Session W03: Early Time Chemistry I

Ultrafast reaction dynamics of nitro-organic molecular cations

Shock initiation of energetic materials produces cations and anions that contribute to the chemical reactions leading to detonation. Despite their potential importance, these transient charged species are difficult to detect in detonation experiments and often ignored in molecular dynamics modeling of shocked energetic materials. The pump-probe technique of femtosecond time-resolved mass spectrometry (FTRMS) and complementary quantum chemistry calculations can unravel initial unimolecular dissociation reactions of transient cationic species on femtosecond-picosecond timescales. This presentation will discuss insights gained into sub-picosecond reaction timescales of ionized energetic molecules including nitro-nitrite rearrangement in nitromethane cation, Coulomb explosion of multiply charged nitrotoluene cations, and direct dissociation of the metastable ethylene glycol dinitrate (EGDN) cation. The potential use of FTRMS measurements and calculations to inform design of laser-initiated energetic materials and advance explosive detection applications will also be discussed.   

Toward Understanding Shock-Synthesis of Nanocarbon Materials from Nitrogen-Containing Precursors

Abstract under review   

Investigating high-explosives electronic structure and radiation induced decomposition by X-ray Raman scattering

Theory and modeling have long preceded experiment in the fundamental physical and chemical kinetic properties of detonation. This technical gap exists because conventional chemical analytic methods are not easily applied to the rapid processes within the tumultuous dense bulk of the detonation. Hard X-ray core-level spectroscopies give access to local electronic structure within the bulk and are sensitive to the bond nature of the X-ray excited atom. X-ray Raman spectroscopy (XRS), a core-level photon-in/photon-out hard X-ray technique, emerges as a unique tool to address these fundamental questions. XRS provides bulk sensitivity and allows investigating absorption edges of low Z elements with hard X-rays avoiding usual constraints inherent to UV/soft X-ray spectroscopies.

We present X-ray Raman spectra of static high explosives measured at the BL-15 beamline of the Stanford Synchrotron Radiation Lightsource¹, along with ab initio density functional theory calculations. By combining experimental results and theoretical calculations, spectral features and trends observed in the near-edge region of the XRS spectra were analyzed, unambiguously assigning spectral fingerprints to different moieties within the samples².  As an application of the developed experiment-theory methodology, we performed systematic XRS studies on high-explosives for different radiation doses at cryogenic temperatures. Such application may enable distinguishing undetonated systems from transient structures originating from decomposition induced by the intense x-ray radiation beam. Understanding the decomposition mechanisms under static conditions, sets the basis for future time-resolved dynamic studies of high explosives under detonation conditions.

[1] D. Sokaras, D. Nordlund, T.-C. Weng, et al , "A high resolution and large solid angle x-ray Raman spectroscopy end-station at the Stanford Synchrotron Radiation Lightsource", Review of Scientific Instruments 83, 043112 (2012) https://doi.org/10.1063/1.4704458

[2] Paredes-Mellone, O.A., Nielsen, M.H., Vinson, J. et al. Investigating the electronic structure of high explosives with X-ray Raman spectroscopy. Sci Rep 12, 19460 (2022). https://doi.org/10.1038/s41598-022-24066-z   

Monitoring Formation of Detonation Nanodiamond and Other Novel Carbon Nanostructures Using Advanced X-ray Scattering and Spectroscopy Techniques

Nanodiamond and other carbon allotropes are pervasive throughout the solid residue produced by the detonation of many common high explosive materials, with the specific composition depending on many factors including the initial chemistry and detonation environment.  Detonation models predict which allotropes may form through computation of Chapman-Jouguet point and subsequent evolution through the size-calibrated carbon phase diagrams; however, formation mechanisms and kinetics are still in need of experimental validation.  In this presentation, we will present the work to date attempting to directly measure the evolution of the diamond phase during high-explosive detonation via time-resolved x-ray diffraction, in comparison to particle size and morphology evolution dynamically measured with small-angle scattering.  We will also present developments in the use of core-level x-ray Raman Spectroscopy and its potential for dynamically measuring detonation chemistry.   

X-ray diffraction of TATB shocked to 100 GPa

We present in situ x-ray diffraction measurements of shocked 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) to determine the solid carbon reaction products produced under shock compression to high pressure. TATB single crystals were shocked between 20 and 100 GPa in experiments at the Omega Laser Facility. VISAR was used to determine the pressure of the compressed TATB during the time of the x-ray exposure.