Session Y1: Detonation and Shock-induced Chemistry IX: Spectroscopic Studies

Ultrafast studies of shock induced chemistry-scaling down the size by turning up the heat

We will discuss recent progress in measuring time dependent shock induced chemistry on picosecond time scales. Data on the shock induced chemistry of liquids observed through picosecond interferometric and spectroscopic measurements will be reconciled with shock induced chemistry observed on orders of magnitude larger time and length scales from plate impact experiments reported in the literature. While some materials exhibit chemistry consistent with simple thermal models, other materials, like nitromethane, seem to have more complex behavior. More detailed measurements of chemistry and temperature across a broad range of shock conditions, and therefore time and length scales, will be needed to achieve a real understanding of shock induced chemistry, and we will discuss efforts and opportunities in this direction.   

Spectroscopic Analysis of Time-Resolved Emission from Shocked Explosive Samples

We report a series of time-resolved spectroscopic measurements that aim to characterize the reactions that occur during shock initiation of high explosives. The experiments employ time- and wavelength-resolved emission spectroscopy to analyze light emitted from shocked and detonating thin explosive films. In this talk we present analysis of optical emission spectra from hexanitroazobenzene (HNAB), hexanitrostilbene (HNS), and pentaerythritol tetranitrate (PETN) samples. The emission features observed in the spectra are assigned to electronic transitions of molecular fragments, and the implications of these findings on our understanding of the underlying reaction mechanisms are discussed.   

Molecular Response of Liquid Nitrogen Multiply Shocked to 40 GPa

Liquid nitrogen was subjected to multiple shock compression to examine its response to pressures (15-40 GPa) and temperatures (1800-4000K) previously unexplored in static and shock compression. Raman spectroscopy measurements (of the 2330 cm$^{-1}$ mode) were used to characterize the molecular bond response and to experimentally determine temperature in the peak P-T state. By extending our analysis of the measured Raman shifts to include Raman spectroscopy measurements from previous studies\footnote{D.S. Moore \textit{et al}, J. Chem. Phys., \textbf{90 }1368 (1989)}$^{,}$\footnote{A. F. Goncharov \textit{et al}, Phys. Rev. Lett.,\textbf{101} 095502 (2008)}, an empirical relation was developed that describes the pressure and temperature dependence of the Raman shifts for both static and shock compression. Examining the P-T dependence of all measured Raman shifts showed that the molecular response of liquid nitrogen is both pressure and temperature dependent, and that the molecular response is best understood by considering three temperature regimes (below 1500K, 1500-4000K, above 4000K). Multiply shocked liquid nitrogen remained a molecular fluid at the pressures and temperatures accessed in our work, and became a greybody emitter at the highest pressures.   

Structural evolution of detonation carbon in Composition B-3 by X-ray scattering

High explosive detonation products are primarily composed of solid carbon products. Prior electron microscopy studies have revealed that detonation carbon can contain a variety of unique carbon particles possessing novel morphologies, including core-shell, onions and ribbons. Despite these observations very little is known on what conditions leads to the production of novel carbon nanoparticles. A fuller understanding on conditions that generate such novel carbon materials would greatly benefit from time-resolved studies that probe particle formation and evolution through and beyond the chemical reaction zone. Here, we report initial experiments employing time-resolved X-ray scattering measurements to monitor the detonation carbon products formed from Composition B-3 (60{\%} TNT, 40{\%} RDX). Time-resolved SAXS (TRSAXS) studies were performed at the Dynamic Compression Sector (DCS, Sector 35) at the Advanced Photon Source (Argonne National Laboratory). In-situ formation of solid carbon behind the detonation front was probed on the nanosecond time scale. Analysis of the scattering patterns using model independent methods (Porod and Guinier) yielded insights into particle morphology and interfaces.   

Photophysics of Fluorescent Probes Under 1-10 GPa Shock Compression

The use of fluorescent probes in shocked microstructured media can permit measurements of local pressures and temperatures with high time and space resolution. Here we focus on the use of a highly-emissive dye, rhodamine 6G (R6G). In order to understand the fundamental mechanisms of R6G photophysics under extreme conditions, we synchronized a femtosecond laser and streak camera with a laser-driven flyer plate shock compression system. We studied R6G emission lifetimes and spectra under shock conditions and under static high pressure when the dye was dissolved in poly-methylacryate (PMMA) or when the dye was encapsulated in silica microspheres, where R6G is superemissive. Under shock compression, the emission spectra of R6G redshifts. It is difficult to measure local pressures using the redshift, because one would have to spectrally resolve the emission from every spot in the shocked material. It would be much easier to measure the emission intensity at each location. We have found that the R6G emission intensity also changes in a shock, so it is useful to relate the intensity changes to local shock conditions. Our measurements show the intensity changes in a shock because the fluorescence lifetime drops from about 3.5 ns at ambient pressure to about 1 ns at 10 GPa.