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Energetic materials, defined as controllable storage systems of chemical energy, have numerous military and industry applications as propellants, fuels, explosives, and pyrotechnics. They can release their entire chemical energy over a very short period of time, often within the fs time scale. Due to this very rapid detonation, decomposition studies of these materials pose considerable challenge for physical chemists. Elucidation of the detailed fundamental steps in the initiation of and the propagation phases of energetic material decomposition reactions is central for better understanding, controlling, and enhancing the performance of these materials for combustion and explosion, and to model the combustion behavior of either pure compounds or simple mixtures.
Unimolecular fragmentation pathways and energy partitioning amongst product species and degrees of freedom depend sensitively on the state of the reactant molecule. What is the first step in dissociation? How does the initial product vary with reactant energy (electronic, vibrational, rotational) content and state? These issues become particularly compelling for the rapid decomposition of highly energetic molecules such as 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), and the modern high nitrogen content systems, such as aromatic azines and azoles.
Ignition processes involving sparks, shock, lasers, and arcs can all initiate the decomposition reaction of energetic material by generating excited electronic states. Decomposition of energetic materials following electronic state excitation has been experimentally proven to play an important role in their overall decomposition mechanisms and kinetics. In our studies thus far, we have followed the excited electronic state decomposition of energetic materials through a photofragmentation/fragment-detection technique. Final decomposition products are characterized by their rotational, vibrational, and electronic state populations.
Since decomposition of energetic materials occurs within the femtosecond time scale, experimental identification of the intermediates and their decomposition dynamics offers a number of challenges. The study of true energetic materials is paralleled by experiments and theory of chemically similar but non energetic species. We anticipate that decomposition mechanisms of the simple model systems will represent some of the complex reactions that are involved in the decompositions of energetic materials. Though limited in their absolute structural resemblance to the energetic materials, the model systems can provide a point of departure and a baseline comparison for the study of the excited electronic state decomposition mechanisms of the complex energetic materials.
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Experimental Methods
Experimental setup consists of laser systems with both ns and fs time duration, a supersonic jet expansion nozzle with a laser desorption attachment, and two vacuum chambers: a time of flight mass spectrometer (TOFMS) chamber and an LIF (laser induced fluorescence) chamber. The energetic materials are placed into gas phase as intact molecules via matrix assisted laser desorption (MALD) technique and then entrained into a molecular beam by supersonic jet expansion.
For fs pump/probe experiments, the sample molecules are excited by the pump beam and dissociate according to their dissociation dynamics. Dissociation products are subsequently ionized by the delayed probe beam and detected via TOFMS. By delaying the probe beam with respect to the pump beam, product appearance times can be determined. The fs laser light is generated by a femtosecond laser system consisting of a self-mode locked Ti:Sapphire oscillator (KM Labs), a home-made ring cavity Ti:Sapphire amplifier, and a commercial traveling optical parametric amplifier of super fluorescence (TOPAS, Light Conversion) system. Pulse duration of the deep UV laser pulse is measured to be 180 fs using a self-diffraction (SD) autocorrelator and off-resonance two-photon absorption of the furan molecule.
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