The diffusion constants of hydrogen adatoms on Cu(111) were measured by using a combination of enabling optical techniques. A steep crossover from a strongly thermally activated hopping (>130K) to a weakly thermally activated ‘hopping’ (<130K) was observed. The diffusion at high temperatures was characterized by a high barrier (0.45eV) and a large pre-exponential factor (1.62 x 102cm2/s); indicating that it was dominated by the kinetics of crossing step edges. The weakly activated diffusion at low temperatures was characterized by a much smaller ‘barrier’ of 0.022eV and a pre-exponential factor of 6.8 x 10-14cm2/s; characteristic of quantum under-barrier tunnelling.

Classical and Quantum Diffusion of Hydrogen Atoms on Cu(111). X.Wang, Y.Y.Fei, X.D.Zhu: Chemical Physics Letters, 2009, 481[1-3], 58-61