3-phonon scattering pathways for vibrational energy transfer in crystalline RDX

TL;DR

This study presents a detailed 3-phonon theoretical analysis of vibrational energy transfer in crystalline RDX, highlighting the dominance of direct pathways and rapid energy redistribution relevant to shock initiation mechanisms.

Contribution

It introduces a comprehensive 3-phonon scattering analysis for RDX, emphasizing the predominance of direct energy transfer pathways over indirect ones.

Findings

Modes up to 90 cm-1 scatter quickly and redistribute energy within 0.16 ps.

Energy is further up-pumped to bond distortion modes within 5.6 ps.

High-frequency modes contribute less to energy transfer due to lower scattering participation.

Abstract

A long-held belief is that shock energy induces initiation of an energetic material through an indirect energy up-pumping mechanism involving phonon scattering through doorway modes. In this paper, a 3-phonon theoretical analysis of energy up-pumping in RDX is presented that involves both direct and indirect pathways where the direct energy transfer dominates. The calculation considers individual phonon modes which are then analyzed in bands. Scattering is handled up to the third order term in the Hamiltonian based on Fermi's Golden Rule. On average, modes with frequencies up to 90 cm-1 scatter quickly and redistribute the energy to all the modes. This direct stimulation occurs rapidly, within 0.16 ps, and involves distortions to NN bonds. Modes from 90 to 1839 cm-1 further up-pump the energy to NN bond distortion modes through an indirect route within 5.6 ps. The highest frequency modes have the lowest contribution to energy transfer due to their lower participation in phonon-phonon scattering. The modes stimulated directly by the shock with frequencies up to 90 cm-1 are estimated to account for 52 to 89\% of the total energy transfer to various NN bond distorting modes.