Homogeneous nucleation in gases: A three-dimensional Fokker–Planck equation for evaporation from clusters

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A theory of homogeneous nucleation in gases is developed, based on independently obtained expressions for the rates of evaporation from and condensation on the clusters. The rate of evaporation was calculated using a diffusion equation in the energy space, derived from the three-dimensional Fokker–Planck equation averaged with respect to the distribution of the angular variables. The obtained rates of nucleation are compared with those obtained previously by the authors on the basis of a diffusion equation in the energy space and a unidimensional Fokker–Planck equation, as well as with the predictions of the classical theory. The present approach predicts rates of nucleation which are closer to the classical theory than those provided by the former approaches. Nevertheless, the difference between the present approach and the classical result is significant for small clusters, i.e., for high supersaturations.