Deep ultraviolet resonance Raman excitation enables explosives detection

Discriminating sensing of explosive molecules using graphene-boron nitride-graphene heteronanosheets

Since the synthesis of graphene-boron nitride heterostructures, their interesting electronic properties have attracted huge attention for real-world nanodevice applications. In this work, we combined density functional theory (DFT) with a Green's function approach to examine the potential of graphene-boron nitride-graphene heteronanosheets (h-NSHs) for discriminating single molecule sensing. Our result demonstrates that the graphene-boron nitride-graphene (h-NSHs) can be used for discriminate sensing of the 2,4-dinitrotoluene (DNT), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), pentaerythritol tetranitrate (PENT), and 2,4,6-trinitrotoluene (TNT) molecules. We demonstrate that as the length of the BN region increases, the sensitivity of the heteronanosheets to the presence of these explosive substances increases.

Standoff Detection System Using Raman Spectroscopy in the Deep-Ultraviolet Wavelength Region for the Detection of Hazardous Gas

This study developed a standoff detection system for Raman spectra in the deep-ultraviolet region to facilitate remote detection of various hazardous materials. Although Raman spectroscopy can distinguish various materials, the measurement of Raman spectra through standoff detection is challenging because of the low scattering cross-section of Raman scattering. The resonance Raman scattering effect in the deep-ultraviolet wavelength region is promising in terms of enhancing the spectral intensity of Raman scattering. A catoptric light receiver system was developed to effectively collect deep-ultraviolet light via a change in the distance from the primary to secondary mirror of the telescope. The experimental results for the standoff detection indicate that the system enables the measurement of the Raman spectrum of SO₂ gas, which was locally present 20 m from the system with a wavelength resolution of 0.15 nm. The gas used in this remote measurement has a relatively simple molecular structure among chemical, biological, radiological, nuclear, and explosive gases. However, the high wavelength resolution of Raman spectroscopy will enable measurement of substances with complex molecular structures, such as bacteria and explosives, without losing the detailed structure of their spectra.

UV laser pulse trains for Raman spectroscopy

The theoretical framework for a novel, to the best of our knowledge, stimulated Raman spectroscopy process using a UV probe laser pulse train is formulated and simulated. The laser pulse train consists of multi-femtosecond micro-pulses separated by a varying time duration, having a fixed carrier frequency. The comb-like probe spectrum undergoes self-beating. By appropriately varying the separation time between the micro-pulses, the full Raman spectrum can be excited. We also show that a Raman wakefield, containing the entire Raman signatures of complex molecules, is induced behind the probe pulse train and can be used for additional classification. Kerr and non-resonant effects are included in our model. As an illustration, simulations of the Raman spectrum of a particular pathogen are presented and discussed.

Detection of Explosives by SERS Platform Using Metal Nanogap Substrates

Detecting trace amounts of explosives to ensure personal safety is important, and this is possible by using laser-based spectroscopy techniques. We performed surface-enhanced Raman scattering (SERS) using plasmonic nanogap substrates for the solution phase detection of some nitro-based compounds, taking advantage of the hot spot at the nanogap. An excitation wavelength of 785 nm with an incident power of as low as ≈0.1 mW was used to excite the nanogap substrates. Since both RDX and PETN cannot be dissolved in water, acetone was used as a solvent. TNT was dissolved in water as well as in hexane. The main SERS peaks of TNT, RDX, and PETN were clearly observed down to the order of picomolar concentration. The variations in SERS spectra observed from different explosives can be useful in distinguishing and identifying different nitro-based compounds. This result indicates that our nanogap substrates offer an effective approach for explosives identification.

