What is surface diffusion?

Surface diffusion is a general process involving the motion of adatoms (adsorbed atom lying on a crystal surface, used in surface chemistry, when describing single atoms lying on surfaces and surface roughness), molecules, and atomic clusters (adparticles) at solid material surfaces. Tunnelling diffusion is a particularly interesting example of an unconventional mechanism wherein hydrogen has been shown to diffuse on clean metal surfaces via the quantum tunnelling effect.