Electron-ion Dissociative Recombination Research Articles

Effect of charged dust grains on the electrojet instabilities

The Farley-Buneman and Gradient Drift instabilities have been investigated using a fluid model, in a partially ionized dusty electrojet region in which dust and neutral particles constitute a uniform static background. The effects of dissociative electron-ion recombination and dust charge fluctuation on the instabilities also have been taken into account. The electron-ion dynamics are considered to derive the perturbed densities which further lead to the generalized dispersion relation. The dispersion relation describes the propagation of electrojet instabilities having frequency within dust ion acoustic range in a magnetized partially ionized dusty plasma. The dispersion relation is separately solved numerically and analytically for the two values of anisotropy parameters which correspond to the two different altitudes in the electrojet region. It is found that Gradient drift instability is unstable at a much longer wavelength as compared to Farley-Buneman instability both with or without dust. At lower altitudes(90 km) the increase of negative charge on dust decreases the threshold electron drift velocity for Farley-Buneman instability while it shows the opposite behavior at higher altitudes(100 km). A much lower electron drift velocity is required to excite the Gradient drift instability than the Farley-Buneman instability at both altitudes. The dissociative electron-ion recombination damps both modes much more than the dust charge fluctuation. A significant changes in threshold drift velocity is observed for the Farley-Buneman mode as compared to the Gradient Drift mode due to the two main damping mechanisms. The present analysis is applicable in the lower ionospheric electrojet region where meteoric ablation processes are dominant.

Cross-comparison of diagnostic and 0D modeling of a micro-hollow cathode discharge in the stationary regime in an Ar/N2 gas mixture

A micro-hollow cathode discharge (MHCD) operated in Ar/N2 gas mixture, working in the normal regime, was studied both experimentally and with a 0D (volume-averaged) model in this work. This source provides high electron densities (up to 1015 cm−3) at low injected power (1 W). To understand the mechanisms leading to the production of N atoms, the densities of electrons, N atoms and argon metastable atoms (Ar*) were monitored over a wide range of experimental conditions. Electrons, N atoms and Ar* densities were probed by means of optical emission spectroscopy, vacuum ultra violet Fourier transform spectroscopy and tunable diode laser absorption spectroscopy, respectively. Measurements showed that using a smaller hole diameter enables to work with less injected power, while increasing the power density inside the hole and, subsequently, increasing the densities of excited species. Varying the percentage of N2 in the gas mixture highlighted that, up to 80%, the density of N atoms increases although the dissociation rate drops. Looking at the processes involved in the production of N atoms with the help of the 0D model, we found that at very low N2 fraction, N atoms are mostly produced through dissociative electron-ion recombination. However, adding more N2 decreases drastically the electron density. The density of N atoms does not drop thanks to the contribution of Ar* atoms, which are the main species dissociating N2 between 5% and 55% of N2 in the gas mixture. A reasonable agreement is found between the experiments and the model results. This study shows that, with this MHCD, it is possible to significantly modify the production of N atoms when modifying the physical parameters, making it particularly relevant for applications requiring a N atoms source, such as nitride deposition.

Comparative study of photo-produced ionosphere in the close environment of comets

Context.TheGiottoand Rosetta missions gave us the unique opportunity of probing the close environment of cometary ionospheres of 1P/Halley (1P) and 67P/Churyumov-Gerasimenko (67P). The plasma conditions encountered at these two comets were very different from each other, which mainly stem from the different heliocentric distances, which drive photoionization rates, and from the outgassing activities, which drive the neutral densities.Aims.We asses the relative contribution of different plasma processes that are ongoing in the inner coma: photoionization, transport, photoabsorption, and electron–ion dissociative recombination. The main goal is to identify which processes are at play to then quantitatively assess the ionospheric density.Methods.We provide a set of analytical formulas to describe the ionospheric number density profile for cometary environments that take into account some of these processes. We discuss the validity of each model in the context of the Rosetta andGiottomissions.Results.We show that transport is the dominant loss process at large cometocentric distances and low outgassing rates. Chemical plasma loss throughe−-ion dissociative recombination matters around 67P near perihelion and at 1P during theGiottoflyby: its effects increase as the heliocentric distance decreases, that is, at higher outgassing activity and higher photoionization frequency. Photoabsorption is of importance for outgassing rates higher than 1028s−1and only close to the cometary nucleus, well below the location of both spacecraft. Finally, regardless of the processes we considered, the ion number density profile always follows a 1∕rlaw at large cometocentric distances.

