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The
Castleman Group
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.
Further information
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).
Featured Product:
Microchannel Plates, Detectors
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Examples of research applications:
Studies of the atomic clusters at the
University of Virginia -
Amber Post
Featured MCP customer:
The Castleman Group at PSU
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