A theoretical study was made of the diffusion pathways of a CaF2 molecule on the terraces of the (111) surface. The surface diffusion could be described as being a staggering movement in which the molecule swung, in every jump, around one of its F ions that was fixed to a Ca ion in the outermost surface triple layer of the crystal. The diffusion could include exchanges of F ions between the ad-molecule and the outermost F layer at the surface. The jump distance was deduced to be a/√2 (= 3.86Å) and the activation energy for diffusion was 0.80eV. By assuming a vibration frequency that was of the order of 1013/s, the surface diffusivity could be described by:

D (cm2/s) = 3.73 x 10-3 exp[-9280/T]

The desorption time from the terrace was described by t(s) = 10-13exp[19720/T], with a mean adsorption energy of -1.70eV. The mean diffusion displacement on the terrace was d(cm) = 3.86 x 10-8exp[5220/T]; with a saddle-point energy of 0.90eV.

Theoretical Study of the Adsorption of a CaF2 Molecule at the (111) Surface of CaF2 - II. Surface Diffusion. H.Dabringhaus, K.Wandelt: Surface Science, 2003, 526[3], 273-81