Building knowledge of metal clusters atom by atom: Are we bulk-like yet

Multi-collision-induced dissociation mass spectrometry is used to measure heat capacities of aluminum clusters, which are found to undergo phase transitions. Aluminum clusters exhibit strongly size-dependent fluctuations in melting temperature, latent heat, and change in entropy upon melting. The latent heats and changes in entropy associated with the phase transitions are correlated. Structures found via calculations by Aguado et al. demonstrate that large changes in melting temperature between clusters of different size are correlated with changes in the geometric structural family of the clusters. Comparison of the heat capacities of aluminum cluster anions and cations, along with theory, indicate the effect of cluster charge on the melting behavior is greatest at cluster sizes where there is an abrupt change in geometric structure with the addition of one atom. Furthermore, the comparison indicates the electronic structure of a cluster can change upon melting.;Annealing and dissociation energy experiments provide the first experimental data on the freezing rates of metal cluster systems as well as the first experimentally measured heat capacity plots with two well-resolved peaks, the lower temperature peak attributed to an entropically-driven solid-solid transition and the higher temperature attributed to a melting transition. A method of calculating the dissociation energy of the clusters that takes into account the latent heat of melting is derived and validated. Furthermore, these experiments demonstrate a correlation between cohesive energy and latent heat in clusters that melt. Dissociation energy measurements and Monte Carlo simulations of liquid clusters find indications of atomic layering in the liquid metal clusters analogous to surface atomic layering in bulk metals.;The internal energy distribution of Al100+ at its melting temperature are probed using multi-collision-induced dissociation mass spectrometry, indicating the cluster undergoes dynamic phase coexistence. Internal energy distributions of larger clusters are also investigated, but the results are less conclusive. The transition to bulk behavior is discussed.;Temperature-controlled reactivity measurements are performed on aluminum clusters of varying size and charge. Changes in reactivity are found upon melting in large clusters and upon solid-solid transitions in other clusters. The smallest clusters studied do not exhibit changes in cluster reactivity with cluster phase.