Diffusion of ammonia on Re(001)

Diffusion of ammonia on Re(001) has been studied utilizing optical second-harmonic diffraction from a surface coverage grating. The diffusion process at initial coverages NH₃/Re > 0.15 could be simulated only if the coverage-dependent diffusion constant had been considered. The resulting diffusivity D(θ) = D₀ exp[-E_m(θ)/RT], is defined by a barrier for diffusion E_m(θ) = E₀ - ωZθ, with activation energy at zero coverage E₀ = 3.4 ± 0.8 kcal mol^-1, D₀ = 2.8 × 10^-3 cm² sec^-1 and the repulsion energy between a pair of nearest neighbour molecules, ω = 0.4 ± 0.06 kcal mol^-1. The coverage effect on the diffusivity is discussed in terms of the thermodynamic factor, based on equilibrium adsorption measurements.