Electron transfer to oriented molecules: K+CF3I and K+CH3I

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Abstract
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.

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