Raman studies of argon dimers in a supersonic expansion. II. Kinetics of dimer formation

Abstract

Results of a systematic study of the formation of dimers in a supersonic free-jet expansion of argon as studied by Raman scattering are presented. By seeding the expansion with ${\mathrm{N}}_{2}$, measurements of temperature and density were obtained in the part of the flow field where dimers were observed. Temperature was also measured in the remaining part of the expansion showing large deviations from the behavior of an isentropic expansion due to condensation. The condensation was observed directly by Rayleigh scattering as well. From the Raman spectra it was shown that the dimers can relax their internal energy rapidly enough to come into thermal equilibrium with the translational degrees of freedom. By relating the intensity of dimer transitions to that of the simultaneously measured ${\mathrm{N}}_{2}$ $J=0\ensuremath{\rightarrow}2$ Raman transition, we were able to measure the concentration of dimers in the expansion. The measurements were compared to the predictions of equilibrium theory for a chemical equilibrium between dimers and monomers. The predictions of two- and three-body collision dimer-formation models were also tested.