Threshold Effects in Electron Transfer to Oriented Molecules

The effects of molecular orientation on electron transfer are explored in collisions between haloalkane molecules oriented in molecular beams and K atoms which have sufficient energy to allow the charged products to separate. For several molecules studied (CF3Br, CF3C1, and CH3Br) attack at the “heads” end of the molecule (the end with the most weakly bound atom) always produces more K+ ions. The effect of orientation is most dramatic at energies near threshold (%.5 eV) and disappears at energies of %20 eV, showing that steric requirements are energy dependent. Heads orientation has a lower energy threshold than tails orientation so there is a limited energy region where reaction occurs only in the heads orientation. For CF3Br, the thresholds are 3.4 and 4.0 eV, corresponding to energies required for formation of CF3Brand CF3 + Br-, respectively. For energies between 3.4 and 4.0 eV, reaction occurs only for attack at the Br end to form only two species, suggesting that the electron is preferentially transferred to the Br end of the molecule.