Caroline McEnnis

Femtosecond and nanosecond laser-induced breakdown spectroscopy of trinitrotoluene

Femtosecond and nanosecond laser-induced breakdown spectroscopy (LIBS) were used to study trinitrotoluene (TNT) deposited on aluminum substrates. Over the detection wavelength range of 200-785 nm, we have observed emission from CN and C(2) molecules as the marker for the explosive with femtosecond LIBS. In contrast, the signal for nanosecond LIBS of TNT is dominated by emission from the elemental constituents of the explosive. Aluminum emission lines from the substrate are also observed with both femtosecond and nanosecond excitation and indicate the role played by the substrate in the interaction.

Ultrafast laser-based spectroscopy and sensing: applications in LIBS, CARS, and THz spectroscopy.

Ultrafast pulsed lasers find application in a range of spectroscopy and sensing techniques including laser induced breakdown spectroscopy (LIBS), coherent Raman spectroscopy, and terahertz (THz) spectroscopy. Whether based on absorption or emission processes, the characteristics of these techniques are heavily influenced by the use of ultrafast pulses in the signal generation process. Depending on the energy of the pulses used, the essential laser interaction process can primarily involve lattice vibrations, molecular rotations, or a combination of excited states produced by laser heating. While some of these techniques are currently confined to sensing at close ranges, others can be implemented for remote spectroscopic sensing owing principally to the laser pulse duration. We present a review of ultrafast laser-based spectroscopy techniques and discuss the use of these techniques to current and potential chemical and environmental sensing applications.

Femtosecond-laser-induced breakdown spectroscopy of explosives

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.

Substrate-related effects on molecular and atomic emission in LIBS of explosives

Femtosecond laser induced breakdown spectroscopy (LIBS) has been shown to be sensitive to a variety of ERCs (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 ERCs. Results from femtosecond and nanosecond LIBS of ERCs 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 ERCs on metal surfaces yield strong atomic emission while nanosecond LIBS of ERCs on glass results in some molecular emission. Molecular emission is also present in femtosecond LIBS spectra of ERCs 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.

Femtosecond laser-induced breakdown spectroscopy of trinitrotoluene

Femtosecond and nanosecond laser-induced breakdown spectroscopy were used to study TNT deposited on aluminum and glass substrates. We have observed emission from CN and C2 molecules depending on excitation conditions.

Laser-Induced Breakdown Spectroscopy of Polymer Matrix Nanocomposites

Laser-induced breakdown spectroscopy was used to study polymer matrix nanocomposites containing metal nanoparticles. We have observed emission from the silver and palladium nanoparticles as well as CN and C2 molecules owing to the polymer matrix.

Investigation of the fragmentation of explosives by femtosecond laser mass spectrometry

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.

Femtosecond Laser-Induced Breakdown Spectroscopy of Trinitrotoluene

Femtosecond laser-induced breakdown spectroscopy (LIBS) has been shown to be effective in detecting TNT, RDX, and other explosive related compounds. We chose to focus on aluminum and glass substrates since these allowed us to study the effects of strong linear absorption for a metal and the multiphoton absorption for a dielectric. For the FLMS experiments only stainless steel substrates were used.

Terahertz Time -Domain Spectroscopy of Aluminum Oxide for Thermal Protection Applications

In this work we show how terahertz time -domain spectroscopy (THz TDS) can be an effective tool for the characterization of optical and microstructural properties of alumin um oxide. Terahertz spectroscopy was performed on aluminum oxide test samples that ranged in mean pore size from 0 to 15 microns. These experiments were performed with a transmission mode terahertz system and show that there is a strong sensitivity to chan ges in the size and number of pores due to scattering . Results show that the magnitude of the THz transmission obtained using standard Fourier transform methods is reduced at higher freq uencies as porosity increases. A t heoretical coherent scattering model ing approach is also discussed that might assist in the interpr etation of these signal losses.