Electron Attachment Rate Coefficients Research Articles

Abundance and excitation of molecular anions in interstellar clouds

We present new observations of molecular anions with the Yebes 40 m and IRAM 30 m telescopes toward the cold, dense clouds TMC-1 CP, Lupus-1A, L1527, L483, L1495B, and L1544. We report the first detections of C3N− and C5N− in Lupus-1A as well as C4H− and C6H− in L483. In addition, we detected new lines of C6H− toward the six targeted sources, of C4H− toward TMC-1 CP, Lupus-1A, and L1527, and of C8H− and C3N− in TMC-1 CP. Excitation calculations using recently computed collision rate coefficients indicate that the lines of anions accessible to radiotelescopes run from subthermally excited to thermalized as the size of the anion increases, with the degree of departure from thermalization depending on the H2 volume density and the line frequency. We noticed that the collision rate coefficients available for the radical C6H are not sufficient to explain various observational facts, thereby calling for the collision data for this species to be revisited. The observations presented here, together with observational data from the literature, have been used to model the excitation of interstellar anions and to constrain their abundances. In general, the anion-to-neutral ratios derived here agree with the literature values, when available, within 50% (by a factor of two at most), except for the C4H−/C4H ratio, which shows higher differences due to a revision of the dipole moment of C4H. From the set of anion-to-neutral abundance ratios derived two conclusions can be drawn. First, the C6H−/C6H ratio shows a tentative trend whereby it increases with increasing H2 density, as we would expect on the basis of theoretical grounds. Second, the assertion that the higher the molecular size, the higher the anion-to-neutral ratio is incontestable; furthermore, this supports a formation mechanism based on radiative electron attachment. Nonetheless, the calculated rate coefficients for electron attachment to the medium size species C4H and C3N are probably too high and too low, respectively, by more than one order of magnitude.

Computational analysis of electrical breakdown of SF6/N2 mixtures

The main aim of the present paper is to clarify the influence of the SF6 fraction in the SF6/N2 mixture on the breakdown voltage. For this, the two-dimensional axisymmetric fluid model coupled with the comprehensive mechanism of plasma chemical reactions is used. In addition, the influence of various parameters such as the voltage rise time and the SF6 fraction in the mixture is analyzed. It is observed that depending on the voltage rise time an admixture of only 1% of SF6 to N2 results in an increase in the breakdown voltage by 7%–43%. The sensitivity of breakdown voltage decreases with decreasing voltage rise time and is caused by the electron attachment time scale becoming comparable to the breakdown time. The results of simulations confirm that the increase in the SF6 fraction in the mixture leads to an increase in the breakdown voltage. This is explained by the influence of the SF6 fraction on the electron attachment rate coefficients rather than on the ionization reactions.

Electron attachment to interstellar grains

ABSTRACT We have performed quantum mechanical calculations of cross-sections and rate coefficients for electron attachment to small, neutral, and positively charged grains. In addition to the polarization potential and – for charged grains – the Coulomb potential, we considered the influence of the interaction between the charge of the electron and a possible permanent electric dipole on the grain. We found large discrepancies between the predictions of classical and quantal models of electron scattering on neutral grains at low energies (temperatures) in the absence of a permanent dipole moment, but much closer agreement when a dipole moment was present. The presence of the long-range Coulomb potential also leads to better agreement between classical and quantal models.

Electron collision cross section set of Cl2 gas and electron transport analysis in Cl2 gas and Cl2/N2 mixtures

A detailed and reliable electron collision cross section set of Cl2 gas is constructed. The present cross section set consists of one elastic momentum transfer, three vibrational excitation, eight dissociative electronic excitation, two electronic excitation, one ion-pair formation, one dissociative electron attachment, and four ionization cross sections. The electron drift velocity, longitudinal diffusion coefficient, ionization coefficient, and electron attachment coefficient in both Cl2 gas and Cl2/N2 mixtures and the electron attachment rate coefficient in mixtures of N2 gas and a minute amount of Cl2 gas are calculated by Monte Carlo simulation using the present cross section set, and then the reliability of the present cross section set is demonstrated by comparing calculated and measured electron transport coefficients and rate coefficients. Furthermore, the rate coefficients for both elastic and inelastic electron collisions and the mean energy of electrons in Cl2 gas are calculated from the present cross section set and are provided.

