Heterogeneous chemical kinetics and particle nucleation in interstellar and atmospheric environments
Dust is believed to play a significant role in the evolution of interstellar clouds and hence in processes such as star formation. The physics involved is similar to that responsible for terrestrial aerosols. Certain chemical reactions in interstellar conditions may only occur on the surface of a host particle and are not viable purely in the gas phase. The traditionally used rate equations approach to describe these reactions fail to account for the statistical fluctuations in the reactant populations, which would be significant in situations where the mean population may be well below unity. This can easily occur in interstellar conditions and quite often in reactions catalysed by terrestrial aerosols. This thesis considers a master equation approach that provides a stochastic description of heterogeneous chemical kinetics and demonstrates that classical kinetics may have been overestimating the reaction rates by one order of magnitude under interstellar conditions. The same idea can be extended to study mantle growth on dust surfaces. Traditionally, this is described using a classical description of nucleation kinetics, generally suitable for large systems. Again, this can be unreliable for heterogeneous nucleation taking place on small particles under low vapour concentration where the mean population of adsorbed nucleating species could be of order unity or less. The the-sis explores a stochastic description of heterogeneous nucleation kinetics and solves the arising equations numerically to demonstrate that the stochastic nucleation rate could be significantly different from that derived using the traditional approach. The chemical composition of interstellar dust has for long puzzled experts. The key to determining this lies in an accurate description of the physical processes underlying the formation of these particles. Magnesium oxide is considered to be one of the major candidates as the primary nucleating material, but recently doubts have been cast over this. However, the models employed in reaching that conclusion seem to be rather inaccurate. The thesis attempts to calculate free energies of molecular clusters using newly designed potential models for MgO. It is found that MgO is probably not the primary nucleating dust species in stellar winds.