This project seeks to understand how the arrival of a steep shock front (< a few picoseconds) transfers energy to molecular materials and causes them to undergo structural and chemical rearrangements. In past years the technology needed to simulate shock-molecule interactions has improved dramatically. The simulations provide information about the position and momentum of every atom with femtosecond time resolution and angstrom space resolution. But there are no experiments that make it possible to evaluate the realism of such simulations in such a detailed manner. In ordinary femtosecond pump-probe measurements, the pump and probe pulses are optical pulses that move at the speed of light. But in shock spectroscopy measurements, the “pump” is a shock wave (usually created by a laser pulse) and the shock velocity is 105 times slower than light. Velocities of shocks in condensed matter are typically a few nanometers per picosecond, so in a picosecond a shock can excite a sample only if the sample is a few nanometers thick. For this reason the Dlott group has developed a shock spectroscopy technique where the shock is launched with a femtosecond laser pulse into a self-assembled monolayer (SAM). Because there are so few molecules to probe in a SAM, a sensitive nonlinear coherent vibrational spectroscopy technique termed nonresonant-suppressed broadband multiplex vibrational sum-frequency generation spectroscopy (SFG) is used. With this SFG method it is possible to obtain high-quality vibrational spectra of shocked monolayers with picosecond time resolution. Several years ago, James Patterson demonstrated the feasibility of this method using alkane thiol self-assembled monolayers. Since that time we have made vast improvements in our shock generation and probe technology and in sample array fabrication. The concept behind these experiments is illustrated in the first figure above. A steep shock is launched with a laser pulse and as it passes over the SAM, molecules in the SAM are probed by SFG. Using standard SAM fabrication methods, the orientation of the shocked molecules relative to the shock propagation axis can be controlled. The improve the likelihood of shock-triggered chemistry, shock compression can be augmented by laser flash-heating to probe high-temperature off-Hugoniot states. In order to interpret this data, we use the paradigm told to us by Yogi Gupta that a shock is “more than just high temperature and high pressure”. So we have developed methods to measure the spectra of SAMs under conditions of static high temperature and high pressure, and we will compare these static spectra to dynamic shock spectra to better identify and understand dynamic shock phenomena. Until our work there did not exist methods to study monolayers under extreme conditions. Our flash-heating SFG technique can be used to study SAMs under conditions of quasistatic high temperature. We have also developed a diamond-anvil static high pressure cell that uses a tiny photonic substrate to amplify the SAM Raman spectra by about one million using surface-enhanced Raman effects. Recently we have shown that we can incorporate nanometer thick layers of high explosives such as HMX into our target arrays. As shown to right, we can use SFG to simultaneously observe the SAM and the HMX layers. The SAM acts as a template and also its response indicates the arrival of the shock front.

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