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The formation of clusters from molecules plays an important role in many industrial and environmental processes: novel material synthesis, catalysis, and atmospheric particle nucleation. Particle formation is often described by the addition and removal of monomers from clusters. The time dependence of the cluster size distribution is then governed by the competition between condensation (forward growth rate) and evaporation (backward growth rate) from the cluster. Classical nucleation theory (CNT) approximates the forward rate with the hard sphere collision rate and unit sticking probability and calculates the backward rate from detailed balance, which assumes the capillarity model to describe cluster free energies. Classical nucleation theory qualitatively predicts experimental trends in particle formation but often fails to quantitatively predict experimental nucleation rates. It is believed that this discrepancy arises from a failure of CNT to adequately describe particle growth rates and free energies for cluster sizes from 1 – 30 monomers. My current research involves the sensitivity of the particle size distribution to parameters that are not taken into account in CNT: cluster-size dependent sticking probabilities, cluster thermodynamics assuming harmonic-oscillators and rigid rotors, and energy accommodation during cluster collisions. These aspects of cluster growth are currently being explored within the context of uni-molecular reaction rate theory and weak collision theory. Rate constants generated from the application of these theories will then be combined with a sectional-discrete particle growth model to generate particle size distributions. These rate constants will also provide a theoretical framework that complements current experimental work involved with measuring cluster evaporation rates. |
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