Study of photo induced charge transfer mechanism of PEDOT with nitro groups of RDX, HMX and TNT explosives using anti-stokes and stokes Raman lines ratios

The paper reports the charge transfer mechanism between poly (3,4-ethylenedioxythiophene) (PEDOT) and high energy materials such as RDX, HMX and TNT, respectively in terms of ratios of anti-stokes (AS) and stokes(S) Raman lines of NO₂ bands. Generally it works as an effective sensing medium for the detection of explosives when mixed in an equal proportion and are subjected to 532 nm wavelength without any chemical treatment [1]. The pristine PEDOT is less sensitive to 532 nm wavelength (2.33 eV) but influences the Raman S and AS lines of explosives in the mixture. The study also reveals that a small quantity (one milligram) of PEDOT is sufficient to initiate the positive charge transfer mechanism between its oxidized state to the lone pairs of electrons on the oxygen atoms of the nitro group of the explosive molecules. Consequently, the intensity of the Raman spectra of RDX, HMX and TNT is dropped by an order of 22.5, 11.45 and 17.2 times, respectively along with the shift of the NO₂ vibrational modes. It is also attributed to Photon-electron-phonon interaction. Finally, we have estimated the reduced mass of the functional group to ascertain the force constant and the intensity ratios of AS /S lines to confirm the charge transfer mechanism. The effect of charge transfer mechanism is also reflected in drastic change in transmission /absorption characteristics of FTIR spectra of same PEDOT and explosive mixtures.

Baseline correction method based on improved asymmetrically reweighted penalized least squares for the Raman spectrum

We present a baseline correction method based on improved asymmetrically reweighted penalized least squares (IarPLS) for the Raman spectrum. This method utilizes a new S-type function to reduce the risk of baseline overestimation and speed up the reweighting process. Simulated spectra with different levels of noise and measured spectra with strong fluorescence background from different samples are used to validate the performance of the proposed algorithm. Considering the drawbacks of the weighting rules for the asymmetrically reweighted penalized least squares (arPLS) method, we adapt an inverse square root unit (ISRU) function, which performs well in baseline correction. Compared with previous penalized least squares methods, such as asymmetric least squares, adaptive iteratively reweighted penalized least squares, and arPLS, experiments with the simulated Raman spectra have confirmed that the proposed method yields better outcomes. Experiments with the measured Raman spectra show that the IarPLS method can improve real Raman spectra within 20 ms. The results show that the proposed method can be successfully applied to the practical Raman spectrum as a strong basis for quantitative analysis.

Optimization of ultraviolet Raman spectroscopy for trace explosive checkpoint screening

Raman spectroscopy has long been considered a gold standard for optically based chemical identification, but has not been adopted in non-laboratory operational settings due to limited sensitivity and slow acquisition times. Ultraviolet (UV) Raman spectroscopy has the potential to address these challenges through the reduction of fluorescence from background materials and increased Raman scattering due to the shorter wavelength (relative to visible or near-infrared excitation) and resonant enhancement effects. However, the benefits of UV Raman must be evaluated against specific operational situations: the actual realized fluorescence reduction and Raman enhancement depend on the specific target materials, target morphology, and operational constraints. In this paper, the wavelength trade-space in UV Raman spectroscopy is evaluated for one specific application: checkpoint screening for trace explosive residues. The optimal UV wavelength is evaluated at 244, 266, and 355 nm for realistic trace explosive and explosive-related compound (ERC) residues on common checkpoint materials: we perform semi-empirical analysis that includes the UV penetration depth of common explosive and ERCs, realistic explosive and ERC residue particle sizes, and the fluorescence signal of common checkpoint materials. We find that while generally lower UV wavelength provides superior performance, the benefits may be significantly reduced depending on the specific explosive and substrate. Further, logistical requirements (size, weight, power, and cost) likely limit the adoption of optimal wavelengths. Graphical abstract.

Deep Ultraviolet Standoff Photoacoustic Spectroscopy of Trace Explosives

We demonstrate deep ultraviolet (UV) photoacoustic spectroscopy (PAS) of trace explosives using a sensitive microphone at meter standoff distances. We directly detect 10 µg/cm² of pentaerythritol tetranitrate (PETN), 2,4,6-trinitrotoluene (TNT), and ammonium nitrate (AN) with 1 s accumulations from a 3 m standoff distance. Large PAS signals for standoff detection are achieved by exciting into the absorption bands of the explosives with a 213 nm laser. We also investigate the impact of the deep UV photochemistry of AN on the PAS signal strength and stability. We find that production of gaseous species during photolysis of AN enhances the PAS signal strength. This deep UV photochemistry can, however, limit the PAS signal lifetimes when detecting trace quantities.

