Noncontact detection of homemade explosive constituents via photodissociation followed by laser-induced fluorescence

Deep ultraviolet laser at 223.8 nm with adjustable repetition rate and narrow pulse width

We presented the first, to our knowledge, demonstration of an ultraviolet (UV) laser at 223.8 nm by six-harmonic generation of an electro-optic Q-switched cavity dumping 1342 nm Nd:YVO4 laser. It offers high power, constant short pulse duration, and adjustable pulse repetition rate. The pulse duration is independent of the pump power and repetition rate compared to classical Q-switched oscillators. The output efficiency of the UV laser is optimized by adjusting the focusing lens. With the incident pump power of 30 W, an maximum average output power of 249 mW was obtained at 13 kHz. The pulse width maintained 3.4-3.5 ns from 5 to 20 kHz. The maximum pulse energy of 28.1 µJ was obtained at 5 kHz, and the corresponding peak power was up to 8.1 kW.

Watt-level deep-UV subnanosecond laser system based on Nd-doped fiber at 229 nm

We report an efficient deep-UV master-oscillator power amplifier (MOPA) laser system at 229 nm that generates 350 ps pulses at 2 MHz repetition rate with an average power of 1.2 W. The use of a polarization-maintaining large mode area neodymium-doped fiber operating on the ⁴F_3/2→⁴I_9/2 transition allows high-power laser emission of up to 28 W near 915 nm in the sub-nanosecond regime with low spectral broadening. Two nonlinear frequency conversion stages (LBO + BBO crystals) in a single-pass configuration directly convert the IR laser emission to deep UV. This laser demonstrates the great potential of Nd³⁺-doped fiber lasers to produce high-power deep-UV emission.

Laser-induced fluorescence of PO-photofragments of dimethyl methylphosphonate

The paper presents the results of calculating the absorption spectrum of a phosphorus monoxide (PO) molecule corresponding to the A²Σ⁺(v^'=0)-X²Π(v^''=0) transition. The efficiency of excitation of the PO molecule is estimated as a function of the spectral parameters of the laser radiation. The positions of the fluorescence bands of PO are calculated. It is shown that the use of excitation wavelengths near the bandheads of the (P₂₂+Q₁₂) and P₁₂ branches of the A²Σ⁺(v^'=0)-X²Π_3/2(v^''=0) band provides spectral separation of the (0, 1) γ-band of the LIF of PO and vibrational-rotational Raman spectrum of atmospheric oxygen. The spectral responses of dimethyl methylphosphonate vapor in air under the action of KrF-laser radiation at a wavelength of 247.78 nm have been experimentally studied. In the wavelength range 252-260 nm, the (0, 1) γ-band of the LIF of PO-fragments and the vibrational-rotational band of spontaneous Raman scattering on oxygen molecules are unambiguously interpreted. It is shown that the results of calculations of the shape and position of the fluorescence spectra are in good agreement with the experimental data.

Evaluation of Limiting Sensitivity of the One-Color Laser Fragmentation/Laser-Induced Fluorescence Method in Detection of Nitrobenzene and Nitrotoluene Vapors in the Atmosphere

The paper presents the results of a numerical evaluation of limiting sensitivity of the method for detecting vapors of nitrocompounds in the atmosphere based on one-color laser fragmentation (LF)/laser-induced fluorescence (LIF) of NO fragments via A2Σ+ (v′ = 0) ← X2Π (v″ = 2) transition. The calculations were performed using the developed kinetic model of the one-color LF/LIF process under consideration. The calculations take into account the influence of ambient nitrogen dioxide as a limiter of the sensitivity of the method when operating in a real atmosphere. It is shown that if the nitrogen dioxide concentration in the atmosphere does not exceed a value of 10 ppb, the maximum detectable vapor concentrations of nitrobenzene and o-nitrotoluene are several ppb. It is also shown that the method of single-frequency one-color excitation usually used for the detection of nitrocompounds does not allow achieving the maximum efficiency of the LF/LIF process.

Two-pulse laser fragmentation/laser-induced fluorescence of nitrobenzene and nitrotoluene vapors

This paper presents the results of an experimental study of the dynamic characteristics of the laser fragmentation/laser-induced fluorescence (LF/LIF) process in nitrobenzene and para-nitrotoluene vapors under synchronized two-pulse laser irradiation. It is shown that if the values of the time delay between the pulses of fragmentation (248.4 nm) and excitation (247.87 nm) of NO fragments are in the range of 20-40 ns, the efficiency of the LF/LIF method can be increased several times.

