Ion Dissociative Recombination Research Articles

A 3D Physics‐Based Particle Model of the Venus Oxygen Corona: Variations With Solar Activity

AbstractDue to Venus not having a substantial planetary magnetic field the fast‐flowing solar wind plasma can propagate to regions close to the planet. Therefore, thermal atomic oxygen in the thermosphere, hot oxygen in the corona, and the resulting pickup oxygen ions are essential for determining the overall interaction of the planet with plasma of the ambient solar wind. To investigate this complex system, we have initiated a project where a combination of Venus Thermosphere General Circulation Model (VTGCM) and Adaptive Mesh Particle Simulator (AMPS) codes are used to determine the variability of the ”hot” O corona depending on the solar conditions. Here we present the results of modeling Venus' oxygen corona using the VTGCM ionosphere/thermosphere and AMPS kinetic particle models. VTGCM produces a self‐consistent calculation of the thermosphere/ionosphere, providing the spatial distributions of the dominant species. That is further used in AMPS’ modeling of Venus' exosphere (a) to specify the source of the newly created hot O atoms produced by dissociative recombination of ions and (b) to account for thermalization of these energetic oxygen atoms as they propagate in the upper thermosphere. The altitude distribution of hot O calculated for the solar maximum conditions agree well with Pioneer Venus Orbiter observations of the oxygen corona. The modeling that we have performed for the solar minimum conditions indicates a decrease of the oxygen density in the corona by almost a factor of six compared to that at solar maximum. That is consistent with the non‐detection of the oxygen corona from Venus Express. As expected, the solar moderate case is between the solar maximum and minimum cases.

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.

Effect of N2 and O2 on OH radical production in an atmospheric helium microwave plasma jet

UV-pulsed laser cavity ringdown spectroscopy of the hydroxyl radical OH(A–X) (0–0) band in the wavelength range of 306–310 nm was employed to determine absolute number densities of OH in the atmospheric helium plasma jets generated by a 2.45 GHz microwave plasma source. The effect of the addition of molecular gases N2 and O2 to He plasma jets on OH generation was studied. Optical emission spectroscopy was simultaneously employed to monitor reactive plasma species. Stark broadening of the hydrogen Balmer emission line (Hβ) was used to estimate the electron density ne in the jets. For both He/N2 and He/O2 jets, ne was estimated to be on the order of 1015 cm−3. The effects of plasma power and gas flow rate were also studied. With increase in N2 and O2 flow rates, ne tended to decrease. Gas temperature in the He/O2 plasma jets was elevated compared to the temperatures in the pure He and He/N2 plasma jets. The highest OH densities in the He/N2 and He/O2 plasma jets were determined to be 1.0 × 1016 molecules/cm3 at x = 4 mm (from the jet orifice) and 1.8 × 1016 molecules/cm3 at x = 3 mm, respectively. Electron impact dissociation of water and water ion dissociative recombination were the dominant reaction pathways, respectively, for OH formation within the jet column and in the downstream and far downstream regions. The presence of strong emissions of the bands in both He/N2 and He/O2 plasma jets, as against the absence of the emissions in the Ar plasma jets, suggests that the Penning ionization process is a key reaction channel leading to the formation of in these He plasma jets.

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.

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.

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.

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.

Detailed H(n = 2) density measurements in a magnetized hydrogen plasma jet

We present detailed spatial density measurements of H(n = 2), i.e. the first excited state of atomic hydrogen, in a hydrogen plasma expansion that is weakly magnetized. At a specific distance from the source of the expansion a sharp transition from a red light emitting plasma (dominated by Hα emission) to a blue light emitting plasma (dominated by Hβ and Hγ emission) occurs. Molecular processes such as dissociative recombination and processes with negative ions are suspected to be key in the understanding of the distinct emissions observed in the two different plasma regions. The relevance of the presented work is to underline these molecular processes in atomic regimes of hydrogen-containing plasmas. The first excited state density, n = 2, is determined with tunable diode laser absorption spectroscopy on the Balmer-alpha transition to investigate how important molecular processes such as dissociative recombination are in the plasma. The density of n = 2 is 1 × 1017 m−3 close to the plasma source and decreases gradually along the plasma column to 1014 m−3 at 20 cm from the plasma source exit. The presented results show that the theoretical possibility to generate a stable hydrogen laser in the visible light, i.e. population inversion of n = 3 with respect to n = 2, is not obtained due to the population of n = 2 by dissociative recombination of ions. The presence of molecular processes in the plasma is further evidenced with the use of a collisional radiative model.

