Electron Attachment Rate Constants Research Articles

Dissociative electron attachment and Ar+ reaction with chromium hexacarbonyl, 296-400 K.

Dissociative electron attachment rate constants have been measured for Cr(CO)6 under thermal conditions, 296-400K, yielding Cr(CO)5- product. At 296K, 2.92 ± 0.70cm3s-1 was measured and a small decrease with temperature was observed (2.72 ± 0.70cm3s-1 at 400K). We additionally determined the cation products of Ar+ reacting with Cr(CO)6.

Parameterization of electron attachment rate constants for impurities in LArTPC detectors

The ability of free electrons to drift long distances at high velocities in pure liquid argon under an applied electric field has been exploited for the past forty years to implement detectors with increasingly larger volumes for high energy physics research. The attachment of free electrons to impurities in the LAr is an important limit on the free instrumented volume of these extremely large detectors, and impurity concentrations as small as 100 ppt can reduce their resolution and efficiency. In this paper, we summarize the electron attachment rate constants as a function of the applied electric field, for common impurities in LArTPCs, obtained from data in the literature. We further provide analytical functions to parameterize the data, which are useful to compare with new measurements, to model and analyze the performance of existing detectors, and to predict the performance of new detectors.

Thermal Electron Attachment to Pyruvic Acid, Thermal Detachment from the Parent Anion, and the Electron Affinity of Pyruvic Acid.

The kinetics of electron attachment to pyruvic acid (CH3COCOOH) and thermal detachment from the resulting parent anion were measured from 300-515 K using a flowing afterglow─Langmuir probe apparatus. An adiabatic electron affinity (EA) for pyruvic acid was derived, 0.84 ± 0.02 eV. Electron attachment rate constants to pyruvic acid of 2.1 × 10-8 and 1.2 × 10-8 were measured at 300 and 400 K, respectively. Rate constants at higher temperatures are less well-defined due to possible contributions from attachment to zymonic open ketone, an endemic impurity in pyruvic acid. Similarly, unimolecular detachment rates are complicated by secondary proton transfer reaction of the pyruvic acid anion with pyruvic acid to yield an 87 Da anion. The possible contributions from these chemistries are considered, and in all cases the equilibrium constant between attachment and detachment remains well-defined, allowing for determination of the EA.

Thermal rate constants for electron attachment to N2O: An example of endothermic attachment.

Rate constants for dissociative electron attachment to N2O yielding O- have been measured as a function of temperature from 400 K to 1000 K. Detailed modeling of kinetics was needed to derive the rate constants at temperatures of 700 K and higher. In the 400 K-600 K range, upper limits are given. The data from 700 K to 1000 K follow the Arrhenius equation behavior described by 2.4 × 10-8 e-0.288 eV/kT cm3 s-1. The activation energy derived from the Arrhenius plot is equal to the endothermicity of the reaction. However, calculations at the CCSD(T)/complete basis set level suggest that the lowest energy crossing between the neutral and anion surfaces lies 0.6 eV above the N2O equilibrium geometry and 0.3 eV above the endothermicity of the dissociative attachment. Kinetic modeling under this assumption is in modest agreement with the experimental data. The data are best explained by attachment occurring below the lowest energy crossing of the neutral and valence anion surfaces via vibrational Feshbach resonances.

Radiative Electron Attachment and Photodetachment Rate Constants for Linear Carbon Chains

Since their recent detection in interstellar clouds and circumstellar envelopes, polyyne and cyanopolyyne anions have raised the question of their possible mechanisms of formation, destruction, and excitation. These anions are observed in the same regions as the corresponding neutral species, with anion-to-neutral abundance ratios of a few percent. It is believed that the abundance ratios are controlled mainly by the radiative attachment processes. We present a quantum study of the radiative electron attachment and photodetachment rate constants for selected linear carbon-chain anions, namely the already detected interstellar anions as well as other potential candidates. The rate constants are calculated within the rigid molecule approximation, with the attached or ejected electron described by a plane wave. A qualitative agreement is obtained with the previous accurate quantum results. For the radiative electron attachment process, the discrepancies between the quantum rate constants calculated here and ...

