Mutual Neutralization Research Articles

Inelastic aluminium-hydrogen collision data for non-LTE applications in stellar atmospheres

Aims. Inelastic processes in low-energy Si + H and Si+ + H− collisions are treated for the states from the ground state up to the ionic state, in order to provide rate coefficients needed for non-LTE modeling of Si in cool stellar atmospheres.Methods. Electronic molecular structure is determined using a recently proposed model approach based on an asymptotic method in combination with available ab initio potentials. Nonadiabatic nuclear dynamics are treated by means of a combination of multichannel formulas and the branching-probability-current method, based on the Landau-Zener model for nonadiabatic transition probabilities.Results. Cross sections and rate coefficients for inelastic processes in Si + H and Si+ + H− collisions for all transitions between 26 low-lying states plus the ionic state are calculated. It is shown that the highest rate coefficient values correspond to the excitation, de-excitation, ion-pair formation, and mutual neutralization processes involving the Si(3p4p 3 D), Si(3p3d 3 F), Si(3p4p 1 D), Si(3p3d 3 P), Si(3p4p 1 S), and the ionic Si+ + H− states. These processes are likely to be important in non-LTE modeling.

Model approach for low-energy inelastic atomic collisions and application to Al+H and Al++H−

A model approach is derived for estimates of cross sections and rate coefficients in low-energy inelastic collisions of hydrogen atoms and negative ions with other atoms and positive ions, which are of astrophysical interests. The approach is based on the asymptotic method for electronic molecular structure determination and on the branching probability current method for a nonadiabatic nuclear dynamical treatment. The derived approach is applied to low-energy Al + H and Al${}^{+}$ + H${}^{\ensuremath{-}}$ inelastic collisions. It is shown that the processes with the largest values of cross sections and rates are the excitation and de-excitation ones between the Al($3d$) and Al($4p$) states in collisions with H, as well as the ion-pair formation and the mutual neutralization processes between these states and the ionic state; the second largest cross sections correspond to the similar processes involving the Al($4s$) state. The mechanisms of the processes are discussed in detail.

Communication: Transfer ionization in a thermal reaction of a cation and anion: Ar+ with Br− and I−

We present experimental evidence that reactions of argon cations Ar(+) with the halogen anions Br(-) and I(-) do not occur exclusively by mutual neutralization, but also produce the cations Br(+) or I(+) ions by transfer ionization (TI). The experiments were carried out in flowing-afterglow plasmas at gas temperatures between and 300 and 500 K, and employed a variant of the Variable Electron and Neutral Density Attachment Mass Spectrometry method. The measured TI rate coefficients are 1.9 ± 0.6 × 10(-9) cm(3) s(-1) and 1.1 ± (0.3)(0.8) × 10(-9) cm(3) s(-1) for the Br(-) and I(-) reactions, respectively. We find that the TI rate coefficients decline with temperature as T(-0.5) to T(-1). No indication of TI was found in the reaction with Cl(-), where it is endoergic.

Identification of the extraction structure of H− ions by Hα imaging spectroscopy

Extraction structure of negative hydrogen ions (H−) (i.e. the regions where H− ions are effectively extracted from the plasma) was obtained using hydrogen Balmer-α (Hα) imaging spectroscopy and cavity ring-down spectroscopy. The H− ion density increases after caesium (Cs) seeding through a surface conversion process on a Cs-covered plasma-grid (PG) surface. We found a reduction in the H− density during beam extraction after Cs conditioning, and the same signal reduction appeared in the Hα intensity caused by the reduction in the excited hydrogen (n = 3) population, which in turn is caused by the decrease in the mutual neutralization process between positive and negative hydrogen ions. We clearly observed the reduction structure of the Hα emissions in the extraction region; the structure expands at optimal Cs conditioning. From this result, the H− ions, which are produced at the PG surface, release to the extraction region and widely distribute during arc discharge. We conclude that the reduction of the H− density is caused by the particle loss due to beam extraction from the PG apertures.

Mutual neutralization in H+ − H− collisions by electron capture

State-selective and total cross-sections for single-electron capture from H− by H+ covering the incident energy range from 10 to 3000 keV are computed by means of the four-body boundary corrected first Born (CB1-4B) approximation. A crucial connection between the Coulomb-distorted asymptotic state in the entrance channel and the pertinent perturbation, which causes the transition in the H+ − H− collisions, is consistently used in our computations of the “prior” version of cross-sections. The obtained results from the CB1-4B method clearly outperform the earlier findings by the close-coupling methods for the same problem. Comparisons with the available measurements are carried out and excellent agreement with the CB1 method is recorded down to impact energies as low as 10 keV.

