Dimers, Clusters, and Condensation in Free Jets. II

Supersonic jets of Ar, Kr, N₂, O₂, NO, CO, SO₂, and CO₂ formed by expansion from sonic orifices and conical nozzles into a vacuum were studied with a mass spectrometer, ionization gauge, and free‐stream velocity analyzer. As the source pressure was raised, the dimer, trimer, etc., signals increased until they attained maxima and then declined. Following the sharp rise of the clusters, the monomer velocity increased, indicating that some of the heat of condensation is transferred into translational energy of the remaining monomers. For source conditions where the dimers, trimers, etc., peak, it is postulated that the rate of growth of supercritical nuclei increases at the expense of subcritical clusters, with the consequence of a relative decline in the smaller cluster concentrations. The source pressure, at which the monomer velocity increases and the dimers peak, shifts with d^−q (d = orifice diameter) and $T_0^r$ (T₀=source temperature), where $\text{q≃ 0.55}$ for most gases and $\text{r=2.2± 0.2}$ for argon and $\text{2.7± 0.2}$ for nitrogen. The scaling law implies termolecular kinetics for dimer formation which may be the rate‐determining process for homogeneous condensation in free jets.