Microclusters and Homogeneous Nucleation Theory.

Nucleation rates determined with the classical theory of nucleation invoking the capillarity assumption are shown to be inapplicable in condensation processes where the critical size nucleus is of the order of 10 atoms. The capillarity assumption is then replaced by the statistical mechanical determination of the free energy of formation of clusters. Methods of calculation and existing results of the statistical mechanical approach to nucleation are reviewed. The calculations yielding the most probable configuration--corresponding to the lowest potential energy and/or lowest free energy of formation of the nuclei--is chosen for comparison with existing condensation experiments on argon nucleation. This application, however, requires the change of the continuum formulation of the nucleation rate to a discrete formalism. In addition a new cluster growth rate equation is derived based on the same model. For the determination of mass fraction of the condensate a new method is found which may replace the Oswatitsch integral approach. The theoretical behavior of the total differential scattering cross section of the condensate is finally investigated in detail using this new method. Thus a direct measure for the evaluation of the validity of the condensation theories by comparison with experimental results based on light scattering since it describes this physically observable quantity. When the above theories are applied to the argon condensation experiments in supersonic nozzle flow by Wu et al. {99} it is found that the molecular dynamics calculations of the free energy of formation by D. McGinty {72} and the noncrystallograhic computation by Hoare and Pal {52} are generally appropriate in describing the observations. McGinty's results {72 } may be more suitable in giving rise to the necessary accumulation of the mass fraction of the condensate l(g(TURN)10(' -5)) and the scattering cross section of the condensate (10('3)% of that of the monomers) at the observation station at the onset of condensation. However, more experiments on argon and other simple vapors are needed for comparison with theories for a firm establishment of the superiority of either of the above models.