Diffusion of hydrogen on Ni(111) over a wide range of temperature: Exploring quantum diffusion on metals

We measured the temperature and azimuthal dependence of the diffusion rates of hydrogen (¹H) and deuterium (²H) on Ni(111) from 65 to 240 K where the diffusion rates vary by 8 orders of magnitude. The diffusion of hydrogen crosses over from an activated behavior above 110 K to another activated behavior below 110 K with a smaller activation energy. By ruling out the possibility of a step effect, we suggest that the crossover is from a classical overbarrier hopping to a quantum mechanical underbarrier tunneling.