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Chemical energy ferroelectrics are generally solid macromolecules showing spontaneous polarization and chemical bonding energy. These materials still suffer drawbacks, including the limited control of energy release rate, and thermal decomposition energy well below total chemical energy. To overcome these drawbacks, we report the integrated molecular ferroelectric and energetic material from machine learning-directed additive manufacturing coupled with the ice-templating assembly. The resultant aligned porous architecture shows a low density of 0.35 g cm−3, polarization-controlled energy release, and an anisotropic thermal conductivity ratio of 15. Thermal analysis suggests that the chlorine radicals react with macromolecules enabling a large exothermic enthalpy of reaction (6180 kJ kg−1). In addition, the estimated detonation velocity of molecular ferroelectrics can be tuned from 6.69 ± 0.21 to 7.79 ± 0.25 km s−1 by switching the polarization state. These results provide a pathway toward spatially programmed energetic ferroelectrics for controlled energy release rates.

The interaction of a laser-induced shock wave with nanoparticles and microparticles of aluminum oxide is investigated through experiments and modeling. The chemistry and physics of the interaction between the particles and plasma generated from laser ablation shows similarities and discrete differences for the two particle sizes. For both particle sizes, early stage (10 μs), oxidation reactions were dominant for both particle sizes, but the same distinctions in flow field were observed and attributed to particle drag. In all stages of interaction, microparticles expand axially with less drag that suppresses their radial expansion. As a result, the dispersion of reactive species was mapped over an up to 80% larger area for nanoparticles relative to microparticles. Results shown here can be applied toward advancing experimental diagnostics and particle-shock wave modeling and simulation efforts for energetic materials.

Laser-induced air shock from energetic materials (LASEM) is a laboratory-scale method for assessing the microsecond and millisecond timescale performance of conventional military explosives. The technique has been combined with optical emission detection schemes to evaluate energy release during detonation and post-detonation timescales from the combustion of metal powders and mixtures of metal powders and explosives. Recently, Al/Zr reactive composite powders have been synthesized via high energy ball milling with the aim of decreasing ignition thresholds while simultaneously increasing overall combustion efficiency and burn time. Here, we utilize LASEM to evaluate micron-sized 3Al:2Zr, Al:Zr, and Al:3Zr fuel powders and compare their behavior to micron-sized Al powder, a commonly used explosive additive. We observed a rapid ignition of Al/Zr powders instead of the delayed ignition of the secondary deflagration cloud that is observed for pure Al. The composites also demonstrated longer burn times compared to Al particles of similar size. We also observed enhanced laser-induced shock velocities for the composite samples, implying increased energy release in the microsecond time regime compared to Al. In addition, we present LASEM data using Al/Zr mixed with trinitrotoluene (TNT), and associated CHEETAH calculations. As expected, adding pure Al to TNT lowers the laser-induced shock velocity, and we observe a further decrease of this velocity when using composite additives. The complex interplay of the metals with each other and the TNT reaction products is yet to be fully understood.

The acoustic responses of a variety of powdered materials including a suite of elemental metals and metalloids (B, Mg, Hf, W, Si, Sn, Cu, Zr, Pb, C, Mo, Al), and metal oxide Al2O3 at the nm and µm scales have been measured following ns-pulsed ablation, excitation, and subsequent reaction in a laser-induced plasma. We track the air shockwave propagation both optically via schlieren imaging and acoustically and find that among Al and Al2O3, the max acoustic amplitude increases and time to peak amplitude decreases as a function of decreasing particle size. Further, this experimental trend can be controlled and enhanced by mixing nm- and µm-sized Al powders relative to the nm-Al powder. We compare Al to Al2O3 at similar sizes to demonstrate the effect of material reactivity on the laser-induced shockwave and acoustic response and find that amplitude is increased and time to peak is decreased when moving from the inert (Al2O3) to reactive (Al) material. A few elemental powders such as Si and nm–Mg perform similarly to nm–Al, while most of the other elemental powders are comparable to µm–Al. The effect of particle size distribution and other material properties on these measurements is discussed. Acoustic measurements are a viable, cheap alternative to expensive optical techniques (such as high-speed schlieren imaging) for measuring the laser-induced shockwave and serve to corroborate such measurements. Such measurements coupled with other diagnostics can characterize material reactions and measure relative energy release between samples, particularly for novel reactive and energetic materials.

