Karola Graupner

Dissociative electron attachment to HCCCN

Dissociative electron attachment to cyanoacetylene (propiolonitrile) HCCCN has been observed in the electron energy range 0-12 eV. Negative ions are formed in two main bands with maxima at ∼1.6 eV (CCCN − ) and ∼5. 3e V (CCCN − ,C N − , HCC − and CC − ). There are also weaker resonances which lead to dissociative electron attachment to form CN − , HCC − and CC − with a maximum intensity at ∼8.1 eV and CCCN − , CN − and CC − at ∼11. 2e V. A trace of CCN − is observed at ∼9. 1e V. The positions of the main dissociative attachment bands observed are close to positions of π ∗ resonances recently calculated by Sommerfeld and Knecht. Calculations have also been performed in this work, which confirm the position of the π ∗ orbitals. The electron affinity of the CCCN radical is determined as 4.59 ± 0.25 eV from the threshold for CCCN − formation at 1.32 ± 0.15 eV. Dissociative electron attachment to this molecule will act as a source of negative ions in extraterrestrial environments where electrons are present with more than 1.3 eV energy.

Metastable Dissociation of Anions Formed by Electron Attachment

A comprehensive analysis of metastable dissociation of 2,4-dinitrotoluene (DNT) parent anions formed by attachment of electrons of controlled energy is presented. We characterize the energy dependence and kinetic energy release of the reaction which competes with autodetachment. A surprising finding is a highly exothermic metastable reaction triggered by the attachment of thermal electrons which we relate to the well-known electrostatic ignition hazards of DNT and other explosives. Quantum chemical calculations are performed for dinitrobenzene in order to elucidate the process of NO abstraction.

Low-Energy Electron Attachment to the Dichlorodifluoromethane (CCl2F2) Molecule†

Results from a joint experimental study of electron attachment to dichlorodifluoromethane (CCl(2)F(2)) molecules in the gas phase are reported. In a high resolution electron beam experiment involving two versions of the laser photoelectron attachment method, the relative cross section for formation of the dominant anion Cl(-) was measured over the energy range 0.001-1.8 eV at the gas temperature T(G) = 300 K. It exhibits cusp structure at thresholds for vibrational excitation of the nu(3)(a(1)) mode due to interaction with the attachment channels. With reference to the thermal attachment rate coefficient k(T = 300 K) = 2.2(8) x 10(-9) cm(3) s(-1) (fitted average from several data), a new highly resolved absolute attachment cross section for T(G) = 300 K was determined. Partial cross sections for formation of the anions Cl(-), Cl(2)(-), F(-), ClF(-), and CCl(2)F(-) were measured over the range 0-12 eV, using three different electron beam experiments of medium energy resolution. The dependence of the attachment rate coefficient k(T(e);T(G) = 300 K) on electron temperature T(e) was calculated over the range 50-15 000 K, based on a newly constructed total cross section for anion formation at T(G) = 300 K. R-matrix calculations for Cl(-) production have been carried out for comparison with the experimental data. The R-matrix results are in line with the main experimental observations and predict the dependence of the DEA cross section on the initial vibrational level nu(3)() and on the vibrational temperature. Furthermore, the cross section for vibrational excitation of the nu(3) mode has been computed.

Metastable anions of dinitrobenzene: Resonances for electron attachment and kinetic energy release

Attachment of free, low-energy electrons to dinitrobenzene (DNB) in the gas phase leads to DNB(-) as well as several fragment anions. DNB(-), (DNB-H)(-), (DNB-NO)(-), (DNB-2NO)(-), and (DNB-NO(2))(-) are found to undergo metastable (unimolecular) dissociation. A rich pattern of resonances in the yield of these metastable reactions versus electron energy is observed; some resonances are highly isomer-specific. Most metastable reactions are accompanied by large average kinetic energy releases (KER) that range from 0.5 to 1.32 eV, typical of complex rearrangement reactions, but (1,3-DNB-H)(-) features a resonance with a KER of only 0.06 eV for loss of NO. (1,3-DNB-NO)(-) offers a rare example of a sequential metastable reaction, namely, loss of NO followed by loss of CO to yield C(5)H(4)O(-) with a large KER of 1.32 eV. The G4(MP2) method is applied to compute adiabatic electron affinities and reaction energies for several of the observed metastable channels.

