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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 fs
time scale. Due to this very rapid detonation, decomposition study of these
materials poses considerable challenge for physical chemists.
Elucidation of
the detailed fundamental steps in the initiation of and the propagation phases
of energetic material decomposition reaction 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) and
1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX).
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 from excited electronic states
has been experimentally proved 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
photofragmentation/fragment-detection technique. Final decomposition product
is characterized by its rotational, vibrational and electronic states.
Since
decomposition of energetic materials happens within the femtosecond time
scale, experimental identification of the intermediates and their
decomposition dynamics measurements are always a difficult job. That is why a
number of simple model systems (prototype molecules with similar energetic
functional group) are selected for the study as well. 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. |