An experimental study of low energy electrons attachment to CH2ClBr using ion mobility spectrometry

The rate constant of low-energy electron attachment to CH2ClBr was determined using atmospheric pressure nitrogen corona discharge electron attachment ion mobility spectrometry at ambient pressure and room temperature in the mean electron range between 0.35 and 0.69eV. The rate constants are in the order of magnitude of ∼10−9cm3molecule−1s−1 and agree well with the reported value. Only one peak was observed in the production of low-energy electron attachment to CH2ClBr and it might be the overlap of hydrated anions Cl−(H2O)n and Br−(H2O)m. In order to clarify the contribution of Cl and Br anions, acetic acid was utilized to react with the hydrated Cl/Br anions. The H2O molecules in the hydrated Cl/Br anions were substituted by CH3COOH molecules, and formed Cl−(CH3COOH)p and Br−(CH3COOH)q, respectively. The contribution of hydrated Cl anions of low-energy electron attachment to CH2ClBr was found to be negligible, whereas the hydrated Br anions were the main product ions.

Evidence for electron-based ion generation in radio-frequency ionization.

Radio-frequency ionization (RFI) is a novel ionization method coupled to mass spectrometry (MS) for analysis of semi-volatile and volatile organic compounds (VOCs). Despite the demonstrated capabilities of RFI MS for VOC analysis in both positive- and negative-ion modes, mechanism of RFI is not completely understood. Improved understanding of the ion generation process in RFI should expand its utility in MS. Here, we studied the possibility of electron emission in RFI using both direct charged particle current measurements and indirect electron detection in a 9.4-T Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer. We show that RF-generated electrons can be trapped in the ICR cell and, subsequently, reacted with neutral hexafluorobenzene (C6 F6 ) molecules to generate C6 F6 (●-) . Intensity of observed C6 F6 (●-) species correlated with the number of trapped electrons and decreased as a function of electron quenching period. We also measured the electron attachment rate constant of hexafluorobenzene using a post-RF electron trapping experiment. Measured electron attachment rate constant of hexafluorobenzene (1.19 (±0.53) × 10(-9) cm(3) molecule(-1) s(-1) ) for post-RF FT-ICR MS agreed with the previously reported value (1.60 (±0.30) × 10(-9) cm(3) molecule(-1) s(-1) ) from low-pressure ICR MS measurements. Experimental results from direct and indirect electron measurements suggest that RFI process involves RF-generated electrons under ultrahigh vacuum conditions.

Rate constants of electron attachment to alkyl iodides measured by photoionization electron attachment ion mobility spectrometry (PI-EA-IMS)

The photoionization electron attachment ion mobility spectrometry (PI-EA-IMS), with photoelectrons formed by photoionization of organic compound like acetone, has been developed to study electron attachment reactions. With this apparatus, the rate constants for electron attachment to alkyl iodides including ethyl iodide (C2H5I), 1-propyl iodide (1-C3H7I), 1-butane iodide (1-C4H9I) and 2-propyl iodide (2-C3H7I) have been determined over the average electron energy from 0.29 to 0.96eV. The rate constants are in the order of magnitude of ∼10−9cm3molecule−1s−1. The experimental measurements show that for straight-chain alkyl iodides, the values of the rate constants follow the order of: k(C2H5I)<k(1-C3H7I)≈k(1-C4H9I) which can be explained by the energy threshold for the formation of iodine anion via dissociative electron attachment. For the isomers, 2-C3H7I has a lower rate constant than 1-C3H7I which may be caused by the effect of branched chain.

Pressure Tuning of Electron Attachment to Benzoquinones in Nonpolar Fluids: Continuous Adjustment of Free Energy Changes

Changing pressure from 1 to 2500 bar continuously tunes free energy changes for electron attachment to molecules in nonpolar liquids by nearly 0.3 eV. Rate constants for electron attachment to substituted benzoquinones were determined over an extended free energy range of nearly 1 eV by a combination of solute, pressure, temperature, and use of solvents with differing energies of the quasifree electron, V0: tetramethylsilane (TMS) and 2,2,4-trimethylpentane (TMP). The rates of attachment to both benzoquinone (BQ) and 2,5-dichlorobenzoquinone in TMS increase as the pressure increases to 2500 bar, while in TMP the rates are higher but change little with pressure; the rate of attachment to fluoranil in TMS is similarly high at 1 bar but decreases with increasing pressure. Together the observed rate constants can be qualitatively interpreted to yield a rate vs free energy relation having both normal and Marcus inverted region behavior. Because the electron attachment reactions yield excited states, quantitative interpretation of the free energy dependence requires knowledge of the excited state energies. The electron enters the second lowest π* orbital to form a π*-π* excited state, which quickly relaxes to the lower n-π* excited state. The rate of attachment to this excited state is low when the free energy of reaction, ΔG°, is positive and increases as ΔG° decreases until near -0.2 eV, after which the rate decreases. While excited state energies are uncertain, reasonable estimates are obtained from absorption, excitation, and fluorescence spectra of the product radical anions measured here. The results are modeled using Marcus theory with inclusion of a high frequency molecular vibration.

