It has long been recognized that impurities adsorbed on the substrate can have a marked influence on the kinetics of heterogeneous nucleation. However, the detailed nature of these effects has not been demonstrated heretofore. In the present work, impurity effects have been characterized for the nucleation of selenium vapor onto Pyrex glass. It is shown that the degree of surface contamination not only affects the critical supersaturation required for nucleation, but may cause a nucleated phase of other than equilibrium structure. It is also shown that the degree of material accommodation for selenium on Pyrex is affected by surface contamination. These results are discussed in terms of the changes in substrate and nucleus interfacial energies accompanying impurity adsorption, and in terms of the possible masking of ordering influences exerted by a clean substrate.
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