The migration of atomic H upon the (111)-(7 x 7) surface was investigated by monitoring optical second-harmonic diffraction from a sub-monolayer grating of adsorbed H. With the aid of sub-micron grating periods, it was possible to measure surface diffusivities of less than 10-14cm2/s. A thermally activated diffusion process was identified which could be described (table 177) by:

D (cm2/s) = 0.001 exp[-1.5(eV)/kT]

G.A.Reider, U.Höfer, T.F.Heinz: Physical Review Letters, 1991, 66[15], 1994-7