Production of neutral species in Titan’s ionosphere through dissociative recombination of ions

The production rates of neutral species by dissociative recombination (DR) of molecular ions with electrons in the ionosphere of Titan are quantified by a new model, including, for the first time, all the available kinetic data on this process. The calculation is based on the ion densities measured by the INMS instrument on Cassini orbiter during flyby T19 at 1100km altitude. These production rates are compared with those predicted by photochemical models: we calculate that for many neutral species, DR has larger production rates than neutral chemistry. Concerning molecular growth in Titan’s ionosphere, DR is shown to have two antagonistic effects: (1) a global chemical lysis of ions through C–C and C–N bond breaking (missed by the “H-loss” DR paradigm); and (2) an enhancement of the neutral chemistry by production of reactive radicals, such as C2H or NH2. Further exploration of this chemistry requires the development of ionospheric coupled models taking explicitly into account the richness of the DR process and the strong impact of ions on the budget of neutral species. This study emphasizes also the urgent need of additional experimental studies about DR of molecular ions, with two priorities: evaluation of the impact of the temperature of ions on the rates and fragmentation patterns, and the systematic study of the fragmentation patterns of CxHyNz+ ions with more than four heavy atoms (m/z>60u).

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.

Breaking bonds with electrons: Dissociative recombination of molecular ions

We discuss exciting recent progress in the theoretical description of molecular ion dissociative recombination, with further applications to related processes such as molecular photoionization and rovibrationally-inelastic electron collisions with molecular ions. The techniques are based on quantum defect theory, an efficient tool for characterizing electron–ion collision processes, and on a generalized adiabatic formulation. When needed, the approximation of adiabaticity in the dissociative coordinate is lifted by taking non-adiabatic couplings explicitly into account. We concentrate on a time-independent theoretical framework that has been applied to describe dissociative recombination in ions such as H 3 + , its isotopologues (H 2D +, D 2H + and D 3 + ), and HCO + (DCO +).

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.

Flowing afterglow studies of the electron recombination of protonated cyanides (RCN)H + and their proton-bound dimer ions (RCN) 2H + where R is H, CH 3, and CH 3CH 2

A study has been made of the electron–ion dissociative recombination of the protonated cyanides (RCNH +, R = H, CH 3, C 2H 5) and their proton-bound dimers (RCN) 2H + at 300 K. This has been accomplished with the flowing afterglow technique using an electrostatic Langmuir probe to determine the electron density decay along the flow tube. For the protonated species, the recombination coefficients, α e(cm 3 s −1), are (3.6 ± 0.5) × 10 −7, (3.4 ± 0.5) × 10 −7, (4.6 ± 0.7) × 10 −7 for R = H, CH 3, C 2H 5, respectively. For the proton-bound dimers, the α e are substantially greater being (2.4 ± 0.4) × 10 −6, (2.8 ± 0.4) × 10 −6, (2.3 ± 0.3) × 10 −6 for R = H, CH 3, C 2H 5, respectively. Fitting of the electron density decay data to a simple model has shown that the rate coefficients for the three-body association of RCNH + with RCN are very large being (2.0 ± 0.5) × 10 −26 cm 6 s −1. The significance of these data to the Titan ionosphere is discussed.

Theoretical study of the H3+ ion dissociative recombination process

We report recent developments in the theory of dissociative recombination of the H3+ ion. The theory is able to explain the surprisingly large dissociative recombination rate observed in storage-ring experiments. We have found that the high rate is caused by Jahn-Teller coupling between the electronic and vibrational degrees of freedom. We compare two theoretical models: A simplified model and an improved one. The two models each produce a theoretical rate that is consistent with the storage-ring observations. The final theoretical dissociative recombination rate presented here accounts for several experimental details that are often not considered.

Radio frequency energy coupling to high-pressure optically pumped nonequilibrium plasmas

