Dissociative Recombination Reactions Research Articles

Necessary Conditions for Development of Inviscid Instabilities in a Vibrationally Excited Dissociating Gas

For a plane flow of a vibrationally excited dissociating diatomic gas the necessary conditions of the existence of growing (neutral) inviscid perturbations, similar to the Rayleigh criterion of a “generalized” inflection point, are obtained. The corresponding formulas are presented for cases with a certain physical interpretation. In particular, the model of a vibrationally excited one-component gas is considered as the initial stage of thermal dissociation, as well as a wide spread model with one dissociation-recombination reaction. The case of a binary molecular-atomic mixture with a vibrationally excited molecular component and a “frozen” gas-phase dissociation-recombination reaction is considered as an intermediate one. Comparative numerical calculations were carried out, which showed, in particular, that under conditions of developed dissociation, the use of the criterion of the “generalized” inflection point does not take into account the specifics of the process. The wave numbers and phase velocities of the I and II inviscid modes calculated on its basis may differ significantly from the results obtained using the new necessary condition.

Low temperature state-to-state vibrational kinetics of O + N2(v) and N + O2(v) collisions

Starting from an equilibrium initial condition at 3000 K, the non-equilibrium vibrational kinetics of a 500 K atmospheric pressure N2/N/O2/O/NO mixture is investigated with a zero-dimensional time-dependent numerical solver. Attention is devoted to the O + N2(v) and N + O2(v) collisions, thanks to new QCT state-to-state rates from the literature. The Zeldovich and the dissociation–recombination reactions are considered; for the N + O2(v) collisions the inelastic vT processes are considered too. Interpolation formulae of the corresponding rates are proposed.The mixture behaviour depends on the combined effects of the different processes, however under the conditions adopted the forward second Zeldovich reaction switches on the kinetics towards a new equilibrium.The aim of the present study is to open the door to more complex applications of plasmas in the fields of the plasma medicine, the fertilizers production and, more in general, of the nitrogen fixation.

A numerical study on high-temperature effects of exploding shock waves

A planar shock of initial strength MS = 3.0 was focused to a tiny region in space using a spherically converging test section. The shock accelerates inside the test section, collides with the focusing end wall, and gets reflected. Numerical studies show that the flow behind reflected shock behaves like an expanding jet moving through a confined area. It was observed that this expansion caused the formation of a mushroom-shaped structure. Thermodynamic characterization of the mushroom structure was made, and it was found that the gas temperature inside the mushroom structure is higher than that across the reflected shock itself. High-temperature effects, such as temperature-dependent Cp variations and dissociation–recombination reactions of the test gas, were added to the simulations to better understand the effect of temperature on the expanding hot gas. A reduction of 39% in the peak temperature value was obtained at the focusing end wall. Also, the flow inside the mushroom structure was observed to be a reactive mixture of a hot gas slug. It is observed that prominent molecular dissociation and recombination take place inside the mushroom structure, which is absent across the reflected shock.

Mathematical modeling of electrodialysis of a dilute solution with accounting for water dissociation-recombination reactions

The paper explores the limitations of electrodialysis (ED) in the range of very dilute feed solutions. The results of first principles modeling of this process are presented, taking into account water dissociation-recombination reactions (without catalytic effects). A 1D and a 2D model are formulated and applied. Both models use the equation for the rate of chemical reactions, the Nernst-Planck and Poisson equations; additionally, the Navier-Stokes equation is used in the 2D model. We show that the dissociation-recombination reactions can cause the appearance of a space charge region (SCR) in the central part of the desalination chamber (DC). In electrochemistry and colloidal science, the formation of a SCR is associated with the formation of an electrical double layer (EDL) at a charged solid surface. Our modeling predicts that in conditions where the concentrations of H+/OH− ions and salt ions in solution are comparable, a SCR can appear in the bulk solution due to the recombination of H+ and OH− ions. The latter are generated at the membrane forming the DC (water splitting). Water splitting makes impossible total desalting of aqueous solutions by conventional ED. An inversion of the salt concentration profiles is observed: the salt ions are accumulated near the membranes; a depletion region occurs in the center of the DC. Interplay between the chemical reactions and space charge is discussed.

