Electron Attachment Cross Sections Research Articles

Electron attachment lineshapes, cross sections and rate constants at ultra-low energies in several halomethyl and haloethyl molecules

Electron attachment lineshapes, cross sections and rate constants are reported for the molecules CH2Br2, CF2Br2, CF3Br, CBrCl3, BrCN, C2H2Br4, CH3I, C2H5I and 1, 1, 1,-C2Cl3F3. Use is made of the krypton photoionisation method. In addition, the signal intercomparison technique is used to obtain individual channel cross sections and rate constants for Br- and CN-- formation from BrCN, and for Br- and Br2- formation from CF2Br2. Present results in 1,1,1,-C2Cl3F3 using a new ion extraction grid geometry are compared with earlier results, and differences discussed in terms of the electric field equipotentials of two extraction systems used in the present work. Approximate potential energy curves for BrCN are presented, and the temperature dependence of the Br- channel cross section discussed.

Attachment of low-energy electrons to HCl

The electron-attachment rate constants of HCl diluted in Ar and N2 were measured as a function of the reduced electric field E/N. These data were converted to the electron-attachment cross section of HCl using the electron-energy distribution functions of pure Ar and N2. The dependence of the electron-attachment rate constant and the mean electron energy on the fraction of HCl in each buffer gas was investigated. A comparison of the current result with both available experimental data and theoretical calculations is made.

Measurement of electron-attachment line shapes, cross sections, and rate constants in HI and DI at ultralow electron energies.

Electron-attachment cross sections are reported in the electron energy range 0--150 meV, at an energy resolution of 6.5 meV (full width at half maximum) for the molecules HI and DI. Use is made of the Kr photoionization method to obtain cross sections for HI, and a signal intercomparison technique to obtain cross sections and the thermal-attachment rate constant for DI. Attachment properties of the two molecules are very similar. The ratio of attachment cross sections is discussed in terms of the reduced-mass dependence of the dissociation width and the survival probability, and in terms of spectroscopic thresholds and rotational populations for attachment at 300 K. Approximate potential-energy curves for the lowest states of the neutral molecule and negative ion are given.

Measurements of electron attachment lineshapes and cross sections at ultralow electron energies for CH2Br2, 1,1,1-C2Cl3H3, 2,3-butanedione and 2,3-pentanedione

Electron attachment cross sections are reported in the electron energy range 0-160 meV, and at energy resolutions of 6.5-7.0 meV (FWHM) for the molecules CH2Br2 (dibromomethane), 1,1,1-C2Cl3H3 (1,1,1-trichloroethane), 2,3-butanedione and 2,3-pentanedione. Use is made of the Kr photoionisation method. Attachment lineshapes, deconvoluted from the spectral slit (electron energy) function, are converted to cross sections by normalisation through thermal attachment rate constants at 300 K. Rate constants as a function of mean electron energy in are calculated from the cross sections using a Maxwellian electron energy distribution function, and a simple expression given for the energy of the peak attachment rate constant.

Plasma instability in the presence of negative ions.

Ion density fluctuations are studied in a diffusion-controlled argon plasma containing, as a dilutant, two electron-attaching species, carbon dioxide and sulfur hexafluoride. It is found that the system becomes increasingly unstable as the concentration of the electron-attaching species is increased. Nonlinear mode-mode couplings have been identified and the coupling coefficients for these interactions have been computed. Turbulent fluctuations are observed to have pronounced three-dimensionality with distinct axial and azimuthal behaviors. The power spectra of these fluctuations are composed of many discrete modes. The importance of changes in the total discharge pressure is dependent on the electron-attachment cross section of the dilutant species. From these results it is shown that the ionization instability is probably responsible for the observed phenomenon.

Control of product channels by addition of vibrational or electronic energy to the reactions of Xe(6s) atoms with CF 3Cl, CF 2Cl 2 and CF 2HCl molecules

An order of magnitude enhancement of the XeCl(B,C) formation rate constants was observed for the 300 K reactions of Xe(6p[ 1 2 ] 0 and [ 3 2 ] 2) atoms with CF 3Cl, CF 2Cl 2 and CF 2HCl, relative to the reactions of Xe(6s, 3P 2) atoms. Similar large XeCl(B,C) formation enhancement factors were found for reactions of the vibrationally excited molecules ( ≈2 eV), produced by CO 2-laser irradiation, with Xe(6s, 3P 2) atoms. These results correlate with the energy dependence of the dissociative electron attachment cross sections of these molecules.

