A. W. Castleman Jr.

Matter of Nanoscale Dimensions

The realm of small dimensions often brings with it new phenomena, sometimes attributable to structure and bonding, while in other cases due to what is commonly called quantum confinement. The Castleman group is striving to bring new understanding to this challenging and important subject by employing the tools and principles from chemical physics to bridge an understanding and develop applications in a number of areas of modern chemical science. The methods employ high technology-molecular beams, flow reactors, ultrafast lasers, and sophisticated new mass spectrometer techniques. The targets are molecular complexes of significance in fields ranging from atmospheric and environmental science to catalysis, microelectronics, cluster assembled nanoscale materials, and hydrogen bonded complexes of biological molecules. Clusters are the media through which the explorations take place.

 Professor Castleman and his students have devised numerous different schemes for producing weakly bound aggregates comprised of molecules, atoms, and/or ions of desired composition and size that can be subjected to detailed investigation. In order to determine the inherent properties and reactivity of these nanoscale systems, they typically study these in an unsupported fashion, either in a molecular beam or suspended in the carrier gas of a flow reactor. The bonding and molecular and optical properties of the cluster systems are ascertained using laser spectroscopy, while their reactivities are determined through a variety of techniques including femtosecond time-resolved laser pump-probe methods in some cases, and through investigations of their surface reactions using specially designed flow-tube reactor methods in others.

 A few years ago, Professor Castleman and his students discovered a new class of molecular clusters termed Metallo-Carbohedrenes or Met-Cars for short. Because of their potential use as new electronic and optical materials, as well as possible value as new catalysts, they have attracted wide interest in the chemistry community. Work is under way to investigate their molecular properties, reactivity, and routes for their synthesis in the solid state.  This aspect of the work has been extended to developing superatoms as mimics of elements that can lead to a 3-D periodic table comprised of unique clusters that can serve as building blocks of new nanoscale materials.

 Along the lines of exploring the physical basis for catalysis, the group is also engaged in a number of studies of the reactivities of metal compound clusters of widely varying composition and types, with particular attention given to oxygen transfer reactions. Investigations are also under way to learn how the small cluster building blocks lead to different morphologies of growing particles that are of interest in wide-ranging areas from photocatalysis to developing new cluster assembled nanoscale materials.

 The vast majority of reactions of practical importance occur in liquids or on surfaces, yet from a molecular point of view they are far less well understood than reactions occurring in the gas phase. The Castleman group is working to lay a foundation for connecting information from the gas to the condensed phase through clusters. In this work, ultrafast lasers are used to excite various constituents of clusters with one laser beam, and probe the course of the ensuing reactions with a second one, all in the femtosecond time domain, thereby enabling actual observation of the making and breaking of bonds. The group has developed a unique method for interrupting and interrogating evolving intermediates in fast reactions using a novel Coulomb explosion technique. Work is in progress on studies of the spectroscopy and reactions of small solvated molecular functional groups, with the objective of learning more about the influence of varying degrees of solvation on their properties and reactivity. In addition, work is under way to develop new analytical techniques for selectively ionizing and sequencing large biological molecules, determining their molecular structures, and investigating the effects of ionizing radiation on matter.

 Another major thrust is learning more about atmospheric chemistry through cluster research. It is well recognized that small aerosol particles, as well as ice crystals and cloud droplets, play an important role in the conversion of many atmospheric molecules. In recent investigations, the group has shed light on the fundamentals of heterogeneous reactions occurring on ice and water cluster surfaces, with attention to problems identified as important in the ozone hole observed in the polar regions of the stratosphere.

 Much of the experimental work involves investigations of cluster dynamics and structures, and related computations into the properties of aggregates of nanoscale dimensions are made in support of the experiments. The promises of developing new materials with tailored properties abound.