Non-Destructive Trace Detection of Explosives Using Pushbroom Scanning Hyperspectral Imaging System

The aim of this study was to investigate the potential of the non-destructive hyperspectral imaging system (HSI) and accuracy of the model developed using Support Vector Machine (SVM) for determining trace detection of explosives. Raman spectroscopy has been used in similar studies, but no study has been published which is based on measurement of reflectance from hyperspectral sensor for trace detection of explosives. HSI used in this study has an advantage over existing techniques due to its combination of imaging system and spectroscopy, along with being contactless and non-destructive in nature. Hyperspectral images of the chemical were collected using the BaySpec hyperspectral sensor which operated in the spectral range of 400–1000 nm (144 bands). Image processing was applied on the acquired hyperspectral image to select the region of interest (ROI) and to extract the spectral reflectance of the chemicals which were stored as spectral library. Principal Component Analysis (PCA) and first derivative was applied to reduce the high dimensionality of the image and to determine the optimal wavelengths between 400 and 1000 nm. In total, 22 out of 144 wavelengths were selected by analysing the loadings of principal components (PC). SVM was used to develop the classification model. SVM model established on the whole spectrum from 400 to 1000 nm achieved an accuracy of 81.11%, whereas an accuracy of 77.17% with less computational load was achieved when SVM model was established on the optimal wavelengths selected. The results of the study demonstrate that the hyperspectral imaging system along with SVM is a promising tool for trace detection of explosives.

Dynamics of the laser fragmentation/laser-induced fluorescence process in nitrobenzene vapors

The paper presents the results of an experimental study of the dynamic characteristics of the laser fragmentation/laser-induced fluorescence (LF/LIF) effects in nitrobenzene vapors under the separate initiation of processes of photofragmentation and fluorescence of fragments by nanosecond laser pulses. It is shown that, due to the inertia of the dissociation mechanism of nitrobenzene molecules, the process of the fragments' formation continues even after letup of excitation. The highest concentration of fragments is reached in a time several times greater than the standard fragmentation pulse duration of 10 ns. A kinetic model is presented that allows one to trace the temporal dynamics of the LF/LIF process of nitrobenzene vapors under separate excitation. A good agreement between the experimental data and the results of calculation indicates the adequacy of application of the developed kinetic model for describing the LF/LIF process. The information obtained in the experiment made it possible to clarify the values of the rate constants of the nitrobenzene dissociation.

Femtosecond laser induced breakdown spectroscopy based standoff detection of explosives and discrimination using principal component analysis

We report the standoff (up to ~2 m) and remote (~8.5 m) detection of novel high energy materials/explosive molecules (Nitroimidazoles and Nitropyrazoles) using the technique of femtosecond laser induced breakdown spectroscopy (LIBS). We utilized two different collection systems (a) ME-OCT-0007 (commercially available) and (b) Schmidt-Cassegrain telescope for these experiments. In conjunction with LIBS data, principal component analysis was employed to discriminate/classify the explosives and the obtained results in both configurations are compared. Different aspects influencing the LIBS signal strength at far distances such as fluence at target, efficiency of collection system etc. are discussed.

Toward wearable sensors: optical sensor for detection of ammonium nitrate-based explosives, ANFO and ANNM

An imine-functionalized polymer displays selective fluorimetric response to the component of ANFO and ANNM, ammonium nitrate and nitromethane!

Use of photoacoustic excitation and laser vibrometry to remotely detect trace explosives

In this paper, we examine a laser-based approach to remotely initiate, measure, and differentiate acoustic and vibrational emissions from trace quantities of explosive materials against their environment. Using a pulsed ultraviolet laser (266 nm), we induce a significant (>100  Pa) photoacoustic response from small quantities of military-grade explosives. The photoacoustic signal, with frequencies predominantly between 100 and 500 kHz, is detected remotely via a wideband laser Doppler vibrometer. This two-laser system can be used to rapidly detect and discriminate explosives from ordinary background materials, which have significantly weaker photoacoustic response. A 100  ng/cm² limit of detection is estimated. Photoablation is proposed as the dominant mechanism for the large photoacoustic signals generated by explosives.