Measurements of electron attachment lineshapes and cross sections at ultra-low electron energies for CF2Cl2, c-C4F6, c-C4F8and c-C7F14

Electron-attachment cross sections are reported in the electron energy range 0-160 meV, and at energy resolutions of 7.0 and 7.5 meV (FWHM), for the molecules CF2Cl2 (dichlorodifluoromethane), c-C4F6 (perfluorocyclobutene), c-C4F8 (perfluorocyclobutane), and c-C7F14 (perfluoromethylcyclohexane). Use is made of the Kr photoionization method. Measured attachment lineshapes are deconvoluted from the spectral slit function, and are converted to cross sections by normalization through thermal attachment-rate constants. Comparisons are made with attachment cross sections derived from several independent sets of swarm-measured rate constants, and with collisional ionization (high-Rydberg attachment) data.

Variation of the electron attachment to n-C4F1 with temperature

The total electron attachment rate constant ka(〈ε〉) for n-C4F10 has been measured using an electron swarm technique in the mean electron energy, 〈ε〉, range 0.76 to 4.81 eV and over the temperature, T, range 300 to 750 K. The measured ka(〈ε〉,T) and the total electron attachment cross section σa(ε,T) determined from these by unfolding are reported. The ka(〈ε〉) first decrease slowly with T between 300 and 400 K, then decrease precipitously between 400 and ∼500 K, and subsequently increase for T>500 K. The overall variation of ka with T depends on 〈ε〉. From the measured dependence of ka(〈ε〉) on the total gas number density and T we determined the contribution of dissociative and nondissociative attachment to ka as a function of 〈ε〉; the contribution of nondissociative attachment decreases with T and the contribution of dissociative attachment increases with T. The ratio Rd/t of the dissociative to the total electron attachment is small at low 〈ε〉 and T, but it increases with both 〈ε〉 and T and reaches unity for T>500 K and 〈ε〉>1.7 eV. These results are discussed in relation to similar earlier work on C2F6 and C3F8.

Electron transmission studies in a free jet: molecular oxygen

A new apparatus incorporating an electron transmission spectrometer and a free jet has been used to study temporary negative-ion formation in O2. The resonant vibrational levels of the ground electronic state of O2- ( nu =4-12) were observed as spin-orbit doublets, consistent with previous experimental results. The vibrational constants of O2-, an upper bound on the autodetachment width of the nu =4 vibrational level and the relative electron attachment cross sections into the vibrational levels ( nu =5-12) have been determined. A velocity broadening term, stemming from the use of an uncollimated free jet, has also been calculated. Measurements of resonances in Ar and He were used to verify the magnitude of this term and permitted the instrument function to be extracted as well.

Comparison of calculated and experimental thermal attachment rate constants for SF6 in the temperature range 200-600 K.

Electron-attachment cross sections are calculated for the process ${e}^{\mathrm{\ensuremath{-}}}$+${\mathrm{SF}}_{6}$\ensuremath{\rightarrow}${\mathrm{SF}}_{6}$${\mathrm{}}^{\mathrm{\ensuremath{-}}}$ in the energy range 1--200 meV. An electron scattering approximation is used in which diatomiclike potential energy curves near the equilibrium ${\mathrm{SF}}_{6}$ ground state are constructed from recent spectroscopic data. Excellent agreement is found over the entire energy range with experimental attachment cross sections at a temperature of 300 K for s-wave (l=0) scattering. The same calculation, with appropriate adjustment of the thermal populations, is used to calculate attachment rate constants k(\ensuremath{\epsilon}\ifmmode\bar\else\textasciimacron\fi{}) in the range 50--600 K for both s- and p-wave scattering. Comparisons are made with four independent sets of measured rate constants in the range 200--600 K, after adding an experimental estimate of the attachment rate for the process ${e}^{\mathrm{\ensuremath{-}}}$+${\mathrm{SF}}_{6}$\ensuremath{\rightarrow}${\mathrm{SF}}_{5}$${\mathrm{}}^{\mathrm{\ensuremath{-}}}$+F. Good agreement is found with three sets of data, and poorer agreement with the fourth. The calculation shows that the true s-wave behavior of the rate constant, in which k(\ensuremath{\epsilon}\ifmmode\bar\else\textasciimacron\fi{}) is independent of \ensuremath{\epsilon}\ifmmode\bar\else\textasciimacron\fi{}, will be realized only at temperatures T less than 115 K, and that the observed constancy in the range 300--600 K is accidental. Theory predicts approximately a 9% increase in k(\ensuremath{\epsilon}\ifmmode\bar\else\textasciimacron\fi{}) as T is lowered from 300 to 200 K.