Portable Spectroscopy

Until very recently, handheld spectrometers were the domain of major analytical and security instrument companies, with turnkey analyzers using spectroscopic techniques from X-ray fluorescence (XRF) for elemental analysis (metals), to Raman, mid-infrared, and near-infrared (NIR) for molecular analysis (mostly organics). However, the past few years have seen rapid changes in this landscape with the introduction of handheld laser-induced breakdown spectroscopy (LIBS), smartphone spectroscopy focusing on medical diagnostics for low-resource areas, commercial engines that a variety of companies can build up into products, hyphenated or dual technology instruments, low-cost visible-shortwave NIR instruments selling directly to the public, and, most recently, portable hyperspectral imaging instruments. Successful handheld instruments are designed to give answers to non-scientist operators; therefore, their developers have put extensive resources into reliable identification algorithms, spectroscopic libraries or databases, and qualitative and quantitative calibrations. As spectroscopic instruments become smaller and lower cost, "engines" have emerged, leading to the possibility of being incorporated in consumer devices and smart appliances, part of the Internet of Things (IOT). This review outlines the technologies used in portable spectroscopy, discusses their applications, both qualitative and quantitative, and how instrument developers and vendors have approached giving actionable answers to non-scientists. It outlines concerns on crowdsourced data, especially for heterogeneous samples, and finally looks towards the future in areas like IOT, emerging technologies for instruments, and portable hyphenated and hyperspectral instruments.

Recent Developments in Spectroscopic Techniques for the Detection of Explosives

Trace detection of explosives has been an ongoing challenge for decades and has become one of several critical problems in defense science; public safety; and global counter-terrorism. As a result, there is a growing interest in employing a wide variety of approaches to detect trace explosive residues. Spectroscopy-based techniques play an irreplaceable role for the detection of energetic substances due to the advantages of rapid, automatic, and non-contact. The present work provides a comprehensive review of the advances made over the past few years in the fields of the applications of terahertz (THz) spectroscopy; laser-induced breakdown spectroscopy (LIBS), Raman spectroscopy; and ion mobility spectrometry (IMS) for trace explosives detection. Furthermore, the advantages and limitations of various spectroscopy-based detection techniques are summarized. Finally, the future development for the detection of explosives is discussed.

Two-Dimensional Titanium Nitride (Ti2N) MXene: Synthesis, Characterization, and Potential Application as Surface-Enhanced Raman Scattering Substrate

We report on the synthesis, characterization, and application of Ti₂N (MXene), a two-dimensional transition metal nitride of M₂X type. Synthesis of nitride-based MXenes (M_n+1N_n) is difficult due to their higher formation energy from M_n+1AN_n and poor stability of M_n+1N_n layers in the etchant employed, typically HF. Herein, the selective etching of Al from ternary layered transition metal nitride Ti₂AlN (MAX) and intercalation were achieved by immersing the powder in a mixture of potassium fluoride and hydrochloric acid. The multilayered Ti₂NT_x (T is the surface termination) obtained was sonicated in DMSO and centrifuged to obtain few-layered Ti₂NT_x. MXene formation was verified, and the material was completely characterized by Raman spectroscopy, XRD, XPS, FESEM-EDS, TEM, STM, and AFM techniques. Surface-enhanced Raman scattering (SERS) activity of the synthesized Ti₂NT_x was investigated by fabricating paper, silicon, and glass-based SERS substrates. A Raman enhancement factor of 10¹² was demonstrated using rhodamine 6G as the model compound with 532 nm excitation wavelength. Detection of trace level explosives with a simple paper-based SERS substrate with Ti₂N (MXene) as active material was also illustrated.

Cost-Effective Plasmonic Platforms: Glass Capillaries Decorated with Ag@SiO2 Nanoparticles on Inner Walls as SERS Substrates

