3-Phonon Scattering Pathways for Vibrational Energy Transfer in Crystalline RDX

A long-held belief is that shock energy induces initiation of an energetic material through an energy up-pumping mechanism involving phonon scattering through doorway modes. In this paper, a Fermi’s golden rule-based 3-phonon theoretical analysis of energy up-pumping in RDX is presented that considers possible doorway pathways through which energy transfer occurs. On average, modes with frequencies up to 102 cm^-1scatter quickly and transfer over 99% of the vibrational energy to other low-frequency modes up to 102 cm^-1within 0.16 ps. These low-frequency modes scatter less than 0.5% of the vibrational energy directly to modes with significant nitrogen-nitrogen (NN) activity. The midfrequency modes from 102 to 1331 cm^-1further up-pump the energy to these modes within 5.6 ps. The highest-frequency modes scatter and redistribute a small fraction of the vibrational energy to all other modes, which last over 2000 ps. The midfrequency modes between 457 and 462 cm^-1and between 831 and 1331 cm^-1are the most critical for vibrational heating of the NN modes and phenomena, leading to initiation in energetics. In contrast, modes stimulated by the shock with frequencies up to 102 cm^-1dominate vibrational cooling of the NN modes.