Classical and quantum diffusion of hydrogen atoms on Cu(1 1 1)

We measured diffusion constants of hydrogen adatoms on Cu(1 1 1) using a combination of enabling optical techniques. We observed a steep crossover from a strongly thermal activated hopping (T > 130 K) to a weakly thermal activated ‘hopping’ (T < 130 K). The diffusion at high temperatures is characterized by a large barrier E diff ( 1 ) = 450 meV and a large pre-exponential factor D 0 ( 1 ) = 162 cm 2 / s , indicating that it is dominated by the kinetics of crossing step edges. The weakly activated diffusion at low temperatures is characterized by a much smaller ‘barrier’ of E diff ( 2 ) = 22 meV and a pre-exponential factor D 0 ( 2 ) = 6.8 × 10 - 14 cm 2 / s , characteristic of a quantum under-barrier tunneling.