This article presents an experimental demonstration of a high-pressure unconditionally stable nonequilibrium molecular plasma sustained by a combination of a continuous wave CO laser and a sub-breakdown radio frequency (rf) electric field. The plasma is sustained in a CO/N2 mixture containing trace amounts of NO or O2 at pressures of P=0.4–1.2 atm. The initial ionization of the gases is produced by an associative ionization mechanism in collisions of two CO molecules excited to high vibrational levels by resonance absorption of the CO laser radiation with subsequent vibration-vibration (V-V) pumping. Further vibrational excitation of both CO and N2 is produced by free electrons heated by the applied rf field, which in turn produces additional ionization of these species by the associative ionization mechanism. In the present experiments, the reduced electric field, E/N, is sufficiently low to preclude field-induced electron impact ionization. Unconditional stability of the resultant cold molecular plasma is enabled by the negative feedback between gas heating and the associative ionization rate. Trace amounts of nitric oxide or oxygen added to the baseline CO/N2 gas mixture considerably reduce the electron–ion dissociative recombination rate and thereby significantly increase the initial electron density. This allows triggering of the rf power coupling to the vibrational energy modes of the gas mixture. Vibrational level populations of CO and N2 are monitored by infrared emission spectroscopy and spontaneous Raman spectroscopy. The experiments demonstrate that the use of a sub-breakdown rf field in addition to the CO laser allows an increase of the plasma volume by about an order of magnitude. Also, CO infrared emission spectra show that with the rf voltage turned on the number of vibrationally excited CO molecules along the line of sight increase by a factor of 3–7. Finally, spontaneous Raman spectra of N2 show that with the rf voltage the vibrational temperature of nitrogen increases by up to 30%. This novel energy efficient approach allows sustaining large-volume high-pressure molecular plasmas without the use of a high-power CO laser. This opens a possibility of using the present technique for high-yield plasma chemical synthesis and plasma material processing.

Optical effects in the aurora caused by ionospheric HF heating

Abstract Ionospheric heating experiments were done by the EISCAT Heater in Tromso on 15–19 November, 1993. A low-light TV camera was installed at the VLF receiving station at Porojarvi about 100 km to the south-east of Tromso. The spectral analysis of the auroral luminosity variations showed that the brightness of the aurora varied at the modulation frequency of the heating wave. The results of this analysis and the numerical simulations of the auroral luminosity variations caused by the HF heating are shown. The variations of the optical emission intensity at the heating frequency occur during the auroral ionosphere modification. The observed intensity variation of the auroral green line during the interval of enhanced electron temperature is explained by a decreasing rate of the O2+ ion dissociative recombination when the electron temperature increases. The brightness variation depends on the characteristic energy and the intensity of the auroral electron flux and the heating wave parameters. The artificial luminosity pulsations caused by HF heating are estimated.

Dissociative recombination of CH+: Cross section and final states.

The cross section as well as the branching ratios for the dissociative recombination of ground-state ${\mathrm{CH}}^{+}$ ions with electrons have been measured using the heavy-ion storage-ring technique and two-dimensional fragment imaging. Although the absolute value of the cross section at thermal energies is found to be in very good agreement with the theory, several unpredicted narrow resonances are also present in the data. These structures are interpreted as due to an indirect recombination process via core-excited Rydberg states. The branching-ratio measurement shows that at low electron energy the 2${\mathrm{}}^{2}$\ensuremath{\Pi} state, producing carbon fragments C${(}^{1}$D), is the most important dissociative state, although transitions during the dissociation to other dissociative potential curves are also present. Anisotropy in the angular distribution of the dissociating fragments is visible for some of the final states. Dissociative recombination of ions in the metastable excited a${\mathrm{}}^{3}$\ensuremath{\Pi} state is also observed, and the lifetime as well as the excitation energy of this state are deduced from the imaging data. \textcopyright{} 1996 The American Physical Society.

The semiclassical—classical path theory of direct electron—ion dissociative recombination and e − + H 3+ recombination

The theory of dissociative recombination (DR) is reviewed. A new semiclassical—classical path theory of DR is presented. It is applicable to DR with or without curve crossing between the potential energy surfaces associated with molecular ion AB − and neutral AB ∗ dissociative states. A new mechanism is proposed for e-H 3 + recombination. Rydberg molecules are first formed and the periodic interaction of the Rydberg electron with the ionic core causes accumulative rotational and vibrational excitation until dissociation occurs. Recombination is completed via a simultaneous one-electron jump to the ion of the dissociating pair.

The fast atomic oxygen corona extent of Mars

The production of fast oxygen atoms in the Martian exosphere as a result of the electron dissociative recombination of ions is re‐evaluated for the time period appropriate to the Phobos 2 mission. Because of the prevalent condition of solar maximum activities in 1989, both the upper atmosphere and ionsphere should have more extended structures than those encountered during the Viking Orbiter (VO) mission in 1976 which was near solar minimum. We compare the distributions of the hot oxygen atoms for these different levels of solar activities and find that the number density of the hot oxygen atoms during the Phobos 2 mission should be a factor of two larger than estimated for the time period of the VO mission. The non‐thermal escape rate of the exospheric oxygen atoms is thus comparable to the ionospheric escape rate measured in the Martian magnetotail. Furthermore, the flux of the oxygen ions picked up in the hot atomic corona should be more intense than previously thought.