Bulletin of the American Physical Society
78th Annual Meeting of the Southeastern Section of the APS
Volume 56, Number 9
Wednesday–Saturday, October 19–22, 2011; Roanoke, Virginia
Session DC: Atomic and Molecular Physics |
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Chair: Leo Piilonen, Virginia Polytechnic Institute and State University Room: Crystal Ballroom C |
Thursday, October 20, 2011 1:30PM - 1:42PM |
DC.00001: Laser Photodetachment Spectroscopy of the S$_{2}^{-}$ Ion John Yukich, Wade Morgan Numerous experiments have investigated the properties and dynamics of single-atom negative ions. Similar experiments can be conducted with molecular negative ions. Laser photodetachment spectroscopy of such ions is more complicated due to rotational and vibrational structure, and often yields spectroscopic benchmarks such as rotational constants. We have conducted low-resolution photodetachment spectroscopy of the S$_{2}^{-}$ ion over a range of roughly 2000 cm$^{-1}$. The ions are created in a Penning ion trap by a two-step dissociative attachment process. The photodetachment is achieved with a tunable ring-cavity titanium:sapphire laser. Our results yield a lower-limit estimate of the minimum detachment threshold energy and exhibit structure that may be due to rotational energy levels. Future experiments will focus on high-resolution detachment spectroscopy of these and other ions with an eye toward measurement of their molecular constants. [Preview Abstract] |
Thursday, October 20, 2011 1:42PM - 1:54PM |
DC.00002: Identification and Analysis of Atomic and Molecular Superposition Spectra Following Laser-Induced Optical Breakdown Alexander C. Woods, Christian G. Parigger Molecular recombination and excitation of atoms following laser-induced optical breakdown provide means for simultaneous detection of atomic and molecular species. Atomic emission spectra may be analyzed to infer electron number and temperature. Careful analysis of select atomic spectra may reveal superposed diatomic molecular spectra. Nonlinear fitting of synthetic molecular spectra, calculated via diatomic quantum theory, provides tools for identification, temperature measurement, and further analyses of the diatomic molecules present. This presentation investigates the presence of C$_{2}$ molecular Swan bands in Balmer Series atomic hydrogen spectra. Combustion plumes are also studied, including comparisons of temperatures obtained using a two-color pyrometer and from data reduction analysis of measured spectroscopic AlO data. [Preview Abstract] |
Thursday, October 20, 2011 1:54PM - 2:06PM |
DC.00003: Highly parallelized detection of single fluorescent molecules: simulation and experiment Brian K. Canfield, Jason K. King, William N. Robinson, William H. Hofmeister, Lloyd M. Davis We are developing an ultrasensitive, fluorescence-based detection system in highly parallel microchannels. Multichannel microfluidic devices have been fabricated by direct femtosecond laser machining of fused silica substrates. We approach single-molecule detection sensitivity by introducing dilute aqueous solutions ($\sim $ pM) of fluorescently labeled molecules into the microchannels. In a custom-built, wide-field microscope, a line-generating red diode laser provides narrow epi-illumination across a 500 $\mu $m field of view. Fluorescence is detected with an electron-multiplying CCD camera allowing readout rates of several kHz. Rapid initial assessment is performed through digital filtering derived from simulations based on experimental parameters. Good agreement has been shown between simulation and experimental data. Fluorescence correlation spectroscopy then provides more detailed analysis of each separate channel. Following optimization, microfluidic devices could easily be mass-produced in low-cost polymers using imprint lithography. [Preview Abstract] |
Thursday, October 20, 2011 2:06PM - 2:18PM |
DC.00004: Terahertz Rotational Spectroscopy of the v5/2v9 Dyad of Nitric Acid Paul Helminger, Douglas T. Petkie, Ivan Medvedev, Frank C. De Lucia Our studies of the terahertz rotational spectrum of nitric acid now include the ground state and the four lowest excited states. We report good progress in the assignment and analysis of the next higher energy states, the v5/2v9 interacting states. This very complex spectrum includes torsional splitting of both states and Fermi and Coriolis type interactions between them. The current analysis includes both microwave and infrared transitions for improved stability. Microwave studies of the rotational spectrum of the nitric acid molecule in the ground and excited vibration states contribute both to a better understanding of this fundamental molecule and to the construction of accurate spectral maps for remote sensing in the atmosphere. [Preview Abstract] |
Thursday, October 20, 2011 2:18PM - 2:30PM |
DC.00005: Microfluidic device for three-dimensional electrokinetic manipulation of single fluorescent molecules Jason K. King, Brian K. Canfield, Lloyd M. Davis, William H. Hofmeister The ability to manipulate and trap single molecules in solution through the application of actively controlled electric fields is a valuable tool for a number of bio-molecular studies of proteins and nucleic acids. Here we report the development of a microfluidic device consisting of four electrodes sputtered onto two glass coverslips and fixed in a tetrahedral arrangement. This geometrical configuration allows for a uniform electric field of any orientation through the application of appropriate voltages. Three-axis control has been demonstrated for micron-sized polystyrene beads and 40 nm fluorescent spheres in phosphate buffered solution. Previous work has characterized planar motion. Recent changes to the experimental setup include the addition of a cylindrical lens in the detection arm to quantify axial position and a National Instruments PCI-7833R to provide precise voltage control. Finally, a real-time tracking algorithm and its use for trapping will be discussed. [Preview Abstract] |
Thursday, October 20, 2011 2:30PM - 2:42PM |