Electron attachment to hexafluoropropylene oxide (HFPO).

We probe the electron attachment in hexafluoropropylene oxide (HFPO), C3F6O, a gas widely used in plasma technologies. We determine the absolute electron attachment cross section using two completely different experimental approaches: (i) a crossed-beam experiment at single collision conditions (local pressures of 5 × 10-4 mbar) and (ii) a pulsed Townsend experiment at pressures of 20-100 mbar. In the latter method, the cross sections are unfolded from the electron attachment rate coefficients. The cross sections derived independently by the two methods are in very good agreement. We additionally discuss the dissociative electron attachment fragmentation patterns and their role in the radical production in industrial HFPO plasmas.

Experimentally derived rate coefficients for electron ionization, attachment and detachment as well as ion conversion in pure O2 and N2–O2 mixtures

We obtain rate coefficients for mixtures of and as well as pure , by fitting pulsed Townsend measurements over a large pressure range from 3 kPa to 100 kPa in a density-reduced electric field range from 40 to 110 Td (1 Td = 10−21 Vm2). The applied model includes ionization, dissociative attachment to and non-dissociative attachment to , and detachment, as well as charge transfer and ion conversion from to and . The results are compared to Bolsig+ and MAGBOLTZ simulations and experimental data, where available. The key novelty is the simultaneously obtained rate coefficients from a single pulsed Townsend experiment including several ion processes over the whole – mixing range.

On the gas-phase formation of the HCO- anion: accurate quantum study of the H- + CO radiative association and HCO radiative electron attachment.

The hydrogen anion has never been observed in the interstellar medium, but it is most likely present in some interstellar regions. Since direct detection appears especially difficult, improving the knowledge of the astrochemical processes involving this anion should be valuable in defining a way of indirect detection. We present the first study of the radiative association of H- and CO to form the HCO- anion within a quantum time-independent approach. We use a state-of-the-art potential energy surface which has been calculated for the present study. The calculated radiative association rate coefficient is monotonically decreasing from 6 × 10-16 to 5 × 10-19 cm3 per molecule per s across the 0.01-1000 K temperature range. At the typical temperature of the cold interstellar medium, ∼10 K, the radiative association rate is ∼2 × 10-17 cm3 per molecule per s. On the other hand, the plane wave approximation is used to calculate the HCO radiative electron attachment rate coefficient. It is found to be almost constant and also equal to 2 × 10-17 cm3 per molecule per s. Setting aside the question of the abundances of the reactants of both processes, these results demonstrate that among the two gas-phase modes of production of the HCO- anion in cold interstellar medium considered in this study, the H- + CO radiative association is dominating below 10 K while the radiative electron attachment rate is larger above 10 K.

