How to break a chemical bond – and how to form a new one

The characterization and controlled manipulation of molecules on a surface gives insight into fundamental physical and chemical processes. We focused on the manipulation of chemical reactions with a scanning tunneling microscope (STM) at low temperatures and under ultrahigh vacuum conditions. First, we have induced the dissociation of atoms from single molecules, analyzing the propagation of the excitation through the molecule as well as the dynamics of the resulting fragments in view of correlation between different fragments. Moreover, in contrast to the bond dissociation, also the formation of covalent bonds was induced. In general, the outcome of a molecular collision is determined by the kinetic energies, relative orientation, and impact parameter of the reactants. We investigated controlled collisions between a mobile 'projectile' and a stationary 'target' on a metal surface, imaging each involved compound before and after the collision. Such a setup allows to precisely control the orientations of molecules as well as the impact parameter during the collision. The experiments give detailed insight into the requirements for a successful bond formation.