DC.00006: Impact of Recent Laboratory N$_2$ Data to our Understanding of Thermospheric Nitric Oxide (NO) Justin Yonker, Karthik Venkataramani, Scott Bailey In spite of its status as a minor species, NO plays key roles in many upper atmospheric processes. As the only heteronuclear molecule, its fundamental, $\Delta$v=1 emission cools the thermosphere (z$>$100 km). Its low ionization potential ensures that NO$^+$ is the end product of the ion-neutral chemistry in the ionospheric E-region. And in the presence of excess atomic oxygen, NO will catalytically destroy ozone. The production of NO is initiated when N$_2$ is ionized, dissociated, or excited by the solar EUV irradiance ($\lambda <$100 nm). In the mesosphere and lower thermosphere (MLT), much of the irradiance is contained in the highly variable soft x-ray region (1$<\lambda <$20 nm). The resulting photoelectrons produce additional ionization as well as excitation of metastable, chemically-reactive species like the first electronically excited N$_2$ state, N$_2$(A$^3\Sigma_u^+$). This talk will incorporate recent laboratory data on the N$_2$ photoabsorption and electron-impact cross-sections into a 1D photochemical reaction-diffusion model of the thermosphere. It is shown that spin-forbidden ($\Delta$S=1) excitation to the N$_2$ triplet manifold enables neutral N$_2$ to participate in the NO production. Additional physical and chemical uncertainties relevant to NO production and loss are also presented. [Preview Abstract] |
Thursday, October 20, 2011 2:42PM - 2:54PM |
DC.00007: Quasibound States of Single-Particle Systems Curt Moyer We have developed a formalism that describes both quasibound and resonant states within the same theoretical framework, and that admits a clean and unambiguous distinction between these states and the states of the embedding continuum. The approach described here builds on our earlier work by clarifying several crucial points and extending the theory to encompass a variety of continuous spectra, including those with degenerate energy levels. The result is a comprehensive and compelling formalism for the study of quasibound states. The difference between `quasibound' and `resonant' states turns out to be largely semantic, inasmuch as both arise from imposing what is arguably the same mathematical rule (a point condition in a novel basis set). Enforcing that rule in a given application is straightforward in principle. The formalism is illustrated by examining several cases pertinent to applications widely discussed in the literature. [Preview Abstract] |
Thursday, October 20, 2011 2:54PM - 3:06PM |
DC.00008: Three-dimensional flow measurements with a four-focus microscope James A. Germann, Brian K. Canfield, Jason K. King, Alexander Terekhov, Lloyd M. Davis The measurement of a one-dimensional flow using a confocal fluorescence microscope with two excitation volumes has been well documented. This technique can be extended to measure flow in all three dimensions simultaneously through a four-focus, two-photon microscope. To this end, an apparatus has been constructed in which the beam from a modelocked Ti-Sapphire laser is passed through a double interferometer configuration to create four displaced focal volumes. Fluorescence is gathered onto a single photon avalanche diode and time-gated by a TimeHarp 200 timer card. Calibration of one-dimensional flow through a square bore capillary has been performed. Flow of adjustable speed and direction in three dimensions is created using a cross-channel microfluidic device. To evaluate flow measurements, Monte Carlo simulations of fluorescence cross-correlation spectroscopy between the four foci were conducted and a LabView program was created to discern the flow parameters from the 16 cross-correlation functions. For simplicity, the model for the correlation functions assumes each focal volume is a three-dimensional Gaussian, but a Gaussian-Lorentzian model may improve fitting. [Preview Abstract] |
Thursday, October 20, 2011 3:06PM - 3:18PM |
DC.00009: Rapid fabrication of long nanochannels with a single femtosecond laser pulse focused to a line Lloyd M. Davis, Alexander Terekhov, Kathleen Lansford, Joshua W. Bradfield, Charles A. Rohde, M. Cather Simpson, Bryon E. Wright We have recently reported the use of tight line-focusing of an amplified femtosecond laser beam to fabricate very long, sub-micron wide features in glass with just a single laser pulse [Davis et al., IQEC/CLEO Pacific Rim, August 2011]. The optical configuration used in these experiments presents distinct advantages and can be expected to have numerous applications, including the rapid creation of micro/nano-fluidic devices and waveguides. Here we review that work and also discuss recent results on imaging features created at the surface or at various depths internal to a substrate using a number of methods, including SEM images of acetate replicas, atomic force microscope, and optical imaging of sections that show the depths of internal features. We also discuss the physical mechanisms that can occur during femtosecond laser-induced plasma formation under different conditions, while emphasizing the non-linear mechanisms that can produce sub-diffraction features and the use of aberrations and spatio-temporal focusing to control the feature depth. [Preview Abstract] |
Thursday, October 20, 2011 3:18PM - 3:30PM |
DC.00010: Application of X-ray Fluorescence Spectroscopy in Analysis of Oil Paint Pigments Cassandra Major, Sarah Formica X-ray Fluorescence (XRF) spectroscopy is a rapid, noninvasive technique for both detecting and identifying chemical elements within a given sample. At North Georgia College and State University, a sealed tube x-ray source and slightly focusing polycapillary optic are used in nondestructive XRF analysis of oil paint pigments. Oil paints contain both organic and inorganic matter, and the inorganic ingredients such as titanium, vanadium, iron, zinc, and other elements are easily detected by XRF, which can be used to uniquely differentiate between various paint pigments. To calibrate the XRF system for paint color identification, six different colors of oil paint were fluoresced and identified based off of their characteristic spectra. By scanning the paint sample in two dimensions, the characteristic XRF spectra obtained were compiled to produce an XRF replica of the painting. [Preview Abstract] |
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