Fragmentation of metastable SF6−∗ ions with microsecond lifetimes in competition with autodetachment

Fragmentation of metastable SF(6)(-*) ions formed in low energy electron attachment to SF(6) has been investigated. The dissociation reaction SF(6)(-*)-->SF(5) (-)+F has been observed approximately 1.5-3.4 micros and approximately 17-32 micros after electron attachment in a time-of-flight and a double focusing two sector field mass spectrometer, respectively. Metastable dissociation is observed with maximum intensity at approximately 0.3 eV between the SF(6)(-*) peak at zero and the SF(5)(-) peak at approximately 0.4 eV. The kinetic energy released in dissociation is low, with a most probable value of 18 meV. The lifetime of SF(6)(-*) decreases as the electron energy increases, but it is not possible to fit this decrease with statistical Rice-Ramsperger-Kassel/quasiequilibrium theory. Metastable dissociation of SF(6)(-*) appears to compete with autodetachment of the electron at all electron energies.

EXPERIMENTAL EVIDENCE FOR RADIATIVE ATTACHMENT IN ASTROCHEMISTRY FROM ELECTRON ATTACHMENT TO NCCCCN

Electron attachment to NCCCCN, dicyanoacetylene (2-butynedinitrile), has been observed. Metastable parent anions, NCCCCN−*, with microsecond or longer lifetimes are formed close to 0 eV electron energy with a cross section of ≥0.25 A2. The stability of NCCCCN−* suggests that radiative attachment to NCCCCN and similar linear carbon chain molecules may be an important mechanism for the formation of negatively charged molecular ions in astrophysical environments. CCCN− and CN− fragment anions are formed at ~3 and ~6 eV.

Dissociative electron attachment to the unstable carbon monosulfide molecule CS

Dissociative electron attachment to the unstable CS molecule, carbon monosulfide, produced in a microwave discharge of carbon disulfide and helium has been observed in the electron energy range 0–10 eV. The S− ion is formed in two electron attachment bands with maxima at 5.43 ± 0.15 eV and ~6.70 ± 0.15 eV and C− is formed in a band with a maximum of ~6.40 ± 0.15 eV. Absolute cross sections for these processes are estimated to be 0.025, 0.003 and 0.002 A2 respectively. These interactions of CS with low energy electrons will play a role in technological plasmas containing carbon and sulfur and extraterrestrial environments where electrons are present with 5–7 eV energy. New results are also presented for the molecules S2O, C3S2 and S2F, which were also formed in the microwave discharge. For S2O the previously reported S− band at 1.8 ± 0.3 eV is confirmed and a new O− attachment band at ~5.4 eV is tentatively assigned to S2O. Ionization thresholds of C3S2 (9.4 ± 0.3 eV) and S2F (10.3 ± 0.4 eV) have been measured, apparently for the first time.

Electron Attachment to Radicals and Unstable Molecules

Experimental investigation of electron attachment to unstable molecules is considered. Recent results of electron attachment to CS, S2O and S2O2 are reviewed. The challenges of this work are highlighted with respect to ongoing experiments to investigate electron attachment to NH and NH2 radicals.

Dissociative electron attachment to unstable molecules and slow fragmentation of metastable molecular anions

Recent experimental investigations of dissociative electron attachment to unstable molecules and free radicals are reviewed along with observations of metastable negatively charged ions. Measurements have been made with a time-of-flight mass spectrometer in Belfast and also, for metastable ions, with a double focussing twin field mass spectrometer in Innsbruck. Electron attachment to unstable CS, for example, was found to be similar to electron attachment to the valence isoelectronic CO molecule with observation of S- and C- ions just above the thermodynamic threshold for S- + C (3P) at 5.43 eV, C-+ S at 6.40 eV and S-+ C(1D) at 6.70 eV with peak cross sections of ~ 0.025 ?2, 0.002 ?2 and 0.003 ?2 respectively. Slow fragmentation of metastable SF-*6 formed in low energy electron attachment to SF6 has been observed on microsecond timescales in competition with autodetachment; processes SF-*6 ? SF-5 + F and SF-*6 ? SF6 + e- respectively. Fragmentation of metastable anions of benzene derivatives, such as 2,4-dinitro-toluene [CH3.C6H3(NO2)2], has also been observed on microsecond timescales.

Electron Attachment to Fluorocarbon Radicals and Unstable Molecules: CF2 and C2F5

Electron attachment to unstable fluorocarbon molecules has been investigated; CF2 - no dissociative electron attachment was observed, C2F5 – electron attachment observed at close to zero electron energy with the observation of C2F5−.