Diffusion of ammonia on Re(001): A monolayer grating optical second harmonic diffraction study

Diffusion of ammonia was studied over the smooth Re(001) surface utilizing optical second harmonic diffraction from surface coverage grating. The decay of the first order diffraction peak as a function of surface temperature for different initial coverages was measured and then simulated by numerical solution of Fick's second diffusion equation, employing a coverage dependent diffusion constant. For the first time the second order diffracted second harmonic signal was monitored during the diffusion process, supporting the calculated initial coverage profile and the diffusion model. The resulting diffusivity D(θ) = D₀exp[-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±0.6 kcal/mol, D₀=2.8×10^-3 cm² sec^-1 and the repulsion energy between the pair of nearest neighbor molecules ω=0.2 ±0.06 kcal/mol. The magnitude and effect of the repulsive interactions on diffusion are compared to their strong effect on the activation energy for desorption.