Mode-Selective Vibrational Energy Transfer Dynamics

The time scale associated with shock-induced detonation is a key property of energetic materials that remains poorly understood. Herein, we test aspects of one potential mechanism, the phonon up-pumping mechanism, where shock compression excites lattice phonon modes, transferring energy to intramolecular vibrations leading to chemical bond cleavage and reaction. We report on subpicosecond to subnanosecond vibrational energy transfer (VET) dynamics of a series of common energetic materials at varios static pressures. In parallel to ultra-fast pump-probe experiments, high-throughput DFT calculations are used to calculate anharmonic couplings that explain scattering pathways of mode-localized energy. The ultrafast appearance of signal at all probed modes in the mid-infrared suggests strong anharmonic coupling of all vibrations in the solid, whereas the long-lived evolution demonstrates that VET is incomplete, and thus thermal equilibrium is not attained, even on the 100 ps time scale. Through these identified key timescales of VET we will discuss the implications on sensitivity measures of energetic materials as it relates to chemical reaction pathways.