After virtù and fortuna: Foucault on the government of economic events

The field of governmentality studies extends the analysis of power relations beyond the state, and has thereby widened the understanding of government. However, the dominant conceptualization of governmentality as conduct of conduct and the empirical focus on subjectivating modes of power limit the potential of the governmentality approach. In this paper I want to explore the possibilities of transcending this perspective by redescribing governmentality as event management. I will therefore draw attention to a couple of remarks on the government of events in Foucault's lectures on the history of governmentality. Foucault identifies a new mode of governing events in the texts of the Physiocrats, which he contrasts with the Machiavellian paradigm of virtù and fortuna. In contrast to Machiavelli's political concept of governing the eventful by relying on the virtuosity of the prince, the Physiocrats develop a genuinely economic rationality of event management based on the assumption that markets establish a nexus in which economic events are mutually neutralized. As I will show, this economic and proto-liberal concept of governing events is still present in neoliberalism, especially in the governmentality of financial markets. However, in the aftermath of the recent financial crisis, critics associated with the neoliberal thought collective have (again) challenged this vision of mutual event neutralization by emphasizing the possibility of event amplification. Understanding governmentality as event management thus helps to analyze and critique current attempts to govern economic and financial crisis.

Flowing afterglow measurements of the density dependence of gas-phase ion-ion mutual neutralization reactions

We have studied the dependence of several ion-ion mutual neutralization (MN) reactions on helium density in the range from 1.6 × 10(16) to 1.5 × 10(17) cm(-3) at 300 K, using the Variable Electron and Neutral Density Attachment Mass Spectrometry method. The rate coefficients of the reactions Ar(+) + Br2(-), Ar(+) + SF6(-), and Ar(+) + C7F14(-) were found to be independent of gas density over the range studied, in disagreement with earlier observations that similar MN reactions are strongly enhanced at the same gas densities. The cause of the previous enhancement with density is traced to the use of "orbital-motion-limit" theory to infer ion densities from the currents collected by ion-attracting Langmuir probes in a region where it is not applicable.

Visualization of H<sup>−</sup> Dynamics in Extraction Region of Negative-Ion Source by H<sub>α</sub> Imaging Spectroscopy

We developed a new imaging spectroscopy diagnostic tool for Hα emission and installed it on a negative hydrogen ion (H−) source to investigate the H− dynamics in the extraction region. During beam extraction, the Hα emission dropped; the same drop also appeared in the H− density (as measured by cavity ring-down spectroscopy). The reduction in the Hα emission results from the reduction in the excited hydrogen population caused by mutual neutralization processes between H+ and H− ions, which in turn are due to a decrease in the H− density. We find a reduction structure in Hα that is observed inside the plasma farther than 20 mm from the plasma grid (PG) surface. The result indicates that H− ions produced at the PG surface accumulate in the extraction region, so we conclude that they flow toward the PG apertures.

Mg-H collision rates for non-LTE determination of stellar atmospheric parameters

Non-LTE (Local Thermodynamical Equilibrium) modeling implies competition between radiative and collisional processes. The influence of inelastic hydrogen atoms collisions, dominant in cold atmospheres has been and remains to be a significant source of uncertainty for stellar abundance analysis. In the particular case of Mg atoms, a large number of electronic states of the MgH molecule as well as the associated couplings that mix the states during the collision were calculated by high level quantum chemical methods and then used in full quantum scattering calculations. This allows to treat the excitation processes between the seven lowest atomic states of magnesium in collision with hydrogen atoms, as well as the ion-pair production and the mutual neutralization processes. The detailed mechanisms involved during the collision process have been analysed in detail. Our calculations show that the usual approximate Drawin formula leads to errors by factors up to 105. As was already found in Li+H and Na+H collisions, excitation processes were found to be less important than charge transfer processes. However, unlike Li and Na, Mg has different spin terms, singlet and triplet, leading both to doublet molecular MgH electronic states. Collisional rates between spin-allowed and optically spin-forbidden atomic states are found to be of the same order of magnitude although optically spin-forbidden states are only collisionally coupled. Thus we may expect consequences on non-LTE calculations.

Merged-beam study of mutual neutralization of H− and H+

Total and partial cross sections have been measured for the mutual neutralization of H− and H+ by means of a merged and inclined beam set-up. The low energy data between 10 meV and 5 eV contradict one previous set of measurements, while above 5 eV the data fall in excellent agreement with previously published results by two other groups.

