Research Projects

Phoenix

    Current research in the Phoenix laboratory involves the combined use of a femtosecond laser facility and a recently constructed tandem time-of-flight mass spectrometer.  Current systems of interest include clusters of SO₂, transition metal oxides, metallocarbohedrenes, and ammonia.  In our experiments, a laser vaporization source is employed in the creation of clusters with various sizes and compositions.  Upon formation, the clusters are directed into the first of two paired TOF mass spectrometers wherein clusters with a particular size and composition are selected and decelerated before entering into the laser interaction region.  Following that arrival, femtosecond laser pulses intersect with the clusters and allow us to perform one of several possible studies.  These experiments can include photodissociation studies, NeNePo and two-color pump-probe investigations, Coulomb explosion imaging, etc.  The products of the laser/cluster interaction are then directed into the second TOF-MS for anaylsis.  The goal of these experiments is to utilize the mass-selectivity of the tandem TOF-MS in conjunction with the versatility of the femtosecond laser system to garner information pertaining to the structure of clusters with known sizes and compositions as well as the dynamics of reactions associated with those systems.

Variable Source Reactor Flow-Tube Cluster Deposition Instrument

    Our studies focus primarily on aluminum clusters, where we attempt to produce small (less than thirty atoms) clusters which satisfy the jellium model and display resistance to oxygen etching. The potential applications for such aluminum clusters include superatom building blocks and alternative fuels. Our current instrumental setup consists of a laser ablation source coupled to a differentially pumped state-of-the-art quadrupole mass spectrometer system. We are currently in the final stages of building a liquid helium-chilled deposition cell which translates into an optical cavity. This will allow us to perform cavity ringdown spectroscopy on clusters suspended in frozen rare gas matrices, thus gaining unprecedented insight into the optical properties of various clusters, their reaction intermediates with oxygen or other etching reactants, and final products.

Insight into the Molecular Level Mechanisms of Heterogeneous Catalytic Reactions

    It is our present goal to uncover possible noble metal and transition metal oxide species responsible for an increased activity and selectivity toward the oxidation of CO and chemical feedstocks such as methanol.  Gas phase clusters may serve as models to probe catalytic reaction steps and to elucidate the electronic characteristics underlying the reactivity of bulk phase catalysts. From these studies, information useful in the design of new catalytic materials may be realized. Utilizing the guided ion beam mass spectrometer, the reactivity of mass selected ionic clusters may be investigated as a function of size, stoichiometry, and ionic charge state.

    Our research is moving toward the study of bimetallic systems with the design of a new disc source. Previously it had been believed that support materials used for heterogeneous catalysis were inert species which did not influence the reactivity of the catalyst. Recent findings are providing clear evidence that charge transfer between Ag-Au bonds in bimetallic nanoparticles can enhance the catalytic activity and provide active sites for effecting oxidation reactions.  We have recently undertaken studies involving mixed metal oxide clusters to probe to effects of catalyst-support interactions and gain a deeper understanding of the principles governing the oxidation of CO and CH₃OH.

Fast Flow Reactor Studies of Heterogeneous Nucleation in Atmospheric Chemistry

    One of our fast flow reactor labs studies environmental chemistry involving ionic water clusters (either H⁺(H₂O)_n or OH^-(H₂O)_n) and their ability to solvate molecules at a temperature range between -140^oC to 120^oC.  Flow tube studies enable us to detect the products of solvation with water clusters; for example, previous studies in our lab have determined the number of water molecules necessary for the uptake of a single acid molecule.  In addition, experiments at different temperatures can be used to determine rate constants for particular chemical reactions.  Very low temperature studies can be used to mimic the conditions of heterogeneous reactions on ice crystals, as in the case of polar stratospheric clouds, which are implicated in the depletion of the ozone layer.  With a fast flow reactor, we can study gas phase reactions taking place at thermal equilibrium.

    Water cluster ions are generated by a discharge ionization of a H₂O/He mixture.  The ions are carried into the flow tube by a continuous flow of 7000 standard cubic centimeters per minute of helium buffer gas.  A predetermined concentration of reactant gas is introduced 30 cm downstream of the source.  The water cluster ions and neutral reactant gas are allowed to react for several milliseconds before a small fraction of reactant and product ions are sampled through an orifice, filtered by a quadrupole mass spectrometer, and detected by an electron multiplier.

