ORIENTATION EFFECTS IN ELECTRON-TRANSFER COLLISIONS

The influence of molecular orientation upon electron transfer has been probed with oriented target molecules in crossed molecular beams. Electron transfer frequently occurs in thermal energy reactive collisions, but at thermal energies charged species can rarely escape their mutual Coulomb attraction, and only neutral products are formed. By increasing the collisional energy to a few eV, the charged species can be separated, 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 molecules, 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 production. 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 the combination of the two. At energies a few volts beyond the threshold, many of the negative ions studied break apart and the orientation dependence 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) orientation is below the threshold for anion fragmentation. The dominant orientation dependence is mostly in the entrance channel, and for this case we believe the electron is preferentially transferred to the Br end of the molecule.