Theoretical study of the adsorption of a CaF 2 molecule at the ( [formula omitted]) surface of CaF 2 II. Surface diffusion

Abstract

This paper is concerned with a theoretical study of the diffusion pathways of a CaF 2 molecule on the terrace of the (1 1 1) surface of a CaF 2 crystal. Altogether, the surface diffusion can be described by a staggering movement, where the molecule swings in every jump around one of its fluorine ions being fixed to a calcium ion of the outermost surface triple layer of the crystal. The diffusion can include exchanges of fluorine ions between the admolecule and the outermost fluorine layer at the surface. The jump distance results as a 0/ 2 =3.86 Å and the activation energy for diffusion as 0.80±0.10 eV. Assuming a vibration frequency in the order of 10 13 s −1 the surface diffusion coefficient becomes D s=3.73×10 −3exp(−9.280×10 3/ T) cm 2 s −1. With the desorption time from the terrace τ des=10 −13exp(1.972×10 4/ T) s with a mean adsorption energy of −1.70±0.03 eV the mean diffusion displacement on the terrace becomes x s =2 D s τ des =3.86×10 −8 exp(5.220×10 3/T) cm with a saddle point energy of E s=0.90±0.13 eV.