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September 10, 2008
Roman Sinreich (Volkamer Lab)
Differential Optical Absorption Spectroscopy - A Method to Trace Gases in the Atmosphere
Differential Optical Absorption Spectroscopy (DOAS) is a well established technique for the detection of trace gases in the atmosphere. The narrow band absorption features (<5nm width) of trace gases in the light spectra are analyzed by applying a modified version of Lambert-Beer’s law. DOAS is a particular form of absorption spectroscopy that relies only on the relative measurement of light intensities, i.e. only one spectrum is needed to apply Lambert Beer’s law; DOAS overcomes the need of measuring a separate I0 spectrum. Thus, the DOAS technique is inherently self-calibrating as well as contact free and identifies the particular molecules (like NO2, O3, SO2, HCHO, NO3, BrO, CHOCHO etc.) unequivocally by their characteristic absorption structures.
We distinguish active DOAS, where artificial light sources are employed, from passive DOAS, which uses natural light sources like the sun (or the moon). A relatively new application of passive DOAS is the Multi-Axis-(MAX-)DOAS which observes scattered sunlight from a variety of viewing directions. MAX-DOAS enables light-weighed instruments which can be deployed from the ground, aboard a ship or a research aircraft. The MAX-DOAS sensitivity is considerably increased at the instrument’s altitude.
Patrick Veres (NOAA)
Acid Man and Negative-Ion Proton-Transfer Reaction Chemical-Ionization Mass Spectrometry (NI-PTR-CIMS):
It’s all about the air we breathe
Negative-Ion Proton-Transfer Reaction Chemical-Ionization Mass Spectrometry (NI-PTR-CIMS) has been developed for real-time measurement of gas-phase organic acids in the atmosphere. The method is based on the non-dissociative proton transfer reactions of CH3COO- with most of the common organic acids. The potential to detect organic acids (monocarboxylic, dicarboxylic, inorganic, and rare acids) has been shown by a number of laboratory investigations. The sensitivity of the instrument to several organic acids has been observed to be on the order of several ion counts per pptv. Detection limits well below the ppbv level are expected for a 1-second integration time. Various instrumental features including zeroing, time response, calibration and validation of the ion chemistry have been investigated and will be discussed. Various field research opportunities have presented themselves and will be presented to show the utility of this technique.
October 1, 2008
Ice Cream Soical
Come meet and network with graduate students and post docs.
October 8, 2008
Carolynn Chin (Sammakia Lab)
The Blonde, the Brunette, or the Redhead: Selectivity in Cross Metathesis with Polyolefins
Carbon-carbon bond formation is at the heart of synthetic organic chemistry, and new methods to form carbon-carbon bonds are highly sought after. One significant advance in this area that has recently seen a substantial increase in application is olefin metathesis, which is the exchange of ends between two alkenes. Olefin cross metathesis (the intermolecular metathesis reaction involving two different alkenes) is of particular significance because it serves as a convenient route to functionalized and higher olefins from simple alkene precursors. With a focus on polyolefins, recently explored insight into the selectivity of cross metathesis along with its powerful utility in total synthesis will be presented.
Danny Bell (Nesbitt Lab)
Velocity Map Imaging: Taking a Look into Gas-Phase Reaction Dynamics
Imaging techniques, which emerged about 20 years ago in the chemical physics community, allow for mass- and quantum-state specific measurement of both the recoil velocities and the angular distribution of reaction products. Any particle that can be ionized is a candidate for detection, and applications span a wide spectrum of systems, from simple unimolecular photodissociation to polyatom-polyatom gas-phase reactions. More recently, refinements in technology (CCDs) and the advent of the Velocity Map Imaging method have increased sensitivity and velocity resolution (<0.1%) in modern experiments, allowing new insight into the details of how molecules react with each other. Important experimental methods presented include Resonance Enhanced Multi-Photon Ionization (REMPI), electrostatic lensing, Micro-Channel Plate (MCP) detectors and supersonic expansions.
October 22, 2008
Mirvat Abdelhaq (Damrauer Lab)
From Starting Materials to Laser Alignment: The design, synthesis and photophysical characterization of Ruthenium polypyridyls and their applications to electron transfer
Ruthenium Donor-Bridge-Acceptor (D-B-A) assemblies have been designed to investigate photoinduced intramolecular electron transfer. Specifically, looking at how slight structural modifications in the bridge can modulate the rates of electron transfer.
Zebuliah Kramer (Skodje Lab)
Theoretical Methods for the Characterization of Reactive Resonances in Gas Phase Reaction Dynamics
The occurrence of reactive resonances, meta-stable intermediate states that form from quantum effects, significantly influence the mechanism and kinetics of a chemical reaction. Traditionally, in nuclear physics, resonances are often observed as salient changes in scattering observables, e.g. the cross section and differential cross section. However, in chemical reaction dynamics, often these resonance features are irresolvable or ambiguous. No unique resonance observable has been discovered that is common to all relevant chemical systems. In order to establish the presence of reactive resonances and deduce the effects of these resonances on the reaction mechanism, theoretical studies are necessary to compliment experimental results. These theoretical studies provide a picture of the dynamics of the chemical reaction at the transition state. The primary emphasis of this talk will be placed on the theoretical methods used for the characterization of resonances.