A cost-effective method for the fabrication of a glass capillary based plasmonic platform for the selective detection and identification of analytes of importance in health, environment, and safety is demonstrated. This was achieved by coating Ag@SiO₂ nanoparticles (Ag ∼ 60 nm) having silica shell of varying thickness (∼2 and ∼25 nm) on the inside walls of glass capillaries, over 2 cm in length, with uniform coverage. It was found that the particle density on the surface plays a decisive role on the enhancement of Raman signals. Multiple hot spots, which are essentially junctions of amplified electric field, were generated when ∼30 Ag@SiO₂ particles/μm² were bound onto the walls of glass capillaries. The pores of the silica shell allow the localization of analyte molecules to the vicinity of hot spots resulting in signal enhancements of the order of 10¹⁰ (using pyrene as analyte; excitation wavelength, 632.8 nm). The applicability of Ag@SiO₂ coated capillaries for the detection of a wide range of molecules has been explored, by taking representative examples of polyaromatic hydrocarbons (pyrene), amino acids (tryptophan), proteins (bovine serum albumin), and explosives (trinitrotoluene). By increasing the thickness of the silica shell of Ag@SiO₂ nanoparticles, an effective filtration cum detection method has been developed for the selective identification of small molecules such as amino acids, without the interference of large proteins.

Ultraviolet Raman Wide-Field Hyperspectral Imaging Spectrometer for Standoff Trace Explosive Detection

We constructed the first deep ultraviolet (UV) Raman standoff wide-field imaging spectrometer. Our novel deep UV imaging spectrometer utilizes a photonic crystal to select Raman spectral regions for detection. The photonic crystal is composed of highly charged, monodisperse 35.5 ± 2.9 nm silica nanoparticles that self-assemble in solution to produce a face centered cubic crystalline colloidal array that Bragg diffracts a narrow ∼1.0 nm full width at half-maximum (FWHM) UV spectral region. We utilize this photonic crystal to select and image two different spectral regions containing resonance Raman bands of pentaerythritol tetranitrate (PETN) and NH₄NO₃ (AN). These two deep UV Raman spectral regions diffracted were selected by angle tuning the photonic crystal. We utilized this imaging spectrometer to measure 229 nm excited UV Raman images containing ∼10-1000 µg/cm² samples of solid PETN and AN on aluminum surfaces at 2.3 m standoff distances. We estimate detection limits of ∼1 µg/cm² for PETN and AN films under these experimental conditions.

Biomimetic Optical-Filter Detection System for Discrimination of Infrared Chemical Signatures

Optical-filter-based chemical sensors have the potential to dramatically alter the field of hazardous materials sensing. Such devices could be constructed using inexpensive components, in a small and lightweight package, for sensing hazardous chemicals in defense, industrial, and environmental applications. Filter-based sensors can be designed to mimic human color vision. Recent developments in this field have used this approach to discriminate between strongly overlapping chemical signatures in the mid-infrared. Reported work relied on using numerically filtered FTIR spectra to model the infrared biomimetic detection methodology. While these findings are encouraging, further advancement of this technique requires the collection and evaluation of directly filtered data, using an optical system without extensive numerical spectral analysis. The present work describes the design and testing of an infrared optical breadboard system that uses the biomimetic mammalian color-detection approach to chemical sensing. The set of chemicals tested includes one target chemical, fuel oil, along with two strongly overlapping interferents, acetone and hexane. The collected experimental results are compared with numerically filtered FTIR spectral data. The results show good agreement between the numerically filtered data model and the data collected using the optical breadboard system. It is shown that the optical breadboard system is operating as expected based on modeling and can be used for sensing and discriminating between chemicals with strongly overlapping absorption bands in the mid-infrared.

Narrow-band 1 W source at 257 nm using frequency quadrupled passively Q-switched Yb:YAG laser

We describe generation of 1.1 W of 257 nm emission by frequency quadrupling the 1030 nm emission from a compact passively Q-switched Yb:YAG laser. The laser utilized a volume Bragg grating to achieve a 0.1 nm linewidth required for UV-Raman spectroscopic applications, generated 100 kW peak power, 250 μJ pulses and 3.6 W of average power at 1030 nm. Fourth harmonic generation (FHG) was carried out using a 10 mm lithium triborate (LBO) crystal to generate 515 nm second harmonic with 70% conversion efficiency, followed by a 7 mm beta-barium borate (BBO) crystal to generate 257 nm fourth harmonic with 45% efficiency, resulting in an overall nonlinear conversion efficiency of 31%. Far-field and near-field of the FHG emission were characterized.

