Ultrahigh time-resolution vibrational spectroscopy of shocked molecular solids

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Abstract
A method is described for obtaining ultrahigh time-resolution vibrational spectra of shocked polycrystalline materials. A microfabricated shock target array assembly is used, consisting of a polymer shock generation layer, a polymer buffer layer, and a thin sample layer. A near-IR pump pulse launches the shock. A pair of delayed visible probe pulses generate a coherent anti-Stokes Raman (CARS) spectrum of the sample. High-resolution Raman spectra of shocked crystalline anthracene are obtained. From the Raman shock shift, the shock pressure is determined to be 2.6 GPa. The rise time of shock loading is 400 ps. This rise time is limited by hydrodynamics of the shock generation layer. The shock velocity in the buffer layer is found to be 3.7 (±0.5) km/s, consistent with the observed shock pressure. As the shock propagates through a few μm of buffer material, the rise time and pressure can be monitored. The rise time decreases from ∼800 to ∼400 ps over the first 6 μm of travel, and the pressure begins to decline after about 12 μm of travel. The high-resolution CARS method permits detailed analysis of the vibrational line shape. Simulations of the CARS spectra show that when the shock front is in the crystal layer the spectral linewidths are inhomogeneously broadened by the distribution of pressures in the layers. When the crystal layer is behind the front, the spectral linewidth can be used to estimate the temperature. The increase of the spectral width from the ambient 4 to ∼6.5 cm−1 is consistent with the expected temperature increase of ∼200°.