Electron angular distributions and attachment rates in o-Benzyne and Phenyl aromatic molecules: the effect of the permanent dipoles

Free, gas-phase polycyclic aromatic hydrocarbons (PAHs) and related species are currently considered to play an important role in the interstellar/circumstellar medium as they are thought to significantly contribute to both Diffuse and Unidentified infrared interstellar bands. They are also considered fundamental blocks of the interstellar dust and several formation mechanisms were proposed with regard to their interstellar/circumstellar synthesis. In this paper we therefore present and discuss the results obtained from ab initio quantum scattering calculations of the response from neutral polar aromatic single-ring species to low-energies electron collisions. Our main purpose is here to provide new values for the rate constants for electron attachment to orthobenzyne and to phenyl molecules by discussing in detail the effects of the long-range dipole interaction in the framework of the Born perturbative approximation at the first order. We shall further discuss the specific behavior of the electrons’ diffusion by such molecules, especially in the low-energy range of the scattered particles’ energies as guided by their permanent dipole moments. We shall also provide accurate numerical fittings for both rates and give explicitly the fitting parameters for their possible use in evolutionary models.

Electron attachment to fluorocarbon radicals

Thermal electron attachment rate constants for a series of small fluorocarbon radicals (CF(2), C(2)F(3), 1-C(3)F(7), 2-C(3)F(7), C(3)F(5), CF(3)O) were measured from 300 to 600 K using the variable electron and neutral density attachment mass spectrometry method. With the exception of CF(2), for which no attachment was observed, all species exclusively underwent dissociative attachment to yield F(-). The magnitude and temperature dependences of the rate constants varied significantly between species; however, attachment was in all cases inefficient, never exceeding 2% of the calculated collisional value. The data are interpreted and extrapolated to conditions inaccessible to the experiment using a kinetic modeling approach to the electron attachment process.

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.

Electron attachment rate constant measurement by photoemission electron attachment ion mobility spectrometry (PE-EA-IMS)

Photoemission electron attachment ion mobility spectrometry (PE-EA-IMS), with a source of photoelectrons induced by vacuum ultraviolet radiation on a metal surface, has been developed to study electron attachment reaction at atmospheric pressure using nitrogen as the buffer gas. Based on the negative ion mobility spectra, the rate constants for electron attachment to tetrachloromethane and chloroform were measured at ambient temperature as a function of the average electron energy in the range from 0.29 to 0.96eV. The experimental results are in good agreement with the data reported in the literature.

Analysis by kinetic modeling of the temperature dependence of thermal electron attachment to CF3Br

Experimental data from the literature for cross sections and rate constants for dissociative electron attachment to CF(3)Br, with separately varied electron and gas temperatures, are analyzed by a kinetic modeling approach. The analysis suggests that electronic and nuclear contributions to the rate constants can be roughly separated, the former leading to a negative temperature coefficient, the latter to a positive temperature coefficient. The nuclear factor in the rate constant is found to be of Arrhenius form with an activation energy which is close to the energy of crossing of the CF(3)Br and CF(3)Br(-) potential curves along the CBr bond.

Pressure and temperature dependence of dissociative and non-dissociative electron attachment to CF3: Experiments and kinetic modeling

The kinetics of electron attachment to CF(3) as a function of temperature (300-600 K) and pressure (0.75-2.5 Torr) were studied by variable electron and neutral density attachment mass spectrometry exploiting dissociative electron attachment to CF(3)Br as a radical source. Attachment occurs through competing dissociative (CF(3) + e(-) → CF(2) + F(-)) and non-dissociative channels (CF(3) + e(-) → CF(3)(-)). The rate constant of the dissociative channel increases strongly with temperature, while that of the non-dissociative channel decreases. The rate constant of the non-dissociative channel increases strongly with pressure, while that of the dissociative channel shows little dependence. The total rate constant of electron attachment increases with temperature and with pressure. The system is analyzed by kinetic modeling in terms of statistical theory in order to understand its properties and to extrapolate to conditions beyond those accessible in the experiment.

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.