Influence of collisions on ion dynamics in the inner comae of four comets.

Collisions between cometary neutrals in the inner coma of a comet and cometary ions that have been picked up into the solar wind flow and return to the coma lead to the formation of a broad inner boundary known as a collisionopause. This boundary is produced by a combination of charge transfer and chemical reactions, both of which are important at the location of the collisionopause boundary. Four spacecraft measured ion densities and velocities in the inner region of comets, exploring the part of the coma where an ion-neutral collisionopause boundary is expected to form. The aims are to determine the dominant physics behind the formation of the ion-neutral collisionopause and to evaluate where this boundary has been observed by spacecraft. We evaluated observations from three spacecraft at four different comets to determine if a collisionopause boundary was observed based on the reported ion velocities. We compared the measured location of the ion-neutral collisionopause with measurements of the collision cross sections to evaluate whether chemistry or charge exchange are more important at the location where the collisionopause is observed. Based on measurements of the cross sections for charge transfer and for chemical reactions, the boundary observed by Rosetta appears to be the location where chemistry becomes the more probable result of a collision between H2O and H2O+ than charge exchange. Comparisons with ion observations made by Deep Space 1 at 19P/Borrelly and Giotto at 1P/Halley and 26P/Grigg-Skjellerup show that similar boundaries were observed at 19P/Borrelly and 1P/Halley. The ion composition measurements made by Giotto at Halley confirm that chemistry becomes more important inside of this boundary and that electron-ion dissociative recombination is a driver for the reported ion pileup boundary.

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.

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.

Experimental electron-ion dissociative recombination rate constant and temperature dependence data for protonated methanol, ethanol, dimethyl ether and diethyl ether

A flowing afterglow with an electrostatic Langmuir probe has been used to determine the electron-ion dissociative recombination (e-IDR) rate constants (αe) and temperature dependencies (300–500K) for a series of small alcohols and ethers. The species covered are protonated methanol (CH3OH2+), protonated ethanol (C2H5OH2+), protonated dimethyl ether ((CH3)2OH+) and protonated diethyl ether ((C2H5)2OH+). The e-IDR rate constants are αe=5.95×10−7 (T/300)−0.57 for protonated methanol, αe=6.20×10−7 (T/300)−0.87 protonated ethanol, αe=5.52×10 −7 (T/300)−0.75 protonated dimethyl ether, and αe=6.48×10 −7 (T/300)−1.05 protonated diethyl ether. The experimental data are discussed in terms of the relevance to the chemical modeling of the interstellar medium.

A study of the electron–ion dissociative recombination rate constants of the isotopomers of protonated carbon monoxide (HCO+) and formaldehyde (H3CO+)

The room temperature electron–ion dissociative recombination (e–IDR) rate constants (αe) for the isotopomers of protonated carbon monoxide (HCO+) and protonated formaldehyde (H3CO+) have been measured using a flowing afterglow equipped with an electrostatic Langmuir probe. The isotopomers included in this study and their corresponding e–IDR rate constants are H312CO+, 7.4×10−7cm3s−1; H313CO+, 6.8×10−7cm3s−1; H12CO+, 2.0×10−7cm3s−1; H13CO+, 1.8×10−7cm3s−1; D12CO+, 1.6×10−7cm3s−1; and D13CO+, 1.4×10−7cm3s−1. The recombination rate constants for the measured isotopomers exhibit a decrease in the rate constant with the addition of a heavy isotope. This effect is greater with a hydrogen/deuterium substitution than with a 12C/13C substitution. The comparison of the current data with previously published data is discussed.

Trends in electron–ion dissociative recombination of benzene analogs with functional group substitutions: Negative Hammett σpara values

An in-depth study of the effects of functional group substitution on benzene’s electron–ion dissociative recombination (e-IDR) rate constant has been conducted. The e-IDR rate constants for benzene, biphenyl, toluene, ethylbenzene, anisole, phenol, and aniline have been measured using a Flowing Afterglow equipped with an electrostatic Langmuir probe (FALP). These measurements have been made over a series of temperatures from 300 to 550K. A relationship between the Hammett σpara values for each compound and rate constant has indicated a trend in the e-IDR rate constants and possibly in their temperature dependence data. The Hammett σpara value is a method to describe the effect a functional group substituted to a benzene ring has upon the reaction rate constant.

