Dimers and Clusters in Free Jets of Argon and Nitric Oxide

Abstract

Molecular cluster abundances of argon and nitric oxide have been measured with a mass spectrometer in free jets with reservoir pressures $p_0$ from 10–10⁴ torr and nozzle diameters $d$ of 0.034 and 0.117 cm, using a phase‐sensitive detection system to eliminate background effects. For a given nozzle diameter, the dimer and larger cluster signals increase with rising reservoir pressure until a peak is reached. The position of the peak scales with $d$ and occurs at a constant factor $p_{0}d$ of 100 ± 10 torr·cm for argon and 78±4 torr·cm for nitric oxide. It is proposed that the peaks of dimer and cluster abundances indicate condensation in the expanding jet. The maximum number of clusters formed depends on nozzle diameter and reservoir temperature. In argon the dimer mole fraction exceeded 0.5, in nitric oxide the maximum detected mole fraction was 0.03. A three‐body dimer formation mechanism is invoked to compare the observed and estimated dimer abundances in an expanding jet.