Threshold Behavior in Electron-Transfer Collisions between Rubidium Atoms and C2F5Cl or C2F5I Molecules

Rubidium atoms are accelerated in a high-temperature expansion of hydrogen to produce beams with energies high enough to observe collisional ionization with a cross beam. The speed of the atoms is directly measured by time-of-flight techniques, and the positive and negative ions produced are detected in separate mass spectrometers and detected in coincidence. Chloroperfluoroethane produces C2F5 - and Cl- ions, whereas iodoperfluoroethane produces I-, C2F5 -, and C2F5I- ions. When the measured speed distributions are used, the signal versus energy may be deconvolved to yield thresholds and electron affinities (EAs). The EA for C2F5I is measured to be 0.96 ± 0.1 eV. Anomalously high EA values result for C2F5 apparently because C2F5 - is produced by parts per million concentrations of Rb2.