November 5, 2008
David Goldstein (George Lab)
The Chemistry of Atomic Layer Deposition
Atomic Layer Deposition (ALD) is a thin-film growth technique offering precise control of film thickness. and the ability to coat high-aspect ratio features such as trenches and nanopowders. Emerging applications for ALD include semiconductor devices, gas sensors, and water-diffusion barriers. The chemistry behind ALD involves understanding how the chemical precursors interact with the surface to deposit the desired material. All ALD precursors need to be stable on the substrate to ensure saturation behavior yet reactive enough to be easily removed with the second reagent. At first, ALD researchers had two main options for precursors: metal halides (e.g. TiCl4 ) and metal alkyls (e.g. AlMe3). Now, life is complicated because many new volatile organometallic compounds for ALD have been synthesized. As the number of precursors increases, proper precursor choice becomes crucial. This is because the film properties, growth rates, and growth temperatures vary widely between the precursors. Many of the above traits can be explained and predicted through knowledge of the precursor reaction mechanisms. In this talk, I explain how I use FT-IR on surfaces to determine the reaction mechanisms of ALD. I will show some representative data on the mechanism for Al2O3 growth by ALD as well as some “tricks of the trade” for IR work.
Michael Cubison (J.-L. Jimenez Lab)
Chemical analysis of atmospheric particles with the Aerodyne Aerosol Mass Spectrometer
The identification of chemical compounds in a gaseous sample through the technique of mass spectrometry was first established in the early 19th century. However, it was not until recent decades that the method was applied for the chemical analysis of particulate matter (PM) in the atmosphere. As the influence of PM remains the least understood impact on anthropogenic climate forcing, and particles are known to cause adverse health effects, the determination of the physical and chemical properties of atmospheric PM is a major thrust of current research within the atmospheric community. The talk will present the engineering and scientific principles behind the operation of one of the most powerful mass spectrometers ever developed for PM analysis. The advantages and disadvantages of the technique will be dicussed, and critically the current limitations and potential future improvements will be explored.
November 12, 2008
Beth Fernandez (Shull Lab)
Observing the Distant Universe
How do astronomers study some of the most distant objects in the universe, and why do we care? I will discuss various methods astonomers use to find the farthest galaxies in the universe - including using the Lyman Alpha line, the Lyman Break Technique, gravitational lensing, and the Near Infrared Background. Then I will discuss what these objects can tell us about themselves and the very early universe.
Katherine Kitney (Jonas Lab)
Introduction to Two-Dimensional Fourier Transform Optical Spectroscopy
This type of nonlinear spectroscopy offers a window onto molecular dynamics with resolution in both time and frequency domains. As in multi-dimensional NMR, there is an advantage in spreading the spectrum into two dimensions. The spectrum is built up from signal fields recorded using spectral interferometry and Fourier transformed. Special attention is paid to the phases of excitation and signal electric fields so that the real and imaginary parts of the spectrum have meaningful physical interpretations. As in other spectroscopies, realistic models are valuable in interpreting the experimental spectra. Approaches to modeling nonlinear signals are outlined
November 19, 2008
Paul Arpin (Kapteyn/Murnane Group)
High Harmonic Generation as a Source of Ultrafast, Coherent Soft X-Rays
Nonlinear optics allows scientists to convert laser light from one color to another. This is a useful technique for applications requiring wavelengths that are not readily available through other lasing techniques, in particular applications extending to very short wavelengths. High harmonic generation can be used to make a laser-like, soft x-ray beam with applications in ultrafast molecular and materials spectroscopy with attosecond time resolution as well as applications in high resolution imaging. In this talk I will discuss the generation process and some of the new phase-matching techniques to improve the conversion efficiency.
Ryan Thalman (Volkamer Lab)
Measurement of Trace Gases by Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS)
Cavity Enhanced Absorption Spectroscopy techniques, Cavity Ring-down (CRDS), Integrated Cavity Output Spectroscopy (ICOS), Incoherent
Broadband Cavity Enhanced Absorption Spectroscopy (IBBCEAS) have been used to resolve path lengths on the order of tens of kilometers to
maximize sensitivity for trace absorbers. Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) combines previous techniques
with the power of the relative retrievals of DOAS fitting to remove the
interferences of aerosols and other broad-band absorbers, leading to a
robust measurement technique for trace gases.
December 3, 2008
Meghan Dunn (Vaida Lab)
Nathan Lemke (Oates Lab)
High-Resolution Spectroscopy of Lattice-Confined Ytterbium Atoms
At NIST we are developing an optical clock based on the 1S0-3P0 transition in neutral Yb atoms. To eliminate frequency shifts related to the motion of the atoms, we use an optical lattice tuned to the “magic wavelength” to tightly confine the atoms during spectroscopy. By comparing the Yb clock with other similar atomic clocks at NIST and JILA, we have measured the frequency of this optical transition with an uncertainty less than .5 Hz, which is a fractional uncertainty below 10-15. In this talk, I will briefly discuss the main components of any optical clock as well as the spectroscopic methods we employ.
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