Temperature Dependence Of Electron Attachment Research Articles

Toward a quantitative analysis of the temperature dependence of electron attachment to SF6.

New flowing afterglow/Langmuir probe investigations of electronic attachment to SF6 are described. Thermal attachment rate constants are found to increase from 1.5 × 10-7 cm3 s-1 at 200 K to 2.3 × 10-7 cm3 s-1 at 300 K. Attachment rate constants over the range of 200-700 K (from the present work and the literature), together with earlier measurements of attachment cross sections, are analyzed with respect to electronic and nuclear contributions. The latter suggest that only a small nuclear barrier (of the order of 20 meV) on the way from SF6 to SF6 - has to be overcome. The analysis shows that not only s-waves but also higher partial waves have to be taken into account. Likewise, finite-size effects of the neutral target contribute in a non-negligible manner.

Temperature dependence of electron attachment to CHCl2Br

Temperature dependence of electron attachment to CHCl2Br

Temperature dependence of electron attachment to CHCLBr2: crossed beams study

Dissociative electron attachment (DEA) to CHClBr2 has been studied in a crossed electron/molecular beams experiment in the electron energy range between 0 and 8eV and in the gas temperature range from 321 to 475K. In this electron energy range we have found two negative fragment ions Cl− and Br− which are formed from a prominent low energy resonant feature (∼0eV) and at resonances at about 0.4 and 4.7eV. The molecular ion ClBr−, in contrast to the swarm experiment, was not observed in the beam experiment. The total absolute cross-section for DEA has a value of 1.4(±0.2)×10−20m2 at 0.4eV and 5(±2)×10−22m2 at 4.7eV. The total DEA cross-section and the DEA cross-section for Br− reaction channel are temperature independent at low electron energies (∼0eV). The cross-section for Cl− reaction channel at ∼0eV increases with the gas temperature and for this reaction channel we have derived activation energy of 42(±15)meV. The enthalpies of the DEA reaction and CCl and CBr bonds have been calculated at G2MP2 and G2 level of theory. The DEA reaction has been found exothermic for both reaction channels.

Temperature dependence of electron attachment to CH 2ClBr: competition between Cl − and Br − formation

Dissociative electron attachment (DEA) to CH 2ClBr has been studied in a crossed electron/molecular beams experiment in the electron energy range between 0 and 2 eV and in the gas temperature range between 328 and 449 K. In this electron energy range we find the two negative fragment ions Cl − and Br − which are both formed from a prominent low energy resonant feature (≈0 eV). While the Br − ion yield (recorded at ≈0 eV) considerably increases with the temperature, that of Cl − slightly decreases resulting in an overall increase of ion formation at low electron energy. The temperature dependence is interpreted by the relative disposition of the involved potential energy surfaces. For the exothermic Br − channel an activation energy of 107 ± 10 meV for the DEA reaction is obtained. Using absolute rates from swarm data the total cross section for the DEA to CH 2ClBr has been calibrated to absolute values leading to 5 × 10 −21 m 2 at 0.5 eV.

Temperature dependence of electron attachment to methylene chloride

Temperature dependence of dissociative electron attachment to methylene chloride in the electron energy range of 0–10 eV was studied in a high-temperature electron swarm apparatus. The measurements were made using N2 and Ar as buffer gases. From the measured electron attachment rate constants, the electron attachment cross sections at 300, 400, and 500 K were determined using an unfolding technique. The maximum electron attachment cross sections at 300, 400, and 500 K were ≈3.1×10−18, ≈8.2×10−18, and ≈1.7×10−17 cm2, and occurred at electron energies of ≈0.8, ≈0.65, and ≈0.55 eV, respectively. The increase in electron attachment to methylene chloride with temperature is attributed to the increase in the vibrational energy of the molecule.

