Hydrogen Surface Diffusion on Homogeneous Metal Surfaces

Hydrogen Surface Diffusion on Homogeneous Metal Surfaces 

For a brief introduction, we refer again to Fig. 25.5(a), which immediately reveals that it requires an activation energy of for an atom (or a molecule) to be transferred from a site A to another, geometrically identical site B on the same periodic surface. In the classical view, this two-dimensional diffusion process can be thought of as a sequence of individual and statistical hopping events of frequency v, each activated with an energy as pointed out, for example, by Roberts and McKee [35]. The inverse of this frequency then yields the residence time, t, of the particle in the respective site. For thermally equilibrated particles, the temperature dependence of the classical surface diffusion is described by the well-known Arrhenius relation 

Another frequently used expression is based on random walk events between fixed sites and relates the pre-exponential factor Dq, to the jump length a and the vibrational frequency parallel to the surface, v. For a surface with fourfold symmetry one has 

The factor A arises from the four possible diffusion probabilities on this surface. Depending on the surface structure and corrugation, there may exist easy and difficult pathways for diffusion with low and high activation barriers, respectively hence, on a crystallographically well defined single crystal surface, the diffu- 

So far we have not considered the most prominent property of an FI atom, viz., its peculiar quantum character. This is based on the light mass of the proton (or 

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