Ultrafast Mid-Infrared Spectroscopy on Shocked Thin Film Explosive Crystals

The chemistry of energetic materials under shocked conditions is complex and poorly understood. Various theories exist that predict reaction mechanisms; however, direct experimental insight into the chemical reactions prevalent during shocks has had little attention. Mid-Infrared radiation measurement were used to investigate the chemistry of ultrafast laser shock loading of thin films of explosive crystals. Spectrally the leading edge of the shock drive beam was clipped to provide a more instantaneous shock to the sample. Generation of the mid-infrared pulse was accomplished via a four wave difference frequency generation (FWDFG) where the compressed pulse (ω_p) was temporally and spatially overlapped in air with the second harmonic of the compressed pulse (ω_2p) to generate a wide spectra of infrared laser energy (ω_p + ω_p – ω_2p = ω_MIR) over ~600 cm^-1 to ~1400 cm^-1. Mid-infrared spectra are collected by an infrared spectrometer and analyzed for chemical bond identification under different shock conditions. This experiment aims to identify the chemistry of shocked explosive crystals to compare to molecular dynamic (MD) simulations. These results should be useful in guiding molecular dynamics simulations that probe the same time and length scales as the experiments.