The Thermal Dissociation-Recombination Reactions of SiF4, SiF3, and SiF2: A Shock Wave and Theoretical Modeling Study.

Monitoring UV absorption signals of SiF2 and SiF, the thermal dissociation reactions of SiF4 and SiF2 were studied in shock waves. Rationalizing the experimental observations by standard unimolecular rate theory in combination with quantum-chemical calculations of the reaction potentials, rate constants for the thermal dissociation reactions of SiF4, SiF3, and SiF2 and their reverse recombination reactions were determined over broad temperature and pressure ranges. A comparison of fluorosilicon and fluorocarbon chemistry was finally made.

Can third-body stabilisation of bimolecular collision complexes in cold molecular clouds happen?

For more than half a century, networks of radiative association, dissociative recombination, and bimolecular reactions have been postulated to drive the low-temperature chemistry of cold molecular clouds. Third-body stabilizations of collision complexes have been assumed to be ‘irrelevant’ due to short lifetime of such complexes. Here, we conduct crossed molecular beam studies of ground state atomic silicon with diacetylene in combination with electronic structure calculations and microcanonical kinetics models operating under cold molecular cloud conditions. Our combined experimental, electronic structure, and microcanonical kinetics modelling investigations provide compelling evidence that three-body collisions of molecular hydrogen with long-lived reaction intermediates accessed through intersystem crossing are prevalent deep inside molecular clouds. This concept might be exportable to reactions involving polycyclic aromatic hydrocarbons thus affording a versatile machinery to complex organics via third-body stabilizations of bimolecular collision complexes deep inside cold molecular clouds.

Kinetic isotope effects for dissociative recombination of tritiated ketenyl ion (3HCCO+): A surface-hopping ab initio molecular dynamics study

Dissociative recombination (DR) reactions are important when modeling charged species in the presence of free electrons. While experimental measurements of DR reaction rates are challenging, surface hopping ab initio molecular dynamics (SH-AIMD) simulations provide an attractive alternative. SH-AIMD is especially well-suited for estimating branching ratios, i.e., the relative rates of competing production channels, for DR reactions. Although the radiolysis of diatomic tritium has been studied experimentally, previous attempts to model these systems have failed to account for isotope effects in DR reactions. Previous SH-AIMD studies have also not investigated tritium isotope effects for the branching ratios of DR reactions. In this study, we compute the DR branching ratios of the protiated and tritiated ketenyl ion. Comparison with literature values for the protiated branching ratios provides confidence in the reliability of our SH-AIMD results. Our simulations predict a significant increase of the HC + CO branching ratio for the tritiated system.

Observation of a significant drop of electron density in cascaded arc argon plasma doped with oxygen gas using laser Thomson scattering

In this work, the electron density (n e) and electron temperature (T e) of cascaded arc argon plasma regulated by adding electronegative oxygen gas have been investigated using laser Thomson scattering diagnostic technique. The results indicate that the addition of O2 gas causes a significant decrease of n e, which drops from 1020 m−3 to 1017 m−3. This is mainly attributed to the dissociative recombination reaction between electrons and O2 + molecular ions. Meanwhile, the formation of negative ions, O2 − and O−, consumes electrons and further makes n e decrease. But, T e remains nearly unchanged with the increase of O2 ratio from 0% to 10%. This is probably due to that the electron energy loss by the electron collisions with O2 molecules in the ground state balances the electron heating induced by the super-elastic collisions with the highly vibrational excited O2 molecules.

