Study Of Dissociative Electron Attachment Research Articles

A study of dissociative electron attachment to CHBr3 and CHI3

Dissociative electron attachment (DEA) has been studied in the systemsBr−/CHBr3 and I−/CHI3 in a crossed electron/molecular beams experiment in theelectron energy range from about 0 to 10 eV. In addition to prominent low-energyresonances (also observed in previous studies), we find a resonance in the systemBr−/CHBr3 at 4.65 eV and we find three additional resonant features in the systemI−/CHI3 at 2.1, 2.9 and 4.1 eV. The absolute partial cross-section for theBr−/CHBr3 DEA reaction was estimated as (3 ± 1) × 10−17 cm2 at 0.5 eV and (2 ± 1) ×10−18 cm2 at 4.65 eV. Analysis of the energy dependence of the I−/CHI3cross-section reveals s-wave attachment near threshold (≈0 eV) and an additionalpronounced feature located at 58 meV. We tentatively assign this featureto vibrational Feshbach resonance associated with excitation of the ν2vibrational quanta of CHI3.

Dissociative electron attachment study to nitromethane

Dissociative electron attachment (DEA) to CH3NO2 in the gas phase was studied in the electron energy range from zero up to 10 eV with an energy resolution of 140 meV. For the most intense negative fragments NO2−, O−, OH−, CN−, CNO− estimates for the absolute partial cross sections were obtained for the first time [σ(NO2)≈10−21 m2 at 0.62 eV, σ(O−)≈10−23 m2 at 5.4 eV, σ(OH−)≈10−24 m2 at 4 eV, σ(CN−)≈10−24 m2 at 1.7 eV, and σ(CNO−)≈10−25 m2 at 4 eV]. In the case of OH−, CN−, and CNO−, ion formation at very low electron energies (≈0 eV) has been observed in contrast to previous studies. The formation of OH− and CNO− at these low electron energies is explained in terms of DEA to vibrationally excited molecules. Analyzing measured partial cross sections, the standard enthalpy of formation of the CH3NO (nitroso-methane) and the CNO radical has been estimated, as ΔfHg∘(CH3NO)=129±30 kJ/mol and ΔfHg∘(CNO)=323±30 kJ/mol, respectively.

High-resolution study of dissociative electron attachment to dipolar molecules atlow energies: CH2Br2 and CCl3Br

Using the laser photoelectron attachment method with an energy width ≤ 1 meV at lowenergies (≤ 172meV) and an electron beam method (energy width about 70 meV) at higherenergies (up to 2 eV), we have determined absolute cross sections σe (E)for dissociative electron attachment to the dipolar molecules CH2Br2 andCCl3Br by normalization to known electron attachment rate coefficients.At thresholds for vibrational excitation of the CH2Br2 molecule,the DA cross section exhibits pronounced structure due to couplingof the attachment process with scattering channels; below the ν3 = 1onset for the symmetric CBr2 stretch vibration, a clear vibrationalFeshbach resonance is observed. At low energies the cross sections σe (E)show an energy dependence between E−1/2 andE−1,in essential agreement with predictions of an extended Vogt–Wannier (EVW)capture model which includes the long-range electron–dipole interaction inaddition to the polarization force. The determined absolute values are, however,substantially smaller than those predicted by the EVW model. SemiempiricalR-matrixcalculations have been carried out which show that at low electron energies(E < 0.15eV) Br− formation from CCl3Br proceeds by s-wave attachment to the anionground state while a broad peak, observed around 0.6 eV and evolvingpredominantly into the Cl− channel, is due to an excited anion state. Comparisonsare made with cross sections and rate coefficients obtained in previousphotoelectron attachment work (TPSA) and in electron beam as well asswarm experiments. Based on our joint experimental results for σe (E),we report the electron temperature dependence of the rate coefficientske (Te) forfree electron attachment involving a Maxwellian electron ensemble and a gas at room temperature(TG = 300K).

Temperature dependencies in dissociative electron attachment to CCl4, CCl2F2, CHCl3 and CHBr3

The studies of dissociative electron attachment to CCl4, CCl2F2, CHCl3, and CHBr3 molecules have been performed in the electron energy range from 0 to 2eV and in the gas temperature range from 300 to 550K using a new crossed electron-molecular beams apparatus with a temperature regulated, effusive molecular beam source. In the case of dissociative electron attachment channels Cl−/CHCl3 and Cl−/CCl2F2 strong enhancements of the negative ion production with the gas temperature at low electron energies has been observed. Activation energies for dissociative electron attachment of 80±20meV for CHCl3 and 130±20meV for CF2Cl2 have been estimated. For the resonances at 0.27eV in CHCl3 and 0.7eV in CF2Cl2 no dependence of the cross-sections on gas temperature have been observed. In the case of the dissociative electron attachment to Cl−/CCl4 and Br−/CHBr3 the cross-sections do not depend on T in the whole measured electron energy range from 0 to 2eV. At the elevated gas temperatures above 400K, unusual temperature effects have been observed for CCl4, CHCl3 and CHBr3 molecules. These effects might be most likely attributed to the thermal decomposition of molecules on the surface of the molecular beam source.

Resonance Processes in e – H2 Collisions: Dissociative Attachment and Dissociation from Vibrationally and Rotationally Excited States

We summarize theoretical studies of dissociative electron attachment to rovibrationally excited H2 molecules by comparing results of earlier local calculations with the results of more recent nonlocal complex potential calculations. The extension of the resonance theory to the near-threshold resonant dissociation is discussed and results are shown for dissociation from vibrationally excited states. Contributions from two negative-ion resonances, 2Σu+ shape resonance and 2Σg+ Feshbach resonance, are analysed. We also demonstrate the isotope effect by presenting the peak values of the dissociative attachment cross section for all five isotopic substitutes of H2.