Less is more: Avoiding the LIBS dimensionality curse through judicious feature selection for explosive detection

Despite its intrinsic advantages, translation of laser induced breakdown spectroscopy for material identification has been often impeded by the lack of robustness of developed classification models, often due to the presence of spurious correlations. While a number of classifiers exhibiting high discriminatory power have been reported, efforts in establishing the subset of relevant spectral features that enable a fundamental interpretation of the segmentation capability and avoid the ‘curse of dimensionality’ have been lacking. Using LIBS data acquired from a set of secondary explosives, we investigate judicious feature selection approaches and architect two different chemometrics classifiers –based on feature selection through prerequisite knowledge of the sample composition and genetic algorithm, respectively. While the full spectral input results in classification rate of ca.92%, selection of only carbon to hydrogen spectral window results in near identical performance. Importantly, the genetic algorithm-derived classifier shows a statistically significant improvement to ca. 94% accuracy for prospective classification, even though the number of features used is an order of magnitude smaller. Our findings demonstrate the impact of rigorous feature selection in LIBS and also hint at the feasibility of using a discrete filter based detector thereby enabling a cheaper and compact system more amenable to field operations.

Deep-UV 236.5 nm laser by fourth-harmonic generation of a single-crystal fiber Nd:YAG oscillator

We demonstrate a deep-UV laser at 236.5 nm based on extracavity fourth-harmonic generation of a Q-switched Nd:YAG single-crystal fiber laser at 946 nm. We first compare two nonlinear crystals available for second-harmonic generation: LBO and BiBO. The best results at 473 nm are obtained with a BiBO crystal, with an average output power of 3.4 W at 20 kHz, corresponding to a second-harmonic generation efficiency of 38%. This blue laser is frequency-converted to 236.5 nm in a BBO crystal with an overall fourth-harmonic generation yield of 6.5%, corresponding to an average output power of 600 mW at 20 kHz. This represents an order of magnitude increase in average power and energy compared to previously reported pulsed lasers at 236.5 nm. This work opens the possibility of LIDAR detection of dangerous compounds for military or civilian applications.

Spectroscopic observation of neutral carbon during photodissociation of explosive-related compounds in the vapor phase

We perform time-resolved laser-induced fluorescence measurements of mononitrotoluenes (MNTs) and dinitrotoluenes (DNTs) in nitrogen and air. We observe the multipeak emission spectrum of NO and find that the emission peak intensity in the 247-248 nm range is stronger than expected compared to the other NO emission peak intensities. This increased emission intensity is believed to be due to neutral carbon [C(I)], which has a strong emission peak at 247.85 nm. By comparing the ratios of integrated emission peak intensities with those expected from the Franck-Condon factors for NO, we are able to identify samples that exhibit C(I) emission. We show that the DNTs exhibit C(I) emission for gate delays of 1500 ns and beyond, while the MNTs exhibit C(I) emission for gate delays of only up to about 500 ns. Carbon deposits in the analysis chamber confirm the presence of C. Ambient NO in air enhances the observed NO+C(I) signal from MNTs and DNTs.

Rapid and specific detection of urea nitrate and ammonium nitrate by electrospray ionization time-of-flight mass spectrometry using infusion with crown ethers

Urea nitrate (UN) and ammonium nitrate (AN) are fertilizer-based explosives that are commonly used in improvised munitions and can have highly destructive effects. Because they are in the form of salts, their relatively low volatility makes them difficult to detect at trace levels. In addition, these salts readily undergo metathetic reactions in water to form urea, ammonium and nitrate, which are ubiquitous in the environment. Thus, selective methods are needed for their detection. In this study, a procedure was developed to detect UN and AN in non-aqueous environments by positive ion electrospray ionization mass spectrometry through the addition of 18-crown-6. The method is sensitive, with detection limits under 2 μM, and selective. The procedure is capable of differentiating urea from uronium ions (protonated urea) and a mixture of urea and AN did not interfere with the UN signal. The procedure is quite versatile and the addition of crown ethers to the sample matrix does not interfere with the detection of high explosives in the negative ionization mode. Experimental results are presented on the utilization of the method in the detection of UN and AN on various surfaces. Semi-quantitative studies showed that AN and UN can be detected at trace levels following finger transfer and a series of studies were performed to demonstrate the effect of various interferences. The results show the method to be a quick and robust procedure for trace detection.

Noncontact optical detection of explosive particles via photodissociation followed by laser-induced fluorescence

High-sensitivity (ng/cm²) optical detection of the explosive 2,4,6-trinitrotoluene (TNT) is demonstrated using photodissociation followed by laser-induced fluorescence (PD-LIF). Detection occurs rapidly, within 6 laser pulses (~7 ns each) at a range of 15 cm. Dropcasting is used to create calibrated samples covering a wide range of TNT concentrations; and a correspondence between fractional area covered by TNT and PD-LIF signal strength is observed. Dropcast data are compared to that of an actual fingerprint. These results demonstrate that PD-LIF could be a viable means of rapidly and remotely scanning surfaces for trace explosive residues.