Measurements of electron attachment lineshapes and cross sections at ultralow electron energies for c-C6F10, c-C6F12, C8F16and 1,1,2-C2Cl3F3

Electron attachment cross sections are reported in the electron energy range 0-160 meV and at energy resolutions of 4.5-7.5 meV (FWHM) for the molecules c-C6F10 (perfluorocyclohexene), c-C6F12 (perfluoro-1,2-dimethylcyclobutane), C8F16 (perfluoro-1,3-dimethylcyclohexane) and 1,1,2-C2Cl3F3 (1,1,2-trichlorotrifluoroethane). Use is made of the Kr photoionization technique, and measured attachment lineshapes are converted to cross sections by normalization through attachment rate constants. Comparisons are made with attachment cross sections derived from swarm-measured rate constants. Similar to previous results in eight other molecules, the present four molecules exhibit resolution-limited onsets at a threshold consistent with an s-wave attachment behavior and with a neutral-negative-ion curve crossing at zero energy.

Dissociative electron attachment to SO2

The total rate constant ka(〈ε〉,T) for dissociative electron attachment to SO2 has been measured as a function of the mean electron energy 〈ε〉 (1.9–4.8 eV) and temperature T (300–700 K) using an electron swarm technique. From the measured ka(〈ε〉,T), the total electron attachment cross sections σda(ε,T) were determined. The peak value (at ∼4.5 eV) of σda increases by more than a factor of 2 when T is increased from 300 to 700 K, but the peak position and onset of σda shift to lower energy only slightly, indicating a dissociative attachment process with a vertical onset. The magnitude of σda(ε,T≂300 K) was also determined in a time-of-flight mass spectrometric study. These measurements resolve the existing disparity in the literature concerning the magnitude of σda(ε) for this molecule.

Calculation of cross sections for vibrational excitation and dissociative attachment in HCl and DCl beyond the local-complex-potential approximation

Cross sections for vibrational excitation and dissociative electron attachment in the collision of low-energy electrons with HCl and DCl are calculated using a resonance model which includes the effects of the long-range dipole potential. The parameters of the model are determined by a least-squares fit of the fixed-nuclei eigenphase sum to recent ab initio data of Padial and Norcross. The nuclear dynamics in the short-lived collision complex is treated without resorting to the local approximation for the decay width and the level shift. The resulting integral cross sections for vibrational excitation and dissociative attachment are in semi-quantitative agreement with the available experimental data, both with respect to the energy dependence and the absolute magnitude of the cross sections. The accuracy of the widely used adiabatic-nuclear-vibration approximation is quantitatively assessed for this prototypical electron-polar-molecule collision system.

Cross sections for H- and Cl- production from HCl by dissociative electron attachment.

Dissociative electron-attachment cross-section measurements for the production of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ from HCl have been performed with use of a crossed target-beam--electron-beam collision geometry and a quadrupole mass spectrometer. The relative-flow technique is employed to determine the absolute values of cross sections. The attachment energies corresponding to various cross-section maxima are 7.1\ifmmode\pm\else\textpm\fi{}0.1 eV and 9.05\ifmmode\pm\else\textpm\fi{}0.1 eV for ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$/HCl; 0.85\ifmmode\pm\else\textpm\fi{}0.02 eV for ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$/HCl. The cross sections at these maxima are 2.07\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$ and 0.93\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$ for ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$/HCl; 26.59\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$ for ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$/HCl, respectively.

Measurements of electron attachment lineshapes and cross sections at ultra-low electron energies for 2-C4F8, c-C5F8, c-C6F6and C7F8

Electron attachment cross sections are reported in the electron energy range 0-150 meV, and at energy resolutions of 5-8 meV (FWHM), for the molecules 2-C4F8 (perfluoro-2-butene), c-C5F8 (perfluorocyclopentene), c-C6F6 (perfluorobenzene) and C7F8 (perfluorotoluene). Use is made of the Kr photoionisation technique, and measured attachment lineshapes are converted to cross sections by normalisation through absolute attachment rate constants. As in previous results on 2-C4F6, CFCl3, CCl4 and SF6, the present four molecules exhibit resolution-limited onsets at threshold consistent with the s-wave nature of the attachment, and a neutral-negative-ion curve crossing at zero energy.

