Session E7: CH.2 Chemistry: Complex Molecules

Probing Physical and Chemical Properties of Laser Shocked Materials using Ultrafast Dynamic Ellipsometry and Spectroscopies

Ultrafast laser techniques allow resolution of shock induced physics and chemistry picoseconds behind the shock front. In this presentation, the 350 ps sustained laser-generated shocks will be shown to combine with ultrafast dynamic ellipsometry to measure the shock state and transient absorption to measure the molecular electronic response to shock loading. Experimental data will be presented on shocked explosive crystals and liquids. Ultrafast dynamic ellipsometry was used to measure the shock and particle velocity as well as the shocked refractive index. Transient absorption spectra of RDX and simple molecular liquids in the spectral region from 440 to 780 nm were measured to map out shock reactivity during the first 350 ps, over shock stress states from 7 to 20 GPa. Additionally, nonlinear spectroscopic probes will be demonstrated to offer the potential to measure even more details of the molecular shock response, such as evolution of chemical species and vibrational temperature. Preliminary results of shocked phenylacetylene obtained using vibrational coherent anti-Stokes Raman spectroscopy (CARS) and the capability of femtosecond stimulated Raman scattering (FSRS) data to measure the nonequilibrium time evolution of mode specific vibrational temperatures on picosecond time scales will be discussed.   

Shock-driven chemical reaction in phenylacetylene

Phenylacetylene (PA) comprises a covalently-linked benzene ring and acetylene moiety, presenting an interesting molecular structure for study of shock driven chemical reactions. In the present work, gas gun-driven embedded electromagnetic gauging experiments produced \textit{in situ} particle velocity wave profiles at multiple Lagrangian positions at several shock input conditions. The input shock wave evolves over time and distance into a complex multiple wave structure, with a fast risetime 2$^{\mathrm{nd}}$ wave, slower risetime 3$^{\mathrm{rd}}$ wave, and unusual wave dynamics in the 1$^{\mathrm{st}}$ wave. From the shock and particle velocities, the Hugoniot reaction condition, and intermediate and final states associated with the chemical reactions have been obtained. For example, at shock inputs just above the cusp condition, an induction time of 200 ns was observed, with the evolved first wave traveling at U$_{\mathrm{s}}=$ 4.2 km/s, P $=$ 5.6 GPa; reaction rates of a few to 10 microsec$^{-1}$ were inferred. A thermodynamically complete unreacted equation of state was calibrated to estimate the temperature rise along the shock locus. Use of this EOS with the measured wave risetimes yielded highly state-sensitive global reaction rates.   

Observations on shock induced chemistry of cyclohexane

We use double pass absorption spectroscopy to examine shock induced reactions in situ in cyclohexane and benzene at pressures up to 33.1 GPa. Reactions in cyclohexane begin by 27 GPa and complete by 33.1 GPa. Absorption spectra indicate that the first reaction occurs within or near the shock front, and that a metastable local equilibrium is reached in the post-shock state. A second process is observed upon reshock at the lower pressures, suggesting a new equilibrium is reached post-reshock as well. Absorption bands are consistent with the formation of short radicals or fragments upon decomposition; however, spectral resolution is too low to confirm this mechanism. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Pressure Induced Phase Transitions of Cycloheptanone and Cycloheptanol up to 30 GPa

High-pressure spectroscopic studies of cycloheptanone (C7H12O) and cycloheptanol (C7H14O) have been carried out up to 30 GPa at room temperature using Raman scattering and infrared absorption combined with diamond anvil cell techniques. Different phase transition behaviors in these derivatives of cycloheptane have been observed. Both Raman and infrared absorption spectra show dramatic changes at 1 GPa including the appearance of new peaks and peak sharping/splitting. These changes can be attributed to the phase transition from liquid phase into truly crystalline phase. With further compression, cycloheptanone gradually turns into amorphous state when pressure higher than 24.0 GPa. In contrast, no truly crystalline phase was observed in cycloheptanol up to 30 GPa. A phase transition from liquid to plastic phase was observed around 1.5 GPa based on the synchrotron angle dispersive x-ray diffraction measurements. The cycloheptanol begins to transform into glass phase around 4.0 GPa as all the Raman, infrared and x-ray diffraction peaks start to broadening. The mechanism of very different phase transitions presented in these derivatives of cycloheptane with two different substituent groups has been discussed.   

High Pressure Raman Spectroscopy of Hydrogen Bonded, Layered Crystal of Squaric Acid

High pressure Raman spectroscopy experiments were carried out on squaric acid (H$_{2}$C$_{4}$O$_{4}$) to understand the role of hydrogen bonding on the structural and chemical stability of layered molecular crystals. Measurements in a diamond anvil cell up to 70 GPa revealed several instances of structural changes: (1) disappearance of some lattice modes at 0.6-0.9 GPa, indicating a change in the crystal structure symmetry from monoclinic to tetragonal, (2) disappearance of some intramolecular modes at 3 GPa, indicating possible symmetrization of hydrogen bonding in crystal layers, and (3) appearance of new intramolecular modes at 13-14 GPa. The latter changes were accompanied by a gradual increase in the Raman intensity and changes in the widths of lattice and intramolecular modes. No chemical changes were observed over the pressure range examined. These results suggest that hydrogen bonding network in layers is preserved to the highest applied pressures. However, the layers could be distorted with respect to each other above 13 GPa. Work supported by DOE/NNSA and ONR/MURI.   

High-Pressure Effects in Benzoic Acid Dimers: Vibrational Spectroscopy

To understand pressure effects on dimer structure stability, Raman and FTIR spectroscopy were used to examine changes in hydrogen bonded dimers of benzoic acid crystals up to 31 GPa. Raman measurements indicated a phase transition around 7-8 GPa. It is proposed that this transition is caused by a rearrangement of molecules within the dimer leading to a symmetry change from C$_{\mathrm{2h}}$ to likely C$_{2}$ or C$_{\mathrm{s}}$. This change was reversible upon pressure release from 15 GPa. Pressures above 15 GPa, induced gradual changes in luminescence and a color change in the crystal from white to brownish. FTIR measurements at 31 GPa revealed the formation of a new broad band centered around 3250 cm$^{-1}$, which was attributed to the stretching vibrations of the O$-$H bond. It is proposed that hydrogen bonded dimers of benzoic acid transform partially to a covalently bonded compound composed of benzoic anhydride-like molecules and H$_{2}$O. This study demonstrates that application of high pressure can lead to significant changes in the H-bonded dimer structure, including formation of chemical bonding. Work supported by DOE/NNSA and ONR/MURI.