A newly upgraded laser-induced air shock from energetic materials (LASEM) system with improved time-resolution and diagnostic capabilities has been used to investigate the influence of aluminum morphology on the energy release rates (microsecond and millisecond timescales) and high-temperature chemistry of military explosives.

The large heat release predicted in the early investigations of energetic aluminum nanoparticles (Al NPs) used in solid-state propulsion and pyrotechnics has been offset by hindered diffusion-limited detonation rates due to excess oxide shell formations and surface area loss from aggregations. We address these challenges by manufacturing graphitic shell coated Al NPs (

Optical ignition of solid energetic materials, which can rapidly release heat, gas, and thrust, is still challenging due to the limited light absorption and high ignition energy of typical energetic materials (e.g., aluminum, Al). Here, we demonstrated that the optical ignition and combustion properties of micron-sized Al particles were greatly enhanced by adding only 20 wt % of graphene oxide (GO). These enhancements are attributed to the optically activated disproportionation and oxidation reactions of GO, which release heat to initiate the oxidization of Al by air and generate gaseous products to reduce the agglomeration of the composites and promote the pressure rise during combustion. More importantly, compared to conventional additives such as metal oxides nanoparticles (e.g., WO3 and Bi2O3), GO has much lower density and therefore could improve energetic properties without sacrificing Al content. The results from Xe flash ignition and laser-based excitation experiments demonstrate that GO is an eff...

Microcrystalline igneous rocks of volcanic origin, such as basalt or rhyolite, were a common source of prehistoric stone tools. This study explores the use of laser-induced breakdown spectroscopy (LIBS) as part of an effort to develop analytical tools to examine geological materials of archaeological interest. LIBS is a method of atomic emission spectroscopy capable of providing rapid, in situ elemental analysis of any material with minimal to no sample preparation. We have utilized both a bespoke laboratory system and a commercial handheld analyzer to acquire broadband LIBS emission spectra for fresh and unaltered volcanic samples of varying age and composition from two tectonic settings. The distinguishing chemical characteristics of the different volcanic regions and centers were identified and their sources differentiated through multivariate chemometric analysis. Classification and discrimination has been achieved with a high degree of success using a pattern recognition approach. These results suggest that the provenance of stone artifacts may be identified in the field using handheld LIBS through the use of algorithmic matching of LIBS emission spectra against a sufficiently robust and representative database previously prepared by analysis of samples of known origin.

Aluminum nanoparticles (nAl) have great potential for energetic applications. However, the native oxide shell (amorphous alumina, Al2O3) inhibits efficient energy release and acts as a barrier for aluminum (Al) oxidation. An energetic oxidizer, aluminum iodate hexahydrate (AIH), has recently been demonstrated as an effective coating for nAl. However, the current chemical synthesis method has led to widely varying AIH concentrations on nAl particles. Plasma surface treatment of nAl is a novel “energy coupled to material” technique which alters the nAl surface properties without changing the bulk active Al core. This work explores a new approach to engineering the nAl surface using atmospheric argon (Ar) plasma to accomplish two objectives: (1) reduce the nAl oxide shell thickness, and (2) synthesize AIH on the treated particle surface. Transmission electron microscopy (TEM) reveals more than 40% reduction in the oxide thickness after 10 min Ar plasma treatment. Laser-induced air shock from energetic materials (LASEM) experiments show significant energy release enhancements for the plasma-treated nAl with AIH coating (PT-nAl-AIH) compared to commercial nAl as well as untreated nAl with AIH coating (UT-nAl-AIH). The results demonstrate the potential of applying atmospheric plasma techniques to modify nAl for enhanced reactivity.

