Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T3: Spectroscopy and Structure |
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Sponsoring Units: DCP Chair: David Chandler, Sandia National Laboratories Room: 107 |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T3.00001: Analysis of new and aged energetic residues using CO$_{2}$ enhanced Laser Induced Breakdown Spectroscopy Jorge Martinez, Charlemagne Akpovo, Staci Brown, Cleon Barnett, Dawn Lewis, Lewis Johnson CO$_{2}$ enhanced LIBS plasmas have several positive attributes such as longer plasma lifetimes and excellent ionic/neutral/molecular emissions relative to the plasma heating and cooling processes. In this experiment, a study of decay constants as related to enhanced CO$_{2}$ plasmas for various elemental and molecular elemental features are studied for frozen (recently obtained) and room temperature stored (3 year old) energetic residues. The difference between these aged residues and new residues will provide insights into the types of elemental profiles significant to energetic detection in both ambient atmosphere and real world environments. 10 milli-Joule nanosecond and femtosecond pulses were combined with 3 J defocused CO$_{2}$ pulses. Four spectrometers were utilized (2 broadband, 2 high resolution) to acquire spectra at 1.5 and 10 microseconds after plasma initiation. The samples consisted of 10 mg/ml concentrations of DNT, TNT, and RDX allowed to dry on aluminum and silicon substrates. Differences in decay of nitrogen and hydrogen emissions as a function of time were observed in aged vs fresh TNT samples. DNT aged and fresh decay constants for ionic and neutral species showed good agreement. RDX (fresh, aged) and TNT (aged) displayed reduced emissions of molecular species of the C$_{2}$ swan band and violet band CN as compared to DNT (fresh, aged) and TNT (fresh). [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T3.00002: Measurement and Analysis of CN Violet System in Laser-Induced Plasma Sultan A. Behery, Christian G. Parigger Pulsed, infrared Nd:YAG laser radiation is utilized to ablate material from carbon-containing samples in air. Time-resolved measurements of the micro-plasma show well-developed diatomic spectra of the CN violet system. Of Interest are interferences from the C$_2$ Deslandres D'Azambuja system in the CN spectra, as previously noted in experiments with CO$_2$ laser radiation focused into CO$_2$ gas expanding into air. The recombination emission spectra from diatomic species, e.g., CN or C$_2$, clearly indicate temperatures in excess of 6000 Kelvin. Studies of the CO$_2$ TEA laser-induced micro-plasmas show these highly excited, high-temperature molecular transitions several tens of microseconds after plasma generation, mixed with signatures of Stark-broadened atomic lines. Spectroscopic fitting with accurate molecular line strengths of superposed emission spectra is of current interest, including study of the C$_2$ Deslandres D'Azambuja system near the 4-4 band of the CN $\Delta$v = 0 sequence of the CN B$^2\Sigma^+ \rightarrow$ X$^2\Sigma^+$ Violet System. In addition, discussed are physics phenomena associated with laser-induced optical breakdown. Laser-induced plasma applications include characterization of carbon and nitrogen containing materials. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T3.00003: Spectroscopic Temperature and Number Density of Nitric Oxide in Laser-Induced Plasma Josef P. Fleischmann, Lauren D. Swafford, Michael J. Witte, David M. Surmick, Alexander C. Woods, Sultan A. Behery, Christian G. Parigger, James O. Hornkohl We report measurements of nitric oxide emission spectra subsequent to infra-red Nd:YAG laser-induced breakdown in air. Plasma is generated by focusing 160 mJ energy per pulse, 13 ns pulse-width, laser radiation at a wavelength of 1064 nm. The NO emissions are recorded for time delays of 25, 50, and 75 $\mu$s after plasma generation, utilizing a 0.64 meter Czerny-Turner type spectrometer with a 3600 grooves/mm grating, and an intensified linear diode array. The analysis utilizes accurate line strengths for selected bands in the ultraviolet region of 205 to 300 nm. Temperatures on the order of 6000 to 7000 Kelvin are inferred from the emission spectra. Comparisons are included with previous experimental studies in 1:1 mixture of N$_2$:O$_2$, where we deduced temperature and species densities using plasma predictions for various conditions and a so-called non-equilibrium air radiation code. The current work elaborates on details of two specific NO bands to evaluate as well accuracy of our line strength data. While the presented spectra, recorded in laser-induced plasma in air, are due to recombination processes following optical breakdown, results of our work on diatomic nitric oxide emissions are expected to be also applicable in chemical physics investigations of combustion. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T3.00004: Time-Resolved Aluminum Monoxide Emission Measurements in Laser-Induced Plasma David Surmick, Christian Parigger Laser-induced plasmas are useful for diagnostic applications in a wide variety of fields. One application is the creation of laser-induced plasmas on the surface of an aluminum sample to simulate an aluminized flame. In this study, aluminum monoxide emissions are measured to characterize the temperature along the laser-induced plasma as a function of time delay following laser-induced optical breakdown. The breakdown event is achieved by focusing 1064 nanometer laser radiation from an Nd:YAG laser onto the surface of an aluminum sample. Light from the plasma is dispersed with the use of a Czerny-Turner spectrograph, and time resolved emission spectra are recorded with an intensified, gated detector. Temperatures are inferred from the diatomic molecular emissions by fitting the experimentally collected to theoretically calculated spectra using a Nelder-Mead algorithm. For computation of synthetic spectra we utilize accurate line strengths for selected AlO molecular bands. Atomic emissions from aluminum are also investigated in our study of laser-induced plasma. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T3.00005: Atomic hydrogen measurements in laser-induced plasma Lauren Swafford, Christian Parigger New temporally and spatially resolved experimental results are presented for laser-induced plasma evolution in laboratory air. The measurements of hydrogen alpha and beta Balmer series line shapes are analyzed using various theory results. Plasma is generated using a typical laser-induced breakdown spectroscopy arrangement that employs focused, Q-switched Nd:YAG laser radiation at the fundamental wavelength of 1064 nm. Stark-broadened emission profiles for hydrogen alpha and beta allow us to determine electron density and temperature. Electron density is primarily inferred from Stark-broadening of experimental records for various time delays from plasma generation. Boltzmann plots are used to infer the electron temperature for well-defined Balmer series lines. Of particular interest is diagnostics of electron density from the asymmetric H beta line shape. The correlation of the hydrogen beta line shape asymmetry and of the full width at half maximum is explored. H alpha and H beta lines emerge only for time delays on the order of 0.5 $\mu $s and 2 $\mu $s, respectively. For earlier time delays we infer electron density from nitrogen emission lines. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T3.00006: Measurement and Analysis of Carbon Swan Emissions using Laser Induced Breakdown Spectroscopy Michael Witte, Christian Parigger Carbon Swan emissions are frequently noticeable in the recorded spectra of laser-generated plasma, for example, at or near biological materials, hydrocarbons and/or during laser ablation of carbon-containing substances. Therefore, it is desirable to accurately model C$_2$ diatomic molecular spectra. Temporally-resolved spectroscopy allows us to explore highly excited carbon Swan spectra, and in turn, we can utilize rotational and vibrational molecular spectra to characterize the laser plasma. In this work, C$_2$ is examined for nanosecond to microsecond time delays from optical breakdown, and for the $\Delta v = +2, +1, 0,$ and $ -1$ transitions. In previous experiments, line-strengths were used to determine vibrational and rotational temperature when assuming local thermodynamic equilibrium. We report new experimental results by exploring the temporal and spatial evolution and decay of laser-plasma generated by focusing 13 nanosecond, 190 mJ energy/pulse Nd:YAG laser radiation onto a carbon containing material, and subsequently dispersing and recording the emitted radiation using a spectrometer and a 2-dimensional gated, array detector. The computed line-strengths for the C$_2$ Swan system are employed as well in our analysis and fitting of the new experimental results. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T3.00007: Molecular Spectroscopy of TiO in Laser-Induced Plasma Alexander C. Woods, Christian G. Parigger Potential energy curves can be calculated for many diatomic molecules due to the symmetries and availability of experimental data for the spectral transitions of diatomic molecules. With accurate potential energy curves for diatomic molecules, line strengths can be determined for allowed spectral transitions. Combined with parameters such as temperature and resolution, line strengths allow us to create the molecular spectra. This investigation explores the fitting of computed spectra for selected titanium monoxide (TiO) molecular transitions to measured spectra collected at various times following the generation of laser-induced plasma. Using gated detection, spectral data is gathered during laser ablation of a titanium sample at rest in laboratory air. A Nelder-Mead fitting routine is applied to infer the temperature of the spectral transitions in the plasma. The result is a temperature versus time profile of the transitions of the TiO molecule along the plume. The error associated with each inference is determined by randomly adjusting the spectral baseline, as the measured spectrum is repeatedly fit. Atomic lines, which dominate the early spectra of laser-induced plasma, are also addressed. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T3.00008: Low-light-level Ladder-type Electromagnetically Induced Transparency and Two-photon Absorption Zong-Syun He, Chin-Chun Tsai, Hong-Ren Chen, Wei-Fu Chen, Jing-Yuan Su, Meng-Huang Sie, Jyun-Ya Ye In this study, we discuss the ladder-type electromagnetically induced transparency (EIT) and two-photon absorption (TPA) under a low-light-level probe regime (0.2 $\mu $W/cm$^{\mathrm{2}}$ (0$.$06$\Gamma_{\mathrm{2}}))$ and a weakcoupling power. The specific reduction of the fluorescence due to TPA in a room-temperature three-level system via EIT interference is first clearly observed, while the probe Rabi frequency is weakened to about 0.3 MHz to avoid the affect of the vicinity hyperfine state. The EIT transparency rate derived from the loss of fluorescence is about 25{\%}. This result proves that the low transparency rate is inevitable when EIT is applied in the thermal vapor. Additionally, the linewidth below $\Gamma (=\Gamma_{\mathrm{2}}+ \quad \Gamma_{\mathrm{3}})$ is obtained, while the coupling Rabi frequency is as large as 1.7 $\Gamma $ ($=$12.6 MHz). According to the tendency of the experimental and simulation results, the subnatural linewidth is still achievable as $\Omega_{\mathrm{c}} $is 2.5 $\Gamma $ ($=$18.5 MHz). The simulation results by solving the optical Bloch equations in the steady state are in good agreement with both EIT and TPA. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T3.00009: Three-body recombination of helium atoms from ultracold to thermal energies: classcial trajectory vs. quantal calculations Jesus Perez-Rios, Steve Ragole, Jia Wang, Chris H. Greene Classical trajectory and quantum calculations of helium three-body recombination are compared. The energies treated range from the ultracold up to the thermal regime. Quantum calculations are performed for the $J^{\Pi}$ = $0^{+}$ symmetry of the three-body recombination rate in order to compare with the classical results for zero angular momentum, yielding a good agreement for $E\sim$ 1 K. The classical calculations are treated as a scattering process in $n=6$-dimensions, and the results emerge from trajectory calculations. The classical threshold law is derived and numerically confirmed for the three-body recombination rate. Finally, a relationship is found between the quantum and classical three-body elastic cross section for a hard hypersphere that resembles the well-known shadow scattering in two-body collisions. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T3.00010: A Theoretical Study of Structural, Electronic and Vibrational Properties of Small Fluoride Clusters Kevin Waters, Ratnesh Pandey, Sandeep Nigam, Haiying He, Subhash Pingle, Avinash Pandey, Ravindra Pandey Alkaline earth metal fluorides are an interesting family of ionic crystals having a wide range of applications in solid state lasers, luminescence, scintillators, to name just a few. In this work, small stoichiometric clusters of (MF$_{\mathrm{2}})_{\mathrm{n}}$ (M$=$ Mg, Ca Sr, Ba, n$=$1-6) $_{\mathrm{\thinspace }}$were studied for structural, vibrational and electronic properties using first-principles methods based on density functional theory. A clear trend of structural and electronic structure evolution was found for all the alkaline earth metal fluorides when the cluster size n increases from 1 to 6. Our study reveals that these fluoride clusters mimic the bulk-like behavior at the very small size. Among the four series of metal fluorides, however, (MgF$_{\mathrm{2}})_{\mathrm{n\thinspace }}$clusters stands out to be different in its preference of equilibrium structures owing to the much smaller ionic radius of Mg and the higher degree of covalency in the Mg-F bonding. The calculated binding energy, highest stretching frequency, ionization potential, and HOMO-LUMO gap decrease from MgF$_{\mathrm{2}}$ to BaF$_{\mathrm{2}}$ for the same cluster size. These variations are explained in terms of the change in the ionic radius and the basicity of the metal ions. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T3.00011: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T3.00012: Molecular Geometry Determination by Atomic Force Microscopy Nikolaj Moll, Leo Gross, Bruno Schuler, Fabian Mohn, Alessandro Curioni, Gerhard Meyer Using functionalized tips, the atomic resolution of a single organic molecule can be achieved by atomic force microscopy (AFM) operating in the regime of short-ranged repulsive Pauli forces while the van-der-Waals and electrostatic interactions only add a diffuse attractive background.\footnote{L. Gross, F. Mohn, N. Moll, P. Liljeroth, and G. Meyer, Science 325, 1110 (2009).} To theoretically describe the atomic contrast a simple model is introduced in which the Pauli repulsion is assumed to follow a power law as a function of the probed charge density. Even, different bond orders of individual carbon-carbon bonds in organic molecules can be distinguished by AFM.\footnote{L. Gross, F. Mohn, N. Moll, B. Schuler, A. Criado, E. Guiti{\'a}n, D. Pe{\~n}a, A. Gourdon, and G. Meyer, Science 337, 1326 (2012).} The adsorption geometry of single molecules with intramolecular resolution were measured. The lateral adsorption position was determined with atomic resolution, adsorption height differences, and tilts of the molecular plane with very high precision.\footnote{B. Schuler, W. Liu, A. Tkatchenko, N. Moll, G. Meyer, A. Mistry, D. Fox, and L. Gross, Phys. Rev. Lett. 111, 106103 (2013).} [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T3.00013: Loop vs ladder delay scanning protocols in multidimensional spectroscopy with entangled light Konstantin Dorfman, Shaul Mukamel Multidimensional optical signals are commonly recorded by varying the delays between time ordered pulses. These control the evolution of the density matrix and are described by ladder diagrams. We propose a new non time ordered protocol based on monitoring the wavefunction and described by loop diagrams. The time variables in this protocol allow to observe different resonances and reveals information about intraband dephasing missed by the standard technique. Coupling to entangled light described naturally by the protocol scrambles the time variables and provides high selectivity and background free measurement of the various resonances. Entangled light can resolve various states even when strong background due to fast dephasing suppresses the resonant features if probed by classical light. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T3.00014: Signatures of Dirac-Weyl fermions in long organic molecules Richard Korytar, Dimitra Xenioti, Peter Schmitteckert, Mebarek Alouani, Ferdinand Evers Oligoacenes are molecules which consist of N linearly fused benzene rings. They have been subject of intensive research since they were suspected to support correlated ground states with charge or spin ordering. In addition, they have been considered promising for technological application in organic electronics. We use ab-intio calculations in order to investigate how the optical gap of the molecule decreases with increasing length N. Intriguingly, we find that the limit of a metallic wire is reached with strong oscillations that exhibit periodicity with several periods that are not commensurate with the lattice symmetries. In particular, at certain magical values N*=10, 21, 32,... the gap is (almost) vanishing and revives again at intermediate values. An explanation will be offered in terms of a band-structure argument. [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T3.00015: A spectroscopic study of graphene nanoribbon formation on gold Dean Cvetko, Arunabh Batra, Olgun Adak, Gregor Kladnik, Alberto Morgante, Latha Venkataraman We study the formation of graphene nanoribbons (GNRs) on Au(110) and Au(111) via the covalent self-assembly of 10,10'-dibromo-9,9'-bianthryl (DBBA), a halogenated precursor molecule. We follow each step of the debromination, polymerization, and dehydrogenation in detail and show that Br-C bonds on the DBBA cleave at temperatures as low as 60C, much lower than that reported in previous STM-based measurements. Through x-ray photoemission spectroscopy (XPS) core-level shifts, we establish that the resulting radicals bind to Au, pointing to the formation of the Au-C and Au-Br bonds. We show that Br desorbs from Au(111) and Au(110) at 230-250C, much lower than previously predicted. Importantly, we find that polymerization and dehydrogenation of precursors proceeds only after removal of halogens from Au, suggesting that the presence of halogens is the limiting factor in this step. Finally, we use angle-resolved ultraviolet photoemission spectroscopy (ARUPS) to study the electronics of the GNR/Au interface and show that the interaction results in a shift in the `surface state' of Au(111) towards E$_{\mathrm{fermi}}$ by 0.2 eV and a broadening due to increased electron effective mass. These experiments allow us to quantify the strength of the GNR-Au interaction. [Preview Abstract] |
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