Profiling astrophysically relevant MgC4H chains. An attempt to aid astronomical observations

ABSTRACT In this paper, we report results of an extensive theoretical study on MgC4H chains conducted at DFT and CCSD(T) levels motivated by the recent discovery of this species in IRC+10216. A detailed characterization of both neutral and charged species is presented, which include structural, chemical bonding and vibrational properties, rotational, centrifugal distortion and Watson l-type doubling constants, dipole moments, Fermi contact, and spin-rotation constants. In addition, we present ab initio estimates needed for subsequent astrochemical evolution modelling (e.g. dissociation energies, acidity, electron attachment, and ionization energies and related chemical reactivity indices). Possible formation pathways are also discussed. They comprise exchange, (radiative) association, dissociative recombination, and ion neutralization reactions. As an important result aiming at stimulating further observational searching, we suggest that MgC4H− anions should also be observable via rovibrational spectroscopy. The reason is twofold: (i) Neutral MgC4H0 chains possess a sufficiently large dipole moment consistent with dipole-bound anion states and large electron attachment cross-sections. (ii) MgC4H− anions possess a dipole substantially larger than MgC4H0 neutrals (and also larger than that estimated earlier for the longest astronomically detected C8H− anion). This makes MgC4H− anion intensities in rovibrational spectrum experimentally accessible even in the unlikely case of a relative abundance MgC4H−/MgC4H0 comparable to that of CH4, whose anion has the lowest relative abundance observed so far in space because weakly polar C4H0 chains do not support dipole-bound anion states. A suggestion on why, counterintuitively, the MgC2H abundance found in IRC+10216 was lower than that of the longer MgC4H is also presented.

Dayside nitrogen and carbon escape on Titan: the role of exothermic chemistry

Context. Atmospheric escape has an appreciable impact on the long-term climate evolution on terrestrial planets. Exothermic chemistry serves as an important mechanism driving atmospheric escape and the role of such a mechanism is of great interest for Titan due to its extremely complicated atmospheric and ionospheric composition. Aims. This study is devoted to a detailed investigation of neutral N and C escape on the dayside of Titan, which is driven by exothermic neutral–neutral, ion–neutral, and dissociative recombination (DR) reactions. It was carried out based on the extensive measurements of Titan’s upper atmospheric structure by a number of instruments on board Cassini, along with an improved understanding of the chemical network involved. Methods. A total number of 14 C- and N-containing species are investigated based on 146 exothermic chemical reactions that release hot neutrals with nascent energies above their respective local escape energies. For each species and each chemical channel, the hot neutral production rate profile is calculated, which provides an estimate of the corresponding escape rate when combined with the appropriate escape probability profile obtained from a test particle Monte Carlo model. Results. Our calculations suggest a total N escape rate of 9.0 × 1023 s−1 and a total C escape rate of 4.2 × 1023 s−1, driven by exothermic chemistry and appropriate for the dayside of Titan. The former is primarily contributed by neutral-neutral reactions, whereas the latter is dominated by ion–neutral reactions; however, contributions from neutral–neutral and DR reactions to the latter cannot be ignored either. Our calculations further reveal that the bulk of N escape is driven by hot N(4S) production from the collisional quenching of N(2D) by ambient N2, while C escape is mainly driven by hot CH3 and CH4 production via a number of important ion–neutral and neutral–neutral reactions. Conclusions. Considered in the context of prior investigations of other known escape mechanisms, we suggest that exothermic chemistry is likely to contribute appreciably to non-thermal C escape on the dayside of Titan, although it plays an insignificant role in N escape.

Effects of non-equilibrium excitation on methane oxidation in a low-temperature RF discharge