Recently, a laboratory-scale method for measuring the rapid energy release from milligram quantities of energetic material has been developed based on the high-temperature plasma chemistry induced by a focused, nanosecond laser pulse. The ensuing exothermic chemical reactions result in an increase in the laser-induced shock wave velocity compared to inert materials. Laser-induced air shock from energetic materials (LASEM) provides a method for estimating the detonation performance of novel organic-based energetic materials prior to scale-up and full detonation testing. Here, the extension of LASEM to non-organic energetic materials is discussed. The laser-induced shock velocities from reactive materials such as Al/PTFE, Al/CuO, Al/Zr alloys, Al/aluminum iodate hexahydrate, and porous silicon composites have been measured; in many cases, the high sensitivity of the samples resulted in propagation of the reaction to the surrounding material, producing significantly higher shock velocities than conventional energetic materials. Methods for compensating for this effect will be discussed. Despite this limitation, the relative comparison of the shock velocities, emission spectra, and combustion behavior of each type of material provides some insight into the mechanisms for increasing the energy release of the material on a fast (µs) and/or slow (ms) timescale.

Laser-induced plasmas and reactions therein depend on the ablated target's chemical composition and morphology. Recently, we have characterized plasma properties such as spatiotemporal morphology, electron density, species temperature within the plasma, ionization degree, etc. for pure Al under variations in morphology. Here, we utilize a bi-metal powder system to study similar characteristics as a function of powder chemistry. Micron-sized ball milled Al/Zr composites of three chemistries (3Al:2Zr, Al:Zr, and Al:3Zr) are compared to similarly-sized pure Al, pure Zr, and a mixture of Al + Zr powder. We find that the introduction of increasing concentrations of Zr has several interesting effects on the plasma compared to pure Al, including a dramatic increase in the electron density (to ~3–5 × 1020 cm−3), and the earlier onset of ZrO emission bands which have a higher disassociation energy and are thermodynamically preferred over AlO at high temperatures. Differences in plume morphology, the spatial distribution of molecular species, and the emission intensities of ZrO, Zr I and Al I were observed for Al + Zr and ball-milled Al:Zr samples, indicating the influence of microstructure on the chemical reactions. We measured plasma temperatures using the Saha-Boltzmann plot method; the composites demonstrate higher temperatures than pure Zr, Al + Zr mixtures, and pure Al. We observe only minor differences in temperature as a function of increasing Zr-content within the composites. This work continues to build on our understanding of the relationships between plasma properties and microsecond-timescale chemical reactions of reactive materials.

Boron (B) is a promising fuel for energetic materials due to its high gravimetric and volumetric energy density. The enhanced ignition and combustion of B in a thermite with CuO and Bi2O3 and their mixtures compared to single metal oxides was recently demonstrated. In this study, we compared the early-time reaction B-thermite mixtures using the laser-induced air shock from energetic materials (LASEM) technique, bomb calorimetry, and differential scanning calorimetry (DSC). The application of LASEM to B/metal oxide samples has enabled the quantification of their microsecond-timescale energy release and excitation temperatures under extremely high heating rates, and the elucidation of their chemistry differences on both the microsecond and millisecond timescales. The time to peak combustion determined by LASEM follows the trend B/CuO/Bi2O3

Unexpected variations in the purity and structure of commercial detonation nanodiamonds (DNDs) is an ongoing issue. Unfortunately, influences of these variations and how they affect DND behaviors are seldom addressed. This work investigates nominally homologous commercial DNDs sold by a single vendor under the same item number but different lots. Paraffin wax and surfactant were discovered as the major organic contaminants in the DND samples along with metallic impurities. The relative quantities of these contaminants were correlated to discrepancies in the structure and crystallinity of DNDs. The DNDs containing more contaminants possessed thicker shells surrounding the diamond cores; the diamond crystallinity was observed to quickly disappear, on the order of several minutes, when exposed to 200 kV electron beam. On the other hand, those DNDs carrying fewer impurities exhibited a stable diamond structure. The differences in metallic impurities were attributed to variations in the DND purification treatments, and did not appear to affect a morphological change under the electron beam. The present work demonstrates the negative effects of impurities in commercial DNDs on the content and stability of diamond carbons. The results also have implications on possible factors that must be considered when using commercial DNDs for advanced applications.

