Bulletin of the American Physical Society
20th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 62, Number 9
Sunday–Friday, July 9–14, 2017; St. Louis, Missouri
Session E8: Spectroscopy and Optical Studies II |
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Chair: Marylesa Howard, National Security Technologies Room: Grand Ballroom C |
Monday, July 10, 2017 3:30PM - 3:45PM |
E8.00001: Temperature dependent of IVR investigated by steady-state and time-frequency resolved CARS for liquid nitrobenzene and nitromethane YanQiang Yang, GangBei Zhu, Lin Yan, XiaoSong Liu Intramolecular vibrational energy redistribution (IVR) is important process in thermal decomposition, shock chemistry and photochemistry. Anti-Stokes Raman scattering is sensitive to the vibrational population in excited states because only vibrational excited states are responsible to the anti-Stokes Raman scattering, does not vibrational ground states. In this report, steady-state anti-Stokes Raman spectroscopy and broad band ultrafast coherent anti-Stokes Raman scattering (CARS) are performed. The steady-state anti-Stokes Raman spectroscopy shows temperature dependent of vibrational energy redistribution in vibrational excited-state molecule, and reveal that, in liquid nitrobenzene, with temperature increasing, vibrational energy is mainly redistributed in NO2 symmetric stretching mode , and phenyl ring stretching mode of $\nu_{\mathrm{CC}}$ . For liquid nitromethane, it is found that, with temperature increasing, vibrational energy concentrate in CN stretching mode and methyl umbrella vibrational mode . In the broad band ultrafast CARS experiment, multiple vibrational modes are coherently excited to vibrational excited states, and the time-frequency resolved CARS spectra show the coincident IVR processes. [Preview Abstract] |
Monday, July 10, 2017 3:45PM - 4:00PM |
E8.00002: Broadband mid-infrared measurements for shock induced chemistry Shawn McGrane, Pamela Bowlan, Kathryn Brown, Cynthia Bolme, Marc Cawkwell Vibrational absorption spectroscopy across the mid-infrared range is a ubiquitous diagnostic of chemical effects due to its sensitivity to small variations in bonding. At the high temperatures and pressures relevant to shock induced chemistry, vibrational spectral peaks become very broad, and accessing as much spectral range as possible with high time resolution can significantly aid in deducing chemical dynamics. Here, we report experiments using broadband (\textless 500 cm$^{\mathrm{-1}}$ to \textgreater 2000 cm$^{\mathrm{-1}})$ mid-infrared femtosecond supercontinua created by four wave mixing in filaments to perform absorption spectroscopy. These broadband mid-infrared supercontinua are detected through upconversion to visible light. Initial efforts to utilize these methods for measurement of chemical dynamics in shocked nitromethane will be reported. [Preview Abstract] |
Monday, July 10, 2017 4:00PM - 4:15PM |
E8.00003: Energy Transfer Between Coherently Delocalized States in Thin Films of the Explosive Pentaerythritol Tetranitrate (PETN) Revealed by Two-Dimensional Infrared Spectroscopy. Joshua Ostrander, Robert Knepper, Alexander Tappan, Jeffery Kay, Martin Zanni, Darcie Farrow Pentaerythritol tetranitrate (PETN) is a common secondary explosive and has been used extensively to study shock initiation and energy propagation in energetic materials. We report 2D IR measurements of PETN thin films that resolve vibrational energy transfer and relaxation mechanisms. Ultrafast anisotropy measurements reveal a sub-500 fs reorientation of transition dipoles in thin films of vapor-deposited PETN that is absent in solution measurements, consistent with intermolecular energy transfer. The anisotropy is frequency dependent, suggesting spectrally heterogeneous vibrational relaxation. Cross peaks are observed in 2D IR spectra that resolve a specific energy transfer pathway with a 2 ps time scale. Measurements of the transition dipole strength indicate that these vibrational modes are coherently delocalized over at least 15$-$30 molecules. We discuss the implications of vibrational relaxation between coherently delocalized eigenstates for mechanisms relevant to explosives. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, July 10, 2017 4:15PM - 4:30PM |
