Coverage-dependent diffusion of ammonia on Re(001)

Diffusion of ammonia on Re(001) was studied utilizing optical second harmonic diffraction from surface coverage grating. The diffusion process at initial coverages NH₃/Re>0.15 could be simulated only if coverage dependent diffusion constant has 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, D₀=2.8·10^-3cm²sec^-1. The repulsion between pair of nearest neighbor molecules of approximately ega=0.4±0.06 kcal/mol is obtained if an average of three nearest neighbors is assumed at the edge of the grating. The coverage effect on the diffusivity is found to correlate quantitatively with dipole-dipole repulsion between nearest neighbors. This results in a constant corrugation ratio E _diff/E_des, over the entire coverage range.