Standard propellant and detonation tests typically performed to characterize the performance of energetic materials require large quantities of material (at least tens of grams) and can be expensive and time-consuming. This work introduces a method for characterizing the deflagration of energetic materials in a laboratory setting, using only 15–20 mg of energetic material. Temperature, energy release and emission signatures were measured and analyzed for the laser-induced deflagration of 8 different conventional military explosives. Graphite nanoparticles and micron-sized aluminum powder were added to the explosive compositions to investigate their effect on the emission signatures. A high-speed color camera recorded the deflagration events and was utilized as a full-color pyrometer to calculate the average temperatures and image hotspots; the temperatures maps were compared to those measured by conventional two-color pyrometry. The laboratory-scale method presented here can be applied to novel energetic materials under development that may be available only in limited quantities to evaluate their potential as propellants or reduced emission signature explosives prior to scale-up.

Since new energetic materials are initially produced in very small quantities for both safety and cost reasons, laboratory-scale methods for characterizing their performance are essential for determining the most promising candidates for scale-up. Laser-induced air shock from energetic materials (LASEM) is a promising new method for estimating the detonation velocity of novel explosives using milligram amounts of material, while simultaneously investigating their high temperature chemical reactions. LASEM has been applied to 6 new explosives for the first time: TKX-50, MAD-X1, BDNAPM, BTNPM, TKX-55, and DAAF. Emission spectroscopy of the laser excited materials revealed the formation of the high pressure bands of C-2 during the ensuing exothermic reactions. The low thermal sensitivity of the materials also led to unusual laser-material interactions, visualized with high-speed video. The estimated detonation velocities for the 6 explosives were compared to predicted values from EXPLO5 and CHEETAH. The LASEM results suggest that TKX-55, BDNAPM, and BTNPM have higher detonation velocities than predicted by the thermochemical codes, while the estimated detonation velocities for MAD-X1 and TKX-50 are slightly lower than those predicted.

Species stratification and local plasma composition can affect microsecond-timescale oxidation reaction rates of metals such as Al in an oxidizing atmosphere. Here, we utilize fast, gated emission spectroscopy and a high-speed framing camera to determine the intensity and spatiotemporal evolution of various Al ablation products within a laser-induced plasma. Using a high-purity Al plate, micron- and nano-sized Al powders, and inert micron Al2O3 powder, we studied the effect of Al morphology and reactivity on the oxidation characteristics and plasma hydrodynamics in air at 1 atm over two temporal regimes (2–10 μs and 20–100 μs). We observed an increase in the spatial distribution and intensity of emission from vaporized Al within the plasma for powder-based samples compared to plate Al due to enhanced material dispersion. In nano-Al, AlO emission forms at, and propagates along, the surface of the powder bed from 2–10 μs, whereas for micron powder, there is a delay in AlO formation within the bulk of the plasma until tens of microseconds. We measured electron densities from a variety of spectral lines, which can range from ~ 2 × 1015 to 2 × 1018 cm−3, and which scale inversely with the rate of plasma expansion across morphologies. The Al I and Al II species temperatures from 2 to 10 μs calculated via Boltzmann plots are similar (from ~ 10,000 to 14,000 K), and we performed a suite of local thermodynamic equilibrium (LTE) validity calculations to establish that these two species are in LTE, while H is not. Using image co-registration, we calculated the thickness of the AlO layer surrounding the expanding Al cloud at times > 20 μs, which can range from ~ 50 to 300 μm. These results allow us to begin to understand the complexities of laser ablated metal powder reactions at microsecond timescales.

Methods for facilitating the fast energy release of aluminum to enhance detonation performance will be discussed. The energy release rates of milligram-quantity samples have been compared by measuring the laser-induced shock wave velocities and tracking the formation of AlO on both the microsecond- and millisecond-timescales.

Herein we present the preparation and characterization of three new bispyrazolyl-based energetic compounds with great potential as explosive materials. The reaction of sodium 4-amino-3,5-dinitropyrazolate (5) with dimethyl iodide yielded bis(4-amino-3,5-dinitropyrazolyl)methane (6), which is a secondary explosive with high heat resistance (Tdec=310 °C). The oxidation of this compound afforded bis(3,4,5-trinitropyrazolyl)methane (7), which is a combined nitrogen- and oxygen-rich secondary explosive with very high theoretical and estimated experimental detonation performance (Vdet (theor)=9304 m s−1 versus Vdet(exp)=9910 m s−1) in the range of that of CL-20. Also, the thermal stability (Tdec=205 °C) and sensitivities of 7 are auspicious. The reaction of 6 with in situ generated nitrous acid yielded the primary explosive bis(4-diazo-5-nitro-3-oxopyrazolyl)methane (8), which showed superior properties to those of currently used diazodinitrophenol (DDNP).

