ELECTRON-TRANSFER TO ORIENTED MOLECULES - K+CF3I AND K+CH3I

K+ ions have been detected from the intersection of a beam of K atoms (5-30 eV) with beams of CH3I and CF3I molecules which had been oriented prior to the collision. Collisional ionization is found to be favored for both molecules when the fast K is incident at the I end of the molecule, even though the electrical polarity of the I end is different for the two molecules. For both molecules, the effect of molecular orientation is most pronounced near threshold (≈5 eV) and almost disappears at higher (30 eV) energies. For CF3I, the threshold for impact at the I end is ≈0.7 eV less than the threshold for impact at the CF3 end. We interpret these results using a "harpoon" mechanism in which the electron jump during the initial approach is probably independent of orientation, but as the charged particles separate, the electron may jump back to the K+. For impact at the I or "head" end, the I- is ejected backwards towards the incoming K+. This increases the final relative velocity of the ions and lowers the probability of neutralization. The ion signal is greater and the electron thus appears to favor the I end of the molecule. © 1990 American Institute of Physics.