Femtosecond laser induced breakdown spectroscopy (LIBS) has been shown to be sensitive to a variety of ERC's
(explosive-related compounds) deposited on substrates. In LIBS, surface material is excited by a high-powered laser
pulse forming a plasma. The optical emission from this plasma is collected and spectrally analyzed to determine the
surface species entrained in the excitation event. The detection of explosive related compounds in the field presents
many challenges, one of these being the wide variety of materials surfaces that might be covered with ERC's. Results
from femtosecond and nanosecond LIBS of ERC's of metal, glass, and polymer substrates show that the optical
properties of the substrate play a large role in the observed emission. Results indicate that nanosecond LIBS of ERC's on
metal surfaces yield strong atomic emission while nanosecond LIBS of ERC's on glass results in some molecular
emission. Molecular emission is also present in femtosecond LIBS spectra of ERC's on all surfaces but is particularly
strong for metal substrates. In particular emission from the CN molecular fragment could provide a means to understand
the effect of the substrate on the excitation event in nitroaromatic compounds since it is present in both nanosecond LIBS
spectra of the TNT/glass system and femtosecond LIBS spectra of the TNT/Al system. The origins of this CN molecular
fragment are currently being studied since fragmentation and reaction processes in LIBS events are not fully understood
at this time.
We use femtosecond laser-induced breakdown spectroscopy (LIBS) to detect trace amounts of TNT and RDX. A high-power pulsed laser is used in LIBS to form a plasma on the material surface and the optical emission from the plasma is spectrally analyzed to determine the material composition. Femtosecond LIBS results for TNT and RDX on aluminum substrates and glass slides are reported. Results are examined in terms of the optical properties of the substrate and the strong linear absorption for aluminum is contrasted with the weaker multiphoton absorption for glass. Optical microscope images of the ablated explosives are shown for femtosecond and nanosecond laser excitation. Fragmentation studies by femtosecond laser mass spectrometry are used to interpret LIBS results.
We use femtosecond laser mass spectrometry (FLMS) to study the fragmentation patterns of solid phase explosive materials subjected to femtosecond laser pulse irradiation. In condensed phase FLMS a compound deposited on a solid substrate is desorbed into vacuum by femtosecond irradiation forming a plume of ionized and neutral species. Positive or negative ions are accelerated by an electric potential, allowed to drift in the field-free region of a time-of-flight (TOF) mass spectrometry instrument, and flight-times are recorded by a micro-channel plate detector and a digital oscilloscope. From the value of the accelerating field and the ion flight time, the mass-to-charge ratio of each ion is obtained. In this paper we report femtosecond laser mass spectra for the positive and negative ions formed by desorbing TNT and RDX with 150 fs pulses centered at 800 nm. The fragmentation pathways for the formation of the observed ions are described and are used to interpret femtosecond laser induced breakdown spectroscopy results.
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