A new laboratory-scale method for predicting explosive performance (e.g., detonation velocity and pressure) based on milligram quantities of material is demonstrated. This technique is based on schlieren imaging of the shock wave generated in air by the formation of a laser-induced plasma on the surface of an energetic material residue. The shock wave from each laser ablation event is tracked for more than 100 μs using a high-speed camera. A suite of conventional energetic materials including DNAN, TNT, HNS, TATB, NTO, PETN, RDX, HMX, and CL-20 was used to develop calibration curves relating the characteristic shock velocity for each energetic material to several detonation parameters. A strong linear correlation between the laser-induced shock velocity and the measured performance from full-scale detonation testing has been observed. The Laser-induced Air Shock from Energetic Materials (LASEM) method was validated using nitrocellulose, FOX-7, nano-RDX, three military formulations, and three novel high-nitrogen explosives currently under development. This method is a potential screening tool for the development of new energetic materials and formulations prior to larger-scale detonative testing. The main advantages are the small quantity of material required (a few milligrams or less per laser shot), the ease with which hundreds of measurements per day can be obtained, and the ability to estimate explosive performance without detonating the material (reducing cost and safety requirements).

Recently, a laboratory-scale method for measuring the rapid energy release from milligram quantities of energetic material has been developed based on the high-temperature chemistry induced by a focused, nanosecond laser pulse. The ensuing exothermic chemical reactions result in an increase in the laser-induced shock wave velocity compared to inert materials; a high-speed camera is used to record the expansion of the shock wave into the air above the sample surface. A comparison of the characteristic shock wave velocities for a wide range of energetic materials revealed a strong linear correlation between the laser-induced shock velocity and the reported detonation velocities from large-scale detonation testing. This has enabled the use of the laser-induced air shock from energetic materials (LASEM) method as a means of estimating the detonation performance of novel energetic materials prior to scale-up and full detonation testing. Here, we report new applications of the LASEM method including conventional energetic materials with metal additives, novel energetic materials, porous silicon impregnated with oxidizers, and aged energetic material samples. We also discuss the challenges and limitations of the technique such as sample preparation, shot-to-shot reproducibility, and reaction propagation. While the extension of LASEM to novel high-nitrogen energetic materials and aged conventional energetic material samples has been quite successful, non-organic and other highly reactive samples present some unique challenges.

A new synthesis approach for aluminum particles enables an aluminum core to be passivated by an oxidizing salt: aluminum iodate hexahydrate (AIH). Transmission electron microscopy (TEM) images show that AIH replaces the Al2O3 passivation layer on Al particles that limits Al oxidation. The new core-shell particle reactivity was characterized using laser-induced air shock from energetic materials (LASEM) and results for two different Al-AIH core-shell samples that vary in the AIH concentration demonstrate their potential use for explosive enhancement on both fast (detonation velocity) and slow (blast effects) timescales. Estimates of the detonation velocity for TNT-AIH composites suggest an enhancement of up to 30% may be achievable over pure TNT detonation velocities. Replacement of Al2O3 with AIH allows Al to react on similar timescales as detonation waves. The AIH mixtures tested here have relatively low concentrations of AIH (15 wt. % and 6 wt. %) compared to previously reported samples (57.8 wt. %) and still increase TNT performance by up to 30%. Further optimization of AIH synthesis could result in additional increases in explosive performance.

The initial evolution of thermal energy transfer into a solid explosive is studied using an indirect femtosecond laser heating technique on a picosecond timescale in order to elucidate the role of temperature in the shock-induced initiation of explosives. The indirect laser heating method is presented; time-resolved visible transient absorption (TA) spectroscopy was used to monitor the energetic material response following heat transfer from the laser-heated gold (Au) layer to the sample. Reported here are visible TA data in the spectral region from 500 to 750 nm for indirect laser-heated thin films of cyclotrimethylene trinitramine (RDX), oxidized polyethylene (OPE), and RDX with 1%, 2.5%, 5%, or 10% OPE prior to decomposition. TA was observed for RDX and RDX with OPE; however, no TA was observed for pure OPE. Compared to pure RDX, the TA intensity of RDX with OPE decreases as the OPE content increases and the time required to observe the TA signal from RDX increases. Our results suggest that the thermal energy produced by a femtosecond laser pulse with an energy of 15 mJ cm-2 is sufficient to induce changes in the electronic structure of RDX, resulting in promotion of the RDX molecules into an excited state. We also determined that the heat transfer rate in RDX depends on its homogeneity and degree of purity.

