We present a combined experimental and theoretical study of the
self-diffusion of ammonia on exfoliated graphite. Using neutron time-of-flight
spectroscopy we are able to resolve the ultrafast diffusion process of adsorbed
ammonia, NH3, on graphite. Together with van der Waals corrected density
functional theory calculations we show that the diffusion of NH3 follows a
hopping motion on a weakly corrugated potential energy surface with an
activation energy of about 4 meV which is particularly low for this type of
diffusive motion. The hopping motion includes further a significant number of
long jumps and the diffusion constant of ammonia adsorbed on graphite is
determined with D=3.9 \cdot 10^{-8}~\mbox{m}^2 /\mbox{s} at 94 K