The kinetic effects of non-equilibrium excitation by direct electron impact on low-temperature oxidation of CH4 were investigated by experiment and simulation. We focused on the vibrational-electronic-chemistry coupling of methane and oxygen molecules under conditions of immediate reduced electric field strengths of 30–100 Td in an RF dielectric barrier discharge. A detailed plasma chemistry mechanism governing the oxidation processes in an He/CH4/O2 combustible mixture was proposed and studied by including a set of electron impact reactions, dissociative recombination reactions, reactions involving vibrationally- and electronically- excited species, and important three-body recombination reactions. A linear increase in reactant consumption with an increase in plasma power was observed experimentally. This suggested the presence of decoupling between the molecular excitation by plasma and the low-temperature chemistry. However, CO formation showed a non-linear trend, with its formation increasing with lower energy inputs and decreasing at higher energy inputs. By modelling the chemical kinetic sensitivity and reaction pathways, we found that the formation of radicals via the chain propagation reactions CH4 + O(1D) → CH3 + OH, and O2(a1Δg) + H → O + OH was mainly accelerated by the electronically excited species O(1D) and O2(a1Δg). The numerical simulation also revealed that under conditions of incomplete relaxation, the vibrational species CH4(v) and O2(v) enhanced chain propagating reactions, such as CH4(v) + O → CH3 + OH, CH4(v) + OH → CH3 + H2O, O2(v) + H → O + OH, thus stimulating the production of active radicals and final products. Specifically, for an E/N value of 68.2 Td in a stoichiometric mixture (0.05 CH4/0.1 O2/0.85 He), O(1D), CH4(v13), and O2(v) were estimated to contribute to 12.7%, 3.6%, and 3.8% of the production of OH radicals respectively. The reaction channel CH4(v13) + OH → H2O + CH3 was estimated to be responsible for 1.6% of the H2O formation. These results highlight the strong roles of vibrational states in a complex plasma chemistry system and provide new insights into the roles of excited species in the low-temperature oxidation kinetics of methane.

Methane pyrolysis with N2/Ar/He diluents in a repetitively-pulsed nanosecond discharge: Kinetics development for plasma assisted combustion and fuel reforming

Experimental and kinetic studies of the non-equilibrium plasma-assisted methane conversion into higher hydrocarbons and H2 with N2/Ar/He diluents are investigated. Experiments of CH4 pyrolysis with N2/Ar/He diluents in a repetitively-pulsed nanosecond discharge are conducted at different applied voltages. The measured results show that H2 and C2H6 are the major products of all mixtures. A plasma-assisted CH4/diluents pyrolysis kinetic model incorporating the key excited species is developed and validated. The evolutions of the excited species, radicals, ions, and electrons at different timescales are revealed by kinetic modeling. A pathway analysis shows that electronically excited N2(A), N2(B), N2(a′), and N2(C) significantly contribute to CH4 dissociation into CH3 and H. As small amounts of CH2 and CH are produced in CH4/N2 mixtures, little C2H4, C3H6, and C3H8, and less C2H2 and C3H4 are experimentally measured and numerically observed than mixtures of Ar or He. The quenching and transitions among N2(A, B, a′, C) and the relaxation of vibrationally excited N2(v) by hydrocarbons provide a thermal contribution to the CH4/N2 mixture, resulting in a higher gas temperature than that of CH4/Ar and CH4/He mixtures. In CH4/Ar mixtures, Ar* and Ar+ are the dominant species for CH4 pyrolysis. Comparatively, electron impact dissociation and ionization reactions are the primary pathways for CH4 consumption with He diluent. A pathway analysis of CH4 pyrolysis further reveals that dissociative recombination reactions between electrons and fuel ions are responsible for carbon formation. This work provides base CH4 pyrolysis mechanisms with different diluents for plasma-assisted CH4 combustion and reforming at low temperatures, as well as an insight into carbon formation pathways of plasma-assisted CH4 pyrolysis.

