Diffusion of ammonia on Re(001)

Abstract

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.