Diffusion of Hydrogen and Deuterium on Ni(111) Over a Wide Range of Temperature: Exploring Quantum Diffusion on Metals

Abstract

From the decay of a periodic modulation of an adsorbate density at spacing ranging from 0.38 to 16.6 μm, we measured the diffusion of hydrogen (¹H) and deuterium (²H) on Ni(111) from 65 to 150 K. In this temperature range, the diffusion rates D(T) of both isotopes are well described by simple Arrhenius functions of temperature. At a surface coverage of θ = 0.3, we find D₀(¹H) = 6.4 × 10⁻⁷ cm²/sec, E_a(¹H) = 2.53 kcal/mol, D₀(²H) = 7.1 × 10⁻⁶ cm²/sec, and E_a(²H) = 3.44 kcal/mol. At θ = 0.06, we find D₀(¹H) = 5.4 × 10⁻⁷ cm²/sec and E_a(¹H) = 2.7 kcal/mol. Our analysis suggests that either the diffusion of both isotopes are classical over-barrier hopping with a large anomalous isotope effect in activation energy originated from the quantum mechanical nature of a barrier-crossing near a saddle point, or the diffusion of hydrogen is already an under-barrier tunneling, while that of deuterium remains a classical over-barrier hopping.