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Overview of Current Research Directions
Exploring the unknown and finding the unexpected is the challenge and excitement that motivates scientific research. It is especially rewarding when the work offers the promise of new knowledge, as well as potential applications. An area in which one can expect to see major advances in the coming decade is research into the behavior of matter of nanoscale dimensions, entities which can display properties unlike those of isolated gas-phase molecules, liquids or solids. Indeed, the realm of small dimensions often brings with it new phenomena, sometimes attributable to unique structures 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 tools involve high technology: -- molecular beams, flow reactors, ultrafast lasers, and sophisticated new mass spectrometer techniques -- while the targets range from atmospheric and environmental science to catalysis, microelectronics, cluster assembled materials and even the interrogation of the effects of solvation and hydrogen bonding on the properties 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 are studied in an unsupported fashion either in a molecular beam or suspended in the carrier gas of a flow reactor. Their bonding, and molecular and optical properties are ascertained using laser spectroscopy, while their reactivities are determined through a variety of techniques including ultrafast (femtosecond time scale) laser pump-probe methods in some cases, and through investigations of their surface reactions using specially designed flow-tube reactor methods in others. Hence, the results also provide insight into the molecular nature of surfaces and extended condensed matter, as well as that of finite size.
Professor Castleman and his group have eight major apparatuses in operation to explore the scientific principles behind the aforementioned phenomena. Currently, particular attention is being directed to studying the formation and properties of a new class of metal-carbon cluster materials discovered in Castleman’s laboratory (termed Met-Cars), and investigating their application in forming cluster assembled materials. Other studies with metal compound clusters are under way to explore the physical basis for catalysis. Research in reactions of water clusters is being conducted to unravel heterogeneous reaction mechanisms of environmental importance. Femtosecond laser techniques are being employed to elucidate the influence of solvation on various classes of reactions, especially those of biochemical significance.
Much of our work involves investigation of cluster dynamics and, in support of the experimental studies, we are also conducting computations on the dynamics and energy exchange involved in the formation and evolution of small particle structures. Quantum mechanical calculations are employed to shed further light on the properties of aggregates of nanoscale dimension. The promises of developing new materials with tailored properties abound.
Cluster research is a new and rapidly growing area in science. A number of problems are being investigated by the Castleman group and a few examples are given here.
Exploring New Concepts in Developing Cluster-Assembled Nanoscale Materials: Met-Cars, Metal and Metal Compound Clusters
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. We are taking a multi-pronged approach to provide new information on their mechanisms of formation, bonding and molecular properties, dynamics and reactivity and even new routes for their synthesis in the solid state. Recent experiments reveal that these clusters grow into multiage structures, adding further excitement to their potential properties and uses. We find that they readily ionize and that molecular aggregates can be formed from a variety of combinations of transition metal atoms. Because of their cage-like structure and delocalized electronic character, they can be expected to function as quantum “particles in a box”. A variety of flow reactor and triple quadrupole mass spectrometer techniques are being employed to investigate their reactivities and potential as catalysts. In order to shed more light on their electronic characteristics and their bonding, we are utilizing laser-induced photodissociation coupled with some sophisticated new ion-beam mass spectrometer techniques. Surprising recent observations show that they can sometimes ionize at very long times after exposure to a source of photons, a process resembling thermionic emission in the solid state.
Gaining Insights into the Physical Basis for Heterogeneous Catalysis
Along the lines of exploring the physical basis for catalysis, the group is engaged in a number of studies of the reactivities of various classes of metal compound clusters of widely varying composition and types, and also of ones to which various species are co-adsorbed. Investigations are underway with transition metal oxides, exploring oxygen transfer reactions with small organic reactions and oxidation-reduction with other molecules such as the oxides of nitrogen. In other studies, we are investigating the evolving structural and electronic properties of a number of metal oxide systems, and conducting laser ionization studies on alkali metals bound to oxides such as MgO, TiO2 and related systems, for example. Alkali metal doping is a common technique to enhance the catalytic behavior of oxide systems and we are exploring the interactions of the electronic energy levels of the metal adsorbate and metal-oxide substrate of the cluster using laser spectroscopic methods. Particularly exciting is the prospect of being able to study metal, non-metal transitions and their influence on the reaction behavior of highly dispersed matter that forms the basis for many industrially important catalytic systems. We are also undertaking investigations 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 new electronic materials.
