Studies of Nanometer Scale Metal Cluster Dissociation in the Gas Phase

Professors Christopher Hogan, Steven Girshick, and Donald Truhlar of the University of Minnesota-Twin Cities are receiving an award from the Macromolecular, Supramolecular and Nanochemistry Program to better understand metal cluster formation and growth. The team is examining the properties of nanometer scale clusters (from as small as 6 atoms up to 1000 atoms) in the gas-phase by a combined experimental and computational approach. Specifically, the dissociation of metal clusters, Mn --> Mn-1 + M (where M is a metal and n is the number of atoms in a metal cluster) is monitored experimentally using tandem differential mobility analysis. These experiments are the first ever measurements of single atom evaporation from gas phase neutral, cationic, and anionic metal clusters at atmospheric pressure. Simultaneously, the dissociation rates of metal clusters are inferred from molecular dynamics trajectory simulations allowing for direct comparison of experimental measurements and computational predictions. Of particular interest in measurements and computations is the identification of anomalous magic number and anti-magic number clusters, which are either anomalously stable or unstable, respectively. The existence of these magic number clusters vastly alters the rate of nucleation for gas phase metal clusters. The team of researchers infers the rate of cluster formation and growth from the measured and computed rates of dissociation for metal clusters, enabling comparison of cluster growth rate predictions to experimental measurements in a newly designed turbulent mixing metal cluster growth reactor.

These experiments and simulations elucidate the behavior of metallic clusters, thus providing the basis for the design of future synthesis systems for multi component metallic materials with nanoscale features. This area of research has technological applications in environmental and materials sciences, e. g., waste incineration and coal burning exhausts. This project is interdisciplinary in nature, bridging the disciplines of physical chemistry and mechanical engineering. It is unique in that it incorporates both newly developed experimental techniques and cutting-edge molecular simulations, where overlap has thus far been sparse. Moreover, these studies are part of a vital effort to link the properties of materials at the molecular scale to properties of bulk materials. Additional broader impacts of these studies include the interdisciplinary education of graduate students and postdoctoral researchers involved in the project, including the development of new interdisciplinary courses focusing on the properties of nanometer scale clusters from both the chemist's and engineer's perspective.