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A rate equation is derived for heterogeneous nucleation of crystals from the vapor on the basis of an adsorption, surface diffusion, and statistical fluctuation mechanism. It should be emphasized that the term surface diffusion as used in this development does not imply the existence of a gradient. Rather it refers to random movement of absorbed atoms on the subtrate surface. It is shown that this mechanism results in a rate at least 102 to 103 times greater than that calculated on the basis of direct addition of atoms to the critical nucleus from the vapor phase. The equation describes the temperature dependence of existing critical supersaturation data for heterogeneous nucleation. Analysis of the existing data by this treatment permits an evaluation of the standard free energy of adsorption. Particular attention is given to heterogeneous nucleation from atomic beams.

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