E8.00004: Initial results from a multiplexed radiance-reflectance diagnostic Thomas Ota, Daniel Eakins, David Chapman Pyrometry is an established diagnostic for the measurement of temperature in high strain rate experiments such as plate impact experiments. By collecting the thermal emission from a shocked sample its temperature may be estimated by applying Planck's Law. This approach is susceptible to significant uncertainty originating from possible changes in emissivity after the sample has been shocked; thus it is desirable to determine emissivity during the experiment. Kirchhoff's Law, which states emissivity is the complement of reflectivity, can be applied to determine emissivity in high strain rate experiments. Ideally the sample's reflectivity would be measured at the same wavelength and on the same experiment as the pyrometry measurement. We describe the development of a diagnostic that produces a modulated reflectivity signal that is insensitive to tilt and surface finish. Following characterisation of the system in the laboratory, dynamic tests were conducted. The reflectivity signal was multiplexed onto a radiance signal allowing simultaneous measurement of radiance and reflectivity. A description of the diagnostic, results from laboratory testing and results and analysis from dynamic experiments are presented. [Preview Abstract] |
Monday, July 10, 2017 4:30PM - 4:45PM |
E8.00005: Emissivity corrected pyrometry of reactive multilayers. Darcie Farrow, Michael Abere, Stephen Rupper, Thomas Conwell, Alexander Tappan, David Adams Ignition of sputter deposited nano-laminates results in rapid, self-propagating reactions. Due to high (10's of m/s) reaction front velocities, temperatures in the 1,000's of \textdegree K, and rapid phase changes occurring during reaction, direct measurement of temperature has proven difficult. This work presents a pyrometry technique with sub-microsecond time resolution, 10$^{\mathrm{-6}}$ m spatial resolution, and real time calculation of emissivity. By modulating a laser at 100 kHz and then Fourier processing the summed signal of emission and modulated reflectance, this emissivity corrected pyrometer overcomes the traditional limitations of two-color pyrometery for samples that do not follow the grey body approximation. The instrument has allowed for the direct measurement of temperature in NiAl and AlPt flame fronts, which allows for a determination of heat loss from an adiabatic condition. Further, a bilayer thickness dependence study has shown the relationship between front propagation velocity and flame temperature. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, July 10, 2017 4:45PM - 5:00PM |
E8.00006: Shock Compression Spectroscopy of Quantum Dots James Christensen, Alexandr Banishev, Dana Dlott We have investigated CdSe quantum dots (QDs) as photoluminescent probes of shocked solids. They could be especially useful for composite materials, where the individual components could be tagged with different color QDs. The QDs are tiny (4 nm) spherical emitters, pumped by a continuous laser during shock or diamond anvil experiments up to 12 GPa. In the diamond anvil the QDs are hydrostatically compressed and the emission blueshifts with increasing pressure. By contrast, in shock experiments the QDs are embedded in a hard glass or a soft polymer matrix and subjected to uniaxial compression, which should mechanically deform them, and the emission redshifts with increasing pressure. We did hundreds of shock experiments with laser-driven flyer plates, measuring time-resolved intensities, spectral shifts and spectral widths with 1 ns time resolution. We also measured the time-dependent strain of the matrix using a fast optomechanical probe. We showed that the QD redshift can measure the strain in the glass or polymer with 1 ns time resolution. In the hard glass above 4 GPa the QDs behave oddly. When the shock arrives, the QDs redshift as the strain increases, but after about 20 ns, the redshift disappears for about 20 ns and then reappears. We think this redshift blinking behavior is related to the shear transients in the matrix, which suggests we might be able to use QDs to measure uniaxial strain and shear. [Preview Abstract] |
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