Electron-ion dissociative recombination rate constants relevant to the Titan atmosphere and the Interstellar Medium.

Following the arrival of Cassini at Titan in 2004, the Titan atmosphere has been shown to contain large complex polycyclic-aromatic hydrocarbons. Since Cassini has provided a great deal of data, there exists a need for kinetic rate data to help with modeling this atmosphere. One type of kinetic data needed is electron-ion dissociative recombination (e-IDR) rate constants. These data are not readily available for larger compounds, such as naphthalene, or oxygen containing compounds, such as 1,4 dioxane or furan. Here, the rate constants for naphthalene, 1,4 dioxane, and furan have been measured and their temperature dependencies are determined when possible, using the University of Georgia's Variable Temperature Flowing Afterglow. The rate constants are compared with those previously published for other compounds; these show trends which illustrate the effects which multi-rings and oxygen heteroatoms substitutions have upon e-IDR rate constants.

Trends in electron–ion dissociative recombination of benzene analogs with functional group substitutions: Positive Hammett σpara constants

An in-depth study of the effects of functional group substitution on benzene's electron–ion dissociative recombination (e-IDR) rate constants has been conducted. The e-IDR rate constants for nitrobenzene, benzonitrile, acetophenone, benzyl methyl ether, and phenyl isothiocyanate have been measured using a flowing afterglow equipped with an electrostatic Langmuir probe (FALP). A trend has been indicated between the associated positive Hammett σpara constant for each functional group substitution and the e-IDR rate constants for the protonated form of the benzene analog. Such a plot has indicated that protonated benzene analogs which have functional groups substitutions with a positive Hammett σpara constant value have approximately the same e-IDR rate constant of 4.6×10−7cm3s−1. Implications of the observed trends to the predictions of e-IDR rate constants for unmeasured benzene analogs are discussed.

Electron-ion dissociative recombination rate constants: Effects for benzene analogs with varying degrees of methylation

An in-depth study of the effects of varying degrees of methyl substitution on protonated benzene's electron-ion dissociative recombination (e-IDR) rate constants has been conducted. The e-IDR rate constants for protonated benzene, toluene, xylene, trimethylbenzene, tetramethylbenzene, pentamethylbenzene, hexamethylbenzene, and all of their corresponding isomers have been measured using a Flowing Afterglow equipped with an electrostatic Langmuir probe (FALP) at 300K. The addition of the first methyl substitution decreases the measured e-IDR rate constant by ∼50%. The addition of successive methyl substituents has little effect on the e-IDR rate constant. Isomeric forms also have no effect on the measured e-IDR rate constant. Implications of the observed trends to the chemistry of the Titan ionosphere are discussed.

Recombinative dissociation and the evolution of a molecular ultracold plasma

We present a kinetic model based on coupled rate-equation simulations that accounts for state-detailed rate processes in the evolution of a molecular Rydberg gas to plasma. Calculations support a steady-state picture of Rydberg predissociation and its contribution to plasma dissipation. Inelastic collisions of electrons with Rydberg states efficiently transfer energy from and to the thermal bath of free electrons, giving rise to quasi equilibria of relatively high temperature. Dissipative effects that remove low-n population change the time course of this heating. Including statistically sampled channels of Rydberg predissociation with n- and ℓ-dependent lifetimes slows the relaxation to quasi equilibrium and retards the initial electron temperature increase. At later times, the dissociative loss of low-n population displaces this quasi equilibrium leading to continuous electron heating. Dissociation products appear at a rate that is controlled initially by the rate of Rydberg relaxation to quasi equilibrium. Thereafter, the overall rate at which the total dissociation consumes neutral molecules determines the decay rate for all levels. Simulations predict a total predissociation rate that is smaller than the initial rate of direct electron–ion dissociative recombination. At later times, residual predissociation displays an apparent first-order decay with a rate constant that fits well with experimental observations.

