Formation of Ammonia Clusters in a Free Molecular Jet of Binary Mixtures

The formation of small ammonia clusters is investigated experimentally with a supersonic free molecular jet of NH3−X binary mixtures (X = Ar, He, N2), with a special attention paid to the effect of second species X on the cluster formation. The ion currents of protonated ammonia clusters (NH3)nH + (n = 2−6) were measured with a combined cluster source system and a double focussing sector field mass spectrometer. Experiments were made both by changing the species concentration at a constant total pressure and by changing the total pressure with a constant composition of each mixture. The NH3 mole fraction in the tested gas mixtures ranges from 10 to 100% (pure ammonia). It is observed that the formation process of ammonia clusters strongly depends on the nature of the carrier gas and its percentage in the gas mixture. The most likely explanation seems to depend on the unique properties of the free jets; the thermal anisotropy, the spatial cooling, and the mass separation effects.