High Spectral Resolution Models of Anharmonic Phonons in Energetic Crystals

Phonons are the primary carriers of energy in many high explosive (HE) crystals near equilibrium. Representations of phonons in second quantization models for energetic materials, however, are relatively scant in the literature. In this talk, we show how such models can determine many properties with high accuracy and very high spectral resolution. Specifically, we show nonequilibrium vibrational energy transfer (VET) under selective phonon excitation and a unique phonon coupling mechanism in thermal transport through HE crystals. The exemplar material is cyclotrimethylene trinitramine (RDX) with an orthorhombic unit cell of 168 atoms.

In the first result, we simulate what happens during mid-infrared pump-probe spectroscopy. Selective excitation of phonons at various frequencies uncovers unique VET pathways and the kinetic evolution of mode populations as the system transitions to a new equilibrium temperature. Owing to the generality of the approach and the complete absence of any empirical parameters, nearly any pump or probe frequency can be studied for any crystalline material. Exceptionally strong agreements are shown with independent experiments. In particular, monitoring the temperatures of the N-N/N-O stretching modes reveals strong coupling with the C-H stretching modes. This coupling could potentially impact both the proton transfer transition state involved in HONO elimination and the N–N stretching mode associated with bond cleavage.

In the second result, we calculate the thermal conductivity and examine thermal transport behaviors. Unlike the particle-like phonon transport in simple crystals, our findings highlight the crucial role of wave-like heat conduction from coupled phonons. The models parse the behaviors by modes to show that the vast majority of heat is transmitted through a mediated effect involving optic modes. Through the careful identification of modes that participate in the transfer of heat, we see opportunities to manipulate related mechanisms using interactions with light.