Magnetic Bottle Photoelectron Spectroscopy

 The main thrust of our photoelectron spectroscopy experiments is to study clusters that are particularly stable in geometry and electronic structure.  One example of our work involves metal carbide clusters.  Metal carbide clusters show excellent potential for the synthesis of cluster-based materials with unique, tailored properties.  Metallocarbohedrenes (Met-Cars), with stiochiometry M₈C₁₂ with M being one or more of several early transition metals, are of particular interest towards these means due to their enhanced stability.  Metallofullerenes and metal carbide nanocrystals have also been shown as important potential precursors to future nanomaterials.  Thus far, the structures and growth mechanisms for many of these metal carbide clusters are not well understood.  In particular, the electronic structure of metal carbide clusters must be characterized in order to understand their unique properties that differ from both their atomic and bulk counterparts.

    Photoelectron spectroscopy (PES) of mass-selected anions is an experimental technique that is well-suited for studying the electronic properties of clusters in the gas phase.  This technique also has the ability to distinguish between isomers not seen though most mass spectrometric methods.  Anion PES involves a single-photon ionization event, and thus allows one to glean information about the energetics of the neutral cluster ground and lower-excited states.  The magnetic bottle photoelectron spectrometer at use in our lab has been designed to study metal, nonmetal, and mixed metal-nonmetal clusters with high sensitivity.  Our supersonic expansion laser vaporization source creates a large distribution of cluster sizes for study.  A tightly-focused Nd:YAG laser, pulsed at 10 Hz, vaporizes a metal target (a rotating and translating rod) while being subjected to a pulse of gas.  In the plasma created from this event, neutral, cation, and anionic clusters undergo a great deal of collisions and cool when expanded into a high vacuum.  The anionic clusters are extracted from the molecular “packet” by a high voltage pulse.  The negatively-charged clusters are directed down a long flight tube (> 1.5 m) towards both the magnetic bottle and a detector.

   The magnetic bottle is made up of a strong magnetic field (1 Tesla) on the underside and a weak field above (1 milliTesla).  The strong field is from a permanent magnet, and the weak field is due to a solenoid formed by a tube wrapped with magnetic wire.  The wire carries a current of 12 amperes, forming the weak magnetic field.  The mass-selected, decelerated cluster anions are interrogated by a high-energy pulse from an excimer laser (> 4 eV).  Photodetached electrons are collected by the “bottle” and are directed towards the electron detector.  The detected electrons are analyzed based on their binding energy, or the energy of the orbital the electron occupied in the anion.  This is found from the relationship
BE = hν - KE  where BE is the electron binding energy, hn is the photon energy, and KE is the electron kinetic energy.  Because we are detaching electrons from anions, the binding energy is approximately equal to the electron affinity of the neutral.  Electron affinity is one major indictor of stability, as a cluster that is particularly stable will have a low affinity for electrons.

    In addition to quantitatively determining electron affinities, our photoelectron spectroscopy experiments can measure HOMO-LUMO gaps, additional electronic transition energies, and vertical detachment energies.  Coupled with high-level theory, the all-important electronic structures and geometries of clusters can be determined.  With this information, clusters that we discover in the gas phase may be used to build cluster-assembled materials in the future.

LOLA

    The molecular beam apparatus known as “Lola” in the Castleman group is a time-of-flight (TOF) mass spectrometer coupled to a photoelectron imaging array for studying the electronic properties of molecules and clusters.  Clusters are created in a laser vaporization source, where a cylindrical metal rod is ablated by the second harmonic (532 nm) of a Nd:YAG pulsed laser forming a highly energetic plasma of electrons and ionized species.  Concomitantly, while the laser impinges upon the rod, a pulsed jet of reactant gas (CH₄, O₂, NH₃, etc.), seeded in He or Ar, discharges across the plasma region forming clusters of various sizes and charge states.  The emergent clusters, after undergoing a supersonic expansion into vacuum, are skimmed through an orifice forming a molecular beam which passes through a three stage electrostatic grid assembly.  Specifically, only cluster anions are detected in the experiment by employing negatively pulsed electrostatic TOF grids.

