The role of surface diffusion in heterogeneous nucleation mechanisms

The theory of heterogenous vapor-solid nucleation is reconsidered from the viewpoint of surface diffusion constraints. It is shown that the conditions for validity of a nucleation mechanism may be phrased in terms of the relationship between mean adsorbed molecule diffusion distance to the mean separation of molecules on the substrate. Within this frame-work a new mechanism of nucleation is evolved. This mechanism involves random creation of nuclei which are promoted to critical nuclei by direct impingement of molecules from vapor. The predicted nucleation rate equation agrees reasonably well with Walton's data for nucleation of silver on sodium chloride.