Dehydrogenation of Ammonia on Ru(0001) by Electronic Excitations

We study the mechanism leading to the breaking of the N–H bonds in ammonia on Ru(0001) by means of scanning tunneling microscopy (STM). Our results support a model where injection of electrons or holes into antibonding (LUMO) and bonding (HOMO) orbitals of the molecule is far more effective than thermal excitations for molecular dissociation. We also found that a critical electric field between tip and surface is necessary to shape the tunneling barrier to obtain efficient rates of electrons from the tip or surface. First principle DFT calculations allow us to explain the observations and show that the applied electric field cannot by itself account for the observed dissociation. Since electron injection is the process governing photo- and electrochemical reactions, our STM study provides a detailed view of the reaction mechanism at the single molecular level in these nonthermal reactions.