Condensation of a supersaturated vapor. VI. The diffusivity and the size of photonucleating clusters in vapors containing CS2

When a supersaturated vapor containing a low concentration of CS2 is illuminated with ultraviolet light, copious nucleation occurs. This paper presents a determination of the diffusivity of the photonucleator by analyzing the decay in the nucleation rate which occurs in a diffusion cloud chamber after blocking the light. Equations describing the mass transfer of clusters of different sizes were derived and solved to obtain the relationship between their diffusivities and the decay of the rate of nucleation. The resultant diffusivities lead to a lower limit for the radius of the CS2 photonucleators of about 5.4 Å. Such a size is much larger than that of any photoproducts of CS2. We conclude that the CS2 photonucleators are stable (or very long-lived) products resulting from the reaction of photoexcited CS2 with other vapor phase molecules (e.g., nonane). The effectiveness of clusters of these photonucleators in causing nucleation is also examined. We show that a generalization of the Becker–Doering version of nucleation theory is consistent with these results.