Chemical Non-equilibrium Simulation of Anode Attachment of an Argon Transferred Arc

The diffuse and constricted arc attachment modes to the anode of an argon wall-stabilized transferred arc are investigated using a 2D chemical non-equilibrium model. Detailed argon chemical kinetic processes and self-consistent effective binary diffusion coefficient approximation treatment of diffusion are coupled with a single-fluid, two-temperature model to investigate the plasma characteristics of the near-anode region. The diffuse and constricted arc attachment modes to the anode are obtained self-consistently by changing the gas flow rates. The results show that the high current density and associated self-induced magnetic field of the constricted arc attachment give rise to a large Lorentz force near the anode, resulting in an anode jet. The convection process dominated by the cathode jet is conducive to the radial transport of electrons in the diffuse arc attachment mode, while in the constricted arc attachment mode the direction of electron transport due to the convection caused by the anode jet is opposite to that in a diffuse arc attachment. The effect of diffusion on electron transport is relatively small in the arc central region, while it becomes important in the arc fringe. The recombination processes, especially the dissociative recombination reactions, restrict the electron radial diffusion in the arc fringe. The diffusion current density is the main component of current density in front of the anode due to the steep gradient of electron pressure. It is found that when the diffusion current is not considered, the electric field and Joule heating in front of anode promote a more constricted arc attachment.

Role of Electron–Ion Dissociative Recombination in $$\hbox {CH}_{4}$$ Microwave Plasma on Basis of Simulations and Measurements of Electron Energy

C–H bond activation was studied in low pressure microwave plasma discharges in methane. Electron energy loss channels were analyzed in view of promoting vibrational excitation. The molecular dissociative recombination (DR) channel is concluded to play multiple roles in the hydrocarbon plasma chemistry. DR increases the electron temperature with input power density and simultaneously breaks the hydrocarbon chains. Depending on the ionic species considered, plasma density $$\hbox {n}_e$$ in the range of $$10^{17}-10^{19}\,\hbox {m}^{-3}$$ ($$10^{-6}-10^{-4}$$ ionization degree) and the electron mean energy $$ $$ in the range of $$2-4\hbox { eV}$$ were estimated on basis of a Boltzman solver. $$ $$ from $$2-3\hbox { eV}$$ measured with Thomson scattering anchored the microwave discharges in a preferential regime for vibrational excitation. The best agreement with experiments was obtained when $$\hbox {C}^{+}$$ is the dominant ion in the $$\hbox {CH}_{4}$$ microwave plasma, formed through successive DR and charge exchange reactions from molecular ions.

Hydronitrogen Molecular Assisted Recombination (HN-MAR) process in ammonia seeded deuterium plasmas

Abstract Ammonia molecules, which will be formed in a nitrogen-seeded divertor plasma, show significant catalytic effects for volumetric recombination of hydrogen ions. This new Molecular Assisted Recombination (MAR), named the Hydronitrogen-MAR (HN-MAR), is based on the charge/ion exchange with NH3 and subsequent volumetric recombination of NH3+ and NH4+. In our PISCES-E RF plasma device, Ar is mixed with ND3/D2 gasses to realize a high electron density plasma [ne ∼ 1017 m−3] which allows us to see the importance of the dissociative recombination reactions which become comparable with wall loss reactions in our chamber. As increasing the plasma density, a calibrated Electrostatic Quadrupole Plasma analyzer measured dropping of ND4+ density fractions which can be explained reasonably by the existence of its dissociative recombination process in the plasma as a dominant reaction.

Comparison of simulations and experiments on the axial distributions of the electron density in a point-to-plane streamer discharge at atmospheric pressure

The axial distributions of the electron density in primary streamer discharge for atmospheric-pressure air are simulated and compared with experimental data obtained by Inada et al (2017 J. Phys. D: Appl. Phys. 50 174005) using a highly temporally and spatially resolved measurement method for Shack–Hartmann-type laser wavefront sensors. The simulation is performed for the same electrode configuration, applied voltage, and gas component as the previously reported experiments. The simulation results show that the dissociative recombination reaction of electrons with cluster ion has an important influence on the axial distributions of the electron density during the primary streamer phase. However, the simulation results also indicate that the effect of the dissociative recombination reaction with cluster ion is limited to the primary streamer phase and the two-body attachment reaction of the electrons with O2 is dominant for the decrease in electron density occurring during the secondary streamer phase. These findings contribute to improving the accuracy of the chemical reaction model of atmospheric-pressure streamer discharge, facilitating the development of atmospheric-pressure plasma applications.

