Ammonia clusters, sulfur dioxide

Gas phase molecular aggregates that contain acid molecules have been produced with free jet expansion techniques and detected by using electron impact ionization mass spectrometry. The clusters of aqueous nitric acid paralleled many properties of the condensed phase. Multiple nitric acid molecules were found in the clusters that were sufficiently dilute. The acid molecule was absent in the ionized clusters involving HC1 and only water was evident. Experiments also demonstrated the reactivity of ammonia with aqueous nitric acid and sulfur dioxide clusters and of sulfur trioxide with water clusters. The natural occurrence of acid cluster negative ions offers a means to probe the gas phase acid loading of the atmosphere through laboratory and field studies of the ion chemistry. 

With free jet expansion techniques, we have produced clusters of aqueous nitric acid (3 ), hydrochloric acid, sulfuric acid (4, pure acetic acid ( 5), and sulfur dioxide (6). For analogy to buffering, the formation of clusters containing ammonia have also been examined. These have included ammonia with aqueous nitric acid (7 ), hydrogen sulfide (7J), and sulfur dioxide (8). The basic experiment involves expansion of vapor through a nozzle, collima-tion of the jet with a skimmer to form a well-directed molecular beam, and detection of clusters via electron impact ionization and quadrupole mass spectrometry. Some variations include the introduction of a reactive gas into vacuum near the expansion as described elsewhere (4, 8) and the implementation of an electrostatic quadrupolar field to examine the polarity of the neutral clusters. The electric deflection technique is described by Klemperer and coworkers (9). 

When sulfur dioxide is introduced through the nozzle and the amount of ammonia behind the annular opening varies from 6 to 50 torr, the extent of ammonia incorporation into the clusters dramatically increases with increasing ammonia pressure. Up to two ammonia molecules were observed to be incorporated into the clusters with 20 torr of ammonia behind the annular opening. With 40 torr, up to four NH3 molecules were observed in the clusters. 

Model examples will be the study of the competitive clustering of ammonia and water on the ammonium ion and the determination of the characteristics of binding of water, carbon dioxide and sulfur dioxide on the nitrite ion, in relation to gas phase experimental measurements. 

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