    The anion clusters, after being accelerated by the TOF grids are intersected by a linearly polarized laser pulse (532 nm YAG), and providing the photon energy of the laser pulse is high enough, serves to photodetach an electron from the anion cluster.  The photoelectron imaging technique based on the principles of Velocity Map Imaging (VMI) is employed to essentially take digitized pictures, via a charged coupled device camera, of expanding electrons from this photodetachment event.  These electron snapshots can then be converted to photoelectron spectra from which electron affinities, HOMO-LUMO gaps, and vibrational separations (useful for structural analysis) can be measured.  Moreover, VMI can also simultaneously measure the angular distribution of the detached electrons, whereby a quantity called the anisotropy parameter can be calculated.  This anisotropy parameter is useful for inferring the nature of the atomic or molecular orbital the ejected electron came from.

RUTH

     In order to study the dynamics and solvation mechanisms of atmospherically relevant molecules and clusters (such as halogen acids and salts), we utilize a technique which relies on the generation of two laser pulses; a pump pulse and a probe pulse (see Laser System below).  To initiate the reaction (pump), a femtosecond laser pulse in the visible or near-ultraviolet region, yet tuned into the absorption band of a molecular electronic state, is employed.  A short while later (femtosecond to picosecond regime), a harmonic wavelength of the tunable femtosecond laser is used to ionize (probe) the intermediate or product species, which can then be detected through the use of time-of-flight mass spectrometry (see Production/Detection System below).  This technique allows us to follow the dynamics and mechanisms of fast ion-molecule reactions with their environment in real-time.

 Laser System
     The laser pulses are generated by a laser system that begins with a diode-pumped Titanium:sapphire oscillator (Spectra-Physics Millennia Pro coupled to a Spectra-Physics Tsunami 3955).  The 80 MHz beam from the oscillator is then intensified in a Titanium:sapphire amplifier (Coherent Hidra-10-10), which itself is being pumped by a 10 Hz Neodinyum:Yttrium-Aluminum-Garnet laser (Quanta-Ray GCR-150-10). The resulting laser light has a wavelength centered on 800nm (tunable between 720nm and 850nm) at a repetition rate of 10Hz. The laser pulse duration is about 100 fs and the pulse energy is >10.0 mJ.  Generation of the second, third, and fourth harmonics centered on 400, 266, and 200 nm, respectively, greatly enhances the ability to access specific atomic and molecular electronic states.

Production/Detection System
     Production of clusters is accomplished by adiabatic expansion of a gas or liquid sample, either neat or seeded in a carrier gas, through a pulsed valve (Parker-Hannefin General Valve Series 9, 10 Hz).  For the formation of mixed clusters, and to initiate reactions of neutral clusters with reactive gases, a pickup source is employed at the exit of the valve.  Subsequently, the neutral cluster beam is skimmed into a differentially-pumped ionization chamber in which it is crossed by the 10 Hz pulsed laser beam at 90°. The electric field in the ion lens (lab-designed quadrupole-type time-of-flight lens) is perpendicular to both the laser beam and the cluster beam, and it is defined by a back plate coupled to two sets of quadrupole-type cylinders.  The potential at the laser focus is typically 2 kV with respect to ground.  Ions formed by multi-photon absorption are accelerated in an electrostatic field from a positive potential (at the back plate) to ground, traverse the drift tube, are nearly reversed in direction by a reflectron lens system, and are detected by a photomultiplier tube coupled to a micro-channel plate (Burle Bipolar Time-of-Flight Detector).

Arc Discharge Nanoparticle Synthesis

Our current research involves the synthesis of colloidal suspensions of early transition metal carbides (TiC and ZrC) and aluminum nanoparticles.  In the experimental setup an arc discharge between two electrodes generates a plasma, the electrodes are made of either titanium, zirconium, or aluminum.  A mixture of helium and methane flows through the plasma to form the metal carbide nanoparticles, and pure helium flows through the form the aluminum nanoparticles.  The particles are then collected in water, and are analyzed and characterized by various methods.  A few of the analysis methods that are used include transmission electron microscopy (TEM), selected area diffraction pattern (SADP), energy dispersive x-ray spectroscopy (EDS), zeta potential, nanosizer, UV-vis, Raman and IR spectroscopy.