The influence of molecular orientation upon electron transfer has been
probed with oriented target molecules in crossed molecular beams. Ele
ctron transfer frequently occurs in thermal energy reactive collisions
, but at thermal energies charged species can rarely escape their mutu
al Coulomb attraction, and only neutral products are formed. By increa
sing the collisional energy to a few eV, the charged species can be se
parated, and the role of orientation on the electron transfer process
can be probed. Collisional ionization of fast ( approximate to 3-20 eV
) neutral K atoms has been observed for a variety of symmetric top mol
ecules, such as CH3I, which were oriented in a molecular beam prior to
collision. In every case studied so far, the heads/tails orientation
of the molecule drastically affects the overall probability of ion pro
duction. Electron transfer is the first step in ion production; in the
second step the ions must get away from one another once formed. Both
of these steps can depend on orientation, and the experiments probe t
he combination of the two. At energies a few volts beyond the threshol
d, many of the negative ions studied break apart and the orientation d
ependence seems mainly to be determined by how the ions get away from
one another. But the thresholds themselves are orientation dependent,
and for the reaction K + CF3Br the threshold for the heads (Br-end) or
ientation is below the threshold for anion fragmentation. The dominant
orientation dependence is mostly in the entrance channel, and for thi
s case we believe the electron is preferentially transferred to the Br
end of the molecule.