The impact of laser-driven flyer plates on energetic materials CL-20, PETN, and TATB has been investigated. Flyer plates composed of 25 μm thick Al were impacted into the energetic materials at velocities up to 1.3 km/s. The flyer plates were accelerated by means of an Nd:YAG laser pulse. The laser pulse generates rapidly expanding plasma between the flyer plate foil and the substrate to which it is adhered. As the plasma grows, a section of the metal foil is ejected at high speed, forming the flyer plate. The velocity of the flyer plate was determined using VISAR, time of flight, and high-speed video. The response of the energetic material to impact was determined by light emission recorded by an infrared-sensitive photodiode. Following post-impact analysis of the impacted energetic material, it was hypothesized that the light emitted by the material after impact is not due to the impact of the flyer itself but rather is caused by the decomposition of energetic material ejected (via the shock of flyer plate impact) into a cloud of hot products generated during the launch of the flyer plate. This hypothesis was confirmed through schlieren imaging of a flyer plate launch, clearly showing the ejection of hot gases and particles from the region surrounding the flyer plate launch and the burning of the ejected energetic material particles.

A series of organic polymers and the military explosive cyclotrimethylenetrinitramine (RDX) were studied using the light emission from a femtosecond laser-induced plasma under an argon atmosphere. The relationship between the molecular structure and plasma emission was established by using the percentages of the atomic species (C, H, N, O) and bond types (C-C, C═C, C-N, and C≡N) in combination with the atomic/molecular emission intensities and decay rates. In contrast to previous studies of organic explosives in which C2 was primarily formed by recombination, for the organic materials in this study the percentage of C-C (and C═C) bonds was strongly correlated to the molecular C2 emission. Time-resolved emission spectra were collected to determine the lifetimes of the atomic and molecular species in the plasma. Observed differences in decay rates were attributed to the differences in both the molecular structure of the organic polymers or RDX and the chemical reactions that occur within the plasma. These differences could potentially be exploited to improve the discrimination of explosive residues on organic substrates with laser-induced breakdown spectroscopy.

The variable pulse width laser at the US Army Research Laboratory can output a maximum of more than 5 J per pulse at pulse widths between 50 s to 10 ms. Maximum energy output is only achieved by proper alignment and laser operation. This report provides tips for safe operation to avoid damage to parts within the laser and provides details of a technique to align the variable pulse width laser that are not included in the operators manual.

The laser induced breakdown spectroscopy (LIBS) technique has been used to analyze a diverse array of materials for several decades. LIBS is ideal for rapid materials analysis since data can be collected in real time with no sample preparation. The coupling of LIBS with multivariate analysis has increased in recent years and allows for rapid processing of spectral information for qualitative or quantitative analysis. We will discuss several examples of how LIBS and multivariate analysis has been used to classify geological and energetic materials at the United States Army Research Laboratory. It is important to understand the parameters that influence the results and the limitations of multivariate analysis for LIBS applications.

Using a series of thirteen organic materials that includes novel high-nitrogen energetic materials, conventional organic military explosives, and benign organic materials, we have demonstrated the importance of variable selection for maximizing residue discrimination with partial least squares discriminant analysis (PLS-DA). We built several PLS-DA models using different variable sets based on laser induced breakdown spectroscopy (LIBS) spectra of the organic residues on an aluminum substrate under an argon atmosphere. The model classification results for each sample are presented and the influence of the variables on these results is discussed. We found that using the whole spectra as the data input for the PLS-DA model gave the best results. However, variables due to the surrounding atmosphere and the substrate contribute to discrimination when the whole spectra are used, indicating this may not be the most robust model. Further iterative testing with additional validation data sets is necessary to determine the most robust model.