Elucidating Mechanisms of Heterogeneous Reactions of Atmospheric Significance
Another major thrust in our group is to learn 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. Acid rain is a good example where we have contributed new knowledge to the formation of sulfuric acid and related sulfate-containing aerosols. In recent investigations we have been shedding light on the fundamentals of heterogeneous reactions on ice and water cluster surfaces with attention to problems identified as important in formation of the ozone hole in the polar regions of the stratosphere. In our work we have identified new structures formed among charged species interacting with water molecules that also provide new information on complexes that exist in the condensed phase, such as well-known clathrate species. These provide new insight into intermolecular interactions that stabilize small complexes, and serve to further elucidate the influence that solvation has on hydrogen bonding networks in complexes. A new Ti-saphire ultrafast laser system, as well as a sophisticated flow reactor facility, are being employed to study the course of important reactions related to these various atmospheric phenomena.
Investigations of Reactions in Simulated Functional Groups and Model Base-Pairs of Biological Molecules, and Studies of Solvation Effects on the Dynamics of Chemical Reactions Using Ultrafast Lasers
The vast majority of reactions of practical importance occur in liquids or on surfaces, yet an understanding of such reactions from a molecular point of view is far more rudimentary than the understanding of reactions occurring the gas phase. Again, using clusters, the Castleman group is working to lay a foundation for connecting information from the gas to the condensed phase using a number of different techniques. Several years ago we assembled a colliding pulsed mode-locked laser system that is being used to excite various constituents of clusters with one laser beam, and probe the course of the ensuing reaction with another, all in the femtosecond time domain. Hence, the making and breaking of bonds and the actual time for which a reaction is occurring can be directly observed. Then by tailoring the composition of the clusters and varying the number of solvent molecules, we are able to explore the effects of caging and of the bonding of solvents as they influence the energy surface, and ultimately the course and rate of a reaction. Along these lines we are currently expending considerable effort to learn more about proton and hydrogen atom transfer reactions that are so important in virtually all reactions which occur in aqueous phases including biological systems. Recently we have contributed to elucidating stepwise versus concerted mechanisms in various photochemical processes, with particular attention to the role of solvation.
In related work, we are investigating the interaction of intense laser pulses with matter to determine the effects of ionizing radiation on molecules in general, and clusters in particular. The work bears on basic questions such as the origin of multicharged centers, and the significance of Coulomb explosion and ensuing ion and free radical reactions, and the findings also pertain to problems in radiation biology and even health issues related to radon distribution in the environment. In the context of basic phenomena, we have recently found that high pulse energies of light can strip away all valence electrons from the heavy atoms contained in a molecular cluster, with I+17 being generated in clusters of HI, for example. Recently, we have developed a new technique using Coulomb explosion to arrest intermediates in a chemical reaction. It has been successfully applied in identifying the competition between concerted and stepwise reactions in model DNA base pairs. The techniques also offer promise of being able to explore various classes of charge- and electron-transfer reactions, for example. Chemical reactions that proceed following either a photophysical or ionizing event, are directly influenced by the mechanisms of energy transfer and dissipation away from the primary site of absorption. Neighboring solvent or solute molecules can affect these processes by collisional deactivation (removal of energy), and also through caging effects and solvation effects described above. Research on clusters offers promise of elucidating the molecular details of these processes.
Work is also in progress on the spectroscopy and reactions of small solvated biological function groups, with the objective of learning more about the influence of hydrogen bonding on their properties and reactivity. In addition, we are developing new analytical techniques for sequencing large biological molecules and determining their molecular structures employing these various laser spectroscopy and ionization methods.
More details about our work can be obtained from the group’s selected publications; a complete listing is also available. A comprehensive overview of the field can be found in an invited article published in the Centennial Issue of The Journal of Physical Chemistry, 100, 12911-12944 (1996).
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