Semiempirical breakdown curves of C2N(+) and C3N(+) molecules; application to products branching ratios predictions of physical and chemical processes involving these adducts

We constructed semiempirical breakdown curves (BDC) for C2N, C3N, C2N+ and C3N+ molecules. These BDC, which are energy dependent dissociation branching ratios (BR) curves, were used to predict products branching ratios for various processes leading to the formation of C2N(+) and C3N(+) excited adducts. These processes, of astrochemical interest, are neutral-neutral and ion-molecule reactions, dissociative recombination and charge transfer reactions with He+. Model predictions of BR are compared to the literature data and to reported values in the kinetic database for astrochemistry KIDA. With the new BR values, the CnN abundances in cold cores were simulated.

Reaction pathways of producing and losing particles in atmospheric pressure methane nanosecond pulsed needle-plane discharge plasma

In this work, a two-dimensional fluid model is built up to numerically investigate the reaction pathways of producing and losing particles in atmospheric pressure methane nanosecond pulsed needle-plane discharge plasma. The calculation results indicate that the electron collisions with CH4 are the key pathways to produce the neutral particles CH2 and CH as well as the charged particles e and CH3+. CH3, H2, H, C2H2, and C2H4 primarily result from the reactions between the neutral particles and CH4. The charge transfer reactions are the significant pathways to produce CH4+, C2H2+, and C2H4+. As to the neutral species CH and H and the charged species CH3+, the reactions between themselves and CH4 contribute to substantial losses of these particles. The ways responsible for losing CH3, H2, C2H2, and C2H4 are CH3 + H → CH4, H2 + CH → CH2 + H, CH4+ + C2H2 → C2H2+ + CH4, and CH4+ + C2H4 → C2H4+ + CH4, respectively. Both electrons and C2H4+ are consumed by the dissociative electron-ion recombination reactions. The essential reaction pathways of losing CH4+ and C2H2+ are the charge transfer reactions.

The interstellar chemistry of C3H and C3H2 isomers.

We report the detection of linear and cyclic isomers of C3H and C3H2 towards various starless cores and review the corresponding chemical pathways involving neutral (C3Hx with x=1,2) and ionic (C3Hx+ with x = 1,2,3) isomers. We highlight the role of the branching ratio of electronic Dissociative Recombination (DR) reactions of C3H2+ and C3H3+ isomers showing that the statistical treatment of the relaxation of C3H* and C3H2* produced in these DR reactions may explain the relative c,l-C3H and c,l-C3H2 abundances. We have also introduced in the model the third isomer of C3H2 (HCCCH). The observed cyclic-to-linear C3H2 ratio vary from 110 ± 30 for molecular clouds with a total density around 1×104 molecules.cm-3 to 30 ± 10 for molecular clouds with a total density around 4×105 molecules.cm-3, a trend well reproduced with our updated model. The higher ratio for low molecular cloud densities is mainly determined by the importance of the H + l-C3H2 → H + c-C3H2 and H + t-C3H2 → H + c-C3H2 isomerization reactions.

A theoretical study of the dissociative recombination of SH+ with electrons through the 2Π states of SH.

A quantitative theoretical study of the dissociative recombination of SH+ with electrons has been carried out. Multireference, configuration interaction calculations were used to determine accurate potential energy curves for SH+ and SH. The block diagonalization method was used to disentangle strongly interacting SH valence and Rydberg states and to construct a diabatic Hamiltonian whose diagonal matrix elements provide the diabatic potential energy curves. The off-diagonal elements are related to the electronic valence-Rydberg couplings. Cross sections and rate coefficients for the dissociative recombination reaction were calculated with a stepwise version of the multichannel quantum defect theory, using the molecular data provided by the block diagonalization method. The calculated rates are compared with the most recent measurements performed on the ion Test Storage Ring (TSR) in Heidelberg, Germany.