Temperature dependence of electron attachment and detachment in SF6 and c-C4F6

In the temperature, T, range of 300–600 K and the mean electron energy range 〈ε〉 of 0.19–1.0 eV, the total electron attachment rate constant for SF6 and c-C4F6 measured in dilute mixtures with N2, is virtually independent of T. Under the same experimental conditions the stabilized SF−6 anion does not undergo autodetachment but the stabilized c-C4F−6 anion undergoes a profound increase in autodetachment (by about four orders of magnitude). This difference between the SF−6 and the c-C4F−6 anions is attributed to the larger electron affinity of the SF6 molecule compared to the c-C4F6 molecule. The heat-activated autodetachment for c-C4F−6 is related to increases in the internal energy content of the c-C4F−6 anion and is found to have an activation energy of 0.237 eV. The ‘‘limiting electric field strength’’ of SF6 was found to increase by ∼11% as T was increased from 300 to 600 K.

Temperature dependence of electron attachment to NO2

Thermal electron attachment to NO2 in He, Ar, and N2 as buffer gases has been studied at various temperatures between 257 and 354 K with the pressure range of a few to 900 Torr. At buffer-gas pressures from a few to several tens Torr, the effective two-body attachment rate constant (keff) initially increases and then shows a saturation at all temperatures for three buffer gases. Further increase of the pressure up to near 1 atm gives a gradual increase of keff for Ar and N2 systems, whereas no such an increase is seen for He at three different temperatures. These behaviors are explained by a mechanism of a combination of the two-step three-body attachment and the attachment to van der Waals molecules containing NO2. A simulation analysis supports this explanation. A remarkable finding is that in all systems studied keff’s become larger at lower temperatures. An Arrhenius plot for the initial two-body attachment rate constant gives an activation energy of −0.17 eV. It has been suggested that an excited triplet state of NO−2 ion may be the species formed by the initial electron attachment.

Density and temperature dependence of electron attachment to O 2 molecules in helium gas

The electron attachment frequency to O 2 molecules in dense helium gas shows a sharp peak at a temperature-dependent density N p. The shape of the peak is simply related to the energy distribution function, and gives information on the tail states.

ChemInform Abstract: ELECTRON ATTACHMENT TO HALOCARBONS OF ENVIRONMENTAL INTEREST: CHLOROETHANES

Electron attachment to the chlorofluoroethanes 1,1‐C2Cl2F4, 1,2‐C2Cl2F4, 1,1,1‐C2Cl3F3, and 1,1,2‐C2Cl3F3 was investigated in dilute mixtures with nitrogen and argon using the electron swarm method. The 1,1,1‐C2Cl3F3 molecule has a very large electron attachment rate constant at thermal energy (2.8×10−7 cm3 s−1); the rate constant for this molecule decreases monotonically with increasing electron energy above thermal. The other chlorofluoroethanes investigated attach electrons less strongly and show maxima in the rate constant vs mean electron energy at ∼0.5 eV. For all four compounds, the measured rate constants are due primarily to dissociative electron attachment producing Cl−. Electron attachment cross sections were determined for these halocarbons which showed the existence of several negative ion states below ∼2 eV. Additionally, the temperature dependence of electron attachment to 1,1‐C2Cl2H4 was investigated from 323–473 K. The measured rate constants and unfolded cross sections indicated electron attachment to a repulsive negative ion state from the v=0 and v=1 levels of the neutral molecule.

Electron attachment to halocarbons of environmental interest: Chloroethanes

Electron attachment to the chlorofluoroethanes 1,1-C2Cl2F4, 1,2-C2Cl2F4, 1,1,1-C2Cl3F3, and 1,1,2-C2Cl3F3 was investigated in dilute mixtures with nitrogen and argon using the electron swarm method. The 1,1,1-C2Cl3F3 molecule has a very large electron attachment rate constant at thermal energy (2.8×10−7 cm3 s−1); the rate constant for this molecule decreases monotonically with increasing electron energy above thermal. The other chlorofluoroethanes investigated attach electrons less strongly and show maxima in the rate constant vs mean electron energy at ∼0.5 eV. For all four compounds, the measured rate constants are due primarily to dissociative electron attachment producing Cl−. Electron attachment cross sections were determined for these halocarbons which showed the existence of several negative ion states below ∼2 eV. Additionally, the temperature dependence of electron attachment to 1,1-C2Cl2H4 was investigated from 323–473 K. The measured rate constants and unfolded cross sections indicated electron attachment to a repulsive negative ion state from the v=0 and v=1 levels of the neutral molecule.

