Bond Dissociation Dynamics of Single Molecules on a Metal Surface.

The breaking of an interatomic bond is at the heart of chemistry yet remains a challenge to be investigated. Molecules on metal surfaces exhibit defined positions and orientations and can be characterized by scanning tunneling microscopy that moreover is able to trigger bond breaking. Until now, the bond dissociation dynamics has been studied in small molecules but not in large ones with various degrees of freedom. Here, we dissociate bromine atoms from single dibromo-terfluorene molecules on Ag(111), identifying not only the displacement but also the rotation of each fragment. It turns out that the molecular excitation that causes dissociation is not locally confined. Instead, it can propagate through the molecule, and the dynamics of the resulting fragments is uncorrelated. The fragment binds to the nearest silver atom after dissociation and dissipates its energy in rotational motion. Our findings could be useful for the precise engineering of chemical reactions with prearranged precursor molecules.