Using supersonic molecular beam techniques we have investigated the dissoci
ative adsorption of NH3 on a Ru(0001) surface. At high incident energies, t
he dissociation increases substantially due to a direct breaking of the N-H
bond on impact with the surface. For low incident translational energies,
the dissociation depends on surface temperature T-s in an unusual manner, p
eaking sharply around 400 K. Increasing the surface defect density by low-f
luence Ar+ sputtering strongly enhances the dissociation probability while
preserving the overall T-s-dependence. We interpret the low incident energy
behavior as due to a mechanism in which a molecular precursor must undergo
diffusion to defects before dissociating. At the lowest surface temperatur
es, dissociation is limited by the diffusion of the reaction products away
from the defects in order to reactivate them. A kinetic model based on this
mechanism is developed which is in good agreement with all experimental ob
servations. (C) 2000 American Institute of Physics. [S0021-9606(00)70340-0]
.