Electric Field-Induced Dissociation of NH$_3$ on Ru(0001) from First Principles*

Recent experimental evidence suggests that an applied electric field from an STM tip can induce rapid dissociation of NH3 on a Ru(0001) surface. Here, density functional theory calculations, within the generalized gradient approximation, are used to understand the experimental observations and develop a clear picture of this complex reaction. The desorption and dissociation thresholds are found to occur at E=-0.06 V/\AA and E=0.12 V/\AA, respectively, in good agreement with experiment. Calculation of the energy barrier along a potential reaction pathway indicates a decrease of 0.50 eV in the barrier at the dissociation threshold. A field-induced softening of the ammonia umbrella mode is found to explain the initial dissociation. It is concluded that an applied electric field greatly enhances the dissociation reaction rate through a reduction of the reaction energy barrier.