Mode-Selective Vibrational Energy Transfer Dynamics in 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) Thin Films

Abstract

The coupling of inter-and intramolecular vibrations plays a critical role in initiating chemistry during the shock-to-detonation transition in energetic materials. Herein, we report on the subpicosecond to subnanosecond vibrational energy transfer (VET) dynamics of the solid energetic material 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) by using broadband, ultrafast infrared transient absorption spectroscopy. Experiments reveal VET occurring on three distinct time scales: subpicosecond, 5 ps, and 200 ps. 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. Density functional theory and classical molecular dynamics simulations provide valuable insights into the experimental observations, revealing compression-insensitive time scales for the initial VET dynamics of high-frequency vibrations and drastically extended relaxation times for low-frequency phonon modes under lattice compression. Mode selectivity of the longest dynamics suggests coupling of the N−N and axial NO 2 stretching modes with the long-lived, excited phonon bath.

Critical Evaluation of Reactive Force Fields for Vibrational Spectra: Case Study of Crystalline Cellulose Iβ

“…Lattice and lattice-molecular vibrations (phonons) are fundamental in vibrational energy transfer that belies the shock response and precedes initiation in crystalline energetic materials [1,2]. Under the harmonic approximation, a structurally stable lattice at equilibrium must have atoms near their equilibrium positions, and a dynamical matrix with only positive eigenvalues.…”

Generation and Implementation of Continuum Infrared Pulses for Broadband Detection in 2D IR Spectroscopy

“…28,29 In transient absorption experiments, Ramasesha and co-workers used broadband IR detection to reveal ultrafast vibrational energy transfer between numerous vibrational modes in energetic materials. 30,31 In addition to TA and 2D IR spectroscopy, the plasma IR source has been used as a probe in mixed 2D VIS-IR spectroscopy, 24 to drive high-order harmonic generation in solids, 32 and as a seed for generating microjoule-scale broadband femtosecond pulses at 3 μm by OPA. 33 In the filamentation process, plasma is generated by focusing the laser harmonics into gas, typically air or nitrogen.…”

“…In our group, this light source has enabled 2D IR studies on the broad resonances in H-bonding systems including liquid water, aqueous protons, concentrated electrolytes, , and confined water in solid acids . Petersen and co-workers have used broadband 2D IR spectroscopy to reveal inter- and intra-molecular couplings across the mid-IR in coupled dimer systems, and to study broad mid-IR electronic transitions in semiconductors and quantum dots. , In transient absorption experiments, Ramasesha and co-workers used broadband IR detection to reveal ultrafast vibrational energy transfer between numerous vibrational modes in energetic materials. , In addition to TA and 2D IR spectroscopy, the plasma IR source has been used as a probe in mixed 2D VIS-IR spectroscopy, to drive high-order harmonic generation in solids, and as a seed for generating microjoule-scale broadband femtosecond pulses at 3 μm by OPA …”