A study of the isomeric dependence of low-energy (<10 eV) electron attachment: Perfluoroalkanes

The attachment of low-energy (<10 eV) electrons to the open-chain i-C4F10, i-C5F12, neo-C5F12, and to the cyclic c-C4F8 and c-C5F10 perfluoroalkanes has been studied using a time-of-flight mass spectrometer (TOFMS) and a high pressure electron swarm technique as well as the TOFMS results reported earlier on c-C4F8 and i-C4F10. Both long-lived parent anions (autodetachment lifetimes τa >10−6 s) and dissociative attachment fragment anions were observed for these molecules in the mass spectrometric study. The relative cross sections for negative ion formation and the τa of the long-lived parent anions were measured as a function of electron energy in the mass spectrometric study. The absolute total electron attachment rate constants as a function of the mean electron energy have been measured in the swarm study and used to calculate the total electron attachment cross sections as a function of electron energy. Both studies have shown a strong dependence of dissociative and nondissociative electron attachment processes on the structure of the molecules studied. For neo-C5F12, the predominant anion is the fragment C4F−9. In contrast, for i-C4F10, i-C5F12, c-C4F8, and c-C5F10, the parent anions are stronger than any of the fragment anions. Energetic considerations have been employed to identify fragmentation mechanisms of the negative ion states (NISs) leading to the formation of the observed fragment anions and to deduce some thermochemical data. The positions of the NISs, the observed differences in the cross sections for parent and fragment anion formation, and the types of the fragment anions formed for the various molecules are discussed.

Effect of temperature on the dissociative electron attachment to CClF3 and C2F6 a)

The total electron attachment rate constant ka(〈ε〉) for CClF3 and C2F6 has been meausured using an electron swarm technique in the mean electron energy range 0.41 to 4.81 eV and over the range of temperature T from 300 to 750 K. At each value of T the total electron attachment cross section σa(ε) was determined from the measured ka(〈ε〉) using the swarm unfolding technique and was compared with the results of a mass spectrometric study. The σa(ε) for C2F6 shows a single peak (due to F− and CF3−) which shifts from 3.9 eV at 300 K to ∼3.3 eV at 750 K. (The onset shifts correspondingly from 2.3 to 1.5 eV.) For CClF3 the σa(ε) shows two peaks: at ∼1.5 eV (due to Cl−) and at ∼4.7 eV (due to Cl−, F−, CClF2−, and ClF−). The peak at ∼1.5 eV is especially sensitive to changes in T. The peak value of σa(ε) increased by a factor of ∼3, and the energy position of the peak and onset shifted to progressively lower energies when T increased from 300 to 700 K. The analysis of these results led us to conclude that the changes in ka(〈ε〉) and σa(ε) for the dissociative attachment processes of these molecules with increasing T result from the increase with T of the total internal (≂ vibrational) energy of the molecule.

Relative electric strengths and polarizabilities of gaseous dielectrics

The relative electric strength (RES) values of eighty dielectric gases have been examined in relation to their polarizabilities. It is found that RES increases with Increasing polarizability values (α), although gases tend to fall into two classes. The significance of these observations is discussed in terms of the basic physico-chemical concepts developed by Christophorou and co-workers; e.g. electron scattering cross sections, ionizatlon cross sections and electron attachment cross sections. The interpretative fallacies of some previous engineering-empirical intuitive approaches to this problem have been indicated. The significance of the foregoing considerations in the choice of a practical dielectric gas is outlined.

S-wave threshold in electron attachment: results in 2-C4F6and CFCL3at ultra-low electron energies

Electron attachment lineshapes and cross sections are reported for the processes 2-C4F6(-)/2-C4F6 and Cl(-)/CFCl3 at electron energies of 0-120 and 0-140 meV, and at resolutions of 6 and 7 meV (FWHM), respectively. As in previous measurements in CCl4 and SF6, the results show resolution-limited narrow structure in the cross section at electron energies below 15 meV. This structure arises from the divergence of the s-wave cross section in the limit of zero electron energy. Comparisons are given with swarm-measured results, and with collisional ionization (high-Rydberg attachment) data in this energy range.

Collisions of Xe(n f) Rydberg atoms with electron attaching targets

This paper reports a study of Xe (nf) destruction in collisions with 1,1,1-C2Cl3F3 and C5F8. The primary Rydberg atom destruction process is collisional ionization that results in the formation of a positive and a negative ion. Rate constants for this reaction have been measured for Xe (nf) over the range 26 ≤n≤43, and the major negative ion products identified. Cross sections for Rydberg electron attachment derived from these and earlier measurements are in good agreement with recent free electron attachment cross sections obtained using swarm techniques. This agreement further confirms that the study of Rydberg atom collisions can provide information on very-low-energy electron interactions.