Study of Dissociative Electron Attachment to HI Molecule by using R-matrix Representation for Green's Function

The new method of calculation of scattering Green's function recently proposed by the authors (G M Mil'nikov, H Nakamura and J Horáček 2001 Comput. Phys. Commun. 135 278) is applied to the process of dissociative attachment of low-energy electrons to HI molecule previously considered by Horáček, Domcke and Nakamura (1997 Z. Phys. D 42 181). The calculation is extended to vibrationally and rotationally excited target gas molecules. The temperature dependence of the dissociative attachment cross section is determined.

A crossed beam high resolution study of dissociative electron attachment to CCl 4

Using a crossed electron/molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process CCl 4 + e → Cl − + CCl 3 (at electron energies between threshold and 2 eV) in a target gas temperature range of 300–500 K. With the available high energy resolution of about 11 meV FWHM from an improved trochoidal electron monochromator it was possible to investigate in a single collision crossed beam experiment the true electron energy dependence of the cross-section of this s-wave electron capture process down to the limiting E −1/2 behavior. At energies above approximately 20 meV the cross-section decreases more strongly towards higher energies, exhibiting around 100 meV the typical E −1 behavior predicted for the s-wave de Broglie cross-section. Additional structure above 200 meV is interpreted in terms of autodetachment and a peak at around 0.8 eV (with a peak height of approximately 0.3% of that of the zero energy peak) is attributed to the triply degenerate 2T 2 excited state resonance. No temperature dependence of the measured cross-section function could be observed in the investigated temperature range of 300–500 K.

An ab initio study of dissociative electron attachment to NaHCO3 and NaCO3, and the role of these reactions in the formation of sudden sodium layers

One of the mechanisms that has been proposed to explain the phenomenon of sudden sodium layers (SSLs) in the upper atmosphere is dissociative attachment of electrons to sodium compounds such as NaHCO3 and NaCO3. In this study, abinitio calculations have been used to estimate the reaction enthalpies (ΔH°0(NaX + e− → Na + X−)) for NaHCO3 and NaCO3, which are found to be 72 ± 20 (0.75) and 21 ± 30 (0.22) kJmol−1 (eV), respectively. NaHCO3 is then shown to be relatively stable above the mesopause. This property, together with its rather small threshold for dissociative electron attachment, makes NaHCO3 a possible source of SSLs under certain conditions.

Studies of dissociative electron attachment to some haloethanes using the falp apparatus: comparisons with data obtained using non-thermal techniques

Measurements are reported of the rate coefficients, β, for the dissociative attachment reactions with electrons of the four haloethanes CH 3CCl 3, CH 2ClCHCl 2, CF 3CCl 3 and CF 2ClCFCl 2, obtained under truly thermal equilibrium conditions at 298, 385 and 470 K using the flowing afterglow/Langmuir probe (FALP) technique. In all four reactions the only product ion observed was Cl −. These values of β are compared with the equivalent β values obtained by the non-thermal swarm and krypton photoionization techniques for which β values were deduced as a function of mean electron/molecule interaction energy, E , at a fixed reactant molecule temperature of ∼ 298 K. The different senses of the variations of β with temperature and with E are discussed and are rationalized in terms of the thermal and non-thermal nature of the different experiments and not as “discrepancies” as described previously.

A study of dissociative electron attachment to CF 3SO 3CH 3 and CF 3SO 3C 2H 5 triflate esters using the FALP apparatus

The rate coefficients, β, for dissociative electron attachment to the triflate esters CF 3SO 3CH 3 and CF 3SO 3C 2H 5, have been determined at 300, 385 and 475 K using the FALP apparatus. The β for both reactions are small (∼10 −10 cm 3 s −1) at 300 K, in contrast to the β for triflic acid, CF 3SO 3H, which is very large (∼ 10 −7 cm 3 s −1) at 300 K. In a parallel study, the reactions of F −, Cl −, Br − and I − with the two esters have been studied at 300 K using the SIFT technique. These ion/molecule reactions proceed at or near their collisional rates, again producing the triflate anion CF 3SO − 3, and reveal that the dissociative electron attachment reactions of these esters are exothermic.

Multiple scattering Xα calculations on the Σ and it electron shape resonances in chloroethylenes and fluoroethylenes

The continuum OS MS Xα method is used to interpret the multiple resonances observed in the electron transmission spectra of tetrachloroethylene and 1,1-dichloroethylene. Good agreement with experiment is found for these molecules and for their fluorine analogues which exhibit one shape resonance. Dissociative electron attachment studies on the halogenoethylenes are interpreted in detail.

Collisional ionization of Na, K, and Cs by CO2, COS, and CS2: Molecular electron affinities

The negative ion products resulting from collisions between orthogonal beams of alkali metal atoms (Na, K, Cs) and the linear triatomic molecules CO2, COS, and CS2 have been studied from threshold to ∼400 eV (lab). Ions with masses corresponding to the parent molecules CO2, COS, and CS2 are detected for all collision permutations except for Na colliding with CO2. The following electron affinities are deduced from measurements of the threshold for the ion pair production reactions: CO2(−0.60±0.2 eV), COS(+0.46±0.2 eV), and CS2(1.0±0.2 eV). The CO−2 ion was found to be metastable with respect to autodetachment. This result is compatible with the negative electron affinity for CO2 and in agreement with our earlier observations of CO−2* and with recent theoretical calculations. The lifetime of CO−2* (9±2×10−5 sec) was measured to be independent of collision energy over the region of energy studied (threshold to ∼20 eV c.m.). The fragment ions O−/CO2, O−/COS, S−/COS, and S−/CS2 were detected at a threshold energy which is consistent with the known energetics of the reactions. These results are compared with recent dissociative electron attachment studies.