Kinetics of molecular association and relaxation in a pulsed supersonic expansion

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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/CO2 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)≫He. The method and results are discussed.