Electron attachment to the interhalogen compounds ClF, ICl, and IBr

Thermal electron attachment rate coefficients for three interhalogen compounds (ClF, ICl, IBr) have been measured from 300 to 900 K at pressures of 1--2 Torr using a flowing afterglow--Langmuir probe apparatus. ClF attaches somewhat inefficiently ($k=7.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$) at 300 K, with the rate coefficient rising to $1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 700 K. At higher temperatures the apparent rate coefficient falls steeply; however, this is interpreted as an artifact due to decomposition on the walls of the inlet line. ICl attaches with even lower efficiency ($k=9.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 300 K) and a less steep increase with temperature. Attachment to IBr is too slow to confidently measure with the present experiment, with an upper limit on the rate coefficient of ${10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ from 300 to 600 K. Both ClF and ICl attach dissociatively to yield $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$, likely exclusively, though ${\mathrm{F}}^{\ensuremath{-}}$ or ${\mathrm{I}}^{\ensuremath{-}}$ may be produced with limits of $l2%$ and $l5%$, respectively. The ClF attachment was further explored through ab initio calculation of the ClF and $\mathrm{Cl}{\mathrm{F}}^{\ensuremath{-}}$ potential energy curves and $R$-matrix calculations of the resonance parameters which were used then for calculations of the dissociative attachment cross sections and rate coefficients. While the magnitude of the attachment rate coefficient for ClF is similar to those for both $\mathrm{C}{\mathrm{l}}_{2}$ and ${\mathrm{F}}_{2}$, the calculated cross sections show qualitatively different threshold behavior due to the $s$-wave contribution allowed by the lack of inversion symmetry. The $v=1$ and 2 vibrational modes of ClF attach about three to four times faster than $v=0$ and 3 at energies lower than $\ensuremath{\sim}0.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The calculated rate coefficients are in good agreement with the experiment at 300 K and increase moderately less steeply with temperature.

Production of and Dissociative Electron Attachment to the Simplest Criegee Intermediate in an Afterglow.

The simplest Criegee intermediate, CH2OO, has been produced in a flowing afterglow using a novel technique. CH2I is produced by dissociative electron attachment to CH2I2, leading to the established reaction CH2I + O2 → CH2OO + I. The presence of CH2OO is established by observation of dissociative electron attachment to yield O(-) using the variable electron and neutral density attachment mass spectrometry (VENDAMS) technique. The measurements establish the electron attachment rate coefficient of thermal electrons at 300 K to CH2OO as 1.2 ± 0.3 × 10(-8) cm(3) s(-1). Thermal electron attachment is solely dissociative and is not a promising route to producing stable CH2OO(-). The results open the possibility of measuring ion-molecule chemistry involving Criegee intermediates, as well as the reactivity of other unstable radicals produced in an analogous manner.

The influence of the temperature on electron attachment to some Br-substituted alkanes

Thermal electron attachment rate coefficients and activation energies for CH3CH2Br, CH3CH2CH2Br, CH3CHBrCH3, CF3CHBrCH3, CH3(CH2)2CH2Br and CH2F(CH2)2CH2Br have been measured using the Pulsed Townsend technique over the temperature range (298–378)K. The corresponding rate coefficients (k’s) at 298K were equal to 9.3±2.10×10−13, 2.9±0.20×10−12, 2.7±0.07×10−12, 1.1±0.06×10−9, 6.6±1.10×10−12 and 2.3±0.3×10−10cm3s−1, respectively. Activation energies (Ea’s) were determined from the fit of the Arrhenius function to the experimental points and were found to be equal to 0.35±0.002, 0.35±0.004, 0.31±0.004, 0.19±0.002, 0.33±0.006 and 0.22±0.002eV, respectively for CH3CH2Br, CH3CH2CH2Br, CH3CHBrCH3, CF3CHBrCH3, CH3(CH2)2CH2Br and CH2F(CH2)2CH2Br molecules.

Electron attachment to propargyl chloride, 305–540 K

Electron attachment to propargyl chloride (HC≡C-CH(2)Cl) was studied in a flowing-afterglow Langmuir-probe apparatus from 305 to 540 K. The sole ion product in this temperature range is Cl(-). Electron attachment is very inefficient, requiring correction for a competing process of electron recombination with molecular cations produced in reaction between Ar(+) and propargyl chloride and subsequent ion-molecule reactions. The electron attachment rate coefficient was measured to be 1.6×10(-10)cm(3) s(-1) at 305 K and increased to 1.1×10(-9)cm(3) s(-1) at 540 K.

