Electron Detachment in Negative Ion-Molecule Collisions.
Absolute total cross sections for electron detachment, reactive scattering, charge transfer and dissociative charge transfer have been measured for collisions of hydrogen and halogen negative ions with various molecular targets. The reactants investigated involve H('-), D('-), F('-), Cl(' -), Br('-) and I('-) ions as projectiles and H(,2), D(,2), HD, N(,2), CO, O(,2), CO(,2), CH(,4) and Cl(,2) molecules as targets. The energy range of these experiments extended from about 1 eV to about 300 eV in the lab. The threshold behavior of the detachment cross sections for the reactants H('-)(D('-)) + H(,2),D(,2) and HD has been determined. The thresholds for detachment for both H('-) and D('-) ions are found to be larger than the electron affinity of hydrogen and isotopic substitution reveals that the detachment cross sections scale with relative collision energy at low collision energies and with relative collision velocity at high collision energies. Upper and lower bounds on detachment-rate constants which are based upon the measurements are presented. Studies of the reactants H('-)(D('-)) and B(,2), CO, O(,2), CO(,2) and CH(,4) reveal that electron detachment is the dominant process for all the molecular targets except O(,2) for which charge transfer dominates. Isotope effects are observed in all the cross sections. The general features of the charge-transfer cross section for the O(,2) target are in agreement with the ideas of a simple two-state collision model. The cross sections for charge transfer (or dissociative charge transfer) are found to be small for all targets except O(,2). In the case of the collisions of F('-) and Cl(' -) with H(,2), D(,2) and HD, reactive scattering is found to be the dominant inelastic channel for F('-) projectile. Electron detachment of F('-) is found to occur by two distinct mechanisms. A striking difference in the detachment and reactive 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. Isotope effects are observed in all the cross sections for both F('-) and Cl('-). Charge transfer and dissociative charge transfer cross sections are found to be the dominant channels for collisions of Cl('-), Br('-) and I('-) with Cl(,2). The electron detachment cross section for I('-) + Cl(,2) is found to be anomalously low. Some energy loss spectra are reported for I('-) + Cl(,2). They exhibit substantial inelastic scattering which is consistent with the calculated potentials of Cl(,2).
- Pub Date:
- Physics: Atomic