Electron Attachment to Fe(CO)n (n = 0–5)

The rate constants of thermal electron attachment at 300 and 400 K to Fe(CO)(n) (n = 0-5) have been measured using a flowing afterglow Langmuir probe apparatus. The stable species Fe(CO)(5) was studied using the traditional method of monitoring electron depletion as a function of reaction time, and the remaining short-lived species were studied using the variable electron and neutral density attachment mass spectrometry (VENDAMS) technique. Attachment to Fe(CO)(5) is purely dissociative and about 20% efficient with a rate constant of (7.9 ± 1.4) × 10(-8) cm(3) s(-1) at 300 K and (8.8 ± 2) × 10(-8) cm(3) s(-1) at 400 K. The attachment rate constants decrease significantly as each CO ligand is removed, with Fe(CO)(n) (n = 4 to 1) attaching with efficiencies on the order of 10%, 1%, 0.1%, and 0.01% respectively. Under the conditions here, attachment to Fe(CO)(4) and Fe(CO)(3) are likely entirely dissociative, whereas attachment to Fe(CO)(2) and Fe(CO) are almost entirely associative. A statistical kinetic modeling approach is used to explain the strong dependence of the attachment rate constant on the number of ligands present in the neutral species through a combination of increasing autodetachment rates and decreasing exothermicities to dissociative attachment. The VENDAMS data also define the 300 K mutual neutralization rate constant of Fe(CO)(4)(-) + Ar(+) to be (5.0 ± 0.8) × 10(-8) cm(3) s(-1) with an upper limit to branching fraction of 0.5 to yield Fe(CO)(4), indicating that significant fragmentation to smaller Fe(CO)(n) occurs.

ChemInform Abstract: Behavior of Rate Coefficients for Ion‐ion Mutual Neutralization, 300‐550 K

AbstractReview: anion neutralization reactions with Ar+ and Kr+; 41 refs.

Behavior of rate coefficients for ion-ion mutual neutralization, 300–550 K

Rate coefficients k(MN) have been measured for a number of anion neutralization reactions with Ar(+) and Kr(+) over the temperature range 300-550 K. For the first time, the data set includes anions of radicals and other short-lived species. In the present paper, we review these results and make note of correlations with reduced mass, electron binding energy of the anion (equivalent to the electron affinity of the corresponding neutral), and temperature, and compare with expectations from absorbing sphere models. An intriguing result is that the data for diatomic anions neutralized by Ar(+) and Kr(+) have k(MN) values close to 3 × 10(-8) cm(3) s(-1) at 300 K, a figure which is lower than those for all of the polyatomic anions at 300 K except for SF(5)(-) + Kr(+). For the polyatomic anions studied here, neutralized by Ar(+) and Kr(+), the reduced mass dependence agrees with theory, on average, but we find a stronger temperature dependence of T(-0.9) than expected from the theoretical E(-0.5) energy dependence of the rate coefficient at thermal energies. The k(MN) show a weak dependence on the electron binding energy of the anion for the polyatomic species studied.

Kinetics of electron attachment to OH and HNO3 and mutual neutralization of Ar+ with NO2− and NO3− at 300 and 500 K

The electron attachment rate constant to nitric acid (HNO(3)) has been measured in a flowing afterglow-Langmuir probe (FALP) apparatus at 300 and 500 K using three independent methods: the traditional FALP technique of monitoring electron depletion, "one-gas" VENDAMS (variable electron and neutral density attachment mass spectrometry), and "two-gas" VENDAMS. The three measurements are in agreement with a 300 K weighted average of 1.4 ± 0.3 × 10(-7) cm(3) s(-1), 2 to 10 times higher than previously reported values. Attachment is primarily dissociative yielding NO(2)(-) as previously reported, but for the first time a small endothermic channel to produce OH(-) was also observed at 500 K. From the one-gas VENDAMS data, associative attachment to the OH produced in the primary attachment was found to occur with an effective two body rate constant of 1.2±(0.7) (3)×10(-11) cm(3) s(-1) at 300 K, the first reported rate constant for this radical species. Finally, ion-ion neutralization rate constants of NO(2)(-) and NO(3)(-) with Ar(+) were determined to be 5.2±(2.5) (1.5) × 10(-8) and 4.5 ± 2.5 × 10(-8) cm(3) s(-1) at 300 K, respectively.

Cross sections for low-energy inelastic Mg + H and Mg++ H−collisions

We report full quantum scattering calculations for low-energy near-threshold inelastic cross sections in Mg + H and Mg+ + H- collisions. The calculations include all transitions between the eight lowest adiabatic MgH((2)Sigma(+)) molecular states, with the uppermost of those diabatically extended to the ionic molecular state in the asymptotic region. This allows us to treat the excitation processes between the seven lowest atomic states of magnesium in collisions with hydrogen atoms, as well as the ion-pair production and the mutual neutralization processes. The collision energy range is from threshold up to 10 eV. These results are important for astrophysical modeling of spectra in stellar atmospheres. The processes in question are carefully examined and several process mechanisms are found. Some mechanisms are determined by interactions between ionic and covalent configurations at relatively large internuclear distances, while others are based on short-range nonadiabatic regions due to interactions between covalent configurations.