Dissociative Electron-Ion Recombination of the Interstellar Species Protonated Glycolaldehyde, Acetic Acid, and Methyl Formate

Recently, methyl formate, glycolaldehyde, and acetic acid have been detected in the Interstellar Medium, ISM. The rate constants, α(e), for dissociative electron-ion recombination of protonated gycolaldehyde, (HOCH(2)CHO)H(+), and protonated methyl formate, (HCOOCH(3))H(+), have been determined at 300 K in a variable temperature flowing afterglow using a Langmuir probe to obtain the electron density. The recombination rate constants at 300 K are 3.2 × 10(-7) cm(3) s(-1) for protonated methyl formate and 7.5 × 10(-7) cm(3) s(-1) for protonated glycolaldehyde. The recombination rate constant of protonated acetic acid could not be directly measured, but it appears to have a rate constant, α(e), on the 10(-7) cm(3) s(-1) scale. Several high- and low-temperature measurements for protonated methyl formate were made. In addition, an α(e) measurement at 220 K for protonated glycolaldehyde was performed. The astrochemical implications of the rates of recombination, α(e), and protonation routes are discussed.

The effect of N-heteroatoms and CH 3 substituents on dissociative electron–ion recombination of protonated single six membered ring compounds at room temperature

Electron–ion dissociative recombination rate constants have been determined for protonated single six-membered hydrocarbon rings with varying N-atom substitution in the rings and varying degrees of methyl substitutions to the rings. The species studied have been protonated forms of the xylenes (C 8H 10; with configurations o, m, and p), the picolines (C 6H 7N; with methyl substitutions located at 2, 3, and 4), mesitylene (C 9H 12) and 2, 5-lutidine (C 7H 9N). By operating at high reactant vapor pressures, ternary association has been made to dominate over recombination to create proton bound dimers and the rate constants of these species have been determined. The studies were made at room temperature in a flowing afterglow with a Langmuir probe to determine the reduction in electron density as a function of distance along the flow tube. All of the data except for mesitylene showed a consistent trend with the numbers of N-atom substitutions and CH 3 attached to the rings. For the protonated species, the rate constants increase with the number of nitrogens and decreased with number of CH 3 substituents attached to the ring. For proton bound dimers, the rate constants increase with both, the number of N-atoms and number of CH 3 substituents. For the xylene and picoline isomer's the measurements showed that the rate constants were independent of isomeric form, and thus, it is not necessary to study all isomeric forms. The relevance of these studies to the ionosphere of Titan is discussed.

Flowing afterglow studies of dissociative electron-ion recombination for a series of single ring compounds at room temperature

A series of dissociative electron-ion recombination reactions has been studied at room temperature using a Flowing Afterglow with Langmuir Probe (FALP) to determine the rate constants. Studied species include protonated five and six membered single rings with the heteroatoms, N, O and S and with or without CH 3 substituent groups attached to the rings. The compounds studied are protonated benzene, C 6H 7 +; toluene, C 6H 5CH 4 +; pyridine, C 5H 5NH +; pyrimidine, C 4H 4N 2H +; 4-picoline, C 6H 7NH +; cyclohexane, C 6H 13 +; 1,4 dioxane, C 4H 8O 2H + all six membered rings (all but the last two have π electrons in the ring) with furan, C 4H 4OH +; pyrrole, C 4H 5NH +; 1-methylpyrrole, C 5H 7NH +; thiophene, C 4H 4SH + and pyrrolidine, C 4H 9NH + all five membered rings (with only the last mentioned having no π electrons in the ring). Comparison is made with literature data on some of the multi-ring systems (PAH's and PANH's). The involvement of π electrons in the rings is discussed. Protonated six membered rings have been shown to recombine ∼2 times faster than five membered rings with the exception of 1,4 dioxane which has no π electrons. The situation is very different for the proton bound dimers, where five membered rings recombine somewhat more rapidly than six membered rings, again with 1,4 dioxane behaving very differently. These data are critically important in chemically modeling the Titan ionosphere which is presently being probed by apparatus onboard the Cassini spacecraft.

Effect of isotopic content on the rate constants for the dissociative electron–ion recombination of N 2H +

The rate constants, α e, for dissociative electron–ion recombination of various isotopic combinations of N 2H + have been determined at 300 and 500 K in a variable temperature flowing afterglow using a Langmuir probe to determine the electron density. The values of α e at 300 K are 2.77 ( 14N 2H +), 2.12 ( 14N 2D +), 2.31 ( 15N 2H +) and 1.98 ( 15N 2D +) × 10 −7 cm 3 s −1. The equivalent values at 500 K are 2.84, 2.33, 2.93, and 2.33 respectively. This has shown that the greatest change occurs between H and D substitution α e( 14/15N 2D +)/ α e( 14/15N 2H +) ∼ 0.765/0.857 at 300 K and 0.820/0.795 at 500 K respectively. Values at 500 K are consistently larger than at 300 K. At both temperatures, the rate constants with H substitution are larger than with D substitution for both 14N and 15N. Values with 14N substitution are larger than with 15N for both H and D. At 500 K, the values are independent of whether the ion contains 14N or 15N. Gas-phase 15N fractionation enhancement is predicted in many regions of the interstellar medium including cold and pre-stellar cores. The effect of this fractionation on the recombination process is investigated. The relevance to storage ring measurements of recombination rate constants is considered and applications to the chemistry of the interstellar medium are discussed.