A series of nitrogen-rich molecules, such as 5-aminotetrazolium nitrate (HAT-NO3) and hydrazinebistetrazole (HBT), bis(2,2,2-trinitroethyl)-hydrazodicarboxylate (BTHC), cyclotrimethylene trinitramine (RDX), trinitrotoluene (TNT), melamine, sucrose, and L-glutamine were studied using laser-induced breakdown spectroscopy (LIBS). The atomic emission intensities and intensity ratios of the constituent elements from the LIBS spectrum were shown to correlate with the mole fractions and stoichiometries of the molecules. In addition, the amount of oxygen present in the molecule influenced the emission intensities of molecular fragments such as C2. Finally, we used principal components analysis to analyze the data from the LIBS spectra and separate the different organic molecules based on the atomic emission intensities and ratios.

A large suite of natural carbonate, fluorite and silicate geological materials was studied using laser-induced breakdown spectroscopy (LIBS). Both single- and double-pulse LIBS spectra were acquired using close-contact benchtop and standoff (25 m) LIBS systems. Principal components analysis (PCA) and partial least squares discriminant analysis (PLS-DA) were used to identify the distinguishing characteristics of the geological samples and to classify the materials. Excellent discrimination was achieved with all sample types using PLS-DA and several techniques for improving sample classification were identified. The laboratory double-pulse LIBS system did not provide any advantage for sample classification over the single-pulse LIBS system, except in the case of the soil samples. The standoff LIBS system provided comparable results to the laboratory systems. This work also demonstrates how PCA can be used to identify spectral differences between similar sample types based on minor impurities.

Laser-induced breakdown spectroscopy (LIBS) is a simple atomic emission spectroscopy technique capable of real-time, essentially non-destructive determination of the elemental composition of any substance (solid, liquid, or gas). LIBS, which is presently undergoing rapid research and development as a technology for geochemical analysis, has attractive potential as a field tool for rapid man-portable and/or stand-off chemical analysis. In LIBS, a pulsed laser beam is focused such that energy absorption produces a high-temperature microplasma at the sample surface resulting in the dissociation and ionization of small amounts of material, with both continuum and atomic/ionic emission generated by the plasma during cooling. A broadband spectrometer-detector is used to spectrally and temporally resolve the light from the plasma and record the intensity of elemental emission lines. Because the technique is simultaneously sensitive to all elements, a single laser shot can be used to track the spectral intensity of specific elements or record the broadband LIBS emission spectra, which are unique chemical ‘fingerprints’ of a material. In this study, a broad spectrum of geological materials was analyzed using a commercial bench-top LIBS system with broadband detection from ∼200 to 965 nm, with multiple single-shot spectra acquired. The subsequent use of statistical signal processing approaches to rapidly identify and classify samples highlights the potential of LIBS for ‘geochemical fingerprinting’ in a variety of geochemical, mineralogical, and environmental applications that would benefit from either real-time or in-field chemical analysis.

In this review we discuss the application of laser-induced breakdown spectroscopy (LIBS) to the problem of detection of residues of explosives. Research in this area presented in open literature is reviewed. Both laboratory and field-tested standoff LIBS instruments have been used to detect explosive materials. Recent advances in instrumentation and data analysis techniques are discussed, including the use of double-pulse LIBS to reduce air entrainment in the analytical plasma and the application of advanced chemometric techniques such as partial least-squares discriminant analysis to discriminate between residues of explosives and non-explosives on various surfaces. A number of challenges associated with detection of explosives residues using LIBS have been identified, along with their possible solutions. Several groups have investigated methods for improving the sensitivity and selectivity of LIBS for detection of explosives, including the use of femtosecond-pulse lasers, supplemental enhancement of the laser-induced plasma emission, and complementary orthogonal techniques. Despite the associated challenges, researchers have demonstrated the tremendous potential of LIBS for real-time detection of explosives residues at standoff distances.