A new technique to study plasma chemistry kinetics

Accurate kinetics of plasma processes are necessary for modeling of the chemistry occurring in the upper atmosphere, re-entry, combustion, and discharges. While a great deal of data exists in the literature for most types of plasma chemistry processes, there remain gaps for reactions less amenable to traditional measurements. In particular ion-ion mutual neutralization reactions have received relatively little study, and essentially no detailed product branching fractions are known. Similarly, while hundreds of electron attachment rate constants to stable species have been reported, only a handful of measurements of electron attachment rate constants to unstable radical species exist in the literature. We report several measurements involving these classes of reactions using a novel flowing afterglow technique: Variable Electron and Neutral Density Attachment Mass Spectrometry (VENDAMS). The technique takes advantage of these processes occurring as secondary and tertiary chemistry in high density plasmas, and uses excess electrons as chemical ionization agents to monitor neutral product concentrations. Systems starting with a variety of neutrals have been studied over a temperature range of 300 to 550 K. We report several novel measurements including: the first temperature dependences of electron attachment rate constants to radical species; the first complete neutral product distributions of ion-ion mutual neutralization reactions along with the associated temperature dependences; evidence of a novel plasma chemistry process (A+ + B− + e− neutrals + e−), which we term electron catalyzed mutual neutralization.

Mutual neutralization in low-energyH++F−collisions

The cross section for mutual neutralization in collisions between ${\mathrm{H}}^{+}$ and ${\mathrm{F}}^{\ensuremath{-}}$ ions at low energies ($E\ensuremath{\leqslant}10$ eV) is calculated using a molecular close-coupling approach. Two different representations of the quasidiabatic potentials and couplings of HF are used. The effect of autoionization on the cross section is investigated. The coupled Schr\"odinger equation for the nuclear motion is solved using a numerical integration of the corresponding matrix Riccati equation and the cross section for mutual neutralization is computed from the asymptotic value of the logarithmic derivative of the radial wave function. The magnitude of the cross section for mutual neutralization in this reaction is small compared to other systems. This can be understood by the lack of avoided crossings at large internuclear distances. Resonant structures are found in the cross section and these are assigned with dominant angular momentum quantum number. The cross section for mutual neutralization in collisions of ${\mathrm{D}}^{+}$ and ${\mathrm{F}}^{\ensuremath{-}}$ ions is also calculated.

Temperature dependences of rate coefficients for electron catalyzed mutual neutralization

The flowing afterglow technique of variable electron and neutral density attachment mass spectrometry (VENDAMS) has recently yielded evidence for a novel plasma charge loss process, electron catalyzed mutual neutralization (ECMN), i.e., A(+) + B(-) + e(-) → A + B + e(-). Here, rate constants for ECMN of two polyatomic species (POCl(3)(-) and POCl(2)(-)) and one diatomic species (Br(2)(-)) each with two monatomic cations (Ar(+)and Kr(+)) are measured using VENDAMS over the temperature range 300 K-500 K. All rate constants show a steep negative temperature dependence, consistent with that expected for a three body process involving two ions and an electron. No variation in rate constants as a function of the cation type is observed outside of uncertainty; however, rate constants of the polyatomic anions (~1 × 10(-18) cm(6) s(-1) at 300 K) are measurably higher than that for Br(2)(-) [(5.5 ± 2) × 10(-19) cm(6) s(-1) at 300 K].

Population inversion in a magnetized hydrogen plasma expansion as a consequence of the molecular mutual neutralization process

A weakly magnetized expanding hydrogen plasma, created by a cascaded arc, was investigated using optical emission spectroscopy. The emission of the expanding plasma is dominated by H{α} emission in the first part of the plasma expansion, after which a sharp transition to a blue afterglow is observed. The position of this sharp transition along the expansion axis depends on the magnetic field strength. The blue afterglow emission is associated with population inversion of the electronically excited atomic hydrogen states n=4-6 with respect to n=3. By comparing the measured densities with the densities using an atomic collisional radiative model, we conclude that atomic recombination processes cannot account for the large population densities observed. Therefore, molecular processes must be important for the formation of excited states and for the occurrence of population inversion. This is further corroborated at the transition from red to blue, where a hollow profile of the excited states n=4-6 in the radial direction is observed. This hollow profile is explained by the molecular mutual neutralization process of H₂+ with H⁻, which has a maximum production for excited atomic hydrogen 1-2 cm outside the plasma center.

Publisher's Note: “Kinetics following addition of sulfur fluorides to a weakly ionized plasma from 300 to 500 K: Rate constants and product determinations for ion–ion mutual neutralization and thermal electron attachment to SF5, SF3, and SF2” [J. Chem. Phys. 133, 234304 (2010)

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