Colleen M. Neal

Improved signal stability from a laser vaporization source with a liquid metal target

The translating and rotating rod or disk of a conventional laser vaporization cluster source is replaced by a liquid metal target. The self-regenerating liquid surface prevents cavities from being bored into the sample by laser ablation. The laser beam strikes a near pristine surface with each pulse, resulting in signals with much better short and long term stabilities. While this approach cannot be used for refractory metals such as tungsten and molybdenum, it is ideal for studies of bimetallic clusters, which can easily be prepared by laser vaporization of a liquid metal alloy.

Substituting a copper atom modifies the melting of aluminum clusters

Heat capacities have been measured for Al(n-1)Cu(-) clusters (n=49-62) and compared with results for pure Al(n) (+) clusters. Al(n-1)Cu(-) and Al(n) (+) have the same number of atoms and the same number of valence electrons (excluding the copper d electrons). Both clusters show peaks in their heat capacities that can be attributed to melting transitions; however, substitution of an aluminum atom by a copper atom causes significant changes in the melting behavior. The sharp drop in the melting temperature that occurs between n=55 and 56 for pure aluminum clusters does not occur for the Al(n-1)Cu(-) analogs. First-principles density-functional theory has been used to locate the global minimum energy structures of the doped clusters. The results show that the copper atom substitutes for an interior aluminum atom, preferably one with a local face-centered-cubic environment. Substitution does not substantially change the electronic or geometric structures of the host cluster unless there are several Al(n) (+) isomers close to the ground state. The main structural effect is a contraction of the bond lengths around the copper impurity, which induces both a contraction of the whole cluster and a stress redistribution between the Al-Al bonds. The size dependence of the substitution energy is correlated with the change in the latent heat of melting on substitution.

Metal clusters that freeze into high energy geometries.

Heat capacities measured for isolated aluminum clusters show peaks due to melting. For some clusters with around 60 and 80 atoms there is a dip in the heat capacities at a slightly lower temperature than the peak. The dips have been attributed to structural transitions. Here we report studies where the clusters are annealed before the heat capacity is measured. The dips disappear for some clusters, but in many cases they persist, even when the clusters are annealed to well above their melting temperature. This indicates that the dips do not result from badly formed clusters generated during cluster growth, as originally suggested. We develop a simple kinetic model of melting and freezing in a system consisting of one liquidlike and two solidlike states with different melting temperatures and latent heats. Using this model we are able to reproduce the experimental results including the dependence on the annealing conditions. The dips result from freezing into a high energy geometry and then annealing into the thermodynamically preferred solid. The thermodynamically preferred solid has the higher freezing temperature. However, the liquid can bypass freezing into the thermodynamically preferred solid (at high cooling rates) if the higher energy geometry has a larger freezing rate.