Spectroscopy

Raúl Hernández Sánchez's research group is interested in combining supramolecular, inorganic, and materials chemistry to synthesize functional systems that bridge the gap between nanoscale materials and molecular chemistry. Their research is focused on developing new synthetic methodologies to access well-defined nanometer-sized clusters where they can investigate surface structure-function relationships relevant in catalytic and magnetic materials. Other efforts in the Hernández Sánchez (HS) group are aimed at designing and synthesizing structural analogues of carbon nanotubes where exquisite control of the resulting framework allows for properties manipulation.

Students in the HS group will engage in synthetic chemistry and develop familiarity with a range of spectroscopic, electrochemical, crystallographic and magnetic techniques. While rooted in synthetic chemistry, research in the HS group will interface with materials, organic, theory, and physical chemistry.

In the Laaser Lab, we are interested in developing a physical understanding of how changes at the molecular level translate to the macroscopic properties of responsive polymeric materials. For example, how does a change in charge spacing affect the interactions between charged polymers, and at what point do the polymers stop behaving like isolated chains in solution and start behaving like part of a bulk material? How do orientational changes in single polymer chains propagate through a material to achieve macroscopic ordering? And how do polymeric networks transduce force, to achieve things like mechanochemical responses?

We explore these questions by a number of optical and spectroscopic methods, such as light scattering and Raman and infrared spectroscopy, along with classical materials characterization methods like rheology and electron microscopy. Together, these methods allow us to develop new understanding of the structure and dynamic properties of responsive polymeric materials, and offer students the opportunity to gain broad experience in both physical chemistry and polymer science.

An urgent problem in protein science is to understand ion uptake and ion recognition (selectivity) by proteins and polypeptides. This impacts proteins ranging from ion channels to ion sensors to metalloregulatory proteins to metallo-enzymes. Why and how are these bio-nanostructures so exquisitely sensitive to particular ions? How do local changes in the binding pockets lead to global conformational change? These questions have major implications for biology, and they need an answer from physical chemistry. In a separate field, that of green chemistry, there is another pressing problem to understand structural and dynamical heterogeneity in ionic liquids. What does this proposed heterogeneity imply about the solvation properties of these new "designer solvents''? Can we measure it? Can we use it to tune the solvent's properties? Better understanding the chemical physics of these systems will aid the rational design of new solvents, which in turn will have industrial consequences. There is an underlying connection between these biophysical and chemical physics questions – to really probe the mechanisms in operation, one must be able to separate static and dynamic heterogeneity. The nonliear spectroscopies developed in the Garrett-Roe lab can do exactly this. Multidimensional infrared spectroscopy can reveal structural dynamics on timescales spanning femtosecond–microseconds, and make molecular movies as reported by vibrational frequencies. These techniques will clarify

1. The mechanism of ion selectivity in selectivity filter of an archetypal biological ion channel (KcsA).
2. The conformational dance of guest and host in the molecular recognition of ionophores such as valinomycin.
3. The binding of calcium ions in a biological ion sensor, EF hand, in a time- and reside-resolved way. Calcium uptake is a key step of many cellular signalling processes and is unexplored on a submicrosecond timescale.
4. The structural and dynamical heterogeneity of ionic liquids. These fluids are a fascinating arena to move towards a deeper understanding of complex fluids.

Each of these experiments requires a combination of experimental expertise in the non-linear spectroscopy as well as skill modelling stochastic dynamics on a complex free energy landscape.

