A. Welford Castleman

From Molecules to Materials: Current Trends and Future Directions

The development, characterization, and exploitation of novel materials based on the assembly of molecular components is an exceptionally active and rapidly expanding field. For this reason, the topic of molecule-based materials (MBMs) was chosen as the subject of a workshop sponsored by the Chemical Sciences Division of the United States Department of Energy. The purpose of the workshop was to review and discuss the diverse research trajectories in the field from a chemical perspective, and to focus on the critical elements that are likely to be essential for rapid progress. The MBMs discussed encompass a diverse set of compositions and structures, including clusters, supramolecular assemblies, and assemblies incorporating biomolecule-based components. A full range of potentially interesting materials properties, including electronic, magnetic, optical, structural, mechanical, and chemical characteristics were considered. Key themes of the workshop included synthesis of novel components, structural control, characterization of structure and properties, and the development of underlying principles and models. MBMs, defined as auseful substances prepared from molecules or molecular ions that maintain aspects of the parent molecular frameworko are of special significance because of the capacity for diversity in composition, structure, and properties, both chemical and physical. Key attributes are the ability in MBMs to access the additional dimension of multiple length scales and available structural complexity via organic chemistry synthetic methodologies and the innovative assembly of such diverse components. The interaction among the assembled components can thus lead to unique behavior. A consequence of the complexity is the need for a multiplicity of both existing and new tools for materials synthesis, assembly, characterization, and

Hund’s rule in superatoms with transition metal impurities

The quantum states in metal clusters bunch into supershells with associated orbitals having shapes resembling those in atoms, giving rise to the concept that selected clusters could mimic the characteristics of atoms and be classified as superatoms. Unlike atoms, the superatom orbitals span over multiple atoms and the filling of orbitals does not usually exhibit Hund’s rule seen in atoms. Here, we demonstrate the possibility of enhancing exchange splitting in superatom shells via a composite cluster of a central transition metal and surrounding nearly free electron metal atoms. The transition metal d states hybridize with superatom D states and result in enhanced splitting between the majority and minority sets where the moment and the splitting can be controlled by the nature of the central atom. We demonstrate these findings through studies on TMMgn clusters where TM is a 3d atom. The clusters exhibit Hund’s filling, opening the pathway to superatoms with magnetic shells.

From SiO molecules to silicates in circumstellar space: atomic structures, growth patterns, and optical signatures of SinOm clusters.

SiO is the dominant silicon bearing molecule in the circumstellar medium; however, it agglomerates to form oxygen-rich silicates. Here we present a synergistic effort combining experiments in beams with theoretical investigations to examine mechanisms for this oxygen enrichment. The oxygen enrichment may proceed via two processes, namely, (1) chemically driven compositional separation in (SiO)(n) motifs resulting in oxygen-rich and silicon-rich or pure silicon regions, and (2) reaction between Si(n)O(m) clusters leading to oxygen richer and poorer fragments. While SiO(2) molecules are emitted in selected chemical reactions, they readily oxidize larger Si(n)O(n) clusters in exothermic reactions and are not likely to agglomerate into larger (SiO(2))(n) motifs. Theoretically calculated optical absorption and infrared spectra of Si(n)O(m) clusters exhibit features observed in the extended red emissions and blue luminescence from interstellar medium, indicating that the Si(n)O(m) fragments could be contributing to these spectra.

A new time-of-flight gating method for analyzing kinetic energy release in Coulomb exploded clusters: application to water clusters

Abstract A new method is presented for the analysis of high energy multicharged cations produced by Coulomb explosion in a time-of-flight mass spectrometer. The technique employs energy gating through a manipulation of electric potentials in the Wiley-McLaren lens assembly, and the use of a reflecting electric field. Water clusters of sizes up to about 20 molecules were irradiated with femtosecond laser pulses to generate protons and multicharged oxygen atoms. Protons with energies in excess of 3 kV are reported as well as O4+ ions with energies in excess of 13 kV.

Recent advances in cluster science.

Small clusters are entities comprised of assemblies of atoms or molecules which often display properties that differ from the individual components and the bulk and, hence, are considered a unique state of matter. Investigating ones of differing sizes provides information that serves to bridge states of matter and, as recently shown, cluster research bridges many disciplines of science. This plenary lecture focused on the varying properties of matter of restricted size, the ability to produce clusters that mimic elements of the periodic table and, hence, behave as super-atoms that can serve as building blocks for new nanoscale matter with designed properties. Mass spectrometry has in the past, and continues in the future, to play a central role in this field.

Comparison of methyl and hydroxyl protons generated in a Coulomb explosion event: application of a time-of-flight gating technique to methanol clusters

Abstract A time-of-flight (TOF) mass spectrometry gating technique is applied to a study of methanol clusters subjected to ionizations via intense femtosecond laser pulses. The resulting high charged species (C2+, C3+/O4+) acquire large amounts of kinetic energy resulting from Coulomb repulsion of multicharged atomic ions that reside in close proximity to one another. Protons which are of two kinds, methyl and hydroxyl, also acquire large amounts of kinetic energy. When compared with protons generated from the Coulomb explosion of water clusters ((H2O)n, n≤20), protons from methanol clusters ((CH3OH)n, n≤10) acquire less overall average kinetic energy, which is in agreement with earlier findings that suggest greater clustering yields higher energy. Interestingly, despite the lower average kinetic energy released, the methanol protons peak at a higher value of energy than those generated in the water cluster system, an effect attributed to the presence of both methyl and hydroxyl groups.