The influence of the temperature on electron attachment to some halocontaining molecules

Thermal electron attachment rate coefficients for CH 2ClCHClCH 2Cl, CH 2ClCH 2Br, CH 2ClCH 2CH 2Br and CF 2ClCFCl 2 have been measured using the Pulsed Townsend technique over the temperature range T = 298–358 K. The corresponding rate coefficients at 298 K are equal to 1.7(4) × 10 −10, 7.9(15) × 10 −10, 3.4(5) × 10 −10 and 3.1(1) × 10 −8 cm 3 molec. −1 s −1, respectively. With rising temperature we have observed an increase of the rate coefficients k( T), which shows an Arrhenius-type behaviour. Activation energies E a of 0.16(1) eV (CH 2ClCHClCH 2Cl), 0.12(1) eV (CH 2ClCH 2Br), 0.20(1) eV (CH 2ClCH 2CH 2Br) and 0.08(1) eV (CF 2ClCFCl 2) were determined from the fit to the experimental data points with a function ln( k) = ln( A) − E a / k B T.

Modeling Carbon Chain Anions in L1527

The low-mass protostellar region L1527 is unusual because it contains observable abundances of unsaturated carbon-chain molecules including CnH radicals, H2Cn carbenes, cyanopolyynes, and the negative ions C4H- and C6H-, all of which are more associated with cold cores than with protostellar regions. Sakai et al. suggested that these molecules are formed in L1527 from the chemical precursor methane, which evaporates from the grains during the heat-up of the region. With the gas-phase osu.03.2008 network extended to include negative ions of the families Cn-, and CnH-, as well as the newly detected C3N-, we modeled the chemistry that occurs following methane evaporation at T~ 25-30 K. We are able to reproduce most of the observed molecular abundances in L1527 at a time of ~5000 yr. At later times, the overall abundance of anions become greater than that of electrons, which has an impact on many organic species and ions. The anion-to-neutral ratio in our calculation is in good agreement with observation for C6H- but exceeds the observed ratio by more than three orders of magnitude for C4H-. In order to explain this difference, further investigation is needed on the rate coefficients for electron attachment and other reactions regarding anions.

On the validity of the Arrhenius equation for electron attachment rate coefficients

The validity of the Arrhenius equation for dissociative electron attachment rate coefficients is investigated. A general analysis allows us to obtain estimates of the upper temperature bound for the range of validity of the Arrhenius equation in the endothermic case and both lower and upper bounds in the exothermic case with a reaction barrier. The results of the general discussion are illustrated by numerical examples whereby the rate coefficient, as a function of temperature for dissociative electron attachment, is calculated using the resonance R-matrix theory. In the endothermic case, the activation energy in the Arrhenius equation is close to the threshold energy, whereas in the case of exothermic reactions with an intermediate barrier, the activation energy is found to be substantially lower than the barrier height.

Modelling low-energy electron–molecule capture processes

Cross sections and rate coefficients for capture of low-energy electrons with polar and polarizable target molecules are calculated in the framework of Fabrikant and Hotop's extended version of the Vogt-Wannier model and an extension of this approach is given in the present article. Analytical approximations are derived in order to facilitate the application to experiments. A comparison with a selection of experimental electron attachment rate coefficients provides insight into the competition between anion formation through electron capture and scattering processes which do not follow this pathway.

High resolution low-energy electron attachment to CF3I

Using several variants of the laser photoelectron attachment method, we have measured the energy-dependent yield for I− formation resulting from dissociative electron attachment (DEA) to CF3I molecules over the energy range 0.5–500 meV. One approach involved a static target gas (TG = 300 K) and pulsed electron production/anion extraction. In another approach, a collimated target was provided by a differentially pumped, seeded supersonic beam (10% CF3I in helium carrier gas, stagnation pressure 1 bar, nozzle temperature 300 K and 600 K). At the onsets for excitation of one and two quanta for the C–I stretching mode ν3, clear downward cusps are detected. With reference to the recommended thermal DEA rate coefficient kA(Te = TG = 300 K) = 1.9 × 10−7 cm3 s−1, a new highly resolved absolute cross section for I− formation has been determined. Our experimental results are well reproduced by a cross section calculated in the framework of the resonance R-matrix theory. The input for the theory includes the known energetic and structural parameters of the neutral molecule and its anion and adopts a revised vertical attachment energy and a surface amplitude chosen to reproduce the thermal DEA rate coefficient. The theory is also applied to predict absolute cross sections for vibrational excitation of the C–I stretching mode ν3. Using our experimental and theoretical DEA cross sections we derive rate coefficients for Rydberg electron transfer (RET) and the dependence of the rate coefficients for free electron attachment of a Maxwellian electron ensemble on the mean electron energy from 0.002 to 2 eV at the constant gas temperature TG = 300 K; in both cases good agreement is observed with direct RET and swarm measurements.

