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Cluster Studies Group

J. Jellinek, M. B. Knickelbein, S. Vajda

Overview

The activities of the Cluster Studies Group encompass studies of a broad variety of physicochemical properties of cluster systems. The immense interest in clusters is fueled by the recognition of the central role they play in many modern technologies and in natural phenomena. The potential of clusters as a means of optimization of existing technologies, developing principally new technological processes, materials, and devices, and in managing natural phenomena, especially those of environmental concern, is truly unparalleled. To make this potential a reality a comprehensive understanding of the properties of clusters as a function of their material(s), size, structure, temperature, etc. is needed. The emphasis in our program is on fundamental studies leading to such an understanding. Its particular strength is that it combines both experimental and theoretical components, which synergistically enhance each other.

Our research focuses on metal clusters because of their particular technological relevance. The elements studied vary from alkali, through transition, to coinage metals and their combinations. The properties investigated include geometrical structures, stability, phases and phase changes, fragmentation mechanisms, electronic features (e.g., ionization potentials, electron affinities), chemical reactivity with a broad variety of molecules, optical and magnetic properties, etc. The cluster features change, in general, with size, and the different properties may be correlated. The central goals include uncovering and understanding the size-dependence of and the correlations between the different features. For example, the chemical reactivity of a cluster of a given metal with a chosen reactant molecule may strongly depend on the cluster size, structure, and temperature. Unraveling the mechanisms through which these latter affect the reaction pathways and rates is crucial for improving, or even fully optimizing, the efficiency and selectivity of real catalysts.

Both the experimental and the theoretical parts of the program represent cutting edge research, which is evidenced by the often unique techniques, approaches and methodologies utilized, and issues and systems studied. The experimental set-ups are based on laser-vaporization cluster sources coupled to continuous or pulsed flow-tube reactors and/or other devices, e.g., an ion trap. The methodologies include time-of-flight mass spectrometry, optical and infrared spectroscopy, photofragmentation measurements, deflection techniques using inhomogeneous magnetic and electric fields, etc. The experimental studies pioneered and perfected in the group include elucidation of cluster structure(s) through their chemical reactivity, separation and quantitative characterization of the thermodynamic and kinetic aspects of cluster-molecule interactions, the effect of temperature on cluster reactivity, electronic spectroscopy of metal clusters through photodissociation spectroscopy of cluster-rare gas complexes, and others.

The theoretical and numerical studies cover a broad range of cluster properties, which include identification of stable and metastable isomeric forms, the dynamics of isomerization transitions, cluster phases and phase changes, multichannel fragmentation, the most detailed to date qualitative and quantitative characterization of cluster-molecule interactions and the mechanisms governing them, structural and electronic properties of cluster-ligand complexes, etc. The theoretical tools used involve first principles-based electronic structure treatments, the density functional theory, many-body semiempirical potentials, large-scale dynamical and statistical simulations, and others. A major part of the theoretical effort is devoted to developing principally new theoretical concepts, methodologies, and techniques, which are especially appropriate and efficient for the small and mesoscale size regime. For example, our group pioneered the theoretical studies of the effects of rotation on the structural, dynamical, and phase properties of clusters. For this a new general scheme for separation of the energy of overall rotation in an arbitrary N-body system has been developed. The scheme has later been adopted and adapted in many different areas outside the cluster field and applied to systems ranging from molecules to continuous media. We have made the first theoretical prediction on the importance of the state of molecular adsorption as a precursor to dissociative adsorption of a molecule on a cluster. The other developments include novel dynamical and statistical analyses schemes, construction of new, more adequate semiempirical potentials, formulating new fitting procedures with a greater degree of insight and control, introducing new notions and concepts for analyses of multicomponent (e.g., alloy) systems, constructing algorithms that combine accuracy with efficiency, etc.

The presence of both the experimental and theoretical components in the program has a truly synergistic effect. Our simulation studies of cluster-molecule reactions were motivated by the ongoing experiments in the group. Our theoretical prediction of the important role of the molecularly adsorbed precursor led to subsequent experiments, which confirmed the prediction. The measurements of the ionization potentials (IPs) and infrared spectra of (Ag)n(C2H4)m systems motivated our density functional studies of these systems. The results of the computations not only confirmed the measured changes in the IPs of the complexes vs those of the corresponding bare Agn clusters, but they also identified the otherwise unknown geometric structure of the dominant species produced in the experiments and pointed out the role of the IP as a possible structural label. Our theoretical studies of bimetallic clusters stimulated a major experimental initiative on these systems within the group.

Our program is alert to changing needs, scientific challenges, and new opportunities. It responds to these by constantly expanding the conceptual, technical, and methodological arsenal and the scope of systems and problems studied. The direction of our evolution can be characterized as "towards more complex systems and phenomena", where the term "complex" is used inclusively to mean larger size clusters, multielement (e.g., alloy) clusters, clusters in confined geometries, cluster-catalyzed reactions, interactions of clusters with electric and magnetic fields, cluster magnetism and its control through chemical means (e.g., ligation), deposited clusters, cluster-assembled designer nanoparticles, nanopores, thin films and, eventually, nanomaterials. The current and planned work involves taking full advantage of the Argonne facilities (e.g., different microscopy probes, Advanced Photon Source) and contributing to the development of planned new facilities such as the Center for Nanoscale Materials. It also involves collaborations both within and outside Argonne. Samples of our recent and current activities are given below.

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