Physical and chemical changes in liquid benzene multiply shocked to 25 GPa

Liquid benzene was confined between two optically transparent windows and multiply shock compressed to peak pressures ranging from 0.6 GPa to 25 GPa. Multiple shock compression produced well-defined, uniform pressure states in the benzene and resulted in lower temperatures than the temperatures in single-shock loading. Time-resolved optical spectroscopy and real-time imaging measurements were performed to examine the physical and chemical changes in liquid benzene over several hundred nanoseconds. A complete and thermodynamically consistent equation of state was developed to calculate temperatures, densities, and energies in the shock compressed benzene.;The liquid-solid phase transition was examined using single-pass light transmission measurements, direct imaging, and high-resolution Raman spectroscopy. The transmission measurements showed no changes in the transparency of the benzene sample. The resulting images at peak pressure were identical to the images at ambient conditions. High-resolution Raman spectroscopy was performed to examine the changes in the nu2+nu7 C-H stretching modes, which show significant splitting in solid benzene. The C-H spectrum of multiply-shocked liquid benzene showed only a single peak, while the C-H spectrum of solid benzene shows significant splitting between the nu 2 and nu7 C-H stretching modes. The results from the transmission, imaging, and Raman experiments show that the liquid-solid phase transition of benzene does not occur on sub-mus time scales even though the benzene is in a pressure-temperature state corresponding to the solid phase.;Time-resolved Raman spectroscopy measurements were performed to 24.5 GPa peak pressures to examine shock-induced chemical changes in benzene. In these measurements, the nu1 ring-breathing mode and the nu 2+nu7 C-H stretching modes were monitored to examine pressure-temperature effects on the C-C and C-H bonds. Up to 20 GPa, the Raman spectra showed shifting and broadening in both the nu2+nu7 C-H stretching and nu1 ring-breathing modes. At 24.5 GPa, the Raman modes become indistinguishable from an increasing background resulting from a chemical change. Analysis of the Raman data indicated these changes were unlikely to be caused by bond dissociation. Comparison to statically compressed solid benzene and theoretical models suggest that multiply-shocked liquid benzene rapidly polymerizes through cycloaddition processes into a network of intermolecularly bonded benzene rings.