A theoretical model of D-region ion chemistry modifications during high power radio wave heating

A simplified D-region ion chemistry scheme is combined with plausible models for the electron temperature dependence of electron attachment and ion-electron recombination processes in order to study plasma density modifications induced by continuous high power radio wave heating. Enhancement of the electron temperature in the D-region by radio wave heating causes increased electron attachment and decreased recombination. The increase in attachment produces a rapid decrease in electron density while enlarging the negative ion concentration on a time scale of several seconds or less. On the other hand decreased recombination leads to increases in the positive ion and electron densities on a time scale of several minutes to an hour. These two effects compete to determine the electron density distribution in the D-region during radio wave heating.

The temperature dependence of electron attachment to N2O in the gas phase

The temperature dependence of electron attachment to N2O has been measured in the gas phase for pure N2O and N2O–C2H6 and N2O–neopentane mixtures. The rate constant found for e−+N2O→N2+O− can be expressed as 2.2×10−7 exp(−5.18×103/T) cm3/molecule⋅sec while that for e− +N2O→N2O−* is 4.2×10−9 exp(−2.65×103/T) cm3/molecule⋅sec. The corresponding activation energies are 0.44 and 0.23 eV. The latter value is discussed by reference to recent calculations on the N2O− energy surface. Formation of N2O−* with such an activation energy seems possible. The temperature dependences of other reactions in the overall mechanism are also determined. Certain rate constants increase with a decrease in temperature. Formation of negative ion complexes is suggested to account for the observation.

529. Temperature dependence of electron attachment at low energies for poly-atomic molecules. (USA): D Spence and G T Schu'z, J Chem Phys, 58 (5), 1 March 1973, 1800–1803

529. Temperature dependence of electron attachment at low energies for poly-atomic molecules. (USA): D Spence and G T Schu'z, J Chem Phys, 58 (5), 1 March 1973, 1800–1803

Temperature dependence of electron attachment at low energies for polyatomic molecules

The cross sections for production of negative ions by attachment of low-energy (<0.2 eV) electrons to selected polyatomic (SF6, CCl4, CFCl3, CH2Br2, CH3I, CHCl3, CF3Br, CH3Br) molecules have been determined as a function of target gas temperature in the range from 300 to 1200°K. For the case of SF6 it is found that the total cross section for negative ion production is independent of gas temperature, although it is known from the experiments of Chen and Chantry that the relative abundances of different species of negative ions produced from SF6 are strongly temperature dependent. In contrast to SF6, the total cross sections for negative ion production in some halogenated hydrocarbons are found to be strongly temperature dependent. It is found that the total cross section for negative ion production in many polyatomic molecules tends towards an absolute maximum as the gas temperature is increased.

The temperature dependence of electron attachment to CCl 4, CHCl 3 and C 6H 5CH 2Cl

The temperature dependences and absolute values of the rate constants for the reaction of electrons with CCl 4, CHCl 3 and C 6H 5CH 2Cl (benzyl chloride) have been measured from 20°C to 200°C. The rate constants are found to be quite well described by the Arrhenius equation k = A exp (−E ∗/RT) . The values of A and E ∗ found are 1·55×10 −7, 8·6×10 −8, 2·0×10 −7 cm 3 s −1 and −0·57, 2·2, 2·9 kcal/mol (−2·4, 9·2, 12·2 kJ/mol) for CCl 4, CHCl 3 abd C 6H 5CH 2Cl respectively. The pressure dependence of the rate constants have also been investigated.

Temperature Dependence of Electron Attachment inI2Vapor

The temperature dependence of electron attachment in ${\mathrm{I}}_{2}$ vapor has been measured over the temperature range from 253 to 467\ifmmode^\circ\else\textdegree\fi{}K. The attachment coefficient was found to increase with temperature over the range studied, varying from 0.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ to 4.2 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ ${\mathrm{cm}}^{3}$/sec. Studies involving selective heating of the electrons indicate that for a gas temperature of 298\ifmmode^\circ\else\textdegree\fi{}K the attachment coefficient increases from 1.8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ to ${\mathrm{cm}}^{3}$/sec as the electron thermal energy was increased from 0.04 to about 0.27 eV.