Time- and space-resolved measurements of Ar(1s5) metastable density in a microplasma using diode laser absorption spectroscopy

Time- and space-resolved measurements of Ar(1s5) metastable (Ar*) density were carried out in a pulsed dc argon microplasma discharge, using diode laser absorption spectroscopy. The temporal behaviour of metastable density after discharge turn off (in the afterglow) depended on their spatial location in the microplasma. In the early afterglow, the Ar* density decayed monotonically with time in the region around the sheath edge, while in the bulk plasma the Ar* density showed a maximum with time. This behaviour was attributed to electron–ion dissociative recombination. Later in the afterglow, the Ar* decay was everywhere monotonic with time, mainly due to three-body collisional quenching by ground state argon atoms. The time evolution of the Ar* density in the afterglow predicted by a kinetic model is in good agreement with the experimental measurements.

Laboratory chemistry relevant to understanding and modeling the ionosphere of Titan

Laboratory data have a dual and critical role in interpreting information obtained from the Cassini spacecraft in its passes through the Titan ionosphere. Firstly, in situ mass spectra are obtained by Cassini and their conversion into atmospheric molecular composition requires chemical modeling to create agreement between the observed mass spectra and those determined from the models. Secondly, once agreement is obtained, then the chemical model can be considered to represent the evolution of the Titan atmosphere. As a contribution to these endeavors in the past, laboratory measurements have been made in the Selected Ion Flow Tube (SIFT) of the reactions of a series of ring molecules with the important ionospheric ion CH3+. These reactions showed that a dominant reaction channel is association. In the present study, this work has been extended to reactions of another important Titan ion C3H3+. These ion-molecule reactions have also been studied at room temperature using a SIFT. Reactions have been studied in detail with benzene, toluene and pyridine and show again that association is very important. The loss of ionization in the ionosphere is then controlled by electron-ion dissociative recombination of the association ions and their progeny. The recombination reactions have been studied as a function of temperature (300 to 550 K) using a flowing afterglow. These combined data have been used to develop a subset of the chemistry and test its viability. They have indicated that association of the important Titan ions with the abundant nitrogen, followed by switching of the nitrogen for the ring compounds, can build up larger species, perhaps resulting in multi-rings. Recombination of such species can affect the ionization balance and provide species which can contribute to the parallel neutral chemistry. Species are suggested that should be looked for in the in situ mass spectra.

Flowing afterglow studies of temperature dependencies for electron dissociative recombination of HCNH +, CH 3CNH + and CH 3CH 2CNH + and their symmetrical proton-bound dimers

A study has been made using a variable temperature flowing afterglow Langmuir probe technique (VT-FALP) to determine the equilibrium temperature dependencies of the dissociative electron-ion recombination of the protonated cyanide ions (RCNH +, where R=H, CH 3 and C 2H 5) and their symmetrical proton-bound dimers (RCNH +NCR). The power law temperature dependencies of the recombination coefficients, α e, over the temperature range 180 to 600 K for the protonated ions are α e( T)(cm 3 s −1)=3.5±0.5×10 −7 (300/ T) 1.38 for HCNH +, α e( T)=3.4±0.5×10 −7 (300/ T) 1.03 for CH 3CNH +, and α e( T)=4.6±0.7×10 −7 (300/ T) 0.81 for CH 3CH 2CNH +. The equivalent values for the proton-bound dimers are α e( T)(cm 3 s −1)=2.4±0.4×10 −6(300/ T) 0.5 for (HCN) 2H + to α e( T)=2.8±0.4×10 −6(300/ T) 0.5 for (CH 3CN) 2H +, and α e( T)=2.3±0.3×10 −6(300/ T) 0.5 for (CH 3CH 2CN) 2H +. The relevance of these data to molecular synthesis in the interstellar medium and the Titan ionosphere are discussed.