KINETICS OF MOLECULAR ASSOCIATION AND RELAXATION IN A PULSED SUPERSONIC EXPANSION

The formation of polar Ar/HCN dimers and trimers in supersonic jets has been studied by observing the intensity of rotational transitions in the expanded pulsed jet as a function of input gas composition at a backing pressure of 1 to 2 atm. A programmable gas flow system was employed for accurate control of the composition of the gas mixture supplied to the pulsed nozzle. The formation of Ar-HCN was found to be first order in [Ar] and in [HCN]; (HCN)2 to be second order in [HCN]; (HCN)3 to be second order in [HCN]; and both Ar2-HCN and (HCN)2-Ar to be first order in [Ar] and in [HCN]. The three trimers are formed in two consecutive bimolecular steps at steady state, with (HCN)2 as the intermediate for (HCN)3, and Ar-HCN as the intermediate for both Ar2-HCN and (HCN)2-Ar. Less extensive measurements were made on the HCN/CO, system, in which formation of the T-shaped HCN-CO2 dimer was found to be first order in [HCN] and in [CO2]. The rare gases He, first-run Ne (70% Ne, 30% He), and Ar were used as carrier gases. The carrier gas plays a major role as a third body in the formation steps. The relative effectiveness of the carrier gases as a relaxant in forming the dimers and trimers is Ar > Ne (first run) much greater than He. The method and results are discussed.