Portable Deep-Ultraviolet (DUV) Raman for Standoff Detection

Alakai Defense Systems has recently developed a man-portable ultraviolet Raman spectrometer system. The portable Raman improvised explosives detector was designed to provide rapid, standoff detection of chemicals of interest to the end user, including, but not limited to explosives, narcotics, toxic industrial chemicals, and toxic industrial materials. In this paper, we discuss general aspects of the system design and user interface. Spectral and instrument performance data are shown for several common materials involved in narcotics manufacture, as well as cocaine and heroin, with comparisons to currently marketed handheld Raman instruments.

Detection of Explosives Using Differential Laser-Induced Perturbation Spectroscopy with a Raman-based Probe

Explosives detection is carried out with a novel spectral analysis technique referred to as differential laser-induced perturbation spectroscopy (DLIPS) on thin films of TNT, RDX, HMX, and PETN. The utility of Raman spectroscopy for detection of explosives is enhanced by inducing deep ultraviolet laser perturbation on molecular structures in combination with a differential Raman sensing scheme. Principal components analysis (PCA) is used to quantify the DLIPS method as benchmarked against a traditional Raman scattering probe, and the related photo-induced effects on the molecular structure of the targeted explosives are discussed in detail. Finally, unique detection is observed with TNT samples deposited on commonly available background substrates of nylon and polyester. Overall, the data support DLIPS as a noninvasive method that is promising for screening explosives in real-world environments and backgrounds.

Single Broadband Phase-Shaped Pulse Stimulated Raman Spectroscopy for Standoff Trace Explosive Detection

Recent success with trace explosives detection based on the single ultrafast pulse excitation for remote stimulated Raman scattering (SUPER-SRS) prompts us to provide new results and a Perspective that describes the theoretical foundation of the strategy used for achieving the desired sensitivity and selectivity. SUPER-SRS provides fast and selective imaging while being blind to optical properties of the substrate such as color, texture, or laser speckle. We describe the strategy of combining coherent vibrational excitation with a reference pulse in order to detect stimulated Raman gain or loss. A theoretical model is used to reproduce experimental spectra and to determine the ideal pulse parameters for best sensitivity, selectivity, and resolution when detecting one or more compounds simultaneously.

Proximal Detection of Traces of Energetic Materials with an Eye-Safe UV Raman Prototype Developed for Civil Applications

A new Raman-based apparatus for proximal detection of energetic materials on people, was developed and tested for the first time. All the optical and optoelectronics components of the apparatus, as well as their optical matching, were carefully chosen and designed to respect international eye-safety regulations. In this way, the apparatus is suitable for civil applications on people in public areas such as airports and metro or railway stations. The acquisition software performs the data analysis in real-time to provide a fast response to the operator. Moreover, it allows for deployment of the apparatus either as a stand alone device or as part of a more sophisticated warning system architecture made up of several sensors. Using polyamide as substrate, the apparatus was able to detect surface densities of ammonium nitrate (AN), 2-methyl-1,3,5-trinitrobenzene (TNT), 3-nitrooxy-2,2-bis(nitrooxymethyl)propyl] nitrate (PETN) and urea nitrate (UN) in the range of 100–1000 μg/cm² at a distance of 6.4 m using each time a single laser pulse of 3 mJ/cm². The limit of detection calculated for AN is 289 μg/cm². AN and UN provided the highest percentages of true positives (>82% for surface densities of 100–400 μg/cm² and fingerprints) followed by TNT and PETN (17%–70% for surface densities of 400–1000 μg/cm² and fingerprints).

Advances in explosives analysis--part II: photon and neutron methods

The number and capability of explosives detection and analysis methods have increased dramatically since publication of the Analytical and Bioanalytical Chemistry special issue devoted to Explosives Analysis [Moore DS, Goodpaster JV, Anal Bioanal Chem 395:245-246, 2009]. Here we review and critically evaluate the latest (the past five years) important advances in explosives detection, with details of the improvements over previous methods, and suggest possible avenues towards further advances in, e.g., stand-off distance, detection limit, selectivity, and penetration through camouflage or packaging. The review consists of two parts. Part I discussed methods based on animals, chemicals (including colorimetry, molecularly imprinted polymers, electrochemistry, and immunochemistry), ions (both ion-mobility spectrometry and mass spectrometry), and mechanical devices. This part, Part II, will review methods based on photons, from very energetic photons including X-rays and gamma rays down to the terahertz range, and neutrons.

