Molecular activation by surface coordination model: Insight into heterogeneous atmospheric chemistry through cluster-ion molecule reactions

One specific discipline for which cluster work has particular relevance is atmospheric science. The original application of the fast flow reactor technique employed in this thesis was atmospheric science. Coincidentally, fast flow reactors are readily adaptable to cluster studies and in some cases the optimal methodology. Successive studies of a chemical reaction over a range of pressures using a fast flow reactor can allow the experimentalist to distinguish whether a reaction product is formed by a bimolecular or termolecular mechanism. This type of study applied to the HCl water cluster system revealed two distinct processes for HCl uptake. The bimolecular and termolecular HCl uptake processes, noted along with the specific ratios of water molecules to HCl molecules lead directly to the Molecular Activation by Surface Coordination model for heterogeneous processes involving HCl on water-ice polar stratospheric clouds. The efficiency of the termolecular uptake process indicated a strong interaction with the cluster. Since the association product is only observable on clusters where one HCl has already been incorporated by a bimolecular process, and this bimolecular process is interpreted as ionic dissolution, the logical conclusion is that the efficient association process must involve an ion-induced dipole interaction. The ramifications of this conclusion are that it is possible that the efficient heterogeneous chemical conversion processes which are required to explain polar stratospheric ozone loss, are in part due to the surface coordination of HCl (making it readily available to participate in reactions) and also a chemical activation aspect resulting from an ion-induced dipole. A noteworthy feature of this model is that it ties together the results of many studies of the HCl/ice system, which previously did not appear to be in accord with one another. Further exploration of the HCl protonated water cluster system reveal striking similarities to surface chemistry results and unique insights which are facilitated by the capabilities afforded by the ion-cluster-fast flow reactor methodology employed. The determination that dissolved HCl, and ion-associated HCl will display chloride ion like reactivity also raises questions regarding the chemistry of other halogen species present in the atmosphere.