Rate constants of electron attachment to chlorobenzenes measured by atmospheric pressure nitrogen corona discharge electron attachment ion mobility spectrometry

Using atmospheric pressure nitrogen corona discharge electron attachment ion mobility spectrometry (APNCD-EA-IMS), the rate constants of electron attachment to 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,4-trichlorobenzene and α,α,α-trichlorotoluene have been determined at ambient temperature as a function of the average electron energy in the range from 0.35 to 0.65 eV based on the experimental measurements of the negative ion mobility spectra. The rate constants are in the order of magnitude of ∼10 −9 cm 3 molecule −1 s −1. The ability of the dichlorobenzene isomers to capture the electrons has been found to decrease following the order of 1,2-chlorobenzene, 1,3-chlorobenzene and 1,4-chlorobenzene, which is different from previously reported results in the literature. For electron attachment to 1,2,4-trichlorobenzene and α,α,α-trichlorotoluene, the experimental measurements show that α,α,α-trichlorotoluene has a higher rate constant than 1,2,4-trichlorobenzene.

Electron Attachment Studies for CHCl3 Using Ion Mobility Spectrometry

The dissociative electron attachment process for CHCl3 at different electric field have been studied with nitrogen as drift and carrier gas using corona discharge ionization source ion mobility spectrometry (CD-IMS). The corresponding electron attachment rate constants varied from 1.26×10−8 cm3/(molecules s) to 8.24×10−9 cm3/(molecules s) as the electric field changed from 200 V/cm to 500 V/cm. At a fixed electric field in the drift region, the attachment rate constants are also detected at different sample concentration. The ion-molecule reaction rate constants for the further reaction between Cl− and CHCl3 are also detected, which indicates that the technique maybe becomes a new method to research the rate constants between ions and neural molecules. And the reaction rate constants between Cl− and CHCl3 are the first time detected using CD-IMS.

Kinetics of electron attachment to SF3CN, SF3C6F5, and SF3 and mutual neutralization of Ar+ with CN− and C6F5−

The additions of two sulfur fluoride derivatives (SF(3)C(6)F(5) and SF(3)CN) to a flowing afterglow were studied by variable electron and neutral density mass spectrometry. Data collection and analysis were complicated by the high reactivity of the neutral species. Both species readily dissociatively attach thermal electrons at 300 K to yield SF(3) + X(-) (X = C(6)F(5), CN). Attachment to SF(3)C(6)F(5) also results in SF(3)(-) + C(6)F(5) as a minor product channel. The determined electron attachment rate constants were 1(-0.6) (+1) × 10(-7) cm(3) s(-1) for SF(3)C(6)F(5), a lower limit of 1 × 10(-8) cm(3) s(-1) for SF(3)CN, and 4 ± 3 × 10(-9) cm(3) s(-1) for SF(3). Mutual neutralization rate constants of C(6)F(5)(-) and CN(-) with Ar(+) at 300 K were determined to be 5.5(-1.6) (+1.0) × 10(-8) and 3.0 ± 1 × 10(-8) cm(3) s(-1), respectively.

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

Rate constants for several processes including electron attachment to SF(2), SF(3), and SF(5) and individual product channels of ion-ion mutual neutralization between SF(6)(-), SF(5)(-), and SF(4)(-) with Ar(+) were determined by variable electron and neutral density attachment mass spectrometry. The experiments were conducted with a series of related neutral precursors (SF(6), SF(4), SF(5)Cl, SF(5)C(6)H(5), and SF(3)C(6)F(5)) over a temperature range of 300-500 K. Mutual neutralization rate constants for SF(6)(-), SF(5)(-), and SF(4)(-) with Ar(+) are reported with uncertainties of 10-25% and show temperature dependencies in agreement with the theoretical value of T(-0.5). Product branching in the mutual neutralizations is temperature independent and dependent on the electron binding energy of the anion. A larger fraction of product neutrals from the SF(6)(-) mutual neutralization (0.9 ± 0.1) are dissociated than in the SF(5)(-) mutual neutralization (0.65 ± 0.2), with the SF(4)(-) (0.7 ± 0.3) likely lying in between. Electron attachment to SF(5) (k = 2.0 × 10(-8) ±(1)(2) cm(3) s(-1) at 300 K) and SF(3) (4 ± 3 × 10(-9) cm(3) s(-1) at 300 K) show little temperature dependence. Rate constants of electron attachment to closed-shell SF(n) species decrease as the complexity of the neutral decreases.