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Excess Thermal Energy and Latent Heat in Nanocluster Collisional Growth

Nanoclusters can form and grow by nanocluster-monomer (condensation) and nanocluster-nanocluster (coagulation) collisions. During growth, product nanoclusters have elevated thermal energies due to potential and thermal energy exchange following a collision. Even though nanocluster collisional heating may be significant and strongly-size dependent, no prior theory describes such phenomenon. We derive a model to describe the excess thermal energy, the kinetic energy increase of the product cluster, and latent heat, the heat released to the background upon thermalization of the non-equilibrium cluster, of collisional growth. Both quantities are composed of an enthalpic term, related to potential energy minimum differences, and a size-dependent entropic term, which hinges upon heat capacity and energy partitioning. Example calculations using gold nanoclusters demonstrate that collisional heating can be important and strongly size dependent, particularly for reactive collisions involving nanoclusters composed of 14-20 atoms. Excessive latent heat release may have considerable implications in cluster formation and growth.