Measurements of thermal electron attachment rate coefficients to molecules using an electron swarm technique

An existing electron swarm apparatus has been redesigned and upgraded. In particular, the new design incorporates a novel planar radioactive foil to form an integral part of the drift tube, allowing us to overcome inherent problems present in our earlier system which used a cylindrical radioactive source. In addition to this, substantial upgrades have been made to improve the gating and amplification electronics and the data acquisition system. This has resulted in a much greater signal to noise ratio and improved accuracy. This paper describes the upgraded apparatus and its use in obtaining thermal (300 K) attachment rate coefficients to a number of molecules. The quality of the measurements and data are illustrated through the measurement of the thermal attachment rate coefficient for SF6 (kth(SF6) = (2.38 ±0.15 ) ×10 -7 cm3 s-1). Thermal electron attachment rate coefficients for four other molecules are presented, namely for two derivatives of SF6, SF5CF3 and SF5Cl, and two perfluorocarbons, c-C4F8 and 2-C4F8.

Analysis of Instabilities in Non-Equilibrium Plasmas

Plasma instabilities with charged particle production processes innon-equilibrium plasma are analysed. A criterion on plasma instabilitiesis deduced by first-order perturbation theory. The relationship betweenplasma instabilities and certain factors (degree of non-equilibrium inplasma, the electron attachment rate coefficient and electrontemperature) are described.

Electron Attachment and Detachment: Cyclooctatetraene

Electron attachment and detachment rate constants were measured for 1,3,5,7-cyclooctatetraene (COT) in a He/Ar buffer gas over the temperature range 295−365 K. A flowing afterglow Langmuir probe apparatus was used for this work. Within experimental uncertainty, the electron attachment rate coefficient is independent of temperature in this range, at a value of 3.5 ± 1.0 ×10-9 cm3 s-1. The electron detachment rate is negligible at room temperature, but climbs to 995 s-1 at 365 K. The attachment/detachment equilibrium constant implies that the electron affinity of COT is 0.57 ± 0.03 eV, in agreement with other studies using different methods. A vertical detachment transition from the planar anion leaves COT in a planar transition state for ring inversion. G2(MP2) and density functional calculations were carried out for COT neutral, anion, dianion, and the ring inversion transition state. The G2(MP2) ring inversion barrier height compares well with the experimental result from NMR studies.

Negative ions in processing plasmas and their effect on the plasma structure

Three fundamental issues: (1) how many negative ions exist in processing plasmas, (2) how negative ions affect the spatial distribution of charged particles, and (3) how negative ions affect the structure of the sheath, are discussed on the basis of knowledge attained in the past decade. Photodetachment techniques for measuring negative ion density are reviewed and results of measurements for low density as well as high-density plasmas containing halogenated gases and oxygen are discussed from a view point of how the negative ion/electron density ratio varies as a function of electron/feedstock gas density ratio. The effect of negative ions on the spatial distributions of charged particles is illustrated by solving plasma balance equations based on an ambipolar diffusion model for various electron attachment rate coefficients and electron densities; some comparisons of the results with other model studies are made. The structure of the sheath in electronegative plasmas recently clarified by a number of workers is illustrated and discussed by giving typical fluid model solutions in the spherical symmetry.