Carrier dynamics in solid–state materials Fundamental electrical, magnetic, and optical properties of solid–state materials are determined  by the dynamical response of carriers to internal and external fields. The near–equilibrium properties of carriers in most materials are well  understood from classical studies of transport and optical conductivity. However, due to strong interactions of carriers among themselves and with the lattice, studies of nonequlibrium dynamics on femtosecond time scales (10 – 15 s) are just emerging. In our group, a particularly versatile and powerful technique, time–resolved two–photon photoemission (TR–2PP) spectroscopy, has been developed for studying the carrier excitation and relaxation processes in solid–state materials. With this technique we are investigating the quantum mechanical phase and carrier population relaxation times in metals, and for intrinsic and adsorbate induced surface states on metals. Of particular interest are the physical processes that induce e–h pair decoherence, since they impose limits on time scales for quantum control of carriers through the optical phase of the excitation light. The manipulation of the carrier phase with light may lead to applications such as ultrafast (>10 THz) switching and information processing, as well as, atomic manipulation of matter, and therefore, it is of great interest for advanced technologies in the 21st century. Ultrafast microscopy Understanding of the carrier dynamics under quantum confinement is a key to advancing nanoscale science and technology. Although with the existing scanning probe techniques we can potentially study dynamics of individual nanostructures, there is also a clear need for ultrafast imaging microscopic techniques in studies of dynamics in complex systems of nanostructures that could comprise ultrafast electronic or optical device. Photoemission electron microscopy (PEEM) is a well–developed surface science technique for imaging nanostructures on metal and semiconductor surfaces. In combination with femtosecond pump–probe excitation, we intend to develop time–resolved PEEM with potentially <1 fs, <20 nm, <100 meV carrier energy resolution. This technique will be applied to fundamental studies of carrier dynamics in individual nanostructures and coupled nanocomposite systems, in order to understand the fundamental physics of hot carriers in low dimensional systems and to develop advanced device concepts. Atomic manipulation with light Electronic excitation of clean or adsorbate covered metal surfaces can impulsively turn–on large mechanical forces that lead to mass transport parallel or perpendicular to the surface. When such forces are harnessed properly, they can be used for atomic manipulation or even atomic switching. Although there are now several examples of atomic manipulation with STM techniques, much less is known about how equivalent, but much larger scale manipulation could be accomplished with light. The recent observation and demonstration of quantum control of motion of Cs atoms above a Cu(111) surface by our group provides a proof–of–principle for the atomic manipulation of surfaces with light. Such studies are being extended to identify the factors that govern the electronic relaxation of adsorbates on metal surfaces, which can effectively quench the nuclear motion.

Undergraduate and graduate students in the van Stipdonk research group use ion trap mass spectrometry, spectroscopy and theory to study a variety of chemical processes in the gas-phase. As summarized below, our current research projects can be grouped into three general areas: (a) fundamental studies of peptide ion dissociation to support application of tandem mass spectrometry (tandem MS) to peptide and protein identification in proteomics; (b) studies of the intrinsic stability and reactivity of metal ion complexes important to biology, energy production and the environment, and (c) vibrational spectroscopy of gas-phase ions using wavelength-selective infrared multiple-photon photodissociation. Besides extensive use of mass spectrometry and tandem MS, work in our laboratory involves the synthesis of model molecules and peptides, including those with isotope labels, and use of density functional theory (DFT) to predict ion structures, energies and vibrational spectra. Our work on tandem MS and peptide dissociation has been funded by the National Science Foundation (NSF). Studies of intrinsic metal ion chemistry have been supported by the U.S. Department of Energy and the Idaho National Laboratory. Work on ion spectroscopy is supported in part by the NSF, the Institue for Molecules and Materials, Radboud University Nijmegen; and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek. 

We measure several fundamental electronic properties of molecules such as charge transfer and electronic delocalization using a technique known as Stark spectroscopy. Stark spectroscopy involves applying large electric fields to molecules in films or matrices and analyzing the effects of the field perturbation on the absorption or emission spectrum. We have recently focused on the properties of MEH-PPV and other molecules used to make to organic light emitting diodes (OLED's). Stark spectroscopy reveals the mechanism by which a large applied electric field, such as those present in OLEDs, produces undesirable emission quenching and suggests strategies for minimizing this loss of device efficiency through modifications of the polymer structure.

Emission properties of Single Molecules and Aggregates

Recent years has seen a huge growth in the use of organic conjugated oligomers and polymers in the design of light emitting diodes (OLEDs) and photovoltaic cells. When these molecules are placed in thin films to fabricate devices, they typically form aggregates. These confer several desirable properties for device function such as protection from oxidative damage and enhanced charge transport. However, they also typically shift the wavelength of emission to lower energies and often significantly reduce its intensity. Our group is using microscopy and spectroscopy to investigate the relationships between molecular structure and the brightness and photo-stability of molecules in the solid state, both in isolation (i.e. as single molecules) and as aggregates. We are also developing methods to image aggregation in films at high resolution and to follow the nucleation of aggregates in the solution phase.