L. D. Doverspike

Collisional electron detachment and decomposition rates of SF−6, SF−5, and F− in SF6: Implications for ion transport and electrical discharges

Measured cross sections for prompt collisional detachment and decomposition of SF−6, SF−5, and F− in SF6 reported in the preceding companion paper are used to calculate detachment coefficients and ion‐conversion reaction coefficients as functions of electric field‐to‐gas density ratio (E/N) for ion drift in SF6. Analysis from a model presented here using these coefficients suggests that prompt electron detachment from SF−6 and SF−5 in SF6 are insignificant processes in such ion‐drift experiments. Calculated rates for ion‐conversion processes indicate the necessity to: (1) reexamine the previously measured rates in SF6 from drift‐tube experiments, and (2) use ion kinetic‐energy distributions with larger high‐energy tails than the standard distributions assumed in earlier calculations. The calculated detachment and reaction coefficients are used in a model which invokes detachment from long‐lived energetically unstable states of collisionally excited SF−6 to explain the pressure dependence of previously mea...

Total electron detachment cross sections for collisions of H- and He and F- with atomic and molecular targets

Absolute total electron detachment cross sections for collisions of H- and D- with He, and F- with He, Ne, Ar, CO, N2 O2 and CO2 have been measured in the energy range from about 1 up to 300 eV. The results for H-(D-)+He are found to be in very good agreement with the recent theoretical calculations of Gauyacq (1980) and Taylor and Delos (1982). The thresholds for electron detachment in F- collisions are found to be approximately 7 eV for all targets except O2, which has a threshold of about 4 eV. For this latter case, evidence of some charge transfer producing O2- (or O-+O) is also observed.

Collisional electron detachment and decomposition cross sections for SF−6, SF−5, and F− on SF6 and rare gas targets

Absolute total cross sections for collisional electron detachment and collision‐induced dissociation (CID) have been measured for binary collisions of SF−6 and SF−5 with rare gas and SF6 targets for laboratory collision energies ranging from about 10 up to 500 eV. The cross sections for electron detachment of SF−6 are found to be surprisingly small, especially for the SF6 target, for relative collision energies below several tens of electron volts. Specifically, detachment onsets are found to occur at around 30 and 90 eV for the rare gas and SF6 targets, respectively. The CID channel which leads to F− as a product is observed to dominate detachment for relative collision energies below 100 eV. The results for the SF−5 projectile are remarkably similar to those exhibited for SF−6. The role of long‐lived excited states in the reactant SF6 ion beam is discussed.

Total cross sections for collisions of O− and S− with hydrogen

Absolute total cross sections for electron detachment and H− (D−) production have been measured for collisions of O− and S− with H2 and D2 for relative collision energies ranging from approximately 0.5 to 25 eV. Electron detachment appears to occur via several mechanisms. Isotope effects are observed in all cross sections. Threshold studies indicate that there is no appreciable potential barrier to H− formation via the ion–molecule reaction O−+H2 → H−+OH, whereas a barrier of about 1 eV is observed for H− production in collisions of S− with H2.

Reactive scattering and electron detachment in collisions of halogen negative ions with isotopic hydrogen molecules

Total cross sections for reactive scattering and collisional electron detachment have been measured for collisions of F− and Cl− with H2, D2, and HD. The collision energy extends from below the energetic thresholds for the processes studied up to a laboratory energy of about 300 eV. Reactive scattering is found to be the dominant inelastic channel for the case of the F− projectile. Isotope effects are observed in all cross sections. Electron detachment of F− is found to occur by two distinct mechanisms. A striking difference in the reactive and detachment cross sections is observed when Cl− is substituted for F− in that the electron detachment cross section is generally larger than that for reactive scattering. As in the F− case, isotope effects are also present for the Cl− projectile. The isotope effects observed for electron detachment of Cl− suggest that detachment cannot be described by any unique mechanism.

Electron detachment in I--rare-gas collisions

Absolute electron detachment cross sections are reported for collisions of I- and the rare gases at energies between threshold and 500 eV in the laboratory frame. The detachment threshold energy and forward rate constants at 4000 and 5000K are reported for each system. The cross sections for I-+Ne are found to be much smaller than those for the other systems studied.

An ion beam study of reactive scattering of halide ions by methyl halides

Measurements of electron detachment cross sections, reactive charge transfer, and differential elastic and inelastic scattering cross sections for Cl−, Br−, and I− ions on CH3I, CH3Br, and CH3Cl targets are reported. The energy range for the experiments is from 3 to 150 eV. Of the very large number of reaction channels open, it appears that most make significant contributions to the total cross section for sufficiently high projectile energies. The SN2 reaction, which may be dominant at lower energies, was not observed and is probably not significant above the lowest energies in this study. Surviving primary ions exhibit a broad spectrum of inelastic loss, suggesting target breakup.

Measurements of absolute total cross sections for charge transfer and electron detachment of halide ions on chlorine

Absolute total cross section measurements are reported for electron detachment and for reactive scattering between the halide ions (Cl−, Br−, and I−) and chlorine gas Cl2. The charge transfer and dissociative charge transfer cross sections are found to be very large at their maxima, which occur at a relative collision energy of about 13 eV. The electron detachment cross section for I−+Cl2 is anomalously low. Some energy loss spectra are reported for I−+Cl2. They exhibit substantial inelastic scattering which is consistent with the calculated potentials of Cl2.

O- scattering by noble gases

O- scattering by Ne, Ar and Kr in the 0.2-1 keV laboratory energy range has been studied. The energy-loss spectra of scattered particles were obtained using the time-of-flight method and electrostatic analysis. Reduced differential cross-sections for elastic and inelastic processes are reported in the 0-5 keV deg. angular range. The dominant inelastic process is found to be single electron detachment leading to the formation of oxygen in the ground 3P and metastable 1D states. A method of forming a metastable 1D oxygen beam is proposed. O+ production was observed for O--Ne collisions, however this process does not appear to be important in the energy and angular range studied. The results are discussed within a quasimolecular framework.

On the reaction He+ +Ar->He+(Ar+)* at low energies

Kinematic studies of the collision system He+ +Ar at low collision energies have confirmed that the charge-transfer process He+ +Ar -> Ar+ +He is highly endothermic and can involve large momentum transfer. Ar2+ ions are also produced and, over the range of collision energies investigated, the cross section for production of Ar2+ is approximately 20% of that for Ar+ formation.