Compact Solid-State 213 nm Laser Enables Standoff Deep Ultraviolet Raman Spectrometer: Measurements of Nitrate Photochemistry

We describe a new compact acousto-optically Q-switched diode-pumped solid-state (DPSS) intracavity frequency-tripled neodymium-doped yttrium vanadate laser capable of producing ~100 mW of 213 nm power quasi-continuous wave as 15 ns pulses at a 30 kHz repetition rate. We use this new laser in a prototype of a deep ultraviolet (UV) Raman standoff spectrometer. We use a novel high-throughput, high-resolution Echelle Raman spectrograph. We measure the deep UV resonance Raman (UVRR) spectra of solid and solution sodium nitrate (NaNO3) and ammonium nitrate (NH4NO3) at a standoff distance of ~2.2 m. For this 2.2 m standoff distance and a 1 min spectral accumulation time, where we only monitor the symmetric stretching band, we find a solid state NaNO3 detection limit of ~100 μg/cm(2). We easily detect ~20 μM nitrate water solutions in 1 cm path length cells. As expected, the aqueous solutions UVRR spectra of NaNO3 and NH4NO3 are similar, showing selective resonance enhancement of the nitrate (NO3(-)) vibrations. The aqueous solution photochemistry is also similar, showing facile conversion of NO3(-) to nitrite (NO2(-)). In contrast, the observed UVRR spectra of NaNO3 and NH4NO3 powders significantly differ, because their solid-state photochemistries differ. Whereas solid NaNO3 photoconverts with a very low quantum yield to NaNO2, the NH4NO3 degrades with an apparent quantum yield of ~0.2 to gaseous species.

A new eye-safe UV Raman spectrometer for the remote detection of energetic materials in fingerprint concentrations: Characterization by PCA and ROC analyzes

We report the results of proximal Raman investigations at a distance of 7 m, to detect traces of explosives (from 0.1 to 0.8 mg/cm(2)) on common clothes with a new eye-safe apparatus. The instrument excites the target with a single laser shot of few ns (10(-9)s) in the UV range (laser wavelength 266 nm) detecting energetic materials like Pentaerythritol tetranitrate (PETN), Trinitrotoluene (TNT), Urea Nitrate (UN) and Ammonium Nitrate (AN). Samples were prepared using a piezoelectric-controlled plotter device to realize well-calibrated amounts of explosives on several cm(2). Common fabrics and tissues such as polyester, polyamide and leather were used as substrates, representative of base-materials used in the production of jackets or coats. Other samples were prepared by touching the substrate with a silicon finger contaminated with explosives, to simulate a spot left by contaminated hands on a jacket or bag during the preparation of an improvised explosive device (IED) by a terrorist. The observed Raman signals showed some peculiar molecular bands of the analyzed compounds, allowing us to identify and discriminate them with high sensitivity and selectivity, also in presence of the interfering signal from the underlying fabric. A dedicated algorithm was developed to remove noise and fluorescence background from the single laser shot spectra and an automatic spectral recognition procedure was also implemented, evaluating the intensity of the characteristic Raman bands of each explosive and allowing their automatic classification. Principal component analysis (PCA) was used to show the discrimination potentialities of the apparatus on different sets of explosives and to highlight possible criticalities in the detection. Receiver operating characteristic (ROC) curves were used to discuss and quantify the sensitivity and the selectivity of the proposed recognition procedure. To our knowledge the developed device is at the highest sensitivity nowadays achievable in the field of eye-safe, Raman devices for proximal detection.

Solution and Solid Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) Ultraviolet (UV) 229 nm Photochemistry

We measured the 229 nm deep-ultraviolet resonance Raman (DUVRR) spectra of solution and solid-state hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). We also examined the photochemistry of RDX both in solution and solid states. RDX quickly photodegrades with a solution quantum yield of φ ~ 0.35 as measured by high-performance liquid chromatography (HPLC). New spectral features form over time during the photolysis of RDX, indicating photoproduct formation. The photoproduct(s) show stable DUVRR spectra at later irradiation times that allow standoff detection. In the solution-state photolysis, nitrate is a photoproduct that can be used as a signature for detection of RDX even after photolysis. We used high-performance liquid chromatography-high-resolution mass spectrometry (HPLC-HRMS) and gas chromatography mass spectrometry (GCMS) to determine some of the major solution-state photoproducts. X-ray photoelectron spectroscopy (XPS) was also used to determine photoproducts formed during solid-state RDX photolysis.