The observed ro-vibrational spectral lines of the simplest polyatomic molecule, H þ 3 , serve as the benchmark to test the most rigorous ab initio theory of intra-molecular dynamics. The ground state equilibrium structure of H þ

The ability to detect trace amounts of explosives and their residues in real time is of vital interest to Homeland Security and the military. Previous work at the US Army Research Laboratory (ARL) demonstrated the potential of laser-induced breakdown spectroscopy (LIBS) for the detection of energetic materials. Our recent efforts have been focused on improving the sensitivity and selectivity of LIBS for residue explosives detection and on extending this work to the standoff detection of explosive residues. One difficulty with detecting energetic materials is that the contribution to the oxygen and nitrogen signals from air can impede the identification of the explosive material. Techniques for reducing the air entrainment into the plasma—such as using an argon buffer gas or a collinear double-pulse configuration—have been investigated for this application. In addition to the laboratory studies, ARL’s new double-pulse standoff system (ST-LIBS) has recently been used to detect explosive residues at 20 m. The efficacy of chemometric techniques such as linear correlation, principal components analysis (PCA), and partial least squares discriminant analysis (PLS-DA) for the identification of explosive residues is also discussed. We have shown that despite the typical characterization of LIBS as an elemental technique, the relative elemental intensities in the LIBS spectra are representative of the stoichiometry of the parent molecules and can be used to discriminate materials containing the same elements. Simultaneous biohazard and explosive residue discrimination at standoff distances has also been demonstrated.

A kinetic model of a laser-induced breakdown spectroscopy (LIBS) plume of cyclotrimethylenetrinitramine (RDX) was developed for the analysis of processes responsible for the LIBS signature of explosives. Air and argon were considered as buffer gases. The model includes a set of processes involving ion chemistry, as well as excitation, ionization, and other processes affecting neutral and ion species. Modeling results show that the overall reaction process can be considered a two-stage process. The first stage corresponds to a fast approach to a quasi-stationary state, while the second stage corresponds to the change of quasi-stationary species concentrations due to the change in temperature. As a result of the two-stage process, the initial mechanism of explosive decomposition is not important in determining its signature in the LIBS measurement time window (1–30 μs). The main processes responsible for generation of excited states for the LIBS emission are electron-excitation impact processes. A mechanism for the appearance of a double peak of the C2 species concentration in the RDX plasma plume was suggested. Double-peak behavior of the C2 species was previously experimentally observed during laser ablation of graphite. Published by Elsevier B.V.

Detecting trace explosive residues at standoff distances in real-time is a difficult problem. One method ideally suited for real-time standoff detection is laser-induced breakdown spectroscopy (LIBS). However, atmospheric oxygen and nitrogen contributes to the LIBS signal from the oxygen- and nitrogen-containing explosive compounds, complicating the discrimination of explosives from other organic materials. While bathing the sample in an inert gas will remove atmospheric oxygen and nitrogen interference, it cannot practically be applied for standoff LIBS. Alternatively, we have investigated the potential of double pulse LIBS to improve the discrimination of explosives by diminishing the contribution of atmospheric oxygen and nitrogen to the LIBS signal. These initial studies compare the close-contact (< 1 m) LIBS spectra of explosives using single pulse LIBS in argon with double pulse LIBS in atmosphere. We have demonstrated improved discrimination of an explosive and an organic interferent using double pulse LIBS to reduce the air entrained in the analytical plasma.

We have developed a double-pulse standoff laser-induced breakdown spectroscopy (ST-LIBS) system capable of detecting a variety of hazardous materials at tens of meters. The use of a double-pulse laser improves the sensitivity and selectivity of ST-LIBS, especially for the detection of energetic materials. In addition to various metallic and plastic materials, the system has been used to detect bulk explosives RDX and Composition-B, explosive residues, biological species such as the anthrax surrogate Bacillus subtilis, and chemical warfare simulants at 20 m. We have also demonstrated the discrimination of explosive residues from various interferents on an aluminum substrate.

We report new ab initio calculations of the three-dimensional potential energy surfaces for the Renner-effect coupled X²A₁ ground electronic state and òB₁ first excited electronic state of the CH₂⁺ molecule. We also make an ab initio calculation of the spin-orbit coupling surface Aso(r₁₂, r₃₂, ρ) between these states. Using these ab initio surfaces in our computer program RENNER, we calculate term values and absorption line intensities, and compare with recently observed high resolution spectra. Adjusting two parameters in the potential surfaces we are able to achieve satisfactory agreement with the experimental results except for those that involve the à state (v₂linear = 8, l = 1) vibronic level. The implication of this disagreement is discussed.