Measurements were made of the diffusivity of H, on the (111) surface, at 108 to 154K. The results for could be described by:

D (cm2/s) = 1.7 x 10-5 exp[-3.4(kcal/mol)/RT]

Non-activated tunnelling was observed following thermally activated hopping at about 125K.

A.Lee, X.D.Zhu, A.Wong, L.Deng, U.Linke: Physical Review B, 1993, 48[15], 11256-9