Mass-Spectrometric Observations of Argon Clusters in Nozzle Beams II. The Kinetics of Dimer Growth

Abstract

Previously reported data on the growth of argon dimer in free‐jet expansions of pure argon are given a simple kinetic interpretation. A third‐order rate constant for recombination, coupled with consideration of the reverse process of dimer dissociation (assuming microscopic reversibility), is chosen for the kinetic description. The temperature–pressure–time history of the expanding gas is based on an ideal isentropic expansion to a terminal Mach number. A stepwise computer solution of the simple differential equation governing dimer growth is carried out with trial temperature‐dependent rate constants. Those rate constants which most nearly reproduce the experimental data are $k_f{\text{\hspace{0.17em}=\hspace{0.17em}0.8\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−30}}{\mathrm{\hspace{0.17em}/\hspace{0.17em}T}}^2$ and $k_f{\text{\hspace{0.17em}=\hspace{0.17em}6.5\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−30}}{\mathrm{\hspace{0.17em}/\hspace{0.17em}T}}^3{\mathrm{cm}}^6{\mathrm{molecules}}^{\text{−2}}·{\sec }^{\text{−1}}$ . The principal conclusions are that the rate constants are strongly temperature dependent, reverse reactions are quite important, dimer continues to be formed throughout the expansion, and the best correlating parameter for dimer growth is $P_0^{2}d$ . An argument is also made that dimers may play a crucial role in homogeneous nucleation by providing a fast, two‐body route for the growth of clusters.