A simplified scheme for generating narrow-band mid-ultraviolet laser radiation

We report on the development and characterization of continuous, narrow-band, and tunable laser systems that use direct second-harmonic generation from blue and green diode lasers with an output power level of up to 11.1 mW in the mid-ultraviolet. One of our laser systems was tuned to the mercury 6(1)S0 → 6(3)P1 intercombination line at 253.7 nm. We could perform Doppler-free saturation spectroscopy on this line and were able to lock our laser to the transition frequency on long time scales.

Dependence of Raman and resonance Raman intensities on sample self-absorption

Resonance Raman cross sections are generally larger than normal or preresonance Raman cross sections. Thus, higher Raman intensities are expected for resonance excitation, especially for backscattering measurements. However, self absorption decreases the observed Raman intensities. In the work here we examine the effect of self absorption on the observed preresonance and resonance Raman intensities. For the simplest case where a single electronic transition dominates the Raman scattering, and where the resonance enhancement scales with the square of the molar absorptivity of the absorption band, theory predicts that for close to resonance excitation the observed Raman intensities monotonically increase as resonance is approached. In the case that an impurity absorbs, the observed Raman intensities may decrease as excitation moves close to resonance for particular conditions of impurity absorption band widths and frequency offsets. Impurity absorption also causes decreases in observed Raman intensities for the more slowly increasing preresonance excitation.

The mechanism of 2,4,6-trinitrotoluene detection with amino acid-capped quantum dots: a density functional theory study

Both a Meisenheimer complex and deprotonated TNT are formed when TNT meets amino acid-capped quantum dots.

Towards deep-UV surface-enhanced resonance Raman spectroscopy of explosives: ultrasensitive, real-time and reproducible detection of TNT

We report ultrasensitive and label-free detection of 2,4,6-trinitrotoluene (TNT) deposited by drop coating using deep-ultraviolet surface-enhanced resonance Raman scattering (DUV-SERRS).

Recent advances and remaining challenges for the spectroscopic detection of explosive threats

In 2010, the U.S. Army initiated a program through the Edgewood Chemical Biological Center to identify viable spectroscopic signatures of explosives and initiate environmental persistence, fate, and transport studies for trace residues. These studies were ultimately designed to integrate these signatures into algorithms and experimentally evaluate sensor performance for explosives and precursor materials in existing chemical point and standoff detection systems. Accurate and validated optical cross sections and signatures are critical in benchmarking spectroscopic-based sensors. This program has provided important information for the scientists and engineers currently developing trace-detection solutions to the homemade explosive problem. With this information, the sensitivity of spectroscopic methods for explosives detection can now be quantitatively evaluated before the sensor is deployed and tested.

Solution and solid trinitrotoluene (TNT) photochemistry: persistence of TNT-like ultraviolet (UV) resonance Raman bands

We examined the 229 nm deep-ultraviolet resonance Raman (DUVRR) spectra of solution and solid-state trinitrotoluene (TNT) and its solution and solid-state photochemistry. Although TNT photodegrades with a solution quantum yield of ϕ ∼ 0.015, the initial photoproducts show DUVRR spectra extraordinarily similar to pure TNT, due to the similar photoproduct enhancement of the -NO2 stretching vibrations. This results in TNT-like DUVRR spectra even after complete TNT photolysis. These ultraviolet resonance Raman spectral bands enable DUVRR of trace as well as DUVRR standoff TNT detection. We determined the structure of various initial TNT photoproducts by using liquid chromatography-mass spectrometry and tandem mass spectrometry. Similar TNT DUVRR spectra and photoproducts are observed in the solution and solid states.

Surface-enhanced Raman scattering detection of ammonium nitrate samples fabricated using drop-on-demand inkjet technology

The United States Army and the first responder community are increasingly focusing efforts on energetic materials detection and identification. Main hazards encountered in theater include homemade explosives and improvised explosive devices, in part fabricated from simple components like ammonium nitrate (AN). In order to accurately detect and identify these unknowns (energetic or benign), fielded detection systems must be accurately trained using well-understood universal testing substrates. These training substrates must contain target species at known concentrations and recognized polymorphic phases. Ammonium nitrate is an explosive precursor material that demonstrates several different polymorphic phases dependent upon how the material is deposited onto testing substrates. In this paper, known concentrations of AN were uniformly deposited onto commercially available surface-enhanced Raman scattering (SERS) substrates using a drop-on-demand inkjet printing system. The phase changes observed after the deposition of AN under several solvent conditions are investigated. Characteristics of the collected SERS spectra of AN are discussed, and it is demonstrated that an understanding of the exact nature of the AN samples deposited will result in an increased ability to accurately and reliably "train" hazard detection systems.

Raman Spectroscopy for Homeland Security Applications

Raman spectroscopy is an analytical technique with vast applications in the homeland security and defense arenas. The Raman effect is defined by the inelastic interaction of the incident laser with the analyte molecule’s vibrational modes, which can be exploited to detect and identify chemicals in various environments and for the detection of hazards in the field, at checkpoints, or in a forensic laboratory with no contact with the substance. A major source of error that overwhelms the Raman signal is fluorescence caused by the background and the sample matrix. Novel methods are being developed to enhance the Raman signal’s sensitivity and to reduce the effects of fluorescence by altering how the hazard material interacts with its environment and the incident laser. Basic Raman techniques applicable to homeland security applications include conventional (off-resonance) Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), resonance Raman spectroscopy, and spatially or temporally offset Raman spectroscopy (SORS and TORS). Additional emerging Raman techniques, including remote Raman detection, Raman imaging, and Heterodyne imaging, are being developed to further enhance the Raman signal, mitigate fluorescence effects, and monitor hazards at a distance for use in homeland security and defense applications.

Comparison of visible and near-infrared Raman cross-sections of explosives in solution and in the solid state

Raman cross-sections of explosives in solution and in the solid state have been measured using visible and near-infrared excitation via secondary calibration. These measurements are valuable for both fundamental scientific purposes and applications in the standoff detection of explosives. The explosive compounds RDX, HMX, TNT, 2,4-DNT, 2,6-DNT, and ammonium nitrate were measured using discrete excitation wavelengths ranging from 532 nm to 785 nm. A comparison of the spectral features and cross-sections between the solid state and solution was performed. Comparison is also made to cross-sections measured with deep ultraviolet excitation.

Resonance-Enhanced Raman Spectroscopy on Explosives Vapor at Standoff Distances

Resonance-enhanced Raman spectroscopy has been used to perform standoff measurements on nitromethane (NM), 2,4-DNT, and 2,4,6-TNT in vapor phase. The Raman cross sections for NM, DNT, and TNT in vapor phase have been measured in the wavelength range 210–300 nm under laboratory conditions, in order to estimate how large resonance enhancement factors can be achieved for these explosives. The results show that the signal is enhanced up to 250,000 times for 2,4-DNT and up to 60,000 times for 2,4,6-TNT compared to the nonresonant signal at 532 nm. Realistic outdoor measurements on NM in vapor phase at 13 m distance were also performed, which indicate a potential for resonance Raman spectroscopy as a standoff technique for detection of vapor phase explosives. In addition, the Raman spectra of acetone, ethanol, and methanol were measured at the same wavelengths, and their influence on the spectrum from NM was investigated.

Refractive-index matching avoids local field corrections and scattering bias in solid-state Na2SO4 ultraviolet Raman cross-section measurements

We report a refractive-index matching method to measure nonabsorbing solid ultraviolet (UV) Raman cross-sections that avoids the local field correction and interface scattering of incident light. We used refractive-index-matched chloroform as an internal standard to determine the solid-state 995 cm(-1) Na(2)SO(4) 244 nm Raman cross-sections. The pure liquid chloroform 668 cm(-1) 244 nm Raman cross-section was determined by using acetonitrile as an internal standard and by calculating the local field corrections for the observed Raman intensities. Our measured 244 nm UV Raman cross-section of the solid-state 995 cm(-1) SO4(2-) band of 1.97 ± 0.07 × 10(-28) cm(2)/(molc·sr) is about half of its aqueous solution Raman cross-section, indicating interactions between the sulfate species in the solid that decrease the Raman polarizability.