation processes is theoretical or even guessed at using analogies to processes in other molecules. Recommended cross sections are identified where feasible.  相似文献   

18.
Intermediate states formed during the dissociative recombination of molecular ions with electrons can play significant roles in determining the magnitude of the total rate coefficient. These resonances are Rydberg states of two types, that is, they can have the ground or excited states of the ion as a core. Those with the excited cores have a fundamentally different excitation mechanism than those with the ground state core. The importance of excited core states in dissociative recombination has received only limited attention in the literature. Theoretical calculations on the dissociative recombination of N2+ are reported which compare the two types of resonances. Potential curves, electronic widths, cross sections, and rate coefficients are calculated for dissociative recombination along the 2(1)Sigma(g)+ state, one of several routes for the dissociative recombination of N2+. The ground core resonances, in this example, are relatively unimportant compared to those with the excited core. Inclusion of the excited core resonances increases the rate coefficient by about a factor of 4 at room temperature, but the increase is not enough to establish 2(1)Sigma(g)+ as the dominant dissociative route.  相似文献   

19.
The chemical kinetics of ammonia borane (AB) in glyme solution is studied using quantum mechanics (QM) based calculations along with experimental results available in the literature. The primary objective of this study is to propose a detailed reaction mechanism that explains the formation of species observed during AB decomposition for temperatures ranging from 323 to 368 K. The quantum mechanics investigation uses transition state theory to identify the relevant reaction pathways. Intrinsic reaction coordinate calculations use the identified transition‐state structure to link the reactants to the products. These calculations were performed using the Gaussian 09 program package, including the solvation model based on density (SMD) with acetonitrile as the solvent. Thermodynamic properties of species at equilibrium or at transition states were computed using the G4(MP2) compound method. Sensitivity analysis was performed using a species conservation model to identify reactions and species that play a critical role. This study confirms the previous experimental observation regarding the initiation of decomposition of AB in glyme. It also elucidates the role of DADB, ammonium borohydride salt ([BH4][NH4]+) and BH2NH2 in hydrogen release and intermediates formed during initial phase of AB decomposition. This work shows how QM calculations along with experimental results can contribute to our understanding of the complex chemical kinetics involved during AB dehydrogenation.  相似文献   

20.
The relative ease with which a low-pressure hydrogen stream may be heated in an electrical discharge suggests that such a system be considered in current efforts to develop thrusters for spacecraft orbit raising purposes. In this work a detailed model of a microwave discharge in flowing, low-pressure hydrogen is used to interpret and clarify experimental measurements of atom concentration, electron energy, and electron density. The radially averaged, constant-pressure model accurately reproduces the experimental data and also calculated the rates of a number of gas-heating and wall-heating processes as well as rates of energy deposition into coolant and working fluid streams. The calculated gas-heating rates indicate that the gas heating is due primarily to the thermalization of the energetic atoms produced by dissociation of H2 via excitation of theb 3 u + state. The calculations also indicate that the energy flux to the quartz tube is significantly influenced by Lyman and Werner band radiation and by heterogeneous atomic recombination processes and, to a much lesser degree, by electron-ion recombination processes. The fraction of power input which is ultimately transferred to the gas stream is a decreasing function of the power input and varies from 0.24 to 0.12.  相似文献   

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