Collisional ionization between alkali atoms and some methane derivatives: Electron affinities for CH3NO2, CF3I, and CF3Br

The negative ion products resulting from collisions between orthogonal beams of alkali atoms (Na, K, Cs) and the methane derivatives CH3CN, CH3NO2, CF3Br, and CF3I have been studied in the energy range from reaction thresholds to ∼40 eV (LAB). Stable negative ions with masses corresponding to the last three molecules were detected and the following electron affinities are derived from measurements of the energy threshold for the ion pair production reactions: E.A.(CH3NO2) =0.44+0.1−0.2 eV; E.A.(CF3Br) =0.91±0.2 eV, and E.A.(CF3I) =1.57±0.2 eV. From measurements of the difference between the energy threshold for the appearance of various fragment ions and the parent ion, the following bond dissociation energies are deduced: D(CH3–NO−2) =0.56±0.2 eV; D(CF3–Br−) =0.54±0.2 eV and D(CF3–I−) =0.32±0.2 eV. An argument is presented which adds further strength to the suggestions of Williams